Tomas Torsvik

Influence of variable Froude number on waves generated by ships in shallow water

Passage through the transcritical speed region of a moving disturbance in a shallow channel, is examined using numerical simulations based on a set of forced Boussinesq equations. The transition is accomplished either by accelerating the wave generating disturbance in a region of constant depth or by moving the disturbance with constant speed over a sloping bottom topography. A series of test cases are examined where the transcritical region is traversed both from subcritical to supercritical speed and vice versa. Results show that the generation of upstream solitary waves depends on the time required for the transition, with large waves being generated for long transition times. It is also apparent that the shape of the wave pattern, and the maximum amplitude of the waves, differ significantly depending on whether the Froude number increase or decrease during the transition of the transcritical region. However, the wave pattern is not determined simply in terms of the Froude number. The strength of the f...

Characteristic properties of different vessel wake signals

ABSTRACT Didenkulova, I., Sheremet, A., Torsvik, T. and Soomere, T., 2013. Characteristic properties of different vessel wake signals. The potential threat in terms of environmental protection and safe navigation posed by wake waves from high-speed ferries and fast conventional ships is well documented. Vessels that travel in the near-critical regime (depth Froude number ~1) at some sections along their ship tracks can generate packets of large, solitonic, very long and long crested waves. However, it is difficult to determine a general characterization of such wakes at the coast, due to the transient and nonlinear nature of this phenomenon, and the fact that wake impact is influenced by the local bathymetry and coastline configuration. This paper investigates the potential benefits of wake analysis by means of a time-frequency technique (windowed Fourier transform), which is a standard signal analysis method that has only recently been applied for wake analysis. Analysis of ship wakes have been performed based on instrumental data of sea surface elevation recorded at different sites in Tallinn Bay, the Baltic Sea, which is characterized by very intense ship traffic and provides a rich collection of vessel-wake signals. Results show that the wake signals are easily identified in spectrograms. The method is particularly useful for identification of low frequency signals that may easily be masked by high frequency noise in the wave record. Furthermore, the spectrogram provides information that makes it possible to associate wake events with individual ships at a given location. This approach also opens a new direction for the statistical description of wakes, applicable to the characterization of the “wake climate” for sites with intense vessel traffic.

Development of a Flexible, Extendable, and Low-Cost Control Unit for Surface Drifters

AbstractAn extensible, low-cost drifter control unit (VELELLA) design is presented, developed for use in drifters that are deployed in inland water bodies or near-coast regions. The control unit is custom built from basic components built around a small footprint microcontroller, making use of a GPS receiver for position tracking, a Global System for Mobile Communications [Groupe SpecialMobile (GSM)] radio for data transmission, and two sensor buses to handle analog and digital data generated by an arbitrary array of external sensors. A cloud-based data collection platform has been implemented to receive and store data transmitted over general packet radio service (GPRS) from the drifter. The control unit was found to perform satisfactorily in operational testing, providing data at subhertz frequency for position and temperature during extended time. However, some issues related to temporary data loss and power consumption spikes were identified, indicating that some further development of the control uni...

Spatial variation in high-speed ferry wakes in the Tallinn Bay area

A large number of high-speed ferry crossings occur in the Tallinn Bay area during the summer season. It is well known that the wakes from such ferries constitute a considerable additional wave load to the coastal region, with different properties from wind generated waves and waves from conventional ships. Waves in this area have been studied with the use of pressure sensors at particular locations, and provide valuable data for assessing the ship wave component of the wave load. Recent numerical simulations show that there is considerable spatial variation in the wave load at the coast due to topographic effects and ship maneuvering. This results in wave focusing and shadow areas on different parts of the coast. It is therefore believed that the previous studies may have missed some ldquohotspotsrdquo for the coastal wave impact. We analyze spatial patterns and far field properties of the long-wave part of wakes of fast ferries in Tallinn Bay with use of the Boussinesq-type shallow-water model COULWAVE forced by realistic ship motions. The calculated heights of ship waves exhibit substantial spatial variability. The largest waves were created when the ship sailing to Tallinn entered into supercritical regime when moving over the coastal slope. The maximum wave height eventually reached 3 m whereas along other sections of the track the wave height was about 1 m. The highest waves hit the area of Pirita Beach that apparently has much larger ship wave loads than the adjacent sections of the Viimsi Peninsula.