Learning Forum

[sam_5195]

Hi, i have an 3d wing in a cuboid enclosure. One side is the velocity inlet and the opposite is the pressure outlet. The remaining have to be declared as pressure far field. While declaring the far field i keep the gauge pressure to be 0. But when the iterations start to occur there is an error which says "Error at Node 0: Mach number at Pressure Far-Field boundary should not be set to zero.

Check the specified mach number on Pressure Far-Field boundary conditions for thread 9.

You may want to consider specifying small mach number value instead of zero." I tried changing the gauge pressure to 101325 at farfield as well as inlet and outlet to see if it works but the error is still there. Any suggestions would be appreciated. Thank you

[DrAmine]

Please check the tutorial of Raef there: 

You need to provide specification which lead to non zero mach number. Actually you do only require to have pressure far-field and no other inflow/outflow boundaries.

[mcoderon]

Hi,

You can estimate the Mach number based on the inlet velocity and insert that number. 

Regards,

Marco

[sam_5195]

Thanks for your suggestions. I don't know how raef was able to declare the farfield without any error. I tried things according to the video but still the error keeps creeping up. I am attaching the .png file of the setting and the error here. Please let me know if you notice something inconsistent.

[DrAmine]

The Gauge Pressure is an input as well as the Mach Number. The Gauge Pressure is the one you get at your stagnation conditions using the isentropic relationship for compressible fluid. Do you need to account to any angle of attack?

[Karthik R]

Hello Sam,

Does this happen at the beginning of your simulation? Have you tried to use a different set of initial conditions? Perhaps from the inlet?

Could you please share a plot of your residuals? I do see that your turbulent viscosity is off-the-charts high is nearly all your cells.

Thanks.

Best Regards,

Karthik

[sam_5195]

I have kept the angle of attack at 0. This error does occur at the beginning of the simulation. At inlet i have put the gauge pressure as 0 and the inlet velocity as 45 m/s with everything else as default. I don't see what else should i change there.

[DrAmine]

Can you please post a screenshot of your operating conditions? Can you please tell me the stagnation pressure outside (I assume far-field is at 1 atm?)

[sam_5195]

These are the operating conditions. Yes the stagnation pressure outside is 1 atm. Just as a separate thought, if i make the enclosure very large compared to the wing and define it as a wall, will the results match with the case when the boundaries are defined as pressure far field? Thanks

[Rob]

Assuming the domain is large then yes, it should give the same result: this is how wind tunnels work. 

The far field boundary is designed for use with compressible gas, so is usually used on it's own: we don't use velocity & pressure boundaries too. I think the tutorial Amine linked will cover this. 

How fast is the wing moving (speed & Mach number)?