### 12.5.1. Visual Inspection

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.

For all examples in this report, visual representations of the flows are included. These serve as a guideline on how to process the results.

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