cell temperature

HRSD · October 24, 2023, 10:36

hi,

we can find cell temperature and boundary cell face temperature with C_T(c,t) and F_T(f,t) command in udf.

what is this temperatures(C_T(c,t) and F_T(f,t))?, total or static temperature?

thanks for your help.

obscureed · October 31, 2023, 07:48

Disclaimer: I'm on shaky ground here, so you should confirm it for yourself by looking at the equations that Fluent is solving, and also by the test case I mention at the end.

When I look at the equation that Fluent solves for energy in compressible cases, I see that the equation solves for E. This quantity (which the manual does not name) is total energy per unit mass of fluid, defined as E = (h - p/rho + v^2/2). Temperature is deduced only from enthalpy, h, which is not dependent on fluid speed, v. I conclude from this that the temperatures that are discussed and reported are static temperatures.

When I last looked, kinetic energy effects are turned off by default if you are using the pressure-based solver. They are typically irrelevant if the fluid is incompressible. In these cases, you should be looking at static temperature results, and this is what C_T should give you as well.

I have to admit that I would find it challenging to devise a test case where I could predict in advance the difference between static and total temperatures. It's easy enough to get some compressible fluid moving fast enough that the two temperatures are different, but these temperatures would also be different from the starting temperature (due to pressure changes as well as kinetic energy), and conduction effects would interfere with any sharp gradients. However, it's easy enough to compare Fluent's postprocessed results with samples from User-Defined Functions, so I would recommend that you do this to be sure. (For example, the UDF could loop over all cells and report the smallest value of C_T.)

October 24, 2023, 10:36	cell temperature	#1
HRSD New Member HRSD Join Date: Jul 2023 Posts: 11 Rep Power: 3	hi, we can find cell temperature and boundary cell face temperature with C_T(c,t) and F_T(f,t) command in udf. what is this temperatures(C_T(c,t) and F_T(f,t))?, total or static temperature? thanks for your help. Last edited by HRSD; October 27, 2023 at 03:25.

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October 31, 2023, 07:48		#2
obscureed Senior Member Join Date: Sep 2017 Posts: 246 Rep Power: 12	Disclaimer: I'm on shaky ground here, so you should confirm it for yourself by looking at the equations that Fluent is solving, and also by the test case I mention at the end. When I look at the equation that Fluent solves for energy in compressible cases, I see that the equation solves for E. This quantity (which the manual does not name) is total energy per unit mass of fluid, defined as E = (h - p/rho + v^2/2). Temperature is deduced only from enthalpy, h, which is not dependent on fluid speed, v. I conclude from this that the temperatures that are discussed and reported are static temperatures. When I last looked, kinetic energy effects are turned off by default if you are using the pressure-based solver. They are typically irrelevant if the fluid is incompressible. In these cases, you should be looking at static temperature results, and this is what C_T should give you as well. I have to admit that I would find it challenging to devise a test case where I could predict in advance the difference between static and total temperatures. It's easy enough to get some compressible fluid moving fast enough that the two temperatures are different, but these temperatures would also be different from the starting temperature (due to pressure changes as well as kinetic energy), and conduction effects would interfere with any sharp gradients. However, it's easy enough to compare Fluent's postprocessed results with samples from User-Defined Functions, so I would recommend that you do this to be sure. (For example, the UDF could loop over all cells and report the smallest value of C_T.)