Showing papers by "Manoochehr Koochesfahani published in 2009"

MTV measurements of the vortical field in the wake of an airfoil oscillating at high reduced frequency

Abstract: We present an experimental investigation of the flow structure and vorticity field in the wake of a NACA-0012 airfoil pitching sinusoidally at small amplitude and high reduced frequencies. Molecular tagging velocimetry is used to quantify the characteristics of the vortex array (circulation, peak vorticity, core size, spatial arrangement) and its downstream evolution over the first chord length as a function of reduced frequency. The measured mean and fluctuating velocity fields are used to estimate the mean force on the airfoil and explore the connection between flow structure and thrust generation.Results show that strong concentrated vortices form very rapidly within the first wavelength of oscillation and exhibit interesting dynamics that depend on oscillation frequency. With increasing reduced frequency the transverse alignment of the vortex array changes from an orientation corresponding to velocity deficit (wake profile) to one with velocity excess (reverse Karman street with jet profile). It is found, however, that the switch in the vortex array orientation does not coincide with the condition for crossover from drag to thrust. The mean force is estimated from a more complete control volume analysis, which takes into account the streamwise velocity fluctuations and the pressure term. Results clearly show that neglecting these terms can lead to a large overestimation of the mean force in strongly fluctuating velocity fields that are characteristic of airfoils executing highly unsteady motions. Our measurements show a decrease in the peak vorticity, as the vortices convect downstream, by an amount that is more than can be attributed to viscous diffusion. It is found that the presence of small levels of axial velocity gradients within the vortex cores, levels that can be difficult to measure experimentally, can lead to a measurable decrease in the peak vorticity even at the centre of the flow facility in a flow that is expected to be primarily two-dimensional.

Molecular Tagging Techniques for Micro-Flow and Micro-Scale Heat Transfer Studies

Abstract: We report recent progresses made in development of novel molecule-based flow diagnostic techniques, named as Molecular Tagging techniques, to achieve simultaneous measurements of multiple important flow variables (such as flow velocity and temperature) for micro-flows and micro-scale heat transfer studies. Instead of using tiny particles, specially-designed phosphorescent molecules, which can be turned into long-lasting glowing molecules upon excitation by photons of appropriate wavelength, are used as tracers for both velocity and temperature measurements. A pulsed laser is used to “tag” the tracer molecules in the regions of interest, and the movements of the tagged molecules are imaged at two successive times within the photoluminescence lifetime of the tracer molecules. The measured Lagrangian displacement of the tagged molecules between the two image acquisitions provides the estimate of the fluid velocity vector. The simultaneous temperature measurement is achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the tagged molecules in the two images. The implementation and application of the MTV&T technique are demonstrated by conducting simultaneous velocity and temperature measurements to qunatify the transient behavior of electroosmotic flow (EOF) inside a microchannel and to reveal the unsteady heat transfer, mass transfer and phase changing process inside micro-sized water droplets pertinent to wind turbine icing phenomena.Copyright © 2009 by ASME

A vortex array model of the unsteady wake of a pitching airfoil

Abstract: In this study we investigate the ability of a simple, vortex-array model to represent the unsteady velocity field in the wake of an airfoil undergoing harmonic as well as non-harmonic pitch oscillation. To this end, the model predictions are compared against existing experimental data of the streamwise velocity in the near wake of an oscillating NACA 0012 airfoil at a chord Reynolds number of 11,000 and a range of reduced frequencies extending up to approximately 10. The results show that the model predictions are in agreement with the experimental observations. Moreover, the model yields the transverse velocity profile, which is not available from the experimental data used here. This enables computation of the mean streamwise force acting on the airfoil from application of the integral momentum equation. Comparison of the model-predicted force coefficient and its dependence on the reduced frequency are found to be consistent with published experimental and computational data.