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HASRUL TAUFIK BIN MOHD ROBI
AM080217
890422-11-5367
3-SMK
DRAG, BOUNDARY LAYER AND HULL RUOGHNESS ON THE HULL SURFACE
27/09/2010
(Signature)
____________________________________
HASRUL TAUFIK BIN MOHD ROBI
INTRODUCTION
The ship needs power to move at the given speed. The power used to overcome the total
drag of the ship. So it is important to know the cause of the drag and how to determine the
value. The drag is the force required to tow the ship at given speed on calm and smooth
water. The total drag has two major components, the frictional drag and residuary drag.
Example of the frictional drag is skin friction and example of the residuary drag is wave
making resistance. The factor that influenced the total drag is the boundary layer, hull
roughness, pressure, shape of the hull and also the weather and route taken of the ship.
The boundary layers show the behaviour of the flow at the surface of the hull. There are 2
type boundary layer, laminar and turbulent flow. They depend mostly on the Reynold
number. The roughness of the hull also effects the formation of the layers.
The hull roughness is important parameter that should not be ignored. Hull roughness
measured by the device called hull roughness gauge. The roughness increase the total drag,
thus increase the cost. Normally the roughness of the ship is increase after the service. paint
coating, antifouling and cleaning at the dry dock can reduce the hull roughness.
DRAG
In fluid dynamic study, the drag or resistance of a moving body in a fluid is refers to the
forces that oppose the motion at the given velocity. The drag of a body can be divides into
two components. One of the components is the drag is always opposite to the movement of
the body and for the perpendicular component called lift and it is important in aerodynamic
study. For a body to move at the given speed, the power system need to be installed to give
the thrust which the force that supplied to overcome the drag of the body. The shape of the
movement body is important factor to determine the drag. There are various type of drag
and can be divided into categories:
Parasitic drag- form drag, skin friction, interference drag
Lift-induced drag
Wave drag
For the high velocity movement or at high Reynold number because from the Reynold
number equation: Rn=v ∙Lν
Therefore Reynold number is proportional to the velocity and the overall drag of the body
can be calculated by the drag equation
FD=12ρV 2CD A
Where FD is drag force,ρ is a density, V is velocity, A is area and CD is the drag coefficient.
The area, A area of the object which is perpendicular to the movement of the motion such
example if the object is sphere, the area is the cross sectional area, that is area of the circle
of radius r same with the radius of the sphere. The drag coefficient is the dimensionless
quantity that pictures the drag of the body. The higher value of the drag coefficient shows
higher the drag force. The drag coefficient also depends on the Reynold number of the
moving body.
The further study of the drag or resistance of the ship is called hydrodynamic study. Based
on the study, many ships can be design to move efficiently with using less power for the
given speed. The study is very important to know the behaviour of the flow and the total
drag of the ship. The result of the study is very important to predict the cost of building and
performance of the ship. Many engineer and ship owner wants to reduce the cost as low as
possible for their own benefit.
To move a ship at the given speed we need to know the effective power of the designed
ship. The effective power of the ship can be calculated by using the formula:
PE=RT ∙V where
RT is t he totaldrag∨resistance and V ist he velocity of t he sh ip .
The drag of the ship that moves at desired speed is the force need to tow the ship at the
given speed at the smooth water. It is assume that there is no interferences form at the
towing ship. But the condition is nearly impossible for the actual ship can move without any
form interferences and sail smoothly. So in order to get the total drag of the actual ship, we
need to do the model test at the towing tank and by using the information of the model
test, the actual resistance of the ship can be determine. The method to determine the
resistance of the ship based on the model test is called ITTC method. The method is firstly
introduce by William Fraude in 1863 where he come out with the derivation and
justification on to extrapolate the model test result to the actual full scaled ship.
The total drag of the ship is divided to two big components that are the drag occur due to
the viscosity of the fluid where the ship move called viscous drag or frictional resistance and
the other component is the drag that formed because the ship running at the surface of the
water and the examples are the wave-making resistance, eddy resistance and air resistance
are called the residuary resistance. Therefore the total drag is sum of all the component of
drag: RT=RF+RR.
At early stage, there is difficulties happens when the total resistance of the model is not
same with the actual scale although using the same formula. This happen because of the
behaviour of the model at the towing tank is not the same with the actual ship at the ship.
