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-   -   Gamma Theta model and underpredicted lift (https://www.cfd-online.com/Forums/cfx/89648-gamma-theta-model-underpredicted-lift.html)

Martin Hegedus July 20, 2011 14:36

Quote:

Originally Posted by Nick R (Post 316695)
Reynolds is 122000. Span/chord = 8 no trips were used...no two bumps is observed in lift diagram ...force measurements and pressure measurements were both done and in good agreement in terms of lift coefficient curve

I'm not sure of the various turbulence models that CFX has, but is there one which provides a dial so that you can tailor it to your case? For example, and this is complete speculation on my part, one of the SA models out in literature has a trip term. One might be able to use such a term to trip the flow at a certain location. Of course that location would be a function of Reynolds number.

Edit: Just wanted to add that the trip term, as far as I understand, does not trip the flow in the sense of rapidly ramping up the eddy viscosity in the region the trip is set.

Josh July 20, 2011 15:36

Quote:

Originally Posted by Martin Hegedus (Post 316825)
Does the statement immediately above refer to the results presented in the paper? The statements don't seem to match. The first two give the sense of all around thumbs up and the last gives a sense of "so-so.".

Sorry. I did word that rather confusingly. Our under-prediction of lift occurred for the SD7003, not the NACA 0012. The NACA 0012 results in the above paper were quite good compared to certain published experimental and DNS data. I specify "certain" because there are large discrepancies between high-fidelity experimental and DNS studies despite having nominally identical setups. As evidenced by this topic, proper prediction of aerodynamic performance criteria, among other parameters, is difficult to achieve in this regime. Experimentally, wind tunnel effects, such as noise and vibration, can cause premature transition, delayed separation, etc. Experimental measurement tools like Pitot tubes and hot wires also cause problems. Numerically, nominally identical DNS setups can produce different results due to discretization errors, averaging techniques, etc. etc. etc. Therefore, slight underestimation of lift by a URANS model for this regime can still be considered as good agreement.

Martin Hegedus July 20, 2011 20:02

Quote:

Originally Posted by Josh (Post 316828)
Sorry. I did word that rather confusingly. Our under-prediction of lift occurred for the SD7003, not the NACA 0012. The NACA 0012 results in the above paper were quite good compared to certain published experimental and DNS data. I specify "certain" because there are large discrepancies between high-fidelity experimental and DNS studies despite having nominally identical setups. As evidenced by this topic, proper prediction of aerodynamic performance criteria, among other parameters, is difficult to achieve in this regime. Experimentally, wind tunnel effects, such as noise and vibration, can cause premature transition, delayed separation, etc. Experimental measurement tools like Pitot tubes and hot wires also cause problems. Numerically, nominally identical DNS setups can produce different results due to discretization errors, averaging techniques, etc. etc. etc. Therefore, slight underestimation of lift by a URANS model for this regime can still be considered as good agreement.

Thanks for the clarification. Do you have a hypothesis of why the SD7003 results did not fair as well as the 0012?

Do you think it is atributed to the wind tunnel test, airfoil geometry, or something else? When I say geometry, I mean that the geometry causes the separation bubble and turbulence to behave differently than the 0012. I'm curious if this is related to the SD7003 being a thinner airfoil. I guess that at some point there may be two separation bubbles, one up front and one in back.

ghorrocks July 20, 2011 20:28

Quote:

I'm not sure of the various turbulence models that CFX has
I am no expert on airfoil modelling but for a thick-ish foil at Re=122000 that sounds like a case when the SST model with turbulence transition model is ideally suited.

Josh July 21, 2011 03:26

Quote:

Originally Posted by Martin Hegedus (Post 316843)
Do you have a hypothesis of why the SD7003 results did not fair as well as the 0012? Do you think it is atributed to the wind tunnel test, airfoil geometry, or something else? When I say geometry, I mean that the geometry causes the separation bubble and turbulence to behave differently than the 0012. I'm curious if this is related to the SD7003 being a thinner airfoil. I guess that at some point there may be two separation bubbles, one up front and one in back.

