Nessun titolo diapositiva

1
AERODYNAMICS OF RACING YACHT APPLICATIONS
A Sail Boat is a complex aerodynamic system
immersed in two fluid in relative motion
The interaction between the air part and the
water part of the system determines the overall
boat performance
2
DESIGN TOOLS
AERODYNAMIC TOOLS
OVERALL PERFORMANCE PREDICTION
HULL AND APPENDAGES ------------------ Tank
Test CFD
SAIL PLAN -------------- Wind Tunnel Full scale
test CFD
VELOCITY PREDICTION PROGRAM
3
VELOCITY PREDICTION PROGRAM
The VPP provides a steady state speed of the
Yacht given the wind direction and magnitude
VPP has a two part structure comprised of the
HYDRODYNAMIC FORCE MODELS and AERODYNAMIC FORCE
MODELS
The JOB of the Solution Algorithm is to find that
combination of Boat Speed and Heel angle so that
aero and hydrodynamic forces are in equilibrium
This is a typical polar plot which shows the boat
speed for a specific true wind and heading
4
VELOCITY PREDICTION PROGRAM
The cores of a VPP are the HYDRODYNAMIC and
AERODYNAMIC FORCE MODELS
The quality of the solution is dependent on the
accuracy of the data of the force models. The
models are usually based on both theory and
empiricism
HYDRODYNAMIC MODEL
AERODYNAMIC MODEL

1. Theoretical relationships
2. Systematic Tank Test (e.g. Delft series)
3. Computational Fluid Dynamics

1. Full scale Test
2. Wind Tunnel
3. Computational Fluid Dynamics

5
HYDRODYNAMIC FORCE MODEL
The model has to provide the total drag as a
function of boat speed and leeway (angle of
attack)
Usually the total hydrodynamic drag of the yacht
is assumed to be the sum of the following
components

1. Wave resistance of the canoe body (systematic
   Tank tests)
2. Appendage and canoe body friction and form drag
   (theoretical relationships, empiric coefficients)
3. Induced drag. Drag associated to the generation
   of lift (theoretical relationships)
4. Added resistance in waves. Unsteady motion due to
   the sea waves (empirical correlations, tank tests)

6
AERODYNAMIC FORCE MODEL
The model has to provide the DRIVING FORCE and
the HEELING FORCE as a function of Apparent Wind
Speed (AWS) and Apparent Wind Angle (AWA). The
polar plot of the sail plan is determined making
use of Tunnel Wind Test or CFD analysis
VPP
The final results of the VPP analyses are
estimates of yacht speeds and sailing times
during a regatta, the best true indicator of
performance. This tool is widely used by the
naval architects in the early stages of the
design process to compare a large number of
configurations with different overall parameters
(such as displacement, length, sail area, .)
7
COMPUTATIONAL FLUID DYNAMICS (CFD)
IMPROVEMENTS IN COMPUTER PERFORMANCE HAVE MADE
THE USE OF Reynolds Averaged Navier-Stokes (RANS)
equations FEASIBLE FOR PRACTICAL DESIGN
APPLICATIONS
LATEST GENERATION OF RACING YACHT HAS GREATLY
BENEFITED OF THESE TOOLS, ORIGINALLY DEVELOPED
FOR AEROSPACE APPLICATIONS
8
COMPUTAIONAL FLUID DYNAMICS (CFD)
CFD methods allows one to investigate in details
the flow fields and to reach a better
understanding of the flow phenomena
While VPP provides an overall performance
prediction, CFD methods allows to reach a refined
aerodynamic optimization
9
COMPUTAIONAL FLUID DYNAMICS (CFD)
Principal CFD methods

1. Panel method, inviscid potential flow equations.
2. RANS, Reynolds Averaged Navier-Stokes equations.
   Capable of solve viscous effects

PANEL METHODS Based on the inviscid potential
flow equation. Panels are distributed over the
model surfaces. Over each panel is distributed a
constant source and/or doublet singularity, which
satisfies the governing equations Low
computaional cost
10

• VISCOUS METHODS
• RANS equations spatially discretized on a
  computational grid.
• Grid points clustered in regions where viscous
  effects are important.
• Spatial Discretization
• Finite element
• Finite volume
• Steady state solution
• Unsteady solution periodic fluctuations of the
  main fluidynamic quantities

Unstructured computational grid
Structured computational grid
11
CFD APPLIED TO HULL AND APPENDAGES
HULL
KEEL AND RUDDER
3D Analysis --Plan form --Bulb,
winglet --Interaction between keel and wave
system
2D Analysis Laminar foil for low Reynolds
number, capable to shift downstream the location
of transition onset
Wave Resistance Prediction
12
CFD APPLIED TO SAILS
13
CFD APPLIED TO RACING YACHTS
THE AERODYNAMIC OPTIMIZATION IS A COMPLEX TASK
REQUIRING KNOWLEDGE OF AERODYNAMICS AND HOW TO
ACT ON THE GEOMETRY IN ORDER TO IMPROVE THE
PERFORMANCE. OPTIMIZATION TECHNIQUES CAN BE
USEFUL IN THIS KIND OF DESIGN PROCESS
14
OPTIMIZATION METHODOLOGIES
Design process as optimization of transfer
function PjPj(Xi) Pj Performance
parameters j1,M Xi Geometrical parameters
i1,N

