Essay: Open-loop subsonic wind tunnel

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ABSTRACT

Wind tunnel is a device used for defining various flow phenomena for various objects like cars, airplanes etc. In automobiles and airplanes various forces affect the efficiency and speed which calls for a testing of new vehicle-scaled models in a wind tunnel testing machine. From this, aerodynamic estimates can be calculated and used for vehicle and component sizing (profile edge rounding, bonnet angle, rear end taper, under body rear end taper etc.), performance estimates and evaluation.

By looking at the way a smaller model acts in the wind tunnel, a pretty good idea of how a real-sized vehicle of the same design will probably work can be obtained.

In this project work, we have designed an open-loop subsonic wind tunnel on Autodesk-INVENTOR. We have also fabricated the experimental setup and successfully visualized the air flow over an Aerofoil model mounted in the test section. This project can be further used by the students to carry out various aerodynamic tests like measuring the lift and drag forces on an automobile model or checking the stability of architecture structures etc.

It is a lot easier, cheaper and safer to build and test a model than to build and test on real models. Also the experiment can be conducted in a well-controlled manner rather than real life environment condition

CHAPTER-1

INTRODUCTION

WIND TUNNEL

History

The earliest wind tunnels were invented towards the end of the 19th century. The wind tunnel was envisioned as a means of reversing the usual paradigm: instead of the air standing still and an object moving at speed through it, the same effect would be obtained if the object stood still and the air moved at speed past it. In that way a stationary observer could study the flying object in action, and could measure the aerodynamic forces being imposed on it. The would-be aeronauts of the nineteenth century closely studied the flight of birds and began building flying machines patterned after avian structures. Their birdlike craft failed miserably. They quickly realized that in reality they knew nothing about the lift and drag forces acting on surfaces cutting through the atmosphere. To fly, man first had to understand the flow of air over aircraft surfaces. This meant that he had to build instrumented laboratories in which wings, fuselages, and control surfaces could be tested under controlled conditions. Thus it is not surprising that the first wind tunnel was built a full 30 years before the Wrights’ success at Kitty Hawk.

English military engineer and mathematician Benjamin Robins (1707–1751) invented a whirling arm apparatus to determine drag and did some of the first experiments in aviation theory. His first machine had an arm 4 feet long. Spun by falling weight acting on a pulley and spindle arrangement, the arm tip reached velocities of only a few feet per second.

Sir George Cayley (1773-1857) also used a whirling arm to measure the drag and lift of various airfoils. His whirling arm was 5 feet long and attained tip speeds between 10 and 20 feet per second.

The whirling arm provided most of the systematic aerodynamic data gathered up to the end of the nineteenth century. Its flaws, however, did not go unnoticed. Test results were adversely influenced as the arm’s eggbeater action which set all the air in the vicinity in rotary motion. Aircraft models on the end of an arm, in effect, flew into their own wakes. With so much turbulence, experimenters could not determine the true relative velocity between the model and air. Something better was needed.

That something better was a “wind tunnel.” This utterly simple device consists of an enclosed passage through which air is driven by a fan or any appropriate drive system. The heart of the wind tunnel is the test section, in which a scale model is supported in a carefully controlled airstream, which produces a flow of air about the model, duplicating that of the full-scale aircraft.

Figure 1.1 Whirling Arm

Fracis H. Wenham (1824-1908), a Council Member of the Aeronautical Society of Great Britain, is generally credited with designing and operating the first wind tunnel in 1871.

In 1941 the US constructed one of the largest wind tunnels at that time at Wright Field in Dayton, Ohio. This wind tunnel starts at 45 feet (14 m) and narrows to 20 feet (6.1 m) in diameter. Two 40-foot (12 m) fans were driven by a 40,000 hp electric motor. Large scale aircraft models could be tested at air speeds of 400 mph (640 km/h).

By the end of World War Two, the US had built eight new wind tunnels, including the largest one in the world at Moffett Field near Sunnyvale, California, which was designed to test full size aircraft at speeds of less than 250 mph.

The advances in computational fluid dynamics (CFD) modelling on high speed digital computers has reduced the demand for wind tunnel testing.

