Airflow Around Buildings

Question 1

What are the two key factors influencing flow around an obstacle?

Answer 1

Air viscosity
Air inertia

Question 2

How does Reynold’s Number affect flow over a curved surface?

Answer 2

Higher Re means flow “sticks”/stays parallel to the surface for longer

Question 3

If wind is flowing normal to the front face of a building, where is the region of positive pressure?

Answer 3

On the incident face

Question 4

Where is suction greatest when flow is normally incident to a face?

Answer 4

Directly around the corner from the incident face

Question 5

What happens on the roof of a building when flow is incident to the corner?

Answer 5

Vortical flow patterns

Question 6

How can parapets affect flow over a roof?

Answer 6

Vortices are lifted away from the roof and hence areas of high suction can be reduced

Question 7

How can apertures or passageways through or under buildings be an issue?

Answer 7

The wind is funnelled creating high speeds and high suction

Question 8

How does flow around a bluff body (rounded) affect downstream structures?

Answer 8

Creates dynamic loading effects due to vortex shedding

Question 9

What can happen if wind flows towards a short building, behind which is a tall building?

Answer 9

A rolling vortex can develop between the buildings

Question 10

How does wind typically damage low pitched tiled rooves?

Answer 10

Uplift on the windward edge

Question 11

How does wind typically damage gables of adjacent buildings

Answer 11

Suction due to channelling

Question 12

According to the Lawson Criteria, when does wind speed become uncomfortable for people? When is it classed as unsafe?

Answer 12

Uncomfortable above 10m/s
Unsafe above 20m/s

Question 13

Where should public seating be placed considering flow around buildings?

Answer 13

In sheltered areas where mean wind speed is less than 4m/s

Question 14

How can wind be designed around to ventilate buildings naturally?

Answer 14

Create stagnant zones outside windows to pressure the air inside
Create suction above plant and stairways to remove warm air

Question 15

How is wind load on a building categorised?

Answer 15

A combination of mean hourly wind speed and turbulent fluctuations

Question 16

What is the equation for mean wind force at a point?

Answer 16

F = 1/2 ρ V^2 A Cf
V is mean velocity
Cf is force coefficient

Question 17

How does strip theory affect building design for wind?

Answer 17

Mean force is found by a sum of forces acting on narrow strips of the whole

Question 18

What does the influence factor β do?

Answer 18

Generalises action on structure to a load effect at some specified point on the structure

Question 19

How is influence factor β defined?

Answer 19

ΔE/δFr
δFr is the wind force at height r
ΔE is the load effect

Question 20

How is mean load effect derived from β?

Answer 20

Emean = L * int(δFrβr)dr

Question 21

How are fluctuating wind forces decomposed?

Answer 21

Along wind and across wind components

Question 22

What is the factor Jb?

Answer 22

A non-dimensional (normalised) load factor

Question 23

How are turbulence intensity (Iu) and Length scale of turbulence (rLu) found?

Answer 23

Using tables and empirical formulae

Question 24

How is maximum wind force determined?

Answer 24

The sum of mean wind force and peak fluctuating wind force

Question 25

τ = 4.5a/meanV. What does this equation describe and what are the terms?

Answer 25

τ is gust averaging time
a is the largest diagonal dimension of a structure face
meanV is mean wind speed

Question 26

What do the symbols - and ^ mean when written above variables?

Answer 26

• signifies mean average of the variable
  ^ signifies peak value of the variable

Question 27

How is pressure on a building characterised?

Answer 27

Using coefficients corresponding to loaded “zones” on a cross-section (tabulated)

Question 28

What is the peak pressure on an area of a building?

Answer 28

q = 1/2 rho Vpeak^2 Cp

Question 29

What is the equation for δFr?

Answer 29

1/2 * rho * Vmean^2 * b * Cf

Question 30

What may you need to do when finding JB?

Answer 30

Convert xLu to yLu or vice versa depending on the wind direction and positions of the axes

Question 31

What is the equation for the expected maximum load effect due to turbulence?

Answer 31

Emax = Emean + gb * sigma EB
gb = 3.5

Question 32

In the Jb equation, what is Bb roughly equal to?

Answer 32

0.95