Flashcards in Model tests Deck (24):
Why are ship model tests so expensive i comparison to other model tests?
- longer waiting period before the water is calm
- numerousness of the parameters to be varied (wave length, wave hight, ship speed etc.)
Why are these tests done?
To investigate nonlinear effects which are not easily accessible by computation
- Non-linear motion
- Roll damping
- Slamming and local loads
- Green water on deck
- Capsizing of intact ships
- Capsizing of damaged ships
Describe a typical model
- mostly self-propelled
- usually free-sailing, steered by automatically or hand ruddered machines
- in some cases guides or wires are used to restrict degrees of freedom
- The model geometry and the wave factor are scaled by the same factor lambda
The gravity force as well as the mass are scaled with what factor?
This means that the ratio between gravity and inertia force is equal for model and ship
What is one impotant consideration when it comes to the test time?
The period of oscillations as well as the time the ship need to travel its own length is sqrt(lambda) longer for the ship. This means that the test videos have to be shown in slow motion - sqrt(lambda) slower - to have the same appearance as for the ship
What is important to consider during the calculation?
The differences in densities for the tank and the sea water
Name the two different tanks that are used for seakeeping investigations
- long and relatively narrow tank
- rectangular shape with an aspect ratio of about 1:1.5
Explain how the HSVA tank works
- A duble flap maker is used for the wave generation, which allows tha approximation of amplitudes of the horizontal component of the orbital velocity as they decrease downwards by e^-kz
- Irregular waves are generated by multiple individual waves being superimposed
- The wave amplitudes are chosen to the according seaway spectrum
- Phases are randomly chosen
- Amplitudes and phases thogether are used to perform an inverse fourier trasnformation
- Hmax for irregular waves is 0.5m, limited by the flap andgle and the power of the hydraulic system
HSVA small towing tank
A plunger is installed, a corpus which radiates waves by being repeatedly sub- and emerged.
All test tanks have a "beach" on the opposite side of the wave maker to absorb the waves, otherwise the waiting time would be increased
- Multiflap wavemaker on the long side, and double flap on the short side
- Multiflap - 50 cm width, activated individually
- Phase shift between the flaps enables different wave propagation directions
What are wave probes?
- record either the absolute water surface deflection viewed from teh carriage or the relative motion viewed from the model
- common types: acoustical, resistive, capacitive
How does acoustical wave probes work?
Works like an echo sounder: ultrasonic sound is emitted by a transmitter and the reflected signal from the water surface is recieved by the same device. The run-time between the output and input signal is reistered and the distance D between the probe and the water surface is measured. This equals D = 0.5*c*t
What can be a challenge with the acoustical probes?
If the waves are too steep then the vertical distrance is not measured but the shortes so the echo might become too weak
Resistive wave probes
An AC voltage is impressed on two wires. Depending on the water level teh wires are wetted up to a certain height.
The resistance measured between the wires changes according to the wetted height. So the water level can be calculated from the resistance measured and the known conductivity of teh water.
What is the disadvantage with a resistive wave probe?
- Calibration requirement before each use because of the dependency on the temperature.
- At high speed wires of the probe superimpose a small wave system and cause spray which affects teh measurement negatively
Capacitive wave probes
An insulated conductor is mounted on the carriage. The water is used as the opposite conductor "plate". Together they work as a capacitor.
Any change in the water level would change the dieelctric effect between the plates, which is then measured.
Disadvantage of the capacitive probe?
Relatively high required distance between conductor and water surface
How is the ship motion recorded - camera?
With a camera system including three high resolution cameras. Cameras trace the vertical and horizontal positions of defined objects (white balls or emitting diodes) which are positioned at the frame of the model.
To prevent concealing, the emitter and the reciever are coupled by an electronic system. The diodes shine one after the other. Active type is suitable for free models.
How is the ship motion recorded - gyroscope?
It contains three accelerometers for measuring accelerations along the three axis and three fiber optic gyroscopes (FOG) serving for measuring absolute rotational speeds. The unit can internally convert the rotational rates into angles of rotation.
How does a gyroscope work?
FOG: based on the interference of light which has passed through a coil of optical fibre.
Two beams from a laser are injected into the same fibre but in opposite directions. Due to the sagnac effect, the beam travelling against the rotation experiences a slightly shorter path delay than the other beam. The resulting difference phase shift is measured through interferometry, thus translating one component of the angular velocity into a shift of the interference pattern which is measured photometrically.
Explain the roll decay test
The model is heeled to a certain angle. After releasing the model the amplitudes of the roll motion are measured. Damping follows from evaluation of the difference between to successive amplitudes.
Model test - slamming
oval holes in the stern to show the location of the pod drives. Round holes for the probes to measure pressure per area. The wood structure is more rigid than the real ship so the pressure values are overestimated.
It is also possible to measure with force transducers.
Test: Green water
Open top design. incoming water due to seaway in the cargo has to be considered to avoid negative effects on ship stability. GM reduction.
Requirement: The pumps in the holds has to be greater than the water intake. Size of pumps are designed to be capable of removing heavy tropical rain fall. Head and following sea are the most severe cases.