Computational and Experimental Investigation of Supersonic Two-dimensional and Axi-symmetric Shallow Open Cavities

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Copyright: Sridhar, Vikram
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Abstract
Time-accurate numerical simulations and experiments were carried out on two-dimensional/planar and axi-symmetric open cavities with length-to-depth ratios of 3, 5, 6 and 8 in a Mach 2 freestream. The study focusses on turbulent boundary layer at entry to cavity. The effect of entry boundary layer thickness on cavity flow dynamics was investigated using two-dimensional models. The axi-symmetric configuration was used mainly to eliminate possible three-dimensional effects that may have been present in two-dimensional cavity configurations and compare the results. The numerical investigation used an in-house code Eilmer-3 developed by Dr. Peter Jacobs and his associates at the University of Queensland. Eilmer-3 basically solves time accurate compressible Navier-Stokes equations using the advection upwind splitting method combining difference and vector splitting flux scheme and incorporates Wilcox k-w turbulence modelling. The experiments involved high speed time-resolved density sensitive flow visualisation such as schlieren, differential interferometry, streak schlieren technique and unsteady surface pressure measurements. The study on two-dimensional/planar configurations showed that beyond a certain length-to-depth ratio of the cavity, strong periodic oscillations become broadband and the flow is less unsteady. The cavity drag, which increased with length-to-depth ratio, however, increased at a faster rate once this threshold was crossed. For the two boundary layer thickness-to-cavity depth ratios considered here, this change in flow behaviour occurred when L/D ≈ 5. This was seen in both simulations and experiments. Use of differential interferometry on two−dimensional/planar cavities showed wave-fronts and closed loop structures (corresponding to vortices) in the shear layer. It also qualitatively confirmed the change in flow behaviour for L/D > 5. With the axi-symmetric cavities, the overall flow physics were similar to that of the planar cavities. Again the change in drag behaviour seemed to occurred when L/D ≥ 5. Experiments carried out in JAXA wind tunnel with a scaled up axi-symmetric model agreed with these observations. Both numerical and experimental data on axi-symmetric cavities showed that the assumption of two-dimensional flow in respect of planar cavities with large aspect ratios is a reasonable one to capture the essential flow physics of cavity flows. The experiments and numerical simulations showed that cavities with thin separating boundary layers are more unsteady than those with a thick separating boundary layer. For the two boundary layer thicknesses investigated, the threshold length-to-depth ratio, indicating change in flow behaviour, did not change and was L/D ≈ 5.The streak schlieren technique applied to time-resolved schlieren images revealed wave motions inside the cavity and enabled quantification of these motions for the first time. Finally, it has been shown that, incorporating the cavity floor in the boundary condition using the linear inviscid flow stability analysis, is necessary in the correct prediction of the growth rate of the cavity shear layer.
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Author(s)
Sridhar, Vikram
Supervisor(s)
Kleine, Harald
Gai, Sudhir
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Publication Year
2014
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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