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Q:

How is the eigenfrequency of a shaftline affected by the loading condition?

A:

- Hull bending and bearing spacing

- Tensioning from loading

Q:

What is the resonance frequency?

A:

An excitation frequency is called resonance frequency if it is close to the eigenfrequency of the excited body. Then, resonance occurs which results in large amplitudes.

Resonance: The amplitudes of the vibrations become particularly large and disruptive if the

excitation frequency is close to an eigenfrequency (natural frequency) of the ship hull

(whole hull or parts of it such as superstructure, deck, plate field).

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 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 measures can reduce the transmission of engine vibrations?

A:

- Elastic mounting of engine beds

- Flexible coupling in the shafting

- Special sandwich panels between engine and structure

Q:

What is an eigenfrequency?

A:

An excited body without forced excitation (-> harmonic free vibration) vibrates with its eigenfrequency.

Wikipedia:

Natural frequency, also known as eigenfrequency, is the frequency at which a system tends to oscillate in the absence of any driving or damping force.

The motion pattern of a system oscillating at its natural frequency is called the normal mode (if all parts of the system move sinusoidally with that same frequency).

If the oscillating system is driven by an external force at the frequency at which the amplitude of its motion is greatest (close to a natural frequency of the system), this frequency is called resonant frequency.

Q:

Is the first derivative of the ansatz-function always used in the stiffness matrix?

A:

No – it depends on how the particular strain is defined

Q:

To what condition(s) is the virtual deformation bound to?

A:

Virtual deformation must comply with boundary conditions.

Q:

What damage can be induced by shaft vibrations?

A:

- Shaft failure

- Bearing damage / failure

- Gear (tooth damage)

Q:

When should you use the spectral method?

A:

high frequency loading fall into the category of **Wave Propagation** problems.

For wave propagation problems wherein the frequency content of the input is very high, many higher order vibrational modes participate in the motion.

At these higher frequencies, the wavelengths are very small and hence to capture these modes effectively, FE meshes need to be very fine.**Spectral Finite Element Method** (SFEM) is a numerical method evolved from the Fourier Transform based method

There are certain advantages that a transform method can offer over conventional FEM:

-FEM, direct problem: one determines the response to the given input,

-inverse problems deal with determining the input history using the measured responses or determining the system as a whole from the known input and output.

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 are the main vibration exciters on a ship?

A:

Propeller, engine, waves

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