External Flows: Master Fluid Dynamics Around Objects

Question 1

Which factor primarily determines the drag force acting on a body submerged in a fluid flow?

Accepted Answer

Body shape

Answer

Fluid velocity

Answer

Fluid viscosity

Answer

Body's angle of attack

Question 2

What is the fundamental principle behind the operation of an airfoil, enabling it to generate lift?

Accepted Answer

Asymmetrical pressure distribution

Answer

Aerodynamic streamlining

Answer

Boundary layer separation control

Answer

Shock wave formation

Question 3

Which characteristic is distinctive of laminar boundary layer flow?

Accepted Answer

Smooth and orderly

Answer

Turbulent and chaotic

Answer

High skin friction

Answer

Low pressure gradients

Question 4

Which dimensionless parameter is primarily used to characterize the flow regime around a body?

Accepted Answer

Reynolds number

Answer

Mach number

Answer

Froude number

Answer

Strouhal number

Question 5

What is the primary goal of boundary layer control techniques?

Accepted Answer

Prevent flow separation

Answer

Reduce drag

Answer

Increase lift

Answer

Enhance heat transfer

Question 6

How does increasing the Reynolds number affect the boundary layer around a body?

Accepted Answer

Decreased boundary layer thickness

Answer

Increased boundary layer thickness

Answer

Increased turbulence intensity

Answer

Decreased pressure gradients

Question 7

Which of the following is NOT a type of drag force encountered in fluid mechanics?

Accepted Answer

Gravitational Drag

Answer

Pressure Drag

Answer

Skin Friction Drag

Answer

Induced Drag

Question 8

Which force is primarily responsible for generating lift on an airfoil?

Accepted Answer

Lift

Answer

Pressure drag

Answer

Friction drag

Answer

Induced drag

Question 9

What is a characteristic of the boundary layer?

Accepted Answer

A thin layer of fluid adhering to the surface

Answer

A turbulent region of flow far from the surface

Answer

A region where fluid velocity is constant

Answer

A region where pressure is constant

Question 10

What parameter does the Reynolds number quantify?

Accepted Answer

Relative importance of inertia and viscous forces

Answer

Drag coefficient

Answer

Viscous forces

Answer

Boundary layer thickness

Question 11

Where does flow separation occur on a bluff body?

Accepted Answer

When the boundary layer detaches from the surface

Answer

When the flow transitions from laminar to turbulent

Answer

When the pressure coefficient reaches its maximum value

Answer

When the velocity gradient is zero

Question 12

What type of drag is associated with skin friction?

Accepted Answer

Friction drag

Answer

Pressure drag

Answer

Wave drag

Answer

Induced drag

Question 13

When is the assumption of inviscid flow typically made in external flow problems?

Accepted Answer

When viscous forces are negligible compared to inertial forces

Answer

When the Reynolds number is very low

Answer

When the flow is turbulent

Answer

When the fluid is incompressible

Question 14

Regarding the flow past a flat plate, what happens to the boundary layer thickness, δ, as we move downstream?

Accepted Answer

It increases

Answer

It decreases

Answer

It remains constant

Answer

It oscillates

Question 15

In potential flow theory, which of the following forces is NOT considered acting on an airfoil?

Accepted Answer

Skin friction

Answer

Lift

Answer

Drag

Answer

Pressure

Question 16

For a given angle of attack, the drag coefficient of a bluff body typically:

Accepted Answer

Exceeds that of a streamlined body

Answer

Is less than that of a streamlined body

Answer

Matches that of a streamlined body

Answer

Varies with the Reynolds number

Question 17

The primary factor driving the transition from laminar to turbulent flow in a boundary layer is:

Accepted Answer

A combination of all these factors

Answer

Surface roughness

Answer

Pressure gradient

Answer

Reynolds number

Question 18

The pressure coefficient, Cp, is defined as:

Accepted Answer

Ratio of local pressure to dynamic pressure

Answer

Local pressure minus free-stream pressure

Answer

Integral of pressure along a body's surface

Answer

Rate of pressure change with distance

Question 19

Which method is NOT used for boundary layer separation control?

