How can you design a wind tunnel for UAV testing?

Introduction

Wind tunnels are facilities that enable real-world simulation of how air passes around an object. Testing models in wind tunnels provide the data to verify or enhance computer simulations. Designers and engineers use such testing to study and assess aerodynamics and fluid flow phenomena. Wind tunnels enable them to validate the efficiency and durability of anything from architectural elements to cars and aircraft.

World map of wind tunnel testing facilities

List of wind tunnel testing facilities by country

What is a wind tunnel?

A wind tunnel is a duct or tube-like test setup in which powerful electric fans can generate air flowing at a known velocity. The consistent airflow is led past models or full-size objects to investigate the flow and effect of airstreams and optimize their design.

The air in the tunnel moves around the stationary test object to simulate the same relative movement in the real life, while the aerodynamic forces acting on the object, such as drag and lift, are measured.

The model or object is often placed on a turntable or a rotating mobile platform to allow studies of wind impact from multiple directions.

The wind tunnels are made for testing and predicting wind load for all kinds of structures moving through air or affected by wind force: bridges, vessels, aircraft, buildings, wind turbines, and numerous offshore structures, or parts and components of these. Even objects like parachutes, vehicle trailers, or tennis balls are tested in wind tunnels.

The characteristics of the airflow around the object tested can be visualized by flow visualization techniques including photographing or video recording injected smoke or dye, or oil flowing on the object.

Wind tunnel tests verify engineers' calculations and identify areas for improvement in their designs. Advantages of the wind tunnel technique include:

• Accurate results to minimize assumptions

• Well-controlled flow circumstances

• Adjustment of variables at will, and economy

• Easy reproduction of the experimental conditions

• Accurate data generation for validating the Computational Fluid Dynamics (CFD) simulations and field measurements.

How do wind tunnels work?

Without going into too much detail it is best to watch this video explanation of how wind tunnels work.

How wind tunnels work. Video courtesy of Sauber Motorsport

Wind tunnel history - when did it start?

In a trial to test windmill sails, the British civil engineer John Smeaton in stated a need for artificial wind. Smeaton developed the concepts and data which became the basis for the Smeaton pressure coefficient, the lift equation used by the Wright brothers. During the late s wind tunnels appeared when many attempted to develop successful flying machines.

Frank H. Wenham (-), a Council Member of the Aeronautical Society of Great Britain, is generally credited with designing and operating the first wind tunnel in . The tunnel was built by John Browning, an optician, and located at Penn's Marine Engineering Works in Greenwich, England. The tunnel was 3.7 meters (12 feet) long and 45.7 centimeters (18 inches) square. Another early documented wind tunnel was built by the Wright brothers at the end of for flight tests of their gliders.

Even if the first wind tunnel in Great Britain was operated by a compressed air ejector the following wind tunnels used the air moved by a fan disposed upstream of the wind tunnel's test section. The change to suction is an important step in wind tunnel development. When the fan is disposed downstream of the test section and it no longer interferes with the model, which guarantees the quality of flow. 

Wind tunnel structure - how does it work?

Three main criteria that are commonly used to define wind tunnels are maximum achievable speed, flow uniformity, and turbulence level. Therefore, the design aim of a wind tunnel, in general, is to get a controlled flow in the test chamber, achieving the necessary flow performance and quality parameters.

There are five basic parts of the wind tunnel: The Settling Chamber, the Contraction Cone, the Test Section, the Diffuser, and the Drive Section. During a test, the test object is placed in the test section of the tunnel, and the air is made to flow past it. Various types of instrumentation are used to determine the forces on the model.

In some wind tunnel tests, the aerodynamic forces and moments on the model are measured directly. The model is mounted in the tunnel on a special machine called a force balance. The output from the balance is a signal that is related to the forces and moments on the model. Balances can be used to measure both the lift and drag forces. 

The balance must be calibrated against a known value of the force before, and sometimes during, the test. Force measurements usually require some data reduction or post-test processing to account for Reynolds number or Mach number effects on the model during testing.

