Wind tunnel testing

Question 1

What are the four basic parts of wind tunnels?

Answer 1

1) A contoured duct to control the passage of the working fluid through the test section where the model is mounted
2) A drive system to move the working fluid through the duct
3) A mode of the test object that is either full size, or more often, a reduced-scale model
4) Instrumentation equipment ( to measure wind force, pressure)

Question 2

What are the two basic types of wind tunnels and two basic test section configurations?

Answer 2

1) Open circuit tunnel: Air follows a straight path
- The tunnel may have a test section with no solid boundaries (open jet)
- The test section has solid boundaries (closed jet)

2) Closed return wind tunnel: This tunnel has a continuous path for air
-Can have both closed or open test section

Question 3

What are the advantaged and disadvantages of each wind tunnel type?

Answer 3

1) The open circuit tunnel
i) Advantages
- Construction cost is less
- There is no purging problem

ii)Disadvantages
- May require extensive screen to produce high quality flow
- More energy required
- Noisy

2) Closed return circuit
i)Advantages
- Flow quaility
- Less energy
- Less noise

ii)Disadvantages
- Higher cost
- Smoke needs to be purged
- It may need an air exchanger for cooling

Question 4

Why are most wind tunnel tests done in reduced scale models?

Answer 4

Because the cost of operating the tunnel, building a model, making tests during the test programme is expensive.

Blockage effects can cause flow distortion if greater than 5%.

b = S/A, where S is the body cross sectional area and A the cross sectional area of the WT

Question 5

What is an important parameter for simulation of ABL?

Answer 5

The correct model test for phenomena in the wind must be carried out in a turbulent boundary layer and the model required that the BL to be scaled are regard the velocity profile.

(h/zo)model = (h/z0)prototype

Question 6

What characteristics are to be simulated in the WT?

Answer 6

1) The variation of the mean wind speed with height
2) Variation of the trbulence intensitied with height
3) Variation of the integral scaled with height
4) Spectra of the turbulence in the along-wind, across wind and vertical directions

Question 7

How to obtain a fully developed ABL?

Answer 7

To obtain a fully developed ABL long roughness sections are required.
1) Spires can be used, where large scale turbulence is generated
2) Rougness elements can be used

Question 8

How to simulate vertical profile wind speed in ABL?

Answer 8

The vertical profile wind speed should be similar to the value expected in full scale. U(z) = U(zref) *(z/zref)^a

Question 9

How to simulate profile turbulence intensity?

Answer 9

The model and the full scale values should be the same

Question 10

How to simulate integral length scale?

Answer 10

It is scaled as the structure.

Question 11

How to measure the flow velocity?

Answer 11

1) Pitot tube is the most common device for determining the flow velocity inside the test section. It must be aligned to the airstream, otherwise there is error in measurement. Generally it is used in free air to measure mean test wind velocity. It has a low frequency response.

Turbulence may influence Pitot readings

2) Flow direction probes, to measure also flow direction and velocity magnitude. High frequency response.

3) Hot wires anemometes. Can measure up three velocity components simultaneously. Heat trasnfer is function of the fluid velocity.

Question 12

What is a method to derive similutide laws for the dimensional analysis?

Answer 12

1) Equations governing the phenomenon can also be used, as the parameters that affect the problem are correctly specified.

2) Buckingham Pi theorem can be used. Knowledge of parameters that influence the phehnomenon is required

Question 13

What are the differenced between Rigid model and aeroelastic model?

Answer 13

1) Rigid or static model
- Reproduced the external features of the prototype
- The tests are used to define the wind forces in terms of static and fluctuating components.

• The dynamic response of the structure is evaluated by using the measured wind forces together with numerical models that simulate the dynamic response.
• Simple to make and is used in the early stage when the structural design is still uknown
• Results can be used if the structural design is unchanged
• This method can be used if the structure mothion doesnt influence the fluid structure interaction, that is no important motion induced forces exist
• Used for very stiff and damped structures like low and medium rise buildings

2) Aeroelastic model
- Reproduces the external features of the prototype but also the structural properties

• The dynamic response is directly measured
• Aeroelastic phenomena are correctly simulated
• Used for studying light, slender and flexible structures

Question 14

What is the Froude number and how it is defined?

Answer 14

It is used when both elastic and gravity forces contribute to stifness

Fr = U^2/gD = Inertia/Gravitational

Question 15

What is Scruton number and how it is defined.

Answer 15

It is the product of the mass ratio and critical damping ratio

Scr = 2p mh/ρD^2

Question 16

What is the governing similitude in rigid model tests?

Answer 16

1) Parameter scale: λp = pmodel/pprototype

Question 17

How are the loads measured?

Answer 17

It is necessary to know both the average and fluctuating part of the force signal. Instruments can measure only one component (load cells) or up to 3 force components and 3 moments (force balance)

1) Overall load acting on the model can be directly measured with a force balance. They are generally strain gauged elements.

2) Load cells is capable of measuring the load along a single axes and is based on strain gauges.

3) Surface pressure measurements. Large number of measured pressures is needee

Question 18

When are sectional models used?

Answer 18

Sectional models are used to study the wind action on slender structures, like long-span bridges, towers, and cables.

The model is a representation of a section of the structure and it is scaled to partially reproduce the dynamic properties of a structure.

• Model can be mounted on springs to have a scaled mass, moment of inertia, damping and frequency to reproduce a free-motion behavior.
• It is also possible to use an external system to drive the motion

Question 19

What are aeroelastic models?

Answer 19

Aeroelasticity described phenomena involving the interaction between aerodynamic forces and structural deformation or motions.
- If strucutre motion may add to or modify aerodynamic forces
-if the aeroelastic effects, such as aerodynamic damping are uncertain and can be neigther neglected nor estimated

The body can produce additioal important forces not experienced by a rigid model

Therefore an aeroelastic model is required.

Question 20

What is a problem that occurs in similitude requirements for aeroelastic modelling?

Answer 20

The reynold number and froude number are in contrast and therefore cannon be scaled simultaneously. Usually this is overcome by checking the Re effect, so that the wind loads are independed of the Re number.

Re: λv = 1/λL
Fr: λv = sqrt(λL)