Browsing by Author "Sreedhar, M."

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    Large-Eddy Simulation of Longitudinal Stationary Vortices
    Sreedhar, M.; Ragab, Saad A. (AIP Publishing, 1994-07-01)
    The response of longitudinal stationary vortices when subjected to random perturbations is investigated using temporal large-eddy simulation. Simulations are obtained for high Reynolds numbers and at a low subsonic Mach number. The subgrid-scale stress tensor is modeled using the dynamic eddy-viscosity model. The generation of large-scale structures due to centrifugal instability and their subsequent breakdown to turbulence is studied. The following events are observed. Initially, ring-shaped structures appear around the vortex core. These structures are counter-rotating vortices similar to the donut-shaped structures observed in a Taylor-Couette flow between rotating cylinders. These structures subsequently interact with the vortex core resulting in a rapid decay of the vortex. The turbulent kinetic energy increases rapidly until saturation, and then a period of slow decay prevails. During the period of maximum turbulent kinetic energy, the normalized mean circulation profile exhibits a logarithmic region, in agreement with the universal inner profile of Hoffman and Joubert [J. Fluid Mech. 16, 395 (1963)].
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    Large-Scale Structures in a Subsonic Mixing Layer
    Ragab, Saad A.; Sreedhar, M.; Mulholland, D. (AIP Publishing, 1994-09-01)
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    Numerical Simulation of Vortices with Axial Velocity Deficits
    Ragab, Saad A.; Sreedhar, M. (AIP Publishing, 1995-03-01)
    Axial velocity deficit is a source of instability in vortices that may otherwise be stable. Temporal large‐eddy simulation is performed to study the response of vortices with axial velocity deficits to random and controlled disturbances at high Reynolds numbers. The qvortex [Batchelor, J. Fluid Mech. 20, 321 (1964)] is used as a model of such vortices. When the vortex is linearly unstable, the disturbances grow and result in the appearance of large‐scale helical sheets of vorticity. Later, these large‐scale helical structures break up into small‐scale filaments. Associated with the formation of the large‐scale structures is a redistribution of both angular and axial momentum between the core and the surroundings. The redistribution weakens the axial velocity deficit in the core while strengthens the rigid‐body‐like rotation of the core. The emerging mean velocity profiles drive the vortex core to a stable configuration. The vortex eventually returns to a laminar state, with an insignificant decay in the tangential velocity, but with a much weakened axial velocity deficit. A direct numerical simulation obtained at a lower Reynolds number confirms the above conclusions.