N Bose

Propulsive performance from oscillating propulsors with spanwise flexibility

The propulsive performance of oscillating foils with spanwise flexibility was studied using a time-domain panel method. The work was done to assess the effects of spanwise flexibility on the propulsive efficiency of these propulsors, especially those employed by relatively fast swimming marine animals. The method is valid for three-dimensional attached flows around the actual planforms found on these animals and was used in the study reported here to assess the performance of the flukes of an immature fin whale (Balaenoptera physalus). It is shown that passive spanwise flexibility reduces propulsive efficiency, but that propulsive efficiency of these planforms can be increased, over the value for an equivalent rigid foil, by careful control of the phase of the spanwise flexibility relative to other motion parameters.

Energy absorption from ocean waves: a free ride for cetaceans

Flukes of cetaceans are capable of absorbing energy from ocean waves for propulsion. The extent of this energy absorption is demonstrated by considering the flukes of an immature fin whale, Balaenoptera physalus. In a fully developed seaway corresponding to a wind speed of 20 knots (around Beaufort force 5) and at a low swimming speed, of 2.5 m s-1, this whale was able to absorb up to 25% of its required propulsive power in head seas and 33% of propulsive power in following seas. Consequences of wave-energy absorption for energetics of cetacean migrations are discussed.

Icing on a small horizontal-axis wind turbine — Part 1: Glaze ice profiles

Abstract A 1.05m diameter, horizontal-axis, battery-charging wind turbine was operated at St. Phillips, Nfld, during winter 1990–1991. Natural icing events were monitored. The most severe icing resulted from glaze ice accretion that formed in freezing rain and drizzle. Glaze ice profiles on one blade of the turbine were measured at several spanwise positions after icing events. These ice formations are presented in the form of scale diagrams and tables of offsets. Icing formed at the leading edge of the blade section in a qualitatively similar way to that formed on fixed aerofoil sections in similar conditions in icing wind tunnels. The rotation of the blades led to a larger amount of ice build up at the blade tips than at the blade roots. Ice covered the pressure side of the sections, but much of the suction side of the sections was clear of ice.

Distribution of heavy metals in sediment pore water due to offshore discharges: an ecological risk assessment

Barite is used as a weighting agent in synthetic and oil based drilling fluids (SBFs and OBFs) to maintain bore hole pressure during offshore oil drilling operations. Substitution of OBFs by SBFs has reduced the risk of ecological impacts. Barite makes up approximately 33% by weight of an SBF and contains traces of heavy metals, which significantly contribute to the toxicity of drilling waste. Due to the hydrophobic nature of SBFs, drilling wastes are not dispersive in the water column. Arsenic, copper and lead are the three important toxic heavy metals, amongst others, found in the drilling waste. The concentrations of heavy metals are determined using a steady state aquivalence-based fate model in a probabilistic mode. Monte Carlo (MC) simulations using Latin Hypercube Sampling (LHS) are employed to determine pore water concentrations at known pollutant loading rates (E) and impact area (AW) conditions. This paper considers a hypothetical case study to determine the water quality impacts for 4 and 10% attached SBFs which correspond to proposed best available technology (BAT) option and current discharge practice in US offshore. The exposure concentration (CE) is a predicted environmental concentration (PEC), which is adjusted for exposure probability (p) and bioavailable fraction (BF) of heavy metals. The probabilistic response of an ecosystem is defined by developing an empirical distribution function (EDF) of predicted-no-effect-concentration (PNEC) derived from LC50 and NOEC data. The lowest 10th percentile value of an EDF is defined as a representative response of an ecosystem, RE. The ratio of exposure concentration (CE) to the representative response (RE) is defined risk quotient (RQ). The ratio CE/RE 1).

PERFORMANCE OF CHORDWISE FLEXIBLE OSCILLATING PROPULSORS USING A TIME-DOMAIN PANEL METHOD

A two-dimensional time-domain constant potential panel method was used for analysis of chordwise flexible oscillating hydrofoils as oscillating propulsors. The oscillatory motions as well as the chordwise deflections were of large amplitude. Results are presented of propulsive thrust coefficient and efficiency over ranges of motion parameters including varied reduced frequency ; pitch amplitude and feathering parameter ; heave amplitude/chord ratio ; pitching axis position ; phase between pitching and heaving motions ; and deflected shape. The ranges were taken about a favourable operating point for a flexible oscillating NACA 0012 foil. A peak in propulsive efficiency was found for each motion parameter variation studied. Propulsive efficiency varied strongly with changes in heave amplitude/chord ratio, pitch amplitude and phase between heaving and pitching motions, but less strongly for variations in pitching axis position and flexibility.

