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 VVJ July 28, 2007 05:11

I hav two questions...

Hi friends... I have two questions..

1. What is Y+ plot?

2. Which contours will give us an idea of existance of turbulence and its level?? (in both internal and external flow problems)...

plz reply / also tel me some references... thank you..

 Gui July 28, 2007 16:38

Re: I hav two questions...

1) Y+ is the dimensionless wall distance :

y+=u'*y/(v) where u' is the friction velocity, y the distance to the wall and v the viscosity.

Thus a Y+ plot can give you the relation between the velocity and the distance to the wall in order to determine the turbulence level. The problem is that I don't exactly know what maximum value y+ should take depending on the problem and I'd really like more informations about that if anybody here knows.

2) I think the contours of the turbulent variables (k and epsilon, or omega, depending on wich turbulence model you used) should be helpfull. Though I'm not 100% sure it's possible cause I never did it (but I don't see why it wouldn't be possible)

 Robin July 30, 2007 09:40

Re: I hav two questions...

Hi VVJ,

A1: Y+ is the dimensionless distance from the wall to the first node (essentially grid Reynolds Number). It doesn't really tell you anything about the turbulence unless your first node height is constant. It is more often used to ensure the first node is sufficiently close to model the boundary layer. The actual recommended value varies by turbulence model and recommendations are given in the documentation (as well as in numerous other posts on this forum).

A2: Plotting the Turbulence Kinetic Energy (ke) will show regions where the turbulence is high; this is the kinetic energy assumed to be contained in the fluctuating component of velocity.

ke=1/2*u mean^2

where u mean is the mean fluctuating component of velocity. Thus the Mean Fluctuating Component of Velocity is given by:

u mean = sqrt(2*ke)

You can get a rough estimate of the turbulence intensity by taking the ratio of the mean fluctuating component to the mean velocity by creating a new variable in Post with the following expression (as %):

tu = sqrt(2*ke)/Velocity*100

This is typically a more meaningful quantity to look at than the turbulence kinetic energy on its own. If you plot tu, limit the range from 0 to 100 % as this is typically the meaningful range.

Another variable worth looking at is the ratio of Turbulent Eddy Viscosity (viscosity due to turbulent mixing) to the Dynamic Viscosity (viscosity due to molecular action). This can also be calculate easily in Post by creating a variable from the following expression:

Eddy Viscosity Ratio = Eddy Viscosity/Dynamic Viscosity

This ratio should be >100 throughout most of your model for the assumptions of the turbulence model to be correct.

For references I recommend reviewing the CFX theory documentation and following the references included there.

Regards, Robin

 VVJ July 31, 2007 00:03

Re: I hav two questions...

Thanks a lot guys... You hav spent ur valuable time in writing answers for the questions... I will get back to you soon...

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