Dear Vikram,
Thank you for the link.
But I think there is mistakes in the first relation there.
When we write the first equation, generally we consider the Newton's second law
applied to the fluid volume, not the solid.
Since conventionally \vec{n} points outwards this control volume, then actually
the pressure force applied on the surface of
fluid control volume by possible solid is minus P other than a positve one
,which then points inside the volume.
I think this is the basis of most CFD packages.
So in that link most sign of P terms should be changed except the last one,
because that is the pressure force exerted by fluid through the surface.
Zhenyu
-----原始邮件-----
发件人: "Vikram Garg" <[email protected]>
发送时间: 2015-04-28 05:39:05 (星期二)
收件人: Zhang <[email protected]>
抄送: libmesh-users <[email protected]>
主题: Re: [Libmesh-users] calculation of aerodynamic forces and moments
Hey Zhang,
Provided you have a no penetration boundary conditions for
the airfoil, the general expression for the fluid force on an airfoil can be
given as:
F^i_R = \int_{\partial R} (P\hat{n} + \nu \nabla v^i) \cdot \hat{n} (where
R is the airfoil domain, v^i is a unit vector in the i direction, google
"Strict general mathematical definition of drag" for the complete derivation)
Thanks.
On Mon, Apr 27, 2015 at 4:15 AM, Zhang <[email protected]> wrote:
Dear Vikram,
Well. Just now I recognized that as for the form pressure force you are right.
Since I want to get the lift and drag which are applied by the air flow onto
the solid airfoil,
the pressure form should be of positive. while in the original N-S equation
there is a negative one which means the force applied from solid to the gas, as
opposite.
In the same way, I think the contribution of viscosity onto the lift and drag
should also be negative, since in the original N-S form it is positive. Right?
-----原始邮件-----
发件人: "Vikram Garg" <[email protected]>
发送时间: 2015-04-27 16:32:10 (星期一)
收件人: Zhang <[email protected]>
抄送: libmesh-users <[email protected]>
主题: Re: [Libmesh-users] calculation of aerodynamic forces and moments
Hey Zhang,
Yes, the get_normals() function will return the outward
normal vectors.
You had mentioned that force on the airfoil was given by:
\vec{F}=\sum_{qpface_bound} {\tau*normal -normal*p_{qp}} *Jxw_face_bound[qp] .
Should that be a \tau*normal + normal*p_{qp}} ?
Thanks.
On Sun, Apr 26, 2015 at 11:07 PM, Zhang <[email protected]> wrote:
I just made some correction, say, since pressure is defined in a space one order
lower than velocity. In the last code I put all quadrature computation in
velocity space by mistake,
now I put the two integrals, $\int \tau * normal$ and $\int (-p)*normal$, into
two separate
sections and finally add their partial integral together.
Here are the comparison (for only pressure forces)
1) np=1 // the old one
step 9, forces: (-1.299297e-01 3.460514e-03 0.000000e+00 )
step 9, moments: (0.000000e+00 0.000000e+00 7.266930e-05 )
2) np=1 // the corrected one
step 9, forces: (-1.799026e-01 -3.255020e-06 0.000000e+00 )
step 9, moments: (0.000000e+00 0.000000e+00 -9.115135e-07 )
3) np=4 // the corrected one
step 9, forces: (-1.799026e-01 2.098503e-06 0.000000e+00 )
step 9, moments: (0.000000e+00 0.000000e+00 5.020619e-07 )
when AOA=0.0 degree, the lift and z-pitching moment are dramatically reduced to
zero.
Now the question is the negative x-force. In the face quadrature, does
fe_pres_face->get_normals()
return all outward normal vector?
Zhenyu
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Vikram Garg
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Predictive Engineering and Computational Science (PECOS)
The University of Texas at Austin
http://web.mit.edu/vikramvg/www/
http://vikramvgarg.wordpress.com/
http://www.runforindia.org/runners/vikramg
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_______________________________________________
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[email protected]
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--
Vikram Garg
Postdoctoral Associate
Predictive Engineering and Computational Science (PECOS)
The University of Texas at Austin
http://web.mit.edu/vikramvg/www/
http://vikramvgarg.wordpress.com/
http://www.runforindia.org/runners/vikramg
------------------------------------------------------------------------------
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Widest out-of-the-box monitoring support with 50+ applications
Performance metrics, stats and reports that give you Actionable Insights
Deep dive visibility with transaction tracing using APM Insight.
http://ad.doubleclick.net/ddm/clk/290420510;117567292;y
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