Minutes of the Meeting on 22^nd October 2001

Present:     Keith Austin (Flowmaster), Sebastien Caillaud (EDF), Christina Giannoppa (King’s College), Chris Greenshields (Nabla), Martin Hamilton (Hamilton Flowservices), Pierre Moussou (EDF) Arris Tijsseling (Eindhoven University), Della Leslie and Alan Vardy (Dundee University).

Chairman:                   Keith Austin

Minutes:                   Della Leslie

Meeting commenced at 09.30 hours.

Apologies:  Jim Brown (Dundee University), Bruno Brunone (Perugia University), Simon Pugh (ESDU), Patrick Vaugrante (EDF), Lixiang Zhang (Kunming University of Science and Technology).

Preliminary Items

• The minutes from the last meeting were approved.

Item 1.                   Progress at Dundee – Objectives 1 and 2

The first two objectives of the project, exactly as specified in the research proposal, are:

Objective 1: To identify the features of pipe systems that cause greatest susceptibility to risk of damage through fluid-structure interaction.

Objective 2: To identify the minimum acceptable capabilities of methods of analysis suitable for assessing fluid-structure interaction.

Della Leslie outlined the recent progress of the project and the contents of the prepared documents, objective 1 and objective 2. An extensive and constructive discussion developed. The results of this will be reported through the development/amendment of the two documents (to be distributed as soon as possible).

Item 2.       An analysis of the ANSI-OM3 standards

Sebastien Caillaud gave a presentation on vibration monitoring of piping networks. The industrial context of the presentation was the French design guide (RCC-M), in which there is no design specification for vibration in nuclear piping networks.

The current method used in EDF is derived from ANSI-OM3, which presents an empirical relationship between maximum stress and lateral velocity. The relationship is dependent on terms describing material, geometries and boundary conditions. These are accounted for through 6 constants: C₀, C₁, C₂, C₃, C₄ and K₂.

Sebastien presented a flow chart illustrating the methodology of vibration monitoring. Outlining the method: velocity measurements are taken; if these are less than 12mm/s, then there is deemed to be no risk; if they are greater than 12mm/s, then the OM3 standard is used. If the velocity is acceptable by this standard, then again no risk is assumed; otherwise expertise is needed. Sebastien reported that there have been a few systems that have not passed the OM3 standard, but there have been no problems so far. The 12mm/s guideline has been obtained from international experience feedback, though this can be shown to contradict the ANSI-OM3 relationship (using extreme values of all coefficients)

Sebastien continued, presenting some analysis on the ANSI-OM3. He asked whether the simultaneous use of max/min values for all coefficients was realistic, giving the example of C₄ realistic>min(C₄). Concluding, Sebastien highlighted that the method was simple and efficient for large and numerous piping system. The method is based on structural analysis with FSI neglected. The ANSI-OM3 is designed for assessment of systems and not for the design of systems.

A discussion followed, in which Sebastien said that the aim of the research was to justify the standard. In particular the use of the 12mm/s guideline was discussed; Martin Hamilton suggested that it might be acceptable given that certain features are not present. He also said that there was a need to decide where to take measurements. Sebastien said that technicians take these measurements, and Pierre Moussou added that the 12mm/s guide was designed to be “idiot” proof, but the OM3 needed more experienced people. It was highlighted that the OM3 guideline is based on 5 systems (single pipe, single elbow, z-system, U-system and 3D elbow pair).

Item 3.                   Experiments at Dundee – results, interpretation and implications

Arris gave a presentation outlining the experimental work carried out during his visit to Dundee in July. He began by outlining the intended programme for experiments. Starting with the existing system, which he labelled the LT-system: a single elbow and T-piece (in-plane), with closed ends, water filled and suspended with wires. Connected to this were 7 dynamic pressure transducers, 1 static transducer, 12 strain gauges and one velocity (LDV). With this set-up, the intended programme was to perform axial rod impacts (transient and free-vibration), steel hammer impacts (transient and free-vibration, both axial and lateral). This would be followed by the LT-system, air-filled, before examining a T-system (water then air-filled), hopefully extending the water filled T-system to include cavitation experiments , and finally a pressure-temperature experiment (recording the variations in pressure as the temperature in the laboratory varies throughout the course of the day).

