Dynamic pressure higher than total pressure?

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  • Last Post 27 August 2019
aerofrost posted this 26 August 2019

Hi,

Any help would be hugely appreciated on this confusing issue.

I'm modelling an external supersonic flow of M=2 around an aircraft. The boundary conditions are a pressure far field.

Using the isentropic flow relations, for my altitude (18,300m) the static pressure is 7,158 Pa, which using the isentropic flow relations gives a total pressure as 56,008 pa. I input this as the gauge pressure in the far field boundary condition.

Operating pressure is set to zero.

I set the simulation to provide me with dynamic pressure as I need to calculate a drag coefficient, however upon viewing it in Post, the dynamic pressure in the far field is ~3x higher than the total pressure I input??

Is there something I'm missing here? I've gone over my setup over and over and it seems fine.

Thanks very much in advance.

Ed.

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abenhadj posted this 27 August 2019

Always check first in Fluent. What you enter under as pressure for this kind of BC is gauge static pressure.

Best regards,

Amine

aerofrost posted this 27 August 2019

Hi Amine,

Thanks very much for posting an answer. from the ansyshelp.ansys.com website, which I accessed by clicking the help button in fluent, it says the following for high Mach numbers:

The pressure changes in such flows are much larger than those in low-Mach-number compressible flows, so there is no real problem with roundoff error and, therefore, no real need to use gauge pressure. In fact, it is common convention to use absolute pressures in such calculations. Since Ansys Fluent always usues gauge pressure, you can simply set the operating pressure to zero, making gauge and absolute pressures equivalent.

So I was under the impression this boundary condition was correct?

Please let me know any further thoughts on this.

Thanks again.

Ed

abenhadj posted this 27 August 2019

I do not understand your post sorry. Your input for Gauge Pressure is static pressure which you get from the isentropic relationship. For Ma=2, you need to get the stagnation pressure at that altitude (7158) and then you get the input for boundary (914 Pa round about)

Best regards,

Amine

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