Showing papers by "M. S. Chandrasekhara published in 1995"

Analysis of low Reynolds number airfoil flows

Abstract: Compressible steady and unsteady flowfields over a NACA 0012 airfoil at transitional Reynolds numbers are investigated. Comparisons with recently obtained experimental data are used to evaluate the ability of a numerical solution based on the compressible thin layer Navier-Stokes approximation, augmented with a transition model, to simulate transitional flow features. The discretization is obtained with an upwind-biased, factorized, iterative scheme. Transition onset is estimated using an empirical criterion based on the computed mean flow boundary- layer quantities. The transition length is computed from an empirical formula. The incorporation of transition modeling enables the prediction of the experimentally observed leading-edge separation bubbles. Results for steady airfoil flows at fixed angles of attack and for oscillating airfoils are presented. HE prediction of steady, inviscid flows over aerodynamic configurations is performed routinely nowadays. The computation of flows with separation bubbles or of fully sep- arated flows, on the other hand, is still a very challenging problem. For many practical applications, the assumption of fully developed turbulent flow yields good predictions of the flowfield. In other circumstances, such as the leading-edge dynamic stall flow, this assumption is not valid and compu- tations of such flows need improved methods. A characteristic feature of the dynamic stall flow is the onset of compressibility effects at a very low freestream Mach num- ber of O.2. 1-2 In addition, at transitional Reynolds numbers, it has been shown3-4 that the dynamic stall events are closely governed by the formation of a laminar separation bubble and its subsequent bursting. In fact, the above-cited studies demonstrate that the failure of the separated shear layer to reattach initiates dynamic stall, leading to the formation of the dynamic stall vortex. Further complications arise when the locally supersonic flow forms shocks that interact with the local boundary layer. It is important to recognize that the scales of the flow are very small here and the flow physics is not very clear. It is obvious that an accurate computational study of the problem demands a proper modeling of the phys- ics of the local compressible flow. In an effort to reach this eventual goal, it is necessary to include the transition physics that plays a key role in the dynamic stall process. The study to be reported represents a step in this direction. It is well known that prediction of the transition point and the transition length in such strongly adverse pressure gradient driven flows with current methods is difficult and involves uncertainties. Although several methods are available, the engineering prediction of transition relies on empirical for- mulation for boundary-layer flows. Whereas these methods have been moderately successful in steady and subsonic flows,

A phase-locked high-speed real-time interferometry system for large amplitude unsteady flows

Abstract: A high speed phase locked interferometry system has been designed and developed for real-time measurements of the dynamic stall flow over a pitching airfoil. Point diffraction interferograms of incipient flow separation over a sinusoidally oscillating airfoil have been obtained at rates of up to 20 KHz and for free stream Mach numbers of 0.3 and 0.45. The images were recorded on ASA 125 and ASA 400 film using a drum camera. Special electronic timing and synchronizing circuits were developed to trigger the laser light source from the camera, and to initiate acquisition of the interferogram sequence from any desired phase angle of oscillation. The airfoil instantaneous angle of attack data provided by an optical encoder was recorded via a FIFO and in EPROM into a microcomputer. The interferograms have been analyzed using software developed in-house to get quantitative flow density and pressure distributions.

Computer-aided analysis of interferometric images of unsteady aerodynamic flows

Abstract: An algorithm was developed for computer-aided analysis of interferometric fringe patterns which relate to the instantaneous pressure coefficient fields of unsteady aerodynamic flows. Fringe centerlines are extracted by locating intensity extrema in the image. Edge detection using a dual threshold is employed as a guide for approximately locating the extrema and data smoothing via Chebyshev polynomial approximation is used to unambiguously locate the extrema. Quantitative pressure coefficient distributions result from the procedure.