Experimental Investigation of High-aspect Ratio 2D Sails

Andrew Crook and Margot Gerritsen


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





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.



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

Pressure Sensitive Paint (PSP)

Oil film interferometry

Particle Image Velocimetry (PIV)











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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