### 1.4.1. Momentum Sources

An immersed solid is a rigid solid object that moves within a fluid domain. The immersed solid object occupies the same volume of space as part of the fluid domain that contains the immersed solid. In order to model the effect of the immersed solid on the surrounding fluid, the fluid volume, which is contained within the immersed solid volume, is forced to move with the solid by applying a source term to the momentum equation Equation 1–84.

| (1–131)

| (1–132)

| (1–133)

, , and are the three components of fluid velocity and , , and are the components of forcing velocity due to the immersed solid. The momentum source coefficient is a large number, and is evaluated as the average of the three diagonal coefficients in the momentum equation. Further, is the Momentum Force Scaling Factor, which is set to the default value of `10.0`. The appropriate setting of the Momentum Source Scaling Factor is a balance between accuracy and robustness. A higher value leads to a more accurate solution but is less robust and can give convergence difficulties.

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.

The solver automatically updates the mesh positions of the immersed solid at the beginning of each time step, and then sets up a list of fluid nodes that lie inside the given immersed solid. The solver applies the immersed solid sources to the fluid nodes that lie inside the immersed solid in order to drive the fluid velocity towards the immersed solid velocity.

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