The aeroservoelastic behaviour of a thin rectangular wing with a controllable trailing edge flap is investigated. A rather high aspect ratio motivates a numerical model based on linear beam theory for the structural dynamics and strip theory for the unsteady aerodynamic loads. Experimental flutter testing shows good agreement with the numerical stability analysis, and the impact of the trailing edge flap on the dynamics is verified by open-loop testing. The problem of stabilizing the wing utilizing the trailing edge flap is posed, and the design of a fixed-structure feedback controller is performed using numerical optimization. The problem of maximizing closed-loop modal damping with constraints on actuator performance is solved for a sequence of flow speeds and the obtained controller is synthesized using gain scheduling. The fairly large predicted increase in critical speed is experimentally verified with satisfactory accuracy.
A comparative Life Cycle Assessment (LCA) has been performed on the hull of a 24 meter long high speed patrol craft. The aim of the study is to compare different structural materials concepts to determine differences and sensitivities in environmental impact, especially in relation to the total impact including fuel burn. The material concepts studied are aluminium and various composite combinations consisting of glass fibre and carbon fibre with vinyl ester resin both as single skins and as sandwich with a Divinycell foam core. For each material concept a unique and weight optimized structural design was employed fulfilling the DNV high speed craft design code [1]. All identical systems and components for the five hull concepts are omitted in the LCA and hence a comparative study is performed focused on the hull structure. The commercial software SimaPro have been utilized for the LCA calculations and the impact assessment method chosen is the CML Baseline 2000.
The structural optimization carried out in [1] show that a weight reduction by to up to 50% (on the hull) could be achieved by switching from aluminium to a carbon fibre sandwich concept. The weight reduction switching from glass fibre single skin to a carbon fibre sandwich concept is roughly 20%.
The LCA study performed herein shows that, regardless of hull material concept, the environmental impact is by large dominated by the usage phase due to relatively large fuel consumption. A lower structural weight will reduce the fuel consumption and hence the environmental impact. This is illustrated in fig. 1 for the aluminum hull. All different phases of the life cycle are plotted for all environmental impact categories assessed herein. The green color is related to the operation phase and the red is the manufacturing phase. As observed the operation phase is dominating.
In fig. 2 are the results from the LCA presented for all hull concepts and for all environmental impact factors. The Al-hull is the concept with highest structural weight (red bars) and hence was found to have the highest environmental impact. The carbon fibre sandwich hull (green bars) had the lowest weight of the five and consequently the lowest environmental impact.
In the normalization analysis performed, in which different impact factors are assessed using a common impact unit, three impact factors stand out as dominating, independent of hull concept; Abiotic depletion, Global warming and Acidification. All these impact factors are herein associated with the usage phase and diesel consumption. If the operation phase is omitted and only the manufacturing phase (including the material extraction/manufacturing) is studied the observation is that the aluminium concept still will have the largest environmental impact for all categories. The most significant environmental impact is now on the marine and the fresh water aquatic ecotoxicity which is associated with the aluminium raw material excavation and manufacturing processes.
A comparative Life Cycle Assessment is performed for different structural material concepts on a 24-m-long high-speed patrol craft. The study is comparative and determines the differences in and sensitivities to environmental impact, especially in relation to the total impact of fuel burn for the different material concepts. The material concepts are aluminium and various composite combinations consisting of glass fibre and carbon fibre with vinyl ester resin both as single skins and as sandwich with a Divinycell foam core. Commercially available standard Life Cycle Assessment software is used for the Life Cycle Assessment calculations. The study shows that regardless of hull material concept, the environmental impact is dominated by the operational phase due to relatively large fuel consumption. In the operational phase, the lightest carbon-fibre concept is shown to have least environmental impact. Considering the manufacturing phase exclusively for the different hull concepts, it is concluded that the manufacturing of the aluminium hull has a somewhat larger environment impact for the majority of Life Cycle Assessment impact categories in comparison to the different composite hulls. The significant impact on the marine and the fresh water aquatic ecotoxicity originates from the aluminium raw material excavation and manufacturing processes. It is shown that the lightest hull, the carbon-fibre sandwich concept, with a 50% structural weight reduction compared to the aluminium design, can be utilized to reduce the fuel consumption by 20% (775 ton of diesel) over the lifetime with significant impact on the dominating environmental aspects considered herein, abiotic depletion, global warming and acidification.