Accepted Answer

Increased surface roughness

Answer

Boundary layer suction

Answer

Vortex generators

Answer

Leading-edge slats

Question 20

The dominant contributor to the drag force on a bluff body is:

Accepted Answer

Pressure drag

Answer

Skin friction drag

Answer

Induced drag

Answer

Wave drag

Question 21

Flow separation occurs around a bluff body when:

Accepted Answer

The pressure gradient along the body's surface becomes unfavorable.

Answer

The flow is laminar and the Reynolds number is low.

Answer

The body's shape is streamlined and the flow is attached.

Answer

The flow velocity is high and the boundary layer is turbulent.

Question 22

Elaborate on the distinct flow characteristics observed around an airfoil and a bluff body. Explain the reasons behind these contrasting behaviors.

Accepted Answer

Airfoils generate lift by utilizing their curved shape to induce a pressure difference, whereas bluff bodies do not generate lift.

Accepted Answer

Bluff bodies exhibit a higher drag coefficient compared to airfoils due to their larger wake region.

Answer

Both airfoils and bluff bodies experience pressure drag due to the adverse pressure gradient.

Question 23

Which factor primarily determines the boundary layer thickness?

Accepted Answer

Flow Reynolds number

Answer

Fluid velocity

Answer

Fluid viscosity

Answer

Object shape

Question 24

Which key parameter is used to characterize external flows?

Accepted Answer

Reynolds number

Answer

Mach number

Answer

Froude number

Answer

Weber number

Question 25

State the primary assumption made in boundary layer theory.

Accepted Answer

Viscosity is negligible

Answer

Flow is laminar

Answer

Fluid is incompressible

Answer

Pressure gradient is zero

Question 26

Explain the main purpose of boundary layer suction.

Accepted Answer

Delay flow separation

Answer

Reduce drag

Answer

Increase lift

Answer

Improve stability

Question 27

An airfoil is designed to generate lift in a moving fluid. Identify the primary mechanism for lift generation on an airfoil.

Accepted Answer

Pressure difference between the upper and lower surfaces

Answer

Boundary layer separation

Answer

Viscous drag

Answer

Wave interference

Question 28

Which dimensionless number is commonly used to characterize the behavior of fluid flow past an object?

Accepted Answer

Reynolds number

Answer

Mach number

Answer

Froude number

Answer

Prandtl number

Question 29

How does increasing the angle of attack typically affect the lift and drag forces acting on an airfoil?

Accepted Answer

Increased lift and drag

Answer

Increased lift and decreased drag

Answer

Decreased lift and increased drag

Answer

No significant change in lift or drag

Question 30

Which of the following is NOT a technique commonly employed to reduce drag on bluff bodies?

Accepted Answer

Increasing the surface roughness

Answer

Streamlining the body shape

Answer

Utilizing vortex generators

Answer

Employing a diffuser

Question 31

What is the fundamental distinction between laminar and turbulent boundary layers?

Accepted Answer

Laminar boundary layers are characterized by smooth, orderly flow, while turbulent boundary layers are chaotic and dissipate more energy.

Answer

Turbulent boundary layers are characterized by smooth, orderly flow, while laminar boundary layers are chaotic and dissipate more energy.

Answer

Laminar boundary layers are generally thicker than turbulent boundary layers.

Answer

Turbulent boundary layers experience lower skin friction drag compared to laminar boundary layers.

Question 32

What is the general relationship between the thickness of a boundary layer and the Reynolds number?

Accepted Answer

The boundary layer thickness decreases as the Reynolds number increases.

Answer

The boundary layer thickness increases as the Reynolds number increases.

Answer

The boundary layer thickness remains unaffected by changes in Reynolds number.

Answer

The relationship between boundary layer thickness and Reynolds number depends on specific flow conditions.

Question 33

Which fundamental principle governs the behavior of external flows?

Accepted Answer

Euler's Equation

Answer

Newton's Second Law of Motion

Answer

Bernoulli's Equation

Answer

Maxwell's Equations

Question 34

What is the importance of the boundary layer in external flows?

Accepted Answer

It influences the flow behavior around objects and determines the pressure distribution and frictional drag.

Answer

It only influences the flow behavior around objects.

Answer

It only determines the pressure distribution on the surface of objects.

Answer

It only affects the frictional drag experienced by objects.

Question 35

Which factors influence the thickness of the boundary layer?

