FLUID STRUCTURE INTERACTION

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CONTENTS

Organizing committee

1. Torbjørn Kristian Nielsen (NTNU), Chair
2. Chirag Trivedi (NTNU), Co-chair
3. Ole Gunnar Dahlhaug (NTNU)
4. Pål-Tore Storli (NTNU)
5. Carl W. Bergan(NTNU)
6. Einar Agnalt (NTNU)

Background

Hydraulic turbines include both stationary and rotating components. The interaction of the components, mainly between the runner blades and distributor vanes, is critical when the frequency of the rotor-stator interaction (RSI) approaches the runner natural frequency . In the recent years, several turbines have been exposed to heavy fatigue loading and development of crack in the turbine blades. The fatigue loading and the failures are associated with the hydrodynamic force and the response from the mechanical structure for the given condition. For safe and reliable design of the turbines, detailed understanding of fluid structure interaction (FSI) is essential. However, hydraulic turbine is a complex structure and extremely challenging to understand the behavior as mechanical response is dependent on the operating condition. While designing the turbine, factor of safety based on traditional design and experience is considered. However, it is not proved reliable all the time. The FSI is dependent on several parameters: (1) hydrodynamic damping, (2) nearby structure and submergence level, (3) mode-shape, (4) freestream velocity and vortex shedding, (5) damping during cavitation, (6) material properties, (7) rotational speed, (8) natural frequency of individual and combined structure, (9) flow compressibility, (10) wave propagation speed, etc.

Scope

Scope of the third workshop is fluid structure analysis under steady state operating conditions. More specifically, parameters such as study of mode-shape, nodal-diameter, deformation, fatigue loading, estimation of fatigue life, individual/combined natural frequencies, hydrodynamic damping, harmonic response, etc. will be investigated

The Hydrofoil test case focuses on fluid structure analysis (one-way or two-way) and to study one/many of the above parameters. Basic/fundamental research is the main objective and how the approach (applied on hydrofoil) can be useful for the complex structure such as turbine blades. CFD analysis of the hydrofoil will be welcomed if sophisticated turbulence modeling approaches such as, detached eddy simulation, large eddy simulation (LES), hybrid RANS/LES, and direct numerical simulation (DNS), are used that will help to understand the mechanics of vortex shedding and the resonance. In addition to the harmonic response, modal analysis, acoustic model, two-way FSI is also encouraged.

The turbine test case focuses on fluid structure analysis (one-way and/or two-way) and to study one/many of the above parameters. CFD analysis in turbine is NOT the focus as it is already covered under the previous Francis-99 workshops.

Test case: Francis-99 turbine

A model Francis (known as Francis-99) turbine available at the Waterpower Laboratory, NTNU, was used for the measurements. Figure 1 shows the hydraulic loop of the test rig. Water from the basement reservoir (9) was pumped to the overhead tank (2) and flowed down to the turbine (7). A feed pump (1) was operated at a selected speed to maintain constant head. The draft tube outlet was connected to a downstream vessel (8), where a constant head was maintained at an atmospheric pressure and the water above the runner centerline was returned to the basement (9). The test rig is capable of producing head up to 16 m for the open-loop and up to 100 m (flow rate ≤ 0.5 m³ s^-1) for the closed loop.

Figure 1. Open-loop hydraulic system of the model Francis turbine at the Waterpower Laboratory, NTNU. 1 – feed pump, 2 – overhead tank-primary, 3 – overhead tank-secondary, 4 – pressure tank, 5 – magnetic flowmeter, 6 – generator, 7 – Francis turbine, 8 – downstream tank and 9 – basement.

The Francis-99 turbine is a reduced scale (1:5.1) model of a prototype operating at the Tokke power plant, Norway. The turbine includes 14 stay vanes integrated inside the spiral casing, 28 guide vanes, a runner with 15 blades and 15 splitters, and an elbow-type draft tube. The runner includes an alternating arrangement of a splitter and a blade. The runner inlet and outlet diameters are 0.63 m and 0.347 m, respectively. The runner is directly coupled to a DC generator, which also operates as a motor in pumping mode. The test facility is often used for the industrial model test according to IEC 60193 in a closed loop (30 m head, Re = 4 ✕ 10⁶), and it is equipped with standard instruments used to measure the head, flow rate, torque, water temperature and rotational speed.