This paper presents a comparison of different energy management strategies (EMS) for autonomous underwater vehicles (AUV) with fuel cell-battery hybrid power systems. Sophisticated EMS can decrease energy consumption, limit fuel cell degradation or increase reliability. EMS for hybrid vehicles have been studied extensively in the automotive industry where standardised drive cycles are applied. As for AUVs, there are no standard drive cycles and power profiles can vary significantly depending on the type of mission. In this study, rule-based and optimization-based EMS are compared. The rule-based strategies rely on deterministic rules and fuzzy logic, the optimization-based strategies minimize a constrained cost function to efficiently split the power demand. The EMS are evaluated against a previously sampled power profile of a Hugin 3000 AUV. The evaluation against real power profiles adds significant value to this study.
This study combines high-fidelity simulation models with experimental power consumption data to evaluate the performance of Energy Management Strategies (EMS) for fuel cell/battery hybrid Autonomous Underwater Vehicles (AUV). The performance criteria are energy efficiency, power reliability and system degradation. The lack of standardized drive cycles is met by the cost-efficient solution of synthesizing power profiles from sampled AUV field trial data. Three power profiles are used to evaluate finite-state machine, fuzzy logic and two optimization-based EMS. The results reveal that there is a trade-off between the objectives. The rigidity of the EMS determines its load-following behavior and consequently the performance regarding the objectives. Rule-based methods are particularly suitable to design energy-efficient operations, whereas optimization-based methods can easily be tuned to provide power reliability through load-following behavior. Both classes of EMS can be best-choice methods for different types of missions.
This paper analyses the transit performance of state-of-the-art underwater vehicles and presents an intermediate-fidelity steady-state flight mechanics model for qualitative performance assessment of underwater vehicles. Focusing on the comparison of underwater gliders and propeller-driven AUVs, a simple glide metric is presented and the transit performance of the legacy underwater gliders Slocum, Spray and Seaglider as well as propeller-modified versions thereof is evaluated. The evaluation is based on various data sets from wind tunnel tests and Computational Fluid Dynamics (CFD) studies, and shows that for the respective hull shapes gliding locomotion proves more efficient in ideal conditions. However, biofouling conditions inflict a double penalty on glider performance, rendering gliders inferior to propeller-driven vehicles. The Slocum data set is used to validate a steady-state flight mechanics model for qualitative performance prediction. It is shown that even simplistic models based on semi-empirical and analytical expressions can be successfully used for design optimization through parametrization. Being computationally efficient, the model can be a useful tool for design engineers in early design phases. The model is used to evaluate the effects of wing span on gliding efficiency, indicating that the current design of the Slocum glider is near-optimal.
Maribot LoLo is an autonomous underwater vehicle (AUV) developed at the KTH Centre for Naval Architecture as part of the Swedish Maritime Robotics Centre (SMaRC). The center's cross-disciplinary activities require an AUV research platform that can be used for data collection and to test and demonstrate novel technologies. The challenge herein is to create a well-performing and yet versatile vehicle. This paper introduces Maribot LoLo and presents the underlying design philosophy which focuses on versatility and endurance. A free-flooded hull offers modularity and modifiability while reliability and robustness are achieved through hardware redundancy and a hierarchical captain-scientist relationship in the embedded system. The vehicle is designed to be operated at moderate water depths and on long-range missions. This leads to challenges in the design of the variable buoyancy system (VBS) which also is presented. The achievable range of the AUV is evaluated with a simple hydrodynamics model based on frictional drag.
This study focuses on the feasibility and benefits of implementing of a fuel cell/battery hybrid power system in the autonomous underwater vehicle LoLo. We highlight the practical implications and challenges related to such a power system conversion and compare the benefits of using a fuel cell system rather than a pure battery system. Storage of hydrogen and oxygen as pressurized gases is considered most suitable for a conversion of this kind. In order to outperform Li-ion battery packs, high-pressure gas storage is required, preferably at pressures of 500 MPa or higher. Through a market analysis, we analyze the performance of commercial gas cylinders in terms of volumetric and gravimetric densities. This information can subsequently be used to compare energy density and effective energy density for the respective power systems. The study exemplifies how fuel cell/battery hybrid systems can provide up to 75% higher effective energy density compared to conventional battery packs. Ongoing developments in the fuel cell and auxiliary system markets are likely to allow for further optimization of the fuel cell system.
