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Lernmaterialien für Ship Vibration an der TU Hamburg-Harburg

Greife auf kostenlose Karteikarten, Zusammenfassungen, Übungsaufgaben und Altklausuren für deinen Ship Vibration Kurs an der TU Hamburg-Harburg zu.

TESTE DEIN WISSEN

What elements are vibrating and what excites those elements?

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TESTE DEIN WISSEN

Vibration Exciters:
- Engine
- Propeller
- [Waves]


Vibrating elements and structures:
- Detail structures (e.g. decks, heli deck)
- Global hull
- Deck houses


Influencing parameters:
- Surrounding fluid
- Waves [in some special cases]

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TESTE DEIN WISSEN

How many distinct eigenfrequecies has a continuous beam on n-supports above the first eigenfrequency?

a) 2n

b) 2n-1

c) n-1

d) infinite

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TESTE DEIN WISSEN

d) infinite


For such continuous beams on various supports practically no resonance-free operation is possible above the lowest natural frequency.

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TESTE DEIN WISSEN

What damage can be induced by shaft vibrations?

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TESTE DEIN WISSEN

- Shaft failure
- Bearing damage / failure
- Gear (tooth damage)

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TESTE DEIN WISSEN

db-unit

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TESTE DEIN WISSEN

10 decibel = 1 Bel means a factor of 10 in the vibration energy; this is
proportional to the square of the amplitude. The weighting factor of e.g. f = -15
dB means therefore that the amplitude has to be multiplied by 10-15/20 = 0.1778
– Energy factor = 10dB/10
– Amp (field) factor = sqrt(10dB/10)

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TESTE DEIN WISSEN

What is the Dunkerley method?

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TESTE DEIN WISSEN

– Dunkerley is originally not an energy method, but almost 80 years after its invention the energy approach is „derived“ with an energy approach.
– Dunkerley can be used for point (descrete) masses as well as distributed masses. The overall limitations and exact procedure of application remain somewhat foggy.
– No global eigenform is derived, but the eigenform for each weight increment is used

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TESTE DEIN WISSEN

Why do vibrations occur on propellers?

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TESTE DEIN WISSEN

• The propeller is often the main exciter of vibrations
• Pressure pulses:
The propeller passing the hull „carries“ a pressure field which changes with
each degree of revolution due to a non-constant shape of the blade and
significant differences between blade and „no-blade“
• Load variations associated to the velocities at the propeller
• Cavitation

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TESTE DEIN WISSEN

What are the influence factors of cavitation?

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TESTE DEIN WISSEN

Air content (dissolved and undissolved),
microscopic solid particles, turbulence,
surface roughness.

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TESTE DEIN WISSEN

Measurement, analysis and evaluation of vibrations - What do you need to concider?

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TESTE DEIN WISSEN
  • Already before the measurement is started one should have an idea of the
    possible frequency range to be investigated (estimates, calculations…)
  • The sampling rate of recording is essetial for the analysis results
    • A too low sampling frequency might reflect the vibration in-accurately
    • A very high sampling rate might capture a lot of noise and will consume data-space
  • Duration of measurements needs to be sufficient to reflect typical exposure and to

    ensure statistical precision

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TESTE DEIN WISSEN

What is the difference between Bernoulli and Timoshenko beam theory?

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TESTE DEIN WISSEN

Euler-Bernoulli
• Known differential equations
• Cross-section remains normal to axis
• Small deflections


Timoshenko
• Accounts for shear deformation and rot. Bending
• Suitable for thick beams

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TESTE DEIN WISSEN

What are the Transmission ways of Cavitation induced vibration on ship stern? What is the consequence??

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TESTE DEIN WISSEN

Propeller shaft: Fluctuating pressure of the propeller blades


Flow field: Pressure waves induced by the cavitation, a temporally variable
thickening of each propeller blade due to the change of cavitation volume.


Result: Increasing the pressure amplitudes by 5 to 20 times


The pressure fluctuation occurs with the wing blade and its multiples.

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TESTE DEIN WISSEN

ISO 6954:2000

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ISO 6954:2000 – Mechanical Vibration – Guidelines for the measurement, reporting
and evaluation of vibration with regard to habitability on passenger and merchant
ships


Measurements according to ISO 6954:
• Straight ahead, minimum sea state, deep water
• Measurement duration 1 min (2 min for frequencies of 1 – 2 Hz)
• Fourier-Transformation of the measurement signals in the frequency range
• Assessment of the effective value of the frequency based accelerations
• Measurement of the direction of the maximum accelerations


-> Report for Evaluation of Habitability of Ships in Accordance with ISO 6954

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TESTE DEIN WISSEN

What is hydrodynamic mass?

