When the Reynolds number approaches infinity, what happens to the boundary layer thickness?

Remains same

Approaches zero

Approaches once

Approaches infinity

Correct Answer : Approaches zero

Explanation : As the Reynolds number increases, the boundary layer thickness decreases when compared to the length of the body. Usually for very large aircrafts, the value of δ/L is around 0.01 which is a very small value. So hypothetically, as Re ? ∞, δ ? 0.

Recently Updated in Aerodynamics Questions

Why are turbulence models used?

Model hypersonic flows

Define reynolds stress for closure problems

Closure of continuity equation

Lack of system to accurately find turbulent flow characteristic

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.

What are the shortcomings of DNS?

Time consuming

Low scale resolution

Possible for high Reynolds number

Possible for low Reynolds number

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.

Which of these models compute large vortexes directly while neglecting small scale eddies?

DNS

LES

FANS

RANS

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.