10,409 research outputs found
Numerical simulation of flow over a rough bed
This paper presents results of a direct numerical simulation (DNS) of turbulent flow over the rough bed of an open channel. We consider a hexagonal arrangement of spheres on the channel bed. The depth of flow has been taken as four times the diameter of the spheres and the Reynolds number has been chosen so that the roughness Reynolds number is greater than 70, thus ensuring a fully rough flow. A parallel code based on finite difference, domain decomposition, and multigrid methods has been used for the DNS. Computed results are compared with available experimental data. We report the first- and second-order statistics, variation of lift/drag and exchange coefficients. Good agreement with experimental results is seen for the mean velocity, turbulence intensities, and Reynolds stress. Further, the DNS results provide accurate quantitative statistics for rough bed flow. Detailed analysis of the DNS data confirms the streaky nature of the flow near the effective bed and the existence of a hierarchy of vortices aligned with the streamwise direction, and supports the wall similarity hypothesis. The computed exchange coefficients indicate a large degree of mixing between the fluid trapped below the midplane of the roughness elements and that above it
Law of the wall in an unstably stratified turbulent channel flow
We perform direct numerical simulations of an unstably stratified turbulent
channel flow to address the effects of buoyancy on the boundary layer dynamics
and mean field quantities. We systematically span a range of parameters in the
space of friction Reynolds number () and Rayleigh number (). Our
focus is on deviations from the logarithmic law of the wall due to buoyant
motion. The effects of convection in the relevant ranges are discussed
providing measurements of mean profiles of velocity, temperature and Reynolds
stresses as well as of the friction coefficient. A phenomenological model is
proposed and shown to capture the observed deviations of the velocity profile
in the log-law region from the non-convective case
Flow organization and heat transfer in turbulent wall sheared thermal convection
We perform direct numerical simulations of wall sheared Rayleigh-B\'enard
(RB) convection for Rayleigh numbers up to , Prandtl number unity, and
wall shear Reynolds numbers up to . Using the Monin-Obukhov length
we identify three different flow states, a buoyancy dominated regime
(; with the thermal
boundary layer thickness), a transitional regime (; with the height of the domain), and a shear dominated
regime (). In the buoyancy dominated regime the flow
dynamics are similar to that of turbulent thermal convection. The transitional
regime is characterized by rolls that are increasingly elongated with
increasing shear. The flow in the shear dominated regime consists of very
large-scale meandering rolls, similar to the ones found in conventional Couette
flow. As a consequence of these different flow regimes, for fixed and with
increasing shear, the heat transfer first decreases, due to the breakup of the
thermal rolls, and then increases at the beginning of the shear dominated
regime. For the Nusselt number effectively scales as
, with while we find
in the buoyancy dominated regime. In the transitional regime the effective
scaling exponent is , but the temperature and velocity profiles
in this regime are not logarithmic yet, thus indicating transient dynamics and
not the ultimate regime of thermal convection
Passive scalars in turbulent channel flow at high Reynolds number
We study passive scalars in turbulent plane channels at computationally high Reynolds number, thus allowing us to observe previously unnoticed effects. The mean scalar profiles are found to obey a generalized logarithmic law which includes a linear correction term in the whole lower half-channel, and they follow a universal parabolic defect profile in the core region. This is consistent with recent findings regarding the mean velocity profiles in channel flow. The scalar variances also exhibit a near universal parabolic distribution in the core flow and hints of a sizeable log layer, unlike the velocity variances. The energy spectra highlight the formation of large scalar-bearing eddies with size proportional to the channel height which are caused by a local production excess over dissipation, and which are clearly visible in the flow visualizations. Close correspondence of the momentum and scalar eddies is observed, with the main difference being that the latter tend to form sharper gradients, which translates into higher scalar dissipation. Another notable Reynolds number effect is the decreased correlation of the passive scalar field with the vertical velocity field, which is traced to the reduced effectiveness of ejection event
Steady streamwise transpiration control in turbulent pipe flow
A study of the the main features of low- and high amplitude steady streamwise
wall transpiration applied to pipe flow is presented. The effect of the two
transpiration parameters, amplitude and wavenumber, on the flow have been
investigated by means of direct numerical simulation at a moderate turbulent
Reynolds number. The behaviour of the three identified mechanisms that act in
the flow: modification of Reynolds shear stress, steady streaming and
generation of non-zero mean streamwise gradients, have been linked to the
transpiration parameters. The observed trends have permitted the identification
of wall transpiration configurations able to reduce or increase the overall
flow rate in -36.1% and 19.3% respectively. A resolvent analysis has been
carried out to obtain a description of the reorganization of the flow
structures induced by the transpiration
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