Using the Benchtop Wind Tunnel in Aircraft and Space Craft Design

Everybody is familiar with wind tunnels and many will have seen films or pictures that depict their use in aircraft, spacecraft design, and automobile adverts showcasing how aerodynamic their cars are. Recently, new uses have been found in the design of bridges to prevent resonance under high winds and in the design of modern super buildings to see how they interact under wind-based natural disasters, such as hurricanes and typhoons. However, there is another world of wind tunnels that many do not know of, with practical uses across everyday engineering, academia, research and development and product design – the area of benchtop wind tunnels.

Benchtop wind tunnels adhere to the general principles of wind tunnel design, namely controlled air flow, low turbulence, flow accuracy, and repeatability, but use these fundamental properties for more down to earth applications. The most basic of these instruments are used in the calibration of air flow measurement devices known as anemometers and the most complex are used in small scale aerodynamic studies. New applications have come to light recently for tabletop wind tunnels in everyday engineering. These new applications include the thermal characteristic analysis of heat emitting items, such as circuit boards and electronic components, as well as in the measurement of heat sinks, heat exchangers and other heat/cooling transfer devices.

Wind Tunnel Elements

Benchtop wind tunnels are open-loop systems which draw and expel the air in the room rather than recirculating it through a duct, or other closed path, system. Regardless of their size, all wind tunnels have key elements in common. The test section, or air chamber, is commonly the smallest component of the device, but is the most important for the user. These sections generally possess a rounded or square cross-sectional area where the test conditions exist, and measurements are made. The other components of the wind tunnel are involved in controlling the air flow required for the test chamber and can include the fan/blower section to mobilise the air, a series of honeycombs, vanes, filters, and other devices to reduce the air turbulence and produce a laminar air flow, as well as an area of duct cross-sections that shape the flow of the air through the test chamber.

Figure 1. Laboratory-Grade benchtop wind tunnel shown with control panel (Omega Engineering, Inc.)

Applications

The simplest application is anemometer air flow calibration. Anemometers are devices which measure the air velocity and are used across many scientific and engineering disciplines. There are two common types, both of which employ different technologies. These are the vane type air velocity anemometer and the hot-wire anemometer, which employ small fans spins in response to air flow and resistive elements to cool the air, respectively. Across industry, the anemometers are used for a multitude of air flow tests and adjustments in general workspace ventilation systems, air flow velocity monitoring in spray booths, fume hoods, clean rooms and in laminar flow workstations. They are also used to measure the flow through large filters and cooling or heating coils used in industrial processes. Anemometers are widespread throughout the scientific community and find use in weather measurement and analysis, in environmental studies and for research. The most common use for these devices is during the installation and maintenance of HVAC systems, where they are used for balancing and measuring the air flow, and troubleshooting.

In the calibration of anemometers, a wind tunnel is very easy to use and requires no specialist knowledge. The test chamber is often configured to accept specific models and the air flow is pre-calibrated against a NIST standard. To operate, simply mount the unit being tested, choose the air flow and read the output. The unit in Figure 2 is controlled by the instrument panel shown – A multiposition switch selects the air flow rate.

Figure 2. Benchtop Calibration Wind Tunnel (Omega Engineering, Inc.)

For more general applications, a model like that shown in Figure 1 offers greater flexibility. This device can be used in laboratory calibrations, as well as applications suitable for the larger units, including aerodynamic studies using models. Common uses include product design and development, R&D projects, and academic experiments. The test chamber can accommodate custom mounting fixtures and instrumentation. It also includes the ability to measure temperature, humidity, and barometric pressure. Laboratory-grade wind tunnels have much larger flow rate ranges compared to calibration wind tunnels – 25 to 9000 fpm (feet per minute) vs. 500 to 3000 fpm. Also, the flow rates are continuously variable and are not preset, due to them being in a calibration wind tunnel. The lowest flow rates are achieved using specially designed restrictive plates that cut down the air while maintaining a high uniformity and low turbulence.

The most recent application to surface for these compact air flow devices is in everyday engineering design, such as in the thermal evaluation of electrical and electronic components. This includes active devices such as circuit boards and powered components, and passive devices such as heat sinks and heat exchangers

In appearance, these devices are a far cry from the familiar designs used for air flow studies and calibration. As you can see from the example on the left, unlike its traditional counterparts, the air flow chamber is the largest part of the unit. The fans can be individually controlled and draw air through the honeycomb filter to suppress the turbulence and produce a uniform flow. The unit being tested is suspended in the air chamber on a universal, and adjustable, mounting fixture which can accept a wide range of test objects. There are several openings in the test chamber that allow for other instrumentation, such as sensors and anemometers. The air flow is continuously variable so that different test conditions can be utilized, and temperature profile measurements made. This unit can also be operated using a small control box or through a PC interface.

Figure 3. Benchtop Thermal Evaluation Wind Tunnel (Omega Engineering, Inc.)

Conclusion

Wind tunnels are not a common tool for the average engineer. However, the little-known category of benchtop wind tunnels offers great benefits in test, measurement, and design effectiveness for the right applications. For those that routinely use anemometers, a benchtop calibration wind tunnel is an easy way of increasing your in-house calibration capability. For those in research and product development, a laboratory-grade unit can be a decisive data resource. For those who design circuit boards, heat-generating components, heat sinks, or other cooling devices, the thermal evaluation wind tunnel presents a novel way of creating safer, more reliable, and higher quality products. If you fit into any of these categories and have never heard of benchtop wind tunnels, welcome to a new path to productivity.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.