On the model tests
Kul-24.3200 Introduction of Marine Hydrodynamics
Lecturer: Satu Hänninen, VTT, [email protected]
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Outline: On the model tests
• Towing tank in Otaniemi
• Open water tests (propeller)
• Resistance tests (bare ship hull)
• Self-propulsion tests (hull and propeller)
• Preparations of the model and measurement setup
• Some hints
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
2
Outline: On the model tests
• Towing tank in Otaniemi
• Open water tests (propeller)
• Resistance tests (bare ship hull)
• Self-propulsion tests (hull and propeller)
• Preparations of the model and measurement setup
• Some hints
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Towing tank in Otaniemi
Basic information
• Length 130 m
• Width 11 m
• Depth 5.5 m
• Max. forward speed 6.0 m/s
• Plunger-type wave maker
• max. wave height ~0.32 m (regular)
• max. Hs~0.21 m (irregular)
• PMM mechanism for captive manoeuvring
model tests
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Towing tank in Otaniemi
Typical measurements
• Resistance & propulsion in calm water &
waves.
• 3-D wake measurements, flow visualization.
• Wave induced motions & forces on ships and
floating structures.
• Hydrodynamic forces on submerged bodies.
• PMM tests for floating and submerged bodies
etc.
[pics removed]
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Towing tank in Otaniemi
[pics of the towing carriage removed]
Aalto University 16/11/2015
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Towing tank in Otaniemi
Practical example
Videos of propulsion tests
• A general view
• Results on the wave generation
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Outline: On the model tests
• Towing tank in Otaniemi
• Open water tests (propeller)
• Resistance tests (bare ship hull)
• Self-propulsion tests (hull and propeller)
• Preparations of the model and measurement setup
• Some hints
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Propeller: open water tests
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Aim
• To study the performance of
a propeller in open water
What is obtained?
• Open water curves (thrust
and torque coefficients and
efficiency as a function of
advance number
Relation to propulsion tests
• Needed e.g. when defining the wake factor
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Propeller: open water tests
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Principles on studying the open water characteristics of a
propeller in model scale
• In open water tests, the geometry of the model propeller is the same as that of
the propeller in full scale.
• Propeller pitch and the advance number determine the angles of flow at each
blade section ( , i, g, e)
• If angle of attack e does not exceed the critical stall value, the forces and
moments have the same relation both in model- and in full-scale
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Propeller: open water tests
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Effect of scale on the viscous flow?
• Reynolds number is different between model and full scale.
• The frictional resistance coefficient of the propeller depends on the Reynolds
number.
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Propeller: open water tests
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How to conduct open water tests?
• Tests are conducted in still water
• Propeller dynamometer rotates the model and measures thrust and torque
• Rotation speed is set at high value and kept constant
• Flow velocity VA (speed of the towing carriage) is set according to the desired
advance number: =
=
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Introduction of Marine Hydrodynamics
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Propeller: open water tests
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Results
• Advance number
•
=
/
=
• Thrust coefficient
•
=
• Torque coefficient
•
=
• Efficiency
•
=
Aalto University 16/11/2015
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B4.70.P/D=1
KQ
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KT
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KQ
KT
0.4
0.3
0.2
0.1
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0.2
0.4
Introduction of Marine Hydrodynamics
0.6
0.8
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Propeller: open water tests
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Before model tests, define
• Rotation speed (constant)
• Propagation speed for each advance number to be tested.
Measurement gives the time histories of
• Propagation speed VA [m/s]
• Propeller: rotation speed n [1/s], thrust T [kg] and torque Q [kgm]
Analysis of the results
• Calculate the average of each measured quantity. (VA, n, T, Q)
• Define the open water curves. (See previous page.)
• ITTC-57: no corrections due to the scale. ITTC-78: corrections due to the scale.
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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How to select the rotational speed n?
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Aalto University 16/11/2015
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Outline: On the model tests
• Towing tank in Otaniemi
• Open water tests (propeller)
• Resistance tests (bare ship hull)
• Self-propulsion tests (hull and propeller)
• Preparations of the model and measurement setup
• Some hints
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
16
Hull: resistance tests
Aim To study the performance of the bare ship hull in ideal conditions
What is obtained?
• As a function of speed
• Resistance and effective power
• Running position
• Observations / recorded
• Wave pattern
• Streamlines, flow separation (requires e.g. strings and underwater camera)
Relation to propulsion tests
• Resistance of the model is needed when defining the thrust deduction factor.
