Meccanica最新文献

In order to study the dynamic compressive mechanical properties of hail, artificial ice specimens were prepared by adding cotton wool into deionized-distilled water. Dynamic compression experiments of ice specimens under different strain rates were carried out by using the Split Hopkinson Pressure Bar (SHPB). Furthermore, based on finite element software LS-DYNA, numerical simulations were conducted to evaluate applicability of mechanical parameters of ice under impact load. The results show that the specimens have a secondary loading phenomenon under the action of stress wave, and this phenomenon is more obvious with the increase of cotton content; under a given content of cotton and low strain rate, the compressive strength of ice is increased with the increasing of the strain rate; under high strain rate, the compressive strength of ice with 0% and 0.3% cotton is maintained as a constant, but a discrete result is observed for ice with 0.5% cotton; when the velocity of impact bar is in the range of 1.35–11.83 m/s, the compressive strengths of the specimens with cotton content of 0%, 0.3% and 0.5% are 3.3 MPa, 4 MPa and 8.1 MPa, respectively. Beyond that, morphology analysis shows that the edge of the specimen is damaged during impact process and the flying velocity of fragments increases with external force.

This paper presents the design process and the virtual testing of a lab equipment for earthquake simulations. The machine object of study is a redundantly actuated platform with parallel kinematics topology. As known, this kind of architecture owns extremely interesting features which make PKMs (Parallel Kinematics Machines) particularly feasible for recreating seismic events, in terms of displacements, accelerations and, consequently, inertial solicitations. Nonetheless, the use of a parallel over-actuated structure makes the design process challenging under many points of view: the mechanics, the sensorisation, and even the control of the shaking device must be tackled carefully from the design stage to come out with effective solutions. The paper provides details around the choices performed and shows the results of a dynamic simulations campaign to demonstrate the performance offered by the simulator in an actual use case.

This study examines the relationship between the nonlinear behavior of the flexible joints of a fourth-degree-of-freedom robot and the controllability and parametric uncertainties of the system. The initial section of the paper presents the dynamic modeling of the robot and the proposed control strategies. This is followed by a parametric sensitivity analysis, which defines the optimal control strategy to be applied. Finally, the configuration of the experimental robot with flexible joints is presented. The SDRE and LQR strategies are related in the study, both in discrete mode and intended for use in the control unit. The final phase of the study involved the presentation of the results obtained when the robot was controlled using flexible joints. The findings demonstrated a positive outcome for the SDRE control strategy.

The replacement of mechanical planetary gears with magnetic gears (MGs) is an emerging transmission solution for electric vehicles that reduce shock and vibration. This article proposes magnetic two-speed transmission and examines the influence of the noncontact torque of MGs, the meshing stiffness of mechanical gears, and the system load on the vibration of magneto-mechanical combined transmission systems. First, the noncontact torque of an MG is analysed on the basis of the magnetic field modulation principle. A set of dynamic equations was subsequently constructed on the basis of the centralized parameter method. Thereafter, with the principle of harmonic balance, a rapid method for constructing a nonlinear vibration harmonic balance solution for a magneto-mechanical combined vibration system in matrix form is obtained. Finally, simulations and prototype tests were conducted. The results show that noncontact torque consists of stable torque and ripple torque. They also interact with mechanical gears to affect system vibration. he vibration amplitude of the inner rotor is sensitive to the pulsating torque, whereas the vibration of the ferromagnetic pole-pieces is sensitive to the meshing stiffness. The stable torque and load dynamic balance affect the natural frequency of the system. The system demonstrates “load‒stiffness adaptive” characteristics compared with those of pure mechanical gearing. The proposed Harmonic Balance Method can quickly calculate this characteristic, and it offers a theoretical basis for dynamic analysis and vibration reduction in magnetic two-speed transmissions.

We assess the suitability of Reynolds-Averaged Navier–Stokes (RANS) simulation using the Spalart-Allmaras (SA) turbulence model as a closure in analysing the performance of fluidic Active Flow Control (AFC) applications. In particular, we focus on the optimal set of actuation parameters found by Tousi et al. (Appl Math Model, 2021) and Tousi et al. (Aerospace Sci Technol 127:107679, 2022) for a SD7003 airfoil at a Reynolds number (Re=6times 10^4) and post-stall angle of attack (alpha =14^circ) fitted with a Synthetic Jet Actuator (SJA). The Large Eddy Simulation (LES) presented in that work is taken as the reference to identify the best choice of boundary conditions for the turbulence field (tilde{nu }) at both domain inlet and jet orifice in two-dimensional SA-RANS computations. Although SA-RANS is far less accurate than LES, our findings show that it can still predict macroscopic aggregates such as lift and drag coefficients quite satisfactorily and at a much lower computational cost, provided that turbulence levels of the actuator jet are set to a realistic value. An adequate value of (tilde{nu }) is instrumental in capturing the correct flow behaviour of the reattached boundary layers for close-to-optimal actuated cases. This validates the use of RANS-SA as a reliable and cost-effective simulation method for the preliminary optimisation of SJA parameters in AFC applications, provided that thorough sensitivity analysis on turbulence-related boundary conditions is performed. Given the strong sensitivity of flow detachment on the laminar or turbulent nature of boundary layers, our results suggest that such analyses are particularly indispensable for vastly separated flow scenarios in general, notably for bluff bodies at moderate transcritical Reynolds numbers.

In a dual-rotor (DR)-blade-casing system, the squeeze film damper (SFD) plays a crucial role in vibration suppression to reduce the vibration level. This paper investigates the impact of SFD on the vibration responses, specifically considering blade-casing rubbing. The Timoshenko beam element is employed to simulate the casing and rotor, while the blade and disk are established using the lumped-mass element. A dynamic model of the DR-blade-casing system with SFD is then developed, incorporating the effects of rubbing. The influence of parameters such as unbalance and blade-casing clearance on the dynamic response is analyzed for systems with and without SFD. The results indicate that SFD effectively reduces the amplitude of the characteristic frequency corresponding to 4-blade rubbing. Additionally, the rubbing fault also induces the combined frequencies between rotating frequencies, integer frequencies, and fractional frequencies. These findings offer valuable insights for fault diagnosis and aero-engine system design.

In this paper, a five-layered viscoelastic composite laminate is proposed for the constrained layer damping (CLD) treatment of axially loaded beam structures. The CLD arrangement is taken in the conventional form of a sandwich beams. But, the main focus is to investigate the change in the static and dynamic stability characteristics of the axially loaded sandwich beam while the conventional pure viscoelastic core layer is replaced by the present five-layered composite laminate. First, the design of the five-layered composite laminate using three viscoelastic and two meatal-ceramic functionally graded (FG) material layers is presented. Next, an incremental nonlinear finite element model of the axially loaded sandwich beam is formulated based on the fractional Zener constitutive relation and harmonic balance method (HBM). The HBM is implemented with an arbitrary number of harmonic terms. The corresponding complexity in the formulation of the nonlinear system matrices/vectors is handled using a special factorization of the nonlinear strain–displacement matrix and an analytical time-integration strategy. The numerical results mainly illustrate the influence of the geometrical and graded material properties of the FG layers on the critical buckling load and damping in the axially loaded sandwich beam. These results reveal that the five-layered composite core provides not only an augmented damping for attenuation of vibration through the parametric resonance but also a significantly improved static stability of the axially loaded sandwich beam in comparison to the conventional pure viscoelastic core. Therefore, the present five-layer composite laminate may be a potential material for an improved CLD treatment of axially loaded beam structures.