Minutes of the Meeting on 15^th April 2002

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

Chairman:  Keith Austin

Minutes:      Della Leslie

Meeting commenced at 09.15 hours.

Apologies:     Anton Bergant (Litostroj E.I. d.o.o.), Jim Brown (Dundee University), Bruno Brunone (Perugia University), Pierre Moussou (EDF) Simon Pugh (ESDU), Patrick Vaugrante (EDF), Lixiang Zhang (Kunming University of Science and Technology). 

Preliminary Items

• The minutes from the last meeting were approved.
• New members to the group were welcomed and participants introduced themselves.

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 gave a presentation outlining the progress of the current project. It was emphasised that this was the last Group Meeting that could strongly influence the technical direction of the current project, which has only about 6 man months to run. The next meeting will focus on the content and presentation of guidelines.

The most recent progress in the project includes development of the ALT in-house software (Axial/lateral/torsional FSI software for the time-domain). This is currently in working order, although development will be on-going as necessary (for the addition of extra features). It has been used for the first set of parameter variations of System 1 (single pipe). There has also been progress in the experimental program (details given in second presentation). Della reported that the FSI web-site is now on-line and that it is intended that this will be updated regularly and emphasised the need for feedback.. Address: www.dundee.ac.uk/civileng/FSI 

Della described the overall approach to the problem. A large piping system is too complex to analyse, therefore it is considered as a collection of sub-systems. Five sub-systems have been identified for use within the project (as defined in previous meetings). Della pointed out that system 4, the 3-D elbow pair, was of particular interest because this configuration is recommended as an expansion group (although it is commonly found to promote excessive vibration).

For Objective 1, six generic issues have been identified. These are:

(1) Long lengths of unsupported or poorly supported pipe work

(2) Unsupported/unrestrained elbows

(3) Unsupported/unrestrained valves

(4) Unsupported/unrestrained or poorly supported T-junctions

(5) Combinations of the above

(6) Vibrating machinery

Please see the web-site for details on each of these features. How each of these features will be investigated (using sub-systems 1-5) was described. The aim was to limit the quantity of work required to a manageable amount, with each generic issue being investigate through only one or two sub-systems.

Of particular interest was item (5) T-junctions. A specific example for this is the EDF problem of failure of small-bore pipes (branching from a larger pipe). Sebastien added that EDF is currently trying to find robust configuration (not found yet) and that it is not an easy problem. Arris said support of T-junction is explicitly mentioned in the ASME B31 code.

Next Della described some of the analysis being performed. For each sub-system there are millions of possible cases. For system 1 (single pipe) the first set of variations has been carried out and the process of reducing the number performed was described. We reduce to manageable number by specifying a finite number of boundary conditions (Closed end, imposed pressure, semi-infinite, with(out) support, with(out) added mass). Only certain combinations are realistic for study, e.g. semi-infinite at left-hand boundary and semi-infinite at right-hand boundary is not realistic, also the combination of a rigid support with an added mass is not realistic. Della illustrated the axial impact of a single pipe example, showing that the 486 possible combinations of boundary conditions in fact reduce to 96. For each example the output parameters are hoop stress (and strain), axial stress (and strain), Tresca stress (and strain) axial and lateral displacements and pressure. The maximum value at each location along the pipe system is recorded. Some examples of results were shown and discussed (the results will be displayed on the web-site)

For Objective 2, the assessment of methods, there are many methods that may be considered. Della identified those methods that are being investigated and particular features of each of these. Briefly these were: Joukowsky (basic, simple guide: many cases sufficient); Waterhammer (good for rigid systems and buried systems, but misses structural modes of response); Structural dynamics (misses fluid response). For FSI we have: FSI: Friction Coupling (Significant for very long pipes. Though not expected to contribute to undesirable behaviour); FSI: Poisson Coupling (Highly dependent on material properties (e.g. flexible pipes)); FSI: Junction Coupling (Most significant type. Interactions at junctions can cause significant change in measures pressures/stresses.) Finally uncoupled FSI was mentioned, although investigation of this will not be carried out (possible future work). We intend to acknowledge that it is common practice to use this method and it is expect a significant number of cases where this method is sufficient (e.g. will predict the magnitude of forces.) There was a discussion on uncoupled FSI and its use.

Item 2.                   Low-frequency vibrations of piping

Sebastien Caillaud gave a presentation on the work of his colleague Pierre Moussou. This continues from work presented at the previous meeting (October 2001).