However, CFD results are still not completely reliable and wind tunnels are used to verify the CFD computer codes. For limited applications CFD can supplement or possibly replace the use of wind tunnels. For example, the experimental rocket plane Spaceship One was designed without any use of wind tunnels but where external turbulent flow is present; CFD is not practical due to limitations in present-day computing resources.

Wind tunnel

A wind tunnel is a device designed to generate air flows of various speeds through a test section. Wind tunnels are typically used in aerodynamic research to analyze the behaviour of flows under varying conditions, both within channels and over solid surfaces. The wind tunnel moves air around an object, making it seem like the object is really flying.

Figure 1.2 A wind tunnel test shows how a tennis ball moves through the air

Engineers can use the results obtained from the wind tunnel testing experiments to inexpensively tweak designs for aerodynamic performance without building numerous prototypes.

Types Of Wind Tunnels

Based on the path followed by the drawn air:-

Open return wind tunnel

Closed return wind tunnel

Based on speed of air in the test section:-

Subsonic wind tunnel High Supersonic wind tunnel

Transonic wind tunnel Hypersonic wind tunnel

Supersonic wind tunnel High Hypersonic wind tunnel

Based on the nature of the flow:-

Laminar flow wind tunnel

Turbulent flow wind tunnel

Boundary- layer wind tunnels are used to simulate turbulent flow near and around engineering and manmade structures.

Range of the Mach number, [M] Name of flow or conditions

M<1 Subsonic

M=1, or near 1 Transonic

1<M<3 Supersonic

3<M<5 High supersonic

M>5 Hypersonic

M>> 5 High Hypersonic

Table 1.1 Classification of various wind tunnels according to the Mach number.

PROBLEM STATEMENT

This report will focus primarily on the principle of working of a wind tunnel, the flow visualization inside a wind tunnel, materials required for fabricating a small scale, open loop, subsonic wind tunnel and various advantages, disadvantages and application of the wind tunnel.

OBJECTIVES

To design an open-loop, subsonic, suck-down type wind tunnel on Autodesk-INVENTOR.

To fabricate the experimental setup for educational and research purpose.

To get an impression of fluid flow around a scale model of a real object.

PROJECT SCOPES

To find the design fundamental for a small scale, open loop, subsonic wind tunnel.

Make the research for small wind tunnel background and construction.

To find the best material to be used and estimate the cost for model construction.

To study the flow visualization of an object design.

WORKING OF A WIND TUNNEL

Air is drawn through a duct equipped with a viewing window and instrumentation where models or geometrical shapes are mounted for study. For very large wind tunnels, several meters in diameter, a single large fan is not practical, and so instead a group of multiple fans are used in parallel to provide sufficient airflow.

The airflow created by the fans that is entering the tunnel is itself greatly turbulent due to the fan blade motion (when the fan is blowing air into the test section – when it is drawing air out of the test section downstream, the fan-blade turbulence is not a factor), and so is not directly useful for accurate measurements. The air moving through the tunnel needs to be relatively turbulence-free and laminar. To correct this problem, closely spaced vertical and horizontal air vanes are used to smooth out the turbulent airflow before reaching the subject of the testing.

Due to the effects of viscosity, the cross-section of a wind tunnel is normally circular rather than square, because there will be greater flow constriction in the corners of a square tunnel that can make the flow turbulent. A circular tunnel provides a smoother flow.

The inside facing of the tunnel is typically as smooth as possible, to reduce surface drag and turbulence that could impact the accuracy of the testing. Even smooth walls induce some drag into the airflow, and so the object being tested is usually kept near the center of the tunnel, with an empty buffer zone between the object and the tunnel walls.

Figure 1.3 Schematic of an open wind tunnel with a closed test section

The lighting is usually fixed into the circular walls of the tunnel and shines in through windows. If the light were fixed on the inside surface of the tunnel in a conventional manner, the light bulb would create turbulence as the air blows around it. Similarly, observation is usually done through transparent portholes into the tunnel. Rather than simply being flat discs, these lighting and observation windows may be curved to match the cross-section of the tunnel and further reduce turbulence around the window.

Various methods are used to study the actual airflow around the geometry and compare it with theoretical results, which must also take into account the Reynolds number and Mach for the regime of operation.

FLOW VISUALIZATION

Lift and drag are just two elements of aerodynamics forces that come into play inside a wind tunnel. For aircraft testing in particular, there are dozens of variables (like pitch, yaw, roll and many others), that can affect the outcome of experiments.

