Graduate Seminar in Fluid Mechanics

March 9, 2018 @ 4:00 pm – 5:00 pm
132 Gilman Hall

4:10 pm Presentation

 “Experimental Study of Shock Waves Interaction with Rigid Porous Media”

 Presented by OMRI RAM (Adviser: Prof. Katz)

It is well known that porous obstacles can cause significant diffraction and attenuate a shock wave propagating through them. Various models were proposed in the past to incorporate the microscopic interaction forces between the fluid and the skeleton of the porous sample into a macroscopic solution of the governing equations. However, these models which are usually based on a multiphase solution approach require identifying multiple properties of the fluid, the solid matrix and its geometry, some of which are notoriously difficult to measure. In this study, silicon carbide porous media with various porosities were placed in a shock tube at a fixed distance from the end-wall. The samples were subjected to a shock wave and the pressure build-up at the end-wall was recorded. An analysis methodology was developed to study the effect of various parameters on the pressure build-up in the confined volume. This methodology addresses the porous medium and the gas in the confined volume behind it as a single mechanical system. Assuming that the flow through the porous sample is close to being isentropic, a constitutive model that enables predicting the pressure profile developing on the end-wall was derived. Furthermore, it was shown that all of the experimental results can be represented in a non-dimensional form, thus revealing the similarity between them. The mechanical system perspective enabled us to better understand the physical mechanisms affecting the pressure pulse transformation while passing through the porous medium and through the air gap between the rear face of the porous sample and the end-wall. The modal response of the system revealed that when an arbitrary pressure pulse is imposed on the front face of the porous medium the high frequency spectral components were attenuated. The system acts as a low pass filter on the pressure profile propagating through it and inhibits the propagation of fast changing pressure pulses.

4:35 pm Presentation

Instability of Supersonic Boundary Layers and its Sensitivity to Base-Flow Distortion”

 Presented by JUNHO PARK (Adviser: Prof. Zaki)

The nonlinear parabolized stability equations (NPSE) can accurately and efficiently predict the amplification of finite amplitude instability waves and transition to turbulence in high-speed boundary layers. The base state is obtained from the similarity solution of the boundary-layer equations, and is distorted by the instabilities. While the NPSE fully accounts for this distortion, it does not account for potential uncertainties in the base state due to the flow environment, and boundary and thermal conditions.  These uncertainties alter the transition behavior.  In this work, we examine the sensitivity of finite-amplitude boundary-layer instabilities to base-flow distortions using the NPSE framework. We start with a review of the transition in supersonic boundary layers, and formulate the sensitivity analysis via theoretical (adjoint) and numerical techniques. The sensitivity of instability waves and transition onset to modifications in the base velocity and temperature are analyzed, and the uncertainty in transition due to wall heating is discussed.

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