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20053762

铁虫 (小有名气)


[交流] 非稳态计算

我是用非稳态来计算的,不知道时间步的设置有什么经验没,以及每个时间步收敛怎么回事(第一个就要收敛吗)?
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kxglp

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小木虫(金币+0.5):给个红包,谢谢回帖
臭水沟(金币+1): 谢谢应助~~ 2011-06-25 10:48:47
在计算非稳态流体时:

1.前面的时间步长最好能够取小一点,比如说你想用1e-3进行后面的计算,前面的步长可以取1e-6计算20步左右,给流场一个很好的初始条件

2.另外,如果计算时间足够长,前面的几步不收敛,而后面的所有步都收敛,应该也不会影响最后的计算结果
6楼2011-06-23 09:37:48
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seaharrier

铁杆木虫 (知名作家)


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20053762(金币+1):谢谢参与
臭水沟(金币+1): 谢谢应助~~ 2011-06-20 14:32:40
你做了软件自带的非定常例子没有,可以找找感觉
2楼2011-06-20 11:24:47
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20053762

铁虫 (小有名气)


臭水沟: 请使用“引用回复”~~ 2011-06-22 12:08:31
觉得那些太浅了,走了的,遇到的情况不同,不好比较
3楼2011-06-22 11:12:43
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kxglp

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臭水沟(金币+2): 谢谢应助~~ 2011-06-22 16:05:39
这是fluent官方材料关于implicit solver time step 的介绍

1.        Since the ANSYS FLUENT formulation is fully implicit, there is no stability criterion that needs to be met in determining  t. However, to model transient phenomena properly, it is necessary to set  t at least one order of magnitude smaller than the smallest time constant in the system being modeled. A good way to judge the choice of  t is to observe the number of iterations ANSYS FLUENT needs to converge at each time step.

2.        The ideal number of iterations per time step is 5– 10. If ANSYS FLUENT needs substantially more, the time step is too large. If ANSYS FLUENT needs only a few iterations per time step,  t should be increased. Frequently a time-dependent problem has a very fast “startup” transient that decays rapidly. Therefore, it is often wise to choose a conservatively small  t for the first 5–10 time steps.  

3.        t may then be gradually increased as the calculation proceeds.
4.        For time-periodic calculations, you should choose the time step based on the time scale of the periodicity. For a rotor/stator model, for example, you might want 20 time steps between each blade passing. For vortex shedding, you might want 20 steps per period.

5.        To verify that your choice for  t was proper after the calculation is complete, you can plot contours of the Courant number within the domain. To do so, select Velocity... and Cell Courant Number from the Contours of drop-down lists in the Contours dialog box.

6.        For a stable, efficient calculation, the Courant number should not exceed a value of 20–40 in most sensitive transient regions of the domain
4楼2011-06-22 14:16:29
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