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bash0r July 9, 2011 05:22

Displaying Acceleration Parameter
I'd like to display the acceleration parameter of my flow.
The definition of the acceleration parameter is K = \frac{\nu}{U_\infty^2} \cdot \frac{dU_\infty}{ds}.
How would you manage do display the acceleration parameter in CFD-Post?

thx in advance!

ghorrocks July 10, 2011 06:42

What do all the variables represent?

bash0r July 10, 2011 09:16


Originally Posted by ghorrocks (Post 315521)
What do all the variables represent?

\nu: kinematic viscosity
U_\infty: velocity of the free-flow
s: parameter along the geometry (in this case its an airfoil)

ghorrocks July 10, 2011 19:00

Sounds like a strange definition of acceleration. Isn't the kinematic vis and Uinf constant? Doesn't that mean dUinf/ds = 0, so your equation is always 0?

Far July 17, 2011 10:58

This is definition by volino for the transitional boudary layer (paper of journal of turbomachinery 2002 or 2003)

ghorrocks July 17, 2011 19:43

I see. But still, isn't the dUinf/ds term zero, so it always evaluates to zero?

Far July 18, 2011 06:09

yes you are right. this definition is little bit wrong.

The actual definition is :

The derivative does not contain the free stream velocity, rather it is the rate of change of boundary layer stream wise velocity at onset of transition w.r.t to the local distance (in this case it is the suction side length) and again in first term in denominator i.e the velocity is not the free stream, it is again the boundary layer streamwise velocity just at onset of transition.

Hope now it is clear. Now we are looking for your help. as I was also in search that how to plot the acceleration parameter in CFX.

Best Regards

ghorrocks July 18, 2011 07:01

What boundary layer streamwise velocity? How far out from the wall?

Captain Crossflow July 28, 2011 06:16

The acceleration parameter is: K = \frac{\nu_e}{U_e^2} \frac{dU_e}{ds}
\nu_e: viscosity at boundary layer edge
U_e: velocity at boundary layer edge
s: for 2D: arclength along the airfoil

U_e can be calculated from the surface pressure distribution by assuming constant pressure normal to the wall inside the boundary layer (boundary layer theory, dp/dz = 0) and applying the (compressible) Bernoulli equation.

\nu_e can be assumed constant, I think, or can be calculated again with Bernoulli and applying isentropic relations and the Sutherland law.

ghorrocks July 28, 2011 08:21

Thanks for explaining the equation more fully. I had no idea what Far was talking about.

I cannot see how you will calculate this in CFD-Post. In that case I would export the required variables and calculate it externally in excel or matlab. I assume you wish to plot this function over a foil surface? In that case export the temperature, velocity and any other field from the foil surface and do the calculation of the parameter externally.

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