Setting turbulence scale values

For the Mixing Length and Fixed Turbulent Viscosity models, you must use the Length Scales option on the 3D Flow page in the Solution dialog box to control the turbulence scale values. From the Length Scales list:

• If you select Automatic, the flow solver automatically sets the eddy length and the flow velocity scale. The flow velocity scale is set to the maximum flow speed in the fluid domain. For the Fixed Turbulent Viscosity turbulence model, the eddy length is lt = 1/7 (Volume) 0.33 where the Volume is the total 3D flow domain volume. For the Mixing Length turbulence model, the eddy length is lt = 2 (Volume) / Wetted area where the Wetted area is the surface area of the 3D flow domain.

• If you select Specify, you can use the Characteristic Length Scale and Fixed Viscosity Mean Flow Velocity Scale options to set the values for the characteristic turbulence scales.The Characteristic Length Scale option lets you specify the eddy length scale.The Fixed Viscosity Mean Flow Velocity Scale option lets you specify the mean flow velocity of the fluid domain.

If the calculated values are inappropriate, then you should use the Specify option to set specific values. In practice, the solution is rarely sensitive to these values and only representative values are required. To help you choose appropriate values for turbulent flow in a channel or duct, the eddy length scale is approximately 1/4 to 1/7 the width of the channel or the smallest dimension of the duct. Larger length scales result in more turbulent mixing of the mean flow; smaller values result in less turbulent mixing.

For example, in an enclosure containing PC boards the eddy length scale is approximately 1/4 the gap between boards and the mean flow velocity scale is the mean velocity between boards. Typical values for electronics enclosures containing racks of PC boards are 0.005 m or 1/4 in for the eddy length scale and 2.0 m/s or 400 ft/min for the mean flow velocity scale.

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