In light of the stochastic nature of wind, the main obstacle to reliable penetration of wind power into the power grid is its variability. An accurate multi-step prediction of wind power is the most efficient way to address this issue, along with challenges in wind power management and maintenance. Several strategies have been presented in the literature for multi-step ahead wind power prediction, however, a comprehensive comparison of these strategies has not been performed to determine how outliers in power datasets influence the accuracy of multi-step ahead predictions at different forecasting horizons. To fill this gap, in this study, by reviewing the existing strategies, three main approaches including the recursive, direct, and multi-input multi-output (MIMO) strategies are investigated using two real-world datasets from two wind turbines in Turkey and Scotland. A hybrid prediction method based on application of the Isolation Forest for outlier treatment, long short-term memory (LSTM) as the core of the prediction model and a new hyperparameter optimization algorithm for the tuning of the LSTM model is used for predictions in different strategies. According to the results of the experiments (1) in two-step ahead wind power forecasting, all strategies produce similar results, in both wind turbines; (2) in all forecast horizons of more than two steps ahead, the MIMO approach is best when the dataset does not contain any outliers – however, when there are outliers, the direct approach performs better; (3) in both datasets, the recursive approach to wind power forecasting produces the highest error rates.

During the final metres of the powered descent of Apollo 11, astronauts Neil Armstrong and Buzz Aldrin lost sight of the lunar surface. As the retro-rockets fired towards the lunar dust – or regolith – to decelerate the spacecraft, soil erosion occurred and the blowing dust led to severe visual obstruction. After a successful landing, the presence of dust continued to impact the mission with adverse effects including respiratory problems and difficulty in performing tasks due to clogging of mechanisms, amongst others. As these effects were observed in subsequent missions, the “dust problem” was identified as one of the main challenges of extra-terrestrial surface exploration. In this work, the focus is placed on dust dispersal, which arises from the interaction between a rocket exhaust flow – or plume – and the planetary surface. Termed plume-surface interactions (PSI), this field of study encompasses the complex phenomena caused by the erosion and lofting of regolith particles. These particles, which are ejected at high-speeds, can lead to damage to the spacecraft hardware or a reduction in functionality. Moreover, plumes redirected back towards the landers can induce destabilising loads prior to touch-down, risking the safety of the landing. To achieve a sustained presence on the Moon, as planned by NASA’s Artemis programme, it is essential that PSI are well understood and mitigating measures are put in place, particularly if spacecraft are to land in the vicinity of lunar habitats. Although experimental work began in the 1960s and mission PSI were first recorded in 1969, a fundamental understanding of this phenomena has not yet been achieved. In this paper, a compendium of experimental PSI is presented, identifying the main challenges associated with the design of tests, stating important lessons learnt and the shortcomings of available experimental data and findings. Lastly, recommendations for future experimental work are presented.

Scour is a significant impact caused by climate change on infrastructure, while also being the most common cause of bridge failure worldwide. Approximately 60% of bridge collapses are a result of scour (Briaud and Hunt, 2006; Wardhana & Hadipriono, 2003).Climate change has resulted in the increase of extreme weather events, such as wildfires and floods among others. Global warming is evident, sea levels are rising, and the frequency and magnitude of flood events is increasing. As the climate is changing, the risk of scour is expected to increase further.Monitoring is crucial for the identification of scour taking place around a structure, its magnitude, as well as the rate of deterioration to allow owners and operators to establish when predetermined thresholds are at risk of being reached. Scour monitoring is crucial to safeguard infrastructure that could be exposed to scour action.According to the Design Manual for Roads and Bridges BD 97/12 Standard entitled ‘The assessment of scour and other hydraulic actions at highway structures’, scour monitoring techniques can be divided in the following categories (Highways Agency, 2012):Measuring the maximum scour level that has taken place; Measuring scour development adjacent to a structure during high flow events; Methods correlating with scour development, such as water level monitoring, flow velocity monitoring and weather warnings. Scour monitoring techniques are mainly reactive. This study compares existing and emerging scour monitoring methods, exploring a combination of scour monitoring sensors at structures at risk of scour. The introduction of a new, innovative sensing platform for scour monitoring is discussed, linking the new sensor package to the asset health management platform using telematics, enhancing the understanding of scour taking place through accurate visualisation. This method facilitates more proactive monitoring of scour, the collection of data necessary for the design and implementation of scour protection measures, and innovative, more accurate scour prediction.References:Briaud JL and Hunt BE (2006) Bridge scour and the structural engineer. Structure Magazine, December: pp. 57–61.Highways Agency, Transport Scotland, Welsh Government and Department for Regional Development Northern Ireland, UK (2012) Design Manual for Roads and Bridges. Highway Structures: Inspection and Maintenance. Volume 3, Section 4, Part 21. BD 97/12. The Assessment of Scour and Other Hydraulic Actions at Highway Structures. The Stationery Office, London, UK.Wardhana K and Hadipriono FC (2003) Analysis of recent bridge failures in the United States. J. Perform. Constr. Facil. 17 (3): 144–150. https://doi.org/10.1061/(ASCE)0887-3828(2003)17:3(144)

