maintain a constant flow rate in channel flow ?
How to maintain a constant flow rate in the DNS of turbulent channel flow? I have taken a long time to look up the appropriate methods from papers, and found that there is two ways, one is to adjust the velocity field, the other one is to adjust the mean pressure gradient. However, the papers have not mentioned how to do it in detail. If you have the experience on this, please tell me the detailed method. Thanks very much.

Re: maintain a constant flow rate in channel flow
Hi,
In both methods you impose a linear pressure gradient (you don't adjust the velocity). This linear pressure gradient can be just a constant (You just add a constant to the momentum equation of the streamwise velocity component.In this case the bulk velocity will fluctuate a little bit around a mean value, but on average the pressure force should be balanced by the wall shear stress.), or you give the pressure gradient a certain value so that the bulk velocity is equal to some constant value (This means that the pressure gradient will fluctuate a bit). The first method is very simple to implement. With the second method you get a bit faster converged turbulent statistics. Tom 
references
Could somebody give references for a comparison between imposing constant flow rate or pressure gradient, please?
Thanks! 
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