77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024;
Salt Lake City, Utah
Session L38: Vortex Dynamics and Vortex Flows: Jets and Wakes
8:00 AM–10:23 AM,
Monday, November 25, 2024
Room: 355 D
Chair: Francisco Huera-Huarte, Universitat Rovira i Virgili
Abstract: L38.00006 : On the extraordinary effect of a specifically designed porous trailing edge in eliminating vortex shedding*
9:05 AM–9:18 AM
Abstract
Presenter:
Thomas Berger
(EPFL)
Authors:
Thomas Berger
(EPFL)
Mohamed Farhat
(École Polytechnique Fédérale de Lausanne)
The present study uncovers how a specifically designed porous extension attached to a hydrofoil trailing edge, may remarkably remove vortex shedding and thus the vortex-induced vibrations (VIV). The case study is a blunt truncated NACA 0009 hydrofoil of 100 mm chord length and 150 mm span, placed in a water stream at high Reynolds number (Re=0.6x106 to 2 x 106). In the absence of our porous extension, as the flow velocity is linearly increased from 6 to 20 m/s, the alternate Karman vortices generated in the wake are responsible of vibrations with a strong torsional lock-in at flow velocities ranging from 15 to 17 m/s. The porous extension, however, largely reduces the flow induced vibrations and the lock-in is effectively suppressed. Specifically, the RMS value of the surface velocity signal is reduced from ARMS = 1.2 to 0.4 mm/s (a 67% reduction) under lock-off conditions, and from ARMS = 269 to 1.3 mm/s (a 99.5% reduction) under lock-in conditions. Laser measurements confirm that the porous insert eliminates the frequency peak associated with the Strouhal shedding frequency and reduces broadband noise excitation. Detailed velocity measurements in the wake using laser Doppler anemometer uncovers how a specific porosity prevents the formation of coherent and periodic Karman vortices. In particular, we found that porous extension is responsible for the generation of streamwise and transverse jets, which extend into the far wake. We believe that this is the key mechanism in suppressing vortex shedding. Interestingly, the measurement of lift and drag forces did not reveal any significant alteration of the hydrodynamic performances of the hydrofoil with the porous extension. These promising results have far-reaching implications for the design of mechanical structures susceptible to VIV, such as aircraft wings, marine propellers, hydraulic pumps, and turbines among others. The potentials benefits include reduced noise emissions and mitigated fatigue risks.
*Swiss National Science Foundation under Grant No. 219723.