What does "direct" turbulence input type mean?

Altair Forum User

Hi all,

 

In BC - surfaces - choose an inflow surface - simple boundary condition, there is a 'turbulence input type'.

 

1) What does the choice of 'direct' mean?

2) Does 'direct' mean direct numerical simulation? (Which means that there is no turbulence)

3) If I choose 'Intensity and length scale', what is the proper dimension for the length scale? ( I am modelling a 2D 4m*4m square in a large rectangular fluid domain, interested in the drag force)

 

Any help is highly appreciated.

 

Thanks all.

 

Fangyuan

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cfdguru

When performing RANS or DES analyses, AcuSolve solves additional partial differential equations to model the impact of turbulence.  So, at inlet boundaries, you need to assign suitable boundary conditions to the variables transported by the turbulence model equations (turbulent kinetic energy, eddy frequency, eddy viscosity, etc.).  When you select 'direct' as the input method, that means you will manually enter the values for each of these variables at the boundary.  However, since it is often uncommon to know what these values should actually be, the options of 'Intensity and Length Scale', etc. are available to compute an estimate of the turbulence variables based on other characteristics of the geometry/flow.  

 

If you are modeling an internal flow, then the length scale corresponds to the largest scale eddy that can be present at the inlet.  For the case of a duct you can use the height of the duct as a first approximation.