Sail Course ® Section 19, Docking and Anchoring. Sail Course ® Figure 19–1 Docking under Sail.
An Explanation of Sail Flow Analysis
Transcript of An Explanation of Sail Flow Analysis
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An Explanation of Sail Flow Analysis
Introduction
Stanford Yacht Research (SYR) is currently doing a study in performance analysis on yacht
sails through experimental and computational methods. This research is being done to study
the flow around sails in a wind tunnel and to validate computer results against experimental
results.
Those involved with the present research include:
Dr. Margot Gerritsen, Stanford Yacht Research
Dr. Andrew Crook, NASA Ames Research Center
Tyler Doyle, Ph.D. student at Stanford University
Sriram Shankaran, Ph.D. student at Stanford University
Steve Collie, Ph.D. student at University of Auckland
Jean-Edmond Coutris, Graduate student at Stanford University
Brendan Abbott, SYR intern - undergraduate at Webb Institute
Daniela Hanson, SYR intern - undergraduate at Webb Institute
Center for Turbulence Research (CTR)
Aero-Astro Department
NASA Ames Research Center
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Sail Design
Working toward designing more and more efficient sails throughout history, many methods
have been employed. From the beginning of sail design,intuition and experience have been
the primary means of designing sails. Intuition and experience still play a major role in sail
design because presently no database exists to tell a sailboat designer what sail will work bestwith a given hull shape!
From intuition,prototype sails can be built to test a design and measure its effectiveness.
Also, wind tunnel testingcan be done with model sails to observe and study the flow around
sails on a smaller scale.
Today, computers have become a means of calculating a lot of information in short amounts
of time. Computer programs that employ the Navier-Stokes fluid mechanics equations can
provide answers to very complicated and long flow equations in relatively short amounts of
time. Through experimental testing, flow results calculated by computers can be validated. In
the near future, it is possible that computers will be able to fully simulate flow on a sail.Today, the power and speed of computers limits what work can be done to analyze flow.
How Does Air Flow Past a Sail
BASIC FUNCTION OF SAILS
Sails are instruments that use the wind to propel a vessel through the water. Trimming the
sails differently allows a vessel to sail at different angles to the wind.
AIR FLOW AT DIFFERENT SAIL ANGLES
Upwind Sailing- In upwind sailing, sails act similarly to foils. The forces generated by the
sail result in lifting forces generated by the keel, and forward motion is produced. (A
FURTHER EXPLANATION OF THE BASIC PHYSICS OF UPWIND SAILING CAN BE
FOUND INTHE PHYSICS OF SAILING )
Reaching
http://www.phys.unsw.edu.au/~jw/sailing.html/http://www.phys.unsw.edu.au/~jw/sailing.html/http://www.phys.unsw.edu.au/~jw/sailing.html/http://www.phys.unsw.edu.au/~jw/sailing.html/ -
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Downwind Sailing - Sails are used to catch the wind. The wind is used to "push" the boat along.
Twist
To describe how air flows past a sail, we must describe how air flows over the water. To
simplify things, let's first assume that the water is not moving. At the water's surface, the air is
moving at the same speed as the water. Thus, the air cannot be moving at the water's surface.
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However, if there is wind, we know that the air must be moving as we move away from the
water. Because the true wind speed is greater further up the mast, the apparent wind created
by the boat's forward velocity causes a "twisting" phenomenon:
Upwind Twist Diagram
The effect of twist is more apparent in downwind sailing. An International Americas CupClass (IACC) yacht, because of its ability to sail downwind close to the speed of the wind,
never really sails downwind at all. The IACC yachts are always experiencing apparent wind
angles that simulate reaching conditions (about 90 degrees), rather than downwind conditions.
Downwind Twist Diagram
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Difficulty of Modeling Actual 3D Sails
Modeling of sails is difficult because the conditions that a sail experiences fluctuate
tremendously. Trim, wind speed, boat speed, heel angle, and weather all change over time.
These factors change the way a sail will perform. Also, flow patterns due to twist are hard to
model in 3-D computer applications because of the complexity of eddies shedding from thehead and foot of the sail.
In This Study, SYR is Using 2D Sections
Rather than modeling 3-D sections, a 2-D section of a sail is used to reduce computational
cost. 2-D sections are representative of flow at a given height on the sail.
The 2-D sections are optimized with computer programs. The flow patterns generated from a
2-D section, although simplified, are not completely irrelevant. Sail-makers still use
optimized 2-D sections to create a 3-D sail. Generally, ten to twenty 2-D sections are used in
making a sail.
Computational Fluid Dynamics
The computer application that SYR uses to analyze flow is a Computational Fluid Dynamics
(CFD) program. CFD is a method that employs fluid mechanics equations to describe flow.
A shape, such as a sail section, is created in the CFD program. It is the computer's task to
calculate the flow field around the shape. However, there are an infinite amount of points
around the shape. Calculating the flow in an undefined region would take decades. Therefore,
a grid must be created around the shape to break down the computational process into a finite
number of calculations. The grid is a crucial part of the solution process. If it is not dense
enough, the flow solution will be lacking. If the grid is too dense, calculations will be too
computationally expensive. This is the challenge of producing good CFD results.
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After a satisfactory grid is created, the CFD program can run. A flow analysis on a 2-D
section takes about 5 hours to run on a high quality computer (such as a Pentium 4). The
results obtained from CFD are velocities at all grid points, pressure distributions, and forces
acting on the sail section. Forces acting on the sail are determined by integrating the pressure
distribution over the sail area.
Velocities at All Grid Points
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Pressure Distribution and Streamlines
Forces Derived from Pressure Distribution
Wind Tunnel Testing
To validate the CFD results, 2-D sections are also being tested by SYR in the 7 ft x 10 ft wind
tunnel at NASA Ames. To create a 2-D model for the wind tunnel, a "slice" of a sail is taken.
This slice is scaled down to a chord length of 2/3 of a foot; then, it is stretched to a height of
10 feet. The model is created with a high aspect ratio to try to eliminate vertical flow. A realsail has a flow pattern like this:
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We only want to analyze the sail flow in two dimensions:
The basic wind tunnel setup looks like this:
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What Does This Tell Us About Sailing?
The use of CFD and wind tunnel testing gives insight into flow around sails and improves
CFD codes. Better understanding of flow around sails will hopefully produce more efficiently
designed sails in the future.
Results from sail modeling tell us, for example, about the importance of trim, staying in
"clean air", and how the main and foresail interact.
Importance of Trim:If the sail is not trimmed correctly, it is not efficient. Over-trimming causes excessive turbulence on the leeward side of the sail.Under-trimming greatly reduces the forces produced by the sail. Ideal trim produces the best flow distribution around the sail.
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Effect of Upwind Boat and "Dirty" Air: The downwind boat cannot point as high as the upwind boat because of the effect of the flow around thesails of the upwind boat.
Main and foresail interaction: The flow around the mainsail speeds up the air at the trailing edge of the foresail, making it more efficient. The airflowing past the mainsail helps the jib sail at a smaller angle of attack.
Limitations of Modern Sail Research
In computer and wind tunnel simulations, simplifications are made to the sail's environment.First of all, the sail is analyzed independently of hull and water. A rigid sail is used in an
upright position. To completely model a real sail it would be necessary to use a flexible 3-
dimensional sail in a heeled position, and to consider the effects of the hull and sea state.
However, modeling something like this would be far too computationally expensive.
In the future, with a growing amount of power and speed in the computer, the possibility of
analyzing more complicated systems will be greater.