• Geometrical parameterization
• Bezier Curves
• NURBS

• Optimization techniques
• Gradient Based Methods
• Design of Experiments
• Evolutionary algorithms

15
OPTIMIZATION METHODOLOGIES
Gradient Based Methods

• Exploration of original configuration
  neibourhood
• Evaluation of the gradient of TF
• Solution moved toward maximum gradient
  direction

• Time cost depending on initial configuration
  choice
• Few iterations required
• Widely used for local optimum searching

16
OPTIMIZATION METHODOLOGIES
Design of Experiments (DOE)

• Transfer function approximated with a
  polynomius
• P ß0 ß1X1 ß11X12 ß12X1X2 ß123X1X2X3
• Evaluation of transfer functions on a set of
  configurations
• DOE Theory Determination of ß coefficients
  through least squares regression

17
OPTIMIZATION METHODOLOGIES
Genetic Algorithms

• Based on Darwins evolutionary theory
• Initial set of design configurations
  (population) randomly selected
• Direct evaluation of TF for each configuration
• Three genetic operators selection,
  recombination and mutation
• Sequential generation of improved populations
  

18
OPTIMIZATION METHODOLOGIES
Comparison between different strategies
19
ON THE USE OF CFD TO ASSIST WITH SAIL DESIGN
INTRODUCTION
TRADITIONALLY THE SAIL DEVELOPMENT IS DELEGATED
TO THE SAILMAKERS EXPERIENCE
COMPUTATIONAL FLUID DYNAMICS CAN BE AN INNOVATIVE
TOOL THAT ALLOWS TO TEST AND COMPARE A LARGE
NUMBER OF CONFIGURATIONS IN A RELATIVELY SHORT
AMOUNT OF TIME
20
ON THE USE OF CFD TO ASSIST WITH SAIL DESIGN

• Aerodynamic design of an IACC sail plan in
  upwind condition.
• Optimal Sails profiles depending on
• Hull and appendages features
• Wind speed and angle
• Sailing style and trim

• Critical for
• Manufacturing problems
• Sail shapes not fixed, varying with wind pressure
  and sail trim

21
DESIGN CRITERIA

• Sails are often compared to aircraft wings.
• Unfortunately classic aeronautical design
  criteria are only partially useful
• Contrary to wings, sails must work in a wide
  operative range (i.e. angle of attack)
• The goal in the aeronautical design is to
  minimize the losses (drag). In sail design the
  goal is to maximize the driving force, without
  taking into account losses.
• The classic Lift-Force projection must replaced
  with the sailing aerodynamics terms
  Driving-force Heeling-Force
• The flow is often separated (depending on the
  angle of attack)

22
DESIGN CRITERIA

• Design Operating Conditions
• True Wind Speed
• Apparent Wind Angle

• Input data
• Initial configuration recovered from pictures
  (deformed shape)
• Righting Moment available

Under fundamental hypothesis of a constrained
Heeling Moment, the GOAL is to find a maximum
driving force configuration.
23
GEOMETRY DATA ACQUISITION

• Superimposition of Bezier curves.
• Principal parameters used to describe the
  profile
• CAMBER
• DRAFT
• Entry Angle
• Exit Angle
• Twist Angle

24
NAVIER-STOKES Solver Hydro

• 3D, fully viscous, multi-grid, multi-block code
  developed by the University of Florence
• Acceleration techniques employed
• Local time-stepping
• Residual smoothing
• Multi-grid Full Approximation Storage (FAS)
• Boundary Conditions
• Solid walls no-slip condition
• Inlet total pressure and flow angles
• Outlet static pressure
• The Earths boundary layer can be taken into
  account imposing spanwise variable inlet
  conditions.

25
GRID GENERATION
The first step is to generate a computational
grid around the sail plan. The computational
grid represents the domain where the solver
computes the solution.

• The size (number of points) of the computational
  grid has to fulfill three main requirements
• To solve the flow phenomena of interest (e.g.
  boundary layer)
• To compute solutions sufficiently
  grid-independent
• To match computational times to design needs

