scholarly journals DIRECT NUMERICAL SIMULATIONS OF SMALL PARTICLES IN TURBULENT FLOWS OF LOWDISSIPATION RATES USING ASYMPTOTIC EXPANSION

2020 ◽  
Author(s):  
Sandipan Banerjee ◽  
Orlando M. Ayala ◽  
Lian-Ping Wang
Keyword(s):  
Asymptotic Expansion ◽  
Numerical Simulations ◽  
Turbulent Flows ◽  
Direct Numerical Simulations ◽  
Small Particles

scholarly journals Unravelling turbulence near walls

2009 ◽  
Vol 630 ◽  
pp. 1-4 ◽  
Author(s):  
IVAN MARUSIC
Keyword(s):  
Laminar Flow ◽  
Turbulent Flow ◽  
Numerical Simulations ◽  
Turbulent Flows ◽  
Direct Numerical Simulations ◽  
Aircraft Wing ◽  
Physical Mechanisms ◽  
Drag And Lift

Turbulent flows near walls have been the focus of intense study since their first description by Ludwig Prandtl over 100 years ago. They are critical in determining the drag and lift of an aircraft wing for example. Key challenges are to understand the physical mechanisms causing the transition from smooth, laminar flow to turbulent flow and how the turbulence is then maintained. Recent direct numerical simulations have contributed significantly towards this understanding.

scholarly journals Mixed convection in turbulent channels with unstable stratification

2017 ◽  
Vol 821 ◽  
pp. 482-516 ◽  
Author(s):  
Sergio Pirozzoli ◽  
Matteo Bernardini ◽  
Roberto Verzicco ◽  
Paolo Orlandi
Keyword(s):  
Free Convection ◽  
Numerical Simulations ◽  
Turbulent Flows ◽  
Momentum Flux ◽  
Direct Numerical Simulations ◽  
Mean Values ◽  
Developed Turbulence ◽  
Wide Range ◽  
Channel Thickness ◽  
The Individual

We study turbulent flows in pressure-driven planar channels with imposed unstable thermal stratification, using direct numerical simulations in a wide range of Reynolds and Rayleigh numbers and reaching flow conditions which are representative of fully developed turbulence. The combined effect of forced and free convection produces a peculiar pattern of quasi-streamwise rollers occupying the full channel thickness, with aspect ratio considerably higher than unity; it has been observed that they have an important redistributing effect on temperature and momentum, providing for a substantial fraction of the heat and momentum flux at bulk Richardson numbers larger than$0.01$. The mean values and the variances of the flow variables do not appear to follow Prandtl’s scaling in the free-convection regime, except for the temperature and vertical velocity fluctuations, which are more directly affected by wall-attached turbulent plumes. We find that the Monin–Obukhov theory nevertheless yields a useful representation of the main flow features. In particular, the widely used Businger–Dyer flux-profile relationships are found to provide a convenient way of approximately accounting for the bulk effects of friction and buoyancy, although the individual profiles may have wide scatter from the alleged trends. Significant deviations are found in direct numerical simulations with respect to the commonly used parametrization of the momentum flux in the light-wind regime, which may have important practical impact in wall models of atmospheric dynamics. Finally, for modelling purposes, we devise a set of empirical predictive formulae for the heat flux and friction coefficients, which are within approximately$10\,\%$standard deviation from the numerical results in a wide range of flow parameters.

scholarly journals Dynamical Subgrid-Scale Parameterizations from Direct Numerical Simulations

2006 ◽  
Vol 63 (11) ◽  
pp. 3006-3019 ◽  
Author(s):  
Jorgen S. Frederiksen ◽  
Steven M. Kepert
Keyword(s):  
Numerical Simulations ◽  
Turbulent Flows ◽  
Climate Models ◽  
Prediction Models ◽  
Isotropic Turbulence ◽  
Direct Interaction ◽  
Direct Numerical Simulations ◽  
Subgrid Scale ◽  
Direct Interaction Approximation ◽  
Barotropic Vorticity

Abstract Dynamical subgrid-scale parameterizations of stochastic backscatter, eddy drain viscosity, and net eddy viscosity have been formulated and calculated for two-dimensional turbulent flows on the sphere based on the statistics of direct numerical simulations (DNSs) with the barotropic vorticity equation. A relatively simple methodology based on a stochastic model representation of the subgrid-scale eddies, but which takes into account the memory effects of turbulent eddies, has been employed. The parameterizations have a cusp behavior at the cutoff wavenumber of the retained scales and have closely similar forms to those based on eddy damped quasi-normal Markovian (EDQNM) and direct interaction approximation (DIA) closure models. Large-eddy simulations (LESs) incorporating DNS-based subgrid-scale parameterizations are found to have kinetic energy spectra that compare closely with the results of higher-resolution DNS at the scales of LES for both isotropic turbulence and Rossby wave turbulence. The methodology presented is general and should be equally applicable to parameterizations of baroclinic processes and convective processes. Applications of the parameterizations to climate models and prediction models are discussed.

Bubble Effects on Wall Shear in Vortical Flows

2003 ◽  
Author(s):  
Arturo Ferna´ndez ◽  
Jiacai Lu ◽  
Gre´tar Tryggvason
Keyword(s):  
Surface Tension ◽  
Drag Reduction ◽  
Numerical Simulations ◽  
Turbulent Flows ◽  
Front Tracking ◽  
Direct Numerical Simulations ◽  
Fluid Inertia ◽  
Fundamental Interaction ◽  
Turbulent Structures ◽  
Simplified Models

Direct numerical simulations of the motion of bubbles in turbulent flows are being carried out, using a finite volume/front tracking technique that accounts fully for the effect of fluid inertia, viscosity, bubble deformability, and surface tension. The objective of the simulations is both to address the fundamental interaction mechanisms between the bubbles and the flow and how the bubbles modify the wall turbulent structures, as well as to provide data for validation of simplified models. Results for bubbles placed in the so-called “minimum turbulent channel” show significant drag reduction as the bubbles disrupt the near-wall turbulent flow.

The double-diffusive modon

2008 ◽  
Vol 609 ◽  
pp. 59-85 ◽  
Author(s):  
TIMOUR RADKO
Keyword(s):  
Asymptotic Expansion ◽  
Numerical Simulations ◽  
Density Ratio ◽  
Direct Numerical Simulations ◽  
Density Stratification ◽  
Salt Flux ◽  
Two Dimensional ◽  
Background Density ◽  
Mixing Rates ◽  
Double Diffusive

Fully developed two-dimensional salt-finger convection is characterized by the appearance of coherent dipolar eddies which carry relatively fresh and cold fluid upward and salty and warm fluid downward. Such structures – the double-diffusive modons – are prevalent in the regime in which density stratification is close to neutral and the salt-finger instability is extremely vigorous. The structure and translation velocities of modons are discussed in terms of the asymptotic expansion in which the background density ratio approaches unity. It is argued that the vertical salt flux is driven primarily by double-diffusive modons, which makes it possible to derive explicit expressions for the mixing rates of temperature and salinity as a function of their background gradients. Predictions of the proposed mixing model are successfully tested by direct numerical simulations.

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