CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Conjugate heat transfer with heat source: Transient vs steadystate

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree1Likes
  • 1 Post By FMDenaro

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 26, 2016, 04:51
Default Conjugate heat transfer with heat source: Transient vs steadystate
  #1
Member
 
Bruno
Join Date: Jun 2016
Location: Siegen, Germany
Posts: 59
Rep Power: 5
MBttR is on a distinguished road
Hi all,

I'm trying to simulate a simple problem of a heat source (130W) in an airflow (roughly 10 m/s). I'm doing this in OpenFOAM but this shouldn't matter. My solutions are blowing up and it's driving me into frustration to such a degree that I have to go back and look at the basics.

It is a transient problem, right? I am interested in knowing which temperature the heat source reaches while being subjected to this air flow. When googling definitions of transient vs steady state, I always find the relatively unsatisfying answer of a transient solution changing in time. This is what is happening here, right? As both the temperature of the air and the heating element will change over time.

Then I start doubting, what is NOT a transient solution? Doesn't everything change in time? Why else would we be running simulations.

Hoping someone can clear this up for me,
Thanks,

Bruno
MBttR is offline   Reply With Quote

Old   July 26, 2016, 05:06
Default
  #2
New Member
 
Melvin.
Join Date: Mar 2009
Location: Singapore
Posts: 27
Rep Power: 12
bighead is on a distinguished road
check your mesh quality first. they should be good before running any calculations.

why do you say its a transient problem ?? based on what you described, it does not seem to be it. If it truely is, then you have to change your timesteps to smaller interval as well
bighead is offline   Reply With Quote

Old   July 26, 2016, 05:06
Default
  #3
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 4,950
Rep Power: 52
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
I am not sure about your doubt...

transient is an evolving stage that, starting from an initial condition can drive to either a steady or a ully developed unsteady state. That depends on the physics of the problem, the Reynolds number being an indicator. Furthermore, for turbulent flows that depends also on the adopted formulation.

In your case, given the heat flux and the asymptotic velocity is not sufficient to evaluate the main non-dimensional numbers. Are you considering air but what about the characteristic lenght?
FMDenaro is offline   Reply With Quote

Old   July 26, 2016, 05:08
Default
  #4
Member
 
Bruno
Join Date: Jun 2016
Location: Siegen, Germany
Posts: 59
Rep Power: 5
MBttR is on a distinguished road
If transient vs steady state relates to the FLOW CONDITIONS, then I'm thinking my problem is steady, after all, it will stay at 130W and 10m/s. But I'm also not convinced that's a satisfying answer, as for example a high angle of attack airfoil will be of transient nature at a certain flow velocity, with constant flow conditions. Perhaps there the reason for it being considered transient is because the flow conditions changes over LOCATION (hence location along the chord), but the definition of transient is changing over TIME?

Sorry for thinking aloud, but clearly I'm missing something.
MBttR is offline   Reply With Quote

Old   July 26, 2016, 05:32
Default
  #5
Member
 
Bruno
Join Date: Jun 2016
Location: Siegen, Germany
Posts: 59
Rep Power: 5
MBttR is on a distinguished road
Quote:
Originally Posted by bighead View Post
check your mesh quality first. they should be good before running any calculations.

why do you say its a transient problem ?? based on what you described, it does not seem to be it. If it truely is, then you have to change your timesteps to smaller interval as well
Thanks for the quick reply!

I thought it was transient as I wanted to see the temperature change over time, but as said in my next post (which I posted before seeing your replies, sorry about that) I am starting to doubt that it might be steady state, as the actual flow conditions don't change over time. Then again, I can't think of a problem where flow conditions do change over time (except for example a simulation involving wind gusts or a heater which increases in intensitivity).

My timesteps are automatically adjusted to have a Co number < 0.6.
MBttR is offline   Reply With Quote

Old   July 26, 2016, 05:37
Default
  #6
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 4,950
Rep Power: 52
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by MBttR View Post
If transient vs steady state relates to the FLOW CONDITIONS, then I'm thinking my problem is steady, after all, it will stay at 130W and 10m/s. But I'm also not convinced that's a satisfying answer, as for example a high angle of attack airfoil will be of transient nature at a certain flow velocity, with constant flow conditions. Perhaps there the reason for it being considered transient is because the flow conditions changes over LOCATION (hence location along the chord), but the definition of transient is changing over TIME?

