Design and Construction
INTRODUCTION
The practice of low-speed experimental aerodynamics has continued to evolve and it is a cornerstone in the development of a wide range of vehicles and other devices that must perform their functions in the face of forces imposed by strong flows of air or water. It was believed that in the 1970s and in the early 1980s the use of wind tunnels, especially in the subsonic regime, would rapidly disappear as computational fluid dynamics (CFD) would become a more attractive option to obtain data for many engineering applications, since it would be better in cost-effectiveness (Barlow et al. 1999). Nevertheless, computational simulations improved since then but they have not come close to reaching a level sufficient to replace the need for experimental data in development projects.
In fact, the investigative methods leading to quantitative predictions have been a combination of experiment and theory, with computational methods becoming a new powerful tool in this field. However, experimental explorations remain the mainstay for obtaining data to designers to achieve detailed results and final decisions across a broad range of industrial and academic applications. A primary tool for experimental aerodynamics is the wind tunnel. A well-designed wind tunnel could supply technical information for a large number of engineering applications such as external aerodynamics (flow over terrestrial and aerial vehicles), civil engineering (flow over bridges, buildings, cables, etc.), sport activities (flow over cyclists, design of volley and basket balls, wind sails etc.), fundamental fluid dynamics (laminar and turbulent flow over simple and complex geometries) and an extensive frame of other options in industry and research centers at universities around the world.
To achieve such level of applicability, there are 2 basic types of wind tunnels and 2 basic test section configurations, respectively. The 2 basic types of wind tunnels are open-circuit and closed-circuit. The advantages of open-circuit wind tunnels are the construction cost (less than closed-circuit), the possibility to run internal combustion engines and extensively use of smoke for flow visualization without the need to purge. The disadvantages are: harder than closed-test section circuit to obtain high-quality flow; wind and cold weather might affect operation; requires more energy to run if the tunnel has a high utilization rate; in general, it tends to be noisy. On the other hand, the advantages of closed-circuit wind tunnels are: using corner turning vanes and screens the quality of the flow can be well-controlled; independent of other activities in the building and weather conditions; less energy required for high utilization rates; less environmental noise when operating. The disadvantages are the higher initial costs (due to return ducts and corner turning vanes), the necessity to purge the tunnel if smoke is extensively used and some method of cooling for high utilization of the tunnel (Barlow et al. 1999; Mehta and Bradshaw 1979).
According to wind tunnel design, there are also 2 basic test section configurations which could be open-test section and closed-test section, which are respectively a free boundary test section (opened to atmospheric condition) and an enclosed test section (surrounded by walls), as illustrated in Fig. 1.
This paper aims to describe some aspects of the design and the construction details of a closed-circuit (closed-test section) subsonic low-speed wind tunnel, which has been designed to achieve 90 m/s in the working section with expected low turbulence intensity. To achieve such main goals a very detailed design was carried on through the use of theoretical analyses, CFD simulations and semi-empirical methods to improve the flow quality along the wind tunnel sections. A very careful attention has been focused in the design of the fan blades and the electrical engine assembly. Flow control and stabilization also took place using screens, honeycombs and corner vanes, all of them optimized for the best characteristics of the main flow. In addition to the design, special attention was taken during the fabrication process, not only including the right materials and construction techniques but also controlling the tolerances, gaps, steps and surface quality all over the wind tunnel sections. The main contribution of this study is to provide guidance by sharing experience for designing the wind tunnel components and parts, like test section, contraction, settling chamber and all the other sections, shedding light to the most important design parameters. Moreover, this technical research provides some construction ideas and tips that may apply to those ones interested to build such equipment. Based on these main observations, the design and construction of each wind tunnel section has been presented and discussed based on these aspects, according to the following sections.