One of the concerns measuring blade loading on the Francis-99 runner is the complex assembly. The runner blades are joined to the crown and band with bolts, which means there is a possibility for rotational asymmetry related to the blade mounting. In addition, the blades at the trailing edge joined to the band are free (around 94 mm length). This is marked in Figure 2 and Figure 3. In this area, variation in gap between the blades and the band is observed. By exciting the blades with the impact hammer, it was clear from the sound response that the blades have different preloading (possibly pretension). A second concern is the crown with several holes and cable channels as shown in Figure 4. The holes were created to perform pressure measurements in the runner. It is fact that this will create asymmetrical stiffness, meaning the bending stiffness of the crown is dependent on the bending direction. Hence, there is possibility of high uncertainty in the strain measurements at these locations. Considering these concerns, the turbine test case is limited for specific study of FSI based on pressure measurements only. This time, we provide data of pressure amplitudes pertained to rotor-stator interaction frequencies and the harmonics. The data may be used to prescribe pressure loading on the blades or blade loading of specific frequency, i.e., RSI, and conduct structural analysis.

Figure 2. Francis-99 runner with free end trailing edge joined to band (marked in red color).

Figure 3. Francis-99 runner with free end trailing edge joined to band (marked in red color).

Figure 4. Francis-99 runner with holes on the crown for the pressure sensors and the instrumentation.

Figure 5 shows locations of pressure sensors in the runner. The guide vane passing frequency f_gv and the harmonic are determined at the sensor locations. The sensor coordinates are shown in Table 1, which are identical to the runner geometry provided for the FSI analysis. Measurements are conducted at five different operating points, keeping same opening angle of the guide vanes; however, the rotational speed of the runner and the head values are different, which are presented in Table 2. From these measurements, we found that the amplitudes of RSI frequency varied significantly and at certain operating condition, the amplitudes are relatively high. This may be the indication of resonance in the turbine at that operating condition. This could be interesting to investigate numerically.

Figure 5. Locations of pressure sensors in the Francis-99 runner.

Table 1. Locations of pressure sensors in runner.

Table 2. Operating parameters and range considered for this workshop.

The pressure sensors were initially calibrated statically with the use of dead weight tester as primary reference. All components in the pressure measurement chain, from the sensors to the data acquisition in current setup are stated to have resonance frequencies above 10 kHz, hence it is assumed that the dynamic uncertainty is very low and only repeatability and hysteresis remains in the uncertainty due to covariance. A repeatability test was conducted at 1 Hz with a pressure alternating between 100 kPa and 90 kPa absolute pressure. Uncertainty budget for the RSI amplitudes is presented in Table 3. A vibration test with the runner in air was conducted to analyze the pressure sensors vibration sensitivity. The results did not give any additional uncertainty.

Table 3. Amplitudes pertained to rotor stator interaction (f_GV). The amplitudes are in percentage of head value at the corresponding operating point.

The RSI amplitudes are normalized for head at that operating point. The uncertainty includes the 95% measurement uncertainty and 95% probability of the amplitude variation found from with the use of short time fast Fourier transform. The analysis was performed with window length equal to 100 periods of the RSI signal and with 50% overlap.

Figure 6. Rotor stator interaction fundamental frequency (f_GV). Amplitudes are normalized by the head value of the corresponding operating point.

The second harmonic of the guide vane passing frequency had an increasing trend towards the measurement at 280 Hz as shown in Figure 7. This indicates a resonance situation with ND4. The objective of third workshop is to perform FSI simulation to analyze the pressure in the runner channel during the resonance. Only one CFD analysis may be enough to find the excitation pressure. To avoid CFD analysis, other excitation methods could be used to excite the runner with a ND4 pressure field. The objective is not to find the same resonance frequency due to the uncertainties of the assembly of the model runner. However, one can replicate the shape presented in Figure 7 using FSI analysis; amplitudes are not necessary to match exactly with the experimental data.

