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\def\pcht{70}
\def\HF{Henry Foulon}
\def\JP{Jean Prieur}
\def\SL{Serge Lewy}
\def\DG{Denis Gely}
\def\SP{Simone Pauzin}
\def\FS{Frank Simon}
\def\DB{Daniel Berge}
\def\JJ{Jacques Julliard}
\def\UM{Ulf Michel}
\def\JD{Jan Delfs}
\def\HH{Hanno Heller}
\def\OR{Olaf Recker}
\def\GN{G\"unther Neuwerth}
\def\DJ{Dieter Jacob}
\def\GA{Gerhard Algermissen}
\def\SW{Siegfried Wagner}
\def\GG{Gianfranco Guidati}
\def\PdF{Paolo di Francescantonio}
\def\SI{Sandro Ianniello}
\def\PR{Piergiovanni Renzoni}
\def\AP{Antonio Pagano}
\def\FM{Francesco Marulo}
\def\SdR{Sergio de Rosa}
\def\MG{Massimo Gennaretti}
\def\AdB{Andr\'e de Boer}
\def\TD{Ton Dassen}
\def\FPG{Frank P.\,Grooteman}
\def\JBHMS{Johan B.H.M.\,Schulten}
\def\FJMvdE{Frits J.M.\,van der Eerden}
\def\HEdB{Hans-Elias de Bree}
\def\HT{Henk Tijdeman}
\def\PL{Pedro Luengo}
\def\JLR{Jose-Luis Rioboo}
\def\HC{Hector Climent}
\def\EC{Emilio Campos}
\def\PGF{Pablo Garc\'{\i}a-Fogeda}
\def\FdlI{Fernando de la Iglesia}
\def\SC{Steve Chow}
\def\NP{Nigel Peake}
\def\RAP{Richard A.\,Pinker}
\def\BJT{Brian J.\,Tester}
\def\LI{Len Ilston}
\def\DP{D.E.\,Patience}
\def\RW{Bob G.\,White}
\def\RL{Robin S.\,Langley}
\def\MS{Malcolm G.\,Smith}
\def\KHH{Ken H.\,Heron}
\def\XL{Xiaodong Li}


\begin{document}
\begin{center}
{\LARGE  AEROACOUSTICS RESEARCH IN EUROPE:\\[4mm]
         THE CEAS-ASC REPORT ON 1997 HIGHLIGHTS}
\end{center}

\begin{center}
 {\large \it An Annual Summary of Aeroacoustics Research in the 8
           CEAS-Countries\\
           by the Members of the CEAS Aeroacoustics
           Specialists' Committee}
\end{center}
\begin{center}
       {{\sl edited by:} Sjoerd W.\ Rienstra \\
       \small \it Eindhoven University of Technology, Netherlands}
\end{center}
\begin{minipage}[t]{\textwidth}
  {\sl with contributions from:}
  \HF{ }- {\it CEAT};\quad
  \JP, \SL, \DG, \SP, \FS{ }- {\it ONERA};\quad
  \DB, \JJ{ }- {\it SNECMA};\quad
  \UM, \JD, \XL, \HH{ }- {\it DLR};\quad
  \OR, \GN, \DJ{ }- {\it RWTH~Aachen};\quad
  \GA, \SW, \GG{ }- {\it Stuttgart~University};\quad
  \PdF{ }- {\it AGUSTA};\quad
  \SI, \PR, \AP{ }- {\it CIRA};\quad
  \FM, \SdR{ }- {\it Naples~University};\quad
  \MG{ }- {\it Rome~University~3};\quad
  \AdB, \TD, \FPG, \JBHMS{ }- {\it NLR};\quad
  \FJMvdE, \HEdB, \HT{ }- {\it Twente University};\quad
  \PL, \JLR, \HC{ }- {\it CASA};\quad
  \EC{ }- {\it INTA};\quad
  \PGF, \FdlI{ }- {\it Madrid~Polytechn.~University};\quad
  \SC{ }- {\it BAe~Airbus};\quad
  \LI, \DP{ }- {\it BAe~Defence};\quad
  \NP{ }- {\it Cambridge~University};\quad
  \RAP{ }- {\it DERA~Pyestock};\quad
  \BJT{ }- {\it Rolls-Royce plc};\quad
  \RW, \RL, \MS{ }- {\it Southampton~University}; \quad
  \KHH{ }- {\it DERA~Farnborough}.
\end{minipage}

\bigskip
{\small Key words: \it Aeroacoustics}\\

\section{INTRODUCTION}

In 1995, the Confederation of European Aero\-space Societies
(CEAS)\footnote{CEAS = Aerospace Societies of France (AAAF), Germany
(DGLR), Italy (AIDAA), the Netherlands (NVvL), Spain (AIAE), Sweden (FTF),
Switzerland (SVFW), and the United Kingdom (RAeS)} formed the
CEAS-Aeroacoustics Specialists' Committee (ASC).
The Committee is to serve and support the scientific and industrial
aeroacoustics community in Europe. Here ``Aeroacoustics'' is to encompass
all aerospace acoustics and related areas.
Each year the Committee will highlight some of the research and development
activities in Europe. This is the report of the 1997 highlights.

\section{AIRCRAFT INTERIOR NOISE}
\subsection{Fuselage wall transmission loss analysis}

In the framework of the European (EU) research programme BRAIN (``Basic
Research on Aircraft Interior Noise'') and a national research programme on
``Cabin Noise Reduction'' the Structures Technology department (SC) of the
National Aerospace Laboratory (NLR) developed analysis tools for the
prediction of noise transmission through a double wall structure. The
double wall consists of a stiffened and unstiffened shell structure with in
between a cavity filled with thermal insulation material and air. Both
shell structures are connected to each other mechanically with so-called
connectors.
\begin{figure}[htb]
\hspace{30mm} \psfig{figure=brain3d.eps,width=7.5cm,angle=0}
\caption{
         Response of double wall and acoustic pressure distribution in two
         perpendicular planes in the receiving room at a certain
         frequency.
        }
\label{brain3d}
\end{figure}
\begin{figure}[htb]
\hspace{30mm} \psfig{figure=panel.eps,width=8.5cm,angle=0}
\caption{
         Measured (\blijn{3.5pt}{3pt}) and calculated (\lijn{25pt})
         response of stiffened (upper) and unstiffened (lower) panel for a
         cavity filled with thermal insulation and air.
         }
\label{panel}
\end{figure}

To model the thermal insulation, porous acoustic finite elements and
boundary elements have been developed, while special coupling elements
serve to describe the coupling between air and structure. The elements have
been implemented in the modular analysis programme B2000. Various damping
models for the structure have been implemented. With these numerical tools
several studies have been carried out. The finite element model for
structure and cavity have been compared with the combination of a boundary
element model for the cavity and a finite element model for the structure.
Also, the damping models have been studied. As a specific goal of the BRAIN
project the response to a harmonic force applied to the stiffened panel of
a flat double wall structure with a cavity filled with thermal insulation
and air has been calculated (Fig.\,\ref{brain3d}) and measured. In
Fig.\,\ref{panel} the response of the unstiffened panel due to sine sweep
(force) excitation between 20 and 200 Hz, obtained with the combined
BEM/FEM, is compared with a measured response. ({\fns \AdB, \FPG})

\subsection{active control of propeller nnise}

CASA has been participating in the European (EU) project ASANCA II devoted
to active control of interior noise in turbopropeller aircraft. The task of
CASA was to develop a methodology to implement transfer functions in the
Finite Element Method to compute interior noise levels. The fuselage
structure was modelled with a fine mesh, using traditional design
techniques. The acoustic cavity was also modelled using traditional
techniques. However, instead of modelling the trim panel, blankets, thermal
insulation and so on, results from the tests (transfer functions relating
outside skin pressures with inside trim panel displacements) are embodied
in the problem of acoustic-structure coupling. This methodology has been
successfully developed and documented within the ASANCA project. ({\fns
\HC})

