Experimental and Numerical Study of Flow Structures Associated With Low Aspect Ratio Elliptical Cavities

Abstract

The flow regimes associated with a 2:1 aspect ratio, elliptical planform cavity in a turbulent flat plate boundary layer have been systematically examined for various depth/width ratios (0.1 to 1.0) and yaw angles (0° to 90°), using a combination of wind tunnel experiments (involving Particle Image Velocimetry and helium bubble visualization) and Computational Fluid Dynamics (CFD) simulations (employing threedimensional steady calculations with the Reynolds Stress model turbulence closure).Satisfactory agreement has been found between the results using the two methods, indicating that the steady numerical simulations can be a cost-effective tool to predict the mean flow features.The flow has been found to be highly dependent on yaw angle and cavity depth. For each of the three broad flow categories specified according to yaw angle, which include the symmetric flow regime (yaw angle = 0°), the straight vortex regime (yaw angle = 90°) and the asymmetric flow regime (15° ≤ yaw angle ≤ 60°) different regimes are found to exist, depending on cavity depth. For each combination of yaw angle and depth, the flow has been analyzed through investigation of shear layer parameters, the three-dimensional vortex structure, pressure distribution and drag, wake flow, and vortex oscillations.While the elliptical cavity flows have been found to have some similarities with those of nominally two-dimensional and rectangular cavities, the three-dimensional effects due to the low aspect ratio and curvature of the walls give rise to features exclusive to low aspect ratio elliptical cavities, including formation of cellular structures at intermediate depths and unique vortex structures within and downstream of the cavity.The three-dimensional flow structure of the flow is most pronounced in the asymmetric regimes with large yaw angles (45° and 60°). The dominant feature in this regime is the formation of a trailing vortex that is associated with high drag and flow oscillations within the cavity.iv Keywords

Global Surface Pressure Pattern for a Compressible Elliptical Cavity Flow Using Pressure-Sensitive Paint

“…The value of C p,peak at x * = 0.92 is 0.28, which is slightly less than that for ε = 0. However, there is greater flow expansion near the rear edge because of an increase in the entrainment over the sidewall [15]. For L/H = 14.33, there is less expansion near the rear edge, and the value of C p,peak at x * ≈ 0.91 is 0.32.…”

“…(𝒂) 𝜀 = 0 (𝒃) 𝜀 = 0.66 (𝒄) 𝜀 = 0.87 The region in which Cp is positive moves further upstream, and an arc-shaped pressure pattern is shown in Figure 6b,c for  = 0.66 and 0.87. Hering and Savory [15] demonstrated that there is an increase in entrainment over the sidewall for rectangular cavities with a greater value for L/W. For elliptical cavities, there is early shear layer impingement on the cavity floor as  increases.…”

“…), of zero, where L and W are the respective lengths of the major axis and the minor axis. For an incompressible elliptical cavity flow, Khadivi and Savory [15] (freestream velocity = 18.3 m/s; W = 53.75 mm; W/L = 0.5; Reynolds number based on the length of minor axis, Re W = 8.7 × 10 4 ) showed that the shear layer grows almost linearly downstream of the separation point. There are symmetrical cellular structures inside the cavity volume if the minor axis is aligned with the streamwise direction.…”

Characteristics of Turbulent Flow Past Passive Rectangular Cavity at Large Reynolds Numbers

“…3a that the vortex eye location is almost not shifting with mainstream Reynolds numbers, and to quantify it, an attempt has been made to extract the location of the vortex eye. The vortex eye in the recirculation zone in the cavity can be identified easily by the local pressure minima criterion [15]. Hence, in the present work, the local pressure minima criterion is used to determine the vortex eye location.…”

“…(10) to measure the shear layer thickness as it is a measure of maximum shear [20]. In the present work, the shear layer growth along the axial directions is extracted using the widely used formula, vorticity thickness, d x [6,15,16,21] as given by,…”