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BASIC COASTAL ENGINEERING Part 5

Tham khảo tài liệu 'basic coastal engineering part 5', kỹ thuật - công nghệ, kiến trúc - xây dựng phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 128 Basic Coastal Engineering position and a free oscillation at the system s natural period or frequency is established. A simple example of such a system is a pendulum. The natural frequency of oscillation of the system depends on the geometry of the system. For a pendulum the oscillation period depends on the length of the arm. It is essentially independent of the magnitude of the disturbance which does however establish the magnitude of oscillation of the system. After the initial disturbance has occurred free oscillations continue at the natural frequency but with exponentially decreasing amplitude due to the effects of friction. These systems can also undergo forced oscillations at frequencies other than the natural frequency owing to a cyclic input of energy at other than the natural frequency. Continuous excitation at frequencies equal or close to the natural frequency will usually cause an amplified system response the level of amplification depending on the proximity of the excitation frequency to the natural frequency and on the frictional characteristics of the system. An enclosed or partly open basin of water such as a lake bay or harbor can be set into free oscillation at its natural frequency and harmonic modes or forced oscillation as described above. The result is a surging or seiching motion of the water mass as a wave propagates back and forth across the basin. The speed and pattern of wave propagation and the resulting natural frequency of the basin depend on the basin geometry. Typical sources of excitation energy include 1. Ambient wave motion if the basin has an opening to permit entry of this energy e.g. harbor open to the sea from which the energy comes 2. Atmospheric pressure fluctuations 3. Tilting of the water surface by wind stress and or a horizontal atmospheric pressure gradient with subsequent release 4. Local seismic activity 5. Eddies generated by currents moving past the entrance to a harbor Bay and harbor oscillations are usually