Table 4. Amplitudes pertained to rotor stator interaction (harmonic (2f_GV). The amplitudes are in percentage of head value at the corresponding operating point.

Figure 7. Rotor stator interaction harmonic frequency (2f_GV). Amplitudes are normalized by the head value of the corresponding operating point.

Test case: Hydrofoil

The test facility was developed in the Waterpower laboratory, which consisted of long piping loop and a test-section for the hydrofoil. The aim was to determine the hydrodynamic damping with flow velocity. To measure flow rate (Q) in the test-section, magnetic flowmeter (calibration uncertainty is in the order of 0.1%) was mounted at far downstream of the test-section.

The test rig is comprised of an upstream convergent and downstream diffusor, these change the pipe section from 0.3 m diameter pipe to a square 0.15 m × 0.15 m, measured internally. The test section where the hydrofoil is located along with most of the instrumentation is placed between two straight pipe sections in order to isolate the test section from the effects of the cross section change.

Figure 8. Hydrofoil test rig.

The test section was designed to ensure a sufficient stiffness, in order to provide a grounded support for the test structure, i.e. the hydrofoil, as is good practice for modal testing. This also enables the assumption of stiff walls for boundary conditions when performing structural simulations. The hydrofoil geometry was long and slender, with a thickness of 0.012 m and a cord length of 0.25 m. After 0.15 m from the leading edge, the thickness was tapered down to 4.5 mm at the trailing edge, before being chamfered and rounded on one side, as is normal for runner blades. The hydrofoil was milled from a single piece of aluminum alloy, and grooves were milled for instrumentation and cables. The hydrofoil was mounted without any angle of attack, and was then excited using piezoelectric MFCs actuators from PI Ceramic. Two MFCs were mounted on each side of the foil at the width wise center, close to the trailing edge. By exciting these in a sinusoidal pattern phase-separated by 180°, a bending action is induced in the blade. As the piezoelectric patches are driven by a high voltage signal and is therefore supplied by a E-835 DuraAct Piezo Driver Module.

In order to measure the vibrational amplitude, semiconductor strain gauges from Kulite were mounted on the trailing edge of the hydrofoil, directly downstream the MFCs. In addition to strain gauges, the vibrational amplitude was measured using LDV equipment from Polytech (type OFV 2200 with an optical head of type OFV-303). LDV is a non-intrusive measurement method that measures vibration using the Doppler-shifted reflection of laser light. The LDV measures vibration in a single spot, along the axis of the laser beam, and the measurement point was located at the trailing edge of the foil .

Figure 11. Natural frequency and flow velocity.

Figure 12. Hydrodynamic damping and flow velocity.

The tests were performed using a Stepped-Sine excitation, in which a series of constant-frequency excitations are performed in order to avoid transient effects when moving through a resonant region. Each measurement consisted of around 60 excited frequencies, and each measurement was repeated several times in order to obtain sufficient statistics for an assessment the uncertainty in both damping and natural frequency. The tests were performed with water velocity 0–25 m s^-1, with steps of 5 m s^-1. The test rig was pressurized in order to obtain cavitation-free conditions.

Interpretation of the hydrofoil data
The hydrofoil data is presented as a set of files, with some information at the heading of each. A more detailed explanation on interpreting the data is given here.

File overview: Three types of data files are given.

1. Francis-99.txt: This file contains the main results from the hydrofoil experiments. The results consist of discharge, damping factor, natural frequency, and gauge pressure in the test rig inlet section.
2. FRF_[X]ms.txt: There are seven of these files, each representing measurements at a separate discharge. The filename denotes the bulk velocity in the test section, and discharge is provided in each file. Other than that, the files give data for frequencies tested, along with amplification factor and phase delay. The amplification factor is normalized to unit value (=1) at resonance.
3. noExcitation.txt: This file contains the hydrofoil’s response without MFC excitation, for different discharges. The response is expressed in trailing edge deflection amplitude, as well as frequency of vibration.

Francis-99.text: Most of the parameters presented in this file are self-explanatory. Each line of data is made from a set of 30 repetitions at that discharge. The damping factor is commonly written as zeta, with 0 being undamped, 1 being critical damping, and anything above 1 overdamped. Critical damping is the point at which oscillations will not occur for a step excitation.

