First-order Forces Research Articles

An assessment of Dragonfly-Backshell aerodynamics preparing for the initial flight on Titan

The Dragonfly mission has the scope of studying the surface of Titan, Saturn’s largest moon, using a rotorcraft lander. A mission-critical event in the timeline involves Dragonfly’s initial flight as it descends into Titan’s atmosphere. The preparation for this initial flight involves releasing the heat shield and utilizing the lander’s rotors to damp motion while still attached to the parachute-supported aeroshell to ensure a smooth release. The aerodynamics of this event are studied in Titan conditions using high-fidelity computational fluid dynamics (CFD). The CFD model is benchmarked using available experimental measurements relevant to subcomponents of the event and include: (1) an open backshell, (2) bluff bodies similar to the Dragonfly fuselage, and (3) a rotor-body interaction from a helicopter rotor. The CFD results correlate well with measurements to provide confidence in the present studies focused on Titan. The novel studies presented in this work focus on the rotor-based control of the lander–backshell combination and its ability to mitigate spinning about the main parachute support line. This effort finds that the relatively large fuselage combined with near-fuselage rotors (driven by aeroshell space limitations) drives an unexpected aerodynamic interaction between the rotor and fuselage. Specifically, we observe low-pressure zones on the fuselage adjacent to the rotor that create a “suction” force. This first-order force creates an unexpected aerodynamic interaction that demands careful attention. In this paper, this force is documented and solutions are proposed to mitigate undesirable control character.

First- and Second-Order Forces in the Asymmetric Dynamical Casimir Effect for a Single δ − δ′ Mirror

Here, we consider an asymmetric δ−δ′ mirror undergoing time-dependent interactions with a massless scalar field in 1 + 1 dimensions. Using fluctuation-dissipation theory for a mirror in vacuum, we compute the force on a moving δ−δ′ mirror with time-dependent material properties. We investigate the first-order forces arising from the two distinct fluctuation sources and calculate the linear susceptibility in each case. We then plot the resulting forces. At the second order, we also find the independent contributions to the total force as well as the force that arises from the interference phenomena between the two fluctuation sources.

Improving of the balancing structures of in-line two-cylinder piston engines

The balancing of the overturning reactive torque (RT) in an in-line two-cylinder internal combustion engine with cranks oriented in one direction is considered, using centrifugal forces from two groups of counterweights located on the crankshaft and balancer shaft. It is shown that there are limitations in balancing the RT associated with the layout of the balancer shaft. New schemes are proposed for placing a group of counterweights on the disks of two directly unconnected gears driven by a crankshaft and rotating at the same angular speed, but in the opposite direction, which ensures complete balancing of the first-order inertial forces of reciprocating moving masses and the optimal possible partial balancing of the RT. The balancing of the RT for internal combustion engines of this type are analyzed, and the feasibility of the proposed designs are confirmed. Keywords:in-line two-cylinder internal combustion engine R2, balance, balance shaft, reaction torque gusarov39@yandex.ru, apelinskiy_mami@mail.ru, prvm@rambler.ru

Investigation of the dynamic response of a floating wind-aquaculture platform under the combined actions of wind, waves and current

Floating wind-aquaculture platforms are drawing increasing attention from the academic and engineering communities due to their potential to fully exploit and utilize marine space and its resources. However, these platforms integrate both the hydrodynamics and aerodynamics of floating wind turbines and aquaculture cages, making their mechanical properties more complex. This study aims to evaluate the effects of three different hydrodynamic and aerodynamic damping components on and the contribution of the stochastic environmental loads to the dynamic response of a floating wind-aquaculture platform. A coupled hydro-aero-servo method is established. Decay and forced oscillation tests of the platform in still water are firstly numerically performed, followed by simulation of the dynamic behavior under different combinations of environmental loads, including the fluctuating wind load of the blades, stochastic wave excitation forces on the floating body and viscous force of the aquaculture cage system. The aerodynamic damping of the wind turbine and the hydrodynamic damping of the floating body are dominant in low- and wave-frequency range, respectively. Regarding the environmental load components, the second-order wave force and the turbulent wind load are dominant in the surge direction in the low-frequency range. The dynamic response of the platform in the wave-frequency range is mainly induced by the first-order wave force. Fish net can suppress the low-frequency motion but has almost no influence on the wave-frequency motion.