The research is based on changing the method of assessment. Previously a numerical approach was used (Circus), but this has been found not the answer. The new method based on experimental results (and simulations). Though it was noted that it is expensive to take measurements in Nuclear Power plants. Starting from the basics, the design of a system is standard. The input data is the pressure class and the nominal flow. Pipe cost vs. pumping cost determined pipes diameter; pressure class and diameter determines pipe thickness and weight conception determines support spacing. Experience shows that natural frequencies fit into two ranges 5-30Hz and 50-200Hz. This is a generic result (has been found in several systems). Sebastien showed that the natural frequencies (analytical) on a single span beam with pin-pin condition gives two ranges, 0-50Hz and 50-200Hz, as function of distance between supports. The results found were discussed and the definition of the parameter alpha, which was part of the formula giving support spacing was requested:

alpha = sqrt(density_steel*momentum_inertia_pipe/(lambda*outer_diameter²) )

where lambda* length_pipe = mass of the pipe

which gives 0.2 < alpha < 0.3 for the pipes studied.

The overall idea is to analyse the general behaviour of a pipe system. Sebastien showed show graphs giving the frequency response under nominal and partial regimes (measured on-site). On each figure, the general shape for each regime was given, proving an envelope for acceptable operation of a system. The figures suggested a logarithmic threshold for ‘normal spectra’.

The remainder of the presentation looked at hydroacoustic sources in systems (including work by Gibert). He also presented pressure spectra according to literature, but said that there was a lack of accurate data for industrial use.

Item 3.                   WAHALoads project - two-phase flow for waterhammer and induced loads

Thorsten Neuhaus gave a presentation on the WAHALoads project; task T3.3 –Benchmark Calculations with existing codes. There are seven participants, eight codes involved and three benchmark problems. He described each code being tested: CATHERE V1.5a (CEA); RELAP5/mod3.2 (EA, TBL); EUROPLEXUS 2000 (EDF); WAHA single phase code (IJS); TFTC Two-phase Flow test code (IJS); DELOS (UCL); MONA 2.2 (UMSICHT). Alan suggested it would be useful if different institution used the same code for the same problem (to see if they got the same results). Summarising, there are two types of codes, general-purpose system codes and specialised fast transient codes.

The benchmark problems are outlining below together with some of the conclusions.

BM1.1a: shock tube theoretical test case – cold liquid, single phase. A compression wave is generated by sudden valve closure at pipe downstream end. The lower pressure remains above saturation temperature and no vapour is generated. It was found that most codes were capable of producing acceptable results, provided sufficient nodes and sufficiently small time steps are used.

BM1.1b: shock tube theoretical test case – hot liquid, two-phase conditions. A compression wave is generated by sudden valve closure at pipe downstream end. The pressure drops to saturation temperature, so that vapour generation and condensation can occur, leading to possible condensation shocks. General-purpose codes were not capable of capturing secondary effect due to vapour cavity collapse, but the special purpose codes for fast dynamic transients could. Thorsten played some simulations showing the results provided by the codes.

BM2: Edwards Pipe Blowdown experiment. The pipe is initially filled with pressurised, subcooled water and the transient initiated by the burst of the rupture disk located at one end of the pipe. A rarefaction wave travels towards the closed end of the pipe and consecutive blowdown is controlled by flashing. Each code predicted the rarefaction wave, but there was a problem with flashing model in MONA code. All codes predicted a higher pressure during first phase. It was suggested that the boundary condition might not be formulated correctly, since all code gave the same result.

BM3: PPP tests (power plant pipe work). This system was described and the initiating event was a rapid valve closure. Thorsten presented the results given by MONA and also Flowmaster. The results were discussed. The timing of the pressure pulses were well predicted by MONA, but the magnitudes were incorrect.

During the presentation there was extensive discussion on the definition of bubble flow, column flow and other definitions of two-phase flows according to the value of the void fraction.

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 began by first presentation progress of the experimental program at Dundee. Last year experiments on the LT system (in-plane elbow and T-piece combination) were carried out for an air filled system, with the help and guidance of Arris Tijsseling. There had been several problems with leaks that had prevented the water filled experiments being performed. In January 2002, the experimental work was continued (now working without external assistance, but with the help of an excellent technician who has assisted previously on the system – Colin Stark). Previous problem had been solved, though there were still many new one. These included some new leaks (at a pressure transducer mounting, at the T-junction and a bleed screw). More important was the incorrect fitting of the o-ring in the T-junction, which was discovered only after high-velocity impact testing lead to a leak. Other problems involved updating of equipment and associated incompatibilities. Over 50 test were completed (125kHz and 10KHz sampling rates, with high and low-velocity axial impact and a lateral hammer impact) taking pressure and strain reading at several locations along the system.

Della illustrated some of the results obtained. Consistency in the results was shown to be excellent. She continued by comparing the experimental results with numerical simulations (using in-house software). These appear to give good agreement, although the numerical simulation exhibited some high-frequency oscillations. It was suggested that this may be due to the extremely fine grid used in the numerical work (time step was of the order 10^-7 seconds.