This paper presents the development and experimental evaluation of a self-steering mechanism for an autonomous sailing vessel. The steering mechanism is designed and tailored to relief the commonly used electro-mechanical steering during majority of the mission by the use of pure mechanical coupling between the self-trimming rig and the rudder. This significantly reduces the need for electric power for steering at constant apparent wind angle. Added to the steering, a two-layer navigation system is proposed for path-planning and navigation with algorithms tailored for low-power, low-memory microcontroller. We present experimental results from a total of 19 days of autonomous sailing in Stockholm's Archipelago. The experiments enabled us to compare the sailing performance with both active and self-steering systems for different apparent wind angles. On average, the active steering keeps a heading within ±5.1° of the target while the self-steering is able to maintain the heading within ±8.1° of the target. Another conclusion that can be drawn is that the apparent wind angle doesn't influence the steering performance.
This paper describes the ongoing development of Maribot Vane, an autonomous sailing vessel at the Maritime Robotics Laboratory of KTH, the Royal Institute of Technology, Stockholm. There is an accelerating need for ocean sensing where autonomous vehicles can play a key role in assisting scientists with environmental monitoring and collecting oceanographic data. The purpose of Maribot Vane is to offer a sustainable alternative for these autonomous missions by using wind and an energy efficient self-steering mechanism. The rig is composed of a free-rotating wing fitted with a coupled control surface. A completely novel wind vane self-steering solution has been developed and is being evaluated. A key point in the development of the vessel is robustness, with a goal of being able to sail in open seas for long period of times. The paper discusses some key concepts, the development method and presents initial results of the new systems.
This workshop draws on experience in the international collaboration for engineering education reform, called the CDIO Initiative, where project-based learning is a key part of the concept. The purpose of project-based courses in engineering education is to provide environments where students can develop a deeper working knowledge of technical fundamentals together with the complex skills necessary for engineering practice, or in short: where students can become engineers. In this workshop, the learning perspective is emphasized, by identifying trade-offs where there are inherent tensions between learning outcomes and other factors in project-based courses (such as project goal, product performance, technical sophistication, teacher popularity, student satisfaction). A set of principles are derived for enhancing learning and teaching in project-based courses, using concrete examples to illustrate thought-provoking implications. Each principle aims to improve both student learning outcomes and cost-effectiveness of teaching. Together the principles constitute a framework for learning-driven course design. The aim is to challenge assumptions and common practices in project-based courses, and provoke fruitful discussion among participants.
There is an increasing demand for underwater communication, not least manifested in a need to distribute and retrieve data from networks of underwater sensors. Whilst there are exceptions, acoustic techniques are generally the only viable means of communication. However, transmitting information acoustically is energy-intensive and can limit the lifetime of battery-powered platforms. Through simulations, this paper statistically investigates a recent transmission power controller, developed for underwater networks of static, battery-powered modems. The controller is self-configuring, as the modems' locations are assumed to be unknown. Further, the controller is fully distributed for scalability and adaptability reasons. The method involves a $k$-nearest neighbor approach when selecting transmission power for packet forwarding, i.e., the transmission power is selected such that only the $k$ closest modems will receive a packet. A well-known flooding-based routing protocol suitable for ad-hoc networks is employed to assess the energy consumption with and without the power controller. The evaluation is based on simulations using 16 modems placed randomly in a square area with varying sizes and choices of $k$. The results show that in a small and dense network, up to 61-68\% energy can be saved with a minor 7\% drop in packet delivery ratio.
In this study, we present the first iteration of DPower, an energy conserving method for use in underwater acoustic networks. The method encompasses a straightforward transmission power calibration procedure and adaptive power level selection. The method was evaluated in combination with DFlood, a known and validated constrained flooding protocol developed for underwater applications. Simulations of a network with given prerequisites have shown that, with an acceptable increase in packet loss, the presented method can dramatically reduce the energy consumption and thus improve the life-time of networks.
