18 Jul y+: what is it and how can I use it?
Very often, after the simulation is completed, here in CONSELF we suggest to verify the y+ value to be withing certain ranges. Very often we get asked: what is y+ and why do we need to ensure these ranges? Well, these questions, even though not very simple to reply, deserve a clear answer! Let’s start from the definition of y+, which, from a mathematical point of view, is here reported:
- u* is the friction velocity. The precise definition of this value would be actually long; for the intention of this post we just need to keep in mind it depends on velocity and fluid type
- y is the cell distance from the solid wall
- ν is the kinematic viscosity of the fluid considered
It’s easy to note that the y+ value obtained actually depends on three type of data: geometry (distance), velocity and fluid type. Only after the simulation is completed all these three type of data are available and CONSELF (or in general a CFD software) calculates the y+ distribution and report it as a results to visualise. Only during the Results step is possible for the user to verify the calculated y+ actually is enclosed within the prescribed ranges for the used turbulence model.
This verification is actually done in a very simple way: just double check the legend scale to check minimum and maximum y+ values and, at the same time, verify in the viewer there these two values are calculated
Now the most important question: if we are not within the range values, what can a user do to fix everything? Well the answer is quite simple: given the three types of data influencing the y+ calculation, the user can only act on the geometry of the problem. CONSELF gives a complete control of the first wall cell height (during the mesh phase) where it is possible to increase/decrease the accuracy of the boundary layer.
Obviously everything said in this post is very practical and there is no a complete theoretical description of the problem, but CONSELF is confident these information can be helpful and guide you into the difficult task of obtaining high quality CFD results.Follow Us
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