Direct numerical simulation of vortex structures and turbulent scalar transfer across a free surface in a fully developed turbulence

1999 ◽  
Vol 11 (6) ◽  
pp. 1581-1595 ◽  
Author(s):  
Ryuichi Nagaosa
Keyword(s):  
Numerical Simulation ◽  
Free Surface ◽  
Direct Numerical Simulation ◽  
Vortex Structures ◽  
Fully Developed Turbulence ◽  
Developed Turbulence

Direct numerical simulation of turbulence in a square annular duct

2009 ◽  
Vol 621 ◽  
pp. 23-57 ◽  
Author(s):  
HONGYI XU
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Secondary Flows ◽  
Turbulence Energy ◽  
Fully Developed Turbulence ◽  
Developed Turbulence ◽  
Corner Flow ◽  
Annular Duct ◽  
The Mean ◽  
Flow Similarity

Direct numerical simulation (DNS) is performed to investigate the fully developed turbulence in a straight square annular duct. The mean flow field and the turbulent statistics are compared with existing experiments and numerical results. The comparisons and the analysis of the DNS data led to the discovery of the turbulent boundary layers of concave and convex 90° corners, a corner flow similarity and the scaling characteristics of corner turbulence. Analysis of the mean streamwise velocity near the concave and convex 90° corners resulted in establishing the ‘law-of-the-corner’ formulations. Comparing these formulations with the ‘law-of-the-wall’ relation, both damping and enhancing mechanisms analytically represented by the van Driest damping function, and the enhancement function were revealed for the concave and convex corner turbulence. The investigation captures the distinctive turbulence-driven secondary flows for both convex and concave 90° corners, and a corner flow similarity rule is discovered, which is associated with the pattern of these secondary flows. A turbulence energy spectrum analysis provides the distinctive features of the fully developed turbulence in the wall and corner regions. The validity of the turbulence eddy viscosity concept is evaluated based on these turbulence energy spectra. The turbulence-driven secondary-flow generation mechanisms are investigated by analysing the anisotropy of the Reynolds stresses.

scholarly journals Dissipation of shear-free turbulence near boundaries

2000 ◽  
Vol 422 ◽  
pp. 167-191 ◽  
Author(s):  
M. A. C. TEIXEIRA ◽  
S. E. BELCHER
Keyword(s):  
Numerical Simulation ◽  
Boundary Condition ◽  
Free Surface ◽  
Direct Numerical Simulation ◽  
Dissipation Rate ◽  
Solid Wall ◽  
Tangential Velocity ◽  
Main Role ◽  
Nonlinear Processes ◽  
Viscous Layer

The rapid-distortion model of Hunt & Graham (1978) for the initial distortion of turbulence by a flat boundary is extended to account fully for viscous processes. Two types of boundary are considered: a solid wall and a free surface. The model is shown to be formally valid provided two conditions are satisfied. The first condition is that time is short compared with the decorrelation time of the energy-containing eddies, so that nonlinear processes can be neglected. The second condition is that the viscous layer near the boundary, where tangential motions adjust to the boundary condition, is thin compared with the scales of the smallest eddies. The viscous layer can then be treated using thin-boundary-layer methods. Given these conditions, the distorted turbulence near the boundary is related to the undistorted turbulence, and thence profiles of turbulence dissipation rate near the two types of boundary are calculated and shown to agree extremely well with profiles obtained by Perot & Moin (1993) by direct numerical simulation. The dissipation rates are higher near a solid wall than in the bulk of the flow because the no-slip boundary condition leads to large velocity gradients across the viscous layer. In contrast, the weaker constraint of no stress at a free surface leads to the dissipation rate close to a free surface actually being smaller than in the bulk of the flow. This explains why tangential velocity fluctuations parallel to a free surface are so large. In addition we show that it is the adjustment of the large energy-containing eddies across the viscous layer that controls the dissipation rate, which explains why rapid-distortion theory can give quantitatively accurate values for the dissipation rate. We also find that the dissipation rate obtained from the model evaluated at the time when the model is expected to fail actually yields useful estimates of the dissipation obtained from the direct numerical simulation at times when the nonlinear processes are significant. We conclude that the main role of nonlinear processes is to arrest growth by linear processes of the viscous layer after about one large-eddy turnover time.

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