The presence of contact and backlash often leads to complex and rich dynamic responses in mechanical structures. This typically occurs in systems such as jointed structures and geared mechanisms, where the inherent complexity leads to responses which are difficult to predict. In particular, identifying models capable of performing reliable predictions is extremely challenging, both for the definition of the equations of motion and for the characterisation of their parameters. In this chapter, we present a systematic approach based on the restoring force surface method for analysing and identifying the localised piecewise nonlinear characteristics in multi-degree-of-freedom systems. Our approach builds on the knowledge of the underlying linear system and outlines a procedure to identify equivalent nonlinear characteristics via a graphic representation of the piecewise restoring forces. The approach is validated against experimental measurements of a test rig representative of a two-degree-of-freedom system with a localised nonsmooth characteristic. The reconstruction of the nonsmooth restoring force, identification of the piecewise characteristics, and the validation of the model against experimental time histories are performed following our systematic procedure, proving the flexibility of this practical approach.

Multiphase flow inside of pipes occurs in a wide variety of engineering applications, including offshore deep-water oil and gas transport. Vibrations induced by the flow inside of the pipe can lead to its mechanical failure and thus lead to uncontrolled release of the fluids being transported. In subsea applications, flexible J-risers are often employed to deliver the produced fluids from the seafloor to the host platform. Despite the potentially significant liabilities associated with subsea hydrocarbon leaks, there has been a distinct lack of investigations into how flow induced vibrations in large scale, pressurised flexible J-risers can lead to system integrity loss. Previous investigations have generally focused on the response of rigid pipes or small scale, unpressurised flexible risers. This study presents an investigation into the response of a 10 m long, 50.8 mm internal diameter composite riser containing a tensile armour helical structure to a variety of two-phase, water-nitrogen flows at 10.8 barg of pressure and ambient temperature. High speed cameras were used to investigate the structure of the flow at either end of the flexible riser, whilst synchronised surface mounted strain gauges and accelerometers were used to investigate the response of the pipe. Time-averaged data were acquired to assess the general response of the pipe, whilst a statistical analysis of the fluctuations highlighted the movement of the pipe. One-dimensional and computational fluid dynamics simulations were used to define the experimental test matrix and provide further insight into the structure of the flow inside the J-riser. Single-phase gas flow was found not to cause the J-riser to move significantly, whilst multiphase flow led to significant in-plane movement of the pipe. Increasing the liquid flow rate (or decreasing the gas flow rate) increased the mean strain experienced by the pipe. At low gas flow rates, the pipe oscillated smoothly about its mean position, but at higher gas flow rates a violent intermittent whipping motion was observed. The latter produced large in-plane and out-of-plane movement of the pipe which could pose a threat to system integrity. This work offers new insights into fluid-structure interactions in large scale engineering applications, contributing to improved system design and control.

