Rigid Counterpart Research Articles

Mechanism of enhanced impulse and entrainment of a pulsed jet through a flexible nozzle

The present experimental study shows that a nozzle with optimal flexibility can enhance the impulse and entrainment of a pulsed jet. Near the nozzle exit, vortex rings emanating from the flexible nozzle move faster because of the timely release of the elastic energy (stored during the expansion) to the jet, which is maximized at the structural stiffness that needs to be optimally tuned to the jet acceleration. The total circulation, hydrodynamic impulse and entrained fluid volume are enhanced substantially. Interestingly, we find that the same condition for optimal flexibility to maximize the hydrodynamic impulse and circulation of the primary vortex ring of the continuous jet (Choi & Park, J. Fluid Mech., vol. 949, 2022, A39) holds universally for the pulsed jet, indicating that abrupt jet termination is irrelevant to the impulse augmentation mechanism. Compared to the rigid counterpart, increments of the impulse ( ${\sim }400\,\%$ ) and entrainment ( ${\sim }220\,\%$ ) of a pulsed jet in the present study are considerably larger than those ( $200\,\%$ and $50\,\%$ , respectively) in a continuous jet from previous studies, which is attributed to the significant suppression of negative pressure at the nozzle exit by the collapsing motion of the flexible nozzle in the phase with the jet-driven upstream propagation of the surface wave on the nozzle. This universal mechanism provides a guideline for a novel jet propulsor using a flexible nozzle, for example, for small-scale underwater robots.

Kinematic analysis and experimental validation of a tripod parallel continuum manipulator

A tripod is a classical lower mobility parallel manipulator with three degrees of freedom. The usual geometrical constraints enforce a type of motion with two rotations on a horizontal plane and a translational motion in a vertical direction. These devices are applicable where a task requires a precise orientational motion. Nevertheless, some parasitic motions occur on the constrained degrees of freedom. Parallel Continuum Manipulators are devices where some elements have been replaced by slender rods whose deformation is source of mobility, allowing the elimination of some of the kinematic joints. This type of closed-loop compliant mechanisms have some features that can be useful in certain applications, e.g., the deformed state introduces a certain preload that avoids backslash, they have an inherent compliance that can be useful in some delicate tasks, and a certain control on wrench applied can be obtained. In this article, the comprehensive kinetostatic analysis of a tripod-type parallel continuum manipulator, 3 P ¯ FS , is explained and compared with that of its rigid counterpart 3 P ¯ RS . Workspace and singularity locus is obtained, positioning accuracy and wrench generation are evaluated experimentally.

Influence of Connector Design on Displacement and Micromotion in Tooth-Implant Fixed Partial Dentures Using Different Lengths and Diameters: A Three-Dimensional Finite Element Study.

The literature presents insufficient data evaluating the displacement and micromotion effects resulting from the combined use of tooth-implant connections in fixed partial dentures. Analyzing the biomechanical behavior of tooth-implant fixed partial denture (FPD) prothesis is vital for achieving an optimum design and successful clinical implementation. The objective of this study was to determine the relative significance of connector design on the displacement and micromotion of tooth-implant-supported fixed dental prostheses under occlusal vertical loading. A unilateral Kennedy class I mandibular model was created using a 3D reconstruction from CT scan data. Eight simulated designs of tooth-implant fixed partial dentures (FPDs) were split into two groups: Group A with rigid connectors and Group B with non-rigid connectors. The models were subjected to a uniform vertical load of 100 N. Displacement, strain, and stress were computed using finite element analysis. The materials were defined as isotropic, homogeneous, and exhibiting linear elastic properties. This study focused on assessing the maximum displacement in various components, including the bridge, mandible, dentin, cementum, periodontal ligament (PDL), and implant. Displacement values were predominantly higher in Group B (non-rigid) compared to Group A (rigid) in all measured components of the tooth-implant FPDs. Accordingly, a statistically significant difference was observed between the two groups at the FPD bridge (p value = 0.021 *), mandible (p value = 0.021 *), dentin (p value = 0.043 *), cementum (p value = 0.043 *), and PDL (p value = 0.043 *). Meanwhile, there was an insignificant increase in displacement values recorded in the distal implant (p value = 0.083). This study highlighted the importance of connector design in the overall stability and performance of the prosthesis. Notably, the 4.7 mm × 10 mm implant in Group B showed a displacement nearly 92 times higher than its rigid counterpart in Group A. Overall, the 5.7 mm × 10 mm combination of implant length and diameter showcased the best performance in both groups. The findings demonstrate that wider implants with a proportional length offer greater resistance to displacement forces. In addition, the use of rigid connection design provides superior biomechanical performance in tooth-implant fixed partial dentures and reduces the risk of micromotion with its associated complications such as ligament overstretching and implant overload, achieving predictable prognosis and enhancing the stability of the protheses.

