A new relation of drag force for high Stokes number monodisperse spheres by direct numerical simulation

2014 ◽  
Vol 25 (6) ◽  
pp. 1860-1871 ◽  
Author(s):  
Ali Abbas Zaidi ◽  
Takuya Tsuji ◽  
Toshitsugu Tanaka
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Drag Force ◽  
Stokes Number ◽  
Monodisperse Spheres

Direct numerical simulation of turbulence over systematically varied irregular rough surfaces

2019 ◽  
Vol 862 ◽  
pp. 781-815 ◽  
Author(s):  
Y. Kuwata ◽  
Y. Kawaguchi
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Root Mean Square ◽  
Drag Force ◽  
Rough Surfaces ◽  
Skin Friction Coefficient ◽  
Root Mean Square Roughness ◽  
Mean Square ◽  
Mean Velocity ◽  
Navier Stokes Equation

Lattice Boltzmann direct numerical simulation of turbulent open-channel flows over randomly distributed hemispheres at $Re_{\unicode[STIX]{x1D70F}}=600$ is carried out to reveal the influence of roughness parameters related to a probability density function of rough-surface elevation on turbulence by analysing the spatial and Reynolds- (double-) averaged Navier–Stokes equation. This study specifically concentrates on the influence of the root-mean-square roughness and the skewness, and profiles of turbulence statistics are compared by introducing an effective wall-normal distance defined as a wall-normal integrated plane porosity. The effective distance can completely collapse the total shear stress outside the roughness sublayer, and thus the similarity of the streamwise mean velocity is clearer by introducing the effective distance. In order to examine the influence of the root-mean-square roughness and the skewness on dynamical effects that contribute to an increase in the skin friction coefficient, the triple-integrated double-averaged Navier–Stokes equation is analysed. The main contributors to the skin friction coefficient are found to be turbulence and drag force. The turbulence contribution increases with the root-mean-square roughness and/or the skewness. The drag force contribution, on the other hand, increases in particular with the root-mean-square roughness whereas an increase in the skewness does not increase the drag force contribution because it does not necessarily increase the surface area of the roughness elements. The contribution of the mean velocity dispersion induced by spatial inhomogeneity of the rough surfaces substantially increases with the root-mean-square roughness. A linear correlation is confirmed between the root-mean-square roughness and the equivalent roughness while the equivalent roughness monotonically increases with the skewness. A new correlation function based on the root-mean-square roughness and the skewness is developed with the available experimental and direct numerical simulation data, and it is confirmed that the developed correlation reasonably predicts the equivalent roughness of various types of real rough surfaces.

scholarly journals Inertial drag on a sphere settling in a stratified fluid

2018 ◽  
Vol 855 ◽  
pp. 1074-1087 ◽  
Author(s):  
R. Mehaddi ◽  
F. Candelier ◽  
B. Mehlig
Keyword(s):  
Numerical Simulation ◽  
Perturbation Theory ◽  
Direct Numerical Simulation ◽  
Drag Force ◽  
Stratified Fluid ◽  
Simulation Studies ◽  
Fluid Density ◽  
Settling Particle ◽  
Inertial Terms ◽  
Good Agreement

We compute the drag force on a sphere settling slowly in a quiescent, linearly stratified fluid. Stratification can significantly enhance the drag experienced by the settling particle. The magnitude of this effect depends on whether fluid-density transport around the settling particle is due to diffusion, to advection by the disturbance flow caused by the particle or due to both. It therefore matters how efficiently the fluid disturbance is convected away from the particle by fluid-inertial terms. When these terms dominate, the Oseen drag force must be recovered. We compute by perturbation theory how the Oseen drag is modified by diffusion and stratification. Our results are in good agreement with recent direct numerical simulation studies of the problem.

A STUDY ON CORRELATION MOMENTS OF TWO-PHASE FLUCTUATING VELOCITY USING DIRECT NUMERICAL SIMULATION

2013 ◽  
Vol 24 (10) ◽  
pp. 1350068 ◽  
Author(s):  
BING WANG ◽  
WEI WEI ◽  
HUIQIANG ZHANG
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Phase Velocity ◽  
Turbulent Flows ◽  
Local Equilibrium ◽  
Stokes Number ◽  
Phase Correlation ◽  
Particle Phase ◽  
Velocity Correlation ◽  
Two Phase

Existing models of two-phase fluctuating velocity correlation moments are unsatisfactory because of their inability to clearly identify the dependency of two-phase velocity covariance on fluid- and particle-phase velocity second moments. This is especially true of wall-bounded turbulent flows. In this paper, the statistical fluctuating velocity of both phases in particle-laden turbulent channel flows were obtained numerically by means of direct numerical simulation (DNS) coupled to the Lagrangian particle trajectory method. The effects of particle Stokes number on the scaling of two-phase fluctuating velocity correlation moments were analyzed considering effects of flow inhomogeneity. An improved two-phase correlation closure model of exponential decay with emphasis on the particle-phase kinetic energy was then proposed based on the results of an evaluation of five existing models. This new model was found to be better than previous models, which used local equilibrium assumption. The present investigations may facilitate understanding of two-phase flow physics and the construction of models capable of predicting the movements of particle-laden turbulent flows accurately using Reynolds-averaged Navier–Stokes (RANS) methods.

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