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Time dependent heat flux (as a boundary heat source)
Posted Jan 10, 2012, 6:42 p.m. EST Heat Transfer & Phase Change Version 4.2a 5 Replies
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I am trying to model a 2D axi-symmetric analysis, which consists of a boundary heat source. I wish to apply a time-dependent heat flux for that boundary. The duration of the analysis is 30 days with 1 hour time steps (0,1[h],30[d]). The heat flux should be zero for hours 0-8 and 16-24; and q_boundary for hours 8-16 for each day.
I am setting the heat flux of the boundary heat source (General source, Qb) as follows:
Qb = q_boundary*(mod((t/3600),24)>=8)*(mod((t/3600),24)<16)
When I run the analysis, I see that no heat flux is applied at any time on that specified boundary. Could you please tell me where do I make a mistake?
Best regards,
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Tolga
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I suspect that your solver settings are such that it skips the time varying input flux (but it could be something else.
First of all, with HT I strongly recommend that you use a V4 Waveform "pulse" function (or a Heaviside function in 3.5), to smooth the transition, such steps gives very serious problems to the solver. Then you need to have a fine mesh on the heat source side (I often add a boundary mesh along the constant Q flux boundary, for time dependent studies) and finally you must help the solver with better steps. By default its in "free" stepping mode (far down in the time solver setting nodes), use rather " Intermediate" and then in your time series define time steps, at least 3 per rise and fall times, then longer in between.
The default time stepper settings is for settling i.e. exponential type asymptotic decays, and not good for periodic BC changes
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Good luck
Ivar
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Thank you for your reply and comments.
I tried a different way after I sent the message. I constructed a time varying heat flux input function with a combination of rectangle and analytic functions.
Rectangle Function (rect1):
Lower Limit: 8
Upper Limit: 16
Size of transition zone: 2
Analytic function (q_boundary_operation):
Expression: q_boundary*rect1(mod(time/3600,24))
argument: time
Then, I used q_boundary_operation(t) for the boundary heat source. This way, I could be able to apply the time varying heat flux. However, the results were not as I expected, and I suspected something was wrong about time stepping. At that moment, I saw your reply, and when I changed the stepping mode to "Intermediate", the problem was solved.
However, in your message, there is a point which is not very clear to me, that is; you mentioned: "in your time series define time steps, at least 3 per rise and fall times, then longer in between". Is this the times we define in the time-dependent solver (i.e. range(0,1[h],7[d])) or is it in another node?
Thanks again,
--
Tolga
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you have also the "Definition - Functions - Waveform - Pulse" for repeated pulses
My comment was jsut that you should ensure that your time stepping you define with your "range" has a few time steps during the transition, to help the solver identify the steps, otherwise it might miss some
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Good luck
Ivar
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--
Tolga
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--
Tolga