# Transient problem!

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 February 7, 2005, 06:10 Transient problem! #1 Jamesdclimber Guest   Posts: n/a Dear anyone who can help, I am currently using cfx 5.7, and modelling laminar flow in a 2D domain over a bluff body. I run the simulation at a timestep of 0.05 seconds for about 50 seconds real time. The solution is quite poor but however the velocity and pressure profile looks OK. Using the same setup as above, Here's the problem when I decrease the timestep to 0.001 sec. the solution becomes weird, at about 10 sec of real time the solution is fine, but after that I seem to get massive recirculation at the upward stream of the bluff body and near the inlet and also at the outlet. Looking at the pressure profile, the pressure is sectioned into rectanglar shapes of the domain and goes from a positive pressure to a negative or vice versa, or sometimes negative pressure throughout. Please could someone help me on this problem, is it my setup, global initialisation etc? James

 February 7, 2005, 12:49 Re: Transient problem! #2 Robin Guest   Posts: n/a James, How did you specify your inlet and outlet boundary conditions. What fluid definition did you use? -Robin

 February 8, 2005, 03:52 Re: Transient problem! #3 Jamesd69climber Guest   Posts: n/a Hi Robin, Inlet conditions; 8 m/s, normal to the boundary, Outlet; Static pressure, relative 0 Pa, Ideal Gas @ temperature 293 K; isothermal. Fluid domain reference pressure: 101325 Pa James

 February 8, 2005, 09:06 Re: Transient problem! #4 Robin Guest   Posts: n/a James, How long is your domain and your bluff body? -Robin

 February 8, 2005, 09:10 Re: Transient problem! #5 Jamesd69climber Guest   Posts: n/a Robin, Domain is 250 m Bluff body 31 m James

 February 8, 2005, 14:06 Re: Transient problem! #7 Jamesd69climber Guest   Posts: n/a Robin, Thankyou for your response, I will let you know how I get on! Regards James

 February 10, 2005, 07:03 Re: Transient problem! #8 Akin Guest   Posts: n/a Robin, Is there a rule of tumb in terms of time steps for RANS models ? like using the SST, if a domain is 1m long and the fluid is 1m/s.

 February 11, 2005, 15:43 Re: Transient problem! #9 Robin Guest   Posts: n/a Do you mean URANS (ie transient)? For transient simulations, the timestep will depend on how rapidly the solution is changing. Generally, if your timestep is small enough, you can converge the linear solution within 3 coefficient loops. For a steady state simulation you should use as big a timestep as you can get away with. Don't be shy. A large timestep will get you through the startup transients quickly. If there are sharp variations in the residuals later, you can reduce the timestep to drive the residuals in. A good check of the timestep is the write a backup file and create streamlines from an inlet colored by time. A big timestep would be 10x the lenght averaged time on the streamlines, a small timestep would be 1/10th of the average time. Also have a look at the tech tip on the CFX Community Site titled "Monitoring and Improving Convergence" (http://www-waterloo.ansys.com/cfxcom...onvergence.htm). Regards, Robin