\subsection{Active control of helicopter noise}

ONERA-CERT has tested the feasibility of structural active control to
reduce the noise in a helicopter cabin, generated via the mechanical deck
by the vibrating gearbox. A prediction model of the vibro-acoustic
behaviour of the orthotropic composite plane panel (core in Nomex honeycomb
and fiberglass layers) fitted with actuators (mini shakers and piezo
patches (2 or 3)) is developed. The active control procedure is obtained
with an LMS (Least Mean Square) algorithm and microphones (3 to 5) as error
sensors \cite{Simon1}. Furthermore, it is possible to compute the
structural intensity distribution supporting the active control arrangement
and providing information on the energy redistribution induced by the
active control \cite{Pauzin}. Experimental and theoretical results deliver
the noise level reductions achieved after active control. Noticeable
reductions have been obtained in large areas for the two kinds of
actuators, even for rather high modal densities, with low number of
actuators and error sensors (table \ref{tabel1}, \cite{Simon2}). ({\fns
\SP, \FS})
\begin{table*}[htb]
\hspace{30mm}
 \begin{tabular}{lr@{}c@{}l}
   & & &  \\
 {\bf 282 Hz  (2 shakers)} & \multicolumn{3}{c}{\bf Noise reduction (dB)} \\
  AC with 3 sensors  1,3,5 & \qquad -14&.&9 \\
  AC with 5 sensors        & -15&.&7 \\
 {\bf 1158 Hz  (3 shakers)}&    &&   \\
  AC with 3 sensors  2,3,5 & -10& &  \\
  AC with 5 sensors        & -11&.&2 \\
 {\bf 282 Hz  (2 piezo patches)}& && \\
  AC with 3 sensors  1,3,5 & -10&.&8 \\
  AC with 5 sensors        & -13&.&3
 \end{tabular}
 \caption{
          Comparison of reductions obtained for different actuators, modal
          densities and error sensors.
         }
 \label{tabel1}
\end{table*}

\section{FAN AND JET NOISE}
\subsection{Acoustic response of fan blades to ingested vorticity}

Asymptotic analysis has been used to study the acoustic response of a blade
row to unsteady forcing by upstream vorticity. The aim has been to include
in a systematic way the full effects of the blade camber, thickness and
mean  loading, and thereby extend the wide range of existing methods which
rely purely on flat-plate theory. We have seen that these effects can be
very significant indeed for the high frequencies often encountered in
rotor-stator systems, and an efficient, analytically-based prediction
method for the radiation has now been developed at Cambridge University
\cite{PeakeKerschen}\cite{EversPeake}. Currently, this method is being
extended into the transonic regime.

In a different direction, a model for the noise generated by the
interaction between ingested atmospheric turbulence and the fan has been
developed \cite{MajumdarPeake}. The idea is that the radiation spectrum is
related directly to the turbulence spectrum far upstream, using rapid
distortion theory to account for the distortion through the intake. The
elongation  of eddies by the stream-tube contraction leads to the strong
generation of tonal noise at low forward speed, and the levels of these
tones, and of the broadband, can now be assessed in terms of the incident
turbulence and flight parameters. Extension of this work to swirling intake
flow is now being completed.

The use of scarfed intakes for the preferential modification of far-field
directivities continues to be considered, and the development of predictive
models, using a blend of ray theory and uniform asymptotics, is well
underway. ({\fns \NP})

\subsection{Acoustic resonance}

The phenomenon of acoustic  resonance is thought to occur when some
unsteady process, typically vortex shedding, locks onto an acoustic mode of
the compressor. Theoretical work at Cambridge University has progressed in
two directions. First, models of the vortex shedding by compressor blades
have been developed using hydrodynamic stability theory
\cite{WoodleyPeake1}\cite{WoodleyPeake2}, and predictions of critical
Reynolds numbers for shedding, and of linear onset and nonlinear saturated
frequencies have been made. Second, the resonant frequencies of
rotor-stator systems have been determined using a blend of analytical and
numerical techniques, at least two distinct physical mechanisms for
resonance have been identified, and very satisfactory agreement with
experiment has been achieved \cite{WoodleyPeake3}. ({\fns \NP})

\subsection{significant improvement in fan noise prediction capabilities}

Following the completion of the EU-funded FANPAC project in 1996, a
considerable effort has been made by the partners to utilise the results to
improve our understanding and validate our prediction methods for fan
noise.  For example the effects of steady intake distortion on fan tone
noise have been investigated in detail and used to validate a Rolls-Royce
prediction method (Fig.\,\ref{RR1},\ref{RR2}). ({\fns \BJT})
%\clearpage
\begin{figure}[htb]
\hspace{30mm} \psfig{figure=RR1_30.eps,width=8.0cm,angle=90}
\caption{
         Test fan with complex droop
        }
\label{RR1}
\end{figure}
\begin{figure}[htb]
\hspace{30mm} \psfig{figure=RR2_30.eps,width=8.0cm,angle=90}
\caption{
         Measured vs predicted tone level for the fan with complex intake
         droop. (\lijn{10pt}) complex droop, (\blijn{1pt}{1.5pt}) clean
         intake, ($\!\protect\boldmath\times\protect\unboldmath\!$) complex
         droop (theory), (\lijn{10pt}) $m$=20 cut-on frequency.
        }
\label{RR2}
\end{figure}

\subsection{Favourable effect of vane sweep on rotor/stator interaction
            noise}

A new lifting surface theory \cite{Schulten21} was successfully applied by
NLR to compute the aerodynamic and acoustic response of a swept vane stator
to impinging viscous rotor wakes. Sample calculations showed that vane
sweep can be exceptionally effective in reducing the noise resulting from
this major fan noise mechanism. Although the potential benefits of vane
sweep have been recognized for a long time, its significance for real fans
remained unclear.

The new method avoids the familiar summation of radial modes, which becomes
cumbersome for swept vanes, and can be considered as an extension of a
propeller lifting surface formulation \cite{Schulten22} to include sound
reflections from hub and casing. It was found (Fig.\,\ref{ceasswp})%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=ceasswp.eps,width=8.0cm,angle=0}
 \caption{
          Acoustic intensity of 2nd harmonic, upstream and downstream of
          stator, vs.\ sweep angle.
         }
 \label{ceasswp}
\end{figure}
that at conditions relevant for a modern, very high bypass design,
interaction noise reductions of the order of 8 dB are obtainable with 20
degrees of vane sweep. On the other hand, sweep angles smaller than 10
degrees are ineffective or may even cause an increase of noise level. The
weak coupling of the unsteady vane pressure and the cut-on duct modes was
positively identified as the reason of the sound reduction by sweep. Vane
sweep seems to be a practical way to achieve a substantial fan noise
reduction. In particular the down-stream propagating noise, which is not
shielded by the rotor, may be reduced by swept vanes. The new method will
play a key-role in the 3-year European programme RESOUND (Section
\ref{RESOUND}). ({\fns \JBHMS})

\subsection{Broadband turbofan noise prediction}

Noise from high by-pass ratio turbofans is mainly due to the fan. The tonal
levels have been greatly reduced during the past decades, and therefore the
broadband component has become the main contribution to the Effective
Perceived Noise Level. Prediction and reduction of high-speed rotor
broadband noise is thus a very important challenge in turbofan noise
control. ONERA has developed a prediction method based on first principles
(the Ffowcs Williams-Hawkings equation for a rotating random axial dipole),
because more sophisticated computations require input data which are not
available yet, neither from CFD nor from experiments. The basic equation
has been extended to include tangential dipoles and to non-compact sources
along the blade span. Results show that the sound power varies with the
rotation speed to the fifth power, which is in good agreement with test
data. The shape of the acoustic spectrum is also well predicted. More work
is required on two issues; (i) the computation of the directivity taking
into account the ducted propagation; (ii) the effect of quadrupole terms
for transonic rotors.