AnXplore: a comprehensive fluid-structure interaction study of 101 intracranial aneurysms.

Advances in computational fluid dynamics continuously extend the comprehension of aneurysm growth and rupture, intending to assist physicians in devising effective treatment strategies. While most studies have first modelled intracranial aneurysm walls as fully rigid with a focus on understanding blood flow characteristics, some researchers further introduced Fluid-Structure Interaction (FSI) and reported notable haemodynamic alterations for a few aneurysm cases when considering wall compliance. In this work, we explore further this research direction by studying 101 intracranial sidewall aneurysms, emphasizing the differences between rigid and deformable-wall simulations. The proposed dataset along with simulation parameters are shared for the sake of reproducibility. A wide range of haemodynamic patterns has been statistically analyzed with a particular focus on the impact of the wall modelling choice. Notable deviations in flow characteristics and commonly employed risk indicators are reported, particularly with near-dome blood recirculations being significantly impacted by the pulsating dynamics of the walls. This leads to substantial fluctuations in the sac-averaged oscillatory shear index, ranging from -36% to +674% of the standard rigid-wall value. Going a step further, haemodynamics obtained when simulating a flow-diverter stent modelled in conjunction with FSI are showcased for the first time, revealing a 73% increase in systolic sac-average velocity for the compliant-wall setting compared to its rigid counterpart. This last finding demonstrates the decisive impact that FSI modelling can have in predicting treatment outcomes.

Chorda Dorsalis System as a Paragon for Soft Medical Robots to Design Echocardiography Probes with a New SOM-Based Steering Control.

Continuum robots play the role of end effectors in various surgical robots and endoscopic devices. While soft continuum robots (SCRs) have proven advantages such as safety and compliance, more research and development are required to enhance their capability for specific medical scenarios. This research aims at designing a soft robot, considering the concepts of geometric and kinematic similarities. The chosen application is a semi-invasive medical application known as transesophageal echocardiography (TEE). The feasibility of fabrication of a soft endoscopic device derived from the Chorda dorsalis paragon was shown empirically by producing a three-segment pneumatic SCR. The main novelties include bioinspired design, modeling, and a navigation control strategy presented as a novel algorithm to maintain a kinematic similarity between the soft robot and the rigid counterpart. The kinematic model was derived based on the method of transformation matrices, and an algorithm based on a self-organizing map (SOM) network was developed and applied to realize kinematic similarity. The simulation results indicate that the control method forces the soft robot tip to follow the path of the rigid probe within the prescribed distance error (5 mm). The solution provides a soft robot that can surrogate and succeed the traditional rigid counterpart owing to size, workspace, and kinematics.

Fluid-structure interaction for aerodynamic performance evaluation of flapping wing with passive pitching motion

The aim of this study is to investigate the impact of locust-like wing flexibility which is modelled with Mylar and carbon fibre pultruded rods on aerodynamic performance. An in-house 3D Navier Stokes solver coupled with a structure solver has been used to evaluate passive pitching motion and aerodynamic performance. The study has focused on two effects which are variations in the diameter of root and spar distribution. All wings have been analysed for a prescribed flapping motion in a horizontal plane, with a peak-to-peak stroke amplitude of 120°, flapping frequency of 25 Hz, and Reynolds number of 13,500. The flow has been assumed to be laminar, viscous, and incompressible. The diameter of the wing root segment attached to the leading edge has been varied to control the extent of the passive pitching motion. The effect of spar distribution on the aerodynamic performance of the flexible flapping wing has also been investigated. The results revealed that careful selection of wing root parameters and spars distribution can achieve a passive pitch angle with an amplitude of 45° and the highest power economy of 1.26. Moreover, a comparison of the flexible wing with its rigid counterpart suggested that flexible wings modelled with a simple passive pitching mechanism can be an efficient solution for bio-inspired flapping-wing micro aerial vehicles.

