How can I set a defined pressure drop in a subdomain?
You need to set momentum sinks in the subdomain to represent the pressure loss. The units of the momentum sink are N/m^3, which is equivalent to N/m^2/m = Pa/m i.e. pressure gradient, dp/dx. If you know the thickness in the flow direction of your subdomain (dx) and you know the required pressure drop through the valve (dp), then you need only set dp/dx as your momentum sink term within the subdomain.
This is a rather simplistic summary however. The pressure loss is probably related to the flowrate through the device. If it can be related to the local velocity in the valve then it should be straightforward to set a sink term using either a linear or quadratic loss coefficient. First you will need to define the curve for pressure drop vs. mean velocity as a CEL function. Let's assume that the loss will be represented by:
S_M,x = -C_R1 * U_x (N/m^3) (1)
where C_R1 is the linear resistance coefficient and U_x is the velocity component. As before, S_M,x is like dp/dx, so we can re-write (1) to give us:
C_R1 = -dp/dx/U_x (2)
You need to set the linear resistance coefficient to be a CEL function according to (2), where dp is the measured pressure drop at velocity Uand dx is the axial length of the subdomain region.