This work will be pursued in the framework of a European programme.
({\fns \SL})

\subsection{Noise fan control and reduction systems}

The need for reducing the fan noise (tones and broad-band components)
radiated by turbofan engines has led SNECMA to start investigations into
noise control and other reduction systems. Firstly, a new sound propagation
code has been developed, taking into account both elastic (Biot theory) and
rigid porous materials within a cylindrical duct. In the mean time, the
active noise control technique has been applied successfully to a model fan
by using a new controller unit (multi-processor/16 channels) developed by
LMA Marseille. Configurations with external and in-duct error sensors were
successfully tested. ({\fns \JJ})

\subsection{Acoustic research at BAe Military Aircraft and Aerostructures}

British Aerospace Military Aircraft and Aerostructures have been continuing
to develop prediction methods for the near field acoustic environment on
and around aircraft with high pressure ratio and high temperature jets,
with particular application to STOVL aircraft. Refinement of the vertical
thrust (i.e.\ for jet borne landing) acoustic prediction codes has taken
place to provide a better estimate of the groundsheet flows and to enable
more rapid modelling of specific configurations of interest. Work has been
continuing to explain the near field under prediction at high frequencies
for high velocity jets using the prediction method presented in
\cite{Ilston}.

Research is being conducted on the application of CFD codes to estimate the
acoustic levels inside cavities which are excited by air flow (e.g., weapon
bay cavities on aircraft). Estimates are being validated against model
scale data. ({\fns \LI, \DP})

\subsection{Important improvement in jet exhaust noise test capability}

Until recently, the major thrust of the civil jet noise research at DERA
Pyestock has been concerned with the noise sources associated with buried
or mixed flow configurations usually with forced mixers.  However, the
trend towards the very large engines has meant that on the grounds of
weight and interference drag, the emphasis has shifted towards
consideration of exhaust systems with separate core and fan nozzles.%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=dera1.eps,width=7.5cm,angle=0}
 \caption{
          Jet noise - Measurements of jet structure using laser anemometry
          in DERA Noise Test Facility.
         }
 \label{dera1}
\end{figure}
A model test programme, currently underway, has the overall aim of
identifying potential concepts for jet noise reduction for such
configurations.  A set of relatively simple test models has been produced
using a modular approach to enable the systematic investigation of the
effect of design parameters on the basic noise characteristics of separate
exhausts.  In the initial phase, a number of generic exhaust builds --
representing current practice -- were tested to determine the detailed
effect on the noise of engine core protrusion, core external angle and
centre bullet. This has revealed the presence of only small changes which
in most cases can be related to the anticipated change in jet structure
with build configuration. The quietest design is the co-planar
configuration by about $\frac12$ PNdB.

To improve the understanding and prediction of the processes associated
with the generation of coaxial jet noise a detailed study of the jet
structure in terms of turbulence and velocity profiles is currently
underway using dual focus laser anemometry (Fig.\,\ref{dera1}).
({\fns \RAP})

\subsection{Flight effects on jet exhaust noise test}

The installation of a turbofan engine under an aircraft wing can have a
substantial effect on the character of the radiated jet noise and this has
been the subject of studies at DERA Pyestock for several years.  However,
these were limited to the flyover orientation, whereas the sideline
legislative limit is usually more difficult to meet. A programme of testing
in the DERA Noise Test Facility has recently been carried out to
investigate installation effects in the sideline orientation.  The tests
utilised several wing geometries, from a rectangular flat plate to a
detailed, three-dimensional model of the wing. The installation effect, at
least statically, is observed to consist of two major separate effects -
reflection of the jet noise by the wing, and excess noise produced by the
interaction of the turbulent jet with the wing or flap trailing edge. The
forward motion of the aircraft introduces further complexities to the
problem.  Some limited simulated-flight data was obtained during the tests,
and it is anticipated that once the static data is analysed, improved
prediction of in-flight noise levels should be possible
(Fig.\,\ref{dera2}). ({\fns \RAP})
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=dera2.eps,width=7.5cm,angle=0}
 \caption{
          Installation effects on jet noise - Test rig in DERA Noise Test
          Facility showing wing removal actuator system.
         }
 \label{dera2}
\end{figure}

\subsection{A new nozzle support in the Cepra19 wind tunnel}

A new series of nozzle models, equipped with a balance, has been installed
in the CEPRA19 anechoic wind tunnel.%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=inj-eau.eps,width=7.5cm,angle=0}
 \caption{
          Water injection in a free supersonic jet.
         }
 \label{inj-eau}
\end{figure}
From now on, thrust measurements and the acoustic characterization of a jet
can be performed simultaneously. The thrust balance ranges up to 10\,000 N.
To improve the performance of this new equipment, ONERA carried out
calibration tests both on the acoustics with well-known nozzles and on the
thrust measurements with a standard dynamometer. This equipment will be in
service in future tests to be conducted for SNECMA, especially on
mixer-ejector and Mid Tandem Fan models. ({\fns \DG})

\subsection{MARTEL - an aeroacoustic test facility for rocket noise}

Installed in CEAT Poitiers by CNES in collaboration with French
laboratories (AEROSPATIALE, ECL, EDF, LEA, ONERA), the MARTEL facility for
high speed and high temperature jets can reproduce an aeroacoustic ambiance
similar to that of the space launchers. This facility will be used to study
and reduce jet noise emissions which may induce damage to the payload
during lift-off.%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=carneau.eps,width=7.5cm,angle=0}
 \caption{
          Acoustic characterisation of a launch pad mock-up.
         }
 \label{carneau}
\end{figure}
In operation since May 1996, several acoustic investigations have already
been carried out into the acoustic field of the free jet (LEA), jet
impingement on a plate simulating the launch pad table (ONERA), water
injection effects (ECL, LEA) (Fig.\,\ref{inj-eau}), acoustic field radiated
by a 2.1\% model of half the Ariane 5 ELA3 launch pad table (ONERA)
(Fig.\,\ref{carneau}). Acoustic experiments are now in progress with a new
fully expanded convergent-divergent nozzle; aerothermic measurements
(intrusive or not) and visualizations are planned.

It is also planned to make the facility available for external researchers
who could benefit from this particular aeroacoustic environment.
({\fns \HF})

\subsection{Rocket noise prediction}

Instituto Nacional de Tecnica Aeroespacial (INTA) has been developing in
recent years research activities on the subject of rocket noise (i.e.\ high
speed jet noise).

From a series of missile launchings a rocket noise data base has been
obtained, including data in the geometric near field and in the acoustic
far field. Sound power and power spectrum were derived from the near field
data, disregarding near field effects, and agreement was found with the
predictions of empirical methods designed to be used with far field data.
Likewise the dimensionless power spectrum fits with the values supplied by
well-known prediction curves, although a disagreement appears at very low
frequencies. Thus apparently geometric near field data provide a good
description of sound power and power spectrum, except at very low
frequencies when the wavelength is so large that the distance from the
source to the observer is smaller than the Rayleigh distance, and acoustic
near field effects must be taken into consideration \cite{Campos1}.

Recently, a code has been developed for the prediction of sound pressure
and pressure spectra, by using a spectral decomposition of the total sound
power, based on a dimensional analysis of the sound radiated from a high
Mach number jet. Values provided by this code are similar, although a bit
higher, to corresponding measured values \cite{Campos2}. ({\fns \EC}).

\section{PROPELLER NOISE}
\subsection{Acoustic effect of leading edge vortex of advanced propellers}

The acoustic module of the NLR lifting surface method for advanced
propellers \cite{Schulten11} was successfully extended with the leading
edge suction analogy. This extension has greatly improved the prediction
accuracy for highly loaded take-off conditions.

As pointed out in \cite{Schulten12}, the leading edge suction force  -- a
higher order effect which yields zero drag for a plate under an angle of
attack in a subsonic, inviscid flow -- is not captured by the lifting
surface approximation. Still, this force can be recovered by applying
locally the two-dimensional potential theory of a semi-infinite plate.