Sorry for thinking aloud, but clearly I'm missing something.

I am used to think about "transient" as a time-evolving stage. It can be of physical nature (for example an airfoil that starting from zero angle of attack reaches a certain angle) but it can also be of numerical nature (for example a flow over a cylinder; starting from arbitrary initial condition, the numerical solution has a numerical transient).

In your case 130W and 10m/s said nothing about the physical state of the solution. You can see that your Re number is = 10^6 L. Similarily, from the 130W (maybe W/m^2 ??) you should consider the Rayleigh number.
You can have a numerical transient and then reaching a steady or unsteady solution.
FMDenaro is offline   Reply With Quote

Old   July 26, 2016, 05:48
Default
  #7
Member
 
Bruno
Join Date: Jun 2016
Location: Siegen, Germany
Posts: 59
Rep Power: 5
MBttR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
I am not sure about your doubt...

transient is an evolving stage that, starting from an initial condition can drive to either a steady or a ully developed unsteady state. That depends on the physics of the problem, the Reynolds number being an indicator. Furthermore, for turbulent flows that depends also on the adopted formulation.

In your case, given the heat flux and the asymptotic velocity is not sufficient to evaluate the main non-dimensional numbers. Are you considering air but what about the characteristic lenght?
Hi Filippo, thanks. I'm not entirely sure what you mean with your last paragraph. What do you mean with asymptotic velocity? How are heat flux + velocity not enough to evaluate the main non-dimensional numbers? What exactly do you mean with non-dimensional numbers, I am only interested in temperature.

I am considering air, yes. Characteristic length is a good one.. I hadn't considered this to be a factor here (sorry if that's stupid, I'm used to external flow problems, not CHT). Characteristic length here I suppose is 30mm (length of the heating element in streamwise direction). In OpenFOAM though, you define the kinematic viscosity, after which it will determine the Reynolds number from that. I have the kinematic viscosity of air (1.8e-5), leading to a Reynolds number of (I believe) roughly 17000.

*edit*
You posted a second comment while I was writing this one, will consider that now and come back, thanks!
MBttR is offline   Reply With Quote

Old   July 26, 2016, 06:37
Default
  #8
Member
 
Bruno
Join Date: Jun 2016
Location: Siegen, Germany
Posts: 59
Rep Power: 5
MBttR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
I am used to think about "transient" as a time-evolving stage. It can be of physical nature (for example an airfoil that starting from zero angle of attack reaches a certain angle) but it can also be of numerical nature (for example a flow over a cylinder; starting from arbitrary initial condition, the numerical solution has a numerical transient).

In your case 130W and 10m/s said nothing about the physical state of the solution. You can see that your Re number is = 10^6 L. Similarily, from the 130W (maybe W/m^2 ??) you should consider the Rayleigh number.
You can have a numerical transient and then reaching a steady or unsteady solution.
How do you come at a Re number of 10^6 L ? Is this just a quick estimation? Shouldn't it be (10/1.4e-5) = 7e5 L? Only a factor 2, I know, but want to make sure.

Was not familiar with the Rayleigh number. It is deifnitely W, not W/m2. The heating element is an electric motor, I know the power from measurements, roughly know the efficiency and from there considered how much power (hence, 130W) is lost as heat. Looking up the definition of the Rayleigh number, I'm not sure I can define it. It has the surface temperature in its definition, which is what I'd like to calculate. But the order of magnitude I believe will be (with characteristic diameter 0,08 cause the motor is cylinder shaped and an expected 80deg temp difference) 5.2e6. Does that figure make sense? How does it come into play in determining whether I have a transient or steady case?
MBttR is offline   Reply With Quote