Robotic compliant grinding of curved parts based on a designed active force-controlled end-effector with optimized series elastic component

High-accuracy, fast-response, and low-overshoot force control are important to guarantee the material removal accuracy and surface quality of robotic compliant grinding. In order to achieve the above target, this study develops an active compliant force-controlled end-effector based on a series elastic actuator for the robotic grinding of curved parts. Firstly, a decoupled robotic grinding system composed of an industrial robot and an end-effector is developed, and a novel force-controlled end-effector is designed by adding an elastic component between the servo motor and the load to improve compliance. Secondly, the influences of the elastic component and grinding tool stiffness on the stability of the force-controlled end-effector system are analyzed by establishing a contact model of the compliant end-effector and the transfer function of the entire force control system. Then, the stiffness of the grinding tool and the elastic component are optimized with full consideration of the system cutoff frequency and the end-effector compliance. To improve the force tracking accuracies of the developed compliant end-effector, a proportional-integral (PI) controller with first-order differential force feedforward control is designed. Finally, grinding experiments are conducted for verification. The effects of spring stiffness and grinding tool stiffness on the force control system are tested, which match well with the theoretical analysis. Grinding results show that the maximum force control error of the compliant force-controlled end-effector is decreased by 70% compared to that of the rigid force-controlled end-effector, and the overshoot of the force control is reduced from 30% to almost 0 under the same response speed. The maximum and average absolute grinding depth errors using the designed end-effector are reduced by 57.2% and 58.6%, respectively, compared with those of the traditional rigid end-effector, and the average surface roughness of the finished part is reduced by 19.2%. Which demonstrates the effectiveness and advantages of the proposed compliant force-controlled end-effector.

Particle focusing in a wavy channel

It is known that inertial lift forces can lead to particle focusing in channel flows; yet oscillatory straining effects have also been suggested as a mechanism for particle focusing in wavy channels. To explore the synergy between these two mechanisms, we analytically and experimentally investigate the focusing behaviour of rigid neutrally buoyant particles in a wavy channel. We decompose the particle-free channel flow into a primary Poiseuille flow and secondary eddies induced by the waviness. We calculate the perturbation of the particle on the particle-free flow and the resulting lateral lift force exerted on the particle using the method of matched asymptotic expansions. This yields a zeroth-order lift force arising from the Poiseuille flow and a first-order lift force due to the waviness of the channel. We further incorporate the inertial lift force into the Maxey–Riley equation and simulate particle trajectories in wavy channels. The interactions between the zeroth-order lift force and the particle-free flow largely determine the focusing locations. Experiments in wavy channels with varying amplitudes at channel Reynolds numbers ranging from 5 to 250 are consistent with the predictions of the focusing locations, which are mainly governed by the channel Reynolds number as well as the competition between the inertial lift and the oscillatory straining effects.

Life, Lawfulness, and Contingency: Kant and Schelling on Organic Nature

Abstract In the Critique of the Power of Judgment, Kant calls purposiveness the “lawfulness of the contingent”. I argue that this should be interpreted not as lawfulness assumed in order to remove unacceptable mechanical indeterminacy, but rather as an additional kind of lawfulness which, in the case of organisms, inexplicably coincides with mechanical determination. Schelling adapts Kant’s notion of natural purposiveness in his own conception of the relation between mechanism and organism. He states in his 1798 work, On the World Soul, that nature is “lawless in its lawfulness, and lawful in its lawlessness”. This should be interpreted similarly: there is a coincidence of two orders of lawfulness in nature. However, while Kant maintains that the coincidence or unity of these two orders is inexplicable for beings like us, Schelling explains the unity by assigning these laws to distinct levels of operation in nature. Organic organization is generated by a second-order operation of an organic principle on the first-order mechanical forces that are characteristic of matter. Schelling thereby builds on Kant’s third Critique framework in a creative way in order to offer a more fully unified account of nature. In this account, mechanism and organism maintain distinct roles but are both grounded in a further, higher principle.