It is well known, however, that for the sharp leading edge of airfoils of
advanced propellers a local separation occurs at the suction side which
develops into a leading edge vortex. Similar to swept wings the effect of
this vortex is to rotate the leading edge suction force over 90 degrees to
the suction side of the blade. This effect is known as the leading edge
suction analogy.

A comprehensive validation was carried out \cite{Schulten13} with data
generated in the EU Brite-EuRam project SNAAP (``Study of Noise and
Aerodynamics of Advanced Propellers''), concluded in 1996. A typical result
is shown in Fig.\,\ref{ceasprop}%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=ceasprop.eps,width=8.0cm,angle=0}
 \caption{
          Sound level of 1st harmonic vs.\ axial coordinate.
         }
 \label{ceasprop}
\end{figure}
for the High Speed Propeller (HSP), designed for a cruise speed of Mach =
0.78. It has the characteristic very thin, highly swept blades of advanced
propellers (propfans). The conditions in Fig.\,\ref{ceasprop} are low speed
and high aerodynamic loading. The sound level was measured along a side
line at a distance of 1.22 tip radius from the propeller axis. The acoustic
effect of the inclusion of the suction analogy is a surprisingly large
increase of about 5 dB. As a result the agreement with the measured data
improves substantially. ({\fns \JBHMS})

\section{HELICOPTER NOISE}
\subsection{Helicopter rotor noise}

A new implementation of the Kirchhoff method which uses subsonically as
well as supersonically rotating CFD grids has been developed.

The specially developed time integration technique considers each grid
element as acoustically non-compact. The method works well even with
relatively wide surface elements and large emission time steps. The code
KIM based on this integration technique has been validated by comparison
with analytical calculations and with experiments for the case of a model
rotor in delocalized hover conditions. The method is robust and saves much
computationally time.

Furthermore, an investigation on broadband Blade-Wake Interaction (BWI)
noise has been performed for realistic rotor cases based on experimental
wind tunnel results and on rotor wake numerical simulations. The study
confirms the part played by vortex turbulence during perpendicular
blade-vortex interactions.  It also shows that very close parallel
interactions which generate impulsive noise in descent flight can also
generate BWI noise. ({\fns \JP})

\subsection{Advanced aeroacoustic prediction methods for high-speed rotors}

A new Kirchhoff code, INKA (Impulsive Noise Kirchhoff Algorithm) has been
developed at the Institute for Aerodynamics and Gasdynamics, University of
Stuttgart \cite{Wagner}. In order to predict high-speed impulsive noise
this code has been coupled to the Euler solver INROT which is of the same
parentage. For the calculation, not only are the aerodynamic grids used for
the Euler computation as adapted to the noise radiation characteristics,
but also for the discretisation of the non-rotating Kirchhoff surface.
Together with the application of accurate interpolation schemes and refined
timesteps for the acoustic calculation the efficiency of the
Euler/Kirchhoff computation could be increased significantly. Very good
agreement between computed and experimental results is achieved for the
non-lifting hover case and good agreement for the lifting forward flight
case. ({\fns \GA, \SW})

\subsection{Reducing the noise emission of fenestrons}

In 1995 a new research initiative on fenestron noise reduction started at
the Institut f\"ur Luft- und Raumfahrt (ILR), University of Technology,
Aachen, in cooperation with Eurocopter Germany. It involves the noise
reduction of fenestrons with the help of Helmholtz Resonators integrated
into the rotor shroud. A model of the EC 135 fenestron (scale 0.7:1) was
built for experimental investigations (Fig.\,\ref{fenestron}).
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=ilr.eps,width=7.5cm,angle=0}
 \caption{
          The Fenestron -- a helicopter shrouded tail rotor.
         }
 \label{fenestron}
\end{figure}

One volume was used consisting in a hollow ring in the shroud surrounding
the rotor. First the incident sound pressure level and the velocity
distribution on the shrouding surface were measured. With these data a
suitable distribution of orifices was computed using the theories of Ingard
\cite{Ingard}, Brown \cite{Brown} and Hersh\&{}Walker \cite{HershWalker}.
The computed resonator geometries were investigated experimentally in a
Kundt tube and in a test facility where the influence of the outer flow on
the absorption of a Helmholtz Resonator could be measured. The observed
flow-dependent mass exchange between resonators decreases their ability to
reduce the incident sound pressure because of a partial loss of the
volume's spring effect. The use of resonators with several orifices turned
out to be a compromise between low structural requirements and the ability
to reduce the emitted noise.

After designing a suitable orifice distribution sound power measurements
were made in the wind tunnel of the ILR using the EC 135 model. A maximum
absorption of  3.5 dB at the blade passing frequency was measured in hover.
With increasing forward flight speed the absorption decreases due to
changes of the axial velocity through the fenestron and due to the highly
turbulent flow behind the rotor. In the next step a reduction of the
influence of the outer flow with the help of a sieve dividing the outer
flow and the resonator flow will be attempted. ({\fns \OR, \GN, \DJ})

\subsection{Helicopter noise fuselage effects}

Agusta and CIRA are involved in the Brite-Euram project HELIFLOW,
concerning the aerodynamic and aeroacoustic analysis of the phenomena due
to interaction between the different components of the helicopter (main
rotor, tail rotor, fuselage), and the study of some particular and complex
operating conditions (e.g. sideward and quartering flight). The attention
is presently focused on the evaluation of the fuselage scattering.%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=figure1.eps,width=8.5cm,angle=0}
 \caption{
          Fuselage influence on the noise from an isolated blade.
         }
 \label{figure1}
\end{figure}
A particular configuration (Fig.\,\ref{figure1}) has been analysed, in
conjunction with the Third University of Rome, in order to assess the
fuselage influence on the noise radiated by a rotating, isolated blade.%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=figur2a.eps,width=8.0cm,angle=0}
 \caption{
          Fuselage effect on the waveform from an isolated blade.
         }
 \label{figur2a}
\end{figure}
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=figur2b.eps,width=8.0cm,angle=0}
 \caption{
          Acoustic pressure from: FW-H, Kirchhoff and BEM.
         }
 \label{figur2b}
\end{figure}
Fig.\,\ref{figur2a} shows how the presence of the fuselage affects the
resulting noise waveform arising from the isolated blade for an in-plane
observer location, while Fig.\,\ref{figur2b} reports the comparison
between the three different numerical formulations adopted to compute the
acoustic pressure time history: Ffowcs Williams-Hawkings (CIRA), Kirchhoff
(Agusta) and a Boundary Element Method (Third University of Rome).
({\fns \PdF, \MG, \SI})
%\clearpage

\subsection{New code for helicopter noise}

Over the last few years the main aim of the CIRA aeroacoustic group has
been the achievement of a reliable prediction of the High Speed Impulsive
(HSI) noise through the use of the FW-H equation. A new code has been
developed implementing the far field approximation through an emission
surface algorithm in order to avoid the Doppler singularity and extend the
integration domain outside the sonic cylinder. An excellent numerical
estimation of noise has been achieved for a hovering rotor blade up to a
tip Mach number of 0.95, when a strong delocalisation of the shock occurs
in the flow field.%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=figure4.eps,width=8.0cm,angle=0}
 \caption{
          HSI noise for a hovering blade.
         }
 \label{figure4}
\end{figure}
Fig.\,\ref{figure4} shows the comparison with the available experimental
data for a hovering blade at M$_{\mbox{\fns tip}}=0.90$: the agreement is
very good, both for the predicted negative peak value and the unsymmetrical
shape of the resulting waveform. The new code will be adapted and used in
the APIAN Project, where CIRA is involved in conjunction with Alenia and
the analysis of modern propeller blades (operating at a supersonic tip
speed) is required. ({\fns \SI})

