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Bug in dp_volumeFlowRate_MFLOW() function #4552

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casella opened this issue Feb 26, 2025 · 1 comment · May be fixed by #4553
Open

Bug in dp_volumeFlowRate_MFLOW() function #4552

casella opened this issue Feb 26, 2025 · 1 comment · May be fixed by #4553
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@casella
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bug Critical/severe issue L: Fluid.Dissipation Issue addresses Modelica.Fluid.Dissipation
Milestone
MSL4.1.0

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@casella
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casella commented Feb 26, 2025

While trying to solve #4424, I discovered that the pressure/flow behaviour of the Modelica.Fluid.Fittings.GenericResistances.VolumeFlowRate model when I set a = 0 and b > 0 was not

dp = b*V_flow

as claimed by the documentation, but rather dp = 1/b*V_flow, which of course caused the pressure losses to be wrong by many orders of magnitude. The issue can be traced back to this chunk of code:

ModelicaStandardLibrary/Modelica/Fluid/Dissipation.mo

Lines 4618 to 4625 in 9e8b7fb

M_FLOW := IN_var.rho*(if a>0 and b<=0 then
Modelica.Fluid.Dissipation.Utilities.Functions.General.SmoothPower(
(1/a)*dp,
(1/a)*dp_min,
0.5)
elseif a>0 and b>0 then
sign(dp)*(-b/(2*a) + sqrt((b/(2*a))^2 + (1/a)*abs(dp)))
else b*dp);

where the last line should be dp/b rather than b*dp.

Probably nobody has ever used this component in this way, i.e. to obtain a linear pressure-flow characteristic, otherwise we would have heard about that. This needs of course to be fixed in MSL 4.1.0.
@casella casella self-assigned this Feb 26, 2025
@casella casella added bug Critical/severe issue L: Fluid.Dissipation Issue addresses Modelica.Fluid.Dissipation labels Feb 26, 2025
@casella casella added this to the MSL4.1.0 milestone Feb 26, 2025
@casella casella linked a pull request Feb 26, 2025 that will close this issue
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@casella
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casella commented Feb 26, 2025

@Esther-Devakirubai, once #4533 is merged in, it will need to be back-ported to maint/4.1.0@

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bug Critical/severe issue L: Fluid.Dissipation Issue addresses Modelica.Fluid.Dissipation
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MSL4.1.0
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Fixed wrong expression in dp_volumeFlowRate_MFLOW casella/ModelicaStandardLibrary
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@casella