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Old   March 27, 2008, 17:06
Default density problem
  #1
snowshovel
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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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Old   March 27, 2008, 21:54
Default Re: density problem
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Rogerio Fernandes Brito
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Did u forget to put Cp and K?
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Old   March 27, 2008, 21:59
Default Re: density problem
  #3
Rogerio Fernandes Brito
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When u create a expression for a variable, dont 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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Old   March 27, 2008, 22:01
Default Re: density problem
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Rogerio Fernandes Brito
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t is the time in [s]!!
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Old   March 28, 2008, 09:18
Default Re: density problem
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snowshovel
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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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Old   March 28, 2008, 09:35
Default Re: density problem
  #6
andy2O
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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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Old   March 28, 2008, 09:52
Default Re: density problem
  #7
snowshovel
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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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Old   March 28, 2008, 10:19
Default Re: density problem
  #8
snowshovel
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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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Old   March 28, 2008, 10:31
Default Re: density problem
  #9
CycLone
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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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Old   March 28, 2008, 10:53
Default Re: density problem
  #10
andy2O
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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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Old   March 28, 2008, 10:56
Default Re: density problem
  #11
andy2O
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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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Old   March 28, 2008, 19:44
Default Re: density problem
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Rogerio Fernandes Brito
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Youre welcome!
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Old   March 29, 2008, 14:09
Default Re: density problem
  #13
snowshovel
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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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Old   March 29, 2008, 14:12
Default Re: density problem
  #14
snowshovel
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Good method. Let's see any new thinking around?
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