\subsection{A new acoustic boundary integral formulation}

Concerning the prediction of the noise radiated by a transonic blade, a new
boundary integral formulation has been developed at Agusta \cite{Paolo}.
The new formulation, called Kirchhoff-FWH (KFWH) is obtained by removing
the non-penetration condition in the FW-H equation. From an applicative
point of view the method is very similar to the classical Kirchhoff method
since the sound radiation is evaluated by executing some integrals on a
closed surface surrounding the sound source. The advantages are that it is
no longer necessary to evaluate the pressure normal derivative, simplifying
the coupling with CFD codes, and that the formulation is much less prone to
the large errors that the classical Kirchhoff formulation can produce if
the integration surface is not correctly placed. A comparison of KFWH
formulation with experimental data and classical Kirchhoff results is given
for the UH-1H hovering rotor at a tip Mach number equal to $0.95$
(Fig.\,\ref{figure3}). ({\fns \PdF})
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=figure3.eps,width=8.5cm,angle=0}
 \caption{
          Kirchhoff-FWH code for noise from transonic blade.
         }
 \label{figure3}
\end{figure}

\section{AIRFRAME NOISE}
\subsection{Theoretical investigation of aircraft landing gears airframe
            noise}

Airframe noise is defined as the noise generated as a result of the
airframe moving through the air.  The main airframe components which lead
to airframe radiation are landing gears and high lift devices.  The
continual development of quiet engines over the years have resulted in
airframe noise levels comparable with the engine noise at approach.
Airframe noise will become an equally significant noise source for the
proposed future large aircraft.  During the past five years noise research
has been taken up again in the USA and this includes research into airframe
noise. Airbus Industrie in 1995 has also sponsored two research projects:
(i) noise source localisation for a scaled A320/A321 aircraft in the CEPRA
19 wind tunnel, (ii) determination of full scale A320 landing gear noise
characteristics in the DNW.  The experimental data of the latter form the
basis for the current theoretical investigation in refining the landing
gear airframe noise prediction methods.  The validity of the improved model
has been tested by using it to predict flyover noise from a range of Airbus
aircraft, and has been found to give a good agreement without any
modification to the constants, that was required by the old model.

Part of this improvement has come about through the realisation that the
noise is a function of local flow around each component of the landing gear
rather than simply a function of flight speed of the aircraft.  However,
the new model contains more geometrical details than the existing models,
but it can be used with greater confidence to predict aircraft landing gear
airframe noise.({\fns \MS, \SC})

\subsection{Microphone array technique to identify airframe noise sources
            on real commercial aeroplanes in landing approach configuration}

An acoustic array was developed by DLR Berlin consisting of 111
microphones, sparsely and nonredundantly distributed on an 8$\times$8 m$^2$
plate (Fig.\,\ref{frankfurt}).%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=frankfur.eps,width=8.5cm,angle=0}
 \caption{
          Acoustic array at Frankfurt Main airport.
         }
 \label{frankfurt}
\end{figure}
The array was mounted about 500 m before the threshold of one of the
runways at Frankfurt Main airport. About 160 landings of commercial
aircraft of all sizes were recorded. The exact location and the frequency
spectra of the airframe noise sources and the sound emitted from the engine
inlets and exhaust nozzles can be determined (Fig.\,\ref{aircptrn}).%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=aircptrn.eps,width=8.5cm,angle=0}
 \caption{
          Acoustic pattern of aircraft (flying from right to left, rear
          engines right).
         }
 \label{aircptrn}
\end{figure}
A comparison between the different aircraft types will help to identify
unnecessarily loud noise sources. It was found that relatively quiet
airframe noise sources can be detected even in the presence of strong
additional airframe or engine sources. The spatial resolution at 2 kHz was
found to be about 0.4 m. The data analysis is carried out in cooperation
with the acoustic consulting firm Akustik-Data. The investigation is
supported by the German ministry of Education, Science, Research, and
Technology in cooperation with Daimler Benz Aerospace Airbus,
DeutscheLufthansa AG, and Flughafen Frankfurt AG. ({\fns \UM})

\section{TECHNIQUES AND METHODS IN AEROACOUSTICS}
\subsection{The Noise Test Facility (NTF) at DERA, Pyestock, UK}

A new acoustic data acquisition and analysis system has recently been
commissioned for the NTF anechoic chamber at DERA Pyestock. This system
provides a considerable enhancement of the facility's acoustic data
gathering/processing and analysis with synchronous sampling of 16 channels
at 16 bit resolution at a sampling frequency of up to 250kHz per channel.
The raw acoustic digital data is acquired and stored in a matter of seconds
thereby allowing the user a choice of on-line/post analysis of narrow-band
and third-octave signal processing. The system also incorporates a
comprehensive data processing/analysis software suite for specialist
requirements. ({\fns \RAP})

\subsection{The Microflown, a new acoustic particle velocity sensor,
            tested in an impedance tube.}

Recently, De Bree et al.\cite{Bree} developed a new acoustic sensor at the
University of Twente , the Netherlands, which measures the acoustic
particle velocity. The new sensor, hereafter called the Microflown, can be
used in the frequency range of zero to approximately 20 kHz.

The Microflown consists of two cantilevers of silicon nitride with a
platinum pattern on top of them. The size of the cantilevers is
800$\times$40$\times$1 $\mu$m. The measurement principle of the Microflown
is based on the temperature difference between two resistive sensors, i.e.\
the two cantilevers, which are 40 $\mu$m apart.

Since the Microflown is a new sensor which has not fully been exploited
yet, it was tested in an impedance tube (Kundt's tube). A special aluminium
sample at the end of the tube has an orifice which accounts for the
absorption. Excellent agreement is obtained when comparing the results both
with theory \cite{Tijdeman}, and with microphone measurements
\cite{Eerden}.

The Microflown can be a very attractive alternative to microphones, while
it can also be used to determine sound level or acoustic intensity.
Furthermore, the sensitivity to the direction of the waves can be a useful
feature. The simplicity, the small dimensions, the dynamic range, the
uniformity, the absence of resonant parts, and the expected low price
promise to make the Microflowns a very useful tool in acoustics. ({\fns
\FJMvdE, \HEdB, \HT})

\subsection{CAA-Simulation of trailing edge noise}

In cooperation with BUAA (Beijing University of Aeronautics and
Astronautics) DLR Braunschweig has started a research initiative into
numerical aeroacoustics, aiming at solving the unsteady linearized Euler
equations. The CAA-code was successfully used to simulate the noise
generated by a single vortex with finite core $C$ which is convected past
the trailing edge of a plate with length $L$ and zero thickness. The code,
based on Tam's DRP-scheme was able to show the expected characteristics of
sound generation and radiation pattern. For low Mach numbers of the mean
flow, the results are in very good agreement with the classical analytical
theory of trailing edge noise, including then 3rd-power-law dependence of
$\langle p^2\rangle/\rho_{\infty}^2c_{\infty}^4$ on the flow velocity, or its
linear dependence on the flow Mach number. The figure (\ref{delfs})%
\begin{figure}[hbt]
 \hspace{30mm} \psfig{figure=dir_CAA.eps,width=7.5cm,angle=0}
 \caption{
          CAA-Simulation of trailing edge noise, $L/C=11.25$. Lenghts are
          proportional to rms-value of pressure. \blijn{3.5pt}{3pt}:
          Theory (Howe).
         }
 \label{delfs}
\end{figure}
shows the simulated directivity in comparison with theory, including
convective amplification following Howe. The a-symmetry of the result is
due to the finite size of the vortex. An alternating passage of vortices on
the upper and lower side of the plate would average out the directivity
pattern to almost exactly the theoretical curve. ({\fns \JD, \XL})

\subsection{CFD methods for fan aero-acoustics design}

To predict the fan-OGV (Outlet Guide Vanes) interaction tones, and to be
able to design a quiet fan, CFD methods have been applied at SNECMA. The
methodology consists of performing a steady-state 3D calculation of the
flow around the fan, from 3D Navier-Stokes equations, then an unsteady
compressible 3D Euler calculation on OGV's, by using the description of the
flow coming from the N.S.\ results. The load harmonics spectrum is computed
from the unsteady static pressure field, and is used as input in an
acoustic code to predict the rotor-stator interaction noise. ({\fns \DB})

