大涡模拟计算策略

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12.16.5 LES-Specific Solution Strategies


Large eddy simulation involves running a transient solution from some initial condition, on an appropriately fine mesh, using an appropriate time step size. The solution must be run long enough to become independent of the initial condition and to enable the statistics of the flow field to be determined.


The following are suggestions to follow when running a large eddy simulation:

1.   Start by running a steady state flow simulation using a Reynolds-averaged turbulence model such as standard K- E, K- W, or even RSM. Run until the flow field is reasonably converged and then use the solve/initialize/ init-instantaneous-vel text command to generate the instantaneous velocity field out of the steady-state RANS results. This command must be executed before LES is enabled. This option is available for all RANS-based models and it will create a much more realistic initial field for the LES run. Additionally, it will help in reducing the time needed for the LES simulation to reach a statistically stable mode. This step is optional.
2.   When you enable LES, ANSYS FLUENT will automatically turn on the unsteady solver option and choose the second-order implicit formulation. You will need to set the appropriate time step size and all the needed solution parameters. (See Section  26.12.1 for guidelines on setting solution parameters for transient calculations in general.) The bounded central-differencing spatial discretization scheme will be automatically enabled for momentum equations. Both the bounded central-differencing and pure central-differencing schemes are available for all equations when running LES simulations.
3.   Run LES until the flow becomes statistically steady. The best way to see if the flow is fully developed and statistically steady is to monitor forces and solution variables (e.g., velocity components or pressure) at selected locations in the flow.

4.   Zero out the initial statistics using the solve/initialize/init-flow-statistics text command. Before you restart the solution, enable Data Sampling for Time Statistics in the Run Calculation task page, as described in Section  26.12.1. With this option enabled, ANSYS FLUENT will gather data for time statistics while performing a large eddy simulation. You can set the Sampling Interval such that Data Sampling for Time Statistics can be performed at the specified frequency. When Data Sampling for Time Statistics is enabled, the statistics collected at each sampling interval can be postprocessed and you can then view both the mean and the root-mean-square (RMS) values in ANSYS FLUENT.
5.   Continue until you get statistically stable data. The duration of the simulation can be determined beforehand by estimating the mean flow residence time in the solution domain ( L/U, where L is the characteristic length of the solution domain and U is a characteristic mean flow velocity). The simulation should be run for at least a few mean flow residence times. Instructions for setting the solution parameters for LES are provided below.

这是帮助里面的LES的求解策略。我现在对这个过程还是有些疑问，希望跟大家讨论一下。
1、我理解的计算过程，
第一种，直接计算第二步，不计算稳态。  
第二种，按第一步中说的先计算稳态，然后加命令solve/initialize/ init-instantaneous-vel，再计算瞬态，计算过程检测进出口的流量。  
瞬态计算一段后，比如接近一个流量水平后，如果有些波动，再加命令solve/initialize/init-flow-statistics ，再接着计算瞬态，重复加第                            第二 个命令，计算瞬态的过程，一直到检测的物理量比较稳定。
2、我的疑惑：  
第一个问题：这两个命令是什么作用，看命令大概是第一个是初始化速度的意思，第二个是，将统计变量置零。如果是将统计变量置零的话，对计算有什么意义。
第二个问题： Continue until you get statistically stable data，这句话是指怎么个过程，是我理解的那种吗？还是有其他的理解？  
其实就是每一个过程在什么样的情况下结束进行下一个过程。比如说计算瞬态到什么程度，才能加第二个命令，加完第二个命令是再算瞬态，还是怎么办？