Figure 1.4 Smoke provides flow visualization so scientists can see how air is moving around the test object

Because air is transparent it is hard to directly observe the air movement itself. Several methods of visualizing the flow have been developed. Some of them include-

Tufts

Tufts are applied to a model and remain attached during testing. Tufts can be used to gauge air flow patterns and flow separation.

Figure 1.5 Wing with mini-tuft

Evaporating suspensions

Evaporating suspensions are simply a mixture of some sort or fine powder, talc, or clay mixed into a liquid with a low latent heat of evaporation. When the wind is turned on the liquid quickly evaporates leaving behind the clay in a pattern characteristic of the air flow.

Figure 1.6 China clay on a wing

Oil

When oil is applied to the model surface it can clearly show the transition from laminar to turbulent flow as well as flow separation.

Figure 1.7 Oil flow visible on a straight wing

Fog

The fog is transported inside the wind tunnel (preferably of the closed circuit & closed test section type). An electrically heated grid is inserted before the test section which evaporates the water particles at its vicinity thus forming fog sheets. The fog sheets function as streamlines over the test model when illuminated by a light sheet.

Figure 1.8 Fog wind tunnel

Sublimation

If the air movement in the tunnel is sufficiently non-turbulent, a particle stream released into the airflow will not break up as the air moves along, but stay together as a sharp thin line. Multiple particle streams released from a grid of many nozzles can provide a dynamic three-dimensional shape of the airflow around a body. As with the force balance, these injection pipes and nozzles need to be shaped in a manner that minimizes the introduction of turbulent airflow into the airstream.

High-speed turbulence and vortices can be difficult to see directly, but strobe lights and film cameras or high-speed digital cameras can help to capture events that are a blur to the naked eye.

High-speed cameras are also required when the subject of the test is itself moving at high speed, such as an airplane propeller. The camera can capture stop-motion images of how the blade cuts through the particulate streams and how vortices are generated along the trailing edges of the moving blade.

EXPERIMENTAL RIG

Figure 1.9 Wind tunnel available in the market

The unit is equipped with a bench, control panel, wind tunnel including an inlet cone, clear experiment section, outlet cone and screen; manual traverse unit, linear track with carrier; and main AC circuit breaker.

The lift and drag option on the H-6910 allows measurement of lift and drag forces on various shapes placed in the wind tunnels airstream. The readings are displayed on a digital meter and the selection between lift and drag is accomplished with the toggle switch located on the meter front panel.

DIFFERENT PARTS OF A WIND TUNNEL

Test Section

The test section is the “heart” of the wind tunnel where the model to be tested is located. The test section is the most delicate part of the tunnel, because it houses the model and includes the lift-and-drag sensory system. This part of the wind tunnel will have the highest air velocity. It will look like the following:

Figure 1.10 Test Section

Diffuser Assembly

This assembly houses the fan and the wind speed sensor. It is perhaps the easiest of the three assemblies to build. This is the largest assembly, and it is the only one that is made up of electronic components and wiring. It will look like the following:

Figure 1.11 Diffuser Assembly

Contraction Cone Assembly

This assembly will be at the forward end of the tunnel, into which the air will flow as it is drawn in by the fan at the back. This assembly consists of the Contraction Cone and the Settling Chamber. It has a provision for increasing the speed of air, taken from the room, before it enters the contraction cone. It will look like the following:

Figure 1.12 Contraction Cone Assembly

Honeycomb

It straightens the flow of air before it enters the contraction cone. A honeycomb naturally produces some turbulence of its own. The early wind tunnels which had a honeycomb but no screens (and usually a very small contraction ratio also) suffered from a very high turbulence intensity in the test section. Most of the modern tunnels have both honeycomb and screens.

Figure 1.13 Honeycomb

DEVELOPMENT OF PROTOTYPE

In this project work we have designed an open-loop, subsonic, suck-down type wind tunnel, fabricated the experimental setup and visualized the air-flow over an aerofoil model mounted in the test section.

We have used Autodesk-Inventor (CAD software) to develop the design of the wind tunnel. We have also used Autodesk-CFD software to analyze the design made on Inventor. This software was a great help to ensure the stability of the design through the various static-stress, pressure and velocity profile which could easily be obtained form it.

