In order to study the dynamic behavior of stiffened cylindrical shells with composite material sandwiched by co-cured damping films under the clamped boundary condition,the specimens of stiffened cylindrical shell with composite material sandwiched by co-cured damping films were prepared,and the dynamic modal test platform was set up.The fundamental frequency,damping ratio and modal shapes of stiffened cylindrical shell specimens were solved,and the accuracy of finite element model was verified.The influence of geometric parameters on structural vibration characteristics was further discussed by the numerical simulation method.The results show that,the fundamental frequency,damping ratio and modal shape of the structure will change abruptly when the height of stiffeners changes,and there is a suitable height value before the abrupt change to make the whole structure consider both damping and stiffness requirements;when the thickness of composite materials is constant,the fundamental frequency of the whole structure decreases gradually and the damping ratio increases gradually with the increase of the damping thickness or damping layer number of single layer;for the stiffened cylindrical shell of single layer damping composite materials,the closer the damping layer is to the inner skin,the higher the stiffness is,and vice versa,the damping capacity is better.
Short-term power load forecasting plays an important role in power system dispatching. To improve forecasting accuracy, a short-term load forecasting model based on stacking ensemble learning was proposed. Firstly, add effective multi-feature variables, and establishes a Stacking ensemble learning model for the load data and feature, which was ensembles by Light Gradient Boosting Machine (abbr. LightGBM) and eXtreme Gradient Boosting (abbr. XGBoost) for prediction. Finally, the comparison and experimental results show that the forecasting error of the proposed model is less than that of the comparative model.
Quadrotors possess traits such as under-actuation, nonlinearity, and strong coupling. Quaternions are primarily used for attitude calculations in drones, with error quaternions seldom being employed directly in the control of specific quadcopter drones. This paper focuses on the low tracking accuracy and weak anti-interference ability of quadcopter drones in trajectory-tracking control. By establishing the quadcopter quaternion model, a controller based on quaternion error is designed through a combination of fractional-order PID control with S-plane control. Trajectory-tracking experiments demonstrate that, in comparison with fractional-order PID, this method exhibits strong wind disturbance resistance and high tracking accuracy.
The problems of leakage, frictional wear, and flow heat transfer caused by the hysteresis characteristics of brush seals are prominent, and existing models for solving the hysteresis characteristics of brush seals have convergence difficulties owing to the extreme degree of nonlinearity caused by additional contacts. In this study, numerical models based on the finite element software ABAQUS were proposed to address the hysteresis characteristics of brush seals with three different structures: basic, low-lag hysteresis, and low-wear high-pressure bearing. The models accounted for the contacts between: the brush, brush and the baffle, and brush and the rotor, while damping was introduced to improve the convergence of the models. The hysteresis of the three structures under differential pressure was investigated and compared with theoretical calculations and experimental test results. The hysteresis of the bristles was normalised by the contact force, the shift of the contact area, and the contact friction force when the hysteresis effect occurred in the brush seal. The results show that the basic model has the most obvious hysteresis characteristics in the presence of differential pressure. Although the low-lag hysteresis model and the low-wear high-pressure bearing model have the weakest hysteresis effect, the deformation variables of the last row of bristles of these two structures are larger than those of the basic model; therefore, the contact position is prone to fatigue fracture. Simultaneously, the hysteresis effect was verified by varying the contact area and contact friction force, and it was found that the larger the contact area and contact friction force, the larger the hysteresis.
In order to investigate the effect of tension–compression asymmetry of propellant mechanical properties on the structural integrity of a Nitrate Ester Plasticized Polyether (NEPE) propellant grain, the unified constitutive equations under tension and compression were established, a new method for grain structural integrity assessment was proposed and the structural integrity of the NEPE propellant grain under the combined axial and transverse overloads was evaluated. The results indicate that the mechanical state of the NEPE propellant grain is in the coexistence of tension and compression under the combined axial and transverse overloads, and the tension and compression regions in the propellant grain is independent of the propellant constitutive behavior. The tension–compression asymmetry of the propellant mechanical properties has a certain impact on its mechanical response. The maximum equivalent stress and strain considering the tension–compression asymmetry falls between that obtained through the tension and compression constitutive model, and is the same as damage coefficient. The safety factor of the NEPE propellant grain considering the tension–compression asymmetry of its mechanical properties is larger than that non-considering, and the traditional method of structural integrity assessment is conservative.