FRF_[X]ms.txt: Each of these files contain the frequency response function, made from 30 repetitions. The frequency response is made from three parameters: frequency, amplification and phase delay. These can be used to construct bode plots (frequency vs amplitude, frequency vs. phase), or to construct nyquist plots (real vs. imaginary). The damping and natural frequency can be extracted from the shape of the curve, and the absolute values are therefore not relevant. As such, the amplitude has been normalized to the value at resonance, and is referred to in the file as “Relative Amplitude Response”

noExcitation.txt: This file contains measurements of trailing edge movement for different discharges, without the MFC excitation. This is therefore a good indication of the vortex shedding interaction. The data shows the maximum amplitude with a corresponding frequency of vibration, for different discharges.

Numerical model

Two distinct geometries of hydrofoil are given: (1) with fillet and (2) without fillet. The geometry without the fillet may be used for the CFD analysis and the geometry with the fillet can be considered for the FSI analysis. The geometry is provided in four different format types, i.e., *.iges, *.parasolid, *.stl, and *.step. We believe that these format types are enough to work with different software to create mesh and conduct the simulations.

Figure 9. Two dimensional view of hydrofoil.

Figure 10. Three-dimensional view of the hydrofoil.

Author centre

Important dates

01 APRIL 2019 Last date for the full-length paper submission.

01 MAY 2019 Notification of the full-length paper.

12 MAY 2019 Last date for the full-length revised paper submission.

15 MAY 2019 Notification of the final formatting, editing and proofread corrections, if any, with deadline of 48 hours.

28-29 MAY 2019 Third Francis-99 workshop.

Abstract and manuscript submission

Accepted papers after the peer-review will be published in the Journal of Physics: Conference Series (JPCS).

Guide for the abstract submission: Abstract should be 150–300 words and it should reflect the actual work, which authors are going to present as results in the paper. Paper size A4 (2 cm margin all sides), double line spacing, 12 point font size, Times New Roman font, and the abstract should be left aligned.

The paper title must include phrase/keyword "Francis-99." This help to index the database, citation and searching papers associated with the Francis-99 test cases. The paper title should less than 100 characters with spaces.

The third Francis-99 workshop includes two different test cases, i.e., Hydrofoil and Turbine. If the authors want to work on both test cases, they may submit two separate papers. Authors are also permitted to combine both test cases into one paper. However, methodology, results and conclusion should be clearly distinguishable in the paper.

Guide for the full-length paper submission:

Abstract should be 100–200 words. IOP Conference Series uses author-supplied PDFs for all online and print publication. Authors are requested to prepare their papers using Microsoft Word or LaTeX, according to the guidelines and templates, and then convert these files to PDF. It is important to ensure that when you submit your paper, it is in its final form ready for publication, and has been thoroughly proofread. IOP do not copy edit or reformat papers and will not send out author proofs prior to publication. Post-publication changes are not permitted, so please ensure that your paper has been checked for errors. By submitting a paper an author and all co-authors are assumed to agree with the terms of the IOP Proceedings Licence.

As a summary, please ensure the following:

1. Paper size is European A4.
2. Margins are 4cm (top), 2.5cm (left and right) and 2.7cm (bottom).
3. The paper includes the author name and affiliation (full address including country).
4. There are no page numbers, or headers and footers, within the paper.
5. The PDF is free of formatting errors (e.g. corrupt equations, missing or low-resolution figures), since conversion from Word to PDF can introduce formatting errors.
6. Text is single spaced, not double spaced.
7. The PDF file is editable and not password protected.
8. All pages are portrait (landscape pages should be rotated).
9. Reference lists are checked for accuracy. References can only be linked via Crossref if they are correct and complete.
10. Figures are placed within the text, not collected at the end of the document.
11. A thorough proofread is conducted to check the standard of English and ensure wording is clear and concise.

Submit Abstract and Manuscript by Email: Chirag Trivedi.