Old   July 26, 2016, 06:38
Default
  #9
Member
 
Bruno
Join Date: Jun 2016
Location: Siegen, Germany
Posts: 59
Rep Power: 5
MBttR is on a distinguished road
Quote:
Originally Posted by bighead View Post
check your mesh quality first. they should be good before running any calculations.

why do you say its a transient problem ?? based on what you described, it does not seem to be it. If it truely is, then you have to change your timesteps to smaller interval as well
Are there any rules of thumb for the domain size around a cht problem? I have a pretty small domain right now, with zeroGradient boundaries on the walls.
MBttR is offline   Reply With Quote

Old   July 26, 2016, 07:02
Default
  #10
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 4,950
Rep Power: 52
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
I would think to a turbulent flow condition
FMDenaro is offline   Reply With Quote

Old   July 26, 2016, 07:04
Default
  #11
Member
 
Bruno
Join Date: Jun 2016
Location: Siegen, Germany
Posts: 59
Rep Power: 5
MBttR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
I would think to a turbulent flow condition
But transient or steady state? And why?
MBttR is offline   Reply With Quote

Old   July 26, 2016, 08:10
Default
  #12
Member
 
Bruno
Join Date: Jun 2016
Location: Siegen, Germany
Posts: 59
Rep Power: 5
MBttR is on a distinguished road
If I go back to the textbook definition, it must be transient. If I keep my eyes fixed at a certain point with a certain position x,y,z, the properties there (T, V) are still a function of time. Sure, it might reach an equilibrium at some point and be steadystate then, but I think it still requires me to solve with a transient solver. If anyone disagrees please do tell me.

By the way, I have just tried solving with a steady state solver and my solution still blows up, so it must be due to my boundary conditions, mesh, ... Something I will ask further in the OpenFOAM forum. Thanks all for the quick replies.
MBttR is offline   Reply With Quote

Old   July 26, 2016, 10:33
Default
  #13
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 4,950
Rep Power: 52
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Turbulence is ..turbulence, it IS unsteady by definition.
You can only think to a STATISTICAL steady state in case of energy equilibrium (production equal to dissipation). In such a case, you can use the statistally averaged form of the equations, the so-called RANS that are steady and solve only for the zero-th order statistic part of the flow (i.e. averaged variables).
FMDenaro is offline   Reply With Quote

Old   July 26, 2016, 10:54
Default
  #14
Member
 
Bruno
Join Date: Jun 2016
Location: Siegen, Germany
Posts: 59
Rep Power: 5
MBttR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
Turbulence is ..turbulence, it IS unsteady by definition.
You can only think to a STATISTICAL steady state in case of energy equilibrium (production equal to dissipation). In such a case, you can use the statistally averaged form of the equations, the so-called RANS that are steady and solve only for the zero-th order statistic part of the flow (i.e. averaged variables).
I've always learned that a steady flow can be fully turbulent as well, as it only refers to the transient nature of the flow. Anyway! I got what I came for; reassurance that I should stick to transient flow and hence my problems are of an entirely different nature. Thanks for all the help
MBttR is offline   Reply With Quote

Old   July 26, 2016, 11:01
Default
  #15
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 4,950
Rep Power: 52
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by MBttR View Post
I've always learned that a steady flow can be fully turbulent as well, as it only refers to the transient nature of the flow. Anyway! I got what I came for; reassurance that I should stick to transient flow and hence my problems are of an entirely different nature. Thanks for all the help
A turbulent flow can be only STATISTICALLY steady ... that means that the statistical averaged of any variable does not vary in time.
MBttR likes this.
FMDenaro is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Conjugate Heat Transfer transient Sara28 FLUENT 0 June 1, 2016 05:59
"parabolicVelocity" in OpenFoam 2.1.0 ? sawyer86 OpenFOAM Running, Solving & CFD 21 February 7, 2012 12:44
Inverse and Transient Heat Transfer Problem on commercial software: is it possible? rogbrito CFX 1 January 29, 2012 18:48
open source code for conjugate heat transfer? cchou Main CFD Forum 0 December 7, 2006 20:26
Convective Heat Transfer - Heat Exchanger Mark CFX 6 November 15, 2004 16:55


All times are GMT -4. The time now is 00:21.