CPCE CPCE Turbulence Modeling & Numerical Methods 3

Question 1

What is the Boussinesq hypothesis used for in RANS turbulence modeling?

Accepted Answer

Relating Reynolds stresses to the mean rate of strain via turbulent viscosity

Answer

The Boussinesq hypothesis approximates Reynolds stresses as proportional to the mean strain rate tensor through an eddy (turbulent) viscosity, simplifying turbulence closure.

Question 2

In which scenario is a Detached Eddy Simulation (DES) approach most appropriate?

Accepted Answer

Flows with large separated regions where RANS is inaccurate but full LES is too costly

Answer

DES uses RANS near walls and LES in detached regions, making it ideal for massively separated flows where RANS fails but full LES would be prohibitively expensive.

Question 3

What is the significance of the y+ (y-plus) value in wall-bounded turbulent CFD simulations?

Accepted Answer

It is a non-dimensional wall distance indicating whether the first cell is in the viscous sublayer

Answer

y+ is a dimensionless wall distance; a y+ ≈ 1 indicates the first mesh cell is within the viscous sublayer, which is required for low-Reynolds-number turbulence model formulations.

Question 4

Which finite volume method property ensures that the sum of fluxes across all faces of a control volume equals zero, maintaining mass conservation?

Accepted Answer

Conservativeness

Answer

Conservativeness ensures that the flux leaving one control volume face exactly equals the flux entering the adjacent cell, guaranteeing global conservation of mass (and other quantities).

Question 5

What is 'false diffusion' in CFD and under what conditions is it most pronounced?

Accepted Answer

Numerical diffusion error from upwind schemes; most pronounced when flow is oblique to the mesh

Answer

False diffusion is a truncation error from first-order upwind discretization that is most significant when the flow direction is oblique (not aligned) to the mesh lines.

(CPCE) Certified Professional CFD Engineer Practice Test

(CPCE) Certified Professional CFD Engineer Practice Test

CPCE CPCE Turbulence Modeling & Numerical Methods 3