Accepted Answer

Velocity and viscosity of the fluid, and the length of the object

Answer

Only the velocity of the fluid

Answer

Only the viscosity of the fluid

Answer

Only the length of the object

Question 36

How does the shape of an airfoil affect the flow behavior around it?

Accepted Answer

It determines the lift and drag forces and influences the pressure distribution, as well as affects boundary layer formation and separation.

Answer

It only determines the lift and drag forces.

Answer

It only influences the pressure distribution on the surface of the airfoil.

Answer

It only affects the boundary layer formation and separation.

Question 37

What is the main difference between an airfoil and a bluff body?

Accepted Answer

Airfoils are streamlined while bluff bodies are not, and airfoils generate lift while bluff bodies do not. Additionally, bluff bodies experience a more turbulent boundary layer than airfoils.

Answer

Only airfoils are streamlined, and only bluff bodies generate lift.

Answer

Only bluff bodies are streamlined, and only airfoils generate lift.

Answer

Only bluff bodies experience a more turbulent boundary layer than airfoils.

Question 38

Which methods are commonly used to visualize the flow behavior around objects?

Accepted Answer

Smoke visualization, particle image velocimetry, and computational fluid dynamics

Answer

Only smoke visualization

Answer

Only particle image velocimetry

Answer

Only computational fluid dynamics

Question 39

What is the impact of Reynolds number on the flow behavior around an object?

Accepted Answer

It determines the transition from laminar to turbulent flow, affects the boundary layer thickness and drag force, and influences the pressure distribution on the surface of the object.

Answer

It only determines the transition from laminar to turbulent flow.

Answer

It only affects the boundary layer thickness and drag force.

Answer

It only influences the pressure distribution on the surface of the object.

Question 40

What is the main purpose of wind tunnel testing in external flow analysis?

Accepted Answer

To measure aerodynamic forces and moments on objects, visualize the flow behavior around objects, and calibrate numerical models.

Answer

Only to measure aerodynamic forces and moments on objects.

Answer

Only to visualize the flow behavior around objects.

Answer

Only to calibrate numerical models.

Question 41

Which software packages are commonly used for computational fluid dynamics analysis of external flows?

Accepted Answer

ANSYS Fluent, OpenFOAM, and STAR-CCM+

Answer

Only ANSYS Fluent

Answer

Only OpenFOAM

Answer

Only STAR-CCM+

Question 42

How do CFD simulations aid in the analysis of external flows, providing valuable insights into flow phenomena?

Accepted Answer

CFD simulations visualize flow patterns and quantify forces on immersed objects.

Answer

CFD simulations are restricted to simple external flow configurations.

Answer

CFD simulation results are always highly accurate and reliable.

Answer

CFD simulations are solely used for research purposes.

Question 43

The thin region surrounding an airfoil where viscous effects are significant is known as:

Accepted Answer

Boundary layer

Answer

Laminar layer

Answer

Turbulent zone

Question 44

For a bluff body in a fluid stream, the primary contributor to drag force is:

Accepted Answer

Pressure drag

Answer

Induced drag

Answer

Wave drag

Answer

Skin friction drag

Question 45

The lift force generated by an airfoil results from:

Accepted Answer

Bernoulli's principle and angle of attack

Answer

Newton's third law of motion

Answer

Magnus effect

Question 46

The Reynolds number is a dimensionless parameter that quantifies:

Accepted Answer

The ratio of inertial forces to viscous forces

Answer

The ratio of inertial forces to pressure forces

Answer

The ratio of pressure forces to viscous forces

Answer

The ratio of gravitational forces to viscous forces

Question 47

Laminar flow with negligible boundary layer effects characterizes which flow regime?

Accepted Answer

Creeping flow

Answer

Transitional flow

Answer

Turbulent flow

Answer

Supersonic flow

Question 48

The concept of dynamic similarity is crucial for:

Accepted Answer

Extrapolating experimental results to full-scale applications

Answer

Designing efficient aircraft wings only

Answer

Predicting the performance of turbines only

Question 49

Which factor significantly influences the drag coefficient of a sphere?

Accepted Answer

Reynolds number

Answer

Mach number

Answer

Prandtl number

Answer

Froude number

Question 50

A technique used to reduce drag on bluff bodies is:

Accepted Answer

Streamlining

Answer

Flow separation enhancement

Answer

Turbulence promotion

Answer

Boundary layer thickening