Effect of Hydrofoil Trailing Edge Geometry on the Wake Dynamics

In the present study, the effect of a hydrofoil trailing edge shape on the wake dynamic and its interaction with the mechanical structure is investigated. This would help better describe the physical reasons for vibration reduction when using oblique and Donaldson trailing edges in comparison to a truncated trailing edge and subsequently allow its further optimization. Thus, hydrofoils with oblique and Donaldson trailing edges are tested in a high-speed cavitation tunnel at zero angle of attack and high Reynolds numbers, ReL = 5·105 – 3·106. The truncated trailing edge hydrofoil is selected as reference. A velocity survey is performed via Laser Doppler Velocimetery, LDV, and Particle Image Velocimetry, PIV. Proper-Orthogonal-Decomposition, POD, is used to extract coherent structures from PIV data. In addition, flow induced vibration measurements and high-speed visualizations are performed. Finally, the effects of a tripped boundary layer transition on the wake are investigated and compared with the natural boundary layer transition. Vortex-induced vibration is found to decrease significantly for oblique and Donaldson trailing edges in comparison to the truncated case, specially under lock-off condition. However, minimum vibration corresponds to the Donaldson trailing edge. The high-speed videos clearly show that for three tested hydrofoils the alternate vortices clearly detach from suction and pressure sides of the trailing edge. However, for the oblique and Donaldson trailing edges the location of the lower vortex detachment is obviously shifted upstream with respect to the upper one. As a result, when the upper vortex rolls up, it coincides with the passage of the lower vortex, leading to their collision. This strong interaction leads to a redistribution of the vorticity, which does not concentrate within the core of Karman vortices any more. However, the spatial phase shift between the separation point of the upper and the lower vortices is different in the case of oblique and Donaldson trailing edges due to the being free the separation point on the Donaldson curve. LDV phase-locked averaging under lock-in condition is performed for truncated, oblique and Donaldson trailing edges. The truncated trailing edge exhibits a symmetric wake. However, in the case of the oblique and Donaldson trailing edges, an asymmetric thickening of the downward near wake is observed. The stream wise velocity fluctuation shows two peaks of different amplitudes. In the case of the truncated trailing edge, the upper and lower vortices have the same core diameter, contrary to the oblique trailing edge, where a larger vortex core diameter is found for the lower vortex. In the case of Donaldson trailing edge, the LDV phase-locked averaging is performed for the tripped transition where the vibration amplitude is high enough to perform the phase-locked average. The measurements show the passage of one vortex after the collision in the near wake contrary to the oblique one