Periodic Boundary Condition
 

I am modelling air flow through a tube at 1273K. The wall is maintained under constant temperature at 293K. With periodic boundary conditions after solution convergence the highest temperature in the periodic zone is 1343K which is apparently more than the flow temperature. Is this 70K overshoot pretty normal with the mentioned temperature range under periodic conditions?

Re: Periodic Boundary Condition
 

No, it's definitely not normal.

Sincerely, Jinwook

Re: Periodic Boundary Condition
 

Thanks. Well then if you could please help me more I would be thankfull. I am computing Nusselt number for Re between 10000 and 50000 through a helically ribbed tube of outer diameter 50mm using the RNG, standard wall functions, swirl dominated flow , such that y+ of grid lies between 30 and 60. I am using discretisation schemes of first order upwind for energy and second order for the rest. I have set the values for Cp, density and viscocity for air at 783K (average)under the material panel.

According to fluent, in the post processing for periodic heat transfer section its said that the displayed temperature my exhibit values outside the range defined by the inlet bulk temperature and the wall temperature.

Re: Periodic Boundary Condition
 

Dear Chinmay

Sometimes, Fluent's reporting may not be correct in the boundary. So, you can check that the 70K overshoot is only reprting value(interpolation or extrapolation eroor) or real calculated value. Make plane just near the periodic plane, say 0.01m beside the periodic plane, and report or display contour. If 70K overshoot is still observed at that plane(near the periodic plane), I think that it is physically unrealistic and something in your calculation or grid generation may be wrong. If 70K overshoot is not observed at that plane, I think that you may consider the overshoot as only reporting problem.

Sincerely, Jinwook

Re: Periodic Boundary Condition
 

Thanks Jinwook The 3D tube model is only 24.4 mm long. As you suggested I checked the tempearute in the periodic region. The overshoot temperature region covers the entire central part( 30% of the area) over the periodic walls. This is like a block placed on the central axis.

Do you think there is something wrong with my grid structure ? Will it pose problems even though I check the y+ values?

Re: Periodic Boundary Condition
 

Sorry, but I can not predict what is your problem. Anyway, 70K overshoot is, as you know, can not be acceptable. In general, Fluent reports error if there is mistake in the periodic boundary.

Sincerely, Jinwook

Re: Periodic Boundary Condition
 

Dear Jinwook To cross check if my grid was wrong, I tried out with a 2D rectangular model with the same periodic boundary conditions. To my surprise the overshoot is even more now and exceeds the inlet temperature by 200°K. Regards Chinmay

Re: Periodic Boundary Condition
 

Just before, I my self have done test of periodic boundary condition, with inlet temperature 1200K and wall temperature 300K. I do not have any problem. I recommend that you would check your problem setting.

Sincerely, Jinwook

Re: Periodic Boundary Condition
 

Dear Jinwook I am working with the Fluent release 5.5.14. My colleague says that while he was working with the older release he didnt face such problems.

Are you using the earlier version? If the versions are same, I suppose, they should produce the same results.

Meanwhile would you mind sending me the case and data file of the one you mentioned earlier. Well, if you do not wish to do so, then its perfectly alright.

You could mail me at erk@eckrohrkessel.com or to my personal mail when the file is small at chinmay2010@yahoo.co.in

Re: Periodic Boundary Condition
 

O.K. But wait moment. I am out of office now.

Sincerely, Jinwook