Several references are known for wind tunnel design in open literature. However, special attention must be taken when considering aspects of low turbulence intensity, flow control and uniformity of the flow inside the wind tunnel as well as cost requirements, manufacturing process and future possible improvements in the equipment. According to these references, some traditional rules may apply with subtle corrections and/or adjustments for each specific design and application. The following main references have been consulted for designing the object of this study - a low-speed wind tunnel: Muncey and Pote (1956), Mehta and Bradshaw (1979), Barlow et al. (1999), Cattafesta et al. (2010), Bell and Mehta (1988, 1989), Lindgren and Johansson (2002), and Calautit et al. (2014). In the next sections, the wind tunnel design's requirements and the detailing of each component will be provided including the main technical aspects related.
WIND TUNNEL DESIGN REQUIREMENTS
The main requirements for this specific wind tunnel were defined at the beginning as: low-speed (subsonic at Mach 0.26) wind tunnel for research and educational purposes, closed-circuit/closed-test section with passive flow control (corner vanes and stabilization chamber). The design criteria have been set to enable accurate measurements of steady or unsteady flow with low turbulence intensity to facilitate the study of the physical phenomena of interest. Moreover, provisions were considered in the wind tunnel design for further boundary-layer transitions experiments and aeroacoustics analysis. According to these requirements, the main characteristics of the wind tunnel were defined as:
Figure 2 shows a schematic view of the closed-circuit wind tunnel and its parts (sections). Some aspects of the wind tunnel design are described in the following sections.
Figure 2 Closed-circuit wind tunnel - sketch and part description.
According to Barlow et al. (1999), the first step in the design of a wind tunnel is to determine the size and shape of the test section. This choice depends on the intended uses of the facility and, as will be discussed, is intrinsically linked to financial resources available to build the equipment. What we called scale factor was considered at the initial design stage, considering flight vehicles cruising below Mach number of 0.3 and with span close to 17 m, giving a scale factors in the range of 1:10 up to 1:20. The test section was originally designed with 1.70 m (width), 1.20 m (height) and 3.0 m (length). An approximation to the available Reynolds number in atmospheric conditions is in the range of 300,000 up to 7 × 105, providing satisfactory speed limits for the models that could be accommodated inside the closed-test section. Figure 3 illustrates the geometric sizing of the test section. It was constructed according to the requirements; including a large access door made of acrylic for making easy the application of visualization techniques such as smoke's rake or particle image velocimetry (PIV). The other lateral panels could be replaced at any time with acrylic panels and/or other specific material such as laminated wood. The test section illumination was completed with light emitting diode (LED) lamps over the floor and ceiling along its length. At the floor, there was the access platform to the aerodynamics balance, which is a circular disk of 1 m in diameter. A common rule of thumb has been set in accordance with Mehta and Bradshaw (1979), were the test section rectangular dimension sizing is around 1.4:1. The results for the boundary layer estimates in the test section will be shown in the CFD analysis section.
Figure 3 Geometric sizing of the closed test section.
The diffuser of a closed-circuit wind tunnel extends from the downstream end of test section (small diffuser, according to Fig. 2) to the third corner of the tunnel, being divided in 2 parts by the tunnel fan (Barlow et al.1999). The second diffuser (big diffuser as seen in Fig. 2) is often named the return duct since it directs the flows towards the test section. Since the power losses at any point in the tunnel are expected to vary as the speed cubed, the main purpose of the diffuser is to reduce the speed with as little energy loss as possible which means maximum pressure recovery, reducing the load of the drive system. Also, the pattern of the flow leaving the test section influences the flow field inside the diffuser. The flow properties like orientation, size and wake development from models affect the diffuser entrance flow. The primary design parameters for a diffuser are the equivalent conical expansion angle and the area ratio (Barlow et al. 1999).
