4:10 pm Presentation
“Cavitation Inception and Associated Pressure Field in a Turbulent Shear Layer”
Presented by KARUNA AGARWAL (Adviser: Prof. Katz)
Cavitation inception in the near field of the shear layer occurs in the core of quasi-streamwise vortices that develop between the main spanwise vortices. These located between 45 to 75% of the reattachment length and form 1-2mm diameter and 5-7 mm long cavities that form and collapse in less than 200 µs. The frequency of events at the same cavitation index increases rapidly with velocity, suggesting yet-unknown scaling trends. Hence, it is essential to measure and characterize the instantaneous pressure fields generated in the core of the quasi-streamwise vortices. Tomographic imaging followed by 3D particle tracking using the Shake-the-Box method are used for calculating the instantaneous velocity and acceleration fields. The experimental results are interpolated using Constrained Cost Minimization to generate divergence-free velocity and curl-free material acceleration at a spatial resolution of 250 µm. The pressure is obtained by spatially integrating the material acceleration. The procedure is tested for synthetic data based on the Johns Hopkins Turbulence Database. The measurements are performed at Reynolds numbers based on separating boundary layer height of Re=7100 and 17700 to characterize the effect of Reynolds number on the frequency, time evolution, size, strength and the pressure in the quasi streamwise vortices. Ultra-high speed imaging is being used to characterize the explosive growth and collapse of the cavities and the effect of nuclei.
4:35 pm Presentation
“Simulation Prediction and Adjoint-Based Sensitivity of Laser-Based Ignition in High-Speed Flows”
Presented by DAVID BUCHTA (Adviser: Prof. Zaki)
Large-scale numerical simulations are starting to enable the prediction of multi-physics phenomena. Their interactions are usually so complex that theory and models alone have a limited predictive capacity. Thus, the Center for Exascale Simulation of Plasma-coupled Combustion (XPACC), a Multidisciplinary Simulation Center within the Predictive Science Academic Alliance Program II (PSAAP II), is developing high-fidelity numerical simulations in conjunction with Exascale-aimed computer science tools and uncertainty quantification (UQ) analyses, such as adjoint-based sensitivity, to provide a route to prediction. Specifically, our prediction is space-time transient ignition kernel (TIK) characteristics in plasma-coupled, high-speed flow turbulence applications, like those anticipated on scramjets. Ignition is seeded using the breakdown of a focused laser beam in fuel-oxidizer mixtures, which dissociates the local mixture into elemental species, generates vorticity, and heats the gas above 10000 Kelvin, activating combustion within 1 nanosecond. Depending on the flow, the kernel can develop into a sustained flame or extinguish completely. To help identify mechanisms of successful or failed ignition, the solution of the discrete-exact adjoint equations for the corresponding compressible reacting flow equations provides input-output sensitivity to our quantity of interest, like the TIK. This sensitivity guides optimization and reduces parameter space to support UQ.