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May 3, 2012, 10:31 
Pressure coefficient larger than 1

#1 
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Zhe Lu
Join Date: Apr 2012
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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 

May 3, 2012, 10:39 

#2 
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Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
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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. 

May 3, 2012, 12:23 

#3 
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Zhe Lu
Join Date: Apr 2012
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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?


May 3, 2012, 12:34 

#4 
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Lucky Tran
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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.


May 4, 2012, 06:39 

#5 
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Zhe Lu
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It's nice talking with you, LuckyTran! Thanks again!


May 17, 2014, 00:02 

#6 
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Harry
Join Date: May 2013
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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.?


May 17, 2014, 04:26 

#7 
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Zhe Lu
Join Date: Apr 2012
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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. 

May 17, 2014, 05:43 

#8 
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Harry
Join Date: May 2013
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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


March 18, 2016, 08:54 

#9 
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Goksu Soydan
Join Date: Dec 2015
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Hello everyone!!
I have a question about pressure coefficient. I am modeling the fluid flow around a horizontal cylinder in open channel. I have a trouble about pressure and pressure coeffiecnt. I couldnt get the value of Cp equal to 1. All the values are small than 1. The inlet velocity is 0.17 m/s and the water height is 45 cm in the inlet. for reference value of pressure ı gave 4406 Pa which is equal to (static pressure=gamaxh). Do ı have to give pressure for the outlet boundary conditions. Also we use transient simulation. While using VOF method we get larger pressure values than the transient simulation. Is it because of the density value?. 

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