Correct Answer : Option (B) :   Define reynolds stress for closure problems

Explanation : While solving Navier’s Stokes equation for turbulent flow which governs the velocity, pressure of the fluid, the quantity is decomposed into mean and fluctuating components. While solving these using RANS equation, we get a reynolds stress term that needs to be closed in order to solve it. Hence we make use of turbulence modeling which defines these reynolds stresses in terms of the known averaged quantities.

Correct Answer : Option (D) :   Possible for low Reynolds number

Explanation : The DNS solves the time – dependent Navier’s Stokes equation by resolving eddies of all scales for a sufficient time before reaching statistical equilibrium. The only shortcoming is that DNS is only applicable for low Reynolds number flow which has simple geometry.

Correct Answer : Option (B) :   LES

Explanation : In Large Eddy simulation, large eddies are computed by resolving large time and length scales. In LES, the smaller length scales are ignored making it an economical and less time consuming than DNS.