Wk2 - Flow Measurement

Question 1

Briefly describe (list) 3 key factors that should be considered when selecting an appropriate method of flow measurement. (6 marks)

(5 possible answers)

Answer 1

Any 3 from:
- Volume of flow
- Conditions of measurement
- Degree of accuracy required
- Cost
- Frequency of measurement

Question 2

Select 2 examples of direct discharge flow measuring methods. (4 marks)

Answer 2

• Contracted weir
• Venturi meter

Question 3

Describe two alternative methods that might be used to calculate flow discharge within a broad-crested weir lab flume. (6 marks)

Answer 3

• Use of float
• Volumetric method

Question 4

What is the formula for Q_Ideal?

Answer 4

Q_Ideal = 1.705.b.h1^1.5

Question 5

what is ‘b’ in flow measurement calcs?

Answer 5

b = width of channel (mm)

Question 6

what is ‘h1’ in flow measurement calcs?

Answer 6

h1 = U/S flow depth above the top of the weir (mm)

Question 7

What units do you need to convert ‘b’ and ‘h1’ from and to?

What is this calc?

Answer 7

From ‘mm’ to ‘m’.

(/1000)

Question 8

What is the formula for Q_Real?

Answer 8

Q_Real = 1.705.Cd.b.h1^1.5

Question 9

what is ‘Cd’ in flow measurement calcs?

what value is this normally?

Answer 9

Discharge coefficient

0.75

Question 10

How and why do Q_Ideal and Q_Real differ?

Answer 10

• Q_Ideal represents an ideal case (i.e. one with no energy losses), whereas in practice there are some energy losses.
• Q_Real incorporates a discharge coefficient which reduces the flow rate to account for these energy losses.

Question 11

How do you calculate the Hydraulic Radius, ‘R’ when working towards the flow classification upstream of the weir.

Answer 11

R=A/P

Question 12

How is the cross-sectional area, ‘A’, calculated when working towards the hydraulic radius?

Answer 12

Cross-sectional area U/S of the weir, ‘A’

A = (h1 + p) . (b)

Question 13

what does ‘p’ represent in flow measurement calcs?

Answer 13

‘p’ = height of the weir (mm)

Question 14

How is the wetted perimeter, ‘P’, calculated when working towards the hydraulic radius?

What is the wetted perimeter equal to?

Answer 14

Wetted perimeter, ‘P’

‘P’ = (h1 + p) + b + (h1 + p)

The wetted perimeter of the channel upstream of the weir.

Question 15

How is the velocity, ‘V’, calculated when working towards the Reynolds number for a flow upstream of a weir?

Answer 15

Velocity, ‘V’ = Q_Real / A

Question 16

What is the formula for calculating the Reynold’s number, Re(channel), for the flow upstream of a weir?

Answer 16

Re_(channel) = ( density x R x V ) / (coeff. of viscosity)

Question 17

What are the Reynold’s number classifications in a weir?

Answer 17

Laminar : Re < 500
Transitional : Re 500 < x < 1000
Turbulent : Re > 1000

Question 18

How do you calculate whether flow upstream of the weir is super-critical or sub-critical?

Answer 18

Calculate the Froude number

Question 19

What is the formula for ‘Froude number’?

Answer 19

Fr = V / (g.Dm)^0.5

Question 20

What is ‘Dm’ equal to?

Answer 20

h1 + p

Question 21

How is the Froude number used to classify flow?

Answer 21

Fr < 1 = Sub-critical

Fr > 1 = Super-critical

Question 22

Explain the significance of the discharge coefficient, Cd, in relation to the broad-crested weir equation.

Answer 22

The discharge coefficient, Cd, can be determined by dividing the actual flow rate, Qa (or Q_Real), by the theoretical flow rate, Qt (or Q_Ideal).

As such, the Cd parameter accounts for energy losses that occur at the weir.

Question 23

Provide two examples of flow measuring methods that utilise the Velocity-Area approach.

Answer 23

Current meter

Floats