Lastly William Fraude has come with the solution by introducing two laws that very
important to the study named the Reynold’s law and Fraude’s law. Other law that introduce
by Fraude is law of comparison which stated that:
RR proportional ¿∆whenV is proportional ¿ t he √L
Then the Fraude’s formulation to determine the frictional resistance:
RF=SV n
From Fraude Hypothesis, CD=CF+CR and CRS=C RM
Where CF=0.075
(log10RN−2 )2
CDS=CFS+CRS+allowances and RT=12V S2 ρSSSCDS
PE=V S ∙ RT
TYPE OF RESISTANCE AT HULL SURFACE
Friction drag
Friction drag occurs due to the friction of water with the surface of the hull. The friction drag
is depends on the roughness of the hull, the wetted surface area and the type of boundary
layer form along the hull. The water has the viscosity, when the ship is moving, the water at
the surface of the hull will attracted to the motion of the hull and form the sheer stress
along the body. The stress is opposing the hull to move freely. As the wetted surface area
increase, which mean the bigger the hull, the bigger the drag. This is because more water
molecule attracted to the hull surface creates higher shear stress. The type boundary layer
affected the frictional drag. Most of the ship has turbulent flow which is thicker than the
laminar flow. The turbulent flow tends to has larger frictional drag compare to the laminar
flow. The friction drag is higher at the high hull roughness because more water is dragged to
the hull. The situation is similar when we walk on the slippery floor compare to roughen
floor. The friction is higher at the roughen floor.
Pressure drag
The pressure drag is related to the condition of the streamline at the surface of the hul. If
the streamline nicely stream, the pressure resistance is low. If the streamline is break and
has a lot of disturbance, the pressure resistance is higher. The pressure resistance is
depends on the shape of the hull. For a hull surface, the streamline crowded at the middle
of the hull and the streamline will shift at the stern. So, the pressure resistance is higher at
the stern and lead to formation of eddies thus increasing the total drag. The pressure
resistance also depends on the flow characteristic. The pressure resistance is lower when
the flow is turbulent but for the hull this condition is can be true because of the size the
turbulent flow. Example of the application is on the golf ball. The golf ball is design to have
the turbulent flow to lower the pressure resistance so the ball can go further. The
application is not applied to the hull because it will increase the building cost.
Wave making resistance.
When the ship is running, there will be distribution on water level will increase the energy
power usage and fuel. Because of the shape of the hull, the hull will generate the pressure
to the water level when running. The pressure will cause the waves to disturb the
distribution of the pressure at the hull thus increasing the effective power. The wave making
is largely depends on the shape of the hull. There are devices that can help the hull
influenced the wave making resistance in favourable ways called bulbous bow. The bulbous
bow help to create good wave that disturb the wave system that create by the hull, thus it
can lower the wave making resistance and lower the cost.
Air resistance
Air resistance caused by the interaction between the air and the above water surface part
and the structure. The air resistance contribute 2% of the total resistance.
BOUNDARY LAYERS
Based on the fluid dynamic, every motion body in the fluid will behave the dynamic flow
behaviour with different characteristic. The flow behaviour that forms at the surface of the
motion body is called boundary layer. As for the hull surface of the ship, the boundary layer
will react at along the surface of the hull transversely depend on the speed of the motion
and viscosity of the fluid. The speed and the viscosity of the fluid can be showed by the
Reynold’s number (Rn). So, the type of layer at the surface of hull is strongly depends on the
Reynold’s number.
The flow will give the force to the body of the hull against the motion of the ship that is the
shear force. Every real fluid has the viscosity, due to the intermolecular force. The bigger the
intermolecular force, the bigger the viscosity. When the ship move at the constant speed,
the fluid molecule near the hull will carried away along with the surface and the fluid that
are close to the hull move in the same way of the direction of the hull. As a result, there will
be layer that act at the hull surface and it become thicker from stern to bow.
The layer become thicker because of the velocity of the fluid is varied alongside the hull. The
layer become thicker and thicker and lastly will leave the hull surface at the end of stern and
form the frictional wake which is one of the components in total ship resistance. If the hull is
a blunt shape at the end, the boundary layer will leave at the separation point which can
lead into the pattern of eddies at back thus further increase the total resistance of the ship.
There are 2 type of boundary layer consist of the laminar and the turbulent layer. The
phenomenon is depends on the speed of the hull surface or the Reynold number of the
fluid. At the slow speed which is almost zero, means at very low Reynold number, the flow
behaviour of the streamlines are laminar flow. At the flow, the thickness is very small and
smooth that gives relatively small resistance or shear stress to the body of the hull surface.
Usually the laminar flow can be found at the Reynold number of 4 to 5^10. As the speed of
the ship increase thus increases the Reynold number, the boundary layer become thicker
and will disturb the laminar flow cause it to break down and the and change to the
turbulent flow. There will be the transition layer in between the laminar and the turbulent
flow where the flow changes its formation called sub laminar layer. There is no specified
place taken of the flow transition. The turbulent flow has higher friction, thicker and gives
more resistance to the body of the hull resulting the increasing of the total resistance of the
ship.
Another reason that the turbulent can grow faster is the roughness of the hull, dirt, wave
current, and even weather. All the reason proves that for a ship to have laminar flow is
impossible. All the ship sailing at the sea experience the turbulent flow, so the model test
that carried at the towing tank need to experience the same as the actual ship in order to
apply the ITTC method for calculating total ship resistance. The turbulence inducer must be
installed to the model to ensure that the model run with the turbulent flow alongside the
hull surface. The installation is at the bow of the model and the example of the turbulence
inducer is trip-wire, sand roughness and stud. Mostly the stud is applied because it easy to
install.