This paper presents a study on the application of a shunt damping circuit for vibration control in electro-mechanically coupled structures. The effectiveness of this design is demonstrated through experimentation with a cantilever beam featuring a magnet at its tip, moving in a wire coil that is connected to the shunt damping circuit. The study showcases the reduction of beam vibrations under both frequency response and free vibration conditions. Results show that the shunt damping circuit significantly reduces the first peak in frequency response during harmonic excitation. Additionally, the settling time of free vibrations is reduced by adjusting the electrical resistance. The paper uses analytical expressions for achieving critical damping for the free response, along with experimental validation. The study also offers a detailed description of the design process of the negative impedance converter used in the electrical circuit. This includes an examination of the influence of the transducer mass and moment of inertia on the converter's design. Finally, this paper offers valuable insights into the design of shunt damping circuits for vibration control in electro-mechanically coupled structures. The experimental results and analytical expressions provided in the study can help guide the realization of such circuits in various applications.

Ocean waves hold great promise as a renewable energy source, yet effectively harnessing this energy necessitates the enhancements of technologies. Despite the advancements in Wave Energy Converters (WECs), several current limitations in wave energy harvesting technologies hinder widespread commercial adoption. WECs have demonstrated their potential for electrical power generation but there is a recognition that a single type of energy harvesting mechanism may not fully exploit the vast energy potential of the marine environment. This study explores Vortex Induced Vibrations (VIV) as a complementary energy har-vesting mechanism for WECs: the basic idea is to exploit wave-induced VIV. The paper investigates the viability of this approach, focusing on clarifying the necessary conditions required to enable this innovative energy harvesting approach. The results provided in this paper contribute to a deeper understanding of the practicality of wave induced VIV harvesting, highlighting some potential challenges associated with deploying such a system.

This paper presents a methodology for accurately incorporating the nonlinearity of boundary conditions (BCs) into the mode shapes, natural frequencies, and dynamic behaviour of analytical beam models. Such models have received renewed interest in recent years as a result of their successful implementation in state-of-the-art multiphysics problems. To address the need for this boundary nonlinearity to be more completely captured in the equations of motion, a nonlinear algebra expansion of the classical linear approach for developing solvability conditions for natural frequencies and mode shapes is presented. The method is applicable to any BC that can be accurately represented in polynomial form, either explicitly or through the application of a Taylor expansion; this is the only assumption made in removing the need for the use of analytical approximations of the dynamics themselves. By reducing the BCs of the beam to a system of polynomials, it is possible to utilise the tensor resultant to develop these solvability conditions analogous to the conditions placed on the matrix determinant in linear, classical cases. The approach is first derived for a general set of nonlinear BCs before being applied to two example systems to investigate the importance of including nonlinear tip behaviour in the BCs to accurately predict the system response. In the first, a theoretical, symmetric system, in which a beam is supported by nonlinear springs, is used to explore both the applicability of the methodology and the improvements it can make to the accuracy of the model. Then, the more practical example of a cantilever beam with repulsive magnetic interaction at the tip is used to more explicitly assess the importance of properly incorporating boundary nonlinearity into multiphysics problems.

There are many examples in nature of travelling waves used for propulsion purposes, e.g., micro-organisms and sea creatures. Structural travelling waves can be used to induce momentum in a surrounding media creating a net propelling force. Recent research has tried to capture this interaction in engineering devices. Nonetheless, some challenges remain to fully exploit this phenomenon so that travelling-waves propelled devices can be optimally designed. One such challenge is that the interaction between the structure and the surrounding fluid heavily influences the amplitude of the waves and how they travel through the structure. This paper proposes a systematic qualitative and quantitative analysis of travelling waves in a slender cantilever beam submerged in water. The novelty of this work is demonstrated through two key aspects: The application of the Euler-Bernoulli beam equation combined with the Galerkin approximation, enabling a deeper understanding of how travelling waves form at resonant frequencies rather than non-resonant ones; and An analytical approach using a Galerkin approximation to characterise the nonlinear fluid-structure interaction, followed by linearisation for a comprehensive parametric study of the problem. In this investigation, the contributions of the first five vibration modes are considered in relation to the travelling waves observed near the resonant peaks. Experimental tests validate the analytical results and assess the accuracy of the proposed models. The results demonstrate that the model presented effectively characterises the travelling waves, making a suitable tool for the design of travelling-wave propelled devices.

