TURBULENCE MODELLING AND MORE ON MESHING

Question 1

What are RANS EQUATIONS?

Answer 1

Reynolds-Averaged Navier-Stokes equations
They take into account the effects of turbulence

Question 2

What are the THREE METHODS OF SIMULATING TURBULENCE?
AND HOW ARE THEY DIFFERENT?

Answer 2

DNS (Direct Numerical Simulation)
* Full resolution
* No modelling required
* Too expensive for practical flows

LES (Large Eddy Simulation)
* Large eddies directly resolved, smaller ones modelled
* Less expensive than DNS - still too expensive in most scenarios

RANS (Reynolds-Averaged Navier-Stokes)
* Solution of time-averaged equations
* Most widely used for industrial flows

Question 3

Why use RANS MODELLING?

Answer 3

• Often unnecessary to resolve the details of the turbulent fluctuations
• Normally only interested in how turbulence affects the mean flow
• Cheaper
• Creates a simpler set of results to interpret

Question 4

What is k – ε TURBULENCE MODELLING?

Answer 4

• Accounts for movement of turbulence through the flow domain
• Uses the TURBULENT KINETIC ENERGY (k) TRANSPORT equation with reference to the TURBULENT DISSIPATION RATE (ε) equation
• Commonly used in commercial CFD codes and is the most widely used and validated method

Question 5

What are the PROS AND CONS OF k – ε TURBULENCE MODELLING?

Answer 5

PROS
* Excellent performance for many industrially relevant flows
* Well established

CONS
* Does not account for the no-slip condition as turbulence is not allowed to form fully
* Poor performance in the following cases;
1) Flows with boundary layer separation
2) Flows with a sudden change in mean strain rate
3) Flows with rotating fluids
4) Flows over curved surfaces

Question 6

What is k-ω TURBULENCE MODELLING?

Answer 6

• Accounts for the transport of turbulence through the flow domain
• Does not require wall-dampening functions for low-Re flows

Question 7

What are the ADVANTAGES AND DISADVANTAGES OF k-ω MODELLING OVER k-ε MODELLING?

Answer 7

ADVANTAGES
* Better near-wall treatment of turbulence
* Takes into account stream-wise pressure gradients better

DISADVANTAGE
* Poor free-stream modelling

Question 8

What is k-ω SST MODELLING?

Answer 8

SST = Shear Stress Transport
* Combines the best features of k-ω and k-ε modelling
* Uses k-ε for free stream modelling but gradually transforms to k-ω model within the near-wall region

Question 9

What are the PROS AND CONS OF k-ω SST MODELLING?

Answer 9

Advantages
* This model is simple and robust and accurate in its near-wall treatment

Disadvantages
* isotropic eddy viscosity is still assumed (μt
is constant in each direction)
* Some flows (curved boundary layers, swirling and rotating flows) don’t
have isotropic eddy viscosity