Questions for compressible flow

Roland R · January 11, 2013, 08:13

Hello,

I would like to complete a simple study. I have a 3D geometry which is a simple tube. There is a small choke in the middle of the tube.
The main settings:
Compressible flow (ideal gas)
Inlet: Total pressure
Outlet: Static pressure
Steady state simulation

Based on the theory of compressible flow, the critical pressure ratio: p_low/p_high=0,53, where the Mach number is 1. I would like to check these values in my simulation but I have some questions:

1. The Mach number is 1 in the choke, this is ok. But locally? Or in the full cross section? If I define a plane in the choke then in case of the critical pressure ratio:

maxVal(Mach Number)@Plane 1 = 1 ?

or

massFlowAve(Mach Number)@Plane 1 = 1 ?

2. Where should I measure the p_low and p_high?

3. The measured pressure is Total Pressure or Static pressure?

Regards
Roland

evcelica · January 12, 2013, 11:22

1: Relief valve and orifice calculations would leave me to believe that it would be the mass flow average, since they tell you how much flow you can flow through an orifice. This could be wrong though, I've never done these types of things with CFD, only hand calcs.

2: Measure them sufficiently far up and downstream from the orifice, past your vena contracta. I would plot a chart of pressure along the length, then choose your points with a low dP/dx that aren't in the immediate zone of the orifice.

3: Static pressure, as total pressure will include your dynamic head.

January 11, 2013, 08:13	Questions for compressible flow	#1
Roland R Senior Member Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 17	Hello, I would like to complete a simple study. I have a 3D geometry which is a simple tube. There is a small choke in the middle of the tube. The main settings: Compressible flow (ideal gas) Inlet: Total pressure Outlet: Static pressure Steady state simulation Based on the theory of compressible flow, the critical pressure ratio: p_low/p_high=0,53, where the Mach number is 1. I would like to check these values in my simulation but I have some questions: 1. The Mach number is 1 in the choke, this is ok. But locally? Or in the full cross section? If I define a plane in the choke then in case of the critical pressure ratio: maxVal(Mach Number)@Plane 1 = 1 ? or massFlowAve(Mach Number)@Plane 1 = 1 ? 2. Where should I measure the p_low and p_high? 3. The measured pressure is Total Pressure or Static pressure? Regards Roland

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January 12, 2013, 11:22		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	1: Relief valve and orifice calculations would leave me to believe that it would be the mass flow average, since they tell you how much flow you can flow through an orifice. This could be wrong though, I've never done these types of things with CFD, only hand calcs. 2: Measure them sufficiently far up and downstream from the orifice, past your vena contracta. I would plot a chart of pressure along the length, then choose your points with a low dP/dx that aren't in the immediate zone of the orifice. 3: Static pressure, as total pressure will include your dynamic head.