In the field of aerospace and mechanical engineering, the identification of mathematical models capable of accurately representing the dynamics of mechanical structures is extremely important to improve the design and accelerate the certification of new systems and structures. Recent developments in the field of machine learning have demonstrated that Neural Networks (NNs) can accurately model the dynamics of linear and nonlinear systems in the time domain. Physics-Informed Neural Networks (PINNs) exploit this property and utilise physics-informed loss functions to identify the parameters of systems. Nonetheless, it is still not completely clear how the loss functions affect the learning process of the NNs and which type of function improves/deteriorates the identification process of the parameters associated with mechanical systems. In this chapter, we investigate the effect of three different loss functions on the learning and identification capabilities of PINNs when mechanical problems are considered. To this end, classic Forward Neural Networks (FNNs) are embedded in a parameter identification scheme based on physics-informed loss functions, and the parameters (natural frequency and damping) of a single-degree-of-freedom (SDOF) mechanical oscillator are identified via numerical experiments. In order to minimise the required training data, the loss functions are built considering the governing equations of motion and a single dynamics response of the oscillator in the form of either acceleration, velocity, or displacement. Their effect is then evaluated in terms of the accuracy of the identified unknown parameters and the capacity of the NN to predict the unknown physical dynamic responses. We demonstrate that loss functions based on the acceleration time series allow the NN to correctly learn the unknown physical dynamic responses, i.e., the velocity and the displacement, with great accuracy; this results in faster and more efficient learning of the dynamic behaviour of the system which, in turn, allows to identify the correct mechanical parameters.

Lightweight structures, once ubiquitous in specific sectors such as aeronautics and space sectors, in recent years, have increasingly attracted the attention of industries that, historically, have not been particularly concerned with structural weight. Ample examples are provided by the civil and automotive industry, in which the paradigm shift towards lower carbon footprint and sustainability prompted new trends characterised by mass reduction, the use of novel materials, and the accounting for large deformations. However, accurately modelling the dynamic behaviour of such structures requires nonlinear mathematical models, which are not widely used in common industrial practices. Reduced-Order Models (ROMs) have emerged as a popular alternative to computationally expensive Finite Element (FE) models, nonetheless, there is still a need to evaluate their effectiveness in accurately modelling strongly nonlinear behaviours. This study investigates the capacity of multiple-degree-of-freedom (MDOF) ROMs to capture and predict the nonlinear behaviour of lightweight structures subjected to large deformations. A novel identification procedure, built on existing linear and nonlinear identification methods, is used to identify an ROM from numerical and experimental data. Being based on the separation of the linear and nonlinear restoring force contributions, the proposed method can be easily embedded in the current industrial practices for the identification of mechanical systems, paving the way to an integrated usage of linear and nonlinear dynamic models. To validate the identified MDOF-ROM, a lightweight structure composed of lumped masses and nonlinear elastic connections is experimentally studied and the numerical and experimental results are compared at different excitation conditions. We demonstrated that the existence of the Nonlinear Restoring Force (NLRF) surface in a reduced subspace corresponds to the presence of local active nonlinearities in the experimental model. This information permits simplifying the nonlinear restoring force function of the ROM, improving the overall identification process. Finally, we showed that the identified ROM accurately represents the nonlinear dynamic behaviour of the experimental test rig and correctly predicts the passage from high-amplitude response to low-amplitude response (jumps) when different levels of excitation are applied to the system, demonstrating the effectiveness of the proposed procedure.

The growing industrial demand for lightweight and low-carbon emission systems is eroding the safety factors adopted in the linear design of vehicles and structures. This exposes the ultimately nonlinear nature of mechanical systems, creating the need for a better understanding of their nonlinear behaviour. In this context, we have experimentally investigated the dynamic behaviour of a nonlinear two-degree-of-freedom mechanical system with piecewise stiffness characteristics. The system is clamped at both ends, and one constraint is directly excited by a shaker. The system allows the adjustment of non-contact gaps and stiffness of the piecewise characteristic and provides a valuable resource for the validation and verification of numerical studies in this field. The experimental results show the very rich dynamics of the system, revealing the presence of quasi-periodic, chaos, and multi-periodic responses as well as branches of bifurcating stable solutions.