\subsection{BEM/FEM method for fluid-structure coupling}

In collaboration with the research project of CASA Space Division on
acoustic loads on very lightweight antennas, a method has been developed at
the Polytechnic University of Madrid to determine the complete coupling of
fluid and structure. The method is based on a FEM for the structure and a
BEM for the fluid. The advantages of the method are the accurate solution
of Kirchhoff's integral formulation of the Helmholtz equation including the
exact Sommerfeld radiation condition at the far field and the computation
of the natural frequencies and normal modes of the coupled fluid-antenna
system. The method has been compared successfully with experimental data
for different support conditions of the antenna. ({\fns \PGF, \FdlI})

\section{MISCELLANEOUS TOPICS IN AEROACOUSTICS}
\subsection{Successful conclusion of EU-funded research project DRAW}

The project DRAW (Development of Design Tools for Reduced Aerodynamic Noise
Wind Turbines) was aimed at improving the prediction of broadband noise
from wind turbine blades. For the dominant trailing-edge noise mechanism a
prediction model was developed at the TNO-Institute of Applied Physics (NL)
which is based on a detailed description of the structure of the turbulent
boundary layer close to the trailing edge. Input quantities can be obtained
either from measurements or from CFD calculations. For the second important
mechanism of inflow-turbulence noise an improved model was created at the
Institute of Aerodynamics and Gasdynamics IAG (D). The model is based on
Howe's acoustic analogy and on the boundary-element method. Both models are
able to consider the true airfoil shape and can therefore serve as a design
tool for low noise airfoils. The models were validated against wind tunnel
measurements on airfoil sections which were carried out in the Small
Anechoic Wind Tunnel of the National Aerospace Laboratory NLR (NL). The
validation revealed that the absolute level of trailing-edge noise can be
computed with a reasonable accuracy. The differences in inflow-turbulence
noise which are due to airfoil shape are predicted accurately. A first
design of a silent airfoil was successful. ({\fns \GG, \SW})

\subsection{High-speed train pantograph measurements}

In the framework of a national Dutch research programme, measurements of
the aerodynamic noise of an Adtranz DSA 350 SEK high-speed train pantograph
were performed by the National Aerospace Laboratory (NLR) in the German
Dutch Wind Tunnel DNW-LLF \cite{Dassen}.

Thirteen configurations --each at four or more different wind speeds-- were
tested, using an acoustic antenna and `single' microphones. The acoustic
antenna consisted of 49 microphones, non-redundantly positioned in a plane
of approximately 4$\times$4 m$^2$, aimed at the localisation of the
pantograph noise sources and the determination of their respective
contributions (Fig.\,\ref{pantograph}).%
\begin{figure}[htb]
 \hspace{30mm} \psfig{figure=pantogra.eps,width=8.5cm,angle=0}
 \caption{
          Overview of experimental set-up showing pantograph (middle),
          acoustic antenna (left) and vortex generators (front).
         }
 \label{pantograph}
\end{figure}
A number of tonal and broadband noise sources were found and spectral data
of these sources were used by the Institute of Applied Physics (TNO-TPD)
for the development and validation of a semi-empirical prediction code.

Analyses revealed that vortex shedding from various sharp and bluff parts
of the pantograph is the most important noise producing mechanism.
Inflow-turbulence noise, generated in the foot region of the pantograph,
may also be important depending on the level and spectrum of the turbulence
in the inflow (i.e.\ in the boundary layer along the train roof). The
results suggest that by optimising the shape of the pantograph top region,
a reduction in the order to 10 dB may be possible.

The findings of the research will be used by Technical Research of the
Netherlands Railways (NS-TO) for defining measures to limit the noise
nuisance caused by future operation of high-speed train operation in the
Netherlands. ({\fns \TD})

\subsection{Acoustic fatigue of aircraft structures}

Although near field jet noise levels have been reduced on civil aircraft
since Concorde, specific components (e.g.\ flaps on aircraft) still
experience high acoustic excitation levels combined with some jet efflux
impingement and consequent thermal loads. The lack of a complete
theoretical treatment lead to the development of design guides and data
sheets starting in the 1970s. However, the introduction of composites, and
other new materials, presents problems in the design process. A three year
research project is therefore being carried out using advanced analytical
procedures, finite element analysis and complementary experimental studies
in order to develop dynamic response prediction procedures which will
result in guides for the design of box-type structures, such as flaps, made
from these materials. It is particularly important that simple procedures
be available for initial design, which could be supported by more
complicated analysis as the final proposed configuration is chosen. ({\fns
\SC, \RW, \RL})

\subsection{Acoustic loads on spacecraft antennas}

The presently available methods to predict the dynamic response and
stresses, induced in antenna reflectors by the strong acoustic loads during
launching, has been reviewed and applied by CASA to the mechanical design
of the reflectors of the IOLA (Inter-Orbit Link Antenna) and LBA (L-Band
Antenna) antennas of the ARTEMIS spacecraft (to be launched by ARIANE V).
Simplified methods have been compared with sophisticated vibro-acoustic
formulations to select the method which correlates best with experimental
results. Also the way to model the environment inside the reverberant
chamber, in which the tests have been carried out, has been one of the
major topics under review. At present, a diffuse acoustic field is assumed
although the measurements suggest that this assumption could not be true
for the low frequencies.

Another point of interest has been the identification of the main
parameters of the structure that control the response, and methods to
characterize the structure using both analytical and experimental
techniques. The modal characterization of the specimen, including modal
shapes and damping in the frequency range of interest, has been found to be
most critical. The correlation between analysis and test must be done such
that the surrounding air and the correct boundary conditions are taken into
account. Finally, design solutions, that guarantee structural integrity
have been implemented and proved by tests after a full and successful
qualification process. ({\fns \PL, \JLR})

\section{PROSPECTS}
\subsection{Future collaboration programmes on fan noise reduction} \label{RESOUND}

Following on from FANPAC, new proposals for collaborative research on
aircraft noise reduction have been submitted to the EC and subsequently
accepted, albeit with some reduction in funding.  These proposals have been
coordinated through the X-NOISE thematic network which has been formed as a
result of the Environmentally Friendly Aircraft study (TEFA).  One of the
three Type I proposals, called
{\topsep3pt
\begin{center}
 RESOUND
\end{center}}
({\bf R}eduction of {\bf E}ngine {\bf SO}urce noise through
{\bf U}n\-der\-stand\-ing and {\bf N}ovel {\bf D}esign), concentrates on
engine noise reduction at source, and in particular on fan noise reduction
as summarised below.

The principal aircraft and engine manufacturers in Europe are facing
increasing pressure to reduce aircraft noise levels.  This arises both from
the community expectations of improved quality of life and from the need to
compensate for the expected growth in air traffic.  A recent EC DGXII
policy statement on aircraft noise declares:\\*
{\em `a co-ordinated strategic approach at European level is essential and
major efforts need to be de\-voted to techniques for further reduction of
ex\-terior aircraft noise to overcome today's technology barrier. Hence the
objective for R\&TD is to enable a breakthrough in noise control
tech\-nol\-ogy.'}

The objective of RESOUND is to acquire the technology necessary to support
the design of derivative and new aero-engines with noise levels that are 4
dB quieter than  those of aircraft currently entering service.  This will
provide the foundation for the achievement of a mid-term (8 years)
objective of reducing aircraft noise levels by at least 6 dB, and allow
European industry to compete on an equal footing with the US.

