14 Jul What is boundary layer and how you can handle it
We all know the importance of CFD in today’s engineer work. It is fundamental when calculating performances in particular: pressure drops, friction forces and viscous phenomena must become as small as possible in order not to affect the overall system efficiency. I am talking about pumps, about airfoils, compressors, turbines, car external aerodynamics, wings: every type of aerodynamic system that has an energy interaction with the fluid. Taking into consideration these effects is on of the fundamentals problem of CFD simulations and we have to take care of it in order to improve our results accuracy.
The first question is: how does the flow interact with the solid walls of the equipment we mentioned before? Looking at the physics it is straightforward noting the development of a velocity gradient in a zone close to the solid wall, as highlighted in this picture. A lot can be said about this velocity gradient (laminar, turbulence) but for the goal of our speech it is important to underling it and keep in mind the gradient highly affects the friction forces according to the well known law:
It is very important the calculation of the velocity gradient at the wall, and any mistake or inaccuracy in this operation may result in a complete underestimation (or overestimation) of the friction forces. We define then the boundary layer as the zone where this velocity gradient has to be calculated accurately.
Looking at this gradient in a logarithmic diagram, it is possible to divide the boundary layer into three parts: a viscous one (very close to the wall), a log-law one (where turbulence become predominant over viscosity) and, obviously, a buffer zone in which occurs a transition between the two. It is possible to define an non-dimentional wall distance, called y+, which in most of the cases defines general accepted limits of these three zones: viscous up to y+ = 5, buffer zone between 5 and 30 and log-law zone between 30 and 300.
It goes beyond the purpose of the current post how you can calculate the y+ and the theoretical approach, but from a practical point of view most of the modern CFD software automatically calculate it for the resolution mesh once the simulation is complete. Well, according to the chosen turbulence model (see table below), the user has to check the mesh wall resolution satisfies the y+ condition to obtain accurate solution, otherwise it is recommendable to modify the mesh in all the zones where the boundary layer in not within the prescribed limits.
The log law and yplus
Boundary layer mesing functionality
From a practical point of view, any user has the availability of the so called “Boundary Layer” meshing feature to handle the situation. This feature extrude a certain number of cells in direction normal to the surface. This feature gives the user a complete control over the mesh resolution close to the wall and by modifying its parameters we can influence the y+ calculated at the end of the simulation, giving us the chance of a try-and-control process to make this value fit inside the acceptable ranges.
In order to predict the possible y+ coming out of a simulation I recommend to use this tool by POINTWISE.
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