However, this programme did not proceed according to plan! Many problems were encountered. The rig had not been used for two years. The result was there was a leak at PT4, LDV not working properly (needs to be sent off for recalibration), theodolite not ok, T-piece leaking and impact-plug leaking (this had been removed in order to re-mill the impact surface – previous experiments had left the surface dented).

As a result of these problems it was decided to proceed with an air-filled system – Arris’ visit was only for 2 weeks, and the main aim of this was to familiarise Della with the workings of the experimental set-up.

In total, 24 LT tests were carried out, 12 with and 12 without end caps. Each set of 12 experiments consists of: 2 transient and 4 free vibration (2 axial rod, 2 lateral hammer), these 6 repeated with different locations for measurements (due to limited number of signals that can be processed). Arris presented some of the results obtained, saying that it takes a long time to analysis ALL results/data obtained. He added that the experimental performed should show the influence of end caps.

With a water filled system, Arris highlighted some of the expected results: Anticipate influence of water, resulting in a lower frequency (about 0.8 factor), a changed spectra if fluid and structural frequency are close, a better match with simulation (because empty pipe is more sensitive).

To conclude his presentation Arris made some historical remarks. Joukowsky, 1898, is usually quoted as being the first to examine waterhammer. However, Arris presented some work by Johannes von Kries from 1883 examining waterhammer and friction.

Arris reported a recent publication, Arris and Dave Wiggert, giving a FSI literature review. This will be distributed with the minutes. He also informed the group of an upcoming FSI symposium, in New Orleans 17-22 November, 2002 (at the 2002 ASME Int. Mechanical Engineering Congress and Exposition).

Finally, Arris informed the group that in the Netherlands, the average consumption of Stroopwafel, which originate from 1784, was 20/year, giving those present a sample of 10, so that regular attendees of the meeting will in fact consume the same as the average Dutchman (partners abstaining)!

Item 4.                   Development of Guidelines – Objective 3

To express these outcomes in a manner that will reduce significantly the uncertainties faced by designers of pipe systems.

Della presented an introduction to objective 3. This outlined a number of points and ideas. Firstly, the results must be simple, with the guidelines aimed at engineers and not mathematicians. The idea of a tree diagram, with potential branches was presented, including:

·        structure induced vs. fluid induced

·        Resonant vs. one-off event

·        Slow vs. rapid event

·        Elbows vs. no elbows

·        Distance of elbow from closed end

·        Position of supports from junction

·        Length and flexibility of supports

·        Ratio of pipe/fluid densities

A discussion developed. Arris asked what the 12-month plan was and what is important, saying that there was too much to do. Alan replied that we intend to do a sensitivity study and not a parameter study: i.e. 10x10 simulations and not 10¹⁰. The question of what are we looking for was made: Pressure: magnitude and duration, Tresca stresses. Chris suggested the rate of pressure rise. Alan added that for the time domain, maximum pressure and Tresca stresses were written into proposal.

The question of static stresses was brought up which resulted in an extensive discussion. The result was to discount static for the current project, ensuring to add a warning statement to this effect in the guidelines developed.

Item 5.                   FIV problems in industrial piping systems: example and general issue

The main focus of a presentation by Pierre Moussou was FIV in Nuclear Safety systems. The types of FIV covered, included: turbulence induced vibration in pipes (e.g. valves, orifices, T-piece), cavitation induced vibration and pumps in partial flow regimes.

