New User of Xfoil: Adjusting Measured Cp for Data Match

[micheljoel200]

i'm a new user of xfoil. I make measurements (Cp) in wind tunnel. But my measured data don't match with the data of xfoil. I use xfoil like my reference.

Which correction can i bring to my measured Cp so that measured and simualation data match together?

Cp = pressure coefficient

thanks

MJ

 

[boneh3ad]

Why do you think that your measures Cp is wrong? It may be that xfoil is not perfect, after all, it is an inviscid panel code with various viscous corrections that can be implemented.

Otherwise it's hard to say much without knowing more about your experiments.

 

[micheljoel200]

some reports compare their measured data with simulation data, and they match very well. But before comparaison they brought some correction of their measured data. The problem is that they don't explain very well what they did.
That is why i'searching now for a very example where almost everything is explain. May be someone can help me here.

thx

 

[boneh3ad]

Again, the problem isn't necessarily experimental. It very well may be, but no one can help you unless you explain your experimental setup a little bit. For example, details of the wind tunnel, te model, your measurement techniques, etc.

 

[micheljoel200]

My wind tunnel has following measure: L= 123 cm, b=61cm, H=31cm.
in the attachment the blue curvature ist from xfoil and the red curvature is from my measurement. There is an offset between the both curvature, i don't know why. I read in book that to match them i need to correct my measured data. But i don't how i can do that.

Please help me.

thx

 

[RandomGuy88]

Assuming you have done nothing wrong with your experiment the correction you need to apply is due to tunnel wall effects. To correct the pressure coefficient you have to correct the dynamic pressure for blockage effects. When you place a model in a tunnel you reduce the effective area through which the air can flow and because mass must be conserved the air will speed up. A similar effect is caused by the wake of your model. You probably measure dynamic pressure upstream of your model so the dynamic pressure your model is experiencing is different from what you measure, therefore you have to correct for it. I will not go into how to do this because it would take to long but you should look at "Low Speed Wind Tunnel Testing" by Pope.

However after looking at your data I can tell you that your only problem is not the blockage effects I just explained, unless your model is really big relative to your tunnel in which case you shouldn't be using that model. The difference between your data and xfoil is too large for it to only be blockage effects. So you either did something wrong in your experiment or xfoil.