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March 27, 2008, 17:06 |
density problem
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#1 |
Guest
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Guys,
I defined a new material in CFX with variable density and viscosity by using CEL. I did plot the density and viscosity in CFX-Pre which look great. the problem is a simple liquid flow in duct. the inlet BC is mass flow rate and static temp. the outlet BC is static pressure. the wall is non-slip with a constant temp. When I tried to run the case, I got a fetal error which is as follows, Fetal Bounds Error Detected ----------------------------- Variable: Density Details of Error ---------------------------- Error detected by routine: MAKDAT CDANAM = LVAR CDTYPE = INTR ISIZE = 95 CRESLT = OLD Anybody knows where the problem is? thanks a lot snowshovel |
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March 27, 2008, 21:54 |
Re: density problem
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#2 |
Guest
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Did u forget to put Cp and K?
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March 27, 2008, 21:59 |
Re: density problem
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#3 |
Guest
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When u create a expression for a variable, don´t forget to put it in the same unit. See below.
Fluxrogerio (B*(19453.29701*(t/A)^3.07678)/(4.73947^3.07678+(t/A)^3.07678)) A 1.0 [s] B 1.0 [W m^-2] Observe that the name "Fluxrogerio" has the unit [W m^-2] or heat flux or rate of energy per area. |
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March 27, 2008, 22:01 |
Re: density problem
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#4 |
Guest
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t is the time in [s]!!
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March 28, 2008, 09:18 |
Re: density problem
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#5 |
Guest
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Rogerio,
Thanks for your help. I defined Cp and K. And, all the units in my expression should be correct since I double checked the values and curves in CFX-Pre. The only thing is I use constant Cp and thermal conductivity since I don't have formula for them. For density and viscosity, I use equations to make the 2 variabls a function of temp. Any thoughts? thanks again. snowshovel |
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March 28, 2008, 09:35 |
Re: density problem
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#6 |
Guest
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Does your definition of density work for all values of pressure and temperature? Does it give sensible values at high and low pressures and temperatures?
I have had problems with this in the past: In the initial stages of a calculation you can get big variations of pressure and temperature - so CFX may be trying to evaluate the density for very high or low pressures and temperatures and your correlation may be failing or giving unrealistic large or small values. For a simple example, imagine if you had used: density = sqrt( T - 273.15 K ) This would work fine for room temperature (T=294.15 K, or 21 deg C), but would fail immediately if T < 273.15 K (T<0 deg C) at *any* intermediate stage of the calculation. It's worth checking, but I cannot guarantee this is the problem. Good luck. andy |
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March 28, 2008, 09:52 |
Re: density problem
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#7 |
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Andy,
That's what I'm worrying. My temperature range is -40 deg F --- 300 deg F. I tried to use a ploynomial equation for the specific heat at constant pressure. Now, the problem is something wrong w/ the specific heat, not density any more. Seems you're right. Do you have any idea to make a quick adjustment in CFX w/o changing my formula since I have to use them? Thanks in advance. Snowshovel |
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March 28, 2008, 10:19 |
Re: density problem
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#8 |
Guest
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After defining the min and max temp under the "table generation" for my equation, CFX solver starts running. Thanks the help from you guys. Have a good weekend.
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March 28, 2008, 10:31 |
Re: density problem
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#9 |
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Hi Snowshovel,
Also make sure your density is a function of the Absolute Pressure (pabs), not Pressure (p). Pressure (p) is relative to the domain pressure and can be negative, which would result in a negative density, whereas the Absolute Pressure (pabs) includes the domain reference pressure. -CycLone |
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March 28, 2008, 10:53 |
Re: density problem
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#10 |
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Hmm... can you define a new variable, Tbound, as:
Tbound = max(min( Temperature, Tmax),Tmin) or something like that and then write your polynomials for cp, density etc. in terms of Tbounded instead of T? I think then: Tbound = T for Tmin<T<Tmax Tbound = Tmin for T<Tmin Tbound = Tmax for T>Tmax so as long as your formula work on the range [Tmin,Tmax], and your final temperatures all lie in this range you should be OK. There are lots of other similar options using CEL.... I bet there's an easier way though - but I can't think of it now. I hope others will post better advice. |
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March 28, 2008, 10:56 |
Re: density problem
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#11 |
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"After defining the min and max temp under the "table generation" for my equation, CFX solver starts running."
That's much better than my min(...,max(...)) solution I just posted. I'll make a note of it myself! Good luck, andy |
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March 28, 2008, 19:44 |
Re: density problem
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#12 |
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You´re welcome!
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March 29, 2008, 14:09 |
Re: density problem
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#13 |
Guest
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CycLone,
Thanks for your comments. In my case, the density and viscosity are function of temp. only. However, your comments really make me know something new. Thanks, snowshovel |
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March 29, 2008, 14:12 |
Re: density problem
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#14 |
Guest
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Good method. Let's see any new thinking around?
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