Direct Numerical Simulation Of Turbulent Channel Flow Research Articles

Does the choice of the forcing term affect flow statistics in DNS of turbulent channel flow?

We seek possible statistical consequences of the way a forcing term is added to the Navier–Stokes equations in the Direct Numerical Simulation (DNS) of incompressible channel flow. Simulations driven by constant flow rate, constant pressure gradient and constant power input are used to build large databases, and in particular to store the complete temporal trace of the wall-shear stress for later analysis. As these approaches correspond to different dynamical systems, it can in principle be envisaged that these differences are reflect by certain statistics of the turbulent flow field. The instantaneous realizations of the flow in the various simulations are obviously different, but, as expected, the usual one-point, one-time statistics do not show any appreciable difference. However, the PDF for the fluctuations of the streamwise component of wall friction reveals that the simulation with constant flow rate presents lower probabilities for extreme events of large positive friction. The low probability value of such events explains their negligible contribution to the commonly computed statistics; however, the very existence of a difference in the PDF demonstrates that the forcing term is not entirely uninfluential. Other statistics for wall-based quantities (the two components of friction and pressure) are examined; in particular spatio-temporal autocorrelations show small differences at large temporal separations, where unfortunately the residual statistical uncertainty is still of the same order of the observed difference. Hence we suggest that the specific choice of the forcing term does not produce important statistical consequences, unless one is interested in the strongest events of high wall friction, that are underestimated by a simulation run at constant flow rate.

DNS of turbulent channel flow with conjugate heat transfer at Prandtl number 0.01

Direct Numerical Simulation (DNS) of the fully developed velocity and temperature fields in a turbulent channel flow coupled with the unsteady conduction in the heated walls was carried out. Simulations were performed with passive scalar approximation at Prandtl number 0.01, which roughly corresponds to the Prandtl number of liquid sodium. DNSs were performed at friction Reynolds numbers 180, 395 and 590. The obtained statistical quantities like mean temperatures, profiles of the root-mean-square (RMS) temperature fluctuations for various thermal properties of wall and fluid, and various wall thicknesses were obtained from a pseudo-spectral channel-flow code. Even for the highest implemented Reynolds number the temperature profile in the fluid does not exhibit log-law region and the near-wall RMS temperature fluctuations show Reynolds number dependence. Conjugate heat transfer simulations of liquid sodium–steel system point to a relatively intensive penetration of turbulent temperature fluctuations into the heated wall. Database containing the results is available in a digital form.

Influence of Density Fluctuation on DNS of Turbulent Channel Flow in the Presence of Temperature Stratification

To investigate the effect of density variation on turbulent heat transfer prediction, we perform a direct numerical simulation (DNS) of turbulent flow in a horizontal channel with varying wall temperature using a new method dealing with the effect of density variation. As a result, asymmetric profiles are observed in turbulence statistics. This tendency cannot be reproduced by the Boussinesq approximation, which is based on the constant-density formulation. A quadrant analysis of the turbulent shear stress finds that the density variation affects ejection events in the vicinity of the walls. It is also revealed that the variable viscosity and thermal conductivity change the dynamic and thermodynamic balances of the flow.

Benchmarks on tera-scalable models for DNS of turbulent channel flow

Due to the extensive requirements of memory and speed for direct numerical simulation (DNS) of channel turbulence, people could only perform DNS at moderate Reynolds number before. However, with the fast development of supercomputers, it has become more and more approachable for researchers to perform DNS of turbulence at high Reynolds number. This makes it imperative to consider the development of tera-scalable DNS codes that are capable of fully exploiting these massively parallel machines. In order to achieve this, three parallel models (1D, 2D and 3D decompositions) have been implemented and benchmarked. All these models have been successfully ported on BlueGene/L. We have benchmarked these models on BG/L at ANL and BGW at IBM Watson center. Details of these models have been described, discussed and presented in this paper. The optimized model can be used to perform DNS at high Reynolds number in the near future.

DNS of Turbulent Channel Flow at High Reynolds Number under a Uniform Magnetic Field

A direct numerical simulation (DNS) of turbulent channel flow with high Reynolds number has been carried out to show the effects of magnetic field. In this study, the Reynolds number for channel flow based on bulk velocity ub, kinematic viscosity ν, and channel width 2δ was set to be constant ; Reb=2δub/ν=45818.A uniform magnetic field was applied in the direction of the wall normal. The Hartman number was Ha=2δB0√σ/ρν=32.5 and 65. Turbulent quantities such as the mean flow turbulent stress, and the turbulent statistics were obtained by DNS. Although the influence of the magnetohydrodynamic dissipation terms in the turbulent kinetic energy budget was small, large-scale turbulent structures, such as vertical structures, low-speed streaks, ejection, and sweep, disappear at the central region of the channel. Consequently, the difference between production and dissipation in the turbulent kinetic energy decreases with increasing Hartmann number at the central region and large-scale structures at this region disappear.

DNS of Turbulent Channel Flow Subjected to the Acceleration

The reverse transition of accelerated turbulent flow is investigated using a direct numerical simulation (DNS). The tendency of the results is consistent with those of the previous experiments. Our major finding is that the response of streaks and quasistreamwise vortices to the acceleration does not coincide qualitatively. The streamwise component of turbulence intensity near the wall is maintained, even in the accelerated flow field. However, the other components of turbulence intensities and the streamwise vorticity are reduced in the whole region of the accelerated flow field because of the termination of the energy redistribution. Furthermore, it is found that the stretching of main flow reduces the regeneration of quasi-streamwise vortices through the decrease in distortion.

DNS of Pulsating Turbulent Channel Flow.

Various direct numerical simulations (DNS) of turbulence are performed hitherto. In most of those DNS's, the mean flow is assumed to be steady. This is because the DNS of the turbulence with an unsteady mean flow requires more computational effort to obtain a stable statistical average. In the present study, DNS of turbulent channel flow driven by temporally-changing pressure gradient is performed. The pressure gradient is so determined that the bulk mean velocity averaged over one cycle is approximately equal to the one with a steady state Reynolds number of Reτs= 180. The each period is divided into twenty phases and statistical average is obtained for various turbulence statistics. Obtained turbulence statistics are compared with steady ones. Number of streamwise vortices decreases in the acceleration period while it increases in the deceleration one.