Engineering topics

Hossain, Murshed

Publications and source records attributed to Hossain, Murshed.

5 records

NASA NTRS1992

Turbulent transport of a passive-scalar field by using a renormalization-group method

A passive-scalar field is considered to evolve under the influence of a turbulent fluid governed by the Navier-Stokes equation. Turbulent-transport coefficients are calculated by small-scale elimination using a renormalization-group method. Turbulent processes couple both the viscosity and the diffusivity. In the absence of any correlation between the passive-scalar fluctuations and any component of the fluid velocity, the renormalized diffusivity is essentially the same as if the fluid velocity were frozen, although the renormalized equation does contain higher-order nonlinear terms involving viscosity. This arises due to the nonlinear interaction of the velocity with itself. In the presence of a finite correlation, the turbulent diffusivity becomes coupled with both the velocity field and the viscosity. There is then a dependence of the turbulent decay of the passive scalar on the turbulent Prandtl number.

Hossain, Murshed↗

NASA NTRS1991

Three-dimensional compressible hydrodynamic convection in the sun and stars

A pseudospectral code is used to obtain numerical solutions of 3D compressible hydrodynamic convection in a stratified medium where the boundaries are open and where radiative losses are strongest at the top. Results are presented for a series of three simulations in progressively deeper boxes. The code does not make the anelastic approximation, but follows sound waves explicitly. Spatial organization is found in the form of vertical 'stacking' of smaller structures on top of larger ones, with each structure being roughly Hp to a vertical extent. Compressibility effects are apparent in the density: the snapshots indicate that, on a horizontal plane, the density at certain points may be of an order of 10 percent larger or smaller than elsewhere on that plane.

Hossain, Murshed↗

NASA NTRS1991

Inverse energy cascades in three-dimensional turbulence

Fully three-dimensional magnetohydrodynamic (MHD) turbulence at large kinetic and low magnetic Reynolds numbers is considered in the presence of a strong uniform magnetic field. It is shown by numerical simulation of a model of MHD that the energy inverse cascades to longer length scales when the interaction parameter is large. While the steady-state dynamics of the driven problem is three-dimensional in character, the behavior has resemblance to two-dimensional hydrodynamics. These results have implications in turbulence theory, MHD power generator, planetary dynamos, and fusion reactor blanket design.

Hossain, Murshed↗

NASA NTRS1990

New numerical solutions of three-dimensional compressible hydrodynamic convection

Numerical solutions of three-dimensional compressible hydrodynamics (including sound waves) in a stratified medium with open boundaries are presented. Convergent/divergent points play a controlling role in the flows, which are dominated by a single frequency related to the mean sound crossing time. Superposed on these rapid compressive flows, slower eddy-like flows eventually create convective transport. The solutions contain small structures stacked on top of larger ones, with vertical scales equal to the local pressure scale heights, H sub p. Although convective transport starts later in the evolution, vertical scales of H sub p are apparently selected at much earlier times by nonlinear compressive effects.

Hossain, Murshed↗

NASA NTRS1988

Renormalization-group theory for the eddy viscosity in subgrid modeling

Renormalization-group theory is applied to incompressible three-dimensional Navier-Stokes turbulence so as to eliminate unresolvable small scales. The renormalized Navier-Stokes equation now includes a triple nonlinearity with the eddy viscosity exhibiting a mild cusp behavior, in qualitative agreement with the test-field model results of Kraichnan. For the cusp behavior to arise, not only is the triple nonlinearity necessary but the effects of pressure must be incorporated in the triple term. The renormalized eddy viscosity will not exhibit a cusp behavior if it is assumed that a spectral gap exists between the large and small scales.

Zhou, YE↗