This chapter discusses:
The first and most important step in any SRS model is the visual inspection of the
turbulence structures. This is typically done using an isosurface of the
-criterion. The definition of 
is:
 | (12–36) |
where in different definitions the constant might be different (for historic
reasons, 
in Ansys Fluent and 
in Ansys CFD-Post). The value of the constant 
is typically unimportant as we are only interested in visual impressions when using
this quantity. In this definition, 
is the absolute value of the Strain Rate and 
is the absolute value of
vorticity.
 | (12–37) |
The rationale behind this definition is that we want to visualize vorticity, which
characterizes turbulence vortices, but also to subtract the mean shear rate in order
to avoid displaying steady shear layers (where 
).
There are different definitions of 
, some of them non-dimensional. Avoid
using non-dimensional 
values as they can be mainly used for visualization of free
vortices and their dynamics (for example, tip vortex of an airplane wing). In
turbulent flows, they can elevate very weak turbulence structures to the same level
as the strong ones and thereby produce an incorrect picture.
In Ansys Fluent, the variable 
is called "
criterion" (under Turbulence)
and in Ansys CFD-Post "Velocity.Invariant 
" in the variable list. Both
codes also have a non-dimensional version of 
(Ansys Fluent: "Normalized q
criterion”," Ansys CFD-Post: "Location / Vortex Core Region, Method =
-Criterion"), which are not very descriptive for turbulence
vortex fields.
The dimensional 
-values can be very large and can vary greatly in the domain.
Frequently, values up to 
can be found in high Re
number flows. In such cases, isosurfaces in the range of 
are typically sensible. You
must experiment with some values for the isosurface before obtaining a suitable
picture. It might be helpful to first plot 
on a fixed surface as a contour
plot and select the correct scaling from that contour plot. Use positive values for
the isosurface. Do not use 
for
visualization, as it will show very weak structures not relevant to turbulence
visualizations.
It is also advisable to color the isosurface of 
with some other variable.
Interesting quantities are the eddy-viscosity ratio (
), or a velocity component that is small or zero in
RANS (such as the spanwise velocity), or the CFL number, and so on. The visual
inspection should be done continuously during the entire start-up and run time of
the simulations (once per day or after every 1000 time steps). It serves the
following purposes (see for example Figure 12.16 and Figure 12.17):
Check if unsteady turbulence develops at all and at the expected locations.
Check large scale symmetries/asymmetries of the flow.
Check the solution for numerical wiggles (odd-even decoupling)
Check the size of the resolved eddies and see if they are as one would expect from the grid resolution.
Check the CFL number on these eddies. It should be smaller than CFL˜
1.
Check the eddy-viscosity ratio. It should be much smaller than RANS.Check for global SRS turbulence models (SAS/DDES/SDES/SBES) if the turbulence structures develop early in the separating shear layer or if a noticeable delay is observed (see Figure 12.26: Isosurfaces of the Q-criterion colored with the velocity magnitude).
Check for ELES/Unsteady inlet conditions, if synthetic turbulence is reasonable and does not decay (such as in Figure 12.45).
Check the progress of the simulation towards a statistically converged solution. This means that the resolved turbulence requires some time until it has developed and has been transported through the domain. Time-averaging has to wait until that stage has been achieved.
Include pictures of turbulence structures in any reports of the test case (slides, reports, publications, service requests).
If possible make animations, which help to understanding of the flow physics and is also helpful for others to understand the flow.
Add monitoring points at interesting locations and plot their development in time to demonstrate statistical convergence.
Unsteady simulations with scale resolution require special care in postprocessing and averaging. Engineers are usually interested only in time-averaged results and not in the details of the unsteady flowfield. It is therefore important to follow a systematic approach when computing such quantities.
The typical process is to start from a RANS solution (or reasonable initial condition). When switching to any SRS model, the flow will require some time to statistically settle into a new state for the following reasons:
The resolved turbulence requires some time to develop and be transported through the domain.
The global flow topology might change from the initial (RANS) solution.
Other physical effects might require longer start-up times (such as multi-phase).
The general strategy is therefore to run the simulation for some start-up
time 
, before activating the averaging process (or
initiating the acquisition of, for example, acoustics information). When should this
process be started and how long does it take until the flow is statistically steady?
This is the stage where any increase in 
would not change the averaged solutions.
Unfortunately 
depends strongly on the flowfield and no general
guidelines can be given. For some flows, the flow develops quickly (in a few
thousand time steps). For others it takes tens of thousands of steps to reach that
point. However, a first estimate can be obtained by estimating throughflow time,
. This
is the time that the mean flow requires to pass one time through the domain
where 
is the
length of the domain and 
is the mean flow velocity. The turbulence statistics
typically require several (3-5) throughflow times to establish themselves. Again,
this is just a rough estimate and can depend on the particular flow.
In order to determine 
more systematically, one must monitor the simulation.
It is advisable to monitor some local and some global quantities.
Continuously inspect the solution visually with the aid of regular images and updated animations.
Inspect solution variables at monitor points in the critical zone of simulation (pressure, velocity, temperature, and so on) as a function of time. The amplitude and frequency of local oscillations should become regular before the averaged statistics can be gathered.
Monitor global quantities (forces on body, massflow, integrated swirl, and so on). Interesting quantities are often those that would be zero for RANS (spanwise forces, and so on) as they are sensitive to the SRS characteristics. They also help to evaluate the overall symmetry of the solution (they should fluctuate around zero) and to determine slow transients (quantities that fluctuate around zero but with low overlaid frequencies).
Only when all indicators show that the flow is no longer changing statistically (meaning only the details of the turbulence structures are a function of time) should the averaging be activated. It is important to document the number of steps that have already occurred when averaging was started and how many steps have been averaged. With respect to averaged quantities:
Monitor time-averaged quantities and ensure that they are not "drifting." They will drift initially, but should then settle to an asymptotic value.
Ensure that they satisfy the symmetry conditions of the flow. Any asymmetry is an indicator of non-convergence (exceptionally, there are flows that develop physical asymmetries despite a symmetric set-up. Example: some symmetric diffusers separate from one side and stay attached on the other).
Ensure that the averaged quantities are smooth.
In zonal/embedded simulations, check if averaged quantities are reasonably smooth across RANS-LES interfaces (they will never be perfectly smooth, but should also not change drastically).