PhD: Experimental investigation on the interactions between freestream turbulence and an isolated bluff body

Research project description : The study of the fluid dynamic behavior around a structure has both fundamental scientific interest, and practical engineering applications, such as in the design of poles, bridges or aircraft wings [1,2,3]. Better understanding the interactions between the surrounding flow and a structure can inform design constraints, such as fatigue or cyclic loading limits. In particular, the interactions can be studied in terms of the downstream wake characteristics, and, depending on the degrees of freedom, the movement of the structure itself due to vortex induced vibrations or galloping, for example.

Research on canonical shapes such as cylinders, and, to a certain extent, simple airfoils that are either rigidly- or elastically-mounted can allow to identify the various physical phenomena involved in their interactions with the incoming flow. These simple cases provide the building blocks to understanding more complex shapes and physics. Additionally, controlling and characterizing the incoming flow allows to observe the resulting effects on the structure’s wake and vibrations. In general, most interaction studies are performed in homogeneous laminar flow conditions; however, recent studies, such as [4] conducted at IMFT, showed that, for an elastically-mounted cylinder, turbulent inflows induced greater vibration amplitudes than
laminar inflows. In the study, only two turbulent inflows were tested using passive grids, while turbulence intensity and the integral length scale were the only parameters used to characterize the inflow. More generally, recent reviews [5,6,7] identified remaining knowledge gaps in the field of bluff body aerodynamics that specifically involve performing experiments in higher Reynolds number ranges, and in comprehensively-characterized inflow conditions that would include freestream turbulence and unsteady inflow velocities.

The present PhD project focuses on experimentally investigating the role of incoming
turbulence on an isolated bluff body. The study will be conducted in the IMFT S2 wind tunnel to allow for a wide range of Reynolds numbers, and an active turbulence grid mounted at the test section inlet will generate the various turbulent inflows (1% < Tu < 15%). A detailed flow characterization downstream different turbulence grid configurations, similar to [8], will enable to determine scenarios of interest, including homogeneous, heterogeneous and unsteady (e.g. gusts) inflows. The effect of these various inflow conditions on a rigidly-mounted bluff body can subsequently be investigated, in terms of resulting aerodynamic forces and velocity
measurements in the wake. By introducing a single degree of freedom to the bluff body’s
motion, fluid-structure interactions, such as vortex induced vibrations, can then be studied
through the bluff body’s displacement, and velocity measurements in the wake.

[1] Timoshenko, S. (1974) Vibrations Problems in Engineering. D. Van Nostrand Company, Inc., 250 Fourth Avenue, New York, Second edition.
[2] Baarholm, G. S., Larsen, C. M., & Lie, H. (2006). ”On fatigue damage accumulation from in-line and cross-flow vortex-induced vibrations on risers,” Journal of Fluids and Structures, 22(1), 109-127. doi:10.1016/j.jfluidstructs.2005.07.013
[3] Cafarelli, I., Liauzun, C., Méry, F., Methel, J., Forte, M., & Lepage, A. (2024). Analysis of the impact of forced pitching oscillations in transonic flow on the transition onset and the aeroelastic behaviour of an airfoil. In IFASD 2024.
[4] Bourguet, R., & Mathis, R. (2023). A wind tunnel investigation of the effects of end and laminar/turbulent inflow conditions on cylinder vortex-induced vibrations. Journal of Fluids and Structures, 123, 104015. doi:
10.1016/j.jfluidstructs.2023.104015
[5] Forouzi Feshalami, B., He, S., Scarano, F., Gan, L., & Morton, C. (2022). A review of experiments on stationary bluff body wakes. Physics of Fluids, 34(1). doi: 10.1063/5.0077323
[6] Williamson, C. H. K., & Govardhan, R. (2008). A brief review of recent results in vortex-induced vibrations. Journal of Wind engineering and industrial Aerodynamics, 96(6-7), 713-735. doi: 10.1016/.jweia.2007.06.019
[7] Bearman, P.W. (2011). Circular cylinder wakes and vortex-induced vibrations. Journal of Fluids and Structures, 27, 648-658 . doi: 10.1016/j.jfluidstructs.2011.03.021
[8] Kurian, T., & Fransson, J. H. (2009). Grid-generated turbulence revisited. Fluid dynamics research, 41(2), 021403. doi:10.1088/0169-5983/41/2/021403