Most of the optimisation studies of Vibration Energy Harvesters (VEHs) account for a single output quantity, e.g. frequency bandwidth or maximum power output, but this approach does not necessarily maximise the system efficiency. In those applications where VEHs are suitable sources of energy, to achieve optimal design it is important to consider all these performance indexes simultaneously. This paper proposes a robust and straightforward multi-objective optimisation framework for Vibration Piezoelectric Energy Harvesters (VPEHs), considering simultaneously the most crucial performance indexes, i.e., the maximum power output, efficiency, and frequency bandwidth. For the first time, a rigorous formulation of efficiency for Multi-Degree of Freedom (MDOF) VPEHs is here proposed, representing an extension of previous definitions. This formulation lends itself to the optimisation of FE and MDOF harvesters models. The optimisation procedure is demonstrated using a planar-shape harvester and validated against numerical results. The effects of changing some structural parameters on the harvester performance are investigated via sensitivity analysis. The results show that the proposed methodology can effectively optimise the global performance of the harvester, although this does not correspond to an improvement of every single index. Furthermore, the optimisation of each performance index individually results in a variety of design configurations that greatly differs from one another. It is here demonstrated that the design obtained with the multi-objective function here proposed is similar to the design obtained when optimising the efficiency.

The finest fraction of the lunar regolith, namely the lunar dust, poses a challenge for hardware design and lunar operations. The Apollo missions experienced equipment malfunctions and failures due to dust interactions with hardware. In this work we focus on the problems related to the clogging of rigid body mechanisms. We explain the causes of the problem and propose a solution consisting of replacing traditional mechanisms with compliant mechanisms. There are multiple methods for synthesizing compliant mechanisms, but two approaches are most commonly used: analytical design and topology optimization. In this paper using a compliant gripper as an example, the suitability of these methods to design compliant mechanisms used in extra-vehicular activities is investigated. In doing so, the feasibility of using complaint mechanisms in the lunar equipment as part of dust mitigation strategies for surface projects is also demonstrated.

Using nature-inspired solutions for propulsion, this work investigates the use of traveling waves to generate thrust in water. A design based on a slender cantilever beam similar to flagella in bacteria is submerged in water and excited with a sinusoidal motion to study the impact of frequency and amplitude of the oscillation on the thrust generation. Structural measurements combined with advanced flow diagnostic techniques are used to characterize the behavior of the fluid–structure system. The structural response and the induced traveling waves are first studied in air and characterized through laser vibrometry and high-speed digital image correlation. This demonstrated the possibility of inducing traveling waves in the structure and permitted to identify the conditions that maximize the traveling versus the standing wave contribution. The characterization of the fluid–structure interaction has been done using Laser Doppler Anemometry (LDA). LDA measurement was carried out downstream from the beam at a fixed distance to measure the velocity of the induced flow at different excitation conditions (amplitude and frequency). The results showed that the coupling between the structural motion and the thrust generated is nonlinear in nature and depends on the tip displacement of the beam. Empirical laws that relate the amplitude and frequency of excitation to the generated thrust are here proposed.

Inspired by the use of travelling waves for propulsion mechanisms adopted by many animals and organisms, this paper investigates the mechanism with which travelling waves in beam-like structures induce velocity in a surrounding fluid. This work focuses on the flow induced around the beam tip and provides an experimental characterisation of the phenomena. A test rig equipped with Laser Doppler Anemometry (LDA) is used to investigate the induced fluid velocity and its dependency on the modal response of the beam. Numerical simulations are performed to complement the experimental tests and provide further insight into the fluid–structure interaction. The numerical model also allows for the beam vibration envelope and vibration patterns to be obtained and correlated with the measured fluid velocity. Several geometrical variations are considered in the analysis over a range of frequencies that encompass resonant responses up to the fourth mode. The results showed that the fluid flow is only marginally affected by the change in the vibration pattern between the first-two resonant mode; the induced fluid velocity and, hence, the thrust are mostly driven by the tip velocity of the beam.

