Santanu Ghosh

A computational study has been done to assess the effectiveness of porous medium in control of normal-shock-wave/boundary-layer interaction at transonic speeds with a view towards application in aircraft wings. Passive control is achieved via re-circulation inside the porous medium, which weakens the shock structure, and hence reduces the wave drag. The study has been done for a Mach 1.3 normal-shock-wave/boundary-layer interaction on a at plate in the presence of a porous medium beneath the region of interaction. The domain used for the computations is adapted from a novel experimental setup, due to Holger Babinsky and his group at Cambridge University, that is capable of stabilizing a normal shock over a control region for fixed inlet parameters. The dependency of the control effectiveness on dimensions of the cavity (length, depth) and porosity has been studied. It is observed that whereas the cavity length has a strong effect on the reduction in total drag, the effects of depth and porosity are less pronounced. The computations are done as steady state RANS calculations using Menter’s k – ω/k – ϵ model for turbulence closure.

A computational study of transonic flow past porous medium has been performed to explore the flow features at the free-stream/porous-medium interface. An immersed boundary (IB) method is used to render the porous medium, modeled as a 2-D matrix of circles (projection of solid circular cylinders in a cavity). Investigation of the flow properties at the interface confirms the presence of a finite (average) slip velocity over the entire interface, which can make the flow more resistant to separation in the presence of an adverse pressure gradient. Lower wall-normal gradients of stream-wise velocity are observed at the interface and in the boundary layer downstream of the porous region, which indicate reduced average shear stress, and hence a lower skin friction drag. However, the results also reveal the formation of an adverse pressure gradient over the porous medium and in the region downstream of it as a result of the flow over porous medium. Parametric studies have also been performed to investigate the effects of changes in the structure of the porous medium on the mean flow properties at the free-stream/porous-medium interface and the boundary layer properties downstream of it. Results indicate that whereas the extents (length and depth) of the porous medium do not have much effect on the value of slip velocity, changes in the porosity and diameter of IB circles result in change of slip velocities. It is further observed that the boundary-layer health worsens as the flow moves past the porous medium and it is affected by the length, porosity, and diameter of circles forming the porous region. Investigation of transonic flow over the DFVLR R-4 airfoil with and without the porous medium indicates that the structure of the flow separation is altered but not attenuated with the introduction of the porous region, at least for the specific porous configuration used in the present study. The simulations are done as steady state Reynolds-averaged Navier-Stokes calculations, using Menter’s k − ω/k − ɛ model for turbulence closure.

Investigations of a turbulent Mach 1.3 flow past a porous medium are presented using computational fluid dynamics. The porous medium is modeled as a square array of circles in a cavity, whose porosity is varied by changing the inter-spacing of the circles. The flow topology within the porous medium near the clear-fluid/porous medium interface is shown to vary with porosity: secondary flows having a repetitive pattern are observed for porosity values less than 0.9, but not for higher values. The determination of the flow properties: streamwise velocity, pressure and temperature, at the clear-fluid/porous-medium interface, show that the properties vary in a repetitive manner between each pair of adjacent circles. Estimation of average values of the properties along the length of the clear-fluid/porous-medium interface shows the presence of a slip velocity, which varies with the porosity of the medium. Non-dimensional gradients normal to the free-stream direction of streamwise velocity and temperature at the clear-fluid/porous-medium interface show smooth variation with porosity; however, the same trend is not observed for pressure. The computations presented here are performed using an existing immersed-boundary method developed for compressible turbulent flows, wherein the porous medium constituted by the circles are discretized as immersed surfaces. Menter’s k −ω/k −ɛ turbulence model is used for the computations.