Test 1 Concept Questions

Question 1

1–1C What is a fluid? How does it differ from a solid?How does a gas differ from a liquid?

Answer 1

A substance in the liquid or gas phase is referred to as a fluid. A fluid differs from a solid in that a solid can resist an applied shear stress by deforming, whereas a fluid deforms continuously under the influence of shear stress, no matter how small. A liquid takes the shape of the container it is in, and a liquid forms a free surface in a larger container in a gravitational field. A gas, on the other hand, expands until it encounters the walls of the container and fills the entire available space.

Question 2

1–3C Define incompressible flow and incompressible fluid. Must the flow of a compressible fluid necessarily be treated as compressible?

Answer 2

Analysis A fluid flow during which the density of the fluid remains nearly constant is called incompressible flow.
A flow in which density varies significantly is called compressible flow. A fluid whose density is practically independent of pressure (such as a liquid) is commonly referred to as an “incompressible fluid,” although it is more proper to refer to incompressible flow. The flow of compressible fluid (such as air) does not necessarily need to be treated as compressible since the density of a compressible fluid may still remain nearly constant during flow – especially flow at low speeds.

Question 3

1–4C Define internal, external, and open-channel flows.

Answer 3

Analysis External flow is the flow of an unbounded fluid over a surface such as a plate, a wire, or a pipe. The flow
in a pipe or duct is internal flow if the fluid is completely bounded by solid surfaces. The flow of liquids in a pipe is
called open-channel flow if the pipe is partially filled with the liquid and there is a free surface, such as the flow of
water in rivers and irrigation ditches.

Question 4

1–9C What is forced flow? How does it differ from natural flow? Is flow caused by winds forced or natural flow?

Answer 4

Analysis In forced flow, the fluid is forced to flow over a surface or in a tube by external means such as a pump or a fan. In natural flow, any fluid motion is caused by natural means such as the buoyancy effect that manifests itself as the rise of the warmer fluid and the fall of the cooler fluid. The flow caused by winds is natural flow for the earth, but it is forced flow for bodies subjected to the winds since for the body it makes no difference whether the air motion is caused by a fan or by the winds.

Question 5

1–10C What is a boundary layer? What causes a boundary

layer to develop?

Answer 5

Analysis The region of flow (usually near a wall) in which the velocity gradients are significant and frictional effects are important is called the boundary layer. When a fluid stream encounters a solid surface that is at rest, the fluid velocity assumes a value of zero at that surface. The velocity then varies from zero at the surface to some larger value sufficiently far from the surface. The development of a boundary layer is caused by the no-slip condition.

Question 6

1–12C What is a steady-flow process?

Answer 6

Analysis A process is said to be steady if it involves no changes with time anywhere within the system or at the
system boundaries.

Question 7

1–13C Define stress, normal stress, shear stress, and pressure.

Answer 7

Analysis Stress is defined as force per unit area, and is determined by dividing the force by the area upon which it acts. The normal component of a force acting on a surface per unit area is called the normal stress, and the tangential component of a force acting on a surface per unit area is called shear stress. In a fluid at rest, the normal stress is called pressure.

Question 8

1–14C When analyzing the acceleration of gases as they flow through a nozzle, what would you choose as your system?
What type of system is this?

Answer 8

Analysis When analyzing the acceleration of gases as they flow through a nozzle, a wise choice for the system is the volume within the nozzle, bounded by the entire inner surface of the nozzle and the inlet and outlet cross-sections. This is a control volume (or open system) since mass crosses the boundary.

Question 9

1–16C You are trying to understand how a reciprocating air compressor (a piston-cylinder device) works. What system would you use? What type of system is this?

Answer 9

Analysis We would most likely take the system as the air contained in the piston-cylinder device. This system is a
closed or fixed mass system when it is compressing and no mass enters or leaves it. However, it is an open system during intake or exhaust.

Question 10

1–22C What is the net force acting on a car cruising at a constant velocity of 70 km/h (a) on a level road and (b) on an uphill road?

Answer 10

Analysis There is no acceleration (car moving at constant velocity), thus the net force is zero in both cases.

Question 11

2–2C What is the difference between intensive and extensive

properties?

Answer 11

Analysis Intensive properties do not depend on the size (extent) of the system but extensive properties do depend on the size (extent) of the system.

Question 12

2–21C Does water boil at higher temperatures at higher

pressures? Explain

Answer 12

Analysis Yes. The saturation temperature of a pure substance depends on pressure; in fact, it increases with pressure. The higher the pressure, the higher the saturation or boiling temperature.

Question 13

2–23C What is vapor pressure? How is it related to saturation pressure?

Answer 13

Analysis The vapor pressure Pv of a pure substance is defined as the pressure exerted by a vapor in phase
equilibrium with its liquid at a given temperature. In general, the pressure of a vapor or gas, whether it exists alone or in a mixture with other gases, is called the partial pressure. During phase change processes between the liquid and vapor phases of a pure substance, the saturation pressure and the vapor pressure are equivalent since the vapor is pure.