The paper compares measurement-based measures for human vibration exposure. Data were collected during sea trials on a 10 m, 50 kn coastguard craft equipped with a three-axial accelerometer at the coxswain seat and with vertically mounted gauges measuring the acceleration of the cockpit floor. The ISO 2631-1:1997 measures of vibration (namely the root-mean-square (r.m.s.) value of the whole-body vibration (determined from the frequency-weighted acceleration signal), the maximum transient vibration value (MTVV), and the vibration dose value), the ISO 2631-5:2004 measure (namely the daily equivalent static compression dose Sed), and also statistically based measures to evaluate the acceleration magnitude are compared and discussed with respect to their ability to identify the mitigating effect of the suspension seat and how the different measures rank the severity of the high-speed craft (HSC) ride. The paper concludes that the r.m.s. value and the MTVV are unsuitable for evaluation of the conditions aboard while the other investigated measures show potential in this respect. Further the approach of ISO 2631-5:2004 taking both the short-term and the long-term perspectives on the human exposure to vibration is concluded to be the most mature method well suited to evaluation of HSC conditions.
The paper describes an experimental study with the major aim to get a detailed picture of thepressure distribution carrying a planning craft at high speed through calm water and waves.The instrumentation, load cases and performed runs are discussed as well as the steps to usethe measurement data for evaluation of numerical models for planing craft in waves.
Previous studies have shown how the use of composite materials and application of sophisticated design methods can give significantly lighter high-speed craft structures than what is normally achieved for traditional aluminium designs. A reduction in structural mass and a corresponding reduction in displacement improve the craft calm water performance but can be unfavourable regarding the rough water performance. Here, the rough water performance of two versions of a fast patrol vessel, one in aluminium and the other in carbon fibre sandwich, is studied with simplified semi-empirical methods and more advanced non-linear time domain simulations. In speeds up to 30 knots, the rough water performance of the two craft versions is found to be practically equal. At higher speeds, the lighter composite craft experiences higher vertical accelerations than the heavier aluminium craft, which implies less operational availability. Using trim ballast tanks, the rough water performance of the lighter craft is improved, and it is shown that the acceleration levels can be reduced and even lowered relative to the heavier aluminium craft. This means that the calm water advantages of a lighter composite vessel can be utilized with the same ride comfort and operational availability as for a heavier aluminium vessel.
An experimental and analytical study was made on the effect of stress singularities on the strength of expanded PVC foam materials of different densities. Experiments were performed on specimens with different wedge geometries ranging from sharp cracks, with the ordinary inverted square root stress singularity, to shallow re-entrant corners with weak singularities. A brittle fracture criterion based on a generalised stress intensity factor, called Q, at the wedge tip was fit to experimental data. The critical stress intensity factor, Qcr, for crack initiation depends on the wedge geometry. This dependence was estimated from simple point-stress criteria and a criterion due to Seweryn [Brittle fracture criterion for structures with sharp notches. Engng Fracture Mech. 47, 673-681 (1994)], and good agreement with experimental data was obtained. When the point-stress criterion was applied to Mode II sharp cracks, poor agreement with published data was found. A critical study of the Mode II crack specimen was therefore initiated, leading to the conclusion that the commonly used specimen gives erroneous values of KIIc and the reason seems to be due to crack surface friction. A new Mode II crack specimen which eliminates crack surface friction was proposed and tested, and good agreement with the point-stress criterion was obtained. A criterion for homogeneous materials proved to be adequate also for the porous PVC foams.
Fjord-terminating glaciers in Svalbard lose mass through submarine melt and calving (collectively: frontal ablation), and surface melt. With the recently observed Atlantification of water masses in the Barents Sea, warmer waters enter these fjords and may reach glacier fronts, where their role in accelerating frontal ablation remains insufficiently understood. Here, the impact of ocean temperatures on frontal ablation at two glaciers is assessed using time series of water temperature at depth, analysed alongside meteorological and glaciological variables. Ocean temperatures at depth are harvested at distances of 1 km from the calving fronts of the glaciers Kronebreen and Tunabreen, western Svalbard, from 2016 to 2017. We find ocean temperature at depth to control c. 50% of frontal ablation, making it the most important factor. However, its absolute importance is considerably less than found by a 2013-2014 study, where temperatures were sampled much further away from the glaciers. In light of evidence that accelerating levels of global mass loss from marine terminating glaciers are being driven by frontal ablation, our findings illustrate the importance of sampling calving front proximal water masses.
