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What is the "discharge coefficient" used in various places in the program, such as with the "Discharge Coefficient" and "Valve Characteristics Curve" Valve Type with a TCV node, or when using the Tank throttling inlet valve option? What should I enter here?
The "Discharge coefficient" is also known as a "valve coefficient" or "Cv", which is defined as: Flow / (Pressure Drop) ^ 0.5. The Discharge Coefficient is used to model the relationship between flow and headloss through the valve being modeled (TCV, tank float valve, etc) The value to enter depends on the valve, so you may be able to obtain it from the manufacturer.
If this is not available, but you have the minor loss coefficient for the valve (K), then you can use the below equation:
K = Cf D^4 / Cv^2
where D = diameter (in., m)
Cv = valve coefficient [gpm/(psi)0.5, (m3/s)/(kPa)0.5]
Cf = unit conversion factor (880 English, 1.22 SI)
Minor Loss Coefficients (K) for common types of valves can be found in places like engineering reference manuals and the included default Minor Loss Library (Components > Engineering Libraries)
Furthermore, if still unsure, you could perform a sensitivity analysis, trying a range of reasonable values and checking the response. If the results that matter to you are not significantly effected, then you may not need to worry about how accurate the discharge coefficient is.
Definition of Discharge CoefficientModeling Reference - Valves Of Various Types