Question 14

2–28C What is flow energy? Do fluids at rest possess any flow energy?

Answer 14

Analysis Flow energy or flow work is the energy needed to push a fluid into or out of a control volume. Fluids at
rest do not possess any flow energy.

Question 15

2–30C What is the difference between the macroscopic and microscopic forms of energy?

Answer 15

Analysis The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame. The microscopic forms of energy, on the other hand, are those related to the molecular structure of a system and the degree of the molecular activity, and are independent of outside reference frames.

Question 16

2–31C What is total energy? Identify the different forms of energy that constitute the total energy.

Answer 16

Analysis The sum of all forms of the energy a system possesses is called total energy. In the absence of magnetic, electrical, and surface tension effects, the total energy of a system consists of the kinetic, potential, and internal
energies.

Question 17

2–32C List the forms of energy that contribute to the internal energy of a system.

Answer 17

Analysis The internal energy of a system is made up of sensible, latent, chemical, and nuclear energies. The sensible internal energy is due to translational, rotational, and vibrational effects.

Question 18

2–56C In which medium does a sound wave travel faster: in cool air or in warm air?

Answer 18

Analysis Sound travels faster in warm (higher temperature) air since c = kRT .
Discussion On the microscopic scale, we can imagine the air molecules moving around at higher speed in warmer air, leading to higher propagation of disturbances.

Question 19

2–58C In which medium does a sound wave travel faster: in air at 20°C and 1 atm or in air at 20°C and 5 atm?

Answer 19

Analysis Air at specified conditions will behave like an ideal gas, and the speed of sound in an ideal gas depends on temperature only. Therefore, the speed of sound is the same in both mediums.
Discussion If the temperature were different, however, the speed of sound would be different.

Question 20

2–73C What is viscosity? What is the cause of it in liquids and in gases? Do liquids or gases have higher dynamic
viscosities?

Answer 20

Analysis Viscosity is a measure of the “stickiness” or “resistance to deformation” of a fluid. It is due to the internal frictional force that develops between different layers of fluids as they are forced to move relative to each other. Viscosity is caused by the cohesive forces between the molecules in liquids, and by the molecular collisions in gases. In general, liquids have higher dynamic viscosities than gases.

Question 21

2–74C What is a Newtonian fluid? Is water a Newtonian

fluid?

Answer 21

Analysis Fluids whose shear stress is linearly proportional to the velocity gradient (shear strain) are called Newtonian fluids. Most common fluids such as water, air, gasoline, and oils are Newtonian fluids.
Discussion In the differential analysis of fluid flow, only Newtonian fluids are considered in this textbook.

Question 22

2–75C How does the kinematic viscosity of (a) liquids and

(b) gases vary with temperature?

Answer 22

Analysis (a) For liquids, the kinematic viscosity decreases with temperature. (b) For gases, the kinematic viscosity increases with temperature.
Discussion You can easily verify this by looking at the appendices.

Question 23

2–76C How does the dynamic viscosity of (a) liquids and

(b) gases vary with temperature?

Answer 23

Analysis (a) The dynamic viscosity of liquids decreases with temperature. (b) The dynamic viscosity of gases
increases with temperature.
Discussion A good way to remember this is that a car engine is much harder to start in the winter because the oil in the engine has a higher viscosity at low temperatures.

Question 24

2–77C Consider two identical small glass balls dropped into two identical containers, one filled with water and the other with oil. Which ball will reach the bottom of the container
first? Why?

Answer 24

Analysis When two identical small glass balls are dropped into two identical containers, one filled with water and the other with oil, the ball dropped in water will reach the bottom of the container first because of the much lower
viscosity of water relative to oil.
Discussion Oil is very viscous, with typical values of viscosity approximately 800 times greater than that of water at
room temperature.

Question 25

2–96C What is surface tension? What is its cause? Why is

the surface tension also called surface energy?

Answer 25

Analysis The magnitude of the pulling force at the surface of a liquid per unit length is called surface tension.
It is caused by the attractive forces between the molecules. The surface tension is also surface energy (per unit area) since it represents the stretching work that needs to be done to increase the surface area of the liquid by a unit amount. Defined as Force/unit length, units of N/M.
Discussion Surface tension is the cause of some very interesting phenomena such as capillary rise and insects that can
walk on water.

Question 26

2–98C What is the capillary effect? What is its cause? How is it affected by the contact angle?

Answer 26

Analysis The capillary effect is the rise or fall of a liquid in a small-diameter tube inserted into the liquid. It is
caused by the net effect of the cohesive forces (the forces between like molecules, like water) and adhesive forces (the forces between unlike molecules, like water and glass). The capillary effect is proportional to the cosine of the contact angle, which is the angle that the tangent to the liquid surface makes with the solid surface at the point of contact.
Discussion The contact angle determines whether the meniscus at the top of the column is concave or convex.

