Aerodynamics Questions and Answers – Pressure Coefficient

This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Pressure Coefficient”.

1. Coefficient of pressure is a ________
a) dimensional quantity
b) dimensionless quantity
c) negligible value
d) cannot be determined

Explanation: Although pressure is dimensional quantity, Coefficient of pressure is dimension less quantity. It is used throughout the aerodynamics from incompressible flow to hypersonic flow. In aerodynamics, it is very easy to find coefficient of pressure rather than pressure.

2. For incompressible flow, Cp is expressed only in terms of ____________
a) pressure
b) density
c) temperature
d) velocity

Explanation: In incompressible flow, the pressure and velocity at two different points can be given by-
P1+0.5*ρ*V12 = P2+0.5*ρ*V22
From here we get, P2-P1 = 0.5*ρ (V12– V22)
Hence, Cp = (P2-P1)/q
Where q-dynamic pressure
On solving the above equation we get, Cp = 1 – (V2/ V1)2.

3. The highest value of Cp is given at ________
a) end points
b) stagnation point
c) everywhere in the flow field
d) at boundaries

Explanation: At stagnation point, the value of velocity is 1 and therefore Cp = 1 – (V2/ V1)2
Gives the value of the coefficient of pressure as 1. This is true only for incompressible flow. Stagnation point gives the highest value of the coefficient of pressure.

4. For compressible flow, the value of Cp at stagnation point is __________
a) 0
b) negative
c) infinity
d) greater than 1

Explanation: In the case of compressible flow, the value of velocity at a stagnation point never equals to 0. The pressure and velocity differ from one point to another. Hence, the Cp value is always greater than 1.

5. The value of Cp for compressible flow can be given as ___________
a) Cp = P2-P1
b) Cp = 0
c) Cp = (P2-P1) /q
d) Cp = 0.5*ρ*V12

Explanation: The pressure is given in terms of coefficient rather than the pressure itself. P2-P1 is the pressure difference at two different points in the flow field. ‘q’ is the dynamic pressure which is given by 0.5*ρ*V12 and Cp is the coefficient of pressure.
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6. If the value of Cp=1, then the local pressure can be given as ___________
a) P = Pfreestream + q
b) P = 0.5*ρ*V12
c) P = Pfreestream
d) P = 0

Explanation: For Cp=1, the flow becomes incompressible and in general Cp tells us how much p differs from Pfreestream in multiples of the dynamic pressure. When Cp=1, it tells that local pressure is one times the dynamic pressure above freestream static pressure.

7. Ram is working on an experiment. He has a beaker in which fluid is moving at a very high velocity. He wants to calculate the relative pressure at each and every point of the flow. Which of the following will help him to do so?
a) Coefficient of lift
b) Coefficient of pressure
c) Drag polar
d) Velocity

Explanation: In fluid dynamics, the coefficient of pressure is used to find the relative pressure at each and every point in a fluid flow. Coefficient of pressure is different at every point in fluid flow and hence has a wide application in aerodynamics and hydrodynamics.

8. Generally, the gliders have Cp as ________
a) 1
b) -1
c) 0
d) infinity

Explanation: Generally the gliders have the pressure coefficient of -1 because it indicates the location of total energy and this total energy is used by variometer (a vertical speed indicator) which reacts to all the vertical movements of the atmosphere.

9. Which of the following is an example of hydrostatic manometer?
a) pressure gauge
b) piston type gauge
c) mercury column manometer
d) spring manometer

Explanation: This type of manometers are used to measure the pressure as well as compare it with the hydrostatic force per unit area at the base of the column. They have a poor dynamic response.

10. In a wing, the coefficient of pressure at the upper surface is greater than the lower surface.
a) true
b) false

Explanation: The pressure on the lower surface of the wing is greater than the lower surface. As the pressure on the lower surface is more, the amount of lift generated increases and we get more lift. The amount of lift depends on the pressure at the lower surface.

11. The shape of the wing is called as ____________
a) geometry
b) wing
c) airfoil
d) wing box

Explanation: The shape of the wing is called airfoil. When the flow passes over an airfoil, it produces aerodynamic forces. Instead of analyzing the complete wing, the analysis is done on an airfoil which gives similar characteristics like a wing.

12. Cl vs Cd is called as _______________
a) drag polar
b) parasitic drag
c) total drag
d) no significance

Explanation: It is called drag polar and it gives the relation between the lift and drag of the aircraft. It describes the amount of lift generated per drag produced. Cl/Cd ratio must be high. The amount of lift generated should be more with relatively less drag.

13. The aspect ratio (AR) is given by __________
a) b/s
b) s/b
c) b2/s
d) s3/b

Explanation: Aspect ratio (AR) is the ratio of span to the mean chord. It is given by the square of wing span divided by wing area. A long and narrow wing has a high aspect ratio and vice versa. It is given by:
AR = b2/S where b is the wing span and S is the wing area.

14. Pressure is _____ proportional to altitude.
a) inversely
b) directly
c) no relation
d) equal

Explanation: Pressure is inversely proportional to altitude. As altitude increases, pressure decreases and vice versa because the air molecules are pulled downwards due to the gravitational force of the earth and hence, all the molecules are close at lower altitude thus generating higher pressure at lower altitudes.

15. The pressure and temperature are ________
a) directly proportional to each other
b) inversely proportional to each other
c) equal
d) independent of each other

Explanation: Gay-Lussac’s law states the relationship between temperature and pressure. They are directly proportional to each other. Increase in temperature increases the pressure and vice versa. When the temperature increases, the gas molecules move faster and hence the pressure increases.

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