Correct Answer : Thermal boundary layer is equal to velocity boundary layer
Explanation : When there’s a viscous flow over a flat plate, there’s a boundary layer formation which has certain properties. On the surface, there’s a no slip condition. Apart from this the temperature of the fluid which is immediately at the surface has the same temperature as the surface which is known as the wall temperature. The velocity profile inside a boundary layer increases along the y – direction until it becomes equal to the freestream velocity. The only property that is incorrect is that the thermal boundary layer is equal to the velocity boundary layer.The boundary of thermal layer is defined as the layer where the outer edge temperature becomes equal to the freestream temperature. Similarly, at the velocity boundary layer, the outer edge velocity is equal to the freestream velocity.
Correct Answer : u = 0.99ue
Explaination : Inside the boundary layer, the velocity increases along the y – direction until it becomes equal to the freestream velocity. The thickness of boundary layer δ is defined as the point from the surface where the velocity is 0.99 times the free stream velocity.
Correct Answer : T = 0.99Te
Explaination : The flow temperature just like velocity varies within the boundary layer. It is a function of y – direction. The temperature ranges from Tw which is the temperature at the wall (y = 0) to T = 0.99Te at y = δt, where δt is the thermal boundary layer thickness. This variation of temperature is known as temperature profile.
Correct Answer : Pr = 1
Explaination : In most of the cases the two thermal and velocity boundary layers are not same except in one exceptional case when the Prandtl number = 1, in which case δt = δ. When Prandtl number is greater than 1, δt < δ and when Prandtl number is less than 1, δt > δ. In real life scenario, the Prandtl number is equal to 0.71 thus the thermal boundary layer thickness is greater than the velocity boundary layer thickness.
Correct Answer : δturbulent > δlaminar
Explaination : In case of turbulent flow, there is a high energy and momentum exchange compared to the laminar flow due to the presence of eddies. This leads to the thermal and velocity boundary layer thickness of the turbulent flow to be higher than that of the laminar flow. Thus, δturbulent > δlaminar and δTturbulent > δTlaminar.
Correct Answer : Viscosity
Explanation : There is shear stress between the adjacent layers of fluid in both laminar and turbulent flow inside the boundary layer. This is due to the viscosity and it is given by the relation:
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.
Correct Answer : True
Explanation : When a flat plate is kept in the freestream flow, there’s a formation of a laminar boundary layer at the leading edge. This boundary layer thickness grows to a point where a transition point is reached. Beyond that point, there’s turbulence due to the presence of eddies and there’s turbulent boundary layer formation whose thickness keeps on increasing.
Correct Answer : Remains constant
Explaination : The y – momentum equation for a boundary layer is given by :According to the formula, at any x point in the boundary layer, pressure remains constant in the direction normal to the surface.
Explanation : Turbulent boundary layer consists of inner and outer layers. There is a viscous – dependent part of the profile very close to the surface and different length scaling parameters are needed for the remaining Reynolds – stress – dependent part of the profile.
Correct Answer : DES model
Explanation : DES model is a hybrid of LES and RANS model and makes use of advantages of both. For treating near – wall regions, DES works like the RANS model and for rest of the region, it works as the LES model.
Correct Answer : K – omega model
Explanation : K – omega is one of the popular turbulence model. It is a two-equation model which solves two transport equations – turbulent kinetic energy and specific dissipation. Turbulent kinetic energy determines the energy whereas the specific dissipation determines the turbulence scale.
Correct Answer : K – Epsilon model
Explanation : K – epsilon model is a two – equation model which includes two transport equations representing the turbulent flow properties. The first transport equation is solved is the turbulent kinetic energy, and the second one is the turbulent dissipation rate.
Correct Answer : 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.
Correct Answer : 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 : 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.