Analytical results for pitching kinematics and propulsion performance of flexible foil

Natural flyers and swimmers employ flexible wings or fins to propel. While the complex interaction between the foil with deformation and the surrounding non-steady fluid environment defines the propulsion performance of the propellers, elucidating the interaction mechanism through theoretical models earns much challenge. Based on elastokinetics and linear potential flow theory, this study proposes a simplified analytical model to clarify the kinematics and the propulsion performance of a flexible thin foil pitching in flow. The dynamical forces, including the inertial force of the foil and the non-steady fluid pressure, are used to determine the averaged deformation angle of the foil. Combining the averaged deformation angle and the prescribed driving pitching motion, the kinematics of the foil is resolved analytically. Based on the analytical expressions for the corresponding pitching motion, analytical relations among the physical parameters of the stiffness and the mass of the foil and the driving frequency are given to these critical conditions, including resonance of the flow–structure system, equal pitching amplitude between the flexible foil and the rigid counterpart, phase angle transition from ${\rm \pi}/2$ to $- {\rm \pi}/2$ . Subsequently, the performance of the foil, including the thrust, the power and the propulsive efficiency, as a function of the flexibility of the foil are derived, together with the introduction of a bluff body type offset drag to the thrust. The formulated analytical theory, which matches nicely with previous reports, will help to interpret the effect of the flexibility and regulate the propulsive performance of the flexible foil when pitching in fluid.

Halide Substituted Ammonium Salt Optimized Buried Interface for Efficient and Stable Flexible Perovskite Solar Cells

AbstractLow‐temperature solution processing of the perovskite layer enables the fabrication of flexible devices. However, the performance of flexible perovskite solar cells (f‐PSCs) lags far behind their rigid counterpart in terms of efficiency and stability. Emerging evidence demonstrates that the quality of the buried interface between perovskite and transporting layer underneath is the key point. Herein, a class of novel halide substituted ammonium salts, i.e., n‐bromophenethylammonium (n‐Br‐PEAX, n = 2 or 4, X = Cl or Br) are designed and synthesized to modify the buried interface as well as the perovskite crystallization of f‐PSCs. It is found that the ammonium salt with rational design molecular structure can modify the crystallization speed of perovskite, leading to the formation of a compact and uniform morphology without nanovoids at the interface. As a result, the efficiency of f‐PSCs is improved from 15.4% to 20.2%. Moreover, the modified devices without encapsulation retain 86% of their initial performance after 1000 h of aging at ambient conditions and 87% after 290 h of continuous operation.

Slamming forces during water entry of a simple harmonic oscillator

When a blunt body impacts an air–water interface, large hydrodynamic forces often arise, a phenomenon many of us have unfortunately experienced in a failed dive or ‘belly flop’. Beyond assessing risk to biological divers, an understanding and methods for remediation of such slamming forces are critical to the design of numerous engineered naval and aerospace structures. Herein we systematically investigate the role of impactor elasticity on the resultant structural loads in perhaps the simplest possible scenario: the water entry of a simple harmonic oscillator. Contrary to conventional intuition, we find that ‘softening’ the impactor does not always reduce the peak impact force, but may also increase the force as compared with a fully rigid counterpart. Through our combined experimental and theoretical investigation, we demonstrate that the transition from force reduction to force amplification is delineated by a critical ‘hydroelastic’ factor that relates the hydrodynamic and elastic time scales of the problem.

