rhoCentralFoam velocity definition

GV-aero · May 2, 2016, 20:07

Hi guys,
I'm new to the forum and openFOAM. I've been following a youtube tutorial on the supersonic wedge case using rhoCentralFoam: https://www.youtube.com/watch?v=6jLn5cSCkd8

It seems they're specifying the airflow velocity in the boundary conditions at 5m/s. However, once the simulation has been executed the resulting Mach number is calculated to be about 5.
My question is, how does 5 m/s translate to Mach 5? Are they manipulating the Reynolds number?

Thanks!

maHein · May 3, 2016, 10:33

Hello,

the local speed of sound depends on the specific gas constant, temperature and isentropic coefficient. As you change the fluid properties, you can achieve Ma = 1 at conditions like U = 1m/s and T = 1K.

Just have a look at the settings in the thermophysicalProperties file in the constant folder.

Kind regards,

Martin

GV-aero · May 3, 2016, 16:10

Thank you Martin- that makes sense.
So, am I correct to assume the specific gas constant is calculated from the gas constant and molar mass; and the isentropic coefficient is 1.4 (for ideal gas)?
Also, does this case simulate an actual real-life gas (molar mass 11.6403 kg/mol; Cp =2.5) or are these numbers just randomly defined to simulate high Mach with low U conditions?
Thanks again!
Greg

May 2, 2016, 20:07	rhoCentralFoam velocity definition	#1
GV-aero New Member Grega Verdenik Join Date: Apr 2016 Posts: 4 Rep Power: 10	Hi guys, I'm new to the forum and openFOAM. I've been following a youtube tutorial on the supersonic wedge case using rhoCentralFoam: https://www.youtube.com/watch?v=6jLn5cSCkd8 It seems they're specifying the airflow velocity in the boundary conditions at 5m/s. However, once the simulation has been executed the resulting Mach number is calculated to be about 5. My question is, how does 5 m/s translate to Mach 5? Are they manipulating the Reynolds number? Thanks!

May 3, 2016, 10:33		#2
maHein Member Join Date: Jun 2012 Posts: 76 Rep Power: 14	Hello, the local speed of sound depends on the specific gas constant, temperature and isentropic coefficient. As you change the fluid properties, you can achieve Ma = 1 at conditions like U = 1m/s and T = 1K. Just have a look at the settings in the thermophysicalProperties file in the constant folder. Kind regards, Martin febriyan91 and tmik like this.

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May 3, 2016, 16:10		#3
GV-aero New Member Grega Verdenik Join Date: Apr 2016 Posts: 4 Rep Power: 10	Thank you Martin- that makes sense. So, am I correct to assume the specific gas constant is calculated from the gas constant and molar mass; and the isentropic coefficient is 1.4 (for ideal gas)? Also, does this case simulate an actual real-life gas (molar mass 11.6403 kg/mol; Cp =2.5) or are these numbers just randomly defined to simulate high Mach with low U conditions? Thanks again! Greg