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Pressure coefficient larger than 1

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Old   May 3, 2012, 10:31
Question Pressure coefficient larger than 1
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Zhe Lu
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Hi everyone:
I am simulating a problem with an airfoil flying above calm water surface. The angle of attack is 6 degrees and the clearance between the trailling edge and the calm water surface is 0.01 chord length(which is 10m). The freestream velocity is 30m/s. The flow is regarded as compressible and unsteady. The flow time now is about 2s, but I notice that the pressure coefficient on the pressure side of the airfoil is at some position slightly larger than 1, though the magnitude is gruadually decreasing. Is this possible, or is there something wrong with my case?
Thanks in advance
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Old   May 3, 2012, 10:39
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Originally Posted by albatross View Post
Is this possible, or is there something wrong with my case?
For incompressible flow, pressure coefficients of 1 corresponds to the stagnation pressure. For compressible flows, the pressure coefficient can be greater than 1. If you calculate the pressure coefficient and it turns out to be larger than 1, that is an indicator that the flow is compressible (but you already knew that).

Last edited by LuckyTran; May 3, 2012 at 11:00.
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Old   May 3, 2012, 12:23
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Thank you very much for your reply! One more question, for incompressible flow, can pressure coefficient be smaller than 1 at the stagnation point? If possible, where is the lost energy?
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Old   May 3, 2012, 12:34
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Quote:
Originally Posted by albatross View Post
Thank you very much for your reply! One more question, for incompressible flow, can pressure coefficient be smaller than 1 at the stagnation point? If possible, where is the lost energy?
For idealized flow with no losses (inviscid flows) the pressure coefficient at stagnation must be exactly 1. For real and practical flows, it will be slightly less owing to dissipation/viscous losses incurred before the stagnation point was reached. The effect of the moving airfoil is felt upstream of it, because the flow is subsonic and the disturbance can propagate upstream.
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Old   May 4, 2012, 06:39
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It's nice talking with you, LuckyTran! Thanks again!
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Old   May 17, 2014, 00:02
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Hi. I know its very old post but i want to ask if for incompressible flow, if pressure coefficient at stagnation point is much lower than 1 (0.7 in my case), will it effect results e.g. lift coefficient, drag coefficient etc.?
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Old   May 17, 2014, 04:26
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Hi, star.
I'm not sure whether your result is correct or not, for the Cp at the stagnation point is a bit too low. According to LuckyTran's opinion above, the Cp is generally only slightly lower than 1 at the stagnation point. Maybe you need to analyze the possible factors that can strongly dissipate the energy of the flow upstream of the stagnation point. If such factors don't exist, the results may be questionable, including Cl and Cd.
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Old   May 17, 2014, 05:43
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Thanks albatross for reply. I will be thankful if someone reply with surety because i did some work and it will be meaningless if Cp has effected my results. Plz help
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