Fluid Dynamics

Question 1

ideal fluid

Answer 1

• steady flow
• incompressible and homogenous
• 1-D velocity profile

Question 2

velocity head

Answer 2

U^2 / 2g

Question 3

pressure head

Answer 3

P / ρ g

Question 4

Hydraulic grade

Answer 4

= pressure head + elevation

= P / ρg + z

Question 5

Total Energy

Answer 5

= Hydraulic grade + velocity head

= P / ρg + z + U^2 / 2g

Question 6

stagnation point

Answer 6

point of no flow

Question 7

reservoir assumptions

Answer 7

fluid velocity inside a reservoir = 0

Question 8

free jet assumptions

Answer 8

pressure within a free jet of water = 0

Question 9

Energy at any point on flow cross section is the same provided:

Answer 9

1. the flow velocity profile can be approximated as uniform

2. The only forces acting on the fluid are weight and pressure (hydrostatic pressure distribution)

Question 10

Two main types of energy loss in pipes

Answer 10

h(f) = friction loss
h(m) = local losses (valves, constrictions)

Question 11

friction loss

Answer 11

h(f) = fL/D * V^2/2g

Question 12

f

Answer 12

friction factor - describing friction resistance of pipe

Question 13

Re

Answer 13

Reynolds number = VD / ѵ

- Re

Question 14

low Re

Answer 14

hydraulically SMOOTH

- roughness height has no effect

Question 15

mid-range Re

Answer 15

Transition

- f varies with Re and roughness

Question 16

high Re

Answer 16

hydraulically ROUGH (loose cell viscous sublayer)
- depends only on roughness

Question 17

f (Laminar flow)

Answer 17

f = 64 / Re

Question 18

f (turbulent flow)

Answer 18

Swarmee and Jain Equation

f = 1.325 /

Question 19

h(m)

Answer 19

local/minor losses

h(m) = k V^2 / 2g

Question 20

k (entry into pipe from reservoir - square entrance)

Answer 20

0.5

Question 21

k (exit into reservoir)

Answer 21

1.0

Question 22

Gate Valve

Answer 22

• low head loss

- plate lifted away (small energy effect)

Question 23

Butterfly Valve

Answer 23

• for shedding head

- plate rotated

Question 24

Problems with Negative Pressure

Answer 24

• Blockage (0

Question 25

System Curve

Answer 25

= Total Energy (B) - Total Energy (A) + h(f) + h(m)

Question 26

Pump efficiency

Answer 26

e = Power (out) / Power (in)
e = ɣQh(p) / Power (in)

Question 27

Pump Selection Procedure

Answer 27

1. Find pump that intersects system curve at flow that client is happy with
2. Ensure pump is working at high efficiency
3. Ensure pump will not cavitate

Question 28

NPSH

Answer 28

net positive suction head

- NPSHA upstream of pump > NPSHR

Question 29

Pump Groups Steps

Answer 29

1. work out total Q and total h(p) relationships
2. combine all pumps into single pump curve
3. solve system like it is single pump using combined pump curve

Question 30

Pumps in Series

Answer 30

Qtot = Q1 = Q2

h(p) tot = h(p,1) + h(p,2)

Question 31

Pumps in Parallel

Answer 31

Qtot = Q1 + Q2

h(p) tot = h(p,1) = h(p,2)