Application of a dynamic subgrid-scale model to turbulent recirculating flows

The dynamic subgrid-scale model of Germano et al. is implemented in a finite volume formulation and applied to the simulation of turbulent flow in a three-dimensional lid-driven cavity at Reynolds number of 7500. The filtering operation is carried out in physical space, and the model coefficient is calculated locally. The computed mean and rms velocities as well as the Reynolds stress are compared with experimental data. It is shown that backscatter from small to large scales is necessary to sustain turbulent fluctuations. The model is being applied to the simulation of turbulent flows in a stratified and rotating environment in complex geometries

Structure of mass and momentum fields over a model aggregation of benthic filter feeders

International audienceThe structure of momentum and concentration boundary layers developing over a bed of Potamocorbula amurensis clam mimics was studied. Laser Doppler velocimetry (LDV) and laser-induced fluorescence (LIF) probes were used to quantify velocity and concentration profiles in a laboratory flume containing 3969 model clams. Model clams incorporated passive roughness, active siphon pumping, and the ability to filter a phytoplankton surrogate from the flow. Measurements were made for two crossflow velocities, four clam pumping rates, and two siphon heights. Simultaneous use of LDV and LIF probes permited direct calculation of scalar flux of phytoplankton to the bed. Results show that clam pumping rates have a pronounced effect on a wide range of turbulent quantities in the boundary layer. In particular, the vertical turbulent flux of scalar mass to the bed was approximately proportional to the rate of clam pumping

Mixing in a stratified shear flow: Energetics and sampling

Direct numerical simulations of the time evolution of homogeneous stably stratified shear flows have been performed for Richardson numbers from 0 to 1 and for Prandtl numbers between 0.1 and 2. The results indicate that mixing efficiency R(sub f) varies with turbulent Froude number in a manner consistent with laboratory experiments performed with Prandtl numbers of 0.7 and 700. However, unlike the laboratory results, for a particular Froude number, the simulations do not show a clear dependence on the magnitude of R(sub f) on Pr. The observed maximum value of R(sub f) is 0.25. When averaged over vertical length scales of an order of magnitude greater than either the overturning or Ozmidov scales of the flow, the simulations indicate that the dissipation rate epsilon is only weakly lognormally distributed with an intermittency of about 0.01 whereas estimated values in the ocean are 3 to 7

Fluid dynamics of double diffusive systems

A study of mixing processes in doubly diffusive systems is being conducted. Continuous gradients of two diffusing components (heat and salinity) are being used as initial conditions, and forcing is introduced by lateral heating, surface shear and sloping boundaries. The goals of the proposed work include: quantification of the effects of finite amplitude disturbances on stable, double diffusive systems, particularly with respect to lateral heating, development of an improved understanding of the physical phenomena present in wind-driven shear flows in double diffusive stratified environments, increasing our knowledge-base on turbulent flow in stratified environments and how to represent it, and formulation of numerical code for such flows. The work is being carried out in a new experimental facility at Stanford and on laboratory minicomputers and CRAY computers. In particular we are focusing on the following key issues. The formation and propagation of double diffusive intrusions away from a heated wall and the effects of lateral heating on the double diffusive system; The interaction between the double diffusively influenced fluxes and the turbulence induced fluxes; The formation of gravitational intrusions; and The influence of double diffusive gradients on mixed layer deepening. The goals of the project were as follows. Physical experiments: Construct experimental facility; Modify and fabricate instrument rakes; Develop sampling and calibration software; Develop stratification techniques; Conduct flow visualization studies; Qualify wind tunnel over a range of wind speeds. Numerical experiments: Adapt REMIXCS to handle turbulent flows; Investigate approaches for specifying wind field; Perform calculations for low wind speeds. With the exception of the wind tunnel qualification, all the tasks have already been completed and we are now conducting quantitative experiments. 2 figs

Fluid dynamics of double diffusive systems

The major accomplishments of our initial research period (August 1, 1987, to March 1, 1990) are as follows; we completed construction of the experimental facility. Originally, it had been our intent to modify an existing facility in our laboratory. When this became impractical we constructed a new stand-alone facility. Modified an existing three-dimensional numerical code developed in our laboratory, SEAFLOS1, by incorporating a salinity transport equation. Developed experimental and analytical techniques, and performed both physical and numerical experiments for a wide range of initial and boundary conditions. Focused our overall research effort to answer the following four questions pertaining to the formation of convective intrusions due to lateral temperature gradients established by sidewall heating. (1) What is the internal structure of the convective intrusions as a function of the initial stratification and sidewall heating rates (2) What is the correct scaling for the initial vertical dimension of the intrusions (3) How does the merging process vary as a function of initial stratification and sidewall heating rate (4) Is the sidewall heating critical for continued propagation of the intrusions, or is it merely a trigger which releases the internal instability in the flui

Fluid dynamics of double diffusive systems

A study of mixing processes in doubly diffusive systems is being conducted. Continuous gradients of two diffusing components (heat and salinity in our case) are being used as initial conditions, and forcing is introduced by lateral heating and surface shear. The goals of the proposed work include: (1) quantification of the effects of finite amplitude disturbances on stable, double diffusive systems, particularly with respect to lateral heating, (2) development of an improved understanding of the physical phenomena present in wind-driven shear flows in double diffusive stratified environments, (3) increasing our knowledge-base on turbulent flow in stratified environments and how to represent it, and (4) formulation of a numerical code for such flows. The work is being carried out in an experimental facility which is located in the Stanford Environmental Fluid Mechanics Laboratory, and on laboratory minicomputers and CRAY computers. In particular we are focusing on the following key issues: (1) the formation and propagation of double diffusive intrusions away from a heated wall and the effects of lateral heating on the double diffusive system; (2) the interaction between the double diffusively influenced fluxes and the turbulence induced fluxes; (3) the measurement of heat and mass fluxes; and (4) the influence of double diffusive gradients on mixed layer deepening. 1 fig

Fluid dynamics of double diffusive systems

A study of mixing processes in doubly diffusive systems is being conducted. Continuous gradients of two diffusing components (heat and salinity in our case) are being used as initial conditions, and forcing is introduced by lateral heating and surface shear. The goals of the proposed work include: (1) quantification of the effects of finite amplitude disturbances on stable, double diffusive systems, particularly with respect to lateral heating, (2) development of an improved understanding of the physical phenomena present in wind-driven shear flows in double diffusive stratified environments, (3) increasing our knowledge-base on turbulent flow in stratified environments and how to represent it, and (4) formulation of a numerical code for such flows. The work is being carried out in an experimental facility which is located in the Stanford Environmental Fluid Mechanics Laboratory, and on laboratory minicomputers and CRAY computers. In particular, our overall goals are as follows: (1) develop more general stability and scaling criteria for the destabilization of doubly-stratified systems; (2) further study the variation of flow structure and scale with Rayleigh ratio and lateral heating ratio; (3) further delineate the mechanisms governing convective layer formation and merging; (4) study the mixing processes within the convective layers and across interfaces, and estimate the heat and mass fluxes in such a system; (5) quantify the effects of turbulence and coherent structures (due to a wind-driven surface shear) on a doubly stratified system; and (6) study the interaction between surface shear and side-wall heating destabilization mechanisms. 5 refs