Lecture 4

Question 1

What is a bluff body?

Answer 1

A body that, as a result of its shape, has separated flow over a substantial part of its surface.

Examples include vehicles and certain components like wheels and mirrors.

Question 2

What is the thickness ratio?

Answer 2

Ratio between thickness and length.

Question 3

What is the slenderness ratio?

Answer 3

Ratio between length and thickness.

Question 4

What distinguishes a streamlined body from a bluff body?

Answer 4

A streamlined body has a small separation region, while a bluff body has a large separation region.

Question 5

What is stagnation pressure?

Answer 5

The static pressure at a stagnation point in a fluid flow, where the local velocity of the fluid is zero.

Question 6

Define pressure coefficient (Cp).

Answer 6

The relative static pressure divided by the dynamic pressure.

Question 7

What happens to the pressure coefficient (Cp) at the stagnation point?

Answer 7

Cp = 1 (U = 0).

Question 8

What is the behavior of Cp along a streamline?

Answer 8

Cp = 1 - (U/U∞)².

Question 9

What is the primary contributor to total drag in the absence of flow separation?

Answer 9

Skin-friction drag.

Question 10

What is pressure drag?

Answer 10

It results from the distribution of forces normal to the surface of the body exposed to a fluid stream.

Question 11

What is skin-friction drag (viscous drag)?

Answer 11

A tangential force that originates when a fluid moves past the surface of a body.

Question 12

What characterizes the wake behind a vehicle?

Answer 12

The shedding of vortices.

Question 13

What is the ratio of friction drag to pressure drag on a typical passenger car?

Answer 13

Approximately 7% friction drag and 93% pressure drag.

Question 14

What is the main goal of improving the rear wake of a vehicle?

Answer 14

Increase the static base pressure.

Question 15

What is boat tailing?

Answer 15

The elongation/curvature of the body that allows the flow to stay attached, reducing pressure drag.

Question 16

What is the purpose of a splitter plate?

Answer 16

To avoid interaction between periodic shedding, creating two independent separation regions.

Question 17

What do cavities/extensions do in vehicle aerodynamics?

Answer 17

Delay separation and improve pressure recovery.

Question 18

What effect do vortex generators have on flow?

Answer 18

They delay separation by creating a tip vortex that draws energy into the boundary layer.

Question 19

True or False: Bluff bodies are dominated by pressure drag.

Answer 19

True.

Question 20

Fill in the blank: The area behind a vehicle that appears as a ‘solid’ due to dead flow is known as the ______.

Answer 20

wake.

Question 21

What is the influence of the near wake compared to the far wake?

Answer 21

The influence of the near wake on the body is strong, while the influence of the far wake is weak.

Question 22

How does pressure recovery occur along the body length?

Answer 22

The disturbance caused by the body volume dissipates in the flow field, reducing velocity and increasing static pressure.

Question 23

What is the purpose of vanes in aerodynamic design?

Answer 23

To direct the flow behind the body for increased base pressure and reduced wake size.

Question 24

What does passive ventilation do?

Answer 24

Inserts energy into the wake air jet.

Question 25

What is the role of vortex generators in aerodynamics?

Answer 25

Delay separation by creating a tip vortex which draws energy into the boundary layer.

Question 26

True or False: A turbulent boundary layer is more likely to separate than a laminar one.

Answer 26

False.

Question 27

What are active flow control systems designed to do?

Answer 27

Delay separation using suction/blowing systems.

Question 28

What is a current drawback of active flow control systems?

Answer 28

Requires high additional energy to reduce drag at high Reynolds numbers.

Question 29

What is the nature of the wake shape of a vehicle?

Answer 29

Highly 3D and unsteady with complex flows.

Question 30

What effect does the ground have on a moving car?

Answer 30

It pulls on the air above it and causes it to move faster, resulting in ground effects.

Question 31

Fill in the blank: Ground effects lead to _______ on a simple shape.

Answer 31

down force.

Question 32

What happens to the cross-sectional area in ground effects?

Answer 32

Reduction of cross-sectional area and compression of the streamlines.

Question 33

What is Couette flow?

Answer 33

The laminar flow of a viscous fluid in the space between two parallel plates, one moving relative to the other.

Question 34

What forces and moments disappear if a vehicle travels at yaw 0°?

Answer 34

Side force, yaw, and rolling moments disappear.

Question 35

What forces are left for a symmetric body traveling directly into a stream?

Answer 35

Only drag, lift, and pitch moment are left.

Question 36

What are force coefficients?

Answer 36

Forces normalized by the dynamic pressure and frontal area.

Question 37

What are moment coefficients?

Answer 37

Moments normalized by dynamic pressure, frontal area, and a characteristic length.

Question 38

What is the formula for calculating force coefficients?

Answer 38

C_d = F_D / (1/2 * ρ * V^2 * A)

Question 39

What is stagnation pressure?

Answer 39

The static pressure at a stagnation point in a fluid flow.

Question 40

What is the pressure coefficient (C_p)?

Answer 40

c_p = 1 - (U / U_∞)

Question 41

What is pressure drag?

Answer 41

Results from the distribution of forces normal to the surface of the body.

Question 42

What is friction drag?

Answer 42

Results from the distribution of forces tangential to the surface of the body.

Question 43

What is the reference frame of a moving car in relation to ground effects?

Answer 43

In the reference frame of a moving car, the ground is moving backwards at some speed.