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Hull: resistance tests
Principle: Model tests of surface vessels are conducted
according to Froude’s scaling law.
• Froude number Fn:
=
=
=
• The wave pattern caused by the gravity force and the convective accelerations
are similar between model and full scale.
• Reynolds number is much smaller in model scale and viscous effects are
overestimated in model scale.
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Hull: resistance tests
How to take into account the too small Reynolds number?
Model hull
• The flow is made turbulent using studs, copper strings, or sand roughning at the
bow. (In full scale, the flow is nearly always turbulent due to large Reynolds
number.)
Analysis
• Applied Froude scaling procedure (ITTC-57)
• So called form factor method (ITTC-78)
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Introduction of Marine Hydrodynamics
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Introduction of Marine Hydrodynamics
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Introduction of Marine Hydrodynamics
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Hull: Resistance tests
Measurement setup
• The guides keep the direction of the
ship xed. No yaw or sway.
• The force gauge
• tows the model ship.
• measures the resistance of the model.
• There is a rope between the force
gauge and the model ship to allow
the pitching of the model
ship.
• A counter weight pulls the ship model
backwards in order to prevent surge.
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Hull: Resistance tests
Before the model tests
• Define speeds of the model according to Froude’s scaling law:
=
=
=
Measurement gives the time histories of
• Speed of the model [m/s]
• Towing force [kg]
• Sinkage at the bow and at the stern
Analysis
• Calculate the average of each measured quantity.
• Extrapolation of the resistance to full scale. ITTC-57 (or ITTC-78)
• Effective power as a function of ship speed
• Trim and sinkage as a function of ship speed
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
23
Outline: On the model tests
• Towing tank in Otaniemi
• Open water tests (propeller)
• Resistance tests (bare ship hull)
• Self-propulsion tests (hull and propeller)
• Preparations of the model and measurement setup
• Some hints
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
24
Self-propulsion tests
Aim
• Determine the performance of the ship hull
and propeller taken together.
What is obtained?
• At a given speed
• Delivered power
• Revolution rate of the ship propeller
• Analysis with the data of resistance and open water tests
• Wake and thrust deduction factors
• Hull efficiency and relative rotative efficiency
[pic removed]
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Self-propulsion tests
Components of the model
Propeller
dynamometer
Hull
Rudder
Propeller
Aalto University 16/11/2015
Propulsion
Motor
Resistance
dynamometer
Introduction of Marine Hydrodynamics
Ballast
Weights
26
Self-propulsion tests
Principle: Model tests of surface vessels are conducted
according to Froude’s scaling law.
• Froude number Fn:
=
=
=
• The wave pattern caused by the gravity force and the convective accelerations
are similar between model and full scale.
• Reynolds number is much smaller in model scale and viscous effects are
overestimated in model scale.
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
27
Self-propulsion tests
How to take into account too small Reynolds number? 1/3
Ship model
Turbulence stimulators
• Bow of the ship
• Rudder (close to the leading edge.)
• (Thrusters)
[pics removed]
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Self-propulsion tests
How to take into account too small Reynolds number? 2/3
The way of conducting the self-propulsion tests
• The frictional resistance coefficient is too large in model scale.
• This would cause propeller overloading in a fully self propulsion condition.
• The model is towed by a small force which compensate the difference between
the model and full scale frictional resistance coefficients.
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Self-propulsion tests
How to take into account too small Reynolds number? 3/3
Analysis of the results
• Extrapolation of the results to full scale
according to
• ITTC-57
• (or ITTC-78).
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Self-propulsion test
Measurement setup: What is similar
to the resistance test?
• The guides keep the direction of the
ship xed. No yaw or sway.
• The force gauge tows the model ship.
The force gauge measures the resistance
of the ship model.
• There is a rope between the force gauge
and the model ship to allow
the pitching of the model
ship.
• A counter weight pulls the
ship model backwards in
order to prevent surge.
Longitudinal cut of the hull and of its structures
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Self-propulsion test
Measurement setup: What is
different to the resistance test?
• There is an acting propeller, which
thrusts the model ship forwards.
• The force gauge measures the force Fm
that pulls the ship model forwards.
• The magnitude of Fm equals the
difference between the thrust and the
resistance of the model hull
with an acting propeller.
Longitudinal cut of the hull and of its structures
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
propeller
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Self-propulsion test
Towing force FM at each speed
• Compensates the difference between the frictional resistance coefficient in
model and full scale.
