Ch. 8 Concept Questions - Fluids

Some pipe flow problems require a trial and error solution technique.

true

What value must the Reynolds number be lower than to be considered laminar for pipe flow?

2,100

What value must the Reynolds number be higher than to be considered turbulent for pipe flow?

4,000

Water flows steadily through a horizontal circular pipe of length L1 and diameter D1. Which curve most correctly describes the pressure drop through the pipe as the diameter is increased with the mass flow rate and friction factor staying the same.

E

Water discharges from a submerged pipe into a reservoir. What eventually happens to the kinetic energy of the flow in the pipe after the fluid enters the reservoir?

It is converted to thermal energy.

What chart gives the friction factor in terms of the Reynolds number and relative roughness?

Moody chart

Is Poisuille's law valid for transitional flow?

No, it is only valid for laminar flow

Water flows through two smooth pipes with the same diameter and length as shown below. Both flows are steady, laminar and fully developed. The flow rate through the second pipe is twice that through the first pipe. For this situation.

Pipe 2 has the higher pressure drop

Water flows steadily through a smooth pipe with a Reynolds number Re1. If the velocity is doubled the relation between the Reynolds number Re2 after the increase to Re1 is:

Re2 = 2Re1

What is Reynolds number?

The Reynolds number is the ratio of the inertia to viscous effects in the flow.

Consider the following steady state laminar flow velocity distributions between two plates. Which of the following are true about the flow?

The pressure is decreasing from left to right.

What parameter of the flow is responsible for head loss?

The shear stress on the wall

Which of the three meter types is the most accurate?

Venturi

Steady, fully developed pipe flow experiences no ____.

acceleration

There is a steady turbulent flow of water in a pipe of length L. As the average water velocity decreases, the pressure drop over the length L

decreases

A straight pipe causes a greater head loss than a bent pipe.

false

What parameter replaces the diameter for non-circular duct calculations?

hydraulic diameter

Water is flowing in a pipe. Which is the correct statement about the effect of an increase in the Reynolds number of the flow:

if the flow is laminar it could become turbulent

There is a steady laminar flow of water in a pipe of length L. As the mass flow rate increases, the pressure drop over the length L

increases

A pipe connects two reservoirs at different elevations as shown in the figure below and water flows from reservoir 1 to 2. The flow is turbulent and the friction factor can be taken to be constant. If the pipe diameter D is doubled and the length and elevation difference remain the same, the relation between the mass flow rate for case B and case A is:

mB = 4√2 mA

For a very rough pipe wall the friction factor is constant at high Reynolds numbers. For a pipe of diameter D1 the pressure drop over the length is p1. If the diameter of the pipe is then reduced to one-half the original value what is the relation of the new pressure drop p2 to the original pressure drop p1 at the same mass flow rate?

p2 = 32p1

For a very rough pipe wall the friction factor is constant at high Reynolds numbers. The pressure drop over the length of the pipe is p1. If the mass flow rate of the fluid in the pipe is then doubled what is the relation of the new pressure drop p2 to the original pressure drop p1 at the original mass flow rate?

p2 = 4p1

What is a general description of the flow rate in laminar flow?

small

The dimensions of head loss can be expressed in dimensions of length (L). As such, the head loss in a measure of:

the energy loss per unit mass due to friction

A pipe connects two reservoirs at different elevations as shown in the figure below and water flows from reservoir 1 to 2. If the elevation difference H were increased,

the mass flow rate would increase

For a steady uniform flow of an incompressible liquid that enters a pipe, which of the following is true for the entrance region:

the maximum velocity increases with distance from the entrance

In the case of fully developed pipe flow, _____.

the velocity profile is the same at any cross section of the pipe

If a dye streak injected into a flow almost immediately becomes blurred and spreads across the entire pipe, the flow is said to be ___.

turbulent

Which results in more efficient heat exchange, laminar or turbulent flow?

turbulent

Which has a "flatter" velocity profile?

turbulent flow