Particle Image Velocimetry Investigation Of Strong Disturbance Effect On Boundary Layer

Strong disturbance is classified as wake vortex disturbance outside the boundarylayer and roughness on the boundary surface, based on the performance interactingwith boundary layer. Cylinder wake, hemisphere, and interacting barchan dunes areinvolved to be as the strong disturbances to effect on boundary layer, and theinvestigation is accomplished by particle image velocimetry (PIV). It is indicated thatthe new coherent structures can be directly generated by these three typicaldisturbances in the boundary layer:(1) transverse secondary vortex induced bycylinder wake;(2) hairpin vortice generated by hemisphere;(3) recirculation zoneexisted between the barchans’ horns, all of these new coherent structures exhibitdifferent ways on the mechanism of generation, existence and evolution by comparingwith the coherent structures obtained from time-resolved tomographic PIV(TRTomo-PIV) data in turbulent boundary layer flow.The hairpin vortex is the universal coherent structure detected from all the cases,however, they are generated by different topology and scenario, so the mainconclusions of the present investigation are given by each case, as follows:The trasverse secondary vortex is induced from the interaction between thecylinder wake vortex and the boundary layer, which indicates the onset of bypasstransition, and the wake vortex performs a key role in the whole transition byaccelerating the generation of coherent structures, and lifting them up to show adifferent topology. Especially, the hairpin head is almost vertical to the wall with anincline angle about90°; the hairpin packets also has a larger statistical scale with anincline angle of17°with the wall and a spanwise width of0.55δ between the hairpins’legs in the streamwise/spanwise plane at the height of0.2δ.TR PIV is utilized to acquire the temporal-spatial vector field behind thehemisphere disturbance in a boundary layer flow. The vector field is analyzed byDynamic Mode Decomposition (DMD) and Proper Orthogonal Decomposition (POD)from different perspectives, respectively. The main dynamic modes representing theshedding hairpin vortices and the near-wall coherent structure induced by hairpins areextracted by identifying their frequencies and associated spatial structures. On the other hand, the energy of the hairpin vortice decreases along the streamwise is provedby proper orthogonal decomposition.In order to quantify the flow structure over interacting barchans, PIVmeasurements are made wherein the dune models are immersed in a flowing fluid thatis refractive-index-matched to the dune material. Doing so provided full optical accessto the obstructed regions of flow and eliminated reflections from the liquid-solidboundaries, allowing the whole-plane data to be collected. Two clear barchan modelswith volumetric ratios of0.125were arranged in tandem, and flow fieldmeasurements are made in multiple streamwise-spanwise planes. Ensemble-averagedflow fields, Reynolds stresses and swirling strength are obtained for co-line andoff-line with different barchans spacing and compared to the reference case of anisolated barchan. For the co-line case, the shedding vortexes from the upstreambarchan directly attach on the stoss-side of the downstream one, it changes the energyof the free flow, which results in the decrease of the Reynolds stresses and swirlingstrength of the lee-side flow of the downstream barchan. For the off-line case, theexistence of the downstream barchan directly destroy the symmetric character of theinteracting flow fields between two barchans, in turn, the inclining flow leads to theun-symmetric lee-side flow of the downstream barchan.Statistical methods, such as, conditional average, power spectra, Taylor’s frozenturbulence hypothesis and spatial correlations, are combined to extract coherentstructures from TR Tomo-PIV data in a turbulent boundary layer flow. It is indicatedthat the elongated low-speed and high-speed streak regions are accompanied byhairpins and hairpin packets structures which plays the part of adjusting andtransferring the momentum inside turbulent boundary layer flow. All of thesestructures work together to make up the characteristic of coherent structures in thelogarithmic region of the turbulent boundary layer.