CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   Main CFD Forum (http://www.cfd-online.com/Forums/main/)

 Tom July 13, 1999 14:21

Hello, I try for the first time an unsteady simulation of fluid flow. Now I do not know how large a typical time-step is. My volume has a length of about 300 mm and the typical velocity is about 15 m/s. My calculations are done with fluent, seperated, implicit. I tried a time-step of 0.001s. The solution converged (residuals smaller than 0.001) in one or two iterations each time-step. 1.) Is the time-step too small, or do I have to reduce my convergence-crit. ? I feel, but I don't know, that the convergence-criterion (residuals) from a steady flow cannot taken for an unsteady one. 2.)What influence has the length of the time step to the result at time t ? I heard, that you get different forces at the same time on your monitors (middled wall-shear-stress) with different time-steps. Thanks a lot

 Sung-Eun Kim July 13, 1999 22:29

Dear client,

An idea of appropriate time step can be obtained if you have a rough estimate of the characteristric time scale of the unsteady phenomena you want to resolve. For example, when you try to simulate vortex-shedding around a circular cyliner, it helps a lot to know in advance approximate shedding frequency. Typically, you need at least several scores of time steps to cover one period or chracteristic time scale, although you may be able to use less number of time steps if you use high-order temporal scheme. Having said this, I mus add that it only gives you, at best, a rough estimate of time increment. To ensure that you properly resolve the characteristic time scale of the flow in question, you need to investigate the effects of time step size by trying several different time steps.

 John C. Chien July 13, 1999 23:37

(1).It sounded like that you were driving a car without a license! (2). On a straight highway, you may be able to hold the position of the steering wheel steady for a few seconds and still be able to stay in the same lane. This is more or less like the running the steady state code. (3). If you are driving down-hill on a winding mountain road, I am sure that you will disappear into the deep valley. To stay alive, you will have to move the steering wheel constantly. That rate of moving the steering wheel depends on the road conditions. (4). So, to be on the sure side, you should move the steering wheel all the time for any road conditions. I can't teach you how to drive, but that is just the principle. (5). I remember that was exactly how I drove when taking the driving lessons many years ago. Later on, when I became familiar with the road conditions, I was able to relax and move the steering wheel only when necessary. (6). You don't try to run a transient code just to get the converged steady state solution. To get accurate transient solution, make the time step as small as possible to avoid running into a tree.

 Tom July 14, 1999 01:26