reynolds number for moving mesh

researcher · March 10, 2016, 12:53

hi everyone
can someone tell me, as to how to estimate the Re Number of a moving mesh problem. Since the Re number will determine whether the flow is laminar or turbulent, and therefore, whether to apply laminar NS equations or turbulent models etc, but the question is how to find it ? I dont think it should be based on the free stream velocity because the air next to the moving body should be moving faster than that !
pls help

agd · March 10, 2016, 14:17

What is the fluid velocity? As far as the Re number is concerned that is the velocity you want. As an example, consider flow over an airfoil. The airfoil is fixed and the fluid velocity is the speed of the fluid relative to the airfoil surface. Via a Galilean transformation the problem can also be solved by moving the airfoil through quiescent air. The fluid velocity used in the Re number calculation would still be the velocity of the fluid relative to the airfoil surface. Regardless of whether you use a moving mesh, there should be some physical velocity in your problem.

researcher · March 10, 2016, 14:25

Thank you agd for ur reply.
The flow field i am studying has a 1m/s inlet velocity, while the airfoil is oscillating at say 1 cps frequency.

agd · March 10, 2016, 16:18

So how would you compute the fluid velocity seen by an observer fixed to the airfoil?

researcher · March 10, 2016, 21:25

Well, it is changing continuously. Since it is moving sinusoidally, the velocity is a function of time. This is what confuses me. How to compute the reynolds number.

agd · March 11, 2016, 10:35

When you talk about the Re number remember that there is not a single Re number for a problem. If you are looking for a figure of merit to compare to other problems, you could use the mean velocity of the flow, or you could use the approach velocity (in this case the inlet velocity). Either of these would be reasonable values for computation of a y+ for grid generation. For comparison to experiment I would defer to how the quantity is defined in the experiment.

researcher · March 11, 2016, 18:49

Thank you for reply agd.
I figure one may make any velocity as reference and then go on refining until mesh independence is achieved. M i right?

March 10, 2016, 12:53	reynolds number for moving mesh	#1
researcher Member Shah Join Date: Aug 2013 Posts: 61 Rep Power: 12	hi everyone can someone tell me, as to how to estimate the Re Number of a moving mesh problem. Since the Re number will determine whether the flow is laminar or turbulent, and therefore, whether to apply laminar NS equations or turbulent models etc, but the question is how to find it ? I dont think it should be based on the free stream velocity because the air next to the moving body should be moving faster than that ! pls help

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March 10, 2016, 14:17		#2
agd Senior Member Join Date: Jul 2009 Posts: 351 Rep Power: 18	What is the fluid velocity? As far as the Re number is concerned that is the velocity you want. As an example, consider flow over an airfoil. The airfoil is fixed and the fluid velocity is the speed of the fluid relative to the airfoil surface. Via a Galilean transformation the problem can also be solved by moving the airfoil through quiescent air. The fluid velocity used in the Re number calculation would still be the velocity of the fluid relative to the airfoil surface. Regardless of whether you use a moving mesh, there should be some physical velocity in your problem.

March 10, 2016, 14:25		#3
researcher Member Shah Join Date: Aug 2013 Posts: 61 Rep Power: 12	Thank you agd for ur reply. The flow field i am studying has a 1m/s inlet velocity, while the airfoil is oscillating at say 1 cps frequency.

March 10, 2016, 16:18		#4
agd Senior Member Join Date: Jul 2009 Posts: 351 Rep Power: 18	So how would you compute the fluid velocity seen by an observer fixed to the airfoil?

March 10, 2016, 21:25		#5
researcher Member Shah Join Date: Aug 2013 Posts: 61 Rep Power: 12	Well, it is changing continuously. Since it is moving sinusoidally, the velocity is a function of time. This is what confuses me. How to compute the reynolds number.

March 11, 2016, 10:35		#6
agd Senior Member Join Date: Jul 2009 Posts: 351 Rep Power: 18	When you talk about the Re number remember that there is not a single Re number for a problem. If you are looking for a figure of merit to compare to other problems, you could use the mean velocity of the flow, or you could use the approach velocity (in this case the inlet velocity). Either of these would be reasonable values for computation of a y+ for grid generation. For comparison to experiment I would defer to how the quantity is defined in the experiment.

March 11, 2016, 18:49		#7
researcher Member Shah Join Date: Aug 2013 Posts: 61 Rep Power: 12	Thank you for reply agd. I figure one may make any velocity as reference and then go on refining until mesh independence is achieved. M i right?