CPCE Turbulence Modeling & Numerical Methods Flashcards
6 cards from real CPCE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
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What is the Boussinesq hypothesis used for in RANS turbulence modeling?
Answer: Relating Reynolds stresses to the mean rate of strain via turbulent viscosity
The Boussinesq hypothesis approximates Reynolds stresses as proportional to the mean strain rate tensor through an eddy (turbulent) viscosity, simplifying turbulence closure.
In which scenario is a Detached Eddy Simulation (DES) approach most appropriate?
Answer: Flows with large separated regions where RANS is inaccurate but full LES is too costly
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.
What is the significance of the y+ (y-plus) value in wall-bounded turbulent CFD simulations?
Answer: It is a non-dimensional wall distance indicating whether the first cell is in the viscous sublayer
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.
Which finite volume method property ensures that the sum of fluxes across all faces of a control volume equals zero, maintaining mass conservation?
Answer: Conservativeness
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).
What is 'false diffusion' in CFD and under what conditions is it most pronounced?
Answer: Numerical diffusion error from upwind schemes; most pronounced when flow is oblique to the mesh
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.
In implicit time integration for unsteady CFD, what is the main computational advantage over explicit schemes?
Answer: It allows larger time steps without violating stability constraints
Implicit schemes are unconditionally stable (or have much weaker stability constraints), allowing much larger time steps than explicit schemes, which reduces overall computational cost for slow-evolving flows.