Question 27

2–99C Consider a soap bubble. Is the pressure inside the

bubble higher or lower than the pressure outside?

Answer 27

Analysis The pressure inside a soap bubble is greater than the pressure outside, as evidenced by the stretch of
the soap film.
Discussion You can make an analogy between the soap film and the skin of a balloon.

Question 28

2–100C Is the capillary rise greater in small- or large-diameter
tubes?

Answer 28

Analysis The capillary rise is inversely proportional to the diameter of the tube, and thus capillary rise is greater in
the smaller-diameter tube.

Question 29

Ch.2 What is specific gravity or relative density?

Answer 29

The ratio of the density of a substance to the density of some standard substance at a specificied temperature.

Question 30

Ch.2 What is specific weight?

Answer 30

The weight of a unit volume of a substance.

Question 31

2) What is a viscometer?

Answer 31

a shaft and a journal bearing used to measure the viscosity of the fluid between them.

Question 32

3) What is pressure?

Answer 32

Normal force exerted by fluid per unit area.

Question 33

3) Define gage pressure.

Answer 33

Pg=Pabs-Patm

Question 34

3) What is Pascal’s Law?

Answer 34

(F1/A1)=(F2/A2)

Question 35

3)What is a barometer?

Answer 35

used to measure atmospheric pressure.

Question 36

3)What is buoyant force?

Answer 36

Force acting on a body of uniform density immersed in a fluid is equal to the weight of the fluid displaced by the body, and it acts upward through the centroid of the displaced volume.

Question 37

3) What does it mean if something is stable?

Answer 37

Any small movement will create a restoring force.

Question 38

3) What does it mean if something is neutrally stable?

Answer 38

if you move the ball, it will not restore itself to the original position. It will stay put in a new location.

Question 39

3) What does it mean if something is unstable?

Answer 39

Any disturbance will cause the ball to fall off.

Question 40

4) What is the Lagrangian Description?

Answer 40

Same approach we learned Dynamics, basically following the path of the particle. Fluid properties are functions of time.

Question 41

4)What is the Eulerian Description?

Answer 41

The is the Control Volume approach, We look at the space where the fluid is moving in and out. Fluid properties are functions of position and time.

Question 42

4)In the CV approach, what do we refer to as field properties?

Answer 42

We can refer to Velocity Field, Pressure Field and Acceleration Field, etc.

Question 43

4) What is a Streamline?

Answer 43

Streamline is a curve that is tangent to the instantaneous local velocity vector in the CV. If you take a snapshot of the CV and draw curves tangent to velocities, you will have streamlines.

Question 44

4)What is a Pathline?

Answer 44

A pathline is the actual path traveled by an individual fluid particle some time period.

Question 45

4) What is Streakline?

Answer 45

A streakline is the locus of the fluid particles that have passed sequentially through a prescribed point in the flow. A streakline is formed by continuous introduction of dye or smoke from a point in the flow.

Question 46

4)What is the connection within the Reynolds Transport Theorem?

Answer 46

RTT draws a connection between the system approach and the control volume approach.

Question 47

5) What is an incompressible flow?

Answer 47

There is no such thing as an incompressible fluid. However, if the changes in density are negligible as pressure and velocity change, we call it an incompressible flow. This works well for a lot of liquids such as water and sometimes gases.

Question 48

What is Mechanical Energy?

Answer 48

The form of energy that can be converted to mechanical work completely and directly by an ideal mechanical device such as an ideal turbine. KE and PE are familiar forms of mechanical energy.

Question 49

Is Thermal Energy, considered mechanical energy?

Answer 49

No, since it cannot be converted to work directly and completely(2nd Law of Thermo).

Question 50

In terms of Mechanical Energy, what is difference between a pump and a turbine?

Answer 50

Pump: Add mechanical energy to fluid. ie: water pump
Turbine: Extract mechanical energy from fluid, ie: Hoover Dam

Question 51

What is Inviscid Flow?

Answer 51

There is no such thing as inviscid fluid, however there are regions of fluid flow (outside of boundary layer) that viscous effects are negligible. For those regions, “Inviscid Flow Model” works well and the Bernoulli Equation is one example of that. That means that you should not use Bernoulli Equation in “Boundary Layer Region” where viscous effects are significant.

Question 52

What is Pitot Tube?

Answer 52

An instrument widely used in airplanes, as it measures the velocity of the flow by measuring the difference between static and stagnation pressure.

Question 53

Internal Flow

Answer 53

Flow inside conduits

Question 54

External Flow

Answer 54

Flow over a surface

Question 55

Laminar Flow

Answer 55

A model for low Reynold’s number flows. In this model, streamlines dont cross.

Question 56

Turbulent Flows

Answer 56

A model for high Reynold’s number flows. A disorganized chaotic flow.