 ## General

#### Q: How do I separate out the pressure and shear force contribution to the force function.

• FAQ
Participant

A: For the shear force (per unit area) there is variable called “Wall Shear”. Wall Shear is a vector variable with components Wall Shear X, Wall Shear Y and Wall Shear Z. For the pressure force there is the scalar variable called Pressure. You can include the reference pressure by using the variable “Absolute Pressure” instead. You can convert the scalar pressure (force per unit area) into a vector by multiplying by the Normal vector variable. So, for example PressureForceX = Pressure * Normal X PressureForceY = Pressure * Normal Y PressureForceZ = Pressure * Normal Z Sets up expressions that can be used to define scalar or vector variables. The normal vector typically points out of the domain and so will be pointing in the right direction (from fluid to wall). There is a built-in Force vector that can be used to verify the accuracy of the pressure force vector or can be used as an alternative way to define the pressure vector using the following expressions PressureForceX = Force X / Area – Wall Shear X PressureForceY = Force Y / Area – Wall Shear Y PressureForceZ = Force Z / Area – Wall Shear Z This ensures the two forces (pressure and shear) sum exactly to the total force whereas the more direct expression for the pressure force only sums to the total to within the discretisation accuracy of the mesh. The force on any given boundary is then given by the area integral of the required force, eg: WallForceX = areaInt(Pressure Force X) A CFD-Post session file is attached which creates the expressions and variables needed. It creates a vector variable called “Pressure Force”. It also creates separate scalar variables for each component since it was discovered at version 2019R1 that posts reports that the components of a vector variable are undefined even though it correctly plots the vector. 