Entire body of the wind tunnel has been made from 18mm thick ply-wood sheets. And for the test section 5mm thick Plexiglass sheet (Acrylic Sheet) has been used. The flow visualization will be done using a High Speed Camera.

Figure 1.14 Wind tunnel design made on INVENTOR

CHAPTER-2

ADVANTAGES & DISADVANTAGES OF WIND TUNNEL

ADVANTAGES OF THE WIND TUNNEL

It is a lot easier, cheaper and safer to build and test a model than to build and test on real model.

Experiment can be conducted in a well-controlled manner rather than real life environment conditions.

Data acquisition and processing is simpler with direct connection to ground base equipment.

Dangerous, uncontrollable flight conditions will be safely investigated in a wind tunnel.

If one intends to run internal combustion engines or do extensive flow visualization via smoke, there is no purging problem provided both inlet and exhaust are open to the atmosphere.

DISADVANTAGES OF THE WIND TUNNEL

The main disadvantage of wind tunnel is that it is seldom possible to reproduce the condition of full scale motion exactly. This is mainly due to the use of scaled models for reason of tunnel cost and power consumption.

Also if located in a room, depending on the size of the tunnel to the room size, it may require extensive screening at the inlet to get high-quality flow. The same may be true if the inlet and/or exhaust is open to the atmosphere, when wind and cold weather can affect operation.

For a given size and speed the tunnel will require more energy to run. This is usually a factor only if used for developmental experiments where the tunnel has a high utilization rate.

In general, open circuit tunnels tend to be noisy. For larger tunnels (test sections of 70 ft. and more) noise may cause environmental problems, limit hours of operations, and/or require extensive noise treatment of the tunnel and surrounding room.

CONCLUSION: Because of the low initial cost, an open circuit tunnel is often ideal for schools and universities where a tunnel is required for classroom work and research and high utilization is not required.

APPLICATIONS/USES OF WIND TUNNEL

To determine aerodynamic loads:-

Wind tunnels are used to determine aerodynamic loads on the immersed body. The loads could be static forces and moments or dynamic forces and moments. Examples are forces and moments on airplane wings, airfoils, and tall buildings.

To study how to improve energy consumption by automobiles:-

They can also be used on automobiles to measure drag forces with a view to reduce the power required to move the vehicle on roads and highways.

To study flow patterns:-

To understand and visualize flow patterns near, and around, engineering structures. For example, how the wind affects flow around tall structures such as sky scrapers, factory chimneys, bridges, fences, groups of buildings, etc. How exhaust gases emitted by factories, laboratories, and hospitals get dispersed in their environments.

Other uses include:-

To teach applied fluid mechanics, demonstrate how mathematical models compare to experimental results, demonstrate flow patterns, and learn and practice the use of instruments in measuring flow characteristics such as velocity, pressures, and torques.

Figure 2.1 Testing of Different Types of Vehicles in a Wind Tunnel

CHAPTER-3

LITERATURE REVIEW

Design Construction and Performance Test of a Low Cost Subsonic Wind Tunnel

Md. Arifuzzaman, Mohammad Meshed

Wind tunnel is a device, by artificially producing airflow relative to a stationary body that measures aerodynamic force and pressure distribution to simulate with actual conditions. Wind Tunnels offer a rapid, economical, and accurate means for aerodynamic research. The most important aspect of wind tunnels is their ability to accurate recreate the full complexity of full fluid flow. In the current study, a low cost subsonic wind tunnel is designed, constructed and its performance is tested. The main focuses were to reduce the cost of construction and to erect it in a laboratory room.

The main design criteria used are listed in the table below:

Open circuit wind-tunnel.

Good flow quality (mean flow variation, turbulence intensities & temperature variation).

Contraction ratio, CR, of 8.

Test section is square and the maximum test section length possible in the available space.

Maximum flow speed in the test section of 40 m/s.

Low noise level.

Low cost.

Various design rules have been provided in here along with proper explanation and calculations for the important parts of a wind tunnel and pressure losses for a particular design that had been followed in this research paper were given.

The exact design on which they simulated the velocity profile at different position of the test section is given below.

Parameters Value

Type Open circuit

Test section length 1.35 m

Test section cross section 0.90 m × 0.90 m

Mean air velocity range 28 m/s

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