Immersed Solid Momentum Sources inside function
 

Hi,

Im trying to understand the Immersed Solid Method in Ansys CFX 14.5

In the Theory Guide under 1.4.1. Momentum Sources they mention the "volume-averaged inside function". What does this function do? Can someone help me to understand it?

They write in the Theory Guide:

"The presence of an immersed solid in the flow field is modeled through a special forcing function. If no near-wall treatment is used near the immersed solid boundary, the function is set to be zero for fluid nodes lying outside the immersed solid domain, and one for fluid nodes inside the immersed solid. If near-wall treatment is activated, the function is set to be volume-averaged inside function so that the values and the corresponding forcing terms near the immersed boundary will be non-zero. The volume-averaged inside function averages the inside function, weighted by the nodal volume."

If the whole control volume is within the immersed solid, then the momentum source (MS) is fully applied. If it is wholly outside the immersed solid the MS is not applied. But what about the control volumes which are partially inside and partially outside the solid? It is just saying it works out the proportion of the control volume which is inside the solid, and weights the MS with the proportion.