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The affect of surface waves on foil performance and tip vortex ventilation

Chapter 6 Wing tip vortex ventilation of a T-foil hydrofoil at low submergence

7.1.2 The affect of surface waves on foil performance and tip vortex ventilation

In chapter 4, a link was made between vortex aperiodicity and the axial velocity of the vortex core and hence the proclivity for tip vortex ventilation. It was concluded that motions due to large scale eddies, waves, and vessel motion would have limited effect on the hydrofoil tip vortex axial velocity due to the stream normal accelerations of these phenomena being orders of magnitude lower than those of the vortex. The effect on localized vortex filament cavitation from these factors would be less significant than the characteristics of the vortex aperiodicity itself.

Surface waves, however, might be said to affect the proclivity for tip vortex ventilation in two manners. Firstly, in Chapter 6 it was shown that considerable vorticity can exist in the vicinity of a breaking wave, and that at low hydrofoil submergence the opportunity arises for interaction between the trailing tip vortex and the wave vortices. Secondly, and perhaps more importantly, surface waves affect the pressure field beneath the waves and consequently impact on the pressure field around a hydrofoil in proximity to the free surface. Oscillations in the pressure field varying the effective AoA of the hydrofoil. AoA has

been shown to determine the type of vortex core – changing from a wake-like core at low AoA to a jet-like core at high AoA - and thus the core velocity and associated proclivity for vortex ventilation. Therefore, changes in the pressure field that trigger this change may be the cause of ventilation intermittency so commonly observed in the field. Techniques for dynamically adjusting the AoA in response to changes in the pressure field would serve to mitigate this problem.

7.2

Future work

To address the hypothesis outlined earlier in this chapter. From a research perspective, an investigation is required to quantitatively verify the qualitative observations of modification to the flow topology surrounding the trailing vortex at low submergence. A numerical estimation might be sought or an experiment using PIV tomography, obtaining the three velocity components might be undertaken to confirm the indications provided in Chapter 4 that the fluctuation in the vortex axial velocity is related to the aperiodicity of the vortex axis normal motion.

Measurement of hydrofoil tip surface wake characteristics would enable determination of the energy transfer from wave making drag into the wave wake through to the tip vortex. Comparison of the wave wake data with the energy in the vortex at each depth of submergence obtained from the data in this study in the current study would improve understanding and enable prediction of the effects of hydrofoil submergence upon vortex filament cavitation.

With respect to the study of tip vortex ventilation, further knowledge could be gained through understanding if submergence has any effect on the aperiodicity of the tip vortex, and whether this factor can be linked to the intermittency of the vortex axial velocity.

Comparison of the SAS SST and BSL EARSM turbulence models with the experimental data presented in Chapter 4 of this thesis to assess the capability of advanced URANS turbulence models to capture the flow topography adjacent to the test geometry.

In conclusion, hydrofoil depth of submersion and free stream velocity, have been shown to contribute to the inception of tip vortex ventilation. Building on the knowledge gained in this study, further insight might be obtained into the physics of the vortex ventilation

mechanisms, using simulation and tomographic PIV to quantify the effect of surface waves on the effective AoA, boundary layer separation and vortex core axial velocity.

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Appendix A. Correction of PIV data for carriage vi-

bration

Artefacts in the time-series data were visible as instantaneous distortions in the velocity field (Figure. B-1), resulting from carriage motions caused by minor irregularities in the rail connections and aperiodic tension release in the alignment mechanism These irregularities in the track of the towing tank carriage resulted in sudden planar accelerations which were captured in the instantaneous velocity fields.

In the absence of accelerometer or other data to aid correction, removal of the artefacts in the time series data was undertaken by calculating the deviations in the instantaneous velocity components from the temporal mean of those components within steady state regions distant from the tip vortex. The velocity components of each instantaneous velocity field were then adjusted according to the frame-wise variation from the temporal mean. Figure B-7-1 presents the modified velocity field.

The process for correcting the deviations in velocity fields that have been obtained perpendicular to the free stream is as follows:

Method

A steady state region (window) within the flow field is identified. A size is estimated with consideration to small scale flow features and turbulence length scale as appropriate.

The mean instantaneous transverse (V) and vertical (W) velocity components within the steady state region are obtained.

