Bug in dp_volumeFlowRate_MFLOW function

[casella]

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]

@Esther-Devakirubai, please back-port #4553 to maint/4.1.x and mention this fix in the release notes.

[Esther-Devakirubai]

Backported by #4580

[casella]

We need to double-check this is mentioned in the release notes