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3.2 Wind tunnels, facilities and instrumentation

3.2.1 A-Tunnel

The A-Tunnel in the Enflo laboratory was used for all of the seven hole probe experiments as well as the hot wire experiments. This is an open return tunnel with a rectangular cross section of 0.6 m × 0.9 m and a working section 5 m long. The freestream velocity was maintained constant to within measurement precision by means of a closed-loop active control system. The control system used a Pitot probe mounted at the test section entrance connected to a Furness micromanometer with a full-scale range of 196 Pa to take velocity averages over 30 s intervals. The wind tunnel has a streamwise turbulence intensity level of less than 0.5%.

The wings were mounted vertically on the floor of the tunnel and were fitted in a purpose-built automated pitch positioning system with a 0.25◦

angular precision (see Appendix A for detailed schematic). The tunnel was also equipped with a five degree-of-freedom spherical-Cartesian traverse on which the probes were mounted. The x, y, z traverse was capable of a precision of ±5 µm, and the yaw and pitch angles had a precision of ±0.2◦

. The experimental setup and wind tunnel coordinate system are illustrated in figure 3.2.1.

U∞ Pitch automation traverse system Reference Pitot probe Transducer Array Analogue Signal Multiplexer Data Acquisition System silicone tube bundle Probe θ ϕ 5-axis (ϕ, θ, x, y, z) traverse system α z x

Figure 3.2.1: A-Tunnel experimental setup schematic

3.2.1.1 Seven hole probe fabrication and instrumentation

Seven hole probes were purpose built for this research. They were constructed by assembling and soldering together lengths of 21-gauge stainless steel needle stock (having an outside diameter of 0.9 mm) and manually finishing with a tip cone angle of 60◦

(see figure 3.2.2). With an overall outside diameter of 2.9 mm, the seven hole probes were connected to a customised array of pressure transducers. Seven Honeywell 163PC01D75 temperature compensated, differential, amplified pressure transducers were assembled into a purpose built array. The array can be installed inside of the wind tunnel, allowing for quicker response due to the shorter connections between probe and transducer. This array can also have an additional eighth transducer attached, which was the configuration required when used with a novel eight hole probe. The transducers have a full-scale error of less than 0.25% and used wind tunnel static pressure as the reference. During wind tunnel use, the pressure transducers were calibrated in situ simultaneously against the micromanometer, by exposing the probe tip to known pressures. The transducers

were calibrated before and after measurements, and the data was discarded if the drift was larger than 1%. A full description of probe calibration is in section 3.3.

Figure 3.2.2: Tip of a seven hole probe

3.2.1.2 Uncertainty analysis

The seven hole probe is intended to capture mean pressure data in the vortex, and one of the primary sources of error is not capturing long enough periods of data for a converged mean value. Convergence tests were carried out to ensure that the standard error in the mean was less than 0.2%.

Detailed error analysis for the seven hole probe data was based on the capa- bilities of the pressure transducers, according to the manufacturer’s specification. The full-scale repeatability of the transducers is 0.25%, so random errors up to this amount were added to the raw pressure data of an existing vortex wake scan and the calibration data associated with it. Percentage errors were then calculated for (u, v, w) values as well as how they propagate through to vortex properties such as core radius and circulation. For example, the error in estimating the vortex core radius was 1%, and the difference between maximum vorticity estimates was 4.7%. The remaining uncertainties are in table 3.1. It is important to note that

Property Mean Error Maximum Error Standard Deviation V 2.6 12.8 2.0 U 1.8 6.1 1.2 W 2.6 12.1 2.0 vθ 6.8 156.2 16.4 Γ 92.4 176.8 54 ζx 3.9 7.1 1.1

Table 3.1: Error estimates for seven hole probe experiments (% of maximum value)

most of the maximum errors quoted occurred well outside of the vortex core in regions prone to high levels of noise.

3.2.1.3 Novel 19 hole probe development

Concurrently, a novel 19 hole probe was designed and built for the purpose of extracting mean vorticity directly from pressure data. The numbering scheme for this probe is depicted in figure 3.2.3. An array of 35 low-cost, high gain Honeywell PCAFA6D differential pressure sensors was also designed and built with a net sensitivity of 0.04P a/V for use with the 19 hole probe. An analogue signal multiplexer was built to use in conjunction with this transducer array. The multiplexer allows for use of as many or as few channels as required at any given time, by cycling through five sets of seven signals. The signals from the transducers were then digitized using existing acquisition systems. This probe has been shown to provide slightly higher precision measurements of velocity than a seven hole probe as well as local measurements of velocity gradients (Shaw-Ward et al., 2014). The size of the 19 hole probe made it impractical for use in the bulk of the wake scans, with a tip diameter of 4 ± 0.08mm diameter. While successfully obtaining some velocity gradients, care must be taken in vorticity measurements near the vortex centre, due to the high degree of sensitivity in measurement error.

Figure 3.2.3: Numbering scheme of a 19 hole probe, alongside the tip geometry schematic.

3.2.1.4 Hot wire setup

Single-wire experiments in the wing wake were carried out in the A-Tunnel. The hot wire was calibrated against the pitot-static probe used for the tunnel speed regulation. During experiments, the hot wire was calibrated every 60 minutes, and the data were discarded if the calibration drift was over 1%. The probes were driven by a Newcastle constant temperature anemometer. The hot wire was mounted on the (x, y, z) tunnel traverse. The hot wire was used to measure across the 2D wing wake 0.25b from the wing tip and 0.3c behind the wing. The data rate was 5600 Hz and data was taken over a time period of 1 minute in blocks 4096 points long.

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