Activated carbon is employed for the adsorption of organic micropollutants (OMPs) from water, typically present in concentrations ranging from ng L⁻¹ to μg L⁻¹. However, the efficacy of OMP removal is considerably deteriorated due to competitive adsorption from background dissolved organic matter (DOM), present at substantially higher concentrations in mg L⁻¹. Interpreting the characteristics of competitive DOM is crucial in predicting OMP adsorption efficiencies across diverse natural waters. Molecular weight (MW), aromaticity, and polarity influence DOM competitiveness. Although the aromaticity-related metrics, such as UV₂₅₄, of low MW DOM were proposed to correlate with DOM competitiveness, the method suffers from limitations in understanding the interplay of polarity and aromaticity in determining DOM competitiveness. Here, we elucidate the intricate influence of aromaticity and polarity in low MW DOM competition, spanning from a fraction level to a compound level, by employing direct sample injection liquid chromatography coupled with ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry. Anion exchange resin pre-treatment eliminated 93% of UV₂₅₄-active DOM, predominantly aromatic and polar DOM, and only minimally alleviated DOM competition. Molecular characterization revealed that nonpolar molecular formulas (constituting 26% PAC-adsorbable DOM) with medium aromaticity contributed more to the DOM competitiveness. Isomer-level analysis indicated that the competitiveness of highly aromatic LMW DOM compounds was strongly counterbalanced by increased polarity. Strong aromaticity-derived π-π interaction cannot facilitate the competitive adsorption of hydrophilic DOM compounds. Our results underscore the constraints of depending solely on aromaticity-based approaches as the exclusive interpretive measure for DOM competitiveness. In a broader context, this study demonstrates an effect-oriented DOM analysis, elucidating counterbalancing interactions of DOM molecular properties from fraction to compound level.

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Low molecular weight (LMW) dissolved organic matter (DOM) is the predominant competitor for adsorption sites against organic micropollutants (OMPs) in activated carbon adsorption. However, top-down approaches using highly complex mixtures of real water DOM do not allow to concisely examine the impacts of specific LMW DOM molecular properties on competitive adsorption. Therefore, we followed a bottom-up approach using fifteen model compounds (mDOM) to elucidate how important DOM characteristics, including hydrophobicity and unsaturated structures (ring, double/triple bond), impact competitiveness. Large concentration asymmetry (~500 μg DOC/μg OMP) made mDOM compounds, which were overall less preferentially adsorbed than OMPs, become competitive against OMPs and inhibit OMP adsorption kinetics by pre-occupation of adsorption sites. Our results revealed that both hydrophobicity interactions and π-interactions increased mDOM competitiveness, while π-interactions outweighed hydrophobic interactions. However, π-interactions could not be satisfactorily evaluated with a parameter such as specific ultraviolet absorbance (SUVA) due to interferences of carboxyl groups in aromatic mDOMs. Instead, mDOM adsorbability, described by mDOM adsorption capacity, proved to be a comprehensive indicator for mDOM competitiveness. To our knowledge, this is the first study that systematically clarifies the impacts of intricately interacting molecular properties on DOM adsorption and the related competition against OMP adsorption. DOM adsorbability may inspire a new fractionation, and assist the further isolation, identification and detailed characterization of LMW DOM competitors in real DOM-containing waters.

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Multi-stable structures are able to achieve significant geometric change and retain specific deformed configurations after the loads have been removed. This reconfiguration property enables, for example, to design metamaterials with tunable features. In this work, a type of multi-stable metastructures exhibiting both level and tilted stable configurations is proposed based on 2D and 3D arrangements of bi-stable elements. The resulting level and tilted configurations are enabled by the rotational compliance, bi-stability and spatial arrangement of unit cells. The bi-stability of the unit cells and multi-stability of the metastructures are demonstrated and characterized by experiments and finite element analysis. Results show that transitions between level stable configurations are symmetric in terms of load–deflection response while switching to the tilted stable configurations leads to asymmetric mechanical responses. The tilted stable configurations are less stable than the level configurations. Moreover, we demonstrate that the level and tilted stable configurations of the metastructure depend on the parallel and serial arrangement of the unit cells.

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Many popular microactuators, including piezoelectric actuators, suffer from incomplete conversion of input energy into mechanical output work. The culprit is their inherent stiffness. Energy used in the elastic deformation limits the output displacement and force and thus the work. At full stroke, all force is used to deform the material itself and all energy is stored as internal elastic energy. Therefore, microactuators with zero internal stiffness (in the direction of actuation) are desired for high energy conversion efficiency. These actuators use either field actuation (electrostatic, magnetic) or zero stiffness statically-balancing structures. Only when the system requires energy storage, e.g. in dynamic systems, should actuators with internal stiffness be considered. These microactuators must be integrated with proper force transfer structures to achieve optimal actuation configurations for different actuation purposes. Distributing these actuators in series and in parallel allows us to achieve the desired large displacement and force outputs resulting in distributed actuator structures with optimal energy performance.@en