The small diffuser was designed with an inlet area (Ai) of 1.2 × 1.7 m2 and having an exit area (Ae) of 1.85 × 1.85 m2 with a length of 8.0 m (Fig. 4a). The expansion ratio (Ae/Ai) is 2.00, being the ratio between the diffusor's length (LD) and the entrance height (Hi) approximately 6.65. The small diffuser half angle's value is 2.5. At the end of this diffuser was installed a safety screen to avoid any loose parts from the test section to be carried to the drive system. The second diffuser is installed after the drive system and has 15 m length with exit dimensions of 3.7 × 3.7 m2. The expansion ratio (Ae/Ai) is 2.10, being the ratio between LD and Hi around 8.11. The big diffuser half angle's value is also 2.5. A second safety screen was placed at the end of this diffuser to retain any part lost from the drive system assembly. The diffuser expansion factor must be well evaluated to avoid flow detachment (Mehta and Bradshaw 1979). Knowing this, the area of the diffuser should increase gradually along its axis to avoid flow separation and its geometry can be optimized. Kline's flat diffuser curves (Runstadler et al. 1975) are often used for diffuser's design (Fig. 4b).
Figure 4 (a) Geometric sizing (small diffuser); (b) Diffuser design curves. Adapted from Runstadler et al. (1975).
The area ratio (AR) between the diffuser's exit and entrance is plotted versus the ratio of diffuser length to the entrance height of the diffuser. Three regions are shown on the plot in Fig. 4b, the design of the diffuser is conducted by selecting a length for the diffuser, that is, within facility size constraints. Given LD/Hi (where the height is dictated by the test section size), the corresponding value of AR is selected from the no-stall regions. Although a greater pressure recovery can be achieved by operating in the "unsteady flow" regime, this can contribute to unwanted noise, as well as poor performance at off-design flow conditions. If facility constraints limit the length of the diffuser or a closed-circuit design is used, a turning section with guide vanes can be used, and the diffuser can be broken into multiple sections. In Fig. 4b, it is shown the design point (in red) for both diffusers defined in this study.
CORNERS 1, 2, 3 AND 4
The corners of a closed-circuit wind tunnel require special attention since they have the important task to turn the flow by 90° and to keep it organized, e.g. without regions of separation and recirculation. To accomplish such task 2 sets of corners have been designed and built, corners 1 and 2 after the small diffuser and corners 3 and 4 just after the big diffuser. The corners 1 and 2 have dimensions of 1.85 × 1.85 m2 in the entry and exit areas. The corners placed after the long diffuser have dimensions of 3.7 × 3.7 m2 in the entry and exit areas (Fig. 5a and 5b).
Figure 5 Geometric sizing of corners. (a) Corners 1 and 2; (b) Corners 3 and 4.
As seen in Fig. 5, both set of corners were integrated with turning or corners vanes specially designed to turn the flow by 90° at the bend. Also, these turning vanes are quite important to avoid large losses and to maintain relatively straight flow inside the channel. According to Barlow et al. (1999), it is usual to keep the corners with constant area. It is possible to reduce the losses in the turning vanes by selecting an efficient cross-sectional shape and by using an appropriate chord-to-gap ratio. The shape of the corner vane has been based on Lindgren and Johansson (2002), Fig. 6, with an increase in the thickness of the airfoil. The set of points that describe the geometry of the corner used is listed in Table 1.
Figure 6 Design of the corner vane based on Lindgren and Johansson (2002).
X | Y | X | Y |
0.00 | 0.00 | 0.36 | 1.00 |
0.00 | 0.06 | 0.46 | 0.97 |
0.01 | 0.12 | 0.54 | 0.88 |
0.02 | 0.21 | 0.65 | 0.68 |
0.07 | 0.44 | 0.72 | 0.49 |
0.16 | 0.75 | 0.85 | 0.14 |
0.23 | 0.88 | 0.90 | 0.00 |
|
| 1.00 | -0.30 |
In this study, the small corners have 7 turning-vanes while the big corners have 12 to keep the ratio of the gap to the chord (h/c) about 0.25. This is an important parameter to reduce the losses at the corners, as described in Barlow et al. (1999) and Lindgren and Johansson (2002).
The final configuration of the wind tunnel will incorporate corner vanes that are screwed with adjustable pitch, which means that any eventual flow correction may also apply by turning the vanes inside the corner, thus improving the flow quality and reducing the losses at the bends of the wind tunnel.
