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Scientific
Investigations |

Scientific investigations mostly involve fundamentals of flow phenomena, e.g. stability and turbulence. A research group has under taken the following investigations in the recent past.
The flow past bluff body involves very complex flow physics as there are three simultaneous instabilities present in such flows, namely boundary layer instability, separated shear layer instability and primary vortex or Karman vortex instability. These three different frequencies interact among themselves and produce a complex flow structure. Two-dimensional calculations are not appropriate at high Reynolds numbers and give non-physical shedding (Saha et al., 1999a) since the flow is three dimensional beyond a Reynolds number of 170. The flow turns out to be chaotic at a Reynolds number of 500 (Saha et al., 2000a) and possibly turbulent at a Reynolds number of 1000. Flows above a Reynolds number of 200, require to be computed by solving the three-dimensional unsteady Navier-Stokes equation using very fine grids (Saha et al., 2000b). At the transitional Reynolds number (transition to three dimensionality), two distinct modes of vortex shedding are observed which are characterized by their spanwise wavelengths of the streamwise vortices. The early modes at the transitional Reynolds number, called Mode-A, has a wavelength of three-cylinder width and Mode-B which occurs at the later part of the transitional regime has a wavelength of one-cylinder width. Mode-A also shows an intermittent low frequency irregularity in the shedding cycle called Vortex Dislocation (Saha et al., 2000b). This particular phenomenon is believed to be partially responsible for transition to three-dimensionality.
In one of the earlier investigations (Mukhopadhyay et al. 1992), the effect of blockage ratio on variation of Strouhal number for different values of Reynolds numbers was determined. Engineering applications at high Reynolds number demand the aerodynamic characteristics of bluff body flows with reasonable accuracy using a very reasonable computer resource and time. Therefore, to economize computation and time, calculations are being carried out at high Reynolds number in two dimensions using various eddy viscosity models of turbulence. The use of model takes into account the energy transfer in the third dimension that is neglected is a usual two dimensional formulation. As far as the simulation for the mean flow at the midplane is concerned, RANS (Reynolds Average Navier-Stokes Solution) calculation has been found to be quite promising (Saha et al., 2001).
The work is under progress. Turbulent flow is being computed for a Reynolds number of 21400. The dynamics of the wake is the prime concern of this study. The evolution of coherent structures and incoherent turbulence is to be investigated. For the small scales of turbulence, a dynamic subrid-scale model is deployed. The Aeronautical Research and Development Board has sponsored the project.
The flow field due to an impinging jet at a moderately
high Reynolds number, emanating from a rectangular slot nozzle is computed
using large eddy simulation (LES) technique. A dynamic subgrid-scale model
has been used for the small scales of turbulence. Quite a few successful
applications of the dynamic subgrid-scale stress model use planer averaging
to avoid ill conditioning of the model coefficient. However, a novel localization
procedure has been attempted herein to find out the spatially varying
coefficient of the flow. The flow field is characterized by entrainment
at the boundaries. Periodic boundary conditions could not be used on all
the boundaries. The results reveal the nuances of the vortical structures
that are characteristic of jet flows. The stress budget also captures
locally negative turbulence production rate. The calibration of the model
has been made through prediction of the normalized axial velocity profile
and heat transfer on the impingement plate. The computed results compare
favorably with the experimental observations, especially in the stagnation
zone (Cziesla, Biswas et al., 2000). ** The references are available in the list of Publications |