Inspired by insect flights, flapping wing micro air vehicles (FWMAVs) keep attracting attention from the scientific community. One of the design objectives is to reproduce the high power efficiency of insect flight. However, there is no clear answer yet to the question of how to design flapping wings and their kinematics for power-efficient hovering flight. In this thesis, we aim to answer this research question from the perspectives of wing modeling, design and optimization. Quasi-steady aerodynamic models play an important role in evaluating aerodynamic performance and designing and optimizing flapping wings. In Chapter 2, we present a predictive quasi-steady model by including four aerodynamic loading terms. The loads result from the wing's translation, rotation, their coupling as well as the added-mass effect. The necessity of including all four of these terms in a quasi-steady model to predict both the aerodynamic force and torque is demonstrated. Validations indicate a good accuracy of predicting the center of pressure, the aerodynamic loads and the passive pitching motion for various Reynolds numbers. Moreover, compared to the existing quasi-steady models, the proposed model does not rely on any empirical parameters and, thus, is more predictive, which enables application to the shape and kinematics optimization of flapping wings. For flapping wings with passive pitching motion, a shift in the pitching axis location alters the aerodynamic loads, which in turn change the passive pitching motion and the flight efficiency. Therefore, in Chapter 3, we investigate the optimal pitching axis location for flapping wings to maximize the power efficiency during hovering flight. Optimization results show that the optimal pitching axis is located between the leading edge and the mid-chord line, which shows a close resemblance to insect wings. An optimal pitching axis can save up to 33% of power during hovering flight when compared to optimized traditional wings used by most of the flapping wing micro air vehicles (FWMAVs). Traditional wings typically use the straight leading edge as the pitching axis. In addition, the optimized pitching axis enables the drive system to recycle more energy during the deceleration phases as compared to their counterparts. This observation underlines the particular importance of the wing pitching axis location for energy-efficient FWMAVs when using kinetic energy recovery drive systems. The presence of wing twist can alter the aerodynamic performance and power efficiency of flapping wings by changing the angle of attack. In order to study the optimal twist of flapping wings for hovering flight, we propose a computationally efficient fluid-structure interaction (FSI) model in Chapter 4. The model uses an analytical twist model and the quasi-steady aerodynamic model introduced in Chapter 2 for the structural and aerodynamic analysis, respectively. Based on the FSI model, we optimize the twist of a rectangular wing by minimizing the power consumption during hovering flight. The power efficiency of the optimized twistable wings is compared with corresponding optimized rigid wings. It is shown that the optimized twistable wings can not dramatically outperform the optimized rigid wings in terms of power efficiency, unless the pitching amplitude at the wing root is limited. When this amplitude decreases, the optimized twistable wings can always maintain high power efficiency by introducing certain twist while the optimized rigid wings need more power for hovering. Considering the high impact of the root stiffness on flapping kinematics and power consumption, we present an active hinge design which uses electrostatic force to change the hinge stiffness in Chapter 5. The hinge is realized by stacking three conducting spring steel layers which are separated by dielectric Mylar films. The theoretical model shows that the stacked layers can switch from slipping with respect to each other to sticking together when the resultant electrostatic force between layers, which can be controlled by the applied voltage, is above a threshold value. The switch from slipping to sticking will result in a dramatic increase of the hinge stiffness (about 9x). Therefore, a short duration of the sticking can still lead to a considerable change in the passive pitching motion. Experimental results successfully show the decrease of the pitching amplitude with the increase of the applied voltage. Flight control based on the electrostatic force can be very power-efficient since there is ideally no power consumption due to the control operations. In Chapter 6, we retrospect and discuss the most important aspects related to the modeling, design and optimization of flapping wings for efficient hovering flight. In Chapter 7, the overall conclusions are drawn and recommendations for further study are provided.@en

Spanwise twist can dominate the deformation of flapping wings and alters the aerodynamic performance and power efficiency of flapping wings by changing the local angle of attack. Traditional Fluid–Structure Interaction (FSI) models, based on Computational Structural Dynamics (CSD) and Computational Fluid Dynamics (CFD), have been used to investigate the influence of twist on the power efficiency. However, it is impractical to use them for twist optimization due to the high computational cost. On the other hand, it is of great interest to study the optimal twist of flapping wings. In this work, we propose a computationally efficient FSI model based on an analytical twist model and a quasi-steady aerodynamic model which replace the expensive CSD and CFD methods. The twist model uses a polynomial to describe the change of the twist angle along the span. The polynomial order is determined based on a convergence study. A nonlinear plate model is used to evaluate the structural response of the twisted wing. The adopted quasi-steady aerodynamic model analytically calculates the aerodynamic loads by including four loading terms which originate from the wing's translation, rotation, their coupling and the added-mass effect. Based on the proposed FSI model, we optimize the twist of a rectangular wing by minimizing the power consumption during hovering flight. The power efficiency of optimized twistable and rigid wings is studied. Comparisons indicate that the optimized twistable wings exhibit power efficiencies close to the optimized rigid wings, unless the pitching amplitude at the wing root is limited. When the pitching amplitude at the root decreases by increasing the root stiffness, the optimized rigid wings need more power for hovering. However, with the help of wing twist, the power efficiencies of optimized twistable wings with a prescribed root stiffness are comparable with the twistable wings with an optimal root stiffness. This observation provides an explanation for the different levels of twist exhibited by insect wings. The high computational efficiency of the proposed FSI model allows further application to parametric studies and optimization of flapping wings. This will enhance the understanding of insect wing flexibility and help the design of flexible artificial wings for flapping wing micro air vehicles.

