# Shall I use ideal gas model or other?

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 January 19, 2006, 10:36 Shall I use ideal gas model or other? #1 summer Guest   Posts: n/a Hello, friends. Now I am trying to solve a 3d problem of a channel at steady state. My boundary condition is that the velocity inlet is 50m/s, and pressure outlet is 1atm. Others are solid wall. I would like to get the pressure drop from inlet to outlet. I try to use the ideal gas model in the materials and realized k-e model. How should I set the k and e at the inlet boundary condition? The default of both are 1. I used the default ones, but I can not get the convergent solution. Could you have any idea that how to solve it? Thanks!

 January 19, 2006, 10:55 Re: Shall I use ideal gas model or other? #2 summer Guest   Posts: n/a By the way, I use the air as the flow and the channel is not staight. The cross section of the channel is semi-circular. Thanks!

 January 20, 2006, 04:59 Re: Shall I use ideal gas model or other? #3 kharicha Guest   Posts: n/a Classical things to verify before changing the model: 1) Grid? y+ at the walls? 2) verify that your velocity profile at the inlet is not too simplistic, compare the velocity profile at some distance form the inlet, with the one at the inlet. 3) Do the same with k and epsilon, it should not be too different.

 January 22, 2006, 16:01 Re: Shall I use ideal gas model or other? #4 summer Guest   Posts: n/a Hello, thanks ! What do you mean that the velocity profile at the inlet is not too simplistic. Does it mean the velocity profile at some distance from the inlet should be similar to that at the inlet? Thanks!

 January 23, 2006, 03:38 Re: Shall I use ideal gas model or other? #5 kharicha Guest   Posts: n/a It depends on the geometry of your duct, but usually it is not so different... take a straight duct, assume you want to simulate the flow development along the duct...if you take a constant velocity at the inlet you will have some convergence problem. Because you have a reorganisation of the flow at the vicinity of your inlet. In addition to that a flat velocity profile never exists in reality, the velocity has to be equal to zero at the walls. I assume you model a laminar flow, if it is the case it is better to impose a parabolique profile. If the flow is turbulent, then take a constant velocity, but be carefull on the wall functions tat you will use (depends on y+).

 January 23, 2006, 11:38 Re: Shall I use ideal gas model or other? #6 summer Guest   Posts: n/a O.K, thanks! I will try. Yesterday I changed the solver from seperated to coupled,and used the velocity of about 60m/s(it should be turbulent, I think). So I used k-e and ideal gas. It seemed to work, but when the residual went to 5*10-3, it oscillated. So do you know why? Thanks again. I would like to plot y+ and have a look at it. summer

 January 24, 2006, 11:43 Re: Shall I use ideal gas model or other? #7 summer Guest   Posts: n/a By the way, the residual of continuty increases. I do not know why and how to make it smaller. Thanks!

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