CFD yplus 1

[alexohnh]

hi everybody
I'm a trainee in CFD and I'd like to have informations about yplus.
Any definitions would do.
Thanks.

 

[Baldy217]

There you go

http://www.cfd-online.com/Wiki/Dimensionless_wall_distance_(y_plus)

 

[btrueblood]

Baldy's link was close, here's a slightly better one:

http://www.cfd-online.com/Wiki/Dimensionless_wall_distance

 

[btrueblood]

Related terms: law of the wall, log layer, boundary layer.

 

[alexohnh]

thanks guys.
some at the desk say that you have to maintain the value of y+ between 30 and 150 to ensure the meshing is done properly. but when asking why they don't have a clue, do you?

 

[btrueblood]

There's more info. at the website posted. Short answer - it depends on what turbulence model you are running. You want to have the y+ at the first point off the wall to be in the sublayer, I believe, and to have "adequate resolution" thru the log layer, to properly resolve the boundary layer...

 

[spongebob007]

In terms of boundary layer theory, y+ is simply a local thickness Reynolds number. In terms of CFD y+ is a nondimensional distance from the wall to the first grid point. In a practical sense, we don't resolve the solutions of turbulent flow by direct numercial simulation. This would require a very fine mesh near the wall in order to resolve the turbulent eddies in the boundary layer. Also, turbulence is time varying and random, so EVERY CFD model would need to be run as transient, even if the mean flow is steady state. Modern computers can't handle this except for the simplest flow, and the most powerful computing available when CFD was developed couldn't even match what most people have on their home PC today. Maybe in the future it will be possible to solve the turbulent Navier Stokes equations by direct numercial simulation right down to the wall.

In order to deal with the temporal fluctuation of turbulence, we time average the governing equations. This is why you often hear CFD referred to as RANS (Reynolds-Averaged-Navier-Stokes) analysis. But this is too good to be true. While on one hand we simplify the equations, on the other, the Reynolds averaging process introduces a new variable, so now we have a steady state problem with more unknowns than equations, and we all know that we can't solve a set of equations if we have more unknowns than we have equations. So we need another equation(s) to close the Reynolds equations. The variable is known as the Reynolds Stress, and the closing equation(s) are known as turbulence models.

Turbulence models deal with the flow in the boundary layer. The boundary layer is divided into an inner and outer region, and the inner region can be further subdivided into a laminar (viscous) sublayer and a fully turbulent region. For flow over a smooth flat plate with no adverse pressure gradients or other funky stuff going on, the inner region stretches from the wall out to about y+=150. The inner region is referred to as the "law of the wall zone" The fully turbulent part of the inner region is known as the "log-law of the wall zone", and is characerized by a log-linear variation of the nondimensional velocity u+. In the viscous sublayer, it is assumed the u+=y+, and when plotted on log-linear graphs, it looks like the familiar Couette flow velocity profile that you see in an undergraduate fluids course.

Now CFD codes assume that this viscous sublayer where u+=y+ happens between the wall and the first grid point. The first grid point is where code switches from the log-law to the viscous sublayer. Generally this switch should occur at a value of y+ somewhere around 30. If your mesh is too fine near the wall, you will get a low value of y+. This will result in overprediction of the near wall velocity. On the other extreme, if y+ is too high, it will cause the code to apply the law of the wall to the outer wake where it is not valid.

Okay, so I gave you a simplistic description. Things such as Reynolds number, surface roughness, adverse pressure gradient, etc will change the value of y+ where the velocity profile swicthes from the viscous sublayer to the log law of the wall. The value of y+ for the transition can range anywhere from about 10.8 up to 50.

Further complicating matters is that some turbulence models employed by modern CFD codes can account for very low or high values of y+ and supposedly still give you reasonable accuracy.