Experimental investigations on the sharp leading-edge separation over a flat plate at zero incidence using particle image velocimetry

Abstract

Leading-edge separated flow field over a sharp flat plate is experimentally investigated in Reynolds numbers ranging from 6.2 × 103 to 4.1 × 104, using particle image velocimetry (PIV) and its statistics. It was observed that the average reattachment length is nearly independent of Reynolds number and the small secondary bubble observed near the leading edge was found to shrink with increasing Reynolds number. The wall-normal profiles of the statistical values of kinematic quantities such as the velocity components and their fluctuations scaled well with average reattachment length lR and freestream velocity U∞. Their magnitudes compare well with previous investigations even though the current triangular shaped sharp leading edge is different from previous flat-faced or semi-circular ones. The shear layer was observed to exhibit 2 different linear growth rates over 2 distinct regions. Instantaneous PIV realizations demonstrate unsteady nature of the separation bubble, whose origins in the upstream portion of the bubble are analysed. Bimodal nature of the probability density function (PDF) of fluctuating streamwise velocity at around x/lR = 0.08–0.15 indicates successive generation and passage of vortices in the region, which subsequently interact and evolve into multiscale turbulent field exhibiting nearly Gaussian PDF. Shedding of vortices with wide range of scales are apparent in most of the instantaneous realizations. Proper Orthogonal Decomposition (POD) of the velocity fluctuation magnitude field revealed that the flow structures of the dominant modes and their relative energies are independent of Reynolds number. In each of the dominant modes (first 3 modes), the length scales corresponding to the large scale structures and their spacing are the same for all Reynolds numbers, suggesting that their Strouhal number (observed to be ~ 0.09–0.2 at Reynolds number of 6.2 × 103) of unsteadiness should also be independent of Reynolds number. A single large structure- comparable in size to lR—was apparent well before reattachment in a few instantaneous realizations, as compared to multiple small-scale structures visible in most realizations; at Reynolds number of 6.2 × 103, realizations with such large-scale structures occurred approximately after every 20–30 realizations, corresponding to non-dimensional frequency of 0.4–0.6, which is identified to be the “regular shedding”. It was possible to reconstruct the large-scale structure during the instances from just the first 3 POD modes, indicating that the Strouhal number of regular shedding too is independent of Reynolds number. Graphic abstract

The safety of surgical technique for ileostomy and colostomy in preventing parastomal hernias: an in vitro experimental simulation study

“…Findings from material physics have identified sharp points including edges of incisions as high pressure zones [ 23 ]. This physical knowledge was investigated in this study.…”

Turbulent flow around circular arcs

“…This is identified as the location where the tangential velocity at the closest measurement point to the wall vanishes. 47 To increase the accuracy of this measurement, the field of view is zoomed to the leading edge.…”

“…Lift (a) and drag (b) coefficients versus angle of attack for the range of investigated Reynolds numbers.The ideal angle of attack is the angle of incidence where the stagnation point is at the leading edge. This is identified as the location where the tangential velocity at the closest measurement point to the wall vanishes 47. To increase the accuracy of this measurement, the field of view is zoomed to the leading edge.Figure9shows the location of the stagnation point around the leading edge of the plate for 𝛼 from 8° to 12° at 𝑅𝑒 = 68,200.…”

Surface Roughness Effects on Cavity Flows

“…In the figure, we have also overlaid the experimental data from the low-speed measurements carried out at the University of Glasgow for comparison. The PIV setup used for the experiments, sampling rate, number of samples chosen for statistical convergence, and the method of processing the raw PIV images using the software package DaVis 8., LaVision (including the number of iterations and the size of interrogation window in each iteration) are the same as that described in Fujiwara et al [21,39]. The uncertainty in velocity was estimated to be less than 2% of the mainstream velocity.…”