Experimental Investigation of Trailing-Edge Separation
Creators: Wang, Sen
Publisher: name: University of Alberta Library
Year: 2024

<p>Flow separation at the trailing-edge (TE) region of wings, known as trailing-edge separation, is a process where the boundary layer detaches from the wing surface under the effect of an adverse pressure gradient (APG). The phenomenon is known for reducing lift, increasing drag and producing unsteady force fluctuations. It is commonly seen on devices with a thick airfoil profile. Addressing the adverse effects of TE separation is essential for improving aviation safety and prolonging the lifetime of aerial vehicles. It is in the interest of the aerodynamic community to develop strategies to detect and mitigate trailing-edge separation. This process, however, requires an adequate knowledge of the flow field regarding its time-averaged and unsteady characteristics. <br>
In aim to gain deeper insights into TE separation, this study employed an experimental approach to investigate the turbulent separated flow near the TE of a two-dimensional (2D) wing with a NACA 4418 profile. The near-wall topology of the separated flow was characterized by performing full-span planar particle image velocimetry (PIV) measurements. The time-averaged near-wall streamline pattern revealed that a unique cellular structure formed at angle-of-attack of 9.7°. The structure is known as stall cell, featuring an arc-shape separation front that both ends spiral into two counter-rotating foci. The pattern of stall cell was observed to expand with increasing α. Its three-dimensional (3D) topology at angle-of-attack of 9.7° was characterized through a large-scale 3D particle tracking velocimetry (PTV) measurement with the help of a novel helium-filled soap bubble seeding system. The results showed that stall cell had a short height and consisted of two wall-normal counter-rotating vortices, which were normal to the wing surface and extended to the edge of the turbulent separation bubble. <br>
Time-resolved planar PIV measurements were then carried out to investigate the unsteadiness in trailing-edge separation. The Strouhal number Stl of the unsteadiness was computed based on the characteristic length l of the turbulent separation bubble and freestream velocity. Spectral analysis of the velocity field from the streamwise-wall-normal plane showed two ranges of Stl that were energetic. The lower range extended from Stl = 0.03 to 0.08, with a spectral peak occurring at Stl = 0.06. The higher range was observed within 0.2 &lt; Stl &lt; 0.8 and the spectral peak was detected at Stl = 0.4. Spectral proper orthogonal decomposition (SPOD) analysis was then performed using velocity data to identify the associated flow motions. It was found that the lower range corresponded to the breathing motion, while the higher range was attributed to the vortex shedding motions. The breathing motion was shown to correlate with the dynamics of shear layers. <br>
Finally, the relation between the breathing motion and wall pressure was investigated using simultaneous wall-pressure and planar PIV measurements. The velocity fields from PIV measurement were later synchronized with wall-pressure data in post-processing. Spatio-temporal cross-correlations between the velocity fields and wall pressure data demonstrated that the breathing motion was well correlated with the low-frequency wall-pressure fluctuations measured at 0.44l upstream and downstream of the mean detachment point. The results indicated that the optimal location for sensing the breathing motion was in the region with low-intermittency flow. To determine the phase relation between the breathing motion and low-frequency wall-pressure fluctuations, SPOD analysis was performed using synchronized velocity fields and wall-pressure data. A reduced-order model of velocity fields and wall pressure fluctuations was reconstructed, based on the leading SPOD mode at Stl = 0.024. The results revealed that the expansion (or contraction) of separation bubble preceded a decrease (or increase) in wall pressure measured 0.44l upstream of mean detachment point by a phase of 0.37π. Conversely, the expansion (or contraction) of separation bubble preceded an increase (or decrease) in wall pressure 0.44l downstream of mean detachment point by a phase of 0.34π. The observations align with the fact that TSB expansion occurs when local APG increases, whereas contraction corresponds to a decrease in APG. In summary, the results provide insights into the simultaneous evolution of breathing motion and wall pressure. The findings of this investigation can be utilized in devising strategies for detecting breathing motion and benefiting the future development of active control strategy.</p>