Effect of vane sweep angle on vortex generator wake

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.