# Low Reynolds Lift&Drag Prediction Problems

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May 9, 2019, 11:33
Low Reynolds Lift&Drag Prediction Problems
#1
New Member

Daniel Barreiro Clemente
Join Date: Feb 2019
Location: Munich
Posts: 15
Rep Power: 7
Good day everyone,

I'm currently running simulations for low reynodls number on a 2D airfoil, SD7062, and validating the obtained data against the values from experimental wind tunnel measurements. The Reynolds numbers of interest are 6*10^4, 1*10^5 and 2*10^5.

I have run the cases for 1*10^5 and 2*10^5 at different angles of attack, from 0º to 10º. The results I have obtained are somehow acceptable, as the mean deviation for the LiftCoefficient is around 5-9% and the mean deviation for the DragCoefficient is around 5-18%.

However, using the same Fluent setup and the same ICEM mesh, problems appear when I begin to run simulations for the 6*10^4 Reynodls number. The lift and drag coefficient are overpredicted and underpredicted respectively by more than 35%, and I have no idea why.

The mesh, which can be seen in the attachments, has the following characterisitcs:
-Airfoil length = 1m
-Upstream domain size = 20 times the chords length
-Downstream domain size = 40 times the chords length
-Sides domain size = 15 times the chords length
-Y+ value is <1. The number of cells from the airfoil wall to the end of the domain is 250, with an expansion ratio from the first cell of 1.05. It's an structured, streamwised mesh.
-Number of cells: 240K
The reported mesh quality in ICEM is very high as well.

The Fluent setup is as follows:
-Turbulence model: Transition SST
-Inlet = velocity inlet with a defined speed corresponding to the desired Re number with respect to the airfoils chord
-Outlet = pressure outlet
-Coupled , Least Squares Cell Based ,Second order upwind, with reduced explicit relaxation factors to favour convergence
-Convergence criteria = 10^-6
-Hybrid Initialization

I do not understand why there's such a big discrepancy when the only thing that changes from one case to another is the inlet flow velocity.
Any help in solving this issue would be very much appreciated

As well, any tips on improving the drag coefficient prediciton would be very much appreaciated as well.

Thank you very much in advance.
Attached Images
 SD7062MESH.jpg (37.8 KB, 13 views) SD7062UPCLOSE.jpg (36.1 KB, 15 views)

 May 13, 2019, 23:25 #2 Senior Member   Behrooz Jamshidi Join Date: Apr 2013 Posts: 110 Rep Power: 13 change your turbulence model to k-omega SST and check your results.

 May 16, 2019, 02:41 #3 New Member   New South Wales Join Date: Feb 2016 Posts: 9 Rep Power: 10 I would first post-process the results between the two meshes and compare. If the lift and drag are so different, you should be able to see differences in your cp distribution for example, which may help identify regions where the meshes are behaving differently. If you go down the road of changing turbulence models maybe try: 1. For your low Reynolds number flows with the k-omega SST, there is a low reynolds number correction check box that may also help. From what i've read though, there is some confusion about upper limits of reynolds number you need to turn this on. Maybe someone else can help you with that... 2. Maybe even try the laminar model.