HULL ROUGHNESS OF SHIP HULL SURFACE
Hull roughness is one important part of study and research of marine engineers because it is
included as one of the factors that relate the total resistance of the ship. Mostly, the hull
roughness cause the boundary layer thickness increases and as the result, increasing the
frictional resistance of the ship thus increase the operating performance and fuel cost of the
ship to keep the desired speed of the ship. Another effect of the hull roughness is the
increasing of the cost to clean the hull and the cost of coating which is done at the dry dock
is expensive and consumes time.
Many engineers and ship owner want to keep the hull as smooth as possible to reduce the
fuel cost of the ship. According to research done by Royal Institute of Naval Architect, for
every 10 micrometers increases in hull surface roughness is an increase of 0.5-1% of the
power to maintain the same speed. There are two main cause of the hull roughness that is
physically and biologically. Examples of the physically hull roughness are corrosion, cracking,
detachment, cold flow, touch-up repair, welds, plate profile, mechanical damage and etc.
Most of the biologically factors is cause by fouling. There are two type of fouling, which are
micro and macro fouling. Macro fouling is caused by animal such as barnacles, tube worms,
and mussels which resulting 40% increase of the drag and by weed such as marine grass,
Ectocarpus and etc can increase up to 10% of the drag. The minor fouling mostly caused by
the slime and can increase 1-2 % of the drag. The fouling can be control by using the
antifouling and can keep the hull smooth as possible. There are various types of antifouling
and the usage are restricted under the law of antifouling system that organized by IMO.
HULL ROUGHNESS AFTER SERVICE
The hull surface will roughen after the service because of:
Mechanical damages
Corrosion
Fouling
Over coated fouling remnants
Coating defect such as flaking and blistering
Uneven areas due to build up of old coating residues at places during maintenance.
The roughness of the mechanical damages, corrosion, fouling, blistering and flaking is higher
than the roughness due to coating applications.
MEASUREMENT OF HULL ROUGHNESS
Hull roughness is measured by the mechanical-electronic device which called hull roughness
analyzer or gauge supplied by British Maritime Technology (BMT). The standard unit of the
hull roughness is the peak to trough height in microns per sample of 50 mm length of
underwater hull (Rt50).
Methods of Measurement:
The will be divided into 10 sections which is same length.
The measurement will be taken 10 times for each section, which is 5 at the port side
and 5 at the starboard side.
There will be 50 readings will be taken at each side, 30 at the vertical side and 20 at
the flat side.
The total 100 reading will be used to plot the roughness distribution graph. The
average hull roughness (AHR) will be calculated to indicate the roughness of the ship.
As mention above, the hull roughness is one of the factors of the frictional resistance of
the ship. The model test in tow tank was carried out at the smooth surface but for the
actual ship, there is no term of smooth hull surface. So, in the calculation of the total
ship resistance, there will be some modification to be made for the allowance of the
roughness. There will be 15% roughness allowance added to the Fraude formulation
which was recommended by NPL for the use in ITTC line.
CONCLUSION
As the conclusion, the drag, the boundary layer and the hull roughness on the hull
surface is related to each other. In order to run the ship at trial speed and service speed
we need the propulsion system that efficient to overcome the drag of the ship.
Therefore it is important to measure the total drag of the ship to estimate the efficient
power needed for the ship. By doing that, we can predict the most suitable cost for the
ship and lower cost can give more profit. In determining the total drag of the ship, it is
important to understand the behaviour of the flow that exists at hull surface and the
behaviour of the flow is showed by the boundary layer that has two components that is
laminar and turbulent flow. Mostly for the ship, the flow is turbulent which is become
the important factor for the total drag because it increase the frictional resistance. The
understanding of the boundary layer and the flow will help during the analyzing of the
model test that carried at the tow tank and transform it to full scale. Without the
understanding we will not get the means of the Fraude’ law, Reynold’s law, law of
similarity and law of comparison and cannot apply the Fraude formulation of total drag.
The roughness of the hull is also one of the factors for the total drag and increases the
cost and reduces the performance and also the factor for the thickness of the boundary
layer increases. So it is important to have smooth hull surface and better coating so the
ship can do her service without any trouble and problems.
REFERENCES
1. http://en.wikibooks.org/wiki/Ship_Resistance
2. http://www.brighthub.com/engineering/marine/articles/60585.aspx
3. http://silver-sails.com/Primer/ResistanceII/
4. http://en.wikipedia.org/wiki/Drag_%28physics%29
5. The Effect of Biocide Free Foul Release Systems on Vessel Performance by John
Willshere, International Paint Ltd., London/UK
6. http://www.answers.com/topic/boundary-layer
7. http://www.informaworld.com/smpp/content~db=all~content=a714006372
8. Ghani, D. M. Ship resistance, johor bharu: UTM.