• You can adjust FM by changing the loading of the propeller (rotational speed n).
• In practice, it would be very difficult to obtain FM = RTM(VM).
• Instead, you need to measure the results both with FM > RTM(VM) and with Fm <
RTM(VM). Then, the values, which corresponds the situation Fm = RTM(VM), can be
solved by interpolation.
• An estimate for the range of the towing force: ± 10% … ± 15% of the resistance of the
model.
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Introduction of Marine Hydrodynamics
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Self-propulsion test
Measured results at the velocity VM
• You have calculated RTM(VM) before the model tests.
• In the model tests, you have measured results for (at least) one Fm when Fm >
RTM(VM) and for (at least) one Fm when Fm < RTM(VM).
• By interpolation, you find rotational speed n, thrust T and torque Q which
corresponds RTM(VM).
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Self-propulsion test
Before the model tests, select
•
Define speeds of the model according to Froude’s scaling law
•
Define the magnitude of the force that compensate too large frictional resistance coefficient. Calculate also the
magnitude of the two target towing forces (one is larger and one smaller than the compensating force) for each velocity.
•
Optional but recommended: have an idea for the first rotation speed of the propeller to be tested at each speed of the
model
Measurement gives the time histories of
•
Speed of the model [m/s]
•
Towing force [kg]
•
Sinkage at the bow and at the stern
•
Propeller: rotation speed n [1/s], thrust T [kg] and torque Q [kgm]
After each test run
•
Calculate the average of each measured quantity.
•
Check whether the towing force is roughly within the target range.
•
Select next revolution rate for the same speed or start measurements with the next speed.
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Introduction of Marine Hydrodynamics
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Self-propulsion test
Analysis (ITTC-57 or -78)
• Extrapolation of the delivered power and speed of rotation
• Analysis with the data of resistance and open water tests
• Wake and thrust deduction factors
• Hull efficiency and relative rotative efficiency
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Introduction of Marine Hydrodynamics
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ITTC-57 (or -78)
Resistance test
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Self-propulsion
test
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Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
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Outline: On the model tests
• Towing tank in Otaniemi
• Open water tests (propeller)
• Resistance tests (bare ship hull)
• Self-propulsion tests (hull and propeller)
• Preparations of the model and measurement setup
• Some hints
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
41
Build-up of the model 1/3
Model hull
• References: stations, waterlines, model number
• Turbulence stimulators
[pics removed]
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Introduction of Marine Hydrodynamics
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Build-up of the model 2/3
Preparing the propulsion system
• Shaft lines
• Propeller hubs
• Pods
[pics removed]
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Build-up of the model 3/3
Assembling the model
[pics removed]
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Calibrations 1/2
Propeller dynamometer
Thrust
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Torque
Introduction of Marine Hydrodynamics
Resistance
dynamometer
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Calibrations 2/2
Measurements
• Measure the response for several loads
• Calculate the average for each response
Defining the calibration coefficient
• Plot the load as a function of response
• Fit a linear polynom
• Calibration coefficient
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
46
Outline: On the model tests
• Towing tank in Otaniemi
• Open water tests (propeller)
• Resistance tests (bare ship hull)
• Self-propulsion tests (hull and propeller)
• Preparations of the model and measurement setup
• Some hints
Aalto University 16/11/2015
Introduction of Marine Hydrodynamics
47
Running position
Waterline at zero speed
Waterline with velocity
Heave at stern
Sinkage (a vertical distance)
Heave at bow
Pitch
(an angle)
Note: in your model tests, the heave is not necessarily measured exactly at fore perpendicular
(FP) and at aft perpendicular (AP). Measure the locations of the potentiometers!
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Introduction of Marine Hydrodynamics
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Self-propulsion tests: First estimate on
the revolution rate 1/2
Starting point: You have the results from open water and resistance tests. You have a
rough idea on the magnitude of the thrust deduction (t) and wake (w) factors.
Step 1: define the magnitude of the needed thrust
• Resistance tests (no effect of the propeller)
resistance of the model RM
• Calculate the force RTM which compensates the too large frictional resistance coefficient
• Needed thrust T:
RM = (1-t)T+ RTM
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=
Introduction of Marine Hydrodynamics
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Self-propulsion tests: First estimate on
the revolution rate 2/2
Step 2: Estimate the revolution rate
• Open water tests gives KT as a function of J.
• At a given speed of the ship V, calculate n as a function of J.
=
=
• Calculate the corresponding values of thrust
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