Learning Forum

[Chun-Wei Tsai]

my initial temperature of jet fuel is set as 15 C with convection boundary condition applied for the walls with heat transfer coefficient of 12 and -55.6 degree celcius. I have set the density with constant, boussinesq approximation and I understand that there is not going to be any mixing due to gravity, however, I would like to confirm that the centre part remains 15 degree throughout the few hours of cooling, is that reasonable? And I have spotted some sudden jump of my residual plot not sure if that means anything wrong.

[Chun-Wei Tsai]

[Rob]

If you're using the Bousinesq approximation (density) and have turned gravity on I'd expect something to happen in the flow. What other boundary or cell zone conditions are set?

[Chun-Wei Tsai]

Hello, thanks for you reply again Rob, really appreciated. So for the whole simulation, firstly I have set the model as Laminar and energy model. The cell zone condition there's I assigned the jet a 1 to the surface body, and for boundary I have used convection for my wall surface body with heat transfer coeff of 12, free stream T of -55.6C, and zero for wall thickness and heat gen. rate. For initialization I set the temeprature15C. I have some confusion about the calculation setting, I have used max iteration of 20 and time step size of 10 and 3600 time steps to simulate a 10 hr of cooling. The model I made is just a square box that's all, and the meshing was 200 division for each side and I used face meshing 

[Rob]

What are you seeing for continuity & velocity residuals? 

[Chun-Wei Tsai]

I dont see anything this time which is weird, I have only seen the energy residual

[Chun-Wei Tsai]

They all have values of zero in the console.

 

[Chun-Wei Tsai]

Just to give you more information, except the density I have set for Bousineq approx, other I have used pieciewise polynomial.

[Rob]

OK. I suspect it's a solver glitch. This assumes you didn't do anything with the Equations under Solver. Because there are no boundaries there are no momentum sources, so a zero flow is not unreasonable. Because there is then no disturbance in the flow there is no "error" to trigger the solver to check and iterate to improve the "error". So the flow remains at zero velocity. This continues. Instead initialise with a tiny cross flow (1e-5m/s is enough) and see how it behaves. 

Note, because all four walls are the same the flow may be very chaotic. 

[Chun-Wei Tsai]

Hello, I have tried adding tiny x velocity with e-5 to e-4, I have seen the velocity and continuity residuals now, I have no idea why the last part has a weird display, during the simulation it sometimes appear and go back to a normal line, so I,m not sure if it's a bug for display or something actually went wrong. The contour plot for temperature i got is still the same as before, the centre part  remains 15.

[Chun-Wei Tsai]

[Rob]

Please can you replot with node values off. Also post screen shots of the Operating Conditions, Cell Zone and Materials panels? I specifically want to see the gravity, density and zone materials values.  

[Rob]

As an aside, the differnce in images between above and original post is that you've not got the interior selected. In 2d you don't pick any surfaces for contours. 

[Chun-Wei Tsai]

[Chun-Wei Tsai]

[Chun-Wei Tsai]

[Chun-Wei Tsai]

[Chun-Wei Tsai]

[Chun-Wei Tsai]

[Chun-Wei Tsai]

[Chun-Wei Tsai]

[Rob]

That looks OK. What's the Thermal Expansion Coefficient? 

[Chun-Wei Tsai]

that one I have left it as 0 as I tried giving it value of 0.00099 which is the typical value for jetfuel the result looks like the result  I previously obtain when the density was not set as  constant.

[Rob]

Have a look here https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_bcs_sec_operating.html%23flu_ug_uns_sec_boussinesq  If you set beta as zero, or near enough zero it's going to cause you a problem. You need to find the correct value for the fuel. 

[Chun-Wei Tsai]

the value 0.001 per celcius was suggest by two jetfuel documents. I have tried applied only heat transfer at the bottom and also heat transfer from top and bottom walls also from all walls , the result all show the similar contour plot patterns that I sent previously, where the cold temp fuel at the bottom and hotter fuel at the top. I wonder how correct is this result as I have looked at some other experiment conducted by cfd where the result shows as this pattern.

[Rob]

Which will explain the lack of motion and dominance of conductivity. 

I can't comment on the above, other than I'd want to see the velocity vector plot that goes with that result. 

[Chun-Wei Tsai]

[Chun-Wei Tsai]