Lecture 02

Question 1

What’s external flow?

Answer 1

Flow over a surface. Like air flow over a car or an air foil.

Question 2

What’s internal flow?

Answer 2

Flow in a conduit like a pipe or vent or duct.

Question 3

What is open channel flow?

Answer 3

A type of internal flow where the fluid only occupies half the inside area of the pipe. We don’t deal with this in class at all.

Question 4

What is the flow driven by?

Answer 4

A pressure difference created by a pump, fan, or gravity.

Question 5

What is the friction at the inside surface of the pipe related to?

Answer 5

The pressure drop and the head loss.

Question 6

What is head loss?

Answer 6

Flow resistance due to friction.

Question 7

The pressure drop in a pipe will determine how we do what?

Answer 7

Size a fan or a pump piping system that’s appropriate.

Question 8

If you need a pump that can generate 8 gpm, and you find one in the store that advertises 8 gpm, why is this not enough to know for sure that it will work properly in your system?

Answer 8

Because everything depends on the load on the pump. If the pump is downstream then there is a load on the pump due to the frictional losses. If these losses are high enough you won’t get the correct gpm. You need to calculate these losses to know for sure.

Question 9

What are some of the things to think about when putting a water fountain on the third floor of a building?

Answer 9

Need enough pressure but can’t have too much pressure. Need to figure our what pressure the utility supplier will give you (usually around 80 psi). You need to calculate the frictional losses in that pipe. Then figure out the pressure losses caused by those frictional losses and by the elevation change. Don’t worry about receiving too much pressure because you can always put a valve on the system (although bigger valves cost more money). But too little pressure would be a problem.

Question 10

Why are liquids transported in a circular pipe?

Answer 10

Because a circular cross-section can withstand large pressure increases with the least amount of distortion. The pipe will distort the least with large pressure increases.

Question 11

Why do ducts in the ceiling usually have rectangular cross-sections and not circular one?

Answer 11

It needs to fit in the ceiling. The pipes are low pressure so distortion is not a big worry. We can make a large cross-sectional area with a rectangular cross-section that wouldn’t be possible with a circular one.

Question 12

What are the frictional losses (shear stress) of a flow proportional to?

Answer 12

The velocity profile of the fluid at the inner wall.

Question 13

How do you calculate frictional loss?

Answer 13

Figure out how the velocity changes (at the wall?) to get the shear stress. Then multiply that by the area to get the frictional loss. This is difficult because we usually don’t know what the velocity profile looks like. (18:20)

Question 14

Why do we usually work with average velocity?

Answer 14

It is easily measured with a flowmeter, which are easy to install in most systems.

Question 15

How do you calculate average velocity through a pipe using the data available in lab?

Answer 15

Divide volumetric flow rate (given by flowmeter) by cross-sectional area of the pipe.

Question 16

In order to be more accurate than this, what can we do?

Answer 16

Take flow measurements at different points. This is more expensive and usually not worth it.

Question 17

Why is it kind of a waste of time to bother being very accurate with flow measurements?

Answer 17

Because different conditions on different days will alter the numbers anyway.

Question 18

What’s the equation for mass flow rate using average velocity? How about for a full velocity profile?

Answer 18

(rho)(Vavg)A. The integral over the cross-sectional area of (rho)*u(r)dA.

Question 19

What are the three types of flow? What is their range of Reynolds numbers? How are each characterized?

Answer 19

Laminar: Re4000, the bulk motion is downstream but their are eddy currents throughout due to friction on the walls. Transition: 2300t know which one it is).

Question 20

Why do we design systems to avoid the transition region?

Answer 20

Because we won’t know if the flow is laminar or turbulent, and each needs special conditions.

Question 21

What is the definition of the Reynolds number?

Answer 21

Inertial forces divided by viscous forces.

Question 22

What is an inertial force?

Answer 22

The force of the fluid by virtue of its own mass and motion (think of momentum). The ‘ma’ portion of F-ma=0. This is proportional to (rho)(L^2)(Vavg^2).

Question 23

Do a proof of “ma is proportional to (rho)(L^2)(Vavg^2).”

Answer 23

Proof in notes.

Question 24

What is a viscous force?

Answer 24

Think of it as the heavy or gluey resisting force caused by the fluid flowing past itself. It’s the resistance due to viscosity.

Question 25

What’s the equation for viscous force?

Answer 25

(Tau)(A)=(mu)(du/dy)*(A)

Question 26

Do a proof of “(Tau)*(A) is proportional to uVL.”

Answer 26

Proof in notes.

Question 27

What does the equation for Reynolds number using velocity reduce to? Use the two equations derived from the inertial force equation and the viscous force equation.

Answer 27

Re=(rho)(V)(L)/(mu)

Question 28

For pipe flow, what is the length used in the Reynolds equation? What is the velocity used?

Answer 28

The length is the hydraulic diameter, Dh. The velocity is the average velocity taken by a flowmeter.

Question 29

What is the physical meaning of the Reynolds number?

Answer 29

The higher the Reynolds number, the more easily the momentum of the flow can overcome the viscous forces.

Question 30

What effect does viscosity have on turbulence?

Answer 30

The higher the viscosity, the less turbulent the flow.

Question 31

What is the critical Reynolds number for internal flow?

Answer 31

2300

Question 32

What is the hydraulic diameter?

Answer 32

It’s an equivalent diameter for use in calculations with flow in non-circular tubes and channels.

Question 33

How do we find the hydraulic diameter for a given system?

Answer 33

Dh=4(A)/P; where P is “wetted perimeter”

Question 34

What is the hydraulic diameter for a rectangular cross-section? Prove it.

Answer 34

Proof in notes. Answer should be (2hw)/(h+w).

Question 35

What is the hydraulic diameter for an annulus? Prove it.

Answer 35

Proof in notes. Answer should be Do-Di.

Question 36

Why is calculating the Reynolds number using volume flow rate more advantageous than using average velocity?

Answer 36

Because the flowmeter gives us flow rate (both mass and volumetric).

Question 37

Derive the equation for Reynolds number using volume flow rate for a circular cross-section.

Answer 37

Answer in notes. Should be (4rhoVdot)/(muDpi)

Question 38

Derive the equation for Reynolds number using volume flow rate for an annulus.

Answer 38

Answer in notes. Should be (4rhoVdot)/(mu[Do+Di]pi)