Four air properties affect the way it flows by an object: viscosity, density, compressibility, and temperature. With the model mounted on a force balance, lift, drag, lateral forces, yaw, roll, and pitching moments over a range of angles of attack can be measured. 

Types of wind tunnels

The type, size, and wind force of a wind tunnel depend on the size and character of the object for testing and the desired flow conditions. Some wind tunnels can accommodate full-scale models but, for practicality and cost reasons, most are built to fit miniaturized models.

Open-circuit wind tunnels (Eiffel type)

Open circuit or open return wind tunnels do not directly re-circulate air. Rather, the air is drawn in from the laboratory environment, passes through the test section, and is returned back to the lab through the tunnel exhaust.

The wind tunnel of the Eiffel type has an open and no air recirculation. The blower is at the end of the tunnel in an outflow diffuser. In this procedure, the flow guidance happens by means of a smoothing chamber with a rectifier and sieves, and a nozzle. Named after Gustave Eiffel, who in built a small wind tunnel at the foot of the Eiffel Tower to study the effect of the wind on more precise fixed points. 

Closed-circuit wind tunnels (Prandtl type)

In the closed return or closed return tunnel, the air is conducted from the exit of the test section back to the fan by a series of turning vanes. Exiting the fan, the air is returned to the contraction section and back through the test section.

Built at the University of Göttingen, in Germany, by Ludwig Prandtl in . After being sucked downstream of the test section, the air is guided through four successive bends and then readmitted upstream of the collector. This type of wind tunnel called the Prandtl wind tunnel &#; or Göttingen wind tunnel &#; leads to better fuel efficiency and provides controlled test conditions (pressure, temperature, humidity).

Modern wind tunnels are often a combination of these types, Prandtl's closed circuit and Eiffel's open circuit with a diffuser.

Wind tunnel classification

Wind tunnels are generally classified by the amount of speed they can produce as either low-speed (Low-Speed Wind Tunnels (LSWT)) or high-speed. They are further classified as:

• Subsonic wind tunnels (80 percent of the speed of sound) - these are furtherly split into &#;Incompressible subsonic&#; (between Mach 0 and about 0.3) and &#;Compressible subsonic&#; (between about Mach 0.3 and about 0.8).

• Transonic wind tunnels &#; between Mach 0.8 and 1.2 - about the speed of sound.

• Supersonic wind tunnels &#; between Mach 1 and 4 - up to 6 times the speed of sound.

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• Hypersonic wind tunnels &#; Mach is greater than 4 - 5 - up to 12 times the speed of sound.

• Hypervelocity wind tunnels &#; more than 12 times the speed of sound. 

Specialized wind tunnels

• High enthalpy wind tunnels are designed to study the flow of air around objects moving at speeds much faster than the local speed of sound (hypersonic speeds). "Enthalpy" is the total energy of a gas stream, composed of internal energy due to temperature, the product of pressure and volume, and the velocity of flow.

• Atmospheric entry wind tunnels or plasma wind tunnels use plasma to simulate aerodynamic heating and a high Mach number state at reentering into the atmosphere. It reproduces re-entry conditions by heating gas to several thousand degrees Celsius, at air flows seven times the speed of sound and with stagnation pressures as high as 50 atmospheres. Varying Reynold numbers indicate the steady or turbulent fluid flow past the object or in the tunnel.

• Climatic wind tunnels have the capability of recreating climatic conditions such as solar radiation, temperature, and humidity, e.g., rainfall, snowfall, and icing. It is used for testing things such as wind turbines, bridge cables, or fans in heating, ventilation, and air conditioning (HVAC).

• Shock tubes or blow-down wind tunnels produce airflow by rapid discharge of a high-pressure storage tank or by suction from an evacuated reservoir. They can replicate and direct blast waves simulating actual explosions and their effects. Shock tubes are used to investigate compressible flow phenomena and gas-phase combustion reactions. 

