Axisymmetric Body
Affiliation : Aeronautical and Automotive Engineering, Loughborough University, UK
✉ m.a.passmore@lboro.ac.uk; admin@nwtf.ac.uk
Introduction
Bluff body wakes are prevalent in many areas of interest including road vehicles, energy systems and environmental flows. The low pressure in the wake creates a pressure deficit on the base and is a large source of aerodynamic drag. The unsteady pressure fluctuations can also be a significant noise source. By adopting a body of revolution with a streamlined nose, the base flow can be isolated from most upstream effects and so allows a fundamental study of the flow dynamics in the wake.
Data is presented for the near-wake of an axisymmetric body using base pressure tappings and large scale Particle Image Velocimetry measurements.
The experimental data aims to answer the key questions: at higher Reynolds number does the very low frequency dynamics influence the three-dimensional structure of the near-wake? What are the factors leading to the selection of either an axisymmetric topology or a reflectional symmetry preserving state as low-drag conditions? A detailed analysis of this dataset is presented in Pavia, G. et al (2019)[1].
In this experiment, Tomographic Particle Image Velocimetery (TPIV) and 2D PIV measurements were conducted on the near-wake region of an axisymmetric body. For the complete dataset, the user can download it from the Loughborough University repository.
Model geometry
The model employed is a cylinder with an elliptical nose cone and a truncated flat base, as shown in Figure 1. The diameter of the cylinder D is 160 mm, and the overall length L is 800 mm, resulting in a length-to-diameter ratio L/D of 5. The Reynolds number based on model diameter is ReD = 3.2 x 105, and ReD = 4.3 x 105 for TPIV and 2D PIV respectively. A carborundum roughness strip, with a width of 10 mm, was placed on the junction between the nose section and the main body to ensure boundary layer transition. The axisymmetric body was supported by a steel shaft, with a diameter of 20 mm, enclosed in a NACA 0021 aerofoil, with a chord, c = 135 mm (or 0.84D). The shaft was centred with the point of maximum thickness of the airfoil (at 0.3 c).
In order to minimise the effects of the supporting system on the wake dynamics, the wing was moved as far upstream of the base as possible, close to the forward joint, with the leading edge of the wing itself located 370 mm (or 2.31D) downstream of the tip of the model nose. The shaft was attached to a turntable equipped with an automated yaw mechanism capable of 360°\degree rotation with ±\pm0.1°\degree accuracy, while the pitch angle α\alpha was manually adjusted with an accuracy of ±\pm0.25°\degree. This was performed using a right-handed reference system located at the intersection between the supporting shaft and the turntable, 400 mm (or 2.5D) below the model axis [X∗^*, Y∗^*, Z∗^*] in Figure. 1b.
A coordinate system with origin in the centre of the model base is used for the presentation of the results; the xx axis is aligned with the flow in the streamwise direction, the zz axis is vertical, positive upwards, and the yy axis follows a right-handed coordinate system, shown in Figure. 1b.
Measurement Locations and Techniques
Experimental Facility
The experiments were carried out in the Loughborough University large wind tunnel. This is an open circuit wind tunnel, with a test section of 1.92 m X 1.32 m X 3.6 m (W X H X L), and a contraction ratio of 7.3:1.
Wind Tunnel Corrections
No corrections have been made to the data.
CAD files
Body.stl | Inlet.stl | Outlet.stl | Support.stl | Tunnel Geometry.stl |
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Available Datasets:
The acquired measurements are divided into two datasets to enhance clarity and ease of understanding.
(i) Datasets- 1 (TPIV & Base pressure)
Flow Conditions
Inlet velocity = 30 m/s.
Free-stream turbulence ≈\approx 0.2 %.
Reynolds number = ReD = 3.2 x 105.
Flow uniformity = ±\pm0.4%.
TPIV settings used in this experiment can be found in Pavia, G. et al (2019)[1].
Schematic of the TPIV and pressure measurement regime is given in figure 2a and 2c.
Drag coefficent, CD‾Base\overline{C_D}_{Base} : 0.189 (time averaged base drag)
Flow field – Tomographic
Instantaneous Velocity field.csv | Mean Velocity Field.csv |
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Pressure measurement
Pressure Tapping Map.csv |
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Instantaneous Pressure.csv | Mean Pressure.csv |
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(ii) Datasets- 2 (2D and Stereo PIV, Base pressure & Force)
Dataset-2 is not presented in any publications so far, and for completeness a short description of the experimental details. For planes yy= 0 mm, zz= 0 mm and zz= 40 mm, a single 5MP LaVision sCMOS camera with a 50 mm fixed f-stop lens was used. For all the stereographic planes at various x∗x^*, two 5MP sCMOS cameras were used with 105 mm fixed f-stop lenses. A minimum angle of 40°\degree between the cameras was maintained for the stereographic planes to improve the through plane velocity results. For the planar measurements, the cameras were located outside of the working section, but for the stereographic measurements the cameras were located downstream of the model in the first diffuser. The RMS error between the dewarped calibration image and a ‘true’ rectilinear grid is, at most, 0.25 for all the planes taken. The raw images had a background subtraction applied to them, using a 7 image long Butterworth filter. These images were then processed using a direct correlation algorithm on a Graphical Processing Unit. The initial pass, at a 64×64 window size and a 50% overlap, was repeated three times before being reduced in size incrementally to the final window size of 32×32 with a 50% overlap.
The force and pressure measurements each have 90,000 samples, taken at 300Hz (300 seconds total), the pressure and force data is not correlated in time. The vertical planes of PIV at yy= 0 mm contain 1000 uncorrelated vector fields taken at 15Hz with all other planes contain 2000 uncorrelated vector fields taken at 15Hz.
Flow Conditions
Inlet velocity = 40 m/s.
Free-stream turbulence ≈\approx 0.2 %.
Reynolds number = ReD = 4.3 x 105.
Flow uniformity = ±\pm0.4%.
2D PIV – All the measurements in yy and zz planes.
Stereo PIV- All the measurements in x∗x^* planes.
Drag coefficent, CD‾Base\overline{C_D}_{Base} : 0.261 (time averaged base drag)
Instantaneous velocity field
xx = 60 mm.csv | xx = 120 mm.csv | xx = 180 mm.csv | xx = 240 mm.csv |
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yy = 0 mm.csv | zz = 0 mm.csv | zz = 40 mm.csv |
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Mean velocity field
xx = 60 mm.csv | xx = 120 mm.csv | xx = 180 mm.csv | xx = 240 mm.csv |
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yy = 0 mm.csv | zz = 0 mm.csv | zz = 40 mm.csv |
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Force measurement
Instantaneous Force Data.csv | Mean Force Data.csv |
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Pressure measurement
Pressure Tapping Map.csv |
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Instantaneous Pressure.csv | Mean Pressure.csv |
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Open Access
This metadata is provided under the Creative Commons Attribution-NonCommercial 4.0 International License https://creativecommons.org/licenses/by-nc/4.0/). This license allows for unrestricted use, distribution, and reproduction in any medium, provided that proper credit is given to the original author(s) and the source. Also provide a link to the license, and indicate if any changes were made. Furthermore, this license does not allow the use of this material for commercial purposes.