Model design

Model requirements

The wind tunnel model’s final configuration depended on a range of diverse require- ments. These requirements include the need to

1. accurately match the geometry of the aerofoil section under investigation,

2. allow variations in the slat position,

3. allow the application of blowing at the desired location,

4. incorporate the pressure taps and flush mounted microphones,

5. allow visual access to the slat cove for PIV,

6. minimize aeroelastic deformation and vibration of the model,

7. avoid excess blockage and loading, and

8. minimize the impact of the endplates and tunnel mountings.

The five main measurement techniques influenced the model design. However, the main restrictions on the model were the need to fit the pressure taps and micro- phones inside the slat along with the blowing system.

The blowing system split the slat into three pressure chambers to distribute the blowing across the full span. These vented to the slat cove via blowholes located at the reattachment location. The slat required a large number of holes to give a smooth distribution across the span and to minimize the impact of the gaps between the blowholes. The size of the air supply tubing limited the size of the holes. The cross-sectional area of the tubes was set at half the size of the blowing tubes so its velocity was double the blowing rate. This was in addition to any losses in the tubing system.

The pressure chambers joined to an external compressor, which provided air compressed up to five bar via an adjustable control valve. This increased internal

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pressure, while the external pressure in the cove region reduced relative to ambient pressure when the wind tunnel is running, creating the driving force for the blowing. A pressure tap located inside each chamber monitored the internal pressure. The external pressure was approximated using pressure taps located both sides of the blowing location 2 mm from the blowholes. A rotometer attached to the air supply monitored the blowing rate. A manometer measured the pressure at this location to record the density and hence mass flow rate of the blowing system.

The inside of the slat contained the pressure tap tubing and microphone cables mounted in the slat in addition to the tubes linking the air supply to the blowing system. The internal size of the slat determines the overall size of the model so this becomes the determining factor in the minimum chord length. The slat was manu- factured using a carbon fibre skin of at least 1.5 mm thick and this meant the slat at the blowing location had to exceed 3 mm in width. An additional offset allowed for the glue in the slat skin to spread during construction without blocking the holes. The blowing location was fixed to coincide with the shear-layer reattachment line in the slat cove, as determined by computational fluid dynamics (Figure 3.33). This gave a minimum size ofc=800 mm, representing a slat chord of 92 mm. This limited the number of sensors that could be fitted inside the slat. The span was limited to 1 m to avoid excessive blockage in the tunnel. An alternative method was to modify the profile in order to reduce the lift and blockage and allow a larger slat cross-sectional area. However, a shortened trailing edge would reduce the circulation around the wing and alter the flow around the slat.

The main element also needed fitting with pressure taps to allow monitoring of the surface pressure distribution. Additionally, the leading edge of the main element needed microphones to measure the unsteady pressure on the other side of the slat cove. The main element was larger than the slat so the sensors did not impose any additional restrictions on the main element design.

PIV had a smaller impact on the design of the model. The model needed end- plates in order to achieve a quasi-2D flow in the centre span of the wing because the model did not completely span the wind tunnel. Hence, the endplates required optical access to the slat region, to allow the camera to see the flow in the cove, for the PIV measurements. PIV also required that the model did not produce strong reflections from the laser requiring the painting of the model black.

The microphone array had little impact on the model because of the array lo- cation away from the model. The main requirement of the array was a direct view of the model and minimization of acoustically reflective surfaces and other sound sources.

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The model cusp extension fitted flush with the slat wall at the cove (Figure 2.2). The extension also matched to the retracted slat setting so in this configuration it would cover the gap between the slat cusp and main element. The extension was manufactured using stereo-lithography so the edges had a minimum thickness of 0.5 mm to avoid damage and vibration.

Model structure

The models 800 mm chord and 1,000 mm span also limited the forces generated to manageable values. Limiting the span had structural advantages by reducing the stiffness needed to prevent wing warping. However, a small aspect ratio required large endplates to produce a near 2D flow at its mid-span. Cherry wood made up the majority of the main element with a carbon fibre trailing edge added to prevent deformation.

The slat and flap were constructed from carbon fibre skins. This enabled the manufacture of the thin trailing edges and restricted the deformation of the slat and flap by the aerodynamic loads. The use of carbon fibre allowed shaping of the trailing edge within 0.1 mm of the design. The final design featured thirty external pressure taps on the slat along with three internal taps to measure the pressure inside the three air supply settling chambers of the slat. These were spaced out based on the pressure distribution found computationally. The distribution was set to give approximately constant change in pressure between adjacent taps around the slat (Figure 2.3). The blowholes were drilled into the slat skin, each was 1 mm in diameter and they were located in two lines each spaced every 5 mm (Figure 2.3). The lines were 2.5 mm apart with the holes alternating between the two lines (Figure 2.4). This gave 400 holes per metre of span . The slat and flap attached to the endplates via 15 mm thick aluminium ribs glued in place inside the skins. Thinner 6 mm ribs divided the slat into three internal plenum chambers.

The hardwood construction of the majority of the main element allowed the cutting of grooves in the surface to mount the sensors. After fitting the sensors, the surface required filling, smoothing and painting to avoiding light reflection during the PIV testing. There were 45 pressure taps on the main element, which were concentrated near to the leading edge along with two microphones mounted in the leading edge to monitor the pressure inside the cove (Figure 2.5).

The primary function of the endplates was aerodynamic. However, they also acted as the mounts for both the slat and the flap. The need to investigate different settings led to fitting adjustable outer windows around the slat. Slots were used to allow the adjustment of the slat gap and of its overlap. These allowed changes of

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0.01c in both the horizontal and vertical directions. Replacing the panels to which the slat attached allowed the slat angle to vary. Glass inner panels for the starboard endplate allow optical access to the slat cove. The endplate construction used 6 mm aluminium sheets machined to accommodate the windows. The sheets had cutouts with honeycomb inserts to reduce their weight and 1 mm aluminium sheets covering both sides to maintain the endplate stiffness. Rounding the edges of the endplates minimized separation (Figure 2.6).

The model was mounted inverted, attached at three points. The two main struts were attached to the pressure surface of the main element at x=0.37cspaced 700 mm apart. They attached to steel top-hat inserts recessed into both surfaces of the wing and distributed the load preventing damage to the main element. The inserts were clamped together using an M12 bolt and covered using plasticine (Figures 2.4 and 2.7). The two main struts provide the pivot point for the model to allow alteration of the aerofoil incidence from -5◦ _{to 22.5}◦_{. The third connection was a tail bar attached} to a structure mounted between the two endplates above the flap. The tail bar construction used aerodynamically shaped steel tubing to minimize its aerodynamic interference. The tubing formed a triangular structure to distribute the loading to the endplates and avoid deformation (Figure 2.8).

The initial tests of the model were in a free transition configuration. Later on, trip strips were added upstream of the slat cusp and trailing edge. The addition of the trip strips prevented laminar flow separation around the cusp [2] and gave a closer match to a higher Reynolds number flow with natural transition. The trip strips were each 12 mm wide and used 80 grit. The strips were located 4-16 mm from the cusp and 44-56 mm from the slat trailing edge on the convex side. The cove side of the trailing edge remained free transition.