26
GRID GENERATION
The boundaries have to be defined far enough from
the sails, where the flow is indisturbed.
27
GRID GENERATION
The Grid consists of three block. Structured
H-type grids were employed in computations. The
grid generation is based on an elliptic procedure
on 2D grids which are subsequently stacked in the
vertical direction.
28
GRID GENERATION
2 200 000 grid point was judged to be a good
compromise (memory requirements of about 650MB)
for mainsail-genoa configuration
For a complete mast model, a viscous grid with
285 x 129 x 93 grid points in the chordwise,
chordwise-orthogonal, and spanwise directions,
was used (3 400 000 grid point, 1000 MB memory
requirements)
Details of the viscous grid around the mast
Grid points clustered near the solid walls in
order to solve the boundary layer
29
VISCOUS SOLUTION
The RANS codes allow to take into account the
viscous effect such as boundary layer, separation
bubble. Compared to other design tools, the RANS
solution allows to investigate in detail the flow
structures
30
PARAMETRIC ANALYSIS
The first analysis presented is obtained through
a parametric variation of the Genoa Camber on
the whole span from the initial value 11 up to
21. The design Apparent Wind Angle (AWA) is 16
deg.
31
PARAMETRIC ANALYSIS
Static pressure distribution on the genoa profile
The results of the computations, for
incompressible flow, are the velocity vectors and
the static pressure field (one value for each
grid point)
LEADING EDGE
TRAILING EDGE
32
PARAMETRIC ANALYSIS
Obviously the Camber variations from its initial
value involve both driving force and heeling
moment changes.
33
PARAMETRIC ANALYSIS
Which is the Optimal Genoa Camber value? -- The
optimal Camber is a comprime solution between
heeling-moment and driving force. -- It is
necessary to evaluate the trade-off between
heeling moment and driving force.
-- To this aim the initial Heeling Moment
(constrained) has to be re-established through
the Mainsail Twist Distribution -- This is a
well-known practice for reducing the heeling
moment used by every yachtsmen
Heeling force
34
PARAMETRIC ANALYSIS
--To re-establish the initial Heeling Moment
(which is the optimization process constraint)
the mainsail twist is varied
--Three parametric curve for three separate
mainsail twist
35
PARAMETRIC ANALYSIS
IT SHOULD BE NOTED THAT THE HEELING MOMENT
COEFFICIENT IS CORRELATED TO THE WIND SPEED
Heeling Moment Coefficient definition
Where Hm Heeling Moment Chm Heeling Moment
Coefficient SA Sail Area dm Distance between
CLR and mast-head
SINCE Hm IS UPPER BOUNDED BY THE BOAT STABILITY,
THE GREATER THE WIND SPEED (AWS), THE LOWER THE
HEELING MOMENT COEFFICIENT (Chm) MUST BE
36
PARAMETRIC ANALYSIS
Light Wind
Moderate Wind
Heavy Wind
Which is the Optimal Genoa Camber value? --
Points A,B and C represent the optimal camber
values for three separate wind magnitude
37
GENOA CAMBER PARAMETRIC ANALYSIS
PLOTTING THE DRIVING FORCE IN A THIRD DIMENSION
WITH THE AID OF THE CONTOUR LINES (lines at
constant driving force)
GIVEN THE RESULT OF THE ANALYSIS, THE ESSENTIAL
FEATURES OF SAILS IS ITS ABILITY TO BE ADJUSTED
TO MATCH THE WIND SPEED CHANGES
38
GENOA CAMBER PARAMETRIC ANALYSIS
SAIL REQUIREMENTS ABILITY TO BE ADJUSTED TO
MATCH THE WIND SPEED CHANGES
NEED FOR MANY SAILS FOR EACH WIND SPEED

• In upwind conditions, an Americas Cup Class has
  2/3 different mainsail and 5/6 different genoa
• Genoa code 1 lt6 kn TWS
• Genoa code 2 8 kn TWS
• Genoa code 3 12 kn TWS
• Genoa code 4 16 kn TWS
• Genoa code 5 20 kn TWS

39
MAINSAIL CAMBER PARAMETRIC ANALYSIS
THE ANALYSIS HAS BEEN REPEATED FOR THE MAINSAIL
CAMBER (AWA 16 deg)
True Wind Speed Optimal camber
8 kn 9.8
13 kn 6
40
MAINSAIL CAMBER PARAMETRIC ANALYSIS
GIVEN THE RESULT OF THE ANALYSIS, MAINSAIL SHOULD
BE FLATTER THAN GENOA.
Main Head
Genoa Head
TWS 8 kn Optimal camber
Mainsail 9.8
Genoa 16
Genoa
Mainsail
This fact can be explained by taking into account
the load distribution The aerodynamic load on
the Genoa is greater than the load on the
mainsail
Main Base
Genoa Base
41
OPTIMUM SPANWISE CAMBER DISTRIBUTION
To go further in the analysis is it possible to
consider the optimum spanwise distribution of the
Genoa camber
CAMBER should increase with increasing
height
42
OPTIMUM SPANWISE CAMBER DISTRIBUTION
To go further in the analysis is it possible to
consider the optimum spanwise distribution of the
Genoa camber
CAMBER should decrease with increasing
height
43
OPTIMUM SPANWISE CAMBER DISTRIBUTION
SPANWISE LOAD DISTRIBUTION
--Light Wind increasing load with
increasing height
--Moderate Wind decreasing load with
increasing height
Genoa Twist distribution fixed
Load on the Genoa is induced by the
Mainsail twist distribution
44
CONCLUSION
From the 1983 turning point in the Americas Cup
hystory to today, the use of CFD in the yacht
design process has quickly increased While no
CFD methos should claim to replace other design
tools (wind tunnel, tank test), CFD play an
important role in a modern design
process Improvements in computer performance
have made the use of RANS the main CFD tool for
practical design applications, opening new
frontiers in racing yacht design