wind tunnel technology

7/27/2019 Wind Tunnel Technology

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WIND TUNNEL


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Research tool used in aerodynamic research-study the effects of air

moving past solid objects.

Instead of the air's standing still and the aircraft/vehicle moving at speed

through it, the same effect would be obtained if the vehicle stood still and

the air moved at speed past it.

Stationary observer could study the aircraft/vehicle in action, and could

measure the aerodynamic forces being imposed on the aircraft/vehicle.

Wind tunnels are used to predict the amount of force generated by solid

objects.

This helps aerodynamicists choose the proper size for things such as

wings, spoilers, and parachutes.

Information obtained in wind tunnels is used to improve the design of

anything affected by wind.


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WIND TUNNELAUTOMOBILES:

To determine not only aerodynamic forces but also to determine ways

to reduce the power required to move the vehicle on roadways at a

given speed.

Unlike Aircraft, the interaction between the road and the vehicle plays

a significant role, and this interaction must be taken into consideration

when interpreting the test results

Moving BeltsApproximate the actual condition

Automotive companies employ wind tunnels to analyze their latest

models-test an entire vehicle, they will also evaluate the aerodynamicsof individual components, such as grilles, side view mirrors, air dams,

rear-deck spoilers and roof racks.

In race car industry- Race teams routinely subject scale models to

wind tunnel testing as well as full-size cars or individual parts


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Wind-Tunnels come in all shapes and sizes

Wind tunnels are either open or closed-return.

They can have open or closed test-sections where the

models are tested.

They can be as large as big buildings or fit on tabletops.

They use fans, compressors, or high-pressure tanks to move

air, or other fluids through the test section.

The test sections are round, square, rectangular, or other

shapes like hexagonal


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EIFFEL TYPE TUNNELopen air returnTakes air from the surroundings & expels it to the surroundings

Designed to return the air with the lowest possible losses

Operation is dependent upon the weather Air flow in test section free from wind

effects More Power Loss Excessive Noise

Economical & Simple Construction


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GOTTINGEN TYPE TUNNELClosed air returnFan drives the air in a closed circuit

Low Power Reduces the Operating Cost

Automobile Model - PlastilinaLoses its stability at higher temperaturesCooler

power lossAir conditioning & Climatic Work


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National Wind Tunnel Facil i ty, I I TK

Special Capabil i ties

Full-model testing with sting support system

2-D model testing using turntables Open jet testing

Flow diagnostic using PIV & visualization

Ground effect simulation with moving belt

Gust and cross-wind simulation

ABL simulation for civil applications

Important features

High Reynolds Number simulation

Very low turbulence

Interchangeable mobile test sections

Automated measurement & control systems


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WIND TUNNELCONSTRUCTION ELEMENTS:

Determine the Size, Performance & Quality of a Wind Tunnel

TEST SECTION

NOZZLE & SETTLING CHAMBER

FAN & DRIVECOOLER

TEST SECTION

Determine the Overall Size of Wind Tunnel

Governing Parameter-Cross- Section Area of the Wind Tunnel NozzleAT

Blockage Ratio (BR) should be as small as possible (on the road it should be zero)

Aircraft Aerodynamics BR < 0.05

GM Automobile wind tunnel only meet this requirement

But Higher BR Automobile Aerodynamics


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Open Test Section:

Free Jet

Air Stream 3 Free boundaries

Air from the test stream mixes with the surrounding air as a

free jetLimits the usable length of the test section

Gradient of the Static Pressure along the Tunnel axis is

negligibleNo errors in measuring drag

Drag of longer bodies with large vertical surfaces at the frontand rear can be measured without errors

Blockage Ratio is less compared to Closed parallel Test Section

Easy Accessible Facilitates Experimenting & Photographing

the flowHigher Loss coefficient of the free jet unimpeded sound

radiation


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Closed Test & Streamlined Section:

Larger usable length of the test section

Air stream is dissipated much more slowly along a closed duct

than in open jetFriction loss along the walls Result in a Pressure Decrease

along the axis of the stream

Pressure Decrease is compensated by slightly widening the

tunnel cross section in the flow direction

Blockage Value is double of free jet Overcome by the

Streamlined walls

Frontal Area of Average Passenger Car 1.85 m - Deviation

between the large & Small cars is no more than +/- 15%

Tunnel Walls are shaped according to the flow pattern of theaverage car in the open airair flow for smaller & larger vehicles

will be slightly distorted


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Slotted Wall:

Combine the Advantages & Eliminate the disadvantages of open &

Closed test sections

Water Tunnels & Wind Tunnels in Marine HydrodynamicsOpen Area Ratio (free surface to Covered surface) must be

calibrated so that the pressure distribution is similar to the

open air


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VEHICLE SCALING

Full-scale testing avoids the scaling problems with models, it requires

large wind tunnels and is expensive.

