# buoyancy driven flow + unsteady

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 May 3, 2004, 15:53 buoyancy driven flow + unsteady #1 co2 Guest   Posts: n/a hi folks, I request some help from you in understanding the rules given on fluent online help for calculation of time step for buoyancy driven flows. 11.5.5 Solution Strategies for Buoyancy-Driven Flows tells us the formula for calculation of time step size for unsteady buoyancy driven flow. For my case the time step size comes out to be 0.74 sec! -- Now the problem is I need to simulate flow over 30 days and I certainly need a bigger time step size -- CAn you help me in understanding how I can increase time step size and still get convergence in each time step? thanks,. CO2

 May 4, 2004, 12:10 Re: buoyancy driven flow + unsteady #2 Evan Rosenbaum Guest   Posts: n/a The formula in the online help is a guideline, not a requirement. Transient evaluations have a maximum number of iterations per time step. If your time steps are converging before maxing out the iterations, you're *probably* OK. We often start with small time steps at first, then increase them gradually as the number of iterations per time stepstarts to drop.

 May 5, 2004, 09:11 Re: buoyancy driven flow + unsteady #4 Evan Rosenbaum Guest   Posts: n/a 1. Increase your step size. 2. The steady-state initial condition gives you well developed flow and temperature gradients. This is a stable starting point. Starting from an initialized solution should work as well, but your time step might have to be really small. You'll probably also have to reduce the underrelaxation on momentum as well. 3. All kind of density formulations will work. We have used ideal gas, Boussinesq, and density versus temperature. 4. No species transport here. 5. Most codes struggle with buoyancy flows. The driving forces are so small that the even small errors can significantly affect the numeric solution. We have had lots of difficulties when trying to do buoyancy driven systems with both liquid and gas circulations, some of which never worked and had to be abandoned.

 May 6, 2004, 11:37 Re: buoyancy driven flow + unsteady #5 co2 Guest   Posts: n/a Many thanks for your post. That certainly helps. One of my concerns is meshing -- I am kind of sure that my mesh is not the best and that is part of the problem (perhaps a big part! ) see, the top part of my 2D axisymmetric model is conical frustum like (there is a vent at the top which is pressure outlet) -- so you can imagine it is hard to fit a quad mesh there. So I use pave scheme there -- Any thought on a better meshing style there? evan, what under relax would you suggest for pressure and momentum ? i have heard that they need to add up to 1.

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