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. However, the passage of two vortices corresponding to the upper and lower vortex is found far from the Donaldson trailing edge. LDV measurements show that the collision of the vortices for the oblique trailing edge, observed under lock-in conditions, also prevails for the lock-off condition in the case of oblique and Donaldson trailing edges. The velocity profile comparison at the vortex formation length for three trailing edges shows that in the case of the Donaldson trailing edge, the wake width increases significantly in comparison to two other trailing edges. Moreover, the minimum stream wise and transverse velocity fluctuation profiles obtained for the three trailing edges correspond to the Donaldson trailing edge. The strong similarity of results obtained for lock-in and lock-off conditions indicates that the collision between upper and lower vortices, clearly observed under lock-in, also occurs for lock-off condition. A thicker boundary layer with laminar-to-turbulent transition occurring further upstream is observed for the pressure side of the Donaldson trailing edge in comparison with the suction side. In contrast to the truncated trailing edge, both sides have a similar boundary layer structure and a similar transition location. Moreover, a thicker boundary layer is found for the Donaldson trailing edge in comparison to the truncated case. The collision between upper and lower vortices is also observed in the case of a tripped transition. However, the vortices are shed with a larger core diameter, greater strength, and lower frequency than for the natural transition. These investigations let us believe that the collision between upper and lower vortices and the resulting vorticity redistribution is the main reason for the vibration reduction obtained with oblique and Donaldson trailing edges. This result opens the way for more effective hydrofoil geometry optimization for further reduction of flow induced vibration.