Hello and Welcome!
In this lesson we will see a brief introduction and overview
of turbulent flow and turbulence modeling.
The objective is to provide context
that will allow you to better understand the practical aspects
of selecting a turbulence model in Ansys Fluent.
Without any further delay, let's get started.
A fluid flow that is unsteady, irregular
and which exhibits chaotic fluctuations of properties
in both space and time is called a turbulent flow.
Turbulence is all around us, in all kinds of engineering devices
spanning a wide range of applications,
aerospace, naval, aerodynamics and combustion systems
to name just a few.
It is important in geophysical applications like oceanography,
meteorology, environmental engineering
and also in biological flows.
The effects of turbulence determine
many important engineering quantities
such as heat transfer, frictional drag,
flow separation, pressure drop, mixing, and others.
So it is critical to be able to predict and analyze its effects.
In order to determine whether or not a flow is turbulent,
we can use the Reynolds number,
which takes its name from Osborne Reynolds,
who performed the famous experiment depicted here on the screen.
In the experiment,
dye was injected into a pipe with water flowing through it.
If the water was moving slowly,
the dye just flowed along with the water.
This is known as laminar flow.
As the speed of the water was increased,
the dye began to exhibit fluctuations,
and as the speed continued to increase,
at a certain point, the dye would quickly mix and disperse.
This is known as turbulent flow.
So in other words, as the speed is gradually increased,
the flow undergoes a transition from laminar to turbulent motion.
Reynolds found that it is not only the velocity
that determines when the flow becomes turbulent.
It also depended on the density and viscosity of the fluid
and the diameter of the pipe.
He combined these into the dimensionless parameter
which is known as the Reynolds number.
The Reynolds number does not only apply to pipe.
The diameter can be replaced with
another appropriate length scale,
such as the distance from the leading edge of a boundary layer
or the cross section of a bluff body.
As you can see, turbulence begins at different Reynolds number
for different kinds of flows.
The important thing to take away from this
is that before running a CFD simulation,
you should calculate the Reynolds number
to see whether your flow is turbulent.
Because the overwhelming majority
of engineering flows are turbulent,
you are almost always going to be simulating a turbulent flow,
and there are different ways to do that.
In theory, if unlimited computing power and speed were available,
you could create an extremely fine grid
and directly simulate all of the unsteady turbulent motions
responsible for causing the die to mix and disperse.
This is called direct numerical simulation, or DNS,
but in reality, it is only currently possible
for low Reynolds number and simple geometry.
The enormous number of grid points
and extremely small time step sizes that are necessary
make this impractical for any engineering application.
As a consequence of this,
in order to actually be able to do a turbulent flow simulation,
you have to use a turbulence model,
and different modeling approaches exist.
One is called large eddy simulation, or LES.
A turbulent flow field is made up of small swirling patterns
known as eddies.
The idea behind LES is to directly solve for the motion
of the largest, most important eddies,
and model the effect of the smaller eddies,
so as the diagram shows,
a lot of the detail is resolved, but not all of it.
Ansys Fluent has several LES models and hybrid approaches,
but this is still very computationally expensive
for most engineering applications
so these are not going to be covered in this course.
That brings us to the most widely used modeling approach,
which is known as the Reynolds Averaged Navier Stokes approach,
or more simply, RANS.
Models based on this approach are called RANS models,
and they are far and away the most widely used turbulence models.
With RANS modeling, no turbulence is resolved in the simulation,
Instead, the mean flow field is calculated
and the role of the model is to predict
the effect of the turbulence on the mean flow field.
The diagram shows an illustration of the mean flow field
predicted by a RANS simulation.
There is not as much detail as LES,
but for most engineering applications,
the mean values are all we are really interested in anyway.
Therefore, the rest of the video will focus on RANS models.
Next, we will talk about how to select
which turbulence model to use in Ansys Fluent.
When you are setting up your CFD simulation in Ansys Fluent,
if your flow is turbulent,
you need to go to the Models section in the Physics tab
and click here where it says “Viscous”
to open the viscous model panel.
The default setting for the viscous model is SST k-omega model
and all the available turbulence models are listed in the panel.
Earlier in this video, we talked about RANS models.
RANS models solve transport equations
for variables that are related to the turbulence,
for instance most of them solve for the turbulent kinetic energy.
Anyway, these models are frequently categorized
by how many transport equations there are.
When you see the panel, all the models
with a number of equations listed in parentheses are RANS models.
It turns out that many of these models
are only used in specialized applications,
so right now,
We will focus on k-epsilon and k-omega models,
which are the ones used most often in general cases.
It is actually useful to think of these
in terms of a k-epsilon family of models
and a k-omega family of models,
because as you can see,
when you select either of these in the upper left,
then you then have to select which variation of k-epsilon,
or which variation of k-omega you want to use.
While there are many possible choices, it is not practical
to consider every single choice in every possible flow.
Although if you do want to learn more information
about all of the models, advanced turbulence training is available.
Anyway, to look at turbulence model selection in a practical light,
Ansys recommends the use of the SST k-omega model
which is the default.
If you experience convergence problems,
there are some alternatives listed here.
When highly accurate resolution of boundary layers is critical,
such as in applications involving flow separation
or finely resolved heat transfer profiles on walls,
then the SST k-omega model is the preferred choice.
There is one corollary to this recommendation.
There can be problems,
for instance certain kinds of multiphase problems,
where the solution depends much more strongly
on other physical models or modeling assumptions
such that only a crude estimate of turbulence is needed.
In such a case, the standard k-epsilon model could be used.
Because it is normally the easiest one to converge.
Of course this only applies to some cases
but it can be helpful to remember that
sometimes the turbulence model is not the biggest assumption
being made in a CFD simulation.
Let's summarize what we learnt in this lesson.
We discussed about turbulence and its applications,
we learnt how to use the Reynolds number
to determine whether the flow is turbulent.
We then discussed about turbulence models
and different modelling approaches
with specific emphasis on RANS models.
Lastly, we discussed the practical aspects
of turbulence model selection in Ansys Fluent.
With that, let’s wrap it up!