CMS-Flow:Eddy Viscosity

In CMS-Flow eddy viscosity is calculated as the sum of a base value $\nu _{0}$, the current-related eddy viscosity $\nu _{c}$ and the wave-related eddy viscosity $\nu _{w}$

$\nu _{t}=\nu _{0}+\nu _{c}+\nu _{w}$

(1)

The base value for the eddy viscosity is approximately equal to the kinematic eddy viscosity can be changed using the advanced cards (Click here for further details).

Current-Related Eddy Viscosity Component

There are four options for the current-related eddy viscosity: FALCONER, PARABOLIC, SUBGRID, and MIXING-LENGTH. The default turbulence model is the subgrid model, but may be changed with the advanced card TURBULENCE_MODEL.

Falconer Equation

The Falconer (1980) equation is the method is the default method used in the previous version of CMS, known as M2D. The first is the Falconer (1980) equation given by

$\nu _{c}=0.575c_{b}|U|h$

(2)

where $c_{b}$ is the bottom friction coefficient, $U$ is the depth-averaged current velocity, and $h$ is the total water depth.

Parabolic Model

The second option is the parabolic model given by

$\nu _{c}=c_{1}u_{*}h$

(3)

where $c_{1}$ is approximately equal to $\kappa /6$.

Subgrid Turbulence Model

The third option for calculating $\nu _{c}$ is the subgrid turbulence model given by

$\nu _{c}=c_{1}u_{*}h+(c_{2}\Delta )^{2}|{\bar {S}}|$

(4)

where $c_{1}$ and $c_{2}$ are empirical coefficients related the turbulence produced by the bed and horizontal velocity gradients, and $\Delta$ is the average grid spacing. $c_{1}$ is approximately equal to 0.0667 (default) but may vary from 0.01-0.2. $c_{2}$ is equal to approximately the Smagorinsky coefficient and may vary from 0.1 to 0.3 (default is 0.2). $|{\bar {S}}|$ is equal to

$|{\bar {S}}|={\sqrt {2{\bar {S}}_{ij}{\bar {S}}_{ij}}}={\sqrt {2{\biggl (}{\frac {\partial U}{\partial x}}{\biggr )}^{2}+2{\biggl (}{\frac {\partial V}{\partial y}}{\biggr )}^{2}+{\biggl (}{\frac {\partial U}{\partial y}}+{\frac {\partial V}{\partial x}}{\biggr )}^{2}}}$

(7)

and

${\bar {S}}_{ij}={\frac {1}{2}}{\biggl (}{\frac {\partial U_{i}}{\partial x_{j}}}+{\frac {\partial U_{j}}{\partial x_{i}}}{\biggr )}$

(8)

The subgrid turbulence model parameters may be changed in the advanced cards EDDY_VISCOSITY_BOTTOM, and EDDY_VISCOSITY_HORIZONTAL. Click here for further details.

Mixing Length Model

The Mixing Length Model implemented in CMS includes a component due to the vertical shear and is given by

$\nu _{c}={\sqrt {(c_{1}u_{*}h)^{2}+(l_{h}^{2}|{\bar {S}}|)^{2}}}$

(9)

where the mixing length $l_{h}$ is determined by $l_{h}=\kappa \min {(c_{2}h,y)}$, with $y$ being the distance to the nearest wall and $c_{2}$ is an empirical coefficient between 0.3-1.2. Eq. (9) takes into account the effects of bed shear and horizontal velocity gradients respectively through the first and second terms on its right-hand side. It has been found that the modified mixing length model is better than the depth-averaged parabolic eddy viscosity model that accounts for only the bed shear effect.

Wave-Related Eddy Viscosity

The wave component of the eddy viscosity is separated into two components

$\nu _{w}=c_{3}u_{w}H_{s}+c_{4}h{\biggl (}{\frac {D_{br}}{\rho }}{\biggr )}^{1/3}$

(10)

where $c_{3}$ and $c_{4}$ are empirical coefficients, $H_{s}$ is the significant wave height and $u_{w}$ is bottom orbital velocity based on the significant wave height. The first term on the R.H.S. of Eq. (10) represents the component due to bottom friction and the second term represents the component due to wave breaking. The coefficient $c_{3}$ is approximately equal to 0.1 and may vary from 0.05 to 0.2. The coefficient $c_{4}$ is approximately equal to 0.08 and may vary from 0.04 to 0.15.

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References

• LARSON, M.; HANSON, H., and KRAUS, N. C., 2003. Numerical modeling of beach topography change. Advances in Coastal Modeling, V.C. Lakhan (eds.), Elsevier Oceanography Series, 67, Amsterdam, The Netherlands, 337-365.

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