Acta Mechanica最新文献

A passive flow control device, Clark-Y airfoil-shaped vortex generator (VG) on NREL Phase VI turbine blade, which has s809 airfoil section, is investigated. Both qualitative oil flow visualization from wind tunnel experiments and quantitative measures of aerodynamic coefficients using steady-state CFD with OpenFOAM are reported. Airfoil-shaped VGs are proposed and compared with traditional rectangular and triangular VGs. The use of airfoil-shaped VGs to delay separation, improving aerodynamic efficiency, inducing local pressure peaks and augmenting vorticity in the flow field are reported in detail. Results show that blades equipped with airfoil-shaped VGs provide a (5%) lift coefficient increase and a (27.68%) drag coefficient reduction compared to clean blades at a stall angle of (alpha = 11^circ ). Airfoil-shaped VGs also generate more vorticity downstream compared to conventional VGs, contributing to maximum increase in peak vorticity inducing an additional momentum to the flow to delay separation without significant drag penalty. Thus, airfoil-shaped VGs offer a promising alternative to traditional VG designs.

The paper deals with the problem of design of unit in auxetic metastructure. The unit is modeled as a two-part spring-like system where each part is with individual stiffness. To overcome the problem of analyzing of each of parts separately, the equivalent spring is suggested to be introduced. In the paper, a method for obtaining the equivalent elastic force of the unit is developed. The method is the generalization of the procedure suggested for substitution of a hard and a soft spring in series with an equivalent one. The nonlinearity of original springs is of quadratic order. As a results, it is obtained that the equivalent elastic force for two equal springs remains of the same type as of the original springs (soft or hard). For two opposite type springs in series with equal coefficients, the equivalent force is soft. The method is applicable for any hard and soft nonlinear springs or spring-like systems. Thus the hexagonal auxetic unit which contains a soft and a hard part in series is analyzed. In the paper, a new analytic method for determination of the frequency of vibration for the unit under action of a constant compression force acting along the unit axis is introduced. The method is applied for units which contain two parts: hard–hard, soft–soft, hard–linear, soft–linear and opposite. The obtained approximate vibration results are compared with numerically obtained ones and show good agreement. The advantage of the method is its simplicity as it does not require the nonlinear equation of motion to be solved.

In this paper, the free vibration analysis of hybrid laminated thin-walled cylindrical shells is investigated based on Sanders’s thin shell theory and artificial spring technology. The hybrid laminated shells are made of graphite fiber reinforced composite (GFRC) and multilayer functionally gradient carbon nanotube-reinforced composite (FG-CNTRC) plies. The influence of Coriolis and centrifugal forces on the shell's strain and kinetic energy, resulting from rotation, has been considered. The orthogonal polynomials are selected as the admissible function and the Rayleigh–Ritz method is employed to obtain the natural frequency. The results of the study are validated against the results of open literature. The influences of CNT distribution patterns, number of plies, stacking sequences, CNT volume fraction, boundary constraints(BCs), rotating speed, and the orientation angle on the natural frequency of the cylindrical shells. The results show that specific stacking configurations can significantly enhance the fundamental frequency of the shell. It provides a reference for the design and optimization of hybrid laminated cylindrical shell structures.

The current work investigates the aerothermoelastic behavior of control fins with nonlinear deployable joints connected to a nonlinear actuator. The fin is attached to a cylindrical body with a hemispherical bow, and the fin–body configuration is subjected to Mach 6 hypersonic flow with a nonzero angle of attack. A Navier–Stokes flow model-based computational fluid dynamics (CFD) solver is coupled to a finite element thermoelastic solver using mapping-based coupling technique. Diffusion function-based smoothing method is used for the CFD grid deformation. The fin is assumed to be connected to a finite stiffness actuator at the root, and the effects of actuator stiffness, actuator freeplay as well as freeplay at the deployable joint are investigated. Flow field, structural and thermal quantities are evaluated and reported for various fin configurations. A complex coupling between different modes of deformation is observed and it is shown that the actuator rotation or angle of attack, and hence torque, strongly depends on the actuator and joint freeplays. The obtained results indicate a significant increase in instabilities in the fin oscillation with increasing joint and root freeplay.

Planar phased array antennas have significant applications in fields such as space communication, electronic reconnaissance, navigation, remote sensing and deep space exploration. This study focuses on the active vibration control of a novel on-orbit assembled large-scale two-dimensional planar phased array antenna. The novel structure and assembly method of the antenna are introduced. Using the finite element method, we develop the dynamic model of antenna structure. Due to the low and dense natural frequency characteristics of the structure, the distributed cable actuators are used to suppress the vibration. Considering the unilateral and saturated constraints of the cable force, the control law is designed by combining the linear quadratic regulator with the Bang–Bang regulator. The controllability criterion and particle swarm optimization algorithm are adopted to optimize the actuator distribution. We validate established dynamic model and control method by numerically simulating. The simulation results indicate that the dynamic model can describe the dynamic behavior as accurately as ABAQUS; the controller can effectively suppress the vibration of the antenna structure.

