Mechanical Engineering Principles

256 MECHANICAL ENGINEERING PRINCIPLES

tall buildings and trees, civil engineers normally
assume that:

Pressurep= 0. 5 ρv^2

This is because the flow of fluid is similar to the
plan view shown in Figure 22.15, where the change
of momentum is much less.

v 1 v 2

Figure 22.15

Problem 3. Determine the wind pressure on

a slim, tall building due to a gale of

100 km/h. Take density of air,

ρ= 1 .2 kg/m^3.

For a tall building, pressure

p= 0. 5 ρv^2

Velocity,v= 100

km

h

×

1000 m

km

×

1h

3600 s

= 27 .78 m/s

Hence,wind pressure,

p= 0. 5 × 1. 2

kg

m^3

×

(

27. 78

m

s

) 2

= 462 .96 N/m^2 = 0 .00463 bar

Problem 4. What would be the wind

pressure of Problem 3, if the gale were

acting on a very wide and flat surface?

For a very wide surface,pressure,

p=ρv^2

= 1. 2

kg

m^3

×

(

27. 78

m

s

) 2

= 926 .1N/m^2 = 0 .00926 bar

(or less than 1/100thof atmospheric pressure!)

Now try the following exercise

Exercise 116 Further problems on the

impact of jets on flat

surfaces

1. A hurricane of velocity 220 km/h blows
   perpendicularly on to a very wide flat
   surface. Determine the wind pressure that
   acts on this surface due to this hurricane,
   when the density of air,ρ= 1 .2 kg/m^3.
   [0.0448 bar]


2. What is the wind pressure for Problem 1
   on a slim, tall building? [0.0224 bar]


3. A tornado with a velocity of 320 km/h
   blows perpendicularly on to a very wide
   surface. Determine the wind pressure that
   acts on this surface due to this tornado,
   when the density of air,ρ= 1 .23 kg/m^3.
   [0.0972 bar]


4. What is the wind pressure for Problem 3
   on a slim, tall building? [0.0486 bar]


5. If atmospheric pressure were 1.014 bar,
   what fraction of atmospheric pressure
   would be the wind pressure calculated in
   Problem 4? [0.0479]

Exercise 117 Short answer questions on

the measurement of fluid

flow

In the flowmeters listed 1 to 10, state typical

practical applications of each.

1. Orifice plate


2. Venturimeter


3. Float and tapered-tube meter


4. Electromagnetic flowmeter


5. Pitot-static tube


6. Hot-wire anemometer


7. Turbine flowmeter


8. Deflecting vane flowmeter


9. Flow nozzles


10. Rotary vane positive displacement meter


11. Write down the relationship between the
    coefficientsCc,CvandCd