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TESTE DEIN WISSEN

A body, which is fully or partially immersed in a fluid, experiences hydrodynamic forces. They can be split up into a part proportional to body accelerations and part containing all other hydrodynamic forces. In analogy to inertial forces („mass *acceleration“), acceleration-dependent hydrodynamic forces per body acceleration are called hydrodynamic masses. They are

in general dependent on the shape of the wetted part of the body and the free surface (frequency dependence?)

Lösung ausblenden
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Q:

What elements are vibrating and what excites those elements?

A:

Vibration Exciters:
- Engine
- Propeller
- [Waves]


Vibrating elements and structures:
- Detail structures (e.g. decks, heli deck)
- Global hull
- Deck houses


Influencing parameters:
- Surrounding fluid
- Waves [in some special cases]

Q:

How many distinct eigenfrequecies has a continuous beam on n-supports above the first eigenfrequency?

a) 2n

b) 2n-1

c) n-1

d) infinite

A:

d) infinite


For such continuous beams on various supports practically no resonance-free operation is possible above the lowest natural frequency.

Q:

What damage can be induced by shaft vibrations?

A:

- Shaft failure
- Bearing damage / failure
- Gear (tooth damage)

Q:

db-unit

A:

10 decibel = 1 Bel means a factor of 10 in the vibration energy; this is
proportional to the square of the amplitude. The weighting factor of e.g. f = -15
dB means therefore that the amplitude has to be multiplied by 10-15/20 = 0.1778
– Energy factor = 10dB/10
– Amp (field) factor = sqrt(10dB/10)

Q:

What is the Dunkerley method?

A:

– Dunkerley is originally not an energy method, but almost 80 years after its invention the energy approach is „derived“ with an energy approach.
– Dunkerley can be used for point (descrete) masses as well as distributed masses. The overall limitations and exact procedure of application remain somewhat foggy.
– No global eigenform is derived, but the eigenform for each weight increment is used

Mehr Karteikarten anzeigen
Q:

Why do vibrations occur on propellers?

A:

• The propeller is often the main exciter of vibrations
• Pressure pulses:
The propeller passing the hull „carries“ a pressure field which changes with
each degree of revolution due to a non-constant shape of the blade and
significant differences between blade and „no-blade“
• Load variations associated to the velocities at the propeller
• Cavitation

Q:

What are the influence factors of cavitation?

A:

Air content (dissolved and undissolved),
microscopic solid particles, turbulence,
surface roughness.

Q:

Measurement, analysis and evaluation of vibrations - What do you need to concider?

A:
  • Already before the measurement is started one should have an idea of the
    possible frequency range to be investigated (estimates, calculations…)
  • The sampling rate of recording is essetial for the analysis results
    • A too low sampling frequency might reflect the vibration in-accurately
    • A very high sampling rate might capture a lot of noise and will consume data-space
  • Duration of measurements needs to be sufficient to reflect typical exposure and to

    ensure statistical precision

Q:

What is the difference between Bernoulli and Timoshenko beam theory?

A:

Euler-Bernoulli
• Known differential equations
• Cross-section remains normal to axis
• Small deflections


Timoshenko
• Accounts for shear deformation and rot. Bending
• Suitable for thick beams

Q:

What are the Transmission ways of Cavitation induced vibration on ship stern? What is the consequence??

A:

Propeller shaft: Fluctuating pressure of the propeller blades


Flow field: Pressure waves induced by the cavitation, a temporally variable
thickening of each propeller blade due to the change of cavitation volume.


Result: Increasing the pressure amplitudes by 5 to 20 times


The pressure fluctuation occurs with the wing blade and its multiples.

Q:

ISO 6954:2000

A:

ISO 6954:2000 – Mechanical Vibration – Guidelines for the measurement, reporting
and evaluation of vibration with regard to habitability on passenger and merchant
ships


Measurements according to ISO 6954:
• Straight ahead, minimum sea state, deep water
• Measurement duration 1 min (2 min for frequencies of 1 – 2 Hz)
• Fourier-Transformation of the measurement signals in the frequency range
• Assessment of the effective value of the frequency based accelerations
• Measurement of the direction of the maximum accelerations


-> Report for Evaluation of Habitability of Ships in Accordance with ISO 6954

Q:

What is hydrodynamic mass?

A:

A body, which is fully or partially immersed in a fluid, experiences hydrodynamic forces. They can be split up into a part proportional to body accelerations and part containing all other hydrodynamic forces. In analogy to inertial forces („mass *acceleration“), acceleration-dependent hydrodynamic forces per body acceleration are called hydrodynamic masses. They are

in general dependent on the shape of the wetted part of the body and the free surface (frequency dependence?)

Ship Vibration

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