stagnation pressure boindary condition

avb

If i have mass flow rate  and pressure provided at inlet for flow through valve, can i use stagnation pressure at inlet boibdary condition

Jagan Adithya Elango

Stagnation pressure boundary condition is used when velocity or flow rate is not known (like to determine the flow rate of fan at certain speed or in buoyancy driven flows).

You can either use mass flow inlet or pressure inlet if you know those values. 

Jagan Adithya Elango

Stagnation pressure boundary condition is used when velocity or flow rate is not known (like to determine the flow rate of fan at certain speed or in buoyancy driven flows).

You can either use mass flow inlet or pressure inlet if you know those values. 

avb

Jagan_21383 said:

Stagnation pressure boundary condition is used when velocity or flow rate is not known (like to determine the flow rate of fan at certain speed or in buoyancy driven flows).

You can either use mass flow inlet or pressure inlet if you know those values. 

I have been told by my prof to analyse flow at different Pressures keeping flow rate same. So am confused what to use the Boundary Conditions

acupro

avb said:

I have been told by my prof to analyse flow at different Pressures keeping flow rate same. So am confused what to use the Boundary Conditions

AcuSolve is essentially solving the pressure difference.  If we assume constant material properties, whether the pressure difference is from 50 Pa to 30 Pa, or 500 Pa to 480 Pa - it is still the same pressure difference of 20 Pa.

If we assume the properties (density usually as in Isentropic density type, but sometimes viscosity also) are a function of Absolute Pressure, then we could have different results based on different Absolute pressure levels - because then the material properties are different.  Then we have to make sure the boundary conditions are also correct.

You could still use constant properties - but obtain what the density and viscosity are for your fluid material at that pressure, and define your material properties accordingly.  You would still only define the mass (or volume) flow rate at the inlet, but the material properties would correspond to that given pressure level.  That would be something like assuming an average pressure in the simulation.

If you do want to use variable properties, I would use Absolute pressures, set the outlet pressure to the specified level, use isentropic density, and use the flow rate at the inlet.  Then the inlet pressure would be the unknown.

avb

thanks for your replies.

So I was thinking of using pressure boundary condition at the inlet while specify the flow rate at the outlet(mass conservation).

Will that be correct ? 

ydigit

avb said:

thanks for your replies.

So I was thinking of using pressure boundary condition at the inlet while specify the flow rate at the outlet(mass conservation).

Will that be correct ? 

As mentioned by Jagan you should use use these BCs separately.

You can used Stagnation Pressure BC at inlet and

- default outlet (pressure) BC at outlet, along with advanced BC called SIC (surface integrated condition) for mass flux at outlet).

If you are using HWCFD, there might be some editing of input file needed, as it will only write out one BC. If you send the model, I can create a video to show the process.

acupro

avb said:

thanks for your replies.

So I was thinking of using pressure boundary condition at the inlet while specify the flow rate at the outlet(mass conservation).

Will that be correct ? 

If you have a single inlet / single outlet, it's typically more robust to specify the mass-flux (really mass flow rate), flow rate (volumetric flow rate), average-velocity - etc - at the inlet and use the standard outflow boundary with static pressure defined.  There will still be conservation - what goes in has to go out.  The boundary conditions (whether mass flux/flow, Surface Integrated Condition on mass flux, average-velocity, etc) yield some sort of 'defined' profile on the boundary - to which the rest of the flow must adjust.  It's more robust to have the rest of the flow adjust in the downstream direction (so those boundary conditions on inflow) versus in the upstream direction (so those boundary conditions on outflow).

In cases with multiple outlets and a known flow-split, you may wish to apply the conditions on the outlets (but I would still suggest leaving one without the defined flow).

avb

acupro_21778 said:

If you have a single inlet / single outlet, it's typically more robust to specify the mass-flux (really mass flow rate), flow rate (volumetric flow rate), average-velocity - etc - at the inlet and use the standard outflow boundary with static pressure defined.  There will still be conservation - what goes in has to go out.  The boundary conditions (whether mass flux/flow, Surface Integrated Condition on mass flux, average-velocity, etc) yield some sort of 'defined' profile on the boundary - to which the rest of the flow must adjust.  It's more robust to have the rest of the flow adjust in the downstream direction (so those boundary conditions on inflow) versus in the upstream direction (so those boundary conditions on outflow).

In cases with multiple outlets and a known flow-split, you may wish to apply the conditions on the outlets (but I would still suggest leaving one without the defined flow).

So how should one model pressurised fluid entering the pipe. should I still go for flow rate boundary condition at the inlet? If yes, then how would the pressure be satisfied at inlet (the one already provided to me). I mean shpuldn't the value by simulation amd provided to me match ?

acupro

avb said:

So how should one model pressurised fluid entering the pipe. should I still go for flow rate boundary condition at the inlet? If yes, then how would the pressure be satisfied at inlet (the one already provided to me). I mean shpuldn't the value by simulation amd provided to me match ?

If using constant fluid material properties - where the properties are not dependent on absolute pressure values - we're just calculating static pressure differences.  (The material properties are then determined to match those at the given 'operating pressure')  The typical outflow boundary is then pressure = 0 (in an integrated sense).  Let's say for the applied known flow rate the calculation yields 400 Pa at the inlet.  This gives a static pressure difference of 400 Pa.  Again, for constant properties, that same 400 Pa difference would also be the same from 5000 Pa to 4600 Pa - just adding a 'delta'.  In this type of simulation, the most robust approach is velocity/flow-rate/etc at the inlet and standard pressure = 0 at the outlet.

If the fluid material properties do depend on absolute pressure, then you have to take more care to match the various known quantities.  Best information to have would be some type of flow rate, plus the absolute pressure at the outlet - then you can still use flow rate type of inlet and that pressure at the outlet.  If you know only inlet absolute pressure and flow rate, then probably use the flow rate at the outlet and a pressure (or stagnation pressure) inlet.