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Old   August 4, 2012, 13:55
Default calculating pressure coefficient on airfoil surface
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Hi every one I am modeling an airfoil in an incompressible low Re flow! I set pressure in my outlet boundary condition as 1. I also set velocity in x and y direction at inlet. my question is when I solve the flow for example for a 10 degree angle of attack and when the flow reach a steady state condition, Are all the far field boundaries and inlet boundary supposed to have a pressure as same as the outlet boundary(it means 1)? if I want to calculate pressure coefficient, am I allowed to use P_out=1 instead of P_infinity or not? otherwise which pressure must be used for P_infinity for calculating pressure coefficient on the airfoil surface? Thanks for any word in advance!
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Old   August 4, 2012, 21:07
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I recommend you to read "Computational Fluid Dynamics Principles and Applications" by J Blazek. in chapter 8, I think you can find answer of your question.
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Old   August 5, 2012, 01:20
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I checked this book, in chapter 8 of this book the method which must be used to set the boundary condition at inlet or outlet for subsonic or supersonic flow is explained! it is not my question?!!! I don't have any problem with setting the boundary condition, my problem is that when I solve the flow over airfoil, and the solution is steady, can I expect the boundaries to have the same pressure as that which is set in outlet or not? if not, why this is so? because in most of the books and papers P_infinity which is needed to calculate Cp is set from upstream, it means inlet pressure! Is inlet pressure different from that of outlet?
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Old   August 5, 2012, 03:55
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Why do you care so much about absolute value of the pressure coefficient? Adding a constant (shift) to the pressure coefficient does not change total forces/ moments.

When your computation domain is infinite, the inlet and outlet pressures are the same. For finite size domains the shift in the pressure coefficient is inverse proportional to the domain size, (or the domain size squared, I am not sure).
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Old   August 5, 2012, 06:43
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Quote:
Originally Posted by truffaldino View Post
Why do you care so much about absolute value of the pressure coefficient? Adding a constant (shift) to the pressure coefficient does not change total forces/ moments.

When your computation domain is infinite, the inlet and outlet pressures are the same. For finite size domains the shift in the pressure coefficient is inverse proportional to the domain size, (or the domain size squared, I am not sure).
I know that this shift doesn't have any effect on lift or drag, but I am comparing my pressure coefficient results. So a shift matters here. My problem is that, because the flow is viscous and low Re(viscosity is important) we have loss in the flow, why the pressure in inlet and outlet is the same? Well, you mean that if for example my domain is 20c farther, I must consider a shift in pressure coefficient which is inversely proportional to domain size?!
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Old   August 5, 2012, 07:38
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Quote:
Originally Posted by mahzad View Post
My problem is that, because the flow is viscous and low Re(viscosity is important) we have loss in the flow, why the pressure in inlet and outlet is the same?
Take as an example viscous flow past flat plate: the flow is viscous but the pressure is constant far away from the plate (or simply constant everywhere for boundary layer approximation/equations).

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Well, you mean that if for example my domain is 20c farther, I must consider a shift in pressure coefficient which is inversely proportional to domain size?!
Yes, and for 20c domain it could be quite negligible. To have an idea about the shift value you could compare values of inlet and outlet pressure in your simulations, i.e. their difference divider by rho v^2/2
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Old   August 5, 2012, 08:02
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Originally Posted by truffaldino View Post
To have an idea about the shift value you could compare values of inlet and outlet pressure in your simulations, i.e. their difference divider by rho v^2/2
So you mean that, in case of any differences I can add (p_out-p_in)/(rho*V^2/2) to the Cp values over the airfoil surface?
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Old   August 5, 2012, 09:22
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Quote:
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So you mean that, in case of any differences I can add (p_out-p_in)/(rho*V^2/2) to the Cp values over the airfoil surface?
No, this will give you an idea of value of relative error if you take p_out as p_infinity
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Old   August 5, 2012, 09:46
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Originally Posted by truffaldino View Post
No, this will give you an idea of value of relative error if you take p_out as p_infinity
ok thanks, another question is that when my solution reach a steady state condition, the pressure is not exactly 1 at inlet, it's for example 1.12 and also it is not 1.12 at every node on inlet boundary, it decreases and reach to 1 as the nodes get farther from the node which has a pressure of 1.12 . If I want to use inlet pressure as P_infinity for calculating Cp,can I average the pressure at inlet nodes? Is it correct? or it is better to use a greater domain, the problem with the use of greater domain is that it increases my grid size and the amount of calculation needed!
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Old   August 5, 2012, 10:49
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It is not a good idea to average pressure at inlet, the error is rather determined by the maximal difference. Try to increase domain (it does not cost much computational time if you are using unstructured grids on outer part of the grid with spacing increacing towards the outer boundaries)
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Old   August 5, 2012, 10:56
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Originally Posted by truffaldino View Post
It is not a good idea to average pressure at inlet, the error is rather determined by the maximal difference. Try to increase domain (it does not cost much computational time if you are using unstructured grids on outer part of the grid with spacing increacing towards the outer boundaries)
My grid is completely structured, and so it would take me a long time specially for unsteady modeling.Anyway thanks, I'm deeply grateful to you for your help dear! your information was really helpful to me!
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Old   August 5, 2012, 20:30
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Hi;
One of my problem is in this area. in Fact, I want to apply constant pressure as a outlet boundary condition in structured grid with collocated arrangement. I used dummy cell to do it, but I can not find out how ap should be calculated? (ap=L/ape; ape=(ap+apE)/2) ap is coefficient of main cell and apE is coefficient of neighbor cell (there is dummy (guest) cell ), my problem is calculating apE, because ap=
Σ an and in guest cell, we don't have an, or I don't know how to calculate it.

Thanks in advance

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Old   August 6, 2012, 03:05
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Hi Mahzad, if your problem is how to calculate the pressure coefficient over the airfoil, why don't you use the inlet velocity as the reference value? Then you'll come up with something like this:

C_p=\frac{p}{0.5\cdot\rho_{\infty}\cdot V_{\infty}^2}

where p is the pressure over the airfoil, and \rho_{\infty} and V_{\infty} are the density and velocity imposed far upstream (which you know becase you have set them).

What do you guys think?
Cheers.
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Old   August 6, 2012, 03:36
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Quote:
Originally Posted by michujo View Post
Hi Mahzad, if your problem is how to calculate the pressure coefficient over the airfoil, why don't you use the inlet velocity as the reference value? Then you'll come up with something like this:

C_p=\frac{p}{0.5\cdot\rho_{\infty}\cdot V_{\infty}^2}

where p is the pressure over the airfoil, and \rho_{\infty} and V_{\infty} are the density and velocity imposed far upstream (which you know becase you have set them).

What do you guys think?
Cheers.
Hi, my problem was not with the calculation of Cp, I just didn't know is it correct to use P_out instead of p_infinity for calculation of Cp or not. because in books P_infinity is referred to P_inlet. Cp=(p-p_infinity)/q which q=0.5rhov^2/2 , I use inlet velocity in calculation of q, but my problem was with the P_infinity! Now I found that when the flow reaches a steady state condition, the pressure on all boundaries must be the same. Because we assume that it is located at infinity. If not, it is better to use a greater domain, in order to reach to this condition!
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