Kinematic Analysis of a Tendon-Driven Hybrid Rigid–Flexible Four-Bar; Application to Optimum Dimensional Synthesis

In design matters, mechanisms with deformable elements are a step behind those with rigid bars, particularly if dimensional synthesis is considered a fundamental part of mechanism design. For the purposes of this work, a hybrid rigid–flexible four-bar mechanism has been chosen, the input bar being a continuum tendon of constant curvature. The coupler curves are noticeably more complex but offer more possibilities than the classical rigid four-bar counterpart. One of the objectives of this work is to completely characterize the coupler curves of this hybrid rigid–flexible mechanism, determining the number and type of circuits as well as constituent branches. Another important aim is to apply optimization techniques to the dimensional synthesis of path generation. Considerable progress in finding the best design solutions can be obtained if all the acquired knowledge about the coupler curves of this hybrid mechanism is integrated into the optimization algorithm.

Modeling water transport properties in carbon nanotubes: Interplay between force-field flexibility and geometrical parameters.

Modeling water and other liquids in computational simulations requires a large set of parameters. Many works have been devoted to finding new, improved water models, with almost all of them designed for bulk systems. Here, we use carbon nanotubes as a play model to investigate the effects of introducing flexibility in water force fields during molecular dynamics simulations of nanoconfined water. We explore six different models to show that viscosity, diffusion, and dipole orientation are vastly influenced by the flexibility and the family of force fields used. Particularly, we found the level of confinement (decreasing the nanotube's diameter) to increase discrepancies in the description of the dipole alignment. In smaller (10,10) nanotubes, the flexible version of the transferable intermolecular potential with three points (TIP3P/Fs) features a high directionality, while its rigid counterpart shows a more distributed dipole orientation. Both viscosity and diffusion are also extremely dependent on the force-field family, with the flexible version of the simple point charge (SPC/Fw) featuring the lower confidence interval.

Gait planning and optimization of an 18 DOF quadruped robot with compliant shanks

Legged robots made of rigid links have disadvantages like poor energy efficiency and large impact forces during foot/terrain contact while walking on 3D uneven terrain. This paper proposes a unique design of an 18 DOF quadruped robot with compliant shanks. Compliance has been added to the quadruped by introducing a “c-section“ in the shank of each leg. The robot dynamics has been modeled using the projected Newton-Euler method, while the “Craig-Bampton Method” has been utilized to model the compliant shanks. Optimal body trajectory and foot placements while maintaining dynamic stability is obtained using an NLP based optimization strategy. A torque-based inverse dynamics control technique ensures that the quadruped follows the desired trajectory. The robot was simulated using Simscape Multibody, which utilizes the robot model’s “.stl,”“.step,” and “.xml” files to follow the optimized joint trajectories. The total energy consumed and joint torques are compared between the complaint and rigid link quadruped robots for walk on different types of terrain with different gaits and C-sections of 3, 4, 6, and 8 mm thickness. The simulation results show a significant reduction in joint torque spikes (more than 70% for all cases) due to the addition of compliance in each leg. This also leads to a reduction in total energy consumption during the walk especially over an uneven terrain. Hence, the results verify that the proposed design of quadruped is functionally more efficient than its rigid counterpart for walking on uneven terrain.

Experimental study on the wake control of a square cylinder mounted with dual rigid/flexible splitter plates in the subcritical regime

The wake control of a bluff body is vital to the safety of structures in the field of coastal and ocean engineering. The quantitative statistics wake control and flow field in the subcritical regime are relatively unexplored. The purpose of this research is to find an optimum arrangement for the wake control of a square cylinder mounted by dual flexible/rigid splitter plates and uncover the physical mechanisms. The Reynolds number based on the side length of the square cylinder is 4.2×103≤Re≤1.0×104. Splitter plates with two thicknesses were adopted, which represents the flexible and rigid splitters, respectively. The splitter length was considered over the range l/L=1∼2 with an increment of Δl/L=0.25. The force exerting on the bluff body and the instantaneous velocity field were measured by the triaxial load cell and particle image velocimetry techniques. The results indicate the modulation of the flapping splitter as well as the splitting effect of the tip vortices determine the lift suppression and the gap flow and streamlined trailing play a vital role in drag reduction. The maximum drag reduction and lift suppression reach ηmax=58.3% and 90.6%, respectively. The flexible splitter outperforms its rigid counterpart, especially at high Reynolds numbers, which is beneficial for those structures in marine engineering. Given the hydrodynamic characteristics and sensitivity to incoming flow conditions, the optimum arrangement is the flexible splitter configuration with splitter length of l/L=1.25∼1.5.

