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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CFD and FEM simulation software is a very powerful tool, with its adoption optimization and innovation can be achieved in every field. To make this instrument accessible to everyone means lowering costs, but mainly to develop an infrastructure that favours a super-easy adoption by market new entrants.
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Ruggero Poletto – email@example.com