Tdyn reference manual
Despite the
great variety of turbulence modelling options available to the user, the k-ε model joint to Law of the Wall
functions, remains the workhorse of the industrial computation. It is therefore
of value to catalogue the major weaknesses associated with this model in
practical application, and if possible, indicate palliative actions which might
be fruitfully considered. These are listed below.
- The turbulent kinetic energy is
over-predicted in regions of flow impingement and re-attachment leading to
poor prediction of heat transfer and the development of boundary layer
flow around leading edges and bluff bodies. The high turbulence levels
predicted upstream of a stagnation point are transported around the body
and the real boundary layer development is swamped by this effect. The
problems depend on the free-stream values of k and ε and do not
occur in all cases.
- Highly swirling flows are often poorly
predicted due to the complex strain fields. Regions of recirculation in a
swirling flow are often under-estimated.
- Mixing is often poorly predicted in flows with
strong buoyancy effects or high streamline curvature.
- Flow separation from surfaces under the action
of adverse pressure gradients is often poorly predicted. The real flow is
likely to be much closer to separation (or more separated) than the
calculations suggest. The SST version of the k-ω model can offer a
considerable improvement.
- Flow recovery following a re-attachment is
often poorly predicted. If possible, avoid the use of wall functions in
these regions.
- The far-field spreading rates of round jets
are predicted incorrectly.
- Turbulence driven secondary flows in straight
ducts of non-circular cross section are not predicted at all. Linear eddy
viscosity models cannot capture this feature.
- Laminar and transitional regions of flow
cannot be calculated with the standard k-ε model.
- If the weakness of the k-ε model has a
great influence in the problem, the simplest and more robust zero
and one-equations turbulence models can be a good option (see Classes of turbulence models available in
Tdyn chapter).