flowRateInletVelocity - unexpected results
 

I'm trying to use flowRateInletVelocity to model a jet-in cross flow using a modified rhoCentralFoam solver (to support multiple gas species). I have a 2d model with a jet width of 0.81mm. I'm trying to specify a mass flow rate of 0.0337kg/(m^2*s), my 2d model has an "empty" depth of 0.01m, so I calculate out an equivalent kg/s (since the units for flowRateInletVelocity are kg/s) to be 2.7317e-7 kg/s and put that into my boundary condition.

I also specify the temperature and total pressure at the jet outlet.

My jet velocity BC looks like:

jetIn
{
type flowRateInletVelocity;
flowRate 2.7317e-7; // Volumetric/mass flow rate [m3/s or kg/s]
value uniform (0 0 0); // placeholder
}

The issue I'm having is that when I look at my results in paraview the mass flow rate is much lower than expected. If I take my species mass fraction * rho * Uy at the boundary over the jet I get a peak value of about 0.0115 kg/(m^2*s) and an average value of about 0.009. This is nowhere near the 0.0337 that I calculated the boundary conditions from. Does anyone have any experience with this BC and able to offer some input/advice? I've attached a plot of my mass flow rate in kg/(m^2*s) in paraView across the jet boundary.

Attachment 18888

Hi Chris,

Can you please explain the "boundary over the jet"? Is it the inlet face of the jet?

If possible, can you also post a sketch of your domain, and where you are calculating the values?

fixedFluxPressure + flowRateInletVelocity to fix mass flow?
 

Hi Chris!

I am in a very similar situation: I aswell have to specify subsonic mass flow at the inlet, and I have modified rhoCentralFoam to support multiple species. However, at the moment I don't have a working KNP "upwinding" predictor for the species transport equation.

When I used fixedFluxPressure and flowRateInletVelocity for a subsonic inlet (using OF 2.1.1), I got precisely the right mass flow through the inlet. In other words,
outputs the flowRate I specify in flowRateInletVelocity.

However, I get an "unnatural" pressure distribution for the cells near the inlet, so I'm not sure if fixedFluxPressure+flowRateInletVelocity is a valid way to fix inlet BC for subsonic inlet in a compressible flow. I think that you should fix the pressure at the inlet, and let the velocity "float" as a zeroGradient. Can anyone verify this? If I understood correctly, fixedFluxPressure + flowRateInletVelocity fixes a nonuniform pressure gradient, and a nonuniform velocity, at the inlet patch to match the flowRate (phi).

Tatu

hello chris,

You are setting a total mass flow rate at inlet, and you evaluate the mass flow of a specie in paraview ... try rho*Uy, not mass_fraction*rho*Uy.

regards,
olivier

Oliver,

This is true, but the jet is blowing a pure gas so along the boundary of interest the mass fraction of interest is equal to 1. I'm interested in controlling the mass flow of said pure gas.

Omkar,

That is correct, I was looking at the inlet face of the jet. My domain is essentially a flat plate (0.2m long) with a small jet blowing perpendicular to the plate at x=0.029m. I'm calculating the values at y=0 in the figure shown (surface of the plate).


Tatu,

Thanks for the input, I'll check out what you're suggesting, although it seems like fixing the pressure with a "floating" velocity would make it difficult to control the mass flow rate.