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Title:Low-frequency unsteadiness in wake of a normal flat plate
Author(s):Najjar, Fady M.; Balachandar, S.
Subject(s):Low-frequency Unsteadiness
Normal Flat Plate
Abstract:The separated flow past a zero-thickness flat plate held normal to a free stream at Re=250 has been investigated through numerical experiments. The long-time signatures of the drag and lift coefficients clearly capture a low frequency unsteadiness with a period of approximately 10 times the primary shedding period. It is observed that the low-frequency unsteadiness is not a simple superposition of a low frequency undulation imposed on the shedding frequency . The amplitude of drag and lift variations during the shedding process is strongly modulated by the low frequency. Furthermore, the low frequency unsteadiness modulates the shedding frequency, with the shedding frequency increasing with increasing mean drag. A simple model is proposed to characterize these variations. A physical interpretation of the low frequency behavior is that the flow oscillates between two different regimes: a regime H of high mean drag and a regime L of low mean drag. The fundamental difference in the Karman vortex dynamics between these two regimes is also explored through a phase-averaging procedure. It is observed that in regime H the shear layer rolls-up closer to the plate to form coherent spanwise vortices, while in regime L the shear layer extends farther downstream and the rolled-up Karman vortices are less coherent. The eduction of the three-dimensional vortical structures is achieved by defining the swirling strength as the imaginary part of the complex eigenvalue of the velocity gradient tensor and visualizing it. The three-dimensional structure of the wake is significantly different between regimes H and L. In the high drag regime three-dimensionality is characterized by coherent Karman vortices which are deformed along the spanwise direction and reasonably well organized streamwise vortices connecting the Karman vortices. With a non-dimensional near wake spanwise wavelength of about 1.2, the three-dimensionality in this regime is reminiscent of mode B three-dimensionality. It is observed that the high degree of spanwise coherence that exists in regime H breaks down in regime L. In the low drag regime the Karman vortices are seen to tear apart and gets tilted along the streamwise direction to form highly incoherent collection of streamwise vortices. It appears that the low frequency unsteadiness might be the result of a beat phenomenon arising from the interaction between the characteristic shedding frequencies in regime H and L. Based on detailed visualization of flow structures we conjecture that the formation of streamwise and spanwise vortices are not in perfect synchronization and that the low frequency unsteadiness is the result of this imbalance ( or phase mismatch).
Issue Date:1997-08
Publisher:Department of Theoretical and Applied Mechanics. College of Engineering. University of Illinois at Urbana-Champaign
Series/Report:TAM R 860
Genre:Technical Report
Sponsor:National Science Foundation 97/08
Rights Information:Copyright 1997 Board of Trustees of the University of Illinois
Date Available in IDEALS:2021-11-04

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  • Technical Reports - Theoretical and Applied Mechanics (TAM)
    TAM technical reports include manuscripts intended for publication, theses judged to have general interest, notes prepared for short courses, symposia compiled from outstanding undergraduate projects, and reports prepared for research-sponsoring agencies.

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