Meccanica最新文献

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.

The paper presents a detailed analysis of the principle of virtual work in its statics and dynamics aspects. Special attention is paid to not exact formulation and interpretation of mechanical system motion equations based upon this principle, what is presented quite often in text books. The general form of the principle of virtual work and variational formulations of the problems in statics are presented and analyzed. Additionally, the dynamic implications of the principle of virtual work and dynamics problem formulation of a system subjected to scleronomic constraints are detailed. The basis for the formulation is the dynamic implication of the principle of virtual work that specifies the relation between the constraint reactions and the body accelerations. This relation takes the form of the variation inequality. As the result, the Gauss principle which determines the body acceleration vector is presented. Furthermore, a description of impact formulation, based upon the Carnot model, caused by the constraints is provided. The theoretical development is illustrated with examples of applications of the principle of virtual work.

In this work, we propose a coordinate method for determining the position of molecular bodies in space that does not use either Euler angles or Hamilton quaternions. The capabilities of the developed high-precision computational algorithm are demonstrated using the example of the Louis Poinsot instability. The change in the nature of this instability when a molecular magnetically susceptible body is exposed to an external magnetic field is also analyzed.

A thermodynamic process is governed by balance equations in field-formulated thermodynamics. Especially the balance equation of entropy takes a prominent role: it introduces the Second Law in the form of a dissipation inequality via the non-negative entropy production. Balance equations and dissipation inequality are independent of the considered material which is described by additional constitutive equations which need the introduction of a state space which is spanned by the state space variables. Inserting these constitutive equations into the balance equations results in the balance equations on state space which include the first order time and position derivatives of the state space variables, called “higher derivatives” wich are directional derivatives in a mathematicle sense. Why do not appear the latter in the Liu Relations which pretend to describe material as well as the equations on state space do ? The answer is that the Liu Relations describe materials whose entropy production does not depend on the higher derivatives. Consequently, the Liu Relations are more specific than the balance equations on state space. A toy example concerning heat conduction in compressible fluids is in two different versions added for elucidation.

After ground de-icing operations, a thin film of anti-icing fluid is sprayed onto the aircraft skin to temporarily prevent the aircraft from icing again. The airworthiness requirements for aircraft takeoff have strict specifications for the rheological properties of the de-icing fluid film and its aerodynamic characteristics. This article conducts numerical studies on the non-Newtonian rheological dynamics of the de-icing fluid film under high shear stress, experimentally verifies the non-Newtonian rheological properties of Type II de-icing fluid, constructs an experimental model of non-Newtonian thin film rheological behavior on the wing surface, and numerically reconstructs the behavior of the de-icing fluid thin film on the wing surface under the effect of high wind speed, including shear-thinning, slip, fragmentation, and detachment. It specifically analyzes the influence mechanisms of aircraft takeoff speed and film thickness on the film detachment behavior. The research results show that lateral winds induce instability fluctuations on the film surface, leading to the accumulation and fragmentation of the film. Changes in takeoff speed alter the airflow shear forces, while film thickness affects internal viscosity and surface tension. As takeoff speed increases, film detachment efficiency improves, but the opposite is true for film thickness.