low speed wind tunnel

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OVER VIEW• Introduction• Structure of wind tunnel• Types of wind tunnel• Design• Advantages• Disadvantages• Applications• Conclusion

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INTRODUCTION Three conditions are present in anemology

(study of winds):1.Specimen is in stationary, wind is in motion. Example: civil structures.2. Specimen is in motion, wind is stationary.Example: An athlete, ships, high speed cars.3.Both the specimen and wind is stationaryExample:Two kinds of forces develop when the above

actions take place, drag and lift force.

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Two kinds of forces develop when the above actions take place, drag and lift force.

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Drag: Force which act on a solid object in the direction of the relative flow velocity.

DF= 0.5 CD ρA V Lift: Force which acts on a solid in a

direction perpendicular to the relative flow velocity direction

LF= 0.5 CL ρA V FD=drag force LF=lift force ρ=mass density of the flow A=reference area v=velocity of object CD=coefficient of drag CL=coefficient of lift

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Testing of scaled down building in a wind tunnel.

A bicycler subjected to test conditions in a wind tunnel.

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Pressure distribution:

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Wind tunnel: A wind tunnel is a machine which can simulate the movement of air around an aircraft in flight. In the wind tunnel, the aeronautical engineer can control the conditions that affect the forces and motion of the aircraft.

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1. HONEY COMB: Outside air enters through the vent.

2. CONTRACTION SECTION: The air moves from wider area into narrow area. Air moves faster here.

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3. Test section: It provides desirable flow condition and spacefor model testing or experiment, where the instrumentation are situated.

4. Guide vanes: To guide the flow through the turning duct, and reduce the extent of secondary flows.

5. Diffuser: A device to lower the air flow speed, consequently reduce the pressure loss due to friction.

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Wind tunnels are classified based according to the following parameters:on

1. Based on type of loop

2.Based on flow Speed

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Based on type of loop

1.Open loop wind tunnel: This type of wind tunnel is open at both ends. The chances of dirt particles entering with air are more so more honeycombs (mesh to clean incoming air) are required.

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2.Closed loop wind tunnel: Outlet of such wind

tunnel is connected to inlet so the same air circulates in the system in a regulated way. The chances of dirt entering the system are also very low. closed wind tunnels have more uniform  flow than open type. This is usually a choice for large wind tunnels as these are more costlier than open type wind tunnels.

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Based on flow speed

1. Transonic wind tunnels:Maximum velocity in test section of transonic wind tunnels can reach upto speed of sound i.e 340m/s or Mach number of 1. These wind tunnels are very common in aircraft industry as most aircrafts operate around this speed.

2. Supersonic wind tunnels:Velocity of air in test section of such wind tunnels can be upto Mach 5. This is accomplished using convergent - divergent nozzles. Power requirements for such wind tunnels are very high.

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3. Hypersonic wind tunnels:Wind velocity in test section of such type of wind tunnels can measure between Mach 5 and Mach 15. This is also achieved using convergent - divergent nozzles.

4. Subsonic or low speed wind tunnels:Maximum flow speed in this type of wind tunnels can be 135m/s. Flow speed in wind tunnels is generally preferred in terms of Mach number which comes out to be around 0.4 for this case. This type of wind tunnels are most cost effective due to the simplicity of the design and low wind speed.

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DESIGN:The first step in the design of a tunnel is to determine the size and shape of the test section ba.sed on the intended uses.

It is also assumed that a major part of the testing will be force testing, where information is sought for performance, fue1 efficiency, stability, or control of a vehicle that may be an aircraft, an automobile, a submarine, a racing yacht, or possibly others.

The air density remains nearly constant and decreasing the cross-sectional area causes the flow to increase velocity and decrease wind pressure.

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Increasing the area causes the velocity to

decrease and the pressure to increase. We want the highest possible velocity in the test section.

We can design the test section to produce a desired velocity or Mach number since the velocity is a function of the cross-sectional area.

We can design the test section to produce a desired velocity or Mach number since the velocity is a function of the cross-sectional area.

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For subsonic wind tunnels

Mach number <1

The plenum is the settling chamber on a closed return tunnel, or the open room of an open return design.

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ADVANTAGES:Today’s architects and engineers challenge the traditional boundaries of their to create buildings and structures as true art expressions.

Progressive designers and engineers understand that significant savings incost of structure and cladding can be obtained by completing a wind tunnel test in the early stage of the design.

Accurate conditions required by the object is achieved.

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DISADVANTAGES:If located in a room, depending on the size of the tunnel to the room.

For a given size and speed the tunnel will require more energy to run.

In general, a tunnel is noisy. For larger tunnels (test sections of 70 sq.ft. and more) noise may cause environmental problems and limits on hours of operation.

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Aerospace: Wind tunnel is utilized to study aircrafts in free flight. Automotive: In the automotive industry, wind tunnels are used to study the effects of moving air on the drag forces and energy consumption of a vehicle moving on the road.Architecture: The structural stability of a tall/large building might be compromised if the forces of the wind arenot taken into consideration. Often, wind tunnels are used to test those structures on a scaled down model.Sporting goods: Several applications are found in field of sporting goods, from bicycles, through the design of swimming suites, game balls that are affected by wind.

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Thus a wind tunnel is one of the main tools in aerospace and automotive industry.

It is used to determine the coefficients of drag and lift which cannot be determined theoretically.