# CFD Thermal Simulation

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 April 7, 2016, 09:20 CFD Thermal Simulation #1 Member   Chaitanya Join Date: Mar 2016 Location: Stuttgart, Germany Posts: 50 Rep Power: 2 Hello Everyone, I am very new to Fluent and very basic knowledge of commercial CFD softwares. I have taken advanced CFD courses during my masters and currently I am doing a CFD thermal simulation for a nozzle. (air flow) My model looks something like this. Nozzle.PNG I have constant temperature at inlet and outlet. My model definition is: inviscid, incompressible, steady state flow. PBCS solver. BC: I want to define temp. at inlet and outlet but this option appears only if I tick the energy equation. But I am dealing with incompressible flows. can some one help me how to impose temp. BC in incompressible flows? Or please send me a link to some tutorial where they have done this analysis. !! I hope I am clear with my question. Any documentation or links or tutorials could be helpful. !! Thankyou in advance, Have a nice day!! Chaitanya

 April 7, 2016, 17:35 #2 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 1,372 Rep Power: 20 Note: you can't specify temperature at the outlet (that would make the problem ill-pose). It's not an incompressible flow problem. Your problem is you are trying to use the inviscid flow model (which disables a lot of energy options). You can already specify the flow inlet temperature using an inviscid model with any equation of state. I'm not sure why you say you cannot do that. What version of Fluent are you using? However, with inviscid flow model, you cannot specify temperature on walls. If you want an inviscid flow with heat transfer on walls, you should use the laminar flow model (which is more general anyway) and then activate energy and apply your boundary conditions. You can specify the viscosity to be 0 and make the walls zero-shear walls to have it produce the desired properties of an inviscid flow. cysanghavi likes this.

 April 7, 2016, 19:12 #3 Member   Chaitanya Join Date: Mar 2016 Location: Stuttgart, Germany Posts: 50 Rep Power: 2 Thank you for your reply. As far as I understand, if the flow velocity is not comparable to Mach number, then the flow is incompressible. My density, momemtum are not changing with time, since the velocity is constant. - steady state. About the inviscid case, the viscosity of air at room temp. is of order 10e-5 which is too minimal. Also, my flow is not laminar, since the Re number for v=10m/s is of order 10e6. About the Fluent version, I am using 16.2, But I guess I am not looking up the options properly. I will look into this again. Thus my conclusion, from your sugesstion is that I should use a turbulent model, (not invsicid, not laminar). But I am not convinced to use energy equation state option. Does this mean my I am using compressible flows. ?

April 7, 2016, 20:33
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Lucky Tran
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Quote:
 Originally Posted by cysanghavi Thank you for your reply. As far as I understand, if the flow velocity is not comparable to Mach number, then the flow is incompressible. My density, momemtum are not changing with time, since the velocity is constant. - steady state. About the inviscid case, the viscosity of air at room temp. is of order 10e-5 which is too minimal. Also, my flow is not laminar, since the Re number for v=10m/s is of order 10e6. About the Fluent version, I am using 16.2, But I guess I am not looking up the options properly. I will look into this again. Thus my conclusion, from your sugesstion is that I should use a turbulent model, (not invsicid, not laminar). But I am not convinced to use energy equation state option. Does this mean my I am using compressible flows. ?
The flow can be considered incompressible if the Mach number is low, yes.

But you need to be very careful with the rest of your arguments. Viscosity being on the order of 10e-05 is only meaningful if you compare it to to a velocity and length scale. If we take this argument, then there should not be any drag on any airplane or any car, or anything. The result of viscosity being very small results in the boundary layer problem (flow properties change rapidly over a small region). Although the viscosity is small, the velocity gradients can become extremely large (because of the thin boundary layer). This was the discovery made by Prandtl. It's up to you how you model your problem.

Why do you insist you cannot use energy equation? The energy equation refers to transport equation for internal energy. It has nothing to do with the equation of state. The equation of state is defined separately. As a fluid particle moves throughout the domain it can gain/loss and exchange heat with other particles. This happens whether the flow is inviscid, laminar, or turbulent.

A consequence of incompressible flow is that there is no equation of state involving pressure. In this case, pressure can be determined from the other parameters (namely velocity). However, you can still have equations of state for density and temperature.

 April 8, 2016, 05:11 #5 Member   Chaitanya Join Date: Mar 2016 Location: Stuttgart, Germany Posts: 50 Rep Power: 2 Problem_setup.PNG The problem setup.

 April 8, 2016, 07:21 #6 Member   Chaitanya Join Date: Mar 2016 Location: Stuttgart, Germany Posts: 50 Rep Power: 2 Thanks for the reply.. You made my thought process clear. I agree to your view that this is an incompressible turbulant flow and not invsicid. The boundary layer problem would be solved since if the fluid is vsicid, then there is a scond order viscous term. Following 2 conditions are unclear for me: 1. What do you mean by equation of state ? Do you mean conservation of energy equations. My CFD process is follows: I have to simulate the air lubrication provided by the nozzle during a drilling process and study the temperature profiles generated at the tip. So I have some fixed oultet temp. at the nozzle tip. 2. So I need to fix the outlet temp at some value, But acc. to your previous post, you said this would make the problem ill-posed. How do I go about this. ? 3. Where does the Flunet software know that I am using incompressible flows and it understands just to use mass and mometum conservation equations. ?

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