Model of a flow resistance with a fixed flow coefficient. The mass flow rate is

ṁ = k √ΔP,

where k is a constant and ΔP is the pressure drop. The constant k is equal to k=m_flow_nominal/sqrt(dp_nominal), where m_flow_nominal and dp_nominal are parameters.


In the region abs(m_flow) < m_flow_turbulent, the square root is replaced by a differentiable function with finite slope. The value of m_flow_turbulent is computed as m_flow_turbulent = deltaM * abs(m_flow_nominal), where deltaM=0.3 and m_flow_nominal are parameters that can be set by the user.

The figure below shows the pressure drop for the parameters m_flow_nominal=5 kg/s, dp_nominal=10 Pa and deltaM=0.3.


Important parameters

The parameter from_dp is used to determine whether the mass flow rate is computed as a function of the pressure drop (if from_dp=true), or vice versa. This setting can affect the size of the nonlinear system of equations.

If the parameter linearized is set to true, then the pressure drop is computed as a linear function of the mass flow rate.

Setting allowFlowReversal=false can lead to simpler equations. However, this should only be set to false if one can guarantee that the flow never reverses its direction. This can be difficult to guarantee, as pressure imbalance after the initialization, or due to medium expansion and contraction, can lead to reverse flow.

If the parameter show_T is set to true, then the model will compute the temperature at its ports. Note that this can lead to state events when the mass flow rate approaches zero, which can increase computing time.


For more detailed models that compute the actual flow friction, models from the package Modelica.Fluid can be used and combined with models from the Buildings library.

For a model that uses the hydraulic parameter and flow velocity at nominal conditions as a parameter, use Buildings.Fluid.FixedResistances.HydraulicDiameter.


The pressure drop is computed by calling a function in the package Buildings.Fluid.BaseClasses.FlowModels, This package contains regularized implementations of the equation

m = sign(Δp) k √ Δp  

and its inverse function.

To decouple the energy equation from the mass equations, the pressure drop is a function of the mass flow rate, and not the volume flow rate. This leads to simpler equations.


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