Registration

Registration Fees

Presenting author(s) Free
Student (M. Sc. and Ph. D.) Free
NTNU Employee Free
Participant 2500/- NOK
Accompanying person (dinner) 1000/- NOK

Registration is compulsory for all, participating in the workshop, and click here to complete the registration and to make online payment. Last date for the registration is 22 May 2019 at 00:00. If you need invitation letter for visa application, please contact Debbie Koreman van den Bergh (debbie.w.koreman@ntnu.no , tel. +47-73593561).

Venue

The workshop will be organized at Norwegian University of Science and Technology (NTNU), Trondheim, 7491-Norway. Exact location for the presentations and registration is Gamle elektro - EL6, Elektro A, O. S. Bragstads Plass, NTNU, Trondheim, 7491-Norway.

Accommodation

A large number of hotels are available around the NTNU.

1. Scandic Lerkendal
2. Trondheim Vandrerhjem
3. Radisson Blu Royal Garden Hotel Trondheim
4. Comfort Hotel Trondheim
5. P-Hotels Brattora
6. Scandic Bakklandet
7. Thon Hotel Trondheim
8. Thon Hotel Prinsen
9. Quality Hotel Augustin

There are frequent flights from Oslo (such as SAS, Norwegian and Wideroe) Trondheim, which take around one hour. You can also travel by train, NSB, which takes around 8 hours. Airport express (Flytoget) in Oslo, which is running between Oslo airport and Oslo Central and takes around 20 minutes.

Programme

-provisional-

Workshop summary

Francis-99 is a series of three workshops, which provides an open access of the complete design and experimental data of a model Francis turbine. It is an open platform for young researchers/students and gives the possibility of exploring their capabilities/skills in numerical modelling. The researchers can use these data and conduct state-of-the-art numerical studies by applying different tools and techniques.

The first workshop was organized during December 15-16, 2014, which focused on steady state operating conditions of Francis turbine, i.e., best efficiency point, high load and part load.

The second workshop was organized during December 14-15, 2016, which focused on transient operating conditions of Francis turbines, i.e., turbine startup, shutdown and ramping.

The third workshop was organized during May 28-29, 2019, which focused on fluid structure interactions in Francis turbine. However, in this workshop, two test cases were provided: (1) Hydrofoil and (2) Francis turbine. The hydrofoil test case aimed to investigate fundamental research, and the turbine test case aimed to investigate applied research. Parameters such as study of mode-shape, nodal-diameter, deformation, fatigue loading, estimation of fatigue life, individual/combined natural frequencies, hydrodynamic damping, harmonic response, etc. were investigated. In total 11 papers were submitted to the workshop and around 35 participants from different countries, Canada, Spain, Germany, Croatia, Sweden, North Macedonia, Norway, etc., joined the workshop.

On the first day, the workshop was started with opening speech by Chairman, Professor Torbjørn K. Nielsen then summary of Francis-99 workshop by Chirag Trivedi. Research and development activities as well as ambitious plans of NTNU in energy sector were presented by Professor Johan Hustad (Director of NTNU Energy). Hege Brende (Executive Director of HydroCen) presented research work in FME-HydroCen. Two keynote presentations were planned for the first day. The first keynote presentation delivered by Professor Eduard Egusquiza from UPC Spain. The professor has presented research activities in his laboratory pertained to fluid structure interactions and real-time monitoring of rotating machinery at the hydropower plant. Second keynote presentation was delivered by Bjarne Børresen (Multiconsult-Norway), “Runner cracking – ten years later.” He emphasized needed research to reduce the risk of runner crack and requirement credible as well as combined effort to overcome the cracking problem. He also presented statistic based on turbine head, size and specific speed and cracking problems in last three decades. After the keynote speech, research from different authors of Francis-99 papers was presented.

Second day was started with keynote speech from two eminent professors from Laval University-Quebec, Professor Claire Deschênes and Professor Sébastien Houde. Research activities in the Laboratory on Hydraulic Machines (LAMH) were presented that included measurements on propeller turbine, Francis turbine, fluid structure interactions, strain gauge measurement, computational fluid dynamic analysis, basic and applied research, etc. Then, research from different authors of Francis-99 papers was presented. After lunch, research in the Waterpower laboratory, especially HiFrancis and H2020 HydroFlex projects, was presented by Professor Ole Gunnar Dahlhaug and led discussion on next Francis-99 workshop series. Should we continue the Francis-99 workshop series and, if yes, what should be test case? The discussion was very much informative extended beyond one hour. Several researchers provided their wishes for next workshop/series. Large part of discussion was focused around hydrofoil type test case for future workshop that will help to understand fluid structure interactions in further detail with robust verification and validation.