Numerical and experimental research on the coupled motion responses of a Spar platform with a tender vessel during drilling scenarios

During the drilling operation of a Spar platform with a tender semisubmersible platform, the motion of the floating platforms is complex and quite different from that of the separate conditions due to the influence of multibody hydrodynamics and the connecting system. This can lead to potential collision accidents. In this paper, the motion characteristics of a tender assistant drilling (TAD) system under drilling scenarios are analyzed numerically and experimentally. Based on three-dimensional potential flow theory, the numerical model of a multiple floating body system is established to obtain the hydrodynamic parameters, the first-order wave force, and the mean drift force, and then a time domain analysis is conducted. Both single-body and coupled multibody conditions are observed to analyze the motion performance of the gangway and the tension response of the connecting lines. The results show that the Spar platform affects the low-frequency motion of the semisubmersible platform more intensely when the platforms work side by side without hawser lines. The phenomenon of hydrodynamic interaction is weakened under fully coupled conditions due to this connection. In addition, the stroke of the gangway is positively related to the relative motion, which reflects the relative motion response of the platforms.

The Effect of the Second-order Difference-frequency Wave Force on the Motions of an Underwater Vehicle Near the Surface

_ When an underwater vehicle performs tasks, such as power recharge, air exchange, deployment, and retrieval near the surface, the low-frequency suction force and pitch moment will interfere with its motion. This paper presents a study on the effect of second-order difference-frequency wave force on the motions of the DARPA SUBOFF model at various submergence depths and environmental conditions. Ignoring the second-order wave force component induced by the second-order difference-frequency force may underestimate the second-order effect in irregular waves in the time domain. Comparisons were made between Newman’s approximation, which sets the off-diagonal difference-frequency quadratic transfer function (QTF) value to the average over the corresponding diagonal values, and the full QTFs based on the Pinkster approximation in the frequency and time domains. The QTFs-based prediction is significantly larger values than Newman’s approximation. Time-domain simulations were conducted, to investigate the effect of the second-order difference-frequency wave loads on motion responses at different nondimensional submergence depths and various sea conditions. The results of the computation indicate that ignoring the second-order difference-frequency wave force may underestimate the motion and dynamic responses of underwater vehicles; in addition, the pitch and heave motion responses are dominated by the low-frequency response. Introduction When an underwater vehicle travels close to the water surface, it experiences an upward force and pitch moment, known as surface suction. This can significantly affect the underwater vehicle’s behavior when power recharge, air exchange, deployment, and retrieval are incorporated into the underwater vehicle simulations. Veillon et al. (1996) noted that for a 10,000-ton submarine at a depth of 50 m, the surface suction due to waves will require around 20–30 tons of compensation to stop the submarine from surfacing. In addition, Hirom (1974) indicated that in waves, the fluctuating forces on a submarine can be on the order of 1000 kN, with a steady component of approximately 10 kN. In calm water, suction occurs owing to a higher flow velocity and a lower pressure above an underwater vehicle when sailing near the surface, and the wave pattern generated by the underwater vehicle complicates the surface effect. The longitudinal position of the center of pressure is after the amidships, which induces the pitch moment. In the presence of waves, the first-order wave force, which is the oscillatory force at the wave frequency, acts on the underwater vehicle. In addition, waves and model motions induce low-frequency suction force and pitch moment. Using any internal compensation and control surfaces to offset the induced suction force and pitch moment requires a good and early understanding of the magnitude of these forces, for designing sufficient-capacity compensation tanks (Crossland 2013). Therefore, the surface suction and pitch moment effects are essential components of the hydrodynamic characteristics of underwater vehicles sailing near the water surface.

Hydroelastic interactions between waves and an array of submersible flexible fish cages

In this study, a semi-analytical solution is developed to investigate the wave interference and corresponding dynamic responses of an array of submersible flexible fish cages. The net chamber is modelled as a perforated cylindrical shell, and the penetration flow through the cage interface is described by a porous medium model. Based on the potential flow theory, the solution to this physical problem is expanded as a series of eigenfunctions in the local coordinate system of each cage, and the scattered wave potentials generated by all cages are superimposed wherein Graf's addition theorem is employed for the coordinate transformation. The mean wave drift effect is also explored, which is a nonlinear phenomenon derivable from the first-order solution. The results indicate that the interference of waves is determined by wavenumbers, cage spacings or net porosities, and significant wave responses are manifested near the top part of the cage. Additionally, the first-order wave force vanishes at a ratio of cage diameter to wavelength = 0.59, and, along the direction of wave incidence, the rear cages experience opposite mean wave drift loads relative to those on the front cages. These studies provide benchmark results for understanding the hydroelastic characteristics of multi-cage systems.

