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To be more specific, I am doing variations on the cylinder flow model presented as an example for comsol. I have changed the geometries of the cylinder and the fluid properties. The cylinder is .4 meters in diameter and the fluid is air at room temperature (density of 1.225 kg/m^3 and dynamic viscosity of 1.81e-7). I received this error for reynolds numbers of both 1e2 and 1e7.

In these cases, some boundaries or properties are not specified properly. I usually try to "Get the Initial Values". Have a look at the results. Does look like your model specification?

David Wenger
www.comsol.de/company/consultants/wenger/

My boundary conditions are all set properly, as are my subdomain settings. I found that by setting the mesh to be coarse and the reynolds number to be extremely low, it solves the problem in seconds...however once i reach a certain reynolds number it decides its missing some initial condition? I am at a loss here.

Dear Michael,
maybe it is a matter of "absolute tolerances" for the time dependent solver.
Try to estimate the magnitude of the physical quantities you are solving for and set an absolute tolerance for each of them which is at least one order of magnitude less than the expected magnitude of that quantity.
Usually pressure and velocities require different tolerances since their order of magnitude are quite different.
I hope this helps.

Alessandro

The attached simple example does the same thing. When solver parameter is set to stationary mode, it converges very quickly. When the domains are set to transient, it comes to a quick halt with an inability to find a good initial start.

I've tried helping it along: run in stationary mode, store the solution, then tell solver manager to use the stored solution as the initial guess.

Nope.

In these cases, some boundaries or properties are not specified properly. I usually try to "Get the Initial Values". Have a look at the results. Does look like your model specification?

Wenger
www.comsol.de/company/consultants/wenger/

Hi David,

Where is "Get the Initial Values"? Where can I find it?

Thanks

It is under the Solve Menu, just above "Solver Parameters".

I've been playing around some more with the file I uploaded. If I run it in Stationary mode, then go to Solver Settings, and click "Store Solution", then go back to Solver parameters and switch everything over to Transient with segrated solution. Then click on "Get Initial Value", then COMSOL does exactly what I want, namely it plots as the initial value exactly the same solution as it has converged to last time.

I can then go to Solver Manger and click "use stored solution as initial value", but it still doesn't converge.

Depending upon the solver parameter setting tolerances, time-step, etc, it still sometimes complains "Can't find consistent initial values", more commonly it just complains about not converging.

Regards, John

In a similar turbulent natural convection case, how do you initialize the turbulent quantities (log of kinetic energy and dissipation rate) in a transient simulation starting from rest? Also, technically the flow will be laminar until a crtitical velocity is reached; is it ok to model it as turbulent throughout?