Three-dimensional Characterization of the Wake Flow behind Different Vortex Generators

  • Author / Creator
    Sen Wang
  • Vortex generators (VGs) are widely applied passive flow control devices that are proven to have significant effect in enhancing the performance of an aerodynamic system. With the recent development in non-intrusive measurement techniques, and computing ability, it has become possible to measure and simulate three-dimensional flow fields behind VGs. The objective of this investigation was to determine the effect that the shape of VGs may have on the wake flow through measuring the turbulence statistics and three-dimensional coherent structures.
    The experiment examined seven VGs including non-conventional and vane-type VGs. Non-conventional VGs referred to wishbone, doublet and ramp which have wedge-element. Vane-type VGs had rectangular, trapezoidal, and delta vane-blades. VGs were installed in a thin laminar boundary layer at Reynolds number of 930, based on the VG height and freestream velocity. For non-conventional VGs, planar particle image velocimetry (PIV) and stereoscopic PIV (stereo-PIV) measurements were conducted to measure the mean flow and turbulence statistics with high spatial-resolution. For both non-conventional VGs and vane-type VGs, time-resolved tomographic PIV (tomo-PIV) measurement was also carried out in a 92×55×13 mm3 volume at 800 Hz frequency to visualize the vortical structures and their evolution.
    The analysis of the non-conventional VGs consisted of characterizing the mean wake flow, determining the coherent structures and visualizing their evolution. The result suggests that the wishbone VG has the best performance in enhancing flow mixing. The wishbone VG also shows the fastest velocity recovery, followed by the doublet VG and the ramp VG. The peak of turbulence production in the wake of the wishbone and doublet VG has a similar magnitude, and is stronger than the peak in the wake of the ramp VG. The ramp VG produces the most coherent vortical structures based on the three-dimensional visualization. The perturbations caused by the second wedge of the doublet VG breaks down the primary vortex and transfer the energy to smaller structures. Although all the VGs being examined in this study have V-shape geometry, the wishbone VG with the least VG wetted area has the best performance in terms of mixing, while the doublet VG produced the highest level of turbulence.
    The investigation of the vane-type VGs focused on evaluating the device drag, comparing the three-dimensional mean flow field, and visualizing the instantaneous coherent structures. The momentum deficit analysis illustrated that rectangular vane VG produced the largest parasitic drag, followed by the trapezoidal vane VG and delta vane VG. The single rectangular vane VG presented a device drag larger than the trapezoidal vane VG and smaller than the rectangular vane VG. The measurements showed that the rectangular VG produced the strongest streamwise vortices and has the highest level of mean vorticity fluctuation and turbulence production. The instantaneous visualizations showed that the dominant primary vortex was a pair of counter-rotating streamwise vortices. The secondary vortices were observed to be wall-normal vortices. The investigation of the instantaneous coherent structure indicated that the coherency and strength of the generated vortex structure is inversely proportional to the sweep angle of the vane leading-edge. The analysis showed that the effective wall-normal mixing distance is a significant factor in selecting VG, regarding the device drag and flow control effectiveness. The supplementary video, MSc Thesis Supp Video_Sen Wang, can be found from ERA General Collection / Supplementary Thesis Materials (ERA General).

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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