Wind tunnel applications

Designers and engineers use wind tunnel modeling and testing to simulate and assess the aerodynamics around objects and to validate the efficiency and durability of anything from architectural elements to cars and aircraft

Ground vehicles testing &#; low-speed (Subsonic)

• Simulate real-world driving conditions to study aerodynamics on vehicles with the aim to reduce aerodynamic drag, increase efficiency, and lower CO2 emissions.

• Study the acoustics of vehicles to reduce the aerodynamic noise, outside as well as inside the vehicle.

• External flow tunnels are used to study the external flow through the chassis, the wind braking force

• Climatic tunnels are used to evaluate engine colling, inside comfort level, the performance of door systems, braking systems, etc., under various climatic conditions.

Aircraft testing &#; low-speed and high-speed

• Low-speed aircraft wind tunnel testing to measure the aerodynamic characteristics of aircraft - lift and drag properties, as well as stability of aircraft and engine components. These wind tunnel tests are used for static stability testing and pressure measurement.

• The aerodynamic characteristics of aircraft at speeds of takeoff and landing etc.

• Shock waves for high-speed aircraft are visualized using water as a working fluid.

• Understanding the aeroacoustics by exploring aerodynamic noise, i.e., turbulent fluid motion, and acoustic pressure fluctuations, i.e., external noise generation via aerodynamic forces interacting with surfaces.

• Water spray systems are applied for icing testing.

Spacecraft testing &#; high-speed (Supersonic, Hypersonic, Hypervelocity)

• Atmospheric entry wind tunnels or plasma wind tunnels characterize the aerodynamic behaviour and heating of materials and structures by entry simulation on space vehicles such as rockets, satellites, launch vehicles, and spacecraft.

• Simulation of the desired burn-up during atmospheric reentry to estimate the risk of debris reaching the ground.

• High-enthalpy wind tunnels for heating testing of thermal protection systems for re-entry vehicles.

Structural testing &#; low-speed (Subsonic)

• Wind tunnel tests are used to predict the wind loads and responses of a structure, structural components, and cladding to a variety of wind conditions.

• Wind engineering to measure the air velocity around or within, forces or pressures upon civil engineering structures as well as the wind-induced responses &#; especially on large, tall, or wind-sensitive structures such as skyscrapers and tall buildings, groups of buildings, long-span roofs, tunnels, bridges, offshore structures.

• Wind tunnel tests for bridges to evaluate girders, pylons, and cables, are conducted on girders, pylons, and cables, separately or together. Wind Tunnel Tests are also conducted for guard rails, barriers, or road signs, respectively.

• Climatic wind tunnels to study the combined effects of wind and other climatic parameters such as rain, sand, sun, snow, or temperature on building elements

Data acquisition and sensors

Wind tunnels are used by engineers to test forces against wind pressure. Making precise measurements of pressures and forces on the test model allows the engineer to predict them on the full-scale aircraft and improve its aerodynamic performance.

In order to perform these measurements, engineers must use sensors and signal conditioning equipment in order to store and analyze the data. In other words, modern data acquisition systems are needed.

Wind tunnel sensors

For wind tunnel testing engineers usually use dynamic sensors such as:

• Pressure sensors

• Microphones 

• Force sensors

• Strain gauges

Wind tunnel sensors are organized by speed classification. From subsonic use of condenser microphones for acoustic holography to shock sensors for studying boundary layer transition of hypersonic re-entry vehicles. Sensors are usually installed inside the wind tunnels or close to the model itself.

Signal conditioning

Signal conditioning is an electronic circuit that manipulates a signal in a way that prepares it for the next stage of processing. Many applications involve environmental or mechanical measurements from sensors, such as temperature and vibration. These sensors require signal conditioning before a data acquisition device can effectively and accurately measure the signal.

Once the signal is correctly conditioned it is ready to be stored by the data acquisition system and prepared for analysis using signal processing software. 

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References