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Experimental Investigation of High-aspect Ratio 2D Sails Andrew Crook and Margot Gerritsen |
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Research
Aims and Objectives · Gain a
deeper physical understanding of the flow past upwind and downwind sails
under various angles of incidence and Reynolds numbers · Create a
comprehensive data base for validation of numerical solvers and turbulence models
that can be used by the (sail) research community and industry at large The objectives
to achieve these aims for a range of two-dimensional sail sections and
apparent wind angles are: · For select upwind and downwind sections use Particle
Image Velocimetry (PIV) to understand the effect of
Reynolds number upon the flow topology which may include the size and
structure of leading edge separation bubbles with and without a mast for the
upwind case, trailing edge separation location if present and the structure
and frequency (measured using a hotwire or Laser Doppler Velocimetry
(LDV)) of the wake. Furthermore, the sensitivity of the pressure distribution
and sectional lift and drag coefficients to Reynolds number will be addressed
using Pressure Sensitive Paint (PSP) and a total pressure tube wake rake · For all six
sail sections use a range of tools including surface oil flow visualization,
PSP, oil film interferometry or shear sensitive
liquid crystals, to understand the main flow topology on both surfaces such
as mean separation locations, transition location if no leading edge
separation is present · To measure
sectional lift and drag coefficients, and the skin friction distribution for
all six sail sections, with and without a mast for the upwind cases, and to
correlate this with the flow topology · For a select upwind case, to investigate in detail the interaction between the mast and sail for a range of sail incidences and effective wind angles with respect to the mast. PIV will be used to measure the flow structure and PSP and the wake rake to measure the section lift coefficient (CL) and drag coefficient (CD), respectively |
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Relevance
of Proposed Research Optimization
of sail designs is challenging. The hull, sails and rig form an integrated
system with many parameters that should be optimized simultaneously. However,
the fluid-structure interaction between the three-dimensional flow around the
sail and the hull, and the effect of the dynamic motion of the boat on the
sail flow and hull hydrodynamics are complex. The flow and
performance of a sail is sensitive to external conditions, aeroelastic effects, sail trim, and it may also be
Reynolds number dependent. A Velocity Prediction Program (VPP) solves a set
of equations that govern the motion of the yacht and is used to take into
account the performance of the yacht when designing a sail. Wind tunnel tests
are the preferred method of research for modeling the aerodynamics of a
yacht. Sail flow
characteristics and the current state of the art State of the
art sail design is different for upwind and downwind sails. For the upwind
conditions the camber of the sail and the angle of incidence of the apparent
wind to the sail are small, resulting in largely attached flow. Upwind sail
performance is highly sensitive to trim because of the small angles of
incidence involved, meaning that small changes affect the performance
significantly. Upwind sail flow analysis is generally performed using panel
methods, and sometimes Euler codes. Physical
understanding of the flow around generic sail sections at representative
Reynolds numbers is low. An enhanced understanding of the flow physics around
sail sections is required as a first step to understanding the more complex
flow around a three-dimensional upwind sail. Three-dimensional
upwind sails may have separated flow at the head of the sail whilst the flow
remains attached elsewhere as a result of the twisted onset flow. This
greatly influences the sail design and trim. To reduce separation near the
head, the sail is usually twisted also. Generally, strong tip vortices are
shed off the head and the foot of the sail. As a result, the induced drag is
large, and may contribute as much as 15% of the total boat drag (including
hull, rigging and wave drag). Heeling of the boat also significantly affects
the performance of the sails. Also important to understand is the sensitivity
of the two-dimensional flow to Reynolds number, wind direction, camber and
the effects of the mast and it’s orientation with respect to the sail. Such
parameters will effects the transition behavior of the flow, the size of the
leading edge separation bubble if any, and the location of the trailing edge
separation. A correlation of these flow features with the corresponding sail
pressure distribution, lift and drag will be invaluable in understanding how
to better design sails to provide the greatest amount of forward thrust
without exceeding a given rolling moment (Wood and Tan (1978)). Flow
simulation requires the use of viscous solvers and turbulence models. When
using windtunnels, the same limitations of many
upwind experiments such as low aspect ratio and purely force and moment data
are also seen for the downwind experiments. For the downwind case, it is
therefore important to provide flow topology and force/pressure data on
realistic camber section sails that have a high aspect ratio for a range of
wind angles and Reynolds numbers. Prior
experiments (Milgram (1978) and Wilkinson (1984))
have been few and their quantitative use is limited. Facilities All models
will be tested in the 7x10 ft closed-return wind tunnel at NASA Ames. This
wind tunnel is capable of a maximum freestream
speed of 200 knots (103 m/s) and has a freestream
turbulence intensity of approximately 0.25%.The tunnel will be run at a speed
of 30-40 m/s, leading to a chord based Reynolds number of approximately half
a million. This velocity is low enough to avoid compressibility effects, but
high enough to utilize techniques such as PSP which need a reasonable dynamic
pressure to work well. The closed-return tunnel also simplifies the process
of seeding the tunnel for optical techniques such as PIV and LDV. After the
experiments are performed, two designs (one upwind, one downwind) will be
tested in the 12ft pressurized tunnel at NASA Ames at higher Reynolds numbers
to conduct Reynolds number sensitivity analyses. Using this tunnel Reynolds
numbers of up to 4 million per foot (6 atm
pressure) can be achieved at a Mach number of 0.1. Designing a thin sail to
withstand such high dynamic pressures will not be an easy task, and
performing the tests at a lower pressure of 2 atm
(Reynolds number of 1.4 million per foot at Mach 0.1) is more feasible. Techniques
Utilized |
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Timeline November
2002……...Complete design of models and commence construction December
2002……...Complete model construction February
2003……….Conduct initial tests on one section to evaluate testing procedures June
2003……………Complete testing on the remaining sail sections June
2003……………Complete turbulence model validation for downwind sail flow analysis September
2003……..Complete optimization study for the rig of the Maltese Falcon October
2003………..Construct models to test the sail sections suggested by the
optimization method November
2003……..Conduct the final windtunnel experiments January 2004…………Improve optimization method based on the windtunnel validations |
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