Papers by Abbas Rahimi, PhD
Direct numerical simulations of weakly and strongly stratified decaying homogenous turbulence are... more Direct numerical simulations of weakly and strongly stratified decaying homogenous turbulence are conducted for two transition-to-turbulence cases with different initial conditions: the non-helical Taylor–Green vortex (TGV) and the helical Arnold–Beltrami–Childress (ABC) configurations. These simulations are carried out using a pseudo-spectral formulation with a 5123 grid resolution at varying Froude numbers, . First, an effort is made to classify the various regimes represented by and 0.08, or alternatively, buoyancy Reynolds number () values from to for non-helical and helical stably stratified turbulence. Second, various quantities including enstrophy, dissipation, vertical shearing, coherent vortical structures, density fields, energy spectra and fluxes in the different directions, and helicity are analysed to understand the effects of stratification on the development of turbulence and the turbulent structure in these different regimes. Consistent with previous studies by Riley and deBruynKops (Phys. Fluids, 2003) and Rorai et al. (Phys. Rev. E, 2013), both the TGV and ABC simulation results showed that stratification slowed down the development of turbulence. At the lowest (0.16 and 0.08) tested here, for which , the fluid motions in TGV were inhibited significantly by the strong stratification so as to eliminate the energy cascade regime and in turn, the inertial subrange. This led to an increased dissipation at the large scales in these cases as also observed in Brethouwer et al. (J. Fluid Mech., 2007). Spectral fluxes also showed an inverse energy cascade for very high stratification. Due to the nature of the ABC's initial configuration, it was observed that for the same values, its energy cascade was less inhibited in comparison to TGV and always forward as opposed to inverse for the very low- TGV cases, a behaviour that was also reflected in the different evolutions between the two flows. So compared to TGV, the large scales were more energetic in ABC for the same . In addition, horizontal velocity and density fluctuation fields of ABC simulation results, both showed the density layering effect from increased stratification.
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Stratified flows, flows where density varies in one direction, have wide applications in some of ... more Stratified flows, flows where density varies in one direction, have wide applications in some of the phenomena occurring in the atmospheric and ocean. Direct numerical simulations (DNS) of transition to turbulence in a stably stratified Boussinesq fluid are presented for the three-dimensional Taylor-Green vortex problem at different stratification and turbulence intensities measured in terms of different Froude (Fr) ($\infty$ and $10^{-2}-10^{-1}$) and Reynolds numbers (Re) (800 and 1600), respectively. Features investigated include temporal variations of the energy spectrum cascade, local Froude numbers, vertical shearing of the velocities, and dissipation of kinetic and potential energy. The results from these simulations demonstrated forward cascade of energy for high Re and revealed the strong anisotropic structure of turbulence and suppression of vertical motion under stratification.
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Helical flows undergoing density stratification have wide applications in meteorological phenomen... more Helical flows undergoing density stratification have wide applications in meteorological phenomena such as dust devils, tornadoes, and hurricanes due to the complexity and disasters caused by them. Direct numerical simulations (DNS) of transition to turbulence in a stably stratified Boussinesq fluid are presented for different rotation and stratification intensities. In order to understand the effect of velocity on the energy cascade, comparisons are made between helicity initiated and non-helical flows. Results show that stratification decelerates the helicity decay and causes velocity and vorticity to align with each other. With respect to the helical and non-helical flow comparisons, the total energy in the presence of stratification decays faster with helicity. In addition, the behavior of length scales were examined by comparing temporal variations of the vertical shearing of velocities. Results showed a growing asymmetry with time in the case of helical flow, while non-helical flow stayed close to begin symmetric.
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Papers by Abbas Rahimi, PhD