This paper addresses the source localization problem of an acoustic fish-tag using the Time-of-Arrival measurement of an acoustic signal, transmitted by the fish-tag. The Time-of-Arrival measurements denote the pseudo-range information between the acoustic receiver and the fish-tag, except that the Time-of-Transmission of the acoustic signal is unknown. Starting with the pseudo-range measurement equation, a globally valid quasi-linear time-varying measurement model is presented that is independent of the Time-of-Transmission of the acoustic signal. Using this measurement model, an Uniformly Globally Asymptotically Stable (UGAS), three stage estimation strategy (eXogenous Kalman Filter) is designed to estimate the position of an acoustic fish-tag and evaluated against a benchmark Extended Kalman Filter based estimator. The efficacy of the developed estimation method is demonstrated experimentally, in presence of intermittent observations using an array of receivers mounted on three Unmanned Surface Vessels (USVs).
Arctic lakes are exposed to warming during increasingly longer ice-free periods and, if located in glaciated areas, to increased inflow of meltwater and sediments. However, direct monitoring of how such lakes respond to changing environmental conditions is challenging not only because of their remoteness but also because of the scarcity of present and previously observed lake states. At the glacier-proximal Lake Tarfala in the Kebnekaise Mountains, northern Sweden, temperatures throughout the water column at its deepest part (50 m) were acquired between 2016 and 2019. This three-year record shows that Lake Tarfala is dimictic and is overturning during spring and fall, respectively. Timing, duration, and intensity of mixing processes, as well as of summer and winter stratification, vary between years. Glacial meltwater may play an important role regarding not only mixing processes but also cooling of the lake. Attribution of external environmental factors to (changes in) lake mixing processes and thermal states remains challenging owing to for example, timing of ice-on and ice-off but also reflection and absorption of light, both known to play a decisive role for lake mixing processes, are not (yet) monitored in situ at Lake Tarfala.
In Arctic alpine regions, glacio-lacustrine environments respond sensitively to variations in climate conditions, impacting, for example,glacier extent and rendering former ice-contact lakes into ice distal lakes and vice versa. Lakefloors may hold morphological records of past glacier extent, but remoteness and long periods of ice cover on such lakes make acquisition of high-resolution bathymetric datasets challenging. Lake Tarfala and Kebnepakte Glacier, located in the Kebnekaise mountains, northern Sweden, comprise a small, dynamic glacio-lacustrine system holding a climate archive that is not well studied. Using an autonomous surface vessel, a high-resolution bathymetric dataset for Lake Tarfala was acquired in 2016, from which previously undiscovered end moraines and a potential grounding line feature were identified. For Kebnepakte Glacier, structure-from-motion photogrammetry was used to reconstruct its shape from photographs taken in 1910 and 1945. Combining these methods connects the glacial landform record identified at the lakefloor with the centennial-scale dynamic behaviour of Kebnepakte Glacier. During its maximum 20(th) century extent, attained c. 1910, Kebnepakte Glacier reached far into Lake Tarfala, but had retreated onto land by 1945, at an average of 7.9 m year(-1).
LOTUS is a bottom landing, Long Term Underwater Sensing node made for the observation of ocean water temperatures. LoTUS is moored to the seafloor and measures temperature according to a specified time schedule until, at the end of the mission, it surfaces to transmit the collected data to on-shore recipients using an Iridium link. The paper presents an extension of the sensing capability which includes water current velocity (speed and direction) using a robust, reliable and inexpensive Eulerian method. The method is based on the "tilting stick" principle where a combination of inertial and magnetic measurement data are used. The paper discusses the principal technique, modeling of the system, practical considerations, optimization of the setup for specific flow conditions, and the verification of experimental data.
This thesis treats various aspects of structural polymercomposites in aircraft applications. The mechanical performanceand quality of resin transfer molded (RTM) carbon fiberreinforced epoxy composites is studied. In a first part, the influence of manufacturing process parameters on the mechanicalbehavior of laminates is experimentally investigated. A number of process parameters are used as variables and performance ismeasured in terms of tensile and compressive strength as wellas interlaminar fracture toughness. The process parameters are concluded to have little affect on the measured properties. In a second part, the quality and structural performance of an entirely co-cured RTM manufactured aircraft control surfacedemonstrator is investigated. A series of quasi staticstructural tests using distributed loading is performed. Experimental results are compared with finite element analysis. Effects of impact damage on the performance are also studied.Good agreement is obtained between the predictions and the experiments.
A nondestructive method for determination of elasticmaterial properties of orthotropic plates using naturalfrequencies is developed and verified. Finite elementcalculations of the natural frequencies of the plate are matched to experimentally determined frequencies using theelastic constants as variables. The method is successfully verified even for nontrivial specimen geometries with cornersingularities. Emphasis is on practical utilization ofknowledge about numerical and modeling errors as well asexperimental uncertainties.
