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1.2.1

Model details

The model used in the current study has been designed to accommodate different nosing geometries. A nosing is defined as having a constant cross-section which is fitted to either the leading or trailing edge surfaces. In this study, 5 different leading edge noses were used in addition to the base model; these noses are shown in Figure 1.2.

The base model is of rectangular cross-section measuring 76.2 mm in thickness, t, with a chord-to-thickness ratio of c/t = 7. This elongation ratio is kept constant across all tests; however, the length of the nose is not considered in the measurement of the chord. The noses used in the current study include an elliptical nose, with a 3:1 axis ratio, and noses of triangular cross-section with interior angles ranging from 60°-180° at 30° increments. The co-ordinate system used throughout the current study is shown in Figure 1.2. The y- axis is centred along the span of the model, the z-axis is centred vertically on the model and in the streamwise direction, the x-axis begins after the nose. This streamwise location corresponds with the fixed separation points for all of the noses except for the one of elliptical cross-section (which has no leading edge separation).

The model is fitted with 512 pressure taps. The basic layout of the taps is shown in Figure 1.2. As is shown in Figure 1.2, the leading edge has 3 rows of 5 taps each. The tap spacing used for the base case is shown therein and the same vertical spacing is projected onto each nosing. The spacing of the leading edge taps is preserved on the trailing edge surface in addition to a row of taps along the span. Each spanwise row of

U

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x

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Figure 1.2. Schematic of the model showing the pressure tap layout (+ symbols) and the different noses used in the experiments. The leading and trailing edges have been folded out to

taps consists of 67 taps spaced 25.4 mm apart towards the edge of the model and spaced at 15.9 mm closer to the centre. To obtain sectional loading data, three streamwise loops are used comprising the taps on the leading and trailing edge surfaces as well as 27 taps on the top and bottom surfaces. These taps are spaced at 15.9 mm within approximately 3.5t of the leading edge and 25.4 mm from this location until the trailing edge. The density of taps was chosen so that the data were better spatially resolved within as much of the leading edge separation bubble as possible. Although not shown in Figure 1.2, the pressure tap layout on the bottom mirrors that on the top.

In the literature on circular cylinders, and short bluff bodies in general, there is considerable discussion about the inherent three-dimensionalities of bluff body experiments. Roshko (1993) noted that certain extrinsic characteristics can alter the three-dimensionality of a given experiment. The two main extrinsic characteristics which are focused on in this regard are aspect ratio and end plates. There is some overlap between these two issues in the design of such an experiment. In the present experiments end plates are used which extend 0.57 m into the wake ensuring that they protrude into the wake for at least one complete vortex shedding cycle. The aspect ratio is relatively high for comparable studies with a span-to-thickness ratio of 24. However, unlike the plethora of data regarding circular cylinders and aspect ratio, there remains little on its effect for elongated bluff bodies. The significant leading edge separation for these bodies and its interaction with the wall boundary layers is an additional complication to the case of shorter bluff bodies and is shown to be sensitive to many parameters (Castro and Epik, 1998). In the present study, the model is instrumented sufficiently to describe the three- dimensional nature of the flow thus this topic will be returned to when the data are presented (§3.1.3).

1.2.2

Wind tunnel tests

The tests were performed in Tunnel II at the Boundary Layer Wind Tunnel Laboratory. This tunnel is of closed-circuit design and includes a test section measuring 1.83 m high by 3.35 m wide. The length of test section is 39 m; however, the testing was all

performed approximately 2 m from the inlet. At this location, the turbulence intensity is measured less than 1% and the velocity is uniform to within 1% away from the walls.

The model was fitted with end plates, as discussed above and was mounted in the middle of the tunnel both vertically and horizontally. The free stream speed was adjusted to yield 8 different Reynolds numbers ranging between 4x104 – 7.5x104 (based on thickness) in increments of 0.5x104.

The pressure taps were connected to multiplexing pressure scanners. A total of 12 taps were connected to each scanner and each tap was sampled at 500 Hz. The tubing system used to connect each tap to the scanners has been tested to have a frequency response which is flat to approximately 200 Hz. Thus, the data herein have been low-pass filtered at 180 Hz. For each of the nose configurations (6 in total) and each Reynolds number, the pressure data are sampled for 120 sec. The pressure data are interpolated in time at each time step, which is necessary due to the multiplexed nature of the measurement system. Phase lag tests have been performed on this system to ensure that effects from this procedure are negligible. The reader is referred to Ho et al. (1999) for more details on the pressure scanning system.