He began by highlighting the research reported in a paper given at the ASME-PVP 2001 conference, based on 2 small-bore piping fatigue failures that occurred at the end of 1999. These were in a residual heat removal system (RHR), an essential cooling system in which failure cannot be tolerated. He showed a diagram of the system, explaining that the small bore pipes were side branches from the main system.

The system is subject to a broadband spectrum of vibration (0-400hz) and its response is dependent on hydraulic conditions, with cavitation particularly important.

The study he performed was based on the allowable velocities at root of small-bore pipe and the influence of hydraulic conditions. It was found that cavitation of a valve (labelled 13vp) appeared to be main source of vibration. The main results showed that piping system vibration increased when the Thoma number decreased or when hydraulic power increased. Therefore 2 valves were opened to reduce the flow in valve 13vp, reducing the hydraulic power and hence the vibration of the piping system. He said that it was an FSI problem, but that FSI was not calculated.

Pierre continued his presentation, reporting on a second paper (Coupling Effects in a Two Elbows piping system, 7^th Int. Conf. On Flow Induced Vibrations, Lucerne, 2000). This was based on research that aimed to design a simple piping system exhibiting coupling effects. What was discovered, was that one effect of coupling is to prevent the coincidence of structural and fluid natural frequencies. Pierre described the proposed system: a z-configuration (2 flexible elbows, clamped at both ends). The system was designed so that the structural and fluid both produced the same natural frequency (14Hz) when the calculations are performed uncoupled. Repeating the analysis using a coupled model produced frequencies of 12 and 18Hz, and further analysis proved that coincidence in the coupled model could not be possible.

Item 6.                   Fluid transients in flexible tubes

Chris (Greenshields) gave an introduction to this presentation, outlining the history leading to the development of the project currently undertaken by Christina Giannoppa (PhD student, Chris is external supervisor).

The idea to model arteries as a single system, not fluid + solid, dates back to 1998, when Chris presented a single set of equations, representing both fluid and solid in terms of pressure and velocity.

In 1999, this almost worked except for a problem of dissipation. It wasn’t until 2001 when funding for a PhD project was obtained to examine this problem of dissipation in the numerical solution for the solid. It was found to be a bigger problem than anticipated.

Christina took over at this point and gave a presentation describing the results found so far within her project.

For FSI in flexible pipes and tubes, there is more interest in the local deformation. In human arteries, E<<K (i.e. solid is softer). How is this modelled? Christina outlined a common method in which pressure from a fluid analysis is input to a solid, and then the velocity from the solid analysis fed back into the fluid analysis, continuing until the solution converges. She said that this was computationally expensive and potentially unstable. Alan said that the instability is result of approach and not method used, saying that you shouldn’t condemn the entire method because of this, highlighting other possibilities, e.g. change the direction of the iteration arrow or use a relationship between pressure and velocity.

Christina continued by outlining the single approach method (one set of equations for solid and fluid). This is a finite volume method, modelling transient behaviour for compressible solid and fluid components with thick walls. Typically, velocity and pressure are used within fluid equations and displacement and stress for the solid. For the single approach method to work, the solid equations must be written in terms of pressure and velocity. Christina gave some results obtained with this approach, showing total power vs. time, comparing force accumulation vs. stress accumulation. It illustrated that there was a problem with stress accumulation results, which exhibited dissipation. This was found to be due to the method of discretisation.

Summarising, Christina said that the project investigated an innovative method for FSI, and that the single solution method was faster (computationally), more intuitive, and whilst the application of interest was blood flow it was a general method (could be used for general application).

A discussion followed, which included the differences between finite-element and finite-volume methods. Arris mentioned that there is currently funding available short-term stays at Eindhoven for post-graduates via a Madame Curie Scheme.

Next meeting: 15^th April, together with a second day, 16^th April, on Unsteady friction.

Item 7.                          Chairman’s Closure

Closure of meeting: at 17:02 hours.

Addendum

For the next meeting:

We propose to have completed the following work

·          Time domain, theoretical treatment completed