The optimal design of a thin orthotropic wing subject toaeroelastic constraints is studied using numerical methods andverified in low speed wind tunnel testing. The flutter speed ofthe wing is maximized using the laminate orientation asvariable. Further, the problem of increasing the flutter speed to a prescribed value using minimal amount of additional concentrated masses on a fixed wing design is investigated. The main objective of the study is to verify that the performance of the optimized design can be achieved also in experiments. It is found that the optimal design is very sensitive to uncertainties in material and structural properties.Consequently, this has to be accounted for in the problemformulation. It is shown, and experimentally verified, that the robustness requirements on the optimal design can be met byreformulating the optimization problem.
The usefulness of an optical motion capture system in aeroelastic wind-tunnel testing is investigated. A system consisting of four infrared charge-coupled device cameras, observing flat passive reflecting markers, is installed in a low-speed tunnel to measure Butter mode shapes. Free vibration and aeroelastic measurements are performed on four wing configurations consisting of thin flat orthotropic composite laminates with varying laminate orientation. The laminate orientations are chosen to result in dissimilar flutter mode shapes. The wings are equipped with up to 20 markers, and the motion is sampled at 240 Wt. Quantitative scalar comparisons between analysis and experiments, with respect to both amplitude and phase are done using the modal assurance criterion (MAC). Measurements of mode shapes on free vibrating wings (ground vibration tests), as well as limit-cycle Butter oscillations, show good agreement with numerical results. MAC ratings consistently exceeding 0.96 are achieved, However, it is clearly seen that the agreement is better for free vibration comparisons than for Butter This is expected considering the higher complexity of the Butter problem. Thus, the cause cannot be attributed exclusively to insufficiencies in the optical system but also to inaccuracies in the modeling. The good quality of the measurements proves the usefulness of such a noncontact positioning system in experimental wind-tunnel testing, not only in the present flutter context, but in a variety of experimental work affected by aeroelastic deformation.
With regards to energy constrained Autonomous Underwater Vehicles (AUVs), and difficulties inherent to the acoustic underwater communication channel, a non-coherent method is investigated in order to improve energy consumption and reliability over traditional Frequency Shift Keying (FSK), without increasing the bandwidth. A proposed method of adapting Trellis Coded Modulation (TCM) to constant amplitude Permutated Frequency Shift Keying (PFSK) symbol constellations is evaluated. A system implementation of two PFSK methods is simulated in an Additive White Gaussian Noise channel, and field tested in an underwater channel in the Stockholm archipelago, where a binary FSK reference method is used as a comparison. The main interest is comparing electrical bit energy and bit error rate (BER) for the methods. Time variability of frequency fading, related to wind speed, is also evaluated from the field tests.
This paper addresses the design of composite craft with respect to non-hydromechanic, local loads not explicitly covered in the basic design standards. The primary aim of this paper is to prompt a discussion on these localized assault loads and the associated robustness issue for composite craft. A background and review of craft robustness and resistance to these types of loads is presented to provide a basis for a more nuanced discussion on inherent differences between different material concepts. Through a literature review and interviews with designers and operators different opinions on what is regarded as robust craft and why are identified. Further, a discussion on how progress can be made with respect to designing more efficient craft out of composites, based on the outlining of a possible design approach, is presented.
In this article, the problem of collaborative tracking of an underwater target using autonomous surface vehicles (ASVs) is studied. Distance-based formation control with a collision-avoidance potential function is employed as a solution. A formation control protocol is devised and applied to the formation tracking problem. With the protocol, the vehicles form a desired formation around a moving target in order to continuously estimate its position, while the centroid of the formation tracks the target. Almost global stability is proved for the case with three tracking agents. A fully operational platform with four ASVs was built to implement the derived algorithms. One of the vehicles was used to simulate a target and the rest to form a triangular formation around it. Power usage of a naval vessel is highly affected by water resistance forces which increases significantly with the velocity. This was accounted for by adding an additional term to the formation tracking protocol, thereby increasing the overall system endurance. Experimental results are presented.
