Computational Fluid Dynamics Multiple Choice Questions on “Free and Wall Turbulence”.
1. The process which is responsible for spreading of turbulent flows in the flow direction is __________
a) Pluming
b) Entrainment
c) Turbulent mixing
d) Turbulent generation
Answer: b
Clarification: While turbulent flows burst out of its region, fluid from the surrounding is drawn into the turbulent region. This is the process of entrainment. This is the cause of spreading of turbulent flows in the flow direction.
2. Which of these terms represent the burst of turbulent activity to the outer region?
a) Crisis
b) Intermittency
c) Turbulent burst
d) Turbulent jumps
Answer: b
Clarification: Intermittency is the irregular alteration of phases. In turbulent flows, it is seen in the irregular alteration between the turbulent and non-turbulent region of jet flow. Intermittency represents the burst of turbulent activity to the surrounding region.
3. A turbulent jet is formed because ___________
a) A region of high-speed flow is surrounded by a stationary fluid
b) An object disturbs the flow
c) Interaction between an object and a fast moving fluid
d) Interaction of fast and slow moving fluids
Answer: a
Clarification: In mixing layers, the interaction of fast and slow moving fluids create turbulence. A turbulent wake is created by an object which disturbs the flow. When a high-speed flow is surrounded by a stationary fluid, the turbulent jet is formed.
4. The velocity of a free turbulent flow at any particular distance in the cross-stream direction is a function of ___________
a) The ratio of the distance in the cross-stream direction from the centreline and half-width at that cross section
b) The source velocity
c) The cross-stream velocity of the source
d) The velocity in the flow direction of the source
Answer: a
Clarification: The velocity at any point in the cross-stream direction at a particular cross section depends on the ratio of the distance from the centreline and the half width of the cross-stream.
5. Turbulent entrainment leads to ___________
a) Increase in the magnitude of the velocity gradients in the flow direction
b) Increase in the magnitude of the velocity gradients in the cross-stream direction
c) Decrease in the magnitude of the velocity gradients in the flow direction
d) Decrease in the magnitude of the velocity gradients in the cross-stream direction
Answer: c
Clarification: Because of the entrainment of the surrounding fluid, the velocity gradients decrease in magnitude in the flow direction. This also decreases the difference between the speed of the wake fluid and its surroundings.
6. Which of these is correct for mixing layer turbulent flows?
Note:
Umax → Maximum velocity at a particular cross-section
Umin → Minimum velocity at a particular cross-section
y → Distance in the cross-stream direction from the centre line at the cross section
b → Cross-stream half width
U → Velocity at the distance ‘y’.
a) (frac{U}{U_{max}-U_{min}}=a(frac{y}{b}))
b) (frac{U_{max}-U}{U_{max}-U_{min}}=b(frac{y}{b}))
c) (frac{U}{U_{max}}=c(frac{y}{b}))
d) (frac{U-U_{min}}{U_{max}-U_{min}}=d(frac{y}{b}))
Answer: d
Clarification: For mixed flows, the velocity is dependent on minimum velocity. This corresponds to the velocity of the slow-moving fluid. Maximum velocity corresponds to the fast-moving fluid. The function is given by
(frac{U-U_{min}}{U_{max}-U_{min}}=d(frac{y}{b})).
7. If y is the distance in the cross-stream direction from the centre line at a particular cross-section b → Cross-stream half width. The mean velocity gradients and all the velocity fluctuations become zero when the value (frac{y}{b}) ___________
a) goes above unity
b) goes below unity
c) goes above zero
d) goes below zero