Displaying Acceleration Parameter

bash0r · July 9, 2011, 05:22

Hey,
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

Quote:

Originally Posted by ghorrocks

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
Far

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

: velocity at boundary layer edge

: for 2D: arclength along the airfoil

can be calculated from the surface pressure distribution by assuming constant pressure normal to the wall inside the boundary layer (boundary layer theory,

) 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.

July 9, 2011, 05:22	Displaying Acceleration Parameter	#1
bash0r New Member Join Date: Apr 2011 Posts: 12 Rep Power: 15	Hey, 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!

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July 10, 2011, 06:42		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	What do all the variables represent?

July 10, 2011, 19:00		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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?

July 17, 2011, 10:58		#5
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	This is definition by volino for the transitional boudary layer (paper of journal of turbomachinery 2002 or 2003)

July 17, 2011, 19:43		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I see. But still, isn't the dUinf/ds term zero, so it always evaluates to zero?

July 18, 2011, 06:09		#7
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	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 Far

July 18, 2011, 07:01		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	What boundary layer streamwise velocity? How far out from the wall?

July 28, 2011, 06:16		#9
Captain Crossflow New Member Join Date: Jul 2011 Posts: 3 Rep Power: 14	The acceleration parameter is: $K = \frac{\nu_e}{U_e^2} \frac{dU_e}{ds}$ $\nu_e$ : viscosity at boundary layer edge : velocity at boundary layer edge : for 2D: arclength along the airfoil can be calculated from the surface pressure distribution by assuming constant pressure normal to the wall inside the boundary layer (boundary layer theory, ) 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.

July 28, 2011, 08:21		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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.