Suitable inlet boundary condition for a compressible flow simulation
 

Hi,

I'm using FLUENT v14.5 to simulate the turbulent flow over a zero pressure gradient flat plate (RANS, Spalart-Allmaras, pressure-based coupled solver, ICEM Hexa mesh with y+=1). I'm using a compressible free-stream Mach number (M = 0.92), standard sea-level free-stream ideal-gas air properties and a 3-D fluid domain with a cross-sectional area of about 0.057m^2 (rather than 2-D as I need the outlet data as a profile for another simulation). From all this I know the mass flow rate (22.05kg/s).

I've run this for an incompressible free-stream Mach number using the velocity-inlet and pressure-outlet boundary conditions (BoCos) and the results matched my hand-calculations.

However, with the compressible Mach number I get different problems depending on which inlet boundary condition I use and none of them give the correct solution.

1. I understand the velocity-inlet should not be used of compressible flows even though the User Guide says it can. I tried it anyway and FLUENT said the mass flow rate (which I monitor on the outlet face) was twice what it should be.
2. I tried the mass-flow-rate BoCo but the velocity outside the boundary layer was too low (284m/s rather than 313m/s).
3. Neither the pressure-inlet or pressure-far-field BoCos would give a converged solution.

This flow has been solved countless times so can someone tell me which inlet BoCo to use?

Thanks.

Hi, did you find out which BC's are suitable for that kind of simulation? Thanks in advance. Gonzalo

I have not looked into this since I posted that above, but I will need to go back to this simulation soon.

OK, I'll try to do some simulations and I'll see what results I get! Regards

Check out this paper, may be it will help you. Regards.
Gonzalo

reply
 

Use

Pressure Inlet and
Pressure oulet as your inlet and outlet conditions

Thanks
Kiran

As a general rule, for compressible flows you can use:
mass flow inlet, pressure inlet, pressure outlet, pressure far field.

Daniele

Thank you ghost82 and kiran for your replies! Best Regards
Gonzalo

HI, i've been tried diferent BC's for the compressible boundary layer simulation over a flapt plate, without pressure gradient, and inlet Mach number under 1. My problem is that, if I put as inlet BC's pressure inlet, I don't get the desired Mach number in the inlet even if I put the total and static pressure according to the isentropic relation. Another BC's that I tried was pressure-far-fiel, and again I didn get the desired Mach number at the inlet. Another BC's I tried was mass-flow-inlet and it also didn't work. The BC's at the top is symmetry and at the outlet is pressure-outlet. How should I fix the inlet Mach number?

I think that's what I basically said in my original post. I tried all the different boundary conditions at the inlet an none of them gave the correct flowfield. I my return to looking at this soon.

External flow
 

hi all, I'm trying to learn the external flow simulation over an airfoil, but I noticed it is considered to be at sea level, what if I want to try it at altitude, should I set the operating pressure at the corresponding altitude? .. in some example it was set to "zero" why is that ?.. what about the operating Temperature? thnx :rolleyes:

Hi ezmirald,

Yes you would need to match the "altitude" pressure and temperature conditions for the density and viscosity to be representative (check you have the Sutherland and Ideal gas laws activated in the materials options). However you can specify pressure and temperature at your boundaries whilst keeping the operating conditions 0. In this way your pressures will be absolute not gauge (i.e. gauge with respect to 0).

About gonzalo and siw problems: have you checked the density and pressure at the boundaries where you aren't getting the desired Mach?

can you fix the pressue values both at the inlet or outlet, or is it over defining the situation?

Hi. I am currently facing a similar situation. Did you solved the problem?