نشریه مهندسی مکانیک ایران
سال بیست و پنجم شماره 1 (پیاپی 70، بهار 1402)

In this paper, the starting and steady-periodic characteristics of a multi-stage looped-branched thermoacoustic cryocooler and its uncertainty with respect to effective parameters are calculated and discussed. In this regard, the start-up and nominal performance of the system are simulated using the numerical solution of Rott's one-dimensional equations. Numerical and experimental results of a similar system confirm the validity of the results of the present article. Simulation outcomes show that the number of stages, mean pressure and loop perimeter can significantly change the system performance. In contrast to previous findings, the results in this paper show that increasing the number of stages or increasing the mean pressure does not necessarily improve thermodynamic performance. Besides, Morris method is implemented to quantify uncertainty of the operating parameters of a four-stage cryocooler in relation to the geometric, modeling and material characteristics. The results reveal that the gas characteristics, solid conductivity and mesh geometry of the engine and refrigerator regenerator have a dominant nonlinear effect on the required hot temperature, exergy efficiency, relative Carnot efficiency, cooling load and temperature. Moreover, by adding additional thermodynamic cores in each stage, the amount of acoustic work and total enthalpy and its uncertainty increase and the effect of solid thermal conductivity decreases. With the developed model and main findings of this paper, one can optimize a reliable thermoacoustic cryocooler based on its onset and steady characteristics.

In this paper, we optimize the arrangement of cells in a Lithium-ion battery pack using an air-cooling thermal management system under operating conditions in transportation applications. The thermal and electrochemical behavior of battery cells is modeled using the quasi-two-dimensional approach by Newman’s method. The effects of inter-cellular distance on the maximum temperature, and on the temperature distribution in the pack as the target variables are thoroughly investigated. A suitable range for the above distances are then determined in order to reduce the overall cooling power and the expected thermal performance of the pack. According to this study, by reducing the distance between the batteries, the temperature of the cells decreases. However, this reduction in the distance also reduces the battery pack's temperature uniformity and cooling efficiency. The results of the temperature distribution are investigated, analyzed, and discussed. Finally, by examining the effect of the number of pack cells on the cooling efficiency, it is shown that increasing the number of cells initially increases the cooling efficiency; however, after a certain limit, it leads to a decrease in the cooling efficiency.

Trapezoidal plates are referred to as simple models for aircraft wings. These wings can be produced from different materials, each of which has its own advantages. One of the materials that has attracted the attention of researchers in recent years is nanocomposites, which with their high strength and low weight can be a good option for making aircraft wings. In fact, when nanocomposites are produced, they are not without weaknesses and have weaknesses that reduce their mechanical properties. One of these weaknesses is the agglomeration of nanotubes. In the present paper, a trapezoidal nanocomposite plate will be modeled and its behavior in the presence or absence of nanotubes agglomeration will be compared. The data extracted from the relations are for bending of trapezoidal plates. For this purpose, the three-dimensional theory of elasticity will be used as the most accurate theory for modeling of thin and thick plates. A review of the research conducted so far concludes that this is the first time that the effect of nanotubes agglomeration on the flexural behavior of trapezoidal plates has been studied.  It is worth mentioning that in the studied cases, it was observed that the agglomeration of nanotubes has no significant effect on the final results and can be neglected by engineering approximation.

Nowadays, industrial systems deal with a wide range of constraints. Input saturation and lack of system model are two types of these constraints. In this paper, a separate model free adaptive data-driven controller for a nonlinear system called a simulator with the conventional configuration of a main single-tailed propeller, twin rotor MIMO system (TRMS) in the presence of input saturation is presented. In the proposed controller, the design of the control input signal only depends on the input and output data of the system and the system model is not used. The purpose of this paper is to control the horizontal and vertical angles of the TRMS system. For this purpose, at first, using the model of TRMS that has been presented for this system in previous articles, a model of the relevant system is expressed and then only using the input and output data obtained from that model, control the defined angles with a separate model free adaptive data-driven control method. Finally, the performed simulations demonstrate the effectiveness of the proposed method for the TRMS, by using two difference reference signals named variable steps and sinusoidal.

This research aims to provide efficient analytical models for computing the aerodynamic forces exerted on the wings and the tail of a flapping-wing robot. To achieve this purpose, at first, the aerodynamic model of the wings is presented based upon the parallel strips theory and by considering the effects of wing flexibility. Afterward, the aerodynamic model of the tail is introduced regarding the effects of pressure difference on the tail surfaces, leading edge vortices and air friction. Next, the optimal coefficient of wing flexibility is obtained utilizing the genetic algorithm. Ultimately, in order to validate, the results of the proposed models are compared with the models presented in the former studies and the measured experimental data. Simulation results demonstrate that the aerodynamic forces reckoned by the suggested strategies are closer to the experimental data in comparison with the previous models.

In this work, free vibrations of a composite beam is investigated. Considered composite beam is assumed with the presence of pre-twist. One of the beam is without twist while the other end of the beam has the maximum twist angle. Twist rate in the beam is assumed to be non-constant where a higher order polynomial variation of twist angle is assumed. Composite beam of this study is composed of a number of layers where each layer of the beam is reinforced with graphene platelets. Layers may have different amount of graphene which leads to the functionally graded composite laminated beam with pre-twist. Elasticity modulus of the beam is estimated by means of the Halpin-tsai rule while the mass density and Poisson's ratio are assumed using the simple rule of mixtures approach. Timoshenko beam theory is adopted to estimate the displacement field in the beam. Due to the present of twist, five degrees of freedom containing three displacements and two cross-section rotations are considered. Using the Hamilton principle and with the aid of the Ritz method, the equations of motion are discretized. Shape functions of the Ritz method are constructed by means of the Chebyshev polynomials. This set of functions are orthogonal and has high rate of convergency. Matrix representation of the governing equations is obtained using the Chebyshev-Ritz method. The governing equations of the free vibration motion are established and solved as an eigenvalue problem. Results of this study cover the case of free vibration of a graphene platelet reinforced composite laminated pre-twisted beam. At first results of this study are validated with the available data in the open literature and also the required number of shape functions in the Ritz method are estimated through the convergence studies. After that using parametric studies, the influences of different parameters such as number of layers of composite, boundary conditions, twist angle, twist rate, graphene weight fraction and their pattern are explored. Numerical results of this study show that by increasing the weight fraction of graphene platelets, natural frequencies of the beam are enhanced. Also through adoption of a proper pattern, the natural frequencies in the beam may be controlled. The effect of twist angle on the frequencies of the beam is shown. It is depicted that increasing the twist angle may increase/decrease the frequencies depending on the mode number.