Lecture 5

Question 1

What are the main sources of drag on a vehicle?

Answer 1

Pressure drag, Viscous drag, Wake, Cooling air flow, Wheels and wheel housing, Underbody, External details

Each of these contributors can vary in impact based on vehicle design.

Question 2

Fill in the blank: The drag of road vehicles is dominated by ______.

Answer 2

[pressure (form) drag]

Question 3

What is the typical distribution of drag contributors in road vehicles?

Answer 3

~50% from the front end, ~25% from A, C, and D pillars, ~25% from underbody and external details

These percentages can vary with vehicle design and conditions.

Question 4

True or False: The rear end of a vehicle creates high pressure that contributes to drag.

Answer 4

False

The rear end creates low pressure due to the wake formed behind the vehicle.

Question 5

What is the effect of frontal pressure on vehicle aerodynamics?

Answer 5

Increases drag

Reducing frontal pressure can help improve overall vehicle efficiency.

Question 6

What design feature can help reduce stagnation area at the front of a vehicle?

Answer 6

Creating a radius at the hood

This design change helps to reduce separation and improve drag.

Question 7

What is a key characteristic of the ideal wake shape of a road vehicle?

Answer 7

Balanced upwash/downwash/sidewash

This balance helps minimize drag and improve vehicle performance.

Question 8

What happens to air pressure as it approaches the front of the vehicle?

Answer 8

Air pressure increases

This increase is due to the compression of air molecules as the vehicle moves.

Question 9

What are the effects of wheel exposure to oncoming airflow?

Answer 9

Increases drag

Proper wheel covers or deflectors can help mitigate this issue.

Question 10

Fill in the blank: The rear wake creates a low pressure that ______ the vehicle in the opposite direction of driving.

Answer 10

[sucks]

Question 11

What is the influence of front-end inclination on drag?

Answer 11

Affects stagnation pressure and airflow direction

Different inclinations can lead to varying drag coefficients.

Question 12

What is the purpose of having an ‘elliptic’ elongated shape in the rear wake of a vehicle?

Answer 12

To ensure smooth closure of the wake and static pressure recovery

This helps in reducing drag and improving fuel efficiency.

Question 13

True or False: The shape and length of the front bumper have no impact on vehicle aerodynamics.

Answer 13

False

The shape and length can significantly affect frontal pressure and drag.

Question 14

What is a common characteristic of modern vehicle design in terms of aerodynamics?

Answer 14

Rounded corners at the front bumper

This design reduces separation and drag compared to older designs with sharp corners.

Question 15

What does Cp distribution at the front end indicate?

Answer 15

Pressure distribution around the vehicle

Understanding this distribution helps in optimizing vehicle shape for reduced drag.

Question 16

What does the closure point define?

Answer 16

The size of the near wake

It shifts with time as vortices are shed.

Question 17

What is the typical shape of the rear end for a vehicle designed for aerodynamics?

Answer 17

Fastback

The fastback has a small wake size and large vortex drag.

Question 18

What is the effect of a roof spoiler on drag and lift?

Answer 18

Decreases drag and downwash

It allows for some pressure recovery.

Question 19

What does a notchback design typically have in terms of wake and vortex drag?

Answer 19

Wake and vortex drag of equal value/size

It typically results in larger wake drag.

Question 20

Which vehicle design has a large wake drag and low vortex drag?

Answer 20

Squareback

The squareback typically has poor aerodynamic performance.

Question 21

Fill in the blank: The ideal rear end shape of a road vehicle is one where separation occurs at the _______.

Answer 21

edges of the base area.

Question 22

What is the relationship between Cd and Clr in vehicle aerodynamics?

Answer 22

They usually have opposite trends

An increase in roof drop leads to an increase in Clr and a decrease in Cd.

Question 23

What contributes to vortex drag in vehicles?

Answer 23

A-, B-, C-, and D-pillars

These pillars form vortex structures that increase drag.

Question 24

What is the primary function of cooling airflow in vehicles?

Answer 24

To provide adequate cooling through the heat exchangers

It impacts fuel economy, performance, and comfort.

Question 25

True or False: Cooling flow drag is the drag resulting from air that passes through heat exchangers.

Answer 25

True

Question 26

What is the effect of the A-pillar on vehicle drag?

Answer 26

Creates a negative contribution to Cd ## Footnote Its effect needs to be minimized.

Question 27

What design features can help reduce drag caused by the A-pillar?

Answer 27

Reduce step height, create a radius, and introduce molding ## Footnote These optimizations should consider contamination attributes.

Question 28

What is the main purpose of boat-tailing in vehicle design?

