This investigation characterized the relation between the breathing motion and wall-pressure fluctuations for a turbulent separation upstream of the trailing edge of a two-dimensional wing with NACA 4418 profile. The experiments were conducted at a freestream velocity U _∞ = 10.2 m/s with a turbulent intensity of 0.4%. The wing had an aspect ratio of 1.2 and an angle of attack of 9.7°. The corresponding chord-based Reynolds number was 620,000. The measurements consisted of simultaneously acquired wall-pressure measurements at various streamwise locations and time-resolved particle image velocimetry (PIV) in a streamwise-wall-normal plane. Both measurements showed unsteadiness related to the breathing motion at low Strouhal number St _l ≈ 0.05. Here, St _l is defined based on the characteristic length, l, of the mean turbulent separation bubble (TSB). Cross-correlation between the measured wall pressures at different streamwise locations revealed that the breathing motion propagated at approximately 0.8U _∞ downstream of the mean detachment (MD) point. The breathing motion was observed to establish a stronger correlation with the low-frequency wall-pressure fluctuations in the low-intermittency regions as opposed to the high-intermittency regions. Spectral proper orthogonal decomposition was performed using the combined PIV velocity fields and wall-pressure measurements. The results revealed that the expansion (or contraction) of TSB preceded a reduction (or increase) in wall pressure measured upstream of MD and an increase (or reduction) in wall pressure measured downstream of MD. The findings align with the fact that TSB expansion occurs when local adverse pressure gradient (APG) increases, whereas contraction corresponds to a decrease in APG.

This investigation examined the flow field generated by a ramp-shaped vortex generator (VG) that underwent active oscillation within a laminar boundary layer. The oscillations were applied through a servomotor, which pivoted the VG around its leading edge. The study evaluated the influence of varying the maximum VG height during the oscillations (h), actuation frequency (f), and the waveform governing the periodic oscillation of the VG. Planar particle image velocimetry (PIV) measurements were conducted to estimate flow mixing and the drag induced by the VG. The height-based Reynolds number (Re _h) ranged from 300 to 600, and the chord-based Strouhal number (St _c) for the oscillations varied from 0.67 to 3.33. The findings of the study indicate that active VGs lead to a greater wall-normal transport of streamwise momentum and result in lower drag compared to static VGs. Furthermore, increasing h results in larger momentum transport and drag of the active VGs. The investigation also revealed that the highest momentum transport and drag occurred when f was close to the instability frequency of the shear layer. The results show the potential of active VGs for separation control under various flow conditions.

The unsteadiness of a turbulent separation bubble (TSB) formed close to the trailing edge of a two-dimensional wing was investigated using time-resolved particle image velocimetry. The angle of attack was set to 9.7° and the chord-based Reynolds number was 720 000. The TSB consisted of two shear layers and formed a triangular shape in the streamwise-wall-normal plane. The vertices of this triangle consisted of an intermittent detachment point, a fixed corner close to the airfoil trailing edge and an intermittent endpoint in the wake region. The velocity field had three energetic regions each with different Strouhal numbers (St _l): (a) an upstream turbulent boundary layer (TBL) with St _l = 0.1 to 4, (b) a TSB with St _l = 0.03 to 0.08 and (c) two shear layers with St _l = 0.4 to 0.8. The low-frequency motions in the TSB consisted of large zones of positive and negative streamwise velocity fluctuation that were several times wider than the large-scale structures of the upstream TBL. These zones forced an undulation of the separation line and were attributed to Görtler structures. They were also correlated with the velocity fluctuations between the two shear layers. The breathing motion of the TSB occurred at St _l = 0.05. This breathing correlated with the location of the TSB endpoint and the flapping of the upper shear layer. The detachment point of the TSB featured broad fluctuations and did not demonstrate a strong correlation with the breathing motion.

