Airflow Around Buildings Flashcards
What are the two key factors influencing flow around an obstacle?
Air viscosity
Air inertia
How does Reynold’s Number affect flow over a curved surface?
Higher Re means flow “sticks”/stays parallel to the surface for longer
If wind is flowing normal to the front face of a building, where is the region of positive pressure?
On the incident face
Where is suction greatest when flow is normally incident to a face?
Directly around the corner from the incident face
What happens on the roof of a building when flow is incident to the corner?
Vortical flow patterns
How can parapets affect flow over a roof?
Vortices are lifted away from the roof and hence areas of high suction can be reduced
How can apertures or passageways through or under buildings be an issue?
The wind is funnelled creating high speeds and high suction
How does flow around a bluff body (rounded) affect downstream structures?
Creates dynamic loading effects due to vortex shedding
What can happen if wind flows towards a short building, behind which is a tall building?
A rolling vortex can develop between the buildings
How does wind typically damage low pitched tiled rooves?
Uplift on the windward edge
How does wind typically damage gables of adjacent buildings
Suction due to channelling
According to the Lawson Criteria, when does wind speed become uncomfortable for people? When is it classed as unsafe?
Uncomfortable above 10m/s
Unsafe above 20m/s
Where should public seating be placed considering flow around buildings?
In sheltered areas where mean wind speed is less than 4m/s
How can wind be designed around to ventilate buildings naturally?
Create stagnant zones outside windows to pressure the air inside
Create suction above plant and stairways to remove warm air
How is wind load on a building categorised?
A combination of mean hourly wind speed and turbulent fluctuations
What is the equation for mean wind force at a point?
F = 1/2 ρ V^2 A Cf
V is mean velocity
Cf is force coefficient
How does strip theory affect building design for wind?
Mean force is found by a sum of forces acting on narrow strips of the whole
What does the influence factor β do?
Generalises action on structure to a load effect at some specified point on the structure
How is influence factor β defined?
ΔE/δFr
δFr is the wind force at height r
ΔE is the load effect
How is mean load effect derived from β?
Emean = L * int(δFrβr)dr
How are fluctuating wind forces decomposed?
Along wind and across wind components
What is the factor Jb?
A non-dimensional (normalised) load factor
How are turbulence intensity (Iu) and Length scale of turbulence (rLu) found?
Using tables and empirical formulae
How is maximum wind force determined?
The sum of mean wind force and peak fluctuating wind force
τ = 4.5a/meanV. What does this equation describe and what are the terms?
τ is gust averaging time
a is the largest diagonal dimension of a structure face
meanV is mean wind speed
What do the symbols - and ^ mean when written above variables?
- signifies mean average of the variable
^ signifies peak value of the variable
How is pressure on a building characterised?
Using coefficients corresponding to loaded “zones” on a cross-section (tabulated)
What is the peak pressure on an area of a building?
q = 1/2 rho Vpeak^2 Cp
What is the equation for δFr?
1/2 * rho * Vmean^2 * b * Cf
What may you need to do when finding JB?
Convert xLu to yLu or vice versa depending on the wind direction and positions of the axes
What is the equation for the expected maximum load effect due to turbulence?
Emax = Emean + gb * sigma EB
gb = 3.5
In the Jb equation, what is Bb roughly equal to?
0.95