Flexible waveguide integrated thermo-optic switch based on TiO2 platform.

Mechanically flexible photonic devices are critical components of novel bio-integrated optoelectronic and high-end wearable systems, in which thermo-optic switches (TOSs) as optical signal control devices are crucial. In this paper, flexible titanium oxide (TiO2) TOSs based on a Mach-Zehnder interferometer (MZI) structure were demonstrated around 1310 nm for, it is believed, the first time. The insertion loss of flexible passive TiO2 2 × 2 multi-mode interferometers (MMIs) is -3.1 dB per MMI. The demonstrated flexible TOS achieves power consumption (Pπ) of 0.83 mW, compared with its rigid counterpart, for which Pπ is decreased by a factor of 18. The proposed device could withstand 100 consecutive bending operations without noticeable degradation in TOS performance, indicating excellent mechanical stability. These results provide a new perspective for designing and fabricating flexible TOSs for flexible optoelectronic systems in future emerging applications.

Adaptive Visual Servoing Shape Control of a Soft Robot Manipulator Using Bézier Curve Features

Soft continuum robot outperforms its rigid counterpart when executing in complicated environments thanks to its high dexterity of the redundant configuration. Shape control is regarded as a prerequisite to applications in unstructured environments. In this article, we develop an adaptive image-based visual servoing (IBVS) algorithm using an uncalibrated camera to realize visual shape control of the cable-driven soft manipulator. A Bézier curve is selected to be the feature providing shape information of the soft robot. An adaptive law is proposed to simultaneously solve the uncalibrated problem and to analytically model the motion of curve control points. The control algorithm is then designed with third-order Bézier curve being the image features, and theoretically proves its stability. The algorithm has been experimentally validated in the cable-driven soft continuum robot. The experiment results demonstrate rapid converging performance to C-type and S-type reference shapes.

Multibody-Dynamics Approach to Study the Deformation and Aerodynamics of an Insect Wing

In this study, a multibody-dynamics simulation approach was developed for a hawkmoth flexible wing. The wing structure is modeled as a chain of rigid bodies connected by elastic springs, and the aerodynamic force is measured by the extended unsteady vortex-lattice method. The multibody-dynamics and aerodynamic solvers are combined by an implicit coupling approach, and the quasi-Newtonian method is adopted to solve the system of nonlinear differential equations of motion. For validation, numerical results were compared with measurement data from a robotic wing and a living insect. A parametric analysis was conducted to study the effects of several kinematic parameters on the deformation and aerodynamic performance of the wing in hover. In most cases, using a flexible wing is far more efficient in terms of force production in comparison with its rigid counterpart. In general, wing deformation may cause considerable differences between the wing-tip and wing-base kinematic parameters. In particular, elevation motion as observed in living insects may be due to the passive oscillations of elastic elements, as opposed to a deliberate motion.

Soft-body dynamics induces energy efficiency in undulatory swimming: A deep learning study.

Recently, soft robotics has gained considerable attention as it promises numerous applications thanks to unique features originating from the physical compliance of the robots. Biomimetic underwater robots are a promising application in soft robotics and are expected to achieve efficient swimming comparable to the real aquatic life in nature. However, the energy efficiency of soft robots of this type has not gained much attention and has been fully investigated previously. This paper presents a comparative study to verify the effect of soft-body dynamics on energy efficiency in underwater locomotion by comparing the swimming of soft and rigid snake robots. These robots have the same motor capacity, mass, and body dimensions while maintaining the same actuation degrees of freedom. Different gait patterns are explored using a controller based on grid search and the deep reinforcement learning controller to cover the large solution space for the actuation space. The quantitative analysis of the energy consumption of these gaits indicates that the soft snake robot consumed less energy to reach the same velocity as the rigid snake robot. When the robots swim at the same average velocity of 0.024m/s, the required power for the soft-body robot is reduced by 80.4% compared to the rigid counterpart. The present study is expected to contribute to promoting a new research direction to emphasize the energy efficiency advantage of soft-body dynamics in robot design.

