Fluids Ch. 6, 8, and 9 conceptual Qs

In the case of fully developed pipe flow, the velocity profile is...

the same at any cross section of the pipe

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

hydraulic diameter

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

the energy loss per unit mass due to friction

Which meter type is the most accurate?

Venturi

Which has a "flatter" velocity profile?

turbulent flow

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

2100

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

4000

How is the acceleration of a fluid particle described?

By using the material derivative

What term of the continuity equation, when held constant, allows the continuity equation to reduce to a relationship involving only velocity gradients for incompressible fluids?

Density

Air flows over flat plates at the same velocity U. The single flat plate in Situation 1 is twice as long as two flat plates in Situation 2. For both situations transition would occur at a position of x = 1.1 L. The correct relation between the total drag force F1 on the single plate of Situation 1 to the sum of the drag forces F2 on the two plates of Situation 2 is

F1 > F2

Air flows over flat plates at the same velocity U. The single flat plate in Situation 1 is twice as long as two flat plates in Situation 2. The boundary layer is turbulent over the entire length for both situations. The correct relation between the total drag force F1 on the single plate of Situation 1 to the sum of the drag forces F2 on the two plates of Situation 2 is

F1 < F2

Water flows over two flat plates. The free stream velocity is the same for both plates, the flow is laminar, and both plates have the same width. The length of the first plate is L and that of the second plate is 2L. The relation between the shear force F2 for the plate of length L2 = 2 L1 relative to the shear force F1 for the plate of length L1 is

F2 < 2F1

What effect does the Reynolds number have on the character of the flow over an object?

If Reynolds number increases, the extent of the region around the object that is affected by viscosity decreases.

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

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 engery.

If a cylinder is rotated in a uniform flow, what happens to the stagnation point?

It moves along the surface of the cylinder, and can even move off of the surface of the cylinder.

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

Moody Chart

Equation that are the basic differential equations describing the flow of Newtonian fluids

Navier-Stokes

What law, when directly applied to a fluid particle moving along a streamline, led to the Bernoulli equation?

Newtons Second Law

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, which pipe has the higher pressure drop?

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 a general description of the flow rate in laminar flow?

Small

How does vorticity relate to the rotation vector?

Vorticity is twice the rotation vector

Yes or no. Inviscid, nonviscous, and frictionless are the three terms for flows where the shearing stresses are negligible.

Yes

Steady, fully developed pipe flow experiences no

acceleration

Parallel plate flow where one plate moves with constant velocity is called

couette flow

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

Lines where the velocity potential is constant are called

equipotential lines

True or false. A straight pipe causes a greater head loss than a bent pipe.

false

True or false. Flows can cross a streamline.

false

True or false. If you measured the pressure while moving across the boundary layer from the body to the boundary layer edge, one would find a significant pressure change.

false

True or false. Pressure is constant along a curved surface.

false

True or false. The actual transition from laminar to turbulent boundary layer always occurs at a specific point on the surface.

false

True or false. The pressure or force of gravity can cause a fluid flow element to rotate.

false

Give an example of a type of flow for which the Navier-Stokes equation gives an exact solution.

fixed plates in steady, laminar flow

a hurricane is an example of a

free vortex

There is 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

What is the Reynolds number?

ratio of the inertia to viscous effects in the flow

What is the value of the vorticity in irrotational flow fields?

zero

How are stresses related to the rate of deformation of incompressible Newtonian fluids?

linearly

The 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 is doubled, the relation between the mass flow rate for case B and case A is:

mb = 4sqrt(2)ma

What shape is the velocity distribution for steady, laminar flow in circular tubes?

parabolic

Which of the following is an example of a blunt body?

parachute

The fluid boundary layer is defined as:

region in the fluid where fluid interacts with the surface

According to the momentum equation, the time rate of change of the linear momentum is equal to

resultant force

Lines along which the stream function is constant

streamlines

The 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, how would the mass flow rate be effected?

the mass flow rate would increase

For a steady uniform flow of an incompressible liquid that enters a pipe, what is true about the velocity of the entrance region?

the maximum velocity increases with distance from the entrance

What parameter of the flow is responsible for head loss?

the shear stress on the wall

The boundary layer theory exists based on the fact that the boundary layer is

thin

What is the purpose of streamlining

to reduce the pressure drag at the cost of increasing the skin friction drag

True or False. For irrotational flow, the vorticity is zero.

true

True or false. If a Reynolds number is small, the viscous effects are relatively strong.

true

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

true

True or false. The accuracy of a boundary layer results using the momentum integral equation depends on how closely the shape of the assumed velocity profile approximates the actual profile.

true

True or false. The fluid outside of the boundary layer and wake region flows as if it were inviscid.

true

True or false. The velocity potential and the stream function are related.

true

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

represents a flow in which the streamlines form concentric circles

vortex

Yes or no. Potential flow theory provides a reasonable approximation when dealing with low viscosity, high velocity flows in regions where the flow is accelerating.

yes

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 Δp1. If he diameter of the pipe is then reduced to one-half the original value what is the relation of the new pressure drop Δp2 to the original pressure drop Δp1 at the same mass flow rate?

Δp2 = 32Δp1

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 Δp1. If the mass flow rate of the fluid in the pipe is then doubled what is the relation of the new pressure drop Δp2 to the original pressure drop Δp1 at the original mass flow rate?

Δp2 = 4Δp1

A valve has a loss coefficient Kc. The valve is improved so that the loss coefficient is reduced to one-half the value. For Kcb = Kca/2. For the same flow rate, the relation between the pressure drop across the valve after the improvement ΔpB relative to that before ( ΔpA) is:

ΔpB = ΔpA/2