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July 20, 2011, 14:36 |
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#21 | |
Senior Member
Martin Hegedus
Join Date: Feb 2011
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Quote:
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. Last edited by Martin Hegedus; July 20, 2011 at 14:48. Reason: add more info |
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July 20, 2011, 15:36 |
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#22 |
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Joshua Counsil
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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.
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July 20, 2011, 20:02 |
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#23 | |
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Martin Hegedus
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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. |
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July 20, 2011, 20:28 |
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#24 | |
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Glenn Horrocks
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July 21, 2011, 03:26 |
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#25 | |
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Joshua Counsil
Join Date: Jul 2009
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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. Agreed! This may allude to the mystery behind our superior NACA 0012 results over our SD7003 results. |
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July 21, 2011, 05:05 |
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#26 |
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Nick
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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?
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July 21, 2011, 18:02 |
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#27 |
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Joshua Counsil
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Location: Halifax, Nova Scotia, Canada
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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. |
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July 22, 2011, 00:32 |
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#28 |
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Nick
Join Date: Nov 2010
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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?
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July 22, 2011, 00:49 |
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#29 |
Senior Member
Joshua Counsil
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Location: Halifax, Nova Scotia, Canada
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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. |
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July 22, 2011, 01:03 |
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#30 |
Senior Member
Nick
Join Date: Nov 2010
Posts: 126
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I see. Excellent. I'll contact you.
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