How do I calculate y+ in fluid mechanics from the relation y+=(ut*y)/v?

In summary, the conversation is about calculating y in the relation y+=(ut*y)/v where y+ is similar to a local Reynolds number and ut is the friction velocity. The problem is not knowing what y+ to choose in order to calculate y. The ultimate goal is to calculate y and the question is whether to assume a y+ and, if so, on what basis to choose it. The attempt at a solution involved assuming y+ and solving for y, but the result did not make sense. The person is confused and is seeking guidance on how to calculate y+ from scratch.
  • #1
firestarter7
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Homework Statement


I am trying to calculate y in the relation y+=(ut*y)/v where y+ is similar to a local Reynolds number, so its magnitude can be expected to determine the relative importance of viscous and turbulent processes. In the listed relation ut is the friction velocity [ut=sqrt(wall shear stress/density)] and v is the viscosity. The problem I am facing is that I don't know what y+ to choose to calculate y, my ultimate goal is thus to calculate y, and I am not sure if I should only assume a y+ so I can calculate y. If so, on what basis should I choose y+? Thanks.

The Attempt at a Solution


I did assume y+ and solved for y, but that just does not make sense. I am confused as I am only trying to calculate the laminar boundary layer y, on a flat plate according to Blasius procedures.
 
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  • #2
I would ideally like to know how to calculate y+ from scratch, but I am not sure if that is possible.
 

Related to How do I calculate y+ in fluid mechanics from the relation y+=(ut*y)/v?

What is y+ in fluid mechanics?

In fluid mechanics, y+ is a dimensionless parameter used to characterize the behavior of a fluid flow near a solid surface or wall. It represents the distance from the wall to the nearest grid point in a computational fluid dynamics (CFD) simulation.

What does the equation y+=(ut*y)/v represent?

This equation is known as the Reynolds similarity law and is used to calculate the value of y+, where u is the mean velocity of the fluid, t is the characteristic length scale of the flow, y is the distance from the wall, and v is the kinematic viscosity of the fluid.

How does y+ affect the accuracy of a CFD simulation?

The value of y+ plays a crucial role in determining the accuracy of a CFD simulation. A value of y+ less than 1 is considered a fully resolved simulation and is generally more accurate. Higher values of y+ may lead to inaccuracies in the results, especially in regions near the wall.

What is the recommended range for y+ in a CFD simulation?

The recommended range for y+ in a CFD simulation varies, but generally a value between 1 to 5 is considered acceptable. A value less than 1 is desirable for accurate results, but it may also lead to a high computational cost. A value greater than 5 is considered problematic and may require further refinement of the mesh.

How can I ensure that I have the correct value of y+ in my CFD simulation?

To ensure that you have the correct value of y+ in your CFD simulation, it is recommended to perform a grid sensitivity analysis. This involves running the simulation with different grid resolutions and comparing the results to determine the optimal value of y+ for your specific case.

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