Join online via Zoom: https://wse.zoom.us/j/435449376
“Large Eddy Simulations of a Developing Turbulent Boundary Layer over Cube Roughened Plate”
Presented by SAMVIT KUMAR
(Advisers: Profs. Meneveau & Mittal)
Turbulent boundary layers are present whenever there is fluid flow over a solid surface or wall. More often than not, these surfaces are rough. The roughness strongly influences flow physics and the drag at the wall. Therefore, it is important to understand the fundamental nature of turbulent boundary layers over rough walls. We first present an improved method for generation of turbulent inflow for simulations of developing boundary layers. The approach is based on prior recycling methods for flow over smooth (Lund et al., 1998) and rough (Yang and Meneveau, 2015) surfaces. In the recycling method, mean and fluctuation velocities on a sample plane are rescaled, combined and recycled back to the inlet, as the inflow velocity. A roughness-length related scale is chosen for rescaling of the inner layer, depending on the surface geometry and the displacement thickness is chosen instead of $delta_{99}$ as the length scale to rescale the outer layer. The blending function, dependent on both the inner and the outer length scales, is used to combine the two profiles, to obtain the inflow velocity.
We then find an appropriate grid size and use the integral wall model for Large Eddy Simulations of flow over a staggered array of cubes. Results obtained are compared with Direct Numerical Simulations carried out by (Lee et al., 2011) and good agreement is shown. We also set up simulations for flow over multi scale roughness elements.
“The Pressure Field and its Relation to Cavitation Inception in a Turbulent Shear Layer”
Presented by KARUNA AGARWAL
(Adviser: Prof. Katz)
We investigate the unsteady pressure field associated with quasi-streamwise vortices in the shear layer behind a backward facing step. The primary objective is to understand the conditions for cavitation inception, which occurs in the core of these vortices, and appears as 1-2 mm wide and 5-7 mm long cavities. The Reynolds numbers based on the step height are 1.6×10^4 and 5.8×10^4. High speed tomographic imaging followed by 3D particle tracking using the Shake-the-Box method is used for calculating the instantaneous velocity and acceleration fields. The data are interpolated using a constrained cost minimization technique, which establishes a divergence free velocity and curl-free material acceleration fields at a spatial resolution of 200µm. The pressure is then calculated by spatially integrating the material acceleration. Statistical analysis provides the probability density functions of the pressure, strength, size, and straining of the quasi-streamwise vortices. Effects of spatial resolution of the measurements are also discussed. With increasing Reynolds number, the pressure minima are more preferentially located within the quasi streamwise vortices, for longer durations, and appear to be strongly influenced by vortex stretching.