Wind-Tunnel Techniques - Wind Engineering - Lecture Slides, Slides of Environmental Law and Policy

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• Original wind tunnel of W.C. Kernot - 1893

Contraction

Propeller

Gas engine

Fly wheel Air jet

Model mounted on 3-wheel carriage

• Simulation of atmospheric boundary layer :

Natural growth method :

Boundary layer is grown naturally over surface roughness elements

Boundary layer thickness is usually too small to model complete atmospheric boundary layer - use auxiliary ‘tripping’ devices

10-15 m

• Simulation of atmospheric boundary layer :

Methods for short test sections :

Other devices : triangular ‘spires’ , graded grids

Counihan method

Fins

Castellated barrier

hT

Roughness

~4hT

• Simulation of hurricane boundary layers :

Near eye wall : steep profile up to about 100 metres - then nearly constant

Can use non-hurricane boundary layer for rougher terrain in wind tunnel simulations

Turbulence is higher in hurricanes (but ‘patchy’)

• Simulation of thunderstorm downburst by impinging jet :

Jet Working section

Contraction Diffusing section

Vertical board Blower

Stationary downbursts only are modelled - continuous not transient

• Modelling rules - dimensional analysis :

For example, reduced frequency :

Non-dimensional numbers may not be independent

i.e. proportional to square root of the Cauchy Number divided by the density ratio

s

a 2 a

2 s

s ρ

ρ . ρ U

E

K

U

L

ρ L

E

K

U

n L  

• Modelling rules - dimensional analysis :

Scaling requirements might be relaxed

Not possible to obtain equality of all non-dimensional groups

Judgement based on experience and understanding of mechanics of the

phenomena

Quality assurance manuals and standards for wind-tunnel testing are

now available - e.g. A.W.E.S. , A.S.C.E.

• Area-averaged pressures :

Discrete averaging overestimates continuous average fluctuating loads

Rd discrete averaging Rc continuous averaging

Rd Rc

Assumed correlation function = exp (-Cr)

B B 2

0 0 2 4 6 8 10 CB

Variance of averaged panel force to variance of point pressure

Overestimation depends on correlation between point pressures on the area

• Frequency response of measurement system :

Require amplitude response ratio equal to 1.0 ( +/- small error) over a

defined frequency range

0

1

0 100 200 300 400 Frequency (Hertz)

Amplitude ratio

System within +/- 15% limits to 150 Hertz

• Types of tubing systems :

Short tube : high resonant frequency but amplitude response rises fast

Restricted tube : restrictor tube damps resonant peak

Leaked tube : high pass filter, mean response is also reduced

Transducer volume (a) Short tube

Restrictor

(b) Restricted tube

Controlled leak (c) Leaked tube

• Overall loads on tall buildings :

Two techniques : aeroelastic models - resonant structural response is scaled

Base-pivotted aeroelastic model :

Gimbals

Springs

Strain gauges Aluminium^ Electromagnet disc

h

motion of building in sway modes of vibration are reproduced - hence aeroelastic (e.g. aerodynamic damping) forces are included

Uses equivalence of rigid body rotation and movement of tall building in first mode with linear mode shape

Model should be scaled to have the same density

• High-frequency base balance :

Support system should be made very stiff, and building model light to keep frequency above measurement range

Frequency relationships :

U 1 (>U 2 ) U 2

Simulated building frequency

Model frequency in wind tunnel

Spectral density

Usable frequency range for measurements

• Full aeroelastic models :

Elastic properties are concentrated in a ‘spine’ to which non-structural segments are attached to give correct aerodynamic shape and mass

Slender structures such as bridges and towers

Length scale ratio and velocity scale ratio chosen to suit size and speed range of wind tunnel

Frequency then obtained by equality of reduced velocity : p

s m

s U

n L U

n L  

  

  

  



Stiffness of spine obtained by requirement to keep frequency of structure equal in model and full scale