Mean values of V and W are obtained from the instantaneous values.

The frame-wise deviation between the V and W instantaneous value and the mean values are obtained

Figure B-1. Raw instantaneous velocity field, distorted due to carriage motion

Figure B-7-1. Corrected instantaneous

1. Raw instantaneous velocity field, distorted due to carriage motion

instantaneous velocity field, normalised to the mean flow in the steady state region

1. Raw instantaneous velocity field, distorted due to carriage motion

Appendix B. Variables of the PIV uncertainty

analysis using the ITTC

Target Flow of Measurement Target flow

Measurement facility Measurement area Uniform flow speed Max flow speed Calibration

3rd order polynomial mapping Magnification factor Flow Visualisation Tracer particle Average diameter

Standard deviation of diameter Average specific gravity Light source

Laser power

Thickness of laser light sheet Time interval

Image Detection Camera

Spatial resolution Sampling frequency Grey scale resolution Cell size

Optical System

Distance from the target Length of focus

Aperture (f number of lens) Perspective angle

Data Processing Image masking Image correction Image pre-processing Pixel unit analysis Multi pass mode Vector post-processing

Variables of the PIV uncertainty

analysis using the ITTC guideline 7.5-01-03

Target Flow of Measurement

2-D water flow

Towing tank (L 100m, W 3.5m wide, D 1.5m)

330 x 280 mm2

2.0 ms-1

2.0 ms-1

order polynomial mapping

400 mm

1492.5 pixels

0.134 mm/pixel

Spherical Carnauba

0.057 mm

Standard deviation of diameter 0.0062 mm

0.999

Double pulse Nd:Yag Laser

130 mJ light sheet 4 mm 0.6 ms 2559 x 2159 pixels 25 Hz 16 bit 6.5 x 6.5 micron 1264 mm 60 mm

Aperture (f number of lens) 8

6.214 o

As required for test geometry 3rd order polynomial mapping None

Cross correlation method (3x16x16 50 % overlap)

Delete vector if its peak ration Q< 1.03 2 x Median filter - remove and replace Remove if diff to average > 2 Denoising: 7x7

Remove groups with < 5 vectors Delete Vector if peak ratio Q<1.25 Fill-up empty spaces (interpolation)

Variables of the PIV uncertainty

03-03

Towing tank (L 100m, W 3.5m wide, D 1.5m)

Delete vector if its peak ration Q< 1.03 remove and replace

Delete Vector if peak ratio Q<1.25 up empty spaces (interpolation)

Appendix C. Image sequence and wake age

Conversion of image number to wake age, for a mean chord length of 0.1175m is presented in Table C-1. The wake age of zero is aligned with the trailing edge of the vertical strut.

Table C-1. Image number and wake age for velocities 2 -12 ms-1

Image No Time [s] Wake age in x/cfor velocity [ms-1]

- - 2 4 6 8 10 11 12 1 0.0 0 0 0 0 0 0 0 2 0.3 5 10 15 20 26 28 31 3 0.6 10 20 31 41 51 56 61 4 0.9 15 31 46 61 77 84 92 5 1.2 20 41 61 82 102 112 123 6 1.5 26 51 77 102 128 140 153 7 1.8 31 61 92 123 153 169 184 8 2.1 36 71 107 143 179 197 214 9 2.4 41 82 123 163 204 225 245 10 2.7 46 92 138 184 230 253 276 11 3.0 51 102 153 204 255 281 306 12 3.3 56 112 169 225 281 309 337 13 3.6 61 123 184 245 306 337 368 14 3.9 66 133 199 266 332 365 - 15 4.2 71 143 214 286 357 - - 16 4.5 77 153 230 306 383 - - 17 4.8 82 163 245 327 - - - 18 5.1 87 174 260 347 - - -

Appendix D. Image Plates

This appendix presents the image plates obtained at each velocity for a NACA 0012 t-foil at an AoA of 8°. The horizontal dashed line in each image marks the static free surface. The grid scale is the geometric mean chord length.

Submergence

Submergence h/c 0.22, AoA 8°, U∞ 6ms

Submergence

Submergence h/c 0.22, AoA 8°, U∞ 6 ms

Submergence

Submergence h/c 0.22, AoA 8°, U∞ 6 ms

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