An experimental study into the fluid–structure interaction of gas–liquid flows as they pass through a pipeline riser, like those utilized in the oil and gas industry, was carried out. In this study, a 10-m-long pipe with inner diameter of 50 mm at 10 bar (gauge) pressure was used with operating fluids of water and nitrogen. A two-phase flow regime diagram is used to determine liquid and gas volume flow rates that fall within the slug flow regime, and this was further refined using PETEX GAP and finally CFD software, STAR-CCM+, to determine the experimental test matrix. Axial strain gauges were positioned close to the flanges at the ends of the riser to measure strain at the cardinal points of the pipe at both the top and bottom of the riser. At approximately 2 m and 3 m horizontal distance from the entrance to the riser, accelerometers were attached to monitor the motion of the pipe at these positions. The motion of the pipe was observed to be almost exclusively in-plane, with only small out-of-plane motion registered on the accelerometers. Strain gauge measurements demonstrated that the increase in mass flow rate of the liquid phase/decrease in mass flow rate of the gas phase served increases the mean values of stain at both the entrance and exit of the riser. A monotonic increase in the magnitude of the rms of the fluctuations of the strain was observed for an increase in the flow rate of either the liquid or the gas phase. The highest flow rates tested introduce an irregular cyclic whipping motion in the flexible pipe in addition to the smooth oscillation motion.

Meta-heuristic optimisation algorithms are high-level procedures designed to discover near-optimal solutions to optimisation problems. These strategies can efficiently explore the design space of the problems; therefore, they perform well even when incomplete and scarce information is available. Such characteristics make them the ideal approach for solving nonlinear parameter identification problems from experimental data. Nonetheless, selecting the meta-heuristic optimisation algorithm remains a challenging task that can dramatically affect the required time, accuracy, and computational burden to solve such identification problems. To this end, we propose investigating how different meta-heuristic optimisation algorithms can influence the identification process of nonlinear parameters in mechanical systems. Two mature meta-heuristic optimisation methods, i.e. particle swarm optimisation (PSO) method and genetic algorithm (GA), are used to identify the nonlinear parameters of an experimental two-degrees-of-freedom system with cubic stiffness. These naturally inspired algorithms are based on the definition of an initial population: this advantageously increases the chances of identifying the global minimum of the optimisation problem as the design space is searched simultaneously in multiple locations. The results show that the PSO method drastically increases the accuracy and robustness of the solution, but it requires a quite expensive computational burden. On the contrary, the GA requires similar computational effort but does not provide accurate solutions.

In the industry field, the increasingly stringent requirements of lightweight structures are exposing the ultimately nonlinear nature of mechanical systems. This is extremely true for systems with moving parts and loose fixtures which show piecewise stiffness behaviours. Nevertheless, the numerical solution of systems with ideal piecewise mathematical characteristics is associated with time-consuming procedures and a high computational burden. Smoothing functions can conveniently simplify the mathematical form of such systems, but little research has been carried out to evaluate their effect on the mechanical response of multi-degree-of-freedom systems. To investigate this problem, a slightly damped mechanical two-degree-of-freedom system with soft piecewise constraints is studied via numerical continuation and numerical integration procedures. Sigmoid functions are adopted to approximate the constraints, and the effect of such approximation is explored by comparing the results of the approximate system with the ones of the ideal piecewise counter-part. The numerical results show that the sigmoid functions can correctly catch the very complex dynamics of the proposed system when both the above-mentioned techniques are adopted. Moreover, a reduction in the computational burden, as well as an increase in numerical robustness, is observed in the approximate case.

Additive manufacturing has become increasingly popular in the last decades and has shown great potential for designing and manufacturing innovative design solutions. Recently it has been demonstrated that additive manufacturing can be used to create monolithic compliant mechanisms that can avoid assembly and relative movement between components, showing considerable advantages in their use in harsh environments (i.e. space applications). In this paper, we explore the possibility of adopting 3D-printed compliant mechanisms as tuned-mass vibration absorbers: the challenge is to identify the characteristics of an equivalent nonlinear oscillator that can be used to assess the performance of the absorber. The experimental and numerical results show that the proposed compliant mechanism offers a complex nonlinear dynamic behaviour and it can effectively act as a vibration absorber for a simple cantilever beam.