Scale model testing, which is comparatively inexpensive and more

convenient for shape modifications, is widely used in the developmentof new products

In the United States, for passenger cars 3/8 scale is widely used, while

in Europe 1/4 scale is the most common, though 1/5 scale is also used in

small wind tunnels For commercial vehicles, a scale of 1/2.5 is recommended


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FUSED DEPOSITION MODELING (FDM)

FDM is a faster, less expensive and more efficient methodfor making detailed and accurate test models

. FDM materials are some of the strongest available in the

additive fabrication market, Polycarbonate (PC), PC-ABS andpolyphenolsulfone (PPSF)

It can withstand the forces and stresses induced as the air

flow strikes the models surface


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Model Testing

Problems With Model Testing Not possible to match Reynolds Number

Wind Tunnel cannot reach necessary speeds

If it could, Mach number would be too large and wed have to worryabout compressibility

Some ways to fix this problem are: A larger wind tunnel with larger models

A different testing fluid with a higher density

Pressurizing and/or adjusting the air temp in the wind tunnel

Or in our case running the wind tunnel at several velocities andextrapolating to determine useful information.


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Basic Problems:

1. Flow Field Similarity:

Similarity between the flow pattern in the wind tunnel and that under

actual driving conditions on the road

Reynolds Number (RN) for the scale model be equal to that for the full-

size vehicle

The Reynolds Number is the ratio of the product of airstream speed and

the characteristic length of the vehicle to the kinematic viscosity of the air.( V x Lc / )

Flow Field Similarity

To satisfy this requirement, a 3/8 scale model should, therefore, be teste

in the wind tunnel at an airstream speed of 8/3 of that of the full-sizevehicle.

Blockage ratio, which is the ratio of the frontal area of the model (or the

full-size test vehicle) to the cross sectional area of the wind tunnel test

section, should be as small as possible- preferably not exceeding 5%


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2.Modeling Of The Ground Plane:

When a vehicle is driven on the road at zero wind speed, the air is at

rest relative to the road.

In a conventional wind tunnel, the air flows with respect to the tunnelfloor and a boundary layer builds up.

This may significantly affect the flow pattern under the scale model

(or full-size test vehicle).

To alleviate this problem, a moving ground plane has been used.Air FlowTwo Fieldsone resulting from the forward motion of the

vehiclethe other from the natural wind

Natural Wind Boundary Layer of the Wind is Turbulent

Flow Field approaching the VehicleLargely InhomogeneousNon

StationaryMore Complex than that of aircraft

Non Homogenous Temperature Field above the road Sunlight will

heat the roadway more than the surrounding air


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The Experiments

Experiment 1- Surface Pressure measurements

Experiment 2- Lift and Drag measurements

Experiment 3- Particle Image Velocimetry, CFD

analysis


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Results - Pressure


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Coefficient of Pressure


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Lift and Drag

The model, was connected to a

dynamometer that measured

force in both the x and y

direction, essentially lift and

drag.

This data was collected using a

data acquisition system as well,

and processed with a PC.

Using these measurements it

was possible to calculate liftand drag on the car, as well as

lift and drag coefficients.

Source: Brad Bruno

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Particle Image Velocimetry

PIV uses the wind tunnel

along with a double pulsed

laser technique to measure

instantaneous velocity and to

map out the flow field. This provides a visual

representation of the flow

along the vehicle, streamlines

and a qualitative

representation of the

velocities.

Source: Brad Bruno


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Results - PIV

Flood Contour of Ford NASCAR

Streamline Contour of Ford NASCAR


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FLOW VISUALIZATION TECHNIQUES


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FLOW VISUALIZATION TECHNIQUES:

Visualize the flow on the Vehicle Body Spatial Flow close to the

vehicleair flow pattern in the passenger compartment

Investigate & Understand the flow field in and around the Vehicle.

Flow Pattern adjacent to vehicle body wool tuftsFlow

Attachment & Flow Separation can be easily detected

Surface Oil Film having colored pigmentsflow separation will

not be indicated clearlySmoke GeneratorEmit Smoke in to the air flowFlow Patterns

will be made visible

Smoke Generator heating a mineral oil derivativeuntil

evaporation occursdense white inoffensive smoke is providedlong thin stem to inject the smoke in to air flow

Bubble Generatorillustrate the flow pattern in separated flow

Helium filled soap bubbles are injected in to the air flow

paths are photographed


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