Answer 28

To increase base pressure and reduce base area ## Footnote It helps in creating a wake that follows the shape of a solid aft extension.

Question 29

What can cause significant losses in cooling airflow?

Answer 29

Gaps in the air guide ## Footnote Proper airflow through heat exchangers is essential for vehicle operation.

Question 30

What is the impact of wheel housing on vehicle aerodynamics?

Answer 30

Negative effects can be reduced but not completely removed ## Footnote Minimizing gaps between the wheelhouse arch and the wheel is important.

Question 31

Fill in the blank: The flow through the heat exchangers can cause losses that can amount to ______ of the total drag of a passenger vehicle.

Answer 31

10%

Question 32

What is the effect of a long rear overhang on vehicle drag?

Answer 32

Good for aerodynamics ## Footnote A long rear overhang with optimized boat-tailing avoids separated flow.

Question 33

What happens when there is increased downwash from the roof?

Answer 33

Increase roof drop or add a roof spoiler ## Footnote This can improve aerodynamic performance.

Question 34

What is the significance of the rear bumper's shape and length?

Answer 34

Important for aerodynamics ## Footnote The rear overhang affects drag performance.

Question 35

What is the drag reduction associated with optimizing air guides?

Answer 35

Reduces cooling flow drag ## Footnote A compromise is needed between cooling efficiency and aerodynamic drag.

Question 36

What is a common requirement for vehicles used in severe winter conditions?

Answer 36

Chains might be needed ## Footnote Further discussion on this topic will occur in later lectures.

Question 37

What causes losses in the wheels and wheel housing?

Answer 37

Air flowing through the spokes ## Footnote Closed rims are not feasible as air flow is necessary for brake cooling.

Question 38

What type of rim shape is preferred by designers for aesthetic reasons?

Answer 38

Concave shapes ## Footnote Although flat or convex shapes give lower drag.

Question 39

Why is a smooth underbody desirable in vehicle design?

Answer 39

To allow air to flow undisturbed to the rear ## Footnote A larger underbody wake may reduce upwash.

Question 40

What can be added to help improve upwash under a vehicle?

Answer 40

Underbody panels and a diffuser ## Footnote These additions can manipulate airflow.

Question 41

Why is the underbody not visible to customers considered easier to improve?

Answer 41

Compared to the top hat design ## Footnote A flat underbody also helps with soiling but may complicate cooling of hot components.

Question 42

What do external mirrors do to the vehicle's drag and frontal area?

Answer 42

Increase both ## Footnote Mirrors create asymmetry, with the driver's side typically being larger.

Question 43

What is the estimated contribution of mirrors to total drag?

Answer 43

Around dCd = +0.010 to +0.025 ## Footnote Current regulations prevent mirrors from being designed too small.

Question 44

Where are modern car mirrors often placed to minimize airflow interference?

Answer 44

At the catwalk ## Footnote This positioning helps reduce disturbances with the a-pillar flow.

Question 45

What happens to the frontal area and energy consumption as height and width increase?

Answer 45

Frontal area increases, leading to increased energy consumption ## Footnote Lengthening the vehicle usually improves drag.

Question 46

What factors influence the frontal area of a vehicle?

Answer 46

Width, height, ground clearance, external details, wheel size, and track width ## Footnote A large frontal area negatively impacts fuel consumption.

Question 47

How does ground clearance affect drag?

Answer 47

An increase in ground clearance often means an increase in drag ## Footnote Lower ground clearance is better for drag reduction.

Question 48

What is the benefit of pitching the car in the front?

Answer 48

Usually beneficial for aerodynamics ## Footnote The optimal pitch angle varies by vehicle.

Question 49

What effect does reducing ground clearance have on wheel area exposure?

Answer 49

Less exposed wheel area ## Footnote This also leads to reduced wheelhouse volume.

Question 50

What should be smooth between the hood and windscreen for optimal aerodynamics?

Answer 50

Transition ## Footnote Wipers should be kept out of airflow to prevent drag.

Question 51

What is the impact of increasing rear overhang on airflow?

Answer 51

Possibility of flow reattachment and increased pressure recovery ## Footnote Smaller wakes lead to lower drag when boat-tailing is applied.

Question 52

What is stated about the coverage of passenger vehicle aerodynamics in the lecture?

Answer 52

Not all aspects can be covered in the time frame ## Footnote More aspects will be discussed later in the course.

Question 53

What is the nature of aerodynamics in vehicle design?

Answer 53

A complete vehicle attribute ## Footnote This includes both the top hat and underbody aspects.