Vibrodynamic parameters of radial piston engines with Pencake mechanism

Introduction (problem statement and relevance). The development of calculation methods at the design stage of the balance and vibration activity of multipurpose converting mechanisms of engines differing in the classical crank mechanism is actual.The purpose of the work was to substantiate such a technique for a FSUE “NAMI” three-cylinder radial engine with a Pencake conversion mechanism designed to drive electric generators.Methodology and research methods. To solve the problem, both classical methods of piston engine dynamics and modern methods based on solid modeling and simulation of forced motion were used. Scientific novelty and results. It was found that in the investigated engines a complete balance of the inertial forces of the first-order translational masses and centrifugal forces could be achieved with the help of two counterweights on the crankshaft. The vibration levels of three-cylinder radial and in-line engines assembled from the same links have been compared and the noticeable advantages of the radial star-shaped engine were marked both in terms of the root-mean-square values of vibration velocity and in its spectral vibration.Practical significance. The results of the work can be used in practical development.

Constraining the Milky Way Halo Kinematics via Its Linear Response to the Large Magellanic Cloud

We model the response of spherical, nonrotating Milky Way (MW) dark matter and stellar halos to the Large Magellanic Cloud using the matrix method of linear response theory. Our computations reproduce the main features of the dark halo response from simulations. We show that these features can be well separated by a harmonic decomposition: the large-scale over/underdensity in the halo (associated with its reflex motion) corresponds to the ℓ = 1 terms, and the local overdensity to the ℓ ≥ 2 multipoles. Moreover, the dark halo response is largely dominated by the first-order forcing term, with little influence from self-gravity. This makes it difficult to constrain the underlying velocity distribution of the dark halo using the observed response of the stellar halo, but it allows us to investigate the response of stellar halo models with various velocity anisotropies: a tangential (respectively radial) halo produces a shallower (respectively stronger) response. We also show that only the local wake is responsible for these variations, the reflex motion being solely dependent on the MW potential. Therefore, we identify the structure (orientation and winding) of the in-plane quadrupolar (m = 2) response as a potentially good probe of the stellar halo anisotropy. Finally, our method allows us to tentatively relate the wake strength and shape to resonant effects: the strong radial response could be associated with the inner Lindblad resonance, and the weak tangential one with corotation.

See the forest not the trees! Ecosystem-based assessment of response, resilience, and scope for growth of global forests

Anthropogenic activities are degrading forest health globally. Detection of changes in forest growth rates is possible with field measures of total net primary productivity (tNpp), however, assessing forest adaptive capacity was historically challenging due to a scarcity of tNpp data, knowledge of carbon allocation to above- and belowground biomass, and inability to calculate forest productive potential. This study used a global data set of 307 published research studies to identify tNpp thresholds and site-level factors constraining growth for forests in boreal, temperate, and tropical biomes. This data makes it possible to: (i) calculate the ratio of measured tNpp to a theoretical maximum tNpp from variables external to the site, its “ecosystem fit,” (ii) identify environmental thresholds by scale and biogeography, and (iii) determine stand level conditions that limit growth. At the global scale, climatic variables explain most of the variance in tNpp whereas, at the biome scale different combinations of climatic/edaphic variables interact with phenological traits to explain productivity. For example, deciduous boreal forests were less resilient if precipitation increased, but deciduous tropical forests were less resilient if minimum annual temperature increased. At the biome scale, boreal forest mean tNpp (7.3 Mg ha−1 yr−1) was significantly lower than tropical forests (19.0 Mg ha−1 yr−1) but the mean tNpp of temperate forests tNpp (14.2 Mg ha−1 yr−1) did not significantly differ from boreal or tropical forests, representing a first-order latitudinal forcing of productivity. Comparison of ranked clusters of tNpp (low, medium, high) indicated that the tropics had a larger proportion of highly productive forests (20%) than temperate (12%) or boreal forests (14%). Tropical forests are highly adapted to small-scale environmental heterogeneity but their unique evolutionary trajectories make them more sensitive to land-use change, habitat fragmentation, and climate disruption. There was minimal overlap among the high tNpp groups between boreal forests (12.0–18.2 Mg ha−1 yr−1) and temperate forests (17.6–37.7 Mg ha−1 yr−1), and moderate overlap between temperate and tropical forests (26.8–45.9 Mg ha−1 yr−1), indicating increased adaptive capacity to environmental variability by climatic zone. All tropical forests had similar ecosystem fit, but Paleotropic forests growing on Inceptisol and Entisol soils were significantly more productive than Neotropical forests revealing similar adaptive capacity although there were less favorable growing conditions in the Neotropics. This study supports including site-level variations in edaphic and climatic factors to understand changes in primary productivity due to disturbance and proposes using Ecosystem fit to identify forest adaptive capacity in response to climate destabilization.