RESOUND addresses the challenge of reducing the noise at source, in
particular turbomachinery noise, through (1) engine component aeroacoustic
design and (2) through novel noise controlling devices that can be
integrated within the engine structure. Innovative technologies to be
evaluated, with the aid of theoretical techniques and experiments at model
and full scale, include:
\begin{list}{}{ \topsep=0.0ex \partopsep=0ex \leftmargin=5em \labelwidth=0.5em
               \labelsep=0.5em \itemsep=0.1ex \parsep=0.0ex }
\item[\blt]fan noise reduction through reduced tip speed and pressure ratio optimisation;
\item[\blt]noise reduction with fan and stator axial sweep and circumferential lean;
\item[\blt]fan noise reduction with variable by-pass nozzle and passive fan tip treatments;
\item[\blt]combustion noise reduction through improved and validated generation model;
\item[\blt]assessment of potential noise hazards of low NOx combustors;
\item[\blt]LP turbine noise reduction through exit guide vane design;
\item[\blt]turbomachinery noise reduction through active stator design;
\item[\blt]turbomachinery noise reduction by means of auxiliary aeroacoustic control devices.
\end{list}
Based on the technology acquired, RESOUND will deliver a full assessment of
the community noise benefits of controlling engine noise at source, through
design and with novel active/passive devices.

The reduction of aircraft noise through improved nacelle technology and
airframe design is being addressed by complementary proposals (RANNTAC and
RAIN respectively), supported by a Type 2 proposal (DUCAT) all of which
will be coordinated through the X-NOISE thematic network that has been
formed as a result of the Environmentally Friendly Aircraft study (TEFA).
Such a combined effort is necessary to meet the challenge of the US
industry, which is backed by a fully funded programme (\$ 200M over 7
years).

RESOUND and the other related projects are due to start on 1 January 1998.
({\fns \BJT})

\subsection{ROSAA project launched}

The Brite-Euram project ROSAA is being launched in 1998 (March 1), in which
a unique rotorcraft simulation system will be developed integrating
advanced aerodynamic codes with the comprehensive aeroelastic codes
available to European manufacturers and with the latest generation European
aeroacoustics codes. An important task of this project will be devoted to
aeroacoustic prediction methods. The activities that will be performed are:
the improvement of DERA's KIRAC code (based on a Kirchhoff formulation) for
HSI noise prediction, the development of the unified
aerodynamic-aeroacoustic BIEM method of the Third University of Rome, the
testing of existing FW-H based codes (Agusta and Westland) and the
development of a common interface to the aerodynamic full-potential code of
ROSAA. ({\fns \PR, \AP})

\subsection{MADAVIC programme on active control}

There is a strong demand in industry for new actuators with smaller size
and improved properties; e.g.\ more power, less weight, higher reliability,
less power consumption, and better compatibility to microelectronics. A new
and expanding area of potential application for these advanced actuators is
that of so-called smart (or intelligent) structures, material and systems.
The target of the MADAVIC programme (Magneto Strictive Actuators for Damage
Analysis and Vibration Control), coordinated by the Dipartimento di
Progettazione Aeronautica of Naples ``Federico II'', is to address the
particular and unique characteristics of magnetostrictive materials in this
respect, aiming at two specific and very important fields of application:
(i) detection and identification of damaged structures and (ii) active
control of vibration.

In the field of active vibration control and insulation, the conventional
actuators are often reported to be strongly limited by narrow frequency
bandwidths, a complicated design, the inability to support static weight,
and lack of robustness. Hence, the entire project covers the following
areas: reliability and quality of materials and products, safety and
reliability of production systems, and technologies for vehicle and
aircraft safety. The results of this multidisciplinary programme will
provide the basic building blocks of the technology required to build
actuator systems for the above mentioned new technologies, and a wide
variety of other industrial applications. ({\fns \FM, \SdR})

\section{SCIENTIFIC EXCHANGE AND INTERACTION}
\subsection{AIAA/CEAS Aeroacoustics Conference}

The 3rd joint AIAA/CEAS Aeroacoustics Conference took place in Atlanta, GA
(USA), May 12--14, 1997. Six Members of the ASC served on the Program
Committee with Dr Andrew Kempton, Rolls Royce, being the European General
and Technical Chairman. Out of approximately 130 papers, about 25 papers
were given by scientists from the CEAS-countries. This number corresponds
to the previous 1996 conference at Penn State, reflecting again the
restricted travel budget situation. However, the next, the 4th joint
Conference, will be held in Toulouse, France, June 2--4, 1998, under the
Chairmenship of Dr G\'erard Fournier (France) and Prof Tim Colonius (USA),
where many more Europeans are expected to attend. ({\fns \HH})

\subsection{ASC-Workshops}

During 5--6 November 1997, at the DNW, the Netherlands, a Workshop on
``Wind Tunnel Testing Techniques'' was held, organized by the Aeroacoustics
Specialists' Committee and the Dutch NVvL. It was the first of a planned
series of dedicated ASC-Workshops. This highly successful meeting with 20
presentations was attended by almost 80 scientists and engineers (including
several from the US), with substantial participation from aviation and
automobile industry.

Directly following the Toulouse Conference, the second ASC-Workshop on
``Aircraft Interior Noise Control'' will be held on 8--9 June 1998, at the
Daimler Benz Aerospace Dornier Company in Friedrichshafen, Germany,
organized by Dr Ingo U.\ Borchers. ({\fns \HH})

