VESSEL WAVE WAKE CHARACTERISATION USING WAVELET ANALYSIS

This work focuses on characterising vessel wave wake (wash) using wavelet analysis when a vessel is operating in the sub-critical and critical zone. Such characterisation complements other wash characteristics: Froude depth number, bow wave angle, solitons and decay coefficient. The examination of experimental results indicates that differences in characteristics with respect to water depth, Froude depth number, vessel displacement, hull form and soliton generation can be identified through wavelet analysis. The results demonstrate “proof of concept” that wavelet analysis is a powerful tool for characterising vessel wash and captures the effects of key operational and vessel changes.

Computational fluid dynamics provide evidence for a compensatory suction feeding like effect in the predatory strike of dragonfly larvae

Most fast-moving aquatic predators face the challenge of bow wave formation. Water in front of predator alarms or even displaces the prey. To mitigate the formation of such a bow wave, a strategy aiming at pressure reduction via suction has evolved convergently in several animal groups: compensatory suction feeding. The aquatic larvae of dragonflies and damselflies (Insecta: Odonata) are likely to face this challenge as well. They capture prey underwater using a fast-moving raptorial appendage, the so-called prehensile labial mask. Within dragonflies (Odonata: Anisoptera) two basic shapes of the prehensile labial mask have evolved, with an either flat and slender or concave distal segment. While the former is a pure grasping device, the latter is also capable of scooping up smaller prey and retaining it inside the cavity by arrays of bristle-like structures. The hydrodynamics of the prehensile labial mask was previously unknown. We used computational fluid dynamic (CFD) simulations of the distal segment of the mask, to investigate for the first time how different shapes of the mask impact their function. Our results suggest that both shapes are highly streamlined and generate a low-pressure area, likely leading to an effect analogous to the compensatory suction feeding. This might be an interesting concept for technical application in small scale grasping devices, e.g. for simple sampling mechanisms in small-sized autonomous underwater vehicles (μAUVs).

Jet-driven bow waves as electron accelerators in the magnetosheath: Monte Carlo simulations

<p>Magnetosheath jets are fast flows of plasma frequently observed downstream of the Earth's quasi-parallel shock. Previous observations have shown that these jets can exhibit supermagnetosonic speeds relative to the background flow and develop their own bow waves or shocks. Such jets have been observed to be able to accelerate ions and electrons. In our study, we model electron acceleration by jet-driven bow waves in the magnetosheath using test-particle Monte Carlo simulations that include magnetic mirroring and pitch-angle scattering of magnetic irregularities. We compare the simulation results to spacecraft observations of similar events to understand the acceleration mechanisms at play. Our preliminary results suggest that the energy increase of electrons can be explained by shock drift acceleration at the moving bow wave. Our simulations allow us to estimate the efficiency of acceleration as a function of different jet and magnetosheath parameters. The acceleration introduced by jet-driven bow waves amplifies shock acceleration downstream of the Earth’s bow shock and may also be applicable to other shock environments.</p>

Effect of Waves on the Behavior of Emergent Buoyantly Rising Submarines Using CFD

Emergent buoyantly rising submarines encounter excess roll problems, partially owing to waves that significantly affect their behavior. This study predicts the behavior of a submarine, including when it rises in static water, beam sea, head wave, following wave, 30∘ bow wave, 60∘ bow wave, 30∘ quartering wave, and 60∘ quartering wave, using the computational fluid dynamics method. The beam sea has a slight effect on pitch prior to the submarine rising to the water surface, but the maximum roll angle in the beam sea is 4.43 times that in static water. After a submarine submerges in water, the pitching oscillation does not decay quickly owing to the yaw angle. The head wave and the following wave have a continuous significant effect on the pitch; the submarine sail remains under the water surface after it submerges from the highest position. The head wave and the following wave have a slight effect on the roll and yaw before the submarine rises to the water surface; however, the roll angle suddenly increases after the submarine submerges from the highest position. As the initial angle between the submarine centerline and wave direction increases, the effect of waves on the longitudinal motion decreases. The amplitude of the pitching oscillation decreases with an increase in the initial angle between the submarine centerline and wave direction, and the waterline when the submarine oscillates on the water surface decreases. The difference in the maximum roll angle between when a submarine rises in an oblique wave and when it rises in beam sea is below 6.3∘. Submarines should try to avoid rising in a head wave and the following wave.

Particle acceleration by transient structures around Earth’s bow shock

<p>Upstream of Earth’s bow shock, the foreshock is filled with particles that have been reflected at the bow shock and are streaming away from it. Interaction of these particles with solar wind particles and discontinuities within this region can cause foreshock transients to form. Downstream of Earth’s bow shock, localized magnetosheath jets with high dynamic pressure are frequently observed. When such a fast magnetosheath jet compresses the ambient magnetosheath plasma, an earthward compressional bow wave/shock can form. Here we present that foreshock transients and magnetosheath jets can accelerate particles through shock drift acceleration, Fermi acceleration, and the betatron acceleration. Foreshock transients and magnetosheath jets therefore can increase the particle acceleration efficiency of the parent shock by providing additional acceleration. The shock environment relevant for particle acceleration is not just the shock itself, but also the nonlinear transient structures both upstream and downstream of it.</p>