Wave-current interaction with three-dimensional bodies in a channel

A frequency-domain numerical model is developed to investigate the hydrodynamic properties of three-dimensional bodies of arbitrary geometry subjected to the action of waves and weak current in a channel based on higher order boundary element method. The problem is divided into a steady problem and an unsteady problem to be solved separately, where the unsteady potentials are given by two sets of integral equations based on linearizing the channel Green function and the velocity potential with the normalized current speed. The first-order wave force is obtained by the pressure integration over the body surface and the second-order drift force is evaluated by the far-field analysis. After examining the validation of the numerical model with some generalized relations derived in the present study, numerical studies for a Lewis-form ship and a four-column structure are performed to investigate the influence of channel walls on the hydrodynamic properties of bodies. Numerical results show that compared with the open-sea results, the side-wall effects become obvious when the oscillating frequencies are close to the transverse resonant frequencies of the channel, and compared with the cases without current, the current effects may enhance the interaction between the body and the reflected waves from the channel walls.

Theoretical, Numerical and Experimental Researches on Time-Varying Dynamics of Telescopic Wing

The stability and vibration of the telescopic wing are studied by the theoretical, numerical and experimental methods when the wing is in the deployment and retraction. The telescopic wing is simplified to a telescopic cantilevered laminated composite rectangular plate subjected to the first-order aerodynamic force and in-plane excitation. The time-varying dynamic characteristics of the telescopic cantilevered laminated composite rectangular plate are investigated by using the analytical, numerical and experimental methods. The approximate analytical solution of the nonlinear time-varying system is obtained by using the improved averaging method. Compared with numerical simulations, the approximate analytical results demonstrate a good agreement. The stability of the telescopic cantilever plate during the deploying process is studied by using the eigenvalue method, which demonstrates the effect of the velocity, acceleration and thickness on the stability of the deployment. The experimental results are consistent with the theoretical results and provide the theoretical basis and technical support for the axial moving wing.

Hydrodynamic Sensitivity of Moored and Articulated Multibody Offshore Structures in Waves

Within the framework of Space@Sea project, an articulated modular floating structure was developed to serve as building blocks for artificial islands. The modularity was one of the key elements, intended to provide the desired flexibility of additional deck space at sea. Consequently, the layout of a modular floating concept may change, depending on its functionality and environmental condition. Employing a potential-flow-based numerical model (i.e., weakly nonlinear Green function solver AQWA), this paper studied the hydrodynamic sensitivity of such multibody structures to the number of modules, to the arrangement of these modules, and to the incident wave angle. Results showed that for most wave frequencies, their hydrodynamic characteristics were similar although the floating platforms consisted of a different number of modules. Only translational horizontal motions, i.e., surge and sway, were sensitive to the incident wave angle. The most critical phenomenon occurred at head seas, where waves traveled perpendicularly to the rotation axes of hinged joints, and the hinge forces were largest. Hydrodynamic characteristics of modules attached behind the forth module hardly changed. The highest mooring line tensions arose at low wave frequencies, and they were caused by second-order mean drift forces. First-order forces acting on the mooring lines were relatively small. Apart from the motion responses and mooring tensions, forces acting on the hinge joints governed the system’s design. The associated results contribute to design of optimal configurations of moored and articulated multibody floating islands.

