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Old   September 27, 2005, 02:04
Default About physics
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DAK565656
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Is it correct to describe airfoil, streamed with air at 8 degrees of attack, with steady state model? Stall begins at approx 15 deg. Why Do I ask? Because I have assumption, that there are vortexes, which exists during separation and following reattachment. These vortexes may dissipate to surrounding area causing transiency
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Old   September 28, 2005, 16:55
Default Re: About physics
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Mani
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You are right to be critical of a "steady state" model. If it can be considered "correct" or not will depend on what exactly you are looking for (lift, drag ...), how significant the unsteady effects are, and what your accuracy requirements are. In other words: It may not be correct in the sense of getting the best possible solution, but it might still be good enough for your purposes.
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Old   September 30, 2005, 03:52
Default Re: About physics
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DAK565656
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It's not good enough for drag calculation. Mani, as you remember I had such problem and still have it. I've analyzed pressure and discovered strange increasing of X-component force (drag) value on 40-50% of chord. I believe this increasing contributes to overstated Cdrag. This can be connected with laminar bubble, which in my case, taking into account geometry of profile, begins at 40-60% of chord (for certain angle of attack). Now I try to solve this problem. As one solution I proposed that stady state model isn't correct. As another solution, given turbulent models are not correct in laminar bubble region. What can you say about that?
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Old   September 30, 2005, 15:03
Default Re: About physics
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Mani
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Yes, I remember. In principle, it is possible that the separation is unsteady, and the experimental drag is really a time averaged value. Ideally, this should be documented in the experiment report. Did you compare your pressure distribution with the experimental one? Before you try to treat the separation region more accurately, you should make sure that separation is really expected. Most likely, if you find a large discrepancy between your drag and the experimental drag, it will be due to the extent and location of the separation, not so much because of unsteadiness or inaccurate shear within the separation region. I can only repeat that the most important thing here is to closely examine the experimental data (pressure distribution), and compare it with your results.
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Old   October 4, 2005, 05:10
Default Re: About physics
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DAK565656
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I've exaimined pressure distribution. Everything fine except one thing. If I use contour plot, I see good distribution. If I use 2D plot (Pressure on surface depending on X coord.) I see a little "explosion" of plot line. It's just stops, then 2-3% of chord length no line at all, and then it continues as nothing happened (without jumps). ForceX parameter (which affects on drag) is indefinitely too high in this "explosion region". By the way, this region is located on top surface of airfoil in 30% of chord (location of laminar bubble as I guess)
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Old   October 4, 2005, 15:46
Default Re: About physics
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Mani
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Ok, so it looks like you have either a serious numerical problem, or a post-processing problem. If it's a numerical problem, you need to quit trying to match the "physics", because it's obviously not working. You need to find out why the solution blows up (does the code even converge?). That's assuming your post-processing is not causing this problem. However, because your problem zone is confined to such small region and doesn't seem to affect the rest, it may well be a post-processing mistake.

Apart from that bad location on the suction surface, how does the rest compare to the experimental pressure distribution?
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Old   October 6, 2005, 01:35
Default Re: About physics
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DAK565656
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The rest compares to experimental pressure distribution is quite well. Maybe I don't understand something, let me explain this situation detailed. I'm using CFX 5.7.1 1) Analyzing pressure distribution (PD) on CONTOUR plot I see plausible PD over all airfoil surface. No sudden jumps or collapses 2) If I analyze 2D plot where pressure on Y axis and airfoil master cross-section (X direction of airfoil) on X axis, I see the situation, mentioned above. 3) Analyzing ForceX parameter on CONTOUR plot I see unexpected increasing along top surface at 30-40% of chord, then decreasing and then again increasing (70-80%) and then decreasing (pushing to the trailing edge).

The more angle of attack the bigger this zone on high ForceX. That is why I suggested that it's a laminar bubble, where transition takes place with following reattachment. But what about the second increasing at 70-80% of chord? I have no idea. I carry out calculation at angle of attack of 8 degree and obtain results of Cdrag about 0.04 (even with no-slip condition around airfoil). Calculating at 0 degree I obtain about 0.009 which is good, but this problem region IS still on the surface.

Code converges very well (about 40 CFX iterations). What is post-processing mistake? And how to handle with this? I believe, that if there is no high ForceX zone on the airfoil surface, I would obtain good Cdrag
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