Weaknesses of the k-ε model with wall functions
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).