A Novel Coaxial Balance Mechanism for Reciprocating Piston Engines

Vibration is an important issue faced by reciprocating piston engines, and is caused by reciprocating inertia forces of the piston set. To reduce the vibration without changing the main structure and size of the original engine, we proposed a novel coaxial balance mechanism design based on a compact unit body. By introducing a second-order balance mass, this mechanism can significantly increase the efficiency of vibration reduction. The proposed mechanism can effectively balance the first-order and second-order inertia forces with the potential of balancing high-order inertia forces. To accurately determine the second-order balance mass, a closed-form method was developed. Simulation results with a single-cylinder piston DK32 engine demonstrate the effectiveness and advantage of the proposed mechanism. At a crankshaft speed of 2350 r/min, compared with the first-order balance device, the average root mean square velocity of the test points on the engine’s cylinder was reduced by 97.31%, and the support reaction force was reduced by 96.54%.

Application of time-domain methods for marine hydrodynamic and hydroelasticity analyses of floating systems

ABSTRACT The application of transient three-dimensional numerical code ITU-WAVE is presented for the prediction of hydrodynamic characteristics of floating bodies. The time histories of unsteady motions are directly presented with respect to Impulse Response Functions (IRFs). First-order steady forces of wave-resistance, sinkage force and trim moment are solved as the steady-state limit of surge radiation IRF. The numerical prediction of second-order mean force is presented using near-field method based on the direct pressure integration. The hydrodynamic and structural parts are fully coupled through modal analysis for the solution of hydroelastic analysis. A stiff structure is then studied assuming that contributions of rigid body modes are much bigger than elastic modes. A discrete control of latching is used to increase the bandwidth of the efficiency of Wave Energy Converters (WEC). ITU-WAVE numerical results for different floating bodies show satisfactory agreements compared to analytical, other numerical and experimental results.

Nonlinear hydrodynamics of freely floating symmetric bodies in waves by three-dimensional fully nonlinear potential-flow numerical wave tank

This study estimated the nonlinear hydrodynamic performance of freely floating structures in nonlinear water waves. For this, a three-dimensional, fully nonlinear numerical wave tank (NWT) was developed based on potential theory and the boundary element method (BEM). The fully nonlinear NWT evaluates the hydrodynamic force acting on the instantaneous body position and free-surface elevation based on the mixed Eulerian and Lagrangian (MEL) method and acceleration-potential approach. The instantaneous nonlinear free surface was traced at each time step, and the far-field outgoing wave condition was satisfied by placing an artificial free-surface damping zone. In addition, the least square gradient reconstruction method, thin-plate spline method, and modified inverse distance weighting method were adopted to re-grid the calculation nodes and evaluate the spatial derivatives of the physical quantities robustly. The developed NWT was used to evaluate the first-order and second-order motion RAOs and excitation forces of a freely floating vertical cylinder. The results were compared systematically with previously published results using perturbation theory. Moreover, the heave-to-pitch Mathieu-instability phenomena on a deep-draft floating cylinder were demonstrated by the fully nonlinear NWT simulations. The results agreed well with the corresponding experimental observations.

Higher-order diffraction forces on vertical circular cylinders by using a three-dimensional fully-nonlinear potential numerical wave tank

The aim of this study is to evaluate higher-order diffraction forces on vertical circular cylinders by using an author-developed three-dimensional fully-nonlinear numerical wave tank (NWT). The nonlinear NWT is based on the potential theory, boundary element method, and mixed Eulerian Lagrangian (MEL) approach. The acceleration-potential method is used to calculate the hydrodynamic pressure accurately. The artificial damping zone is adopted to prevent reflected waves from bodies and side walls. The least square gradient reconstruction method and the thin plate spline method are applied to evaluate the spatial derivatives of physical quantities and updated locations of interpolation points. The third-order stokes monochromatic or bichromatic waves are inputted as incident-wave boundary condition. From the nonlinear NWT simulations with FFT, first-order, second-order, and third-order diffraction forces are simultaneously calculated and the results are systematically compared with published experimental and perturbation-based numerical results. The sensitivity of the nonlinear NWT results with the respective orders of Stokes monochromatic or bichromatic input waves is also investigated. The increase of third-order sum-frequency force in shallow water long wave is noticed.