The workshop was ended with closing remark from the Chairman, and acknowledgement from Chirag Trivedi. The workshop series was contribution of several researchers, authors and universities during 2009-2019. This could have not been very successful without teamwork and dedicated effort as well as financial support from different research projects in the Waterpower Laboratory. Later, laboratory visits were arranged that included smart grid laboratory, smart house and the Waterpower Laboratory.

The Francis-99 organizers are very much thankful to all who joined the workshop series and their direct/indirect contribution to make it successful from December 2009 to May 2019.

Publications

1. E Tengs, J Einzinger and PT Storli, 2019, "Two-way coupled simulation of the Francis-99 hydrofoil using model order reduction," https://doi.org/10.1088/1742-6596/1296/1/012001.
2. P Cupr, D Stefan, V Haban and P Rudolf, 2019, "FSI analysis of Francis-99 hydrofoil employing SBES model to adequately predict vortex shedding," https://doi.org/10.1088/1742-6596/1296/1/012002.
3. CW Bergan, EO Tengs, BW Solemslie, P Østby and OG Dahlhaug, 2019, "Damping measurements on a multi-blade cascade with multiple degrees of freedom," https://doi.org/10.1088/1742-6596/1296/1/012003.
4. KF Sagmo and PT Storli, 2019, "A test of the v²-f k-ε turbulence model for the prediction of vortex shedding in the Francis-99 hydrofoil test case," https://doi.org/10.1088/1742-6596/1296/1/012004.
5. E Tengs, LS Fevåg and PT Storli, 2019, "Francis-99: Coupled simulation of the resonance effects in runner channels," https://doi.org/10.1088/1742-6596/1296/1/012005.
6. P Østby, E Agnalt, B Haugen, JT Billdal and OG Dahlhaug, 2019, "Fluid structure interaction of Francis-99 turbine and experimental validation," https://doi.org/10.1088/1742-6596/1296/1/012006.
7. P Foti and F Berto, 2019, "Francis 99: Evaluation of the strain energy density value for welded joints typical of turbine runner blades," https://doi.org/10.1088/1742-6596/1296/1/012007.
8. M Lazarevikj, F Stojkovski, I Iliev and Z Markov, 2019, "Influence of the guide vanes design on stress parameters of Francis-99 turbine," https://doi.org/10.1088/1742-6596/1296/1/012008.
9. D Platonov, A Minakov and A Sentyabov, 2019, "Numerical investigation of pressure pulsations related to rotor-stator interaction in the Francis-99 turbine," https://doi.org/10.1088/1742-6596/1296/1/012009.
10. G Cvijetic, L Culic and H Jasak, 2019, "Application of the harmonic balance method for regime change prediction using Francis-99 test case," https://doi.org/10.1088/1742-6596/1296/1/012010.
11. C Trieu, X Long and B Ji, 2019, "Vortical structures in cavitating flow on the Francis-99 draft tube cone at off-design conditions with the new omega vortex identification method," https://doi.org/10.1088/1742-6596/1296/1/012010.

Group photograph

Acknowledgement

The experiments for the third Francis-99 workshop have been conducted under the HiFrancis project.

Download data

 Table 5. Click on the corresponding hyperlink to download the file.

Notations

LDV - Laser Doppler vibrometer

MFC - Macrofiber composite

Re - Reynolds number, Re=D⋅u/ν

f_{gv -} Guide vane passing frequency (Hz), f_gv=n⋅z_gv

n - Runner rotational speed (revolutions per second)

A - Cross-sectional area of test-section (m²), (0.15 m × 0.15 m)
Q - Flow rate (m³ s^-1)
v, w - Flow velocity (m s^-1)

ζ - Hydrodynamic damping

ω_n - Natural frequency of hydrofoil (Hz)