KTH har infört en ny utbildningsstruktur, som innebär att de två sista åren på civilingenjörsutbildningen utgörs av masterprogram samt att studenterna ska vara behöriga att ta ut en kandidatexamen efter tre års studier. Man kan därmed säga att både kandidat och master ligger inbäddade i den femåriga civilingenjörsutbildningen. Kombinationen av utbildningsprogram, examina och utbildningsmål väcker några principiellt intressanta frågeställningar med långtgående konsekvenser för ingenjörsutbildningen. Vi har analyserat dessa frågeställningar och identifierat att vi står inför ett strategiskt vägval där det nu gäller att långsiktigt slå vakt om ingenjörsutbildningens värde som professionsutbildning. En väl genomförd implementering av den nya utbildningsstrukturen kan förena vetenskaplighet med ingenjörsmässighet och därmed väsentligt stärka ingenjörsutbildningen.
The paper considers full-scale trials that has been performed to evaluate the structural design of the Visby Class corvette. In particular evaluation of the applied design loads is discussed. A method which enables detailed experimental evaluation of the hydrodynamic loads on high-speed craft hull structures is presented. The method involves pressure measurements, reconstruction of momentary spatial pressure distributions from the discrete point measurements, calculation of panel loads by integration of the reconstructed pressure, and derivation of statistical extreme values in parity with the design loads. The presented method has been successfully used to evaluate the Visby design and it is concluded to be an efficient tool. In the paper application of the method is demonstrated on full-scale trial data. Experimentally derived panel loads are shown to be distinctly higher than the minimum requirements according to classification rules. Future application of the method is discussed, for example how it can be used to evaluate and improve existing semi-empirical design methods as well as direct calculation methods.
Weather routing decisions are typically based on performance predictions, in terms of estimated time of arrival, ETA, fuel consumption, and consideration of various constraints on the vessel operation such as limitations on allowable acceleration levels or slamming frequencies.
This paper presents and discusses the results of a comparison between using deterministic and ensemble weather forecasts for weather routing. The study is based on comparisons between predicted and realised performance of routes suggested by a route optimization method and focuses on two important performance factors, namely, fuel consumption and late arrival. The study is purely qualitative since the simulations do not include re-routing of the vessel as new forecasts become available. To perform the study a multi-objective dynamic programming method is tailored to the problem and implemented to perform the route optimization and a ship performance model is used to calculate the additional fuel consumption due to wind and waves acting on the ship. The results show that route optimization using ensemble weather forecasts has the potential to reduce the risk of late arrival for voyages during periods of harsh weather.
This paper deals with the shortcomings in current design methods for dynamically loaded composite structures in underwater applications. This is done through an experimental study to evaluate the eigenfrequencies of rectangular plates made from metals as well as composites that are tested in air (dry) and completely submerged under water (fully wetted). The eigenfrequencies are studied using forced vibrations. The test series comprises 19 specimens that are made from various materials including aluminium, steel, glass-fibre, and carbon-fibre with aspect ratios varying from 3.7 to 11.2 and breadth to thickness ratios ranging from 2.7 to 20.5. The test method is based on electro-mechanical excitation by random vibrations as well as stepped sine refinements in the vicinity of the identified eigenfrequency. The results clearly show how differently the specimens are affected by the "added mass" from the water when fully wetted compared to the dry condition. Slender and more lightweight configurations are more profoundly affected by water than heavier and more rigid specimens. The results clearly show that for advanced composite materials and more complex geometries the current rule-of-thumb methods used by the industry today are inadequate in predicting the shift in natural frequency due to the effect of the surrounding water.
The paper considers explicit FE-modelling of fluid-structure interaction in hull-water impacts. To minimize the variables studied and to enable comparison with analytical methods, the problem is here idealised as a two-dimensional rigid wedge impacting on a calm water surface. A parameter study is performed, where the sensitivity in the mesh resolution and the contact parameter selection is investigated. It is concluded that a numerically stable non-leaking solution to the hull-water impact problem, with good correlation to analytical results, is achievable. It is however also concluded that application of this modelling technique can be costly and far from trivial. The solution is for example highly dependent on the relation between mesh-density and contact-stiffness. Successful modelling hence requires rational approaches for determination of fluid discretization and contact parameters. By making reference to an analytical solution of the hull-water impact problem, the results from the parameter study are generalised, and a rational approach for determination of fluid discretization and contact parameters in the modelling of arbitrary hull-water impact situations is presented. The generality of the approach is favourably demonstrated for different impact angles and velocities.