\begin{small}
\begin{thebibliography}{99}
\bibitem{Simon1}        Simon, F., Pauzin, S., ``Simulation of an Active Vibration Control Procedure: Simulations-Experiments Comparison'', Proc. of Active'97, Budapest, August 1997.
\bibitem{Pauzin}        Pauzin, S., Simon, F., ``Development and Application of Structural Intensity on Composite Panels'', Proc. of Inter Noise 96, p. 1505, Liverpool, July 1996.
\bibitem{Simon2}        Simon, F., Pauzin, S., ``Active Vibration Control Procedure to Reduce Internal Noise of Helicopter'', Proc. of Technical Specialists' Meeting for Rotorcraft, Williamsburg, October 1997.
\bibitem{PeakeKerschen} Peake, N., Kerschen E.J., ``Influence of Mean Loading on Noise Generated by the Interaction of Gusts with a Flat-Plate Cascade: Upstream Radiation'',  Journal of Fluid Mechanics, 347, 1996, pp.315-346.
\bibitem{EversPeake}    Evers, I., Peake N., ``Forward Noise Generation by a Cascade with Camber and Thickness'', AIAA Paper 97-1633, 1997.
\bibitem{MajumdarPeake} Majumdar, S.J., Peake, N., ``Noise Generation by the Interaction between Ingested Turbulence and a Rotating Fan'', Journal of Fluid Mechanics, 1997, to appear.
\bibitem{WoodleyPeake1} Woodley, B.M., Peake, N., ``Vortex Shedding from a Cascade of Aerofoils'', AIAA Paper 97-1814, 1997.
\bibitem{WoodleyPeake2} Woodley, B.M., Peake, N., ``Global Linear Instability of Aerofoil Wakes'', Journal of Fluid Mechanics, 339, 1997, pp.239-260.
\bibitem{WoodleyPeake3} Woodley, B.M., Peake, N., ``Acoustic Resonance in Blade Rows'', Journal of Fluid Mechanics, 1997, submitted.
\bibitem{Schulten21}    Schulten, J.B.H.M., ``Vane Sweep Effects on Rotor/ Stator Interaction Noise'', AIAA Journal, Vol.35, No.6, June 1997, pp.945-951.
\bibitem{Schulten22}    Schulten, J.B.H.M., ``Effects of Asymmetric Inflow on Near-Field Propeller Noise'', AIAA Journal, Vol. 34, No. 2, February 1996, pp.251-258.
\bibitem{Ilston}        Ilston, L.W., Angel, R.G.A., Patience, D.E., Burns, R.E., ``Assessment Methods for Near Field Noise and Ground Erosion'', AIAA Paper 93-4870 (A94-16447), Proceedings AIAA International Powered Lift Conference, Santa Clara, CA, 1-3 December, 1993.
\bibitem{Campos1}       Campos, E., Mall\'en, F., ``Analysis and Prediction of the Noise Generated by Missile Launching'', CEAS/AIAA paper 95-088, CEAS/AIAA Aeroacoustics Conference, Munich, Germany, 12-15 June, 1995.
\bibitem{Campos2}       Campos, E., Mall\'en, F., ``Prediction Methods for the Noise Generated by Missiles'', Proc. NOISE-CON 96, The 1996 National Conference on Noise Control Engineering,  Seattle, WA, USA, September 29-October 2, 1996, pp.867-872.
\bibitem{Schulten11}    Schulten, J.B.H.M., ``Effects of Asymmetric Inflow on Near-Field Propeller Noise'', AIAA Journal, Vol. 34, No. 2, February 1996, pp.251-258.
\bibitem{Schulten12}    Schulten, J.B.H.M., ``Advanced Propeller Performance Calculation by a Lifting Surface Method'', Journal of Propulsion and Power, Vol.12, No.3, May-June 1996, pp.477-485.
\bibitem{Schulten13}    Schulten, J.B.H.M., ``Comparison of Measured and Predicted Noise of the Brite-EuRam SNAAP Advanced Propellers'', AIAA Paper 97-1709, Proceedings 3rd AIAA Aeroacoustics Conference, Atlanta, Ga, USA, May, 1997.
\bibitem{Wagner}        Algermissen, G., Wagner, S., ``Computation of High-Speed Impulsive Noise by an Enhanced Kirchhoff Method'', Technical Specialists' Meeting for Rotorcraft Acoustics and Aerodynamics, Williamsburg, VA, 28-30 October, 1997.
\bibitem{Ingard}        Ingard, U., ``On the Theory and Design of Acoustic Resonators'', Journal Acoustical Society of America, Vol.25, Nr.6, 1953.
\bibitem{Brown}         Brown, D.,  ``Investigation of the Influence of Duct-mounted Helmholtz Resonators on the Sound Field of a Model Ducted Propeller'', NASA CR-1653, 1970.
\bibitem{HershWalker}   Hersh A., Walker, B., ``Acoustic Behaviour of Helmholtz Resonators, Part I \& II'', CEAS/AIAA-95-078/079, 1995.
\bibitem{Paolo}         di Francescantonio, P., ``A New Boundary Integral Formulation for the Prediction of Sound Radiation'', Journal of Sound and Vibration, 1997, 202(4), pp.491-509.
\bibitem{Bree}          de Bree, H-E., Leussink, P.J., Korthorst, M.T. Jansen, H.V., Lammerink, T., Elwenspoek, M., ``The Microflown: a Novel Device Measuring Acoustical Flows'', Sensors and Actuators A 54, 1996, pp.552-557.
\bibitem{Tijdeman}      Tijdeman, H., ``On the Propagation of Sound Waves in Cylindrical Tubes'', Journal of Sound and Vibration 39, 1975, pp.1-33.
\bibitem{Eerden}        van der Eerden, F.J.M., de Bree, H-E., Tijdeman, H., ``Experiments with a New Acoustic Particle Velocity Sensor in an Impedance Tube'', Sensors and Actuators A (to be published).
\bibitem{Dassen}        Dassen, T., Sijtsma, P., van Haaren, R., Parchen, R., Looijmans, K., Holthusen, H., ``The Noise of a High-Speed Train Pantograph as Measured in the German-Dutch Wind Tunnel DNW'', 2nd International Workshop on the AeroAcoustics of High-Speed Trains (AAT), Berlin, 29-30 April, 1997.
\end{thebibliography}
\end{small}

%\newpage
%\section*{Captions}
%Figure \ref{brain3d}:\\
%         Response of double wall and acoustic pressure distribution in two
%         perpendicular planes in the receiving room at a certain
%         frequency.\\\\
%Figure \ref{panel}:\\
%         Measured (\blijn{3.5pt}{3pt}) and calculated (\lijn{25pt})
%         response of stiffened (upper) and unstiffened (lower) panel for a
%         cavity filled with thermal insulation and air.\\\\
%Figure \ref{RR1}:\\
%         Test fan with complex droop\\\\
%Figure \ref{RR2}:\\
%         Measured vs predicted tone level for the fan with complex intake
%         droop. (\lijn{10pt}) complex droop, (\blijn{1pt}{1.5pt}) clean
%         intake, ($\!\protect\boldmath\times\protect\unboldmath\!$) complex
%         droop (theory), (\lijn{10pt}) $m$=20 cut-on frequency frequency.\\\\
%Figure \ref{ceasswp}:\\
%         Acoustic intensity of 2nd harmonic, upstream and downstream of
%         stator, vs.\ sweep angle.\\\\
%Figure \ref{dera1}:\\
%         Jet noise - Measurements of jet structure using laser anemometry
%         in DERA Noise Test Facility.\\\\
%Figure \ref{dera2}:\\
%         Installation effects on jet noise - Test rig in DERA Noise Test
%         Facility showing wing removal actuator system.\\\\
%Figure \ref{inj-eau}:\\
%         Water injection in a free supersonic jet.\\\\
%Figure \ref{carneau}:\\
%         Acoustic characterisation of a launch pad mock-up.\\\\
%Figure \ref{ceasprop}:\\
%         Sound level of 1st harmonic vs.\ axial coordinate.\\\\
%Figure \ref{fenestron}:\\
%         The Fenestron -- a helicopter shrouded tail rotor.\\\\
%Figure \ref{figure1}:\\
%         Fuselage influence on the noise from an isolated blade.\\\\
%Figure \ref{figur2a}:\\
%         Fuselage effect on the waveform from an isolated blade.\\\\
%Figure \ref{figur2b}:\\
%         Acoustic pressure from: FW-H, Kirchhoff and BEM. \\\\
%Figure \ref{figure4}:\\
%         HSI noise for a hovering blade.\\\\
%Figure \ref{figure3}:\\
%         Kirchhoff-FWH code for noise from transonic blade.\\\\
%Figure \ref{frankfurt}:\\
%         Acoustic array at Frankfurt Main airport. \\\\
%Figure \ref{aircptrn}:\\
%         Acoustic pattern of aircraft (flying from right to left, rear
%         engines right).\\\\
%Figure \ref{delfs}:\\
%         CAA-Simulation of trailing edge noise, $L/C=11.25$. Lenghts are
%         proportional to rms-value of pressure. \blijn{3.5pt}{3pt}:
%         Theory (Howe).\\\\
%Figure \ref{pantograph}:\\
%         Overview of experimental set-up showing pantograph (middle),
%         acoustic antenna (left) and vortex generators (front).
%
%\clearpage  \psfig{figure=panel.eps,width=15cm,angle=0}
%\vfill \hspace{61mm} Figure \ref{panel}
%\clearpage  \psfig{figure=ceasswp.eps,width=15cm,angle=0}
%\vfill \hspace{61mm} Figure \ref{ceasswp}
%\clearpage  \psfig{figure=ceasprop.eps,width=15cm,angle=0}
%\vfill \hspace{61mm} Figure \ref{ceasprop}
%\clearpage  \psfig{figure=figure1.eps,width=15cm,angle=0}
%\vfill \hspace{61mm} Figure \ref{figure1}
%\clearpage  \psfig{figure=figur2a.eps,width=15cm,angle=0}
%\vfill \hspace{61mm} Figure \ref{figur2a}
%\clearpage  \psfig{figure=figur2b.eps,width=15cm,angle=0}
%\vfill \hspace{61mm} Figure \ref{figur2b}
%\clearpage  \psfig{figure=figure4.eps,width=15cm,angle=0}
%\vfill \hspace{61mm} Figure \ref{figure4}
%\clearpage  \psfig{figure=figure3.eps,width=15cm,angle=0}
%\vfill \hspace{61mm} Figure \ref{figure3}
%\clearpage  \psfig{figure=dir_CAA.eps,width=13cm,angle=0}
%\vfill \hspace{61mm} Figure \ref{delfs}

\end{document}