We haven't looked too in-depth as to this reasoning, but certain results have led us to propose certain hypotheses. The pressure and friction curve magnitudes of the SD7003 match published DNS and LES data very well, though the reattachment points are too far downstream. Similarly, the velocity profiles are excellently matched in terms of shape, though the separated flow and reversal regions are too thick downstream of transition. These trends seem to indicate proper resolution of the separation and transition onset locations but mediocre transitional flow and reattachment regions. Increasing the turbulence intensity to near-bypass levels (~ 1%) drastically improves the results compared to data with nominally acquiescent conditions. It seems as though the empirical correlations defining transition length and reattachment, e.g., the Re_theta correlation (which is a function of turbulence intensity), require calibration for this regime. The NACA 0012 results, as well, also showed delayed reattachment for most cases.

The exact reason for the SD7003 results being poorer is unknown. However, we have some ideas. For one, the SD7003 seems to have a much more physically complex flow regime at the Reynolds numbers and angles of attack of interest. Even high-fidelity experiments with nominally identical setups show varying physical phenomena, e.g., one study indicated vortex pairing while another witnessed no vortex interactions. The LSB on the SD7003 is also long, thin, and highly unsteady, making it quite difficult to capture either experimentally or numerically.

Other errors abound. We used the same grid resolution, timestepping scheme, etc. on the SD7003 that we used on the NACA 0012, so it's possible that the modelling was insufficient.

Quote:

Originally Posted by ghorrocks (Post 316847)
I am no expert on airfoil modelling but for a thick-ish foil at Re=122000 that sounds like a case when the SST model with turbulence transition model is ideally suited.

Agreed! This may allude to the mystery behind our superior NACA 0012 results over our SD7003 results.

Nick R July 21, 2011 05:05

Thanks for the paper and the comments. It must have been a lot of hard work. So from what I understand your NACA 0012 stalled past 8, is that why you didn't include the results from that point onwards?

Josh July 21, 2011 18:02

It stalled at about 8 degrees for the Re = 50k case with Tu = 0.25%. What should happen for this case, according to published DNS and LES results of Jones et al. and Almutairi et al., is continual bursting (complete separation) and re-formation (reattachment) of the laminar separation bubble. In other words, it's not fully stalled, but it's close. They also witnessed additional trailing edge separation.

For our case, we saw a laminar separation bubble form at the beginning of the simulation, which produced the proper lift and drag results. A few thousand timesteps later, the reattachment point of the bubble slowly moved toward the trailing edge and eventually merged with the trailing edge separation region, forming completely separated flow. This is good - this is supposed to happen, according to the DNS. Next, the bubble should re-form and move upstream toward the leading edge, but it doesn't. The flow remained fully stalled in our simulation, producing too-low lift.

The DNS and LES authors also experienced the above problem (bursting without re-forming) between 8 and 10 degrees when their domain was too thin (~0.2c, I believe). When they widened the domain (~0.5c), the proper behaviour (continual bursting and re-forming) was captured. So it seems that the shortcoming at Re = 50k, AoA = 8 deg. is caused by 2D modelling.

Our simulations did not stall at 8 deg. for the higher Re cases (100k and 250k).

The choice of angles of attack was fairly arbitrary. We chose 4 degrees because almost every published study seemed to include a 4 deg. angle of attack study. We weren't looking to capture stall. We just wanted to see how the model performed for moderate, i.e., practical angles of attack.

Nick R July 22, 2011 00:32

Sounds very interesting and quite complicated. Kudos to you! How did the unstalled naca0012 past 8 (100k) fair in terms of lift coefficient at higher angles compared to experiments?

Josh July 22, 2011 00:49

We only studied the 0, 4, and 8 degree angles. The agreement at 8 degrees, Re = 100k is excellent compared to the experiments at the same conditions (Fig. 21b in the paper).

For the record, we're currently reformatting the pictures to make them better in quality, so the figure quality will improve in the published paper. If you would like an updated copy, remind me in two weeks.

Nick R July 22, 2011 01:03

I see. Excellent. I'll contact you.


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