PROPULSIVE PERFORMANCE OF THREE NATURALLY OCCURRING OSCILLATING PROPELLER PLANFORMS

Abstract A numerical method, the quasi-vortex-lattice method (QVLM), was applied to predict the propulsive performance of three naturally occurring oscillating propellers. These were cetacean flukes for a fin whale (Balaenoptera physalus); white-sided dolphin (Lagenorhynchus acutus); and white whale (Delphinapterus leucas). The fin whales flukes had the highest aspect ratio (6.1) and moderate sweep angle (31°); the white-sided dolphins flukes had the highest sweep angle (47°) and lowest aspect ratio (2.7); and the white whales flukes had moderate aspect ratio (3.3) and the lowest sweep angle (28°). In the numerical simulations, the planforms were assumed to be rigid both in chordwise and spanwise directions, and to be oscillating harmonically in an irrotational, incompressible fluid. Calculation and comparisons of propulsive efficiency and thrust coefficient vs advance ratio for each of the planforms were made in three cases: varied heave amplitude; different pitching axis positions; and varied angular amplitude of pitch.

Sensitivity of AUV added mass coefficients to variations in hull and control plane geometry

The sensitivity of the added mass coefficients of a typical autonomous underwater vehicle (AUV) to changes in geometric parameters was investigated. Qualitative deductions were made concerning the effect of geometric variations. Then the added mass coefficients for several configurations of body geometry were generated for the Canadian Self-Contained Off-the-shelf Underwater Testbed (C-SCOUT) vehicle using the computer program Estimate Submarine Added Mass (ESAM). The changes in the added mass coefficients have direct relationships to the varied parameter. The results presented here are specific to the C-SCOUT, but may be extended to similar axisymmetric bodies.

Sensitivity of AUV response to variations in hydrodynamic parameters

Abstract The sensitivity of the response of a typical AUV to changes in hydrodynamic parameters is examined. The analysis is primarily performed using a computer model of an axi-symmetric vehicle typical of many AUVs in service today. The vehicle used is the Canadian Self-Contained Off-the-shelf Underwater Testbed (C-SCOUT), designed and built by graduate and work term students. The fully nonlinear computer model is based on Newton–Euler equations of motion, and uses the component build-up method to describe the excitation forces. The hydrodynamic parameters are varied in a series of simulations with the computer model; the response being analyzed for specific performance indicators.

Hydrodynamic characteristics of a lunate shape oscillating propulsor

Research was conducted to study the hydrodynamic efficiency of a foil with aft-swept wing tips. A potential flow based time domain panel method was formulated to predict the performance of a lunate and rectangular foil in large amplitude, unsteady motion. Skin drag was approximated and boundary layer growth and separation were also estimated. Hydrodynamic efficiency was evaluated in terms of propulsive efficiency and thrust coefficient of the foil. Results are presented for a lunate shaped planform and for a rectangular foil. Predictions show that the lunate shaped planform has a substantially higher propulsive efficiency (13% higher) than the rectangular foil under heavy load conditions when the feathering parameter is zero, throughout a range of reduced frequencies (0.2 to 1.8). Under a medium load condition, however, the rectangular foil gave a higher propulsive efficiency at reduced frequencies less than 0.5 and the same efficiency value at a reduced frequency of 1.8. For a practical range of reduced frequencies between 0.5 and 1.0, the lunate tail gave higher propulsive efficiency. The lunate planform gave a lower thrust coefficient at a heavy load and higher thrust at a medium load condition than the rectangular planform for all reduced frequencies.

Icing on a small horizontal-axis wind turbine — Part 2: Three dimensional ice and wet snow formations

Abstract Icing on a small horizontal-axis wind turbine was monitored during winter 1990–1991 in Newfoundland. The three-dimensional form of incident glaze ice accretion on the rotor is described. Variations in ice accretion along the blade span resulted from centripetal forces, gravity, and crossflow over the blades. The dominant forces acting on impinging water droplets were centripetal forces, which caused a greater build up of ice at the blade tips and, in particular, at the leading edge towards the blade tips. Icicles formed beyond the blade tips. Wet snow also led to icing of the wind turbine rotor. The form of this accretion is described; wet snow was found to adhere only to the pressure side of the blades. The power output from the wind turbine was monitored. Icing affected the performance of the wind turbine at this site over winter 1990–1991, for a period equivalent to about 1.5% of a year.