Aerodynamics Questions and Answers – Lifting Flow over Cylinder

This set of Aerodynamics Questions and Answers for Experienced people focuses on “Lifting Flow over Cylinder”.

1. Why the circular cylinder will create more drag?
a) The pressure difference between upstream and downstream direction of flow
b) The pressure difference between relative wind and downstream direction of flow
c) The pressure difference between upstream and direction of flow
d) The pressure difference between downstream of flow

Explanation: A circular cylinder produces large drag due to the pressure difference between upstream and downstream of the flow. The difference in pressure is causes by the periodic separation of flow over the surface of the cylinder. This will increase the drag in the cylinder.

2. How the cylinder will vibrate in the wind tunnel test?
a) Due to high pressure
b) Due to low pressure
c) Due to relative pressure
d) Due to fluctuations in the flow

Explanation: The difference in pressure is caused by the periodic separation of flow over the surface of the cylinder. Periodic separation induces fluctuations in the flow and makes cylinder vibrate in the wind tunnel.

3. What is passive flow control method used to reduce the drag co-efficient over a cylinder?
a) Surface framing
b) Flow visualization
c) Roughened surfaces
d) Wiring surfaces

Explanation: To reduce the amount of drag on a cylinder various active and passive flow control method have been employed and tested successfully.T hese method include dimpled surfaces, trip wires, roughened surfaces.

4. What is the reduction of drag by installing the small cylinder?
a) 48%
b) 55%
c) 30%
d) 75%

Explanation: A 48% drag reduction of a cylinder by installing a much smaller cylinder in the upstream direction of flow. The shear layer coming from the smaller cylinder changes the pressure distribution around the layer cylinder in such a way that the drag is dramatically altered.

5. What are the quantitative flow visualization techniques?
a) Smoke flow visualization
b) Surface framing

Explanation: The visualization technique such as smoke flow visualization, surface oil film technique, particle image velocimetry have been employed to locate the position of transition and separation of a boundary layer.

6. What types of visualization is used to delay the separation of the flow?
a) PVT
b) Surface oil film technique
c) Smoke flow visualization

Explanation: Smoke flow visualization has been used by bakic and peric to visualize the delayed separation of the flow over a smooth sphere. The flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient.

7. How drag is formed on the cylinder?
a) Due to viscous effect
b) Due to friction
c) Due to boundary layer
d) Due to surface layer

Explanation: Drag is due to a viscous effect, which generate a frictional shear stress at the body surface and which causes the flow to separate from the surface on the back of the body. At the leading edge of the cylinder, a stagnation point is formed. Where the oncoming flow is brought to rest. The pressure here is equal to the stagnation pressure. At the adjacent to the cylinder surface, a thin boundary layer is formed. Which causes the drag on the cylinder.

8. What is the lift on the cylinder?
a) One
b) Zero
c) Negative
d) Greater than zero

Explanation: Pressure distribution over the top half of the cylinder is equal to the pressure distribution over the bottom half and hence the lift must be zero. The air flows on the cylinder is deflected the same amount upward as the flow is deflected downwards at the rear of the cylinder. So the up wash equalizes the downwash in terms of resulting reaction forces. In the end, the free stream hasn’t been deflected at all but still, lift exists.

9. What is the position of stagnation point, when a cylinder is placed in the wind tunnel?
a) Lower part
b) Upper part
c) Middle part
d) No stagnation point

Explanation: The stagnation point moves to the lower part of the cylinder, similar to the theoretical flow. If the spin is sufficiently increased the stagnation point lifts off the surface. Position of stagnation point is a strong function of circulation, with zero circulation stagnation point lies at zero.

10. How the finite lift is measured in the cylinder?
a) Moving up and down
b) Moving up
c) Moving left and right
d) Spinning cylinder

Explanation: A finite lift is measured for the spinning cylinder in the wind tunnel.T he friction between the fluid and the surface of the cylinder tends to drag the fluid near the surface in the same direction as the rotational motion.

11. What is the flow of velocity at the top of the cylinder?
a) Higher
b) Lower
c) Constant
d) Does not varies

Explanation: On the top of the cylinder usual non-spinning flow this extra velocity contribution creates a higher than usual velocity at the top of the cylinder and a lower than usual velocity at the bottom. These velocities are assumed to be just outside the viscous boundary layer on the surface from Bernoulli’s equation.

12. Is flow over a circular cylinder can produce a finite lift?
a) True
b) False

Explanation: The pressure imbalance creates a net upward force that is a finite lift. Therefore flow over a circular cylinder can produce a finite lift, from Bernoulli’s equation that as the velocity increases, and the pressure decreases and hence the pressure on the top of the cylinder is lower than on the bottom. This pressure imbalance will create a net upward force, i.e. a finite lift.

Sanfoundry Global Education & Learning Series – Aerodynamics.

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