The velocity field over a two-dimensional wing at Reynolds number of 672,000 was investigated to characterize the three-dimensional topology and evolution of the separate flow with variation of the angle of attack. Planar particle image velocimetry over the full span of the wing demonstrated that with increasing angle of attack, isolated pockets of backflow, which appeared near the trailing edge, merged and formed an asymmetric stall cell at angle of attack of 9.7°. The asymmetry was mainly associated with the dissimilar boundary layers developed on the wind tunnel walls at the spanwise ends of the wing. Secondary structures were also observed between the stall cell and the spanwise end. The stall cell topology was characterized using large-scale three-dimensional particle tracking velocimetry measurements using helium-filled soap bubbles. The results showed that the separation bubble had a small wall-normal height with two wall-normal counter-rotating vortices extended up to the edge of the separation bubble. In addition, the investigations demonstrated that vortex generators can induce a symmetric stall cell by removing the secondary structures and isolating the stall cell from the flows at the spanwise ends.

The wakes of wishbone, doublet, and ramp-type vortex generators (VGs) were investigated. The VGs were placed in the thin laminar boundary layer of a flat plate at a Reynolds number of 930 based on the freestream velocity and VG height. The turbulence statistics of the wake were measured with high spatial resolution using planar particle image velocimetry (PIV) and stereoscopic PIV. Three-dimensional time-resolved tomographic PIV was also carried out to visualize the evolution of vortices. The fastest recovery of the wake deficit was observed for the wishbone VG. The peak of turbulence production in the wake of the wishbone and doublet VGs had a similar magnitude and was 1.5 times stronger than that of the ramp VG. The hairpin vortices generated by the ramp VG formed the largest percentage of the wake turbulent kinetic energy, and their size is about half of the hairpins produced by the wishbone and doublet VGs. The wishbone and ramp VGs had the best overall performance. The wishbone VG generated the strongest mixing in the wake region, whereas the ramp VG had the smallest drag coefficient. The doublet VG had the weakest overall performance due to low mixing and the largest drag.

Abstract: The effect of varying the vane sweep angle of different vortex generators (VGs) on the statistics of the wake flow and the coherent structures was investigated. Pairs of rectangular, trapezoid, and delta vanes with sweep angle varying in ascending order were arranged at an equal but opposite angle with respect to the flow. A single rectangular vane was also investigated to identify the effect of vane pairing. The VGs were installed in the thin laminar boundary layer of a flat plate at Reynolds number of 930, based on VG height and free-stream velocity. Time-resolved tomographic particle image velocimetry (tomo-PIV) was carried out in a volume covering the wake, and stereo-PIV was applied to two cross-flow planes of the wake. The measurements showed two counter-rotating streamwise vortices, which induce a strong upward motion along the centerline of the wake. A pair of secondary counter-rotating streamwise vortices were also observed. The single rectangular vane showed a primary streamwise vortex and a weaker secondary streamwise vortex with opposite rotation. The evaluation of wall-normal momentum transport along the wake centerline showed that the paired configuration enhanced flow mixing. The investigation of the instantaneous coherent structures and proper-orthogonal-decomposition of the three-dimensional velocity fluctuations indicated that the coherence and strength of the vortices were inversely proportional to the sweep angle of the VG. The delta vane VG, with the largest sweep angle, produced more small-scale turbulence while the rectangular VG, with an upswept vane, produced the most coherent streamwise vortices. The investigation of wall-normal momentum transport showed that the rectangular VG had the best performance in improving flow mixing, followed by the trapezoidal, single rectangular, and delta VG. When evaluating the performance of VG using the ratio of mixing enhancement over drag, the trapezoidal VG was the most efficient one. The investigation confirmed the possibility of lowering the device drag while maintaining the effectiveness of VG using an optimum sweep angle. The performance of delta VG was about 60% of the trapezoidal VG, which suggests a large sweep angle can adversely affect the VG performance. Graphical abstract: [Figure not available: see fulltext.].