CFD Tutorial 5 – Boundary Layer Mesh


Hi, welcome to the QuickerSim CFD
Toolbox for MATLAB tutorial number five. This tutorial is devoted to boundary
layer meshing. That’s a special function that’s available in the full version of
our QuickerSim CFD Toolbox. Boundary layers are very important in
fluid dynamics as many key flow features happen over there for instance the drag
generated on surfaces that’s obtained in the fluid simulations strongly depends
on the exact resolution of the flow in the vicinity of the wall. In principle
you should be generating computational mesh that already contains boundary layers in the mesh generator that you use, but if you want just to generate a
simple mesh you can still use some built-in functions that are in our
toolbox. So, we will discuss two cases the first case is the one that you already
know, the circular cylinder let’s just have a look at that and obviously
the flow would pass around the cylinder and what we want to do we’ll want to
generate the boundary layer here at the solid walls. So, what we do is we use a
special structure that called layer and we have to define a few parameters of
this structure. That’s the height of the first element, the growth factor that is
the ratio of consecutive elements like in geometric series and the number of
layers that we want to extrude. After that we just have to define which wall
ID’s should be extruded. Extrude the layers
notice that we have to create at this moment a new version of variables p,e,t as
our mesh gets modified and you have a look at the brand new mesh. Let’s give it
a try and now you can see the difference as there is the zone of boundary layers
created. Since our tool uses only triangular
mesh the boundary layer consists of triangles so this is actually the first
layer, this is the second layer and this is the third layer anyway this is the
process of the boundary layer creation for the cylindrical case. The second case
refers to the urban mesh that will be actually used in the tutorial number
nine let’s just have a look at how it looks about the boundary laye,r so, those
are basically two square obstacles along the flow from left to the right and
since we will be simulating complex turbulent flow we will have to have
really good resolution at the solid walls on the bottom. We will also show
you some extra features as the laplace smoothing of the boundary layer mesh. Okay,
so, definitely we have to define the boundary layers from zones 18 to 23 so
that’s what’s being done in wall IDs. We prescribe new layer of height 0.02 again
the same gowth ratio and again three layers. So, just let’s have a look at the
basic mesh once again there it goes and let’s play with the boundary layer
extrusion. There it goes. Yeah, the one thing that
looks a little bit not according to the best practice is the fact that these
boundary layers gets squeezed when we get closer to the corners this can be
fixed to some extent when we use extra parameters for our function extrude layers
2D, you can always refer to the tutorial to check if any more complex
function contains some extra options. There are also two additional options
available that’s called smooth steps and smooth alpha, basically that means that
we apply extra smoothing for our mesh and smooth alpha is the factor and
smooth step is the number of smooting iterations that are being performed. By
default when the mesh is generated we use ten smoothing steps and smooth alpha
is equal to 0.9 so let’s check out how it looks after different factors are
used. So, we decrease the number of smooth steps to 0.3 and smoothing to 0.8. Let’s
redo things to make sure that member is not corrupted and that’s still what we
get in the vicinity of corners yeah, so, actually that’s even worse because we
use 10 by default and now it’s dropped down to 3, but maybe you can still fix
something. So, yeah let’s use 50 smoothing steps and
yet the same alpha let’s have a look. Okay, so, yeah now it seems to be much
better. This collapse of the boundary layer in the corner is not that strong
anymore. Yeah, here it looks really nice. Obviously
it collapses a little bit it’s pretty simplified function at this moment, but
yeah that’s the tool that you should definitely be using to improving your
boundary layers. You could also actually use that for improving your mesh in
general. Okay, so, this is the thing that we wanted to show in this tutorial. We hope
that you find it useful. Boundary layer generation is really necessary in many
kinds of flows and if you like that please subscribe drop us a comment and
check out other tutorials of QuickerSim CFD Toolbox for MATLAB.
See you next time.

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