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Wing designs for Flapping Wing Micro Air Vehicles (FWMAVs) might use a properly tuned elastic hinge at the wing root to obtain the required passive pitching motion to achieve enough lift production to stay aloft. Practical use of this type of FWMAVs requires some form of control which can be achieved by actively adjusting the elastic hinge stiffness and, thus, the pitching motion and lift production of the wing. This paper studies an elastic hinge design consisting of stacked layers which can be sticked together using electrostatics. This sticking changes the bending stiffness of the hinge. The voltage-dependent behavior of this elastic hinge during the large pitching motion are described in detail. The passive pitching motion is governed by the equation of motion which is a function of the elastic hinge stiffness and the applied control voltage. The lift generated by the passive pitching wings is predicted by a quasi-steady aerodynamic model. Numerical simulations show significant changes of the passive pitching motion and, consequently, of the lift production, if slipping stacked layers stick together. Experiments are conducted to study the practical applicability of this method on FWMAVs. The experiments show similar trends as the numerical simulations in modifying the pitching motion although the effect is less significant which is mainly due to manufacturing difficulties. This approach is, in conclusion, promising to control FWMAV flight.@en

Flapping wings can pitch passively about their pitching axes due to their flexibility, inertia, and aerodynamic loads. A shift in the pitching axis location can dynamically alter the aerodynamic loads, which in turn changes the passive pitching motion and the flight efficiency. Therefore, it is of great interest to investigate the optimal pitching axis for flapping wings to maximize the power efficiency during hovering flight. In this study, flapping wings are modeled as rigid plates with non-uniform mass distribution. The wing flexibility is represented by a linearly torsional spring at the wing root. A predictive quasi-steady aerodynamic model is used to evaluate the lift generated by such wings. Two extreme power consumption scenarios are modeled for hovering flight, i.e. the power consumed by a drive system with and without the capacity of kinetic energy recovery. For wings with different shapes, the optimal pitching axis location is found such that the cycle-averaged power consumption during hovering flight is minimized. Optimization results show that the optimal pitching axis is located between the leading edge and the mid-chord line, which shows close resemblance to insect wings. An optimal pitching axis can save up to 33% of power during hovering flight when compared to traditional wings used by most of flapping wing micro air vehicles (FWMAVs). Traditional wings typically use the straight leading edge as the pitching axis. With the optimized pitching axis, flapping wings show higher pitching amplitudes and start the pitching reversals in advance of the sweeping reversals. These phenomena lead to higher lift-to-drag ratios and, thus, explain the lower power consumption. In addition, the optimized pitching axis provides the drive system higher potential to recycle energy during the deceleration phases as compared to their counterparts. This observation underlines the particular importance of the wing pitching axis location for energy-efficient FWMAVs when using kinetic energy recovery drive systems.@en

Quasi-steady aerodynamic models play an important role in evaluating aerodynamic performance and conducting design and optimization of flapping wings. The kinematics of flapping wings is generally a resultant motion of wing translation (yaw) and rotation (pitch and roll). Most quasi-steady models are aimed at predicting the lift and thrust generation of flapping wings with prescribed kinematics. Nevertheless, it is insufficient to limit flapping wings to prescribed kinematics only since passive pitching motion is widely observed in natural flapping flights and preferred for the wing design of flapping wing micro air vehicles (FWMAVs). In addition to the aerodynamic forces, an accurate estimation of the aerodynamic torque about the pitching axis is required to study the passive pitching motion of flapping flights. The unsteadiness arising from the wing’s rotation complicates the estimation of the centre of pressure (CP) and the aerodynamic torque within the context of quasi-steady analysis. Although there are a few attempts in literature to model the torque analytically, the involved problems are still not completely solved. In this work, we present an analytical quasi-steady model by including four aerodynamic loading terms. The loads result from the wings translation, rotation, their coupling as well as the added-mass effect. The necessity of including all the four terms in a quasi-steady model in order to predict both the aerodynamic force and torque is demonstrated. Validations indicate a good accuracy of predicting the CP, the aerodynamic loads and the passive pitching motion for various Reynolds numbers. Moreover, compared to the existing quasi-steady models, the presented model does not rely on any empirical parameters and thus is more predictive, which enables application to the shape and kinematics optimization of flapping wings.@en