The main part of this presentation was the discussion of Objective 3: How to express outcomes of the project. Della gave an overview of the FSI Web site that has been developed, and will continue to be developed. The site will used for displaying results and act as focus for FSI research. As results and data is obtained they will be put on the site, allowing for immediate feedback.

The development of the site was discussed, including ideas to for items and links on the site. Arris suggested links to Dave Wiggert’s professional course (on FSI) and also items like previous EPSRC reports. Della said that contributions to the site would be welcomed, in particular anyone with access to photographs suitable for the site.

For the development of guidelines, Della described the ideal solution to the problem as a tree-type diagram enabling engineers to determine whether a system would a susceptible to FSI. It was emphasised that the current project will achieve only a small part of this; the aim of the project being to reduce uncertainties, not eliminate them. Alan said that the results should answer the following: 5 subsystems as defined – are these in your system? Will they cause a problem? It is important to keep results found simple and make them accessible to engineers.

Finally the discussion moved on to the future of research at Dundee. The current project finishes this year and Della will be moving to Ireland. Arris asked if the experimental apparatus would survive? Yes (hopefully)! Alan said that future plans would be made once the current project was closer to completion.

Item 5.                   Casto1: Experiments and calculations

Sebastien Caillaud gave a presentation, reporting the experimental and numerical analysis of Castro1: a single elbow system. The system was designed to focus on coupling in the elbow. The system is supported rigidly at one end and is open at the other. Experiments have been carried out with the system empty, i.e. air filled and water filled (the level of the water is varied). The system is excited (using an oscillator) at the remote (open) end, in-plane with the system.

For the air-filed system, experiments were performed and the results compare to simulations by both Circus and Code_Aster. The initial model (using Circus) yielded errors of about 3% and an updated model reduced this to about 1.5%.

For the water filled system there had been many problems, particularly with air in the system. Sebastien said that the system was filled with water, then shook and left for several months before measurements were taken. There was a discussion of other possible solutions for this problem. 

Sebastien presented the results for the water filled system (error level was again about 3%). He showed comparisons between experimental results and different numerical models: Circus coupled, Circus with added mass and Circus acoustic (i.e. comparing FSI and uncoupled model). It showed that uncoupled results and coupled method were very close, particularly for first two modes (higher modes show a greater difference). This implies that the effects of added mass are dominant and coupling effects in the elbow are weak.

Sebastien continued by outlining the future work at EDF. This includes Casto2, which is a Z-shaped pipe (2-elbows in plane). The experiments for the air filled system have been completed and the experiments for the water-filled system will be coming soon.

Item 6.                   Progress in FSI in flexible tubes

Christina Giannoppa gave a presentation reporting the progress in her PhD project on FSI in flexible tubes. She began by giving an overview of the project and the approach used for modelling: a single solution method using a finite volume code (Foam). She described how the equations were derived. This requires the equations for the solid being written in terms of variables traditionally used for fluids (Velocity and pressure). Because this is a new formulation the code needs to be validated for solids and the test case for this is a beam, fixed at one end and subjected to a step load at the remote end.

Typical stress analysis (displacement based) shows dissipation (which shouldn’t occur). Using a velocity-based form with the pressure calculated explicitly, two methods were examined: Euler implicit and backward difference. Both showed less damping, but the backward difference method exhibited oscillations. Question whether this was physical or numerical. Repeating calculations with various time steps altered the frequency of the beat. The group discussed this and it was concluded that this must be numerical. Arris suggested using a forward-difference method. Comparing results between the numerical (3-D) and analytical (1-D) showed a difference of 8.82% for mean and maximum displacement and a 0.9% for the frequency. The group discussed whether comparisons should be made between 1-D and 2-D models, emphasising that comparison must be expressed as % differences and not % error.

Christina continued by describing the method with pressure included implicitly. For the formulation of the boundary conditions there were two options: velocity with a fixed gradient or a fixed value, together with constant pressure. Depending on which is used gave different results, one gives energy loss and the other energy gain. The reason for this was unknown, as both should provide the same result. This was discussed and it was pointed out that the case under consideration was very demanding. It was suggested that a pipe beam might be a useful example to investigate, particularly since the next step would be a flexible tube. For the formulation of a flexible tube it was highlighted that pressure would be discontinuous across the solid/fluid interface. Possible methods to overcome this were discussed.

Any Other Business: A paper by Lixiang Zhang was distributed.

Next meeting: October 7^th 2002

Item 7. Chairman’s Closure

Closure of meeting: at 17:30 approx.