Discussion Forum

I am trying to use comsol 4.4 to solve natural convection problems in 3 very simple geometries (cavities). I have considered two flat geometries and a cylindrical geometry. One causes no problems to solve whereas the other two originate strange results (or no convergence at all).

After setting up the simulation for an initial 2D flat geometry with a Height/Width ratio of 0.5 (please see model number 1) and horizontal active walls (i.e. boundaries where the cold and hot temperatures are imposed), the solution was easy to obtain (after imposing a pressure constrain in one point of the domain).

In a second moment, I rotated the geometry 90 degrees so that the active walls would be vertical (please see model number 2). Despite not changing any other parameter, no convergence is achieved with this 2nd geometry, as the solving procedure stops at the 2nd iteration (with problems apparently in the segregated group dealing with flow variables, i.e. velocity and pressure). Every time a solution is attempted, the following error is reported:

“Undefined value found.
- Detail: Undefined value found in the equation residual vector.
There are 461 degrees of freedom giving NaN/Inf in the vector for the variable comp1.p.
at coordinates: (0,0.0236111), (0,0.0222222), (0,0.0208333), (0,0.0194444), (0.000310276,0.0236165), ...
There are 461 degrees of freedom giving NaN/Inf in the vector for the variable comp1.u.
at coordinates: (0,0.0236111), (0,0.0222222), (0,0.0208333), (0,0.0194444), (0.000310276,0.0236165), ...
There are 461 degrees of freedom giving NaN/Inf in the vector for the variable comp1.v.
at coordinates: (0,0.0236111), (0,0.0222222), (0,0.0208333), (0,0.0194444), (0.000310276,0.0236165), ...”

I did try to change mesh density, damping factors in solvers, initial velocity estimates, but obtained always the above type of error. I also tried to ramp up the volume force acting on the fluid elements and obtained a solution when this force was 10% of the actual value. For bigger values (of volume force) the error estimate of the flow-related segregated group reduces initially and then starts to oscillate over iteration (number) in a periodic way. I thought this could indicate that the flow may be unstable but, for the parameters of the current simulation, I do not expect the flow to be unstable… what could be causing this problem? How to address errors like those mentioned above?

In a third moment I tried to solve the same problem but with a 2D cylindrical geometry, basically one cylinder inside a bigger cylinder, with the inner cylinder having the highest temperature (please see model number 3). In this case, the solution is obtained way too fast (just a couple of seconds), and the obtained results show no natural convection and null velocity in between the two cylinders. The fact that the solution is obtained too fast seems to indicate that the flow problem is not being solved. And the fact that no natural convection appears in the temperature/velocity plots even when I increase the temperature difference up to 40ºC is, again, consistent with the flow problem not being solved (since it is very unlikely that the used configuration will not lead to natural convection between the two cylinders). I also tried to change the portion of the cylinders’ surface where I imposed fixed temperature but the results never showed natural convection flow/temperature patterns, which is again highly questionable. Are these results a consequence of wrong definitions in the simulation? How to capture the likely natural convection occurring in this geometry?

Thank you very much for your input.