setting speed of sound

Kimican · November 11, 2015, 18:45

hi guys,

my question is quite simple. how can i set the speed of sound?

ty

mprinkey · November 11, 2015, 20:10

In what code? The speed of sound is a direct consequence of the choice of state relation. With the ideal gas law, the speed of sound can be tweaked by changing the molecular weight or, even, the ideal gas constant. If you are doing isothermal flow, you could also set the temperature.

Kimican · November 11, 2015, 21:21

I use Fluent. In which menu/tool can i set these? How can i set the speed of sound with changing the molecular weight? Is the speed of sound function of only temperature?

For example i want to set an Mach number where the speed of sound is 340 m/s. Can i define the this value as speed of sound then i set the inlet velocity as 680 m/s to get Mach 2 e.g?

LuckyTran · November 12, 2015, 01:05

The speed of sound is a material property and can determined by the state of the fluid. e.g. a fluid a a given pressure and temperature will have a certain speed of sound.

But in many codes, an equation of state is used rather than directly looking up properties out of some (massive) libraries. If you have invoked a particular equation of state, then you must also determine the speed of sound from this equation of state to be consistent with the rest of your simulation.

For an ideal gas, the speed of sound is dependent only temperature and molecular weight

https://en.wikipedia.org/wiki/Speed_of_sound

You cannot "set" the speed of sound, but you can alter the temperature of the gas to achieve a particular speed of sound. You need to specify the temperature needed to achieve a speed of sound of 340 m/s. Then you can specify velocity. But be careful because at inlets for compressible flows, the inlet boundary condition is the stagnation temperature and not the static temperature. So you'll have to do so basic calculations to get the stagnation temperature from the static temperature and velocity.

Kimican · November 13, 2015, 18:07

first ty all

yes i know that speed of sound is function of molecular wight and temperature. but i was asking about same material e.g. air. for same material (ideal gas) speed of sound is function of only temperatur, isn't it?

however, i have defined the properties of air at -50 C. this operation is shown in the uploadad picture (if i could upload the picture

). that's the question: am i on the true way?

and when i start to analysis i get reversed flow from the 100-110th iterations though i extrude the outlet. and there is another terrible thing. this is Cd value. Cd value is negative and nearly e+06 level. how this terrible things can occur? what is the problem?

November 11, 2015, 18:45	setting speed of sound	#1
Kimican New Member Join Date: May 2015 Posts: 28 Rep Power: 10	hi guys, my question is quite simple. how can i set the speed of sound? ty

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November 11, 2015, 20:10		#2
mprinkey Senior Member Michael Prinkey Join Date: Mar 2009 Location: Pittsburgh PA Posts: 363 Rep Power: 25	In what code? The speed of sound is a direct consequence of the choice of state relation. With the ideal gas law, the speed of sound can be tweaked by changing the molecular weight or, even, the ideal gas constant. If you are doing isothermal flow, you could also set the temperature.

November 11, 2015, 21:21		#3
Kimican New Member Join Date: May 2015 Posts: 28 Rep Power: 10	I use Fluent. In which menu/tool can i set these? How can i set the speed of sound with changing the molecular weight? Is the speed of sound function of only temperature? For example i want to set an Mach number where the speed of sound is 340 m/s. Can i define the this value as speed of sound then i set the inlet velocity as 680 m/s to get Mach 2 e.g?

November 12, 2015, 01:05		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	The speed of sound is a material property and can determined by the state of the fluid. e.g. a fluid a a given pressure and temperature will have a certain speed of sound. But in many codes, an equation of state is used rather than directly looking up properties out of some (massive) libraries. If you have invoked a particular equation of state, then you must also determine the speed of sound from this equation of state to be consistent with the rest of your simulation. For an ideal gas, the speed of sound is dependent only temperature and molecular weight https://en.wikipedia.org/wiki/Speed_of_sound You cannot "set" the speed of sound, but you can alter the temperature of the gas to achieve a particular speed of sound. You need to specify the temperature needed to achieve a speed of sound of 340 m/s. Then you can specify velocity. But be careful because at inlets for compressible flows, the inlet boundary condition is the stagnation temperature and not the static temperature. So you'll have to do so basic calculations to get the stagnation temperature from the static temperature and velocity.

November 13, 2015, 18:07		#5
Kimican New Member Join Date: May 2015 Posts: 28 Rep Power: 10	first ty all yes i know that speed of sound is function of molecular wight and temperature. but i was asking about same material e.g. air. for same material (ideal gas) speed of sound is function of only temperatur, isn't it? however, i have defined the properties of air at -50 C. this operation is shown in the uploadad picture (if i could upload the picture ). that's the question: am i on the true way? and when i start to analysis i get reversed flow from the 100-110th iterations though i extrude the outlet. and there is another terrible thing. this is Cd value. Cd value is negative and nearly e+06 level. how this terrible things can occur? what is the problem?