Experimental and Numerical Investigation of Vortex-Induced Vibration for a Fully Submerged Oscillating Circular Cylinder in a Circulating Water Channel

1. Experimental and Numerical Investigation of Vortex-
Induced Vibration for a Fully Submerged Oscillating
Circular Cylinder in a Circulating Water Channel
H. el Sheshtawy, M. Youssef, O. el Moctar, T. E. Schellin
ISOPE-2021￭ 20-25June.2021

2. Content
1. Motivation
2. EXPERIMENTAL SETUP
3. NUMERICAL METHODS
4. Results
5. Conclusion

3. 3
Motivation
• Flexible cylindrical structures are often subject to Vortex-Induced Vibration (VIV). In marine
engineering, VIV has many applications such as offshore risers, offshore wind turbine, monopile
towers used for offshore wind turbines etc, as shown in the following figure (Liu et al. (2020))
• The lock-in phenomenon occurs when the vortex shedding frequency is close to a natural frequency of
vibration of the cylinder. When this happens, large damaging vibrations can result, leading to a
considerable reduction of fatigue life. Many engineering structures are subject to lock-in phenomena,
such as offshore oil risers, offshore wind turbine towers, heat exchanger tubes, bridges, factory
chimneys, etc.
20-25June.2021
www.uni-due.de

4. EXPERIMENTAL SETUP
• Flexible cylindrical
structures are often
subject to Vortex-
Induced Vibration
(VIV). In marine
engineering, VIV
has many
applications such
as offshore risers,
offshore wind
turbine, monopile
towers used for
offshore wind
turbines etc.
20-25June.2021
www.uni-due.de

5. 5
Experimental Set-Up
• The physical experiment were conducted in a closed loop circulating water channel of the Institute of
Ship Technology, Ocean Engineering and Transport Systems (ISMT), University of Duisburg-Essen.
The channel’s measuring section was 6000 mm long, 1470 mm wide, and 670 mm deep. The water
surface was covered with Perspex plates.
• Three different time steps at at U∞=1.8 m/s, corresponding to Re = 5.6×104.
• The cylinder was made from Teflon and securely fixed at the center of its bottom to the channel’s
bottom plate. Consequently, the tip of the cylinder was free to move in two degrees-of-freedom
motions as a cantilever beam.
• Cylinder’s natural frequencies in air and water are 11.62 and 9.95 Hz, respectively, see. Tödter (2021)
and el Sheshtawy et al. (2021).
20-25June.2021
www.uni-due.de

6. 6
Numerical Method
• The computational domain was based on the geometric configuration of the circulating water tunnel.
The (stream wise) length and (span wise) height of the computational domain were, respectively, LD =
35D and H = water tank height (0.7 m), where Di is the cylinder’ diameter.
www.uni-due.de
Grid ∆x (m)
No. of cell
x106 ∆t (sec) CL (-)
1
0.1 7.882 1.0x10-3 0.037
2
0.08 15.269 8.0x10-4 0.028
3
0.063 30.563 6.3x10-4 0.035
20-25June.2021

7. 7
Numerical Method
• Navier-Stokes equations for the fluid flow, the momentum conservation law of the elastic isothermal
solid structure.
• Mesh morphing approach using the Laplace equation to couple between
• SIMPLE Algorithm solving the pressure velocity coupling equations
• A Detached-Eddy Simulation (DES) technique. The DES model is based on the Spalart-Allmaras (S-A)
one equation turbulence model (Spalart and Allmaras, 1994).
• STAR-CCM+
• Space and Unstructured grid with local refinement around superstructure and on lee side
• Time were discretized with 2nd order approximation
• Time step corresponds to a Courant number less than 0.2 on average
• The y+ value equaled unity in our study.
www.uni-due.de 20-25June.2021

8. 8
Experimental Vs. Numerical Results
• The following figure plots time histories of comparative computed and measured normalized
displacements in the transverse at the cylinder tip, Ay, expressed as follows:
Ay = y/D,
where y is the cylinder’s transverse oscillation and D is the cylinder’s diameter.
www.uni-due.de 20-25June.2021

9. 9
Results
• The following figure plots the Power Spectral Density (PSD) of inline (ax) and transverse (ay)
accelerations determined via a Fast Fourier Transformation (FFT). The VIV-induced vortex shedding
frequency, i.e., the frequency corresponding to the Strouhal number, turned out to be 6.586 Hz. The
second frequency peak occurred at 19.76 Hz and equaled three times the vortex shedding frequency.
20-25June.2021
www.uni-due.de

10. 10
Results
• The following plots our computed inline (Ax) and transverse (Ay) motions as trajectories. As seen, amplitudes of
transverse motions are larger than amplitudes of inline motions. Thus, transverse motions dominated the cylinder
vibration.
• The cylinder oscillating with a Lissajous (figure-eight shaped) trajectory.
• The maximum values of Ay ~ 0.9D and Ax ~ 0.4D.
20-25June.2021
www.uni-due.de

11. 11
Results
• The following counter plot depicts the flow field of VIV our numerical simulations. Vortex-induced
cylinder motions were well captured. At the cylinder’s top, the maximum normalized transverse
displacements were about Ay ~ 0.9. As shown, the highest displacement and consequently the stress
occurred at the tip of the cylinder. The different stress distribution on the cylinder was due to the VIV
had different densities initiates at different cylinder’s depth.
20-25June.2021
www.uni-due.de

12. 12
Summary
• First, the flow around a circular cylinder was experimentally investigated.
• Second, numerical three-dimensional fluid structure interaction simulations
were performed to evaluate the experimental measurements.
• Third, discretization errors were analyzed, and the effect of the y+ wall
treatment on the accuracy of the solutions was determined to verify the
simulations. Compared to the experimental results, the computed oscillations of
the cylinder in the transverse direction nearly equaled the experimentally
measured oscillations. The numerical FSI fluid flow compared favorably to the
experimental benchmark measurements. In future, various turbulent models
shall be implemented to check the validity of this approach.
20-25June.2021
www.uni-due.de

13. 13
References
1- Liu, G., Li, H., Qiu, Z., Leng, D., Li, Z., & Li, W. (2020). A mini review of recent progress on vortex-induced vibrations of
marine risers. Ocean Engineering, 195, 106704.
2- Tödter, S., el Sheshtawy, H., Neugebauer, J., el Moctar, O., & Schellin, T. E. (2021). Deformation measurement of a
monopile subject to vortex-induced vibration using digital image correlation. Ocean Engineering, 221, 108548.
3- El Sheshtawy, H., Tödter, S., Neugebauer, J., el Moctar, O., & Schellin, T. E. (2021). Experiment investigations for vortex-
induced vibration of a circular cylinder oscillating in two degrees-of-freedom with high aspect ratio and small mass ratio for
low reduced velocities, revised manuscript submitted to the the journal of Ocean Engineering
20-25June.2021
www.uni-due.de

14. Thank you for your attention!
Questions!