Air foil lift coefficient validation

jsb12136 · September 17, 2015, 09:10

Hi guys,
I am doing a validation for lift and drag coefficient with NACA0012 air foil in different pitch angles. Chord length is 0.14m with a height of 0.005m. The foil is totally under water with a fluid speed of 3.8m/s. Time step is 0.001s. I got good Cl at first and it is stable, but there is a jumping of Cl at 0.7s. What is the problem of this situation?

fluid23 · September 17, 2015, 13:24

The first problem that I see is that you need to improve mesh resolution in the wake. What is going on downstream will affect what is happening upstream.

How are you initializing the solution? It could be that you aren't to a point where you should consider the transient solution. It is best to run a steady state case, let it converge completely and use that as your initialization for transient study.

For the transient solution, how many inner iterations are you performing? Is each time step converging?

I would also caution you that the NACA0012 isn't the best airfoil for this for 2 reasons. First, symmetric airfoils tend to be a little tricky for some reason. I have had issues in the past trying analyze them. Unless you must do 0012, pick something with a little camber. Second, there is considerable disagreement between published data sets for the 0012. I suggest making sure that you are comfortable with the data you are using for your comparison.

jsb12136 · September 18, 2015, 12:18

Hi, Matt
Thanks for your reply. I agree that downstream will influence the upstream so I increase the mesh cell numbers(tripled cell numbers). The time-step is 0.001s calculated based on the Strouhal number 0.2 and 50 iterations each time-step to make sure its converging. What do you mean by a steady state? I tried to use steady solver at first and then use that file as initialization for unsteady simulation. The results are improved nicely. Am I doing right? Would you please tell me the reason why?

fluid23 · September 18, 2015, 12:24

Basically you are just giving your solution a starting point that is closer to reality so that you don't have to wait for (simulation) time to pass before you start to see a properly developed flow field. If you start with 0 velocity everywhere, then even after 50 iterations your first time step is no where near correct. Then the second time step assumes that the end condition of the first step was correct and uses that as its starting point. This continues for a while until this initialization error has disappeared which is (I think) what you were seeing before. That's maybe not the most technical description of what is happening but should give you an idea of why you are doing it.

fluid23 · September 18, 2015, 12:26

When I said steady state, I mean use the steady state solver. Then you can clear results (history only) then change your physics continua over to transient and you will have a file that will start at iteration 1 of time step 0 with a flow field that is pretty close to accurate.

Maheboob · June 16, 2016, 11:27

I am having problem in opening a new simulation
as a new to star CCM+ i dont know how to start the simulation workbench
whenever i i click on create new simulation it shows"server process ended unexpectedly"
can anyone help in installing or to troubleshoot my problem..?

June 16, 2016, 11:27	installation problem	#6
Maheboob New Member Maheboob Join Date: Jun 2016 Posts: 1 Rep Power: 0	I am having problem in opening a new simulation as a new to star CCM+ i dont know how to start the simulation workbench whenever i i click on create new simulation it shows"server process ended unexpectedly" can anyone help in installing or to troubleshoot my problem..?

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September 17, 2015, 13:24		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	The first problem that I see is that you need to improve mesh resolution in the wake. What is going on downstream will affect what is happening upstream. How are you initializing the solution? It could be that you aren't to a point where you should consider the transient solution. It is best to run a steady state case, let it converge completely and use that as your initialization for transient study. For the transient solution, how many inner iterations are you performing? Is each time step converging? I would also caution you that the NACA0012 isn't the best airfoil for this for 2 reasons. First, symmetric airfoils tend to be a little tricky for some reason. I have had issues in the past trying analyze them. Unless you must do 0012, pick something with a little camber. Second, there is considerable disagreement between published data sets for the 0012. I suggest making sure that you are comfortable with the data you are using for your comparison.

September 18, 2015, 12:18		#3
jsb12136 New Member Xiaosong Zhang Join Date: Oct 2014 Posts: 14 Rep Power: 12	Hi, Matt Thanks for your reply. I agree that downstream will influence the upstream so I increase the mesh cell numbers(tripled cell numbers). The time-step is 0.001s calculated based on the Strouhal number 0.2 and 50 iterations each time-step to make sure its converging. What do you mean by a steady state? I tried to use steady solver at first and then use that file as initialization for unsteady simulation. The results are improved nicely. Am I doing right? Would you please tell me the reason why?

September 18, 2015, 12:24		#4
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	Basically you are just giving your solution a starting point that is closer to reality so that you don't have to wait for (simulation) time to pass before you start to see a properly developed flow field. If you start with 0 velocity everywhere, then even after 50 iterations your first time step is no where near correct. Then the second time step assumes that the end condition of the first step was correct and uses that as its starting point. This continues for a while until this initialization error has disappeared which is (I think) what you were seeing before. That's maybe not the most technical description of what is happening but should give you an idea of why you are doing it.

September 18, 2015, 12:26		#5
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	When I said steady state, I mean use the steady state solver. Then you can clear results (history only) then change your physics continua over to transient and you will have a file that will start at iteration 1 of time step 0 with a flow field that is pretty close to accurate.