On Constraints and Parasitic Motions of a Tripod Parallel Continuum Manipulator

A parallel continuum manipulator (PCM) is a mechanism of closed-loop morphology with flexible elements such that their deformation contributes to its mobility. Flexible hexapods are six-degrees-of-freedom (DoF) fully parallel continuum mechanisms already presented in the literature. Devices of reduced mobility, i.e., lower mobility than six DoF, have not been studied so far. An essential characteristic of lower mobility mechanisms is that reduced mobility is due to kinematic constraints generated by mechanical arrangements and passive joints. In rigid-link parallel manipulators, those constraints are expressed as a set of equations relating to the parameters representing the end effector’s pose. As a consequence, independent output pose variables are controllable with the position equations, while dependent output variables undergo parasitic motions. In this paper, the performance of a tripod-type parallel continuum manipulator, 3PF̲S, is compared with the operation of its rigid counterpart 3P̲RS. We will show that in PCMs there are no such geometric constraints expressible with algebraic equations, but it is difficult to perform some types of motion in the end effector with the input torques. Another goal of this paper is to evaluate such limitation of motion in a tripod-like PCM and compare it with the constraints of the rigid 3P̲RS. Finally, the paper shows that there are strong similarities in the reduced mobility of both mechanisms.

Experimental and Numerical Dynamic Behavior of Bending-Torsion Coupled Box-Beam

Purpose Structural configurations related to new green aircraft design require high efficiency and low weight. As a consequence, moderate-to-large deformation under operating loads arise and aeroelastic instabilities different with respect to rigid counterpart are possible. Coupled structural configurations can provide the right mean to overcome such a critical situations selecting the right coupling parameters and structural performance. In this work, the dynamic behaviour of stiffened box-beam architecture with selected optimal stiffener orientation to emphasize the bending-torsion coupling characteristics has been investigated.MethodsAn extensive experimental activity has been performed for a validation and confirmation of the numerical results. Two cantilever beams produced with different technologies and materials have been tested. Modal performance has been determined by means of a laser Doppler vibrometer (LDV), while Finite-Element Method (FEM) numerical simulation based on solid elements and equivalent single layer approach have been applied and compared. Experimental/numerical comparison have been presented pointing out the specific coupling performance of this architecture with respect to natural frequencies and modal shapes.ResultsThe activity demonstrates a good correlation in natural frequencies that remains mostly under 4%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\%$$\\end{document}. Modal assurance criterion (MAC) has been considered in comparing experimental and numerical modal shapes.ConclusionThe proposed innovative configuration demonstrates its capability to be used in aeroelastic critical problem as a mean to reduce their influence in aircraft design. The numerical procedure used for equivalencing the stiffened parts of the box-beam has also been validated in dynamical response confirming the possibility to be used in design phase.

Numerical and experimental fluid–structure interaction analysis of a flexible propeller

ABSTRACT The increasing interest in using flexible materials to design marine propellers, considering deformations due to flow loads. A numerical procedure for analysing two-way fluid-structure interactions, based on the commercial STAR-CCM+ multiphysics software, is described and applied to predict the hydroelastic response of the flexible marine propeller P1790 to hydrodynamic forces in open water conditions. The influence of the deformation on the performance of the flexible propeller was analysed by comparison with its rigid counterpart. The procedure has been validated by means of experiments performed in the cavitation tunnel K27 of the Technical University Berlin with both, the flexible and the rigid propeller. The predicted performance coefficients and the axial deformation of the blades agree well with measured values. This suggests the feasibility of using the passive bending and twisting behaviour of a flexible propeller to adapt the pressure distribution on the blade to improve the propeller performance over a range of advance ratios.