The nonlinear bending and vibration behaviors of a variable-width piezoelectric nanoplate considering flexoelectric effect are investigated in this paper. The nonlinear Mindlin plate theory and finite element method are applied to derive the governing equations of variable-width piezoelectric nanoplate with flexoelectricity. The influences of geometric nonlinearity, flexoelectricity, and varying width on the bending deflection and natural frequency of the piezoelectric nanoplate with flexoelectricity under four kinds of boundary conditions are explored in detail. The numerical results show that the flexoelectric effect can strongly influence the maximum deflection, the morphology of deformation, and the natural frequency of the variable-width piezoelectric nanoplate. The consideration of geometric nonlinearity becomes necessary for nanoplate exhibiting strong flexoelectricity or subject to significant voltage loads. The boundary conditions not only affect the morphology of deformation but also influence the variation trend of natural frequency with the variable-width ratio of the piezoelectric nanoplate. While the variation trend of maximum deflection is jointly affected by the boundary conditions and flexoelectricity. The closer the shape of the piezoelectric nanoplate is to a triangle, the greater the combined effect of boundary conditions and flexoelectricity on the variation trend of maximum deflection. The results of this study can contribute to the optimization of piezoelectric nanostructures, and they are helpful in enhancing our comprehension of the mechanical behavior of piezoelectric nanostructures.

This study investigates the free vibration behavior of auxetic metamaterial plates using a refined plate theory. The research focuses on the integration of functionally graded graphene origami (GOri) into the plate structures, examining various content levels and folding patterns to enhance dynamic performance. The GOri-reinforced plates are analyzed within the context of a Winkler–Pasternak elastic substrate. Hamilton’s principle is applied to derive the kinetic equations governing the auxetic metamaterial plates, facilitating an analytical solution for the governing equations. A comprehensive comparison of numerous parameters is conducted, including graphene content and dispersion type, GOri folding degree and distribution pattern, temperature effects, and elastic foundation coefficients, all of which influence the vibrational characteristics of the plates. The findings identify critical factors affecting natural frequency, providing a thorough understanding of the relationship between the physical configuration of auxetic metamaterial plates and their dynamic response. This study ultimately aims to leverage these insights to optimize the design of auxetic metamaterials for improved vibrational performance in engineering applications.

Broadband vibration suppression is a major challenge in engineering applications. In this paper, two Bragg bandgaps of a piezoelectric metamaterial beam with a shunted circuit are bridged to form an ultrawide bandgap by using the genetic algorithm. Piezoelectric patches are periodically attached to the host beam. Inductive-capacitive-resistive (LCR) shunted circuits are connected to the piezoelectric patches. A supercell with different LCR shunted circuits is designed. To couple multiple locally resonant bandgaps to Bragg bandgaps, an optimized scheme based on genetic algorithm is designed. The imaginary part of the wavenumber is used as an optimization objective to achieve the maximum attenuation within the target frequency range. The results show that two Bragg bandgaps are bridged to form an ultrawide bandgap and maximum attenuation is achieved. The transmissibility shows that the metamaterial can achieve optimal vibration suppression in the ultrawide frequency range. The finite element results verify that the optimized metamaterial can bridge the two bandgaps into a wide bandgap and can realize optimal vibration suppression at ultrawide frequencies. The pseudo-stochastic vibration of 600–8100 Hz confirms that the optimized metamaterials are more suitable for broadband vibration suppression. This metamaterial has more advantages in complex engineering environments.

Polymers consist of many discrete chains, making them inherently discrete rather than continuous. To analyze polymers (and their composites) using continuum mechanics, it is necessary to establish a bridge between their discrete and continuum models. In this paper, the discrete microsphere model is employed to derive a physics-based strain gradient continuum, where the strain gradient term relies on the concrete geometric structure. This is achieved by connecting the stretch fluctuation field of polymer chains with the strain gradient field through an asymptotic homogenization method. This homogenization method first provides the construction of the Helmholtz free energy density for the microsphere model and then develops the transformation of the free energy density to that strain gradient continuum. Applying the proposed strain gradient continuum to the Euler–Bernoulli beam, the size-dependent effects of the free energy, the bending rigidity, and deflection are investigated in detail. This homogenization method bridges the gap between discrete and continuous polymer mediums. Furthermore, the continuum model retains high-order strain gradient information. This correlation facilitates the application of polymers in nanocomposites, enabling the creation of groundbreaking materials through artificial design.

Glass fiber-reinforced polymer (GFRP) composites exhibit restricted mechanical performance, notably in terms of interlaminar shear strength and fracture toughness, as a consequence of the propensity for fiber/matrix fracturing and delamination when subjected to exterior loading. This study elucidates the enhancement of GFRP composites' mechanical characteristics through the integration of multi-walled carbon nanotubes (MWCNTs). A solution dip coating method was used to deposit 0.3 wt% MWCNTs on the glass fiber fabrics to manufacture the MWCNT-modified GFRP composites. A comprehensive experimental investigation was undertaken to evaluate the impact of MWCNTs on the mechanical attributes of GFRP composites across varying thicknesses and layups. Flexural strength, interlaminar shear strength and fracture toughness were investigated through three-point bending, short beam shear and end notch flexural (ENF) tests, respectively. To further decipher the microstructural enhancement mechanisms of MWCNTs in GFRP composites, fractured surfaces post-ENF testing underwent examination using a field-emission scanning electron microscope. The results revealed that MWCNT-modified GFRP composites with a 4-mm thickness and unidirectional orientation displayed optimal mechanical properties, and the MWCNT-modified GFRP composites with a certain layering angle surpassed the mechanical performance of their unmodified, thinnest unidirectional GFRP counterparts. This research thereby presents engineers with a novel design strategy to address the challenges posed by intricate application scenarios, enhancing the versatility and resilience of GFRP composites in advanced applications.