نشریه علوم کاربردی و محاسباتی در مکانیک
سال سی و یکم شماره 1 (پاییز و زمستان 1398)

The helical gear systems have some distinctions such as more precision, long life and more applications in the industry, and less vibration, noise and transmission error compared to spur gear systems. One of the damages that can affect the operation of these systems is the gear tooth root crack. So, the crack detection in its early stages of growth and development is very important. The residual signal method is one of the common methods to extract the effect of crack from the system vibration signal. Since the residual signal calculation requires the information of the vibration response of the healthy state or the system initial parameters, providing a method to extract the effect of crack from the vibrating response without any more information is very helpful. In this research, at first the calculation of the helical gear pair mesh stiffness is explained, and the effect of tooth root crack is also described. Then, a single-stage helical gear system, with the motor and the load, is modeled and solved numerically to achieve the system dynamic response. Then, the effect of present method on the vibration signal obtained from the dynamic simulation of a system with a small crack is Shown and discussed. Finally, the proposed method is applied to the dynamic response signal of a system that presented in one of the previous researches, and its efficiency is illustrated.

In this research, large-eddy simulation of a turbulent square duct flow is performed at the friction Reynolds number , using the dynamic Samgorinsky (DS) subgrid-scale model and the results are discussed. To assess the accuracy of the DS model, the results are compared with the reference direct numerical simulation data. Moreover, to see the effect of the DS model, a numerical simulation without a subgrid-scale model is also performed and the results are compared with those of the DS model. Simulations are carried out using a second-order finite volume method for discretization of the Navier—Stokes equations. Results from the DS model simulations, for the grid used, are in good agreement with the direct numerical simulation data for the mean velocity and Reynolds stresses and an appreciable improvement is observed with respect to the no subgrid-scale model simulations.

Numerical solution of conjugate heat transfer (CHT) problems in multi-region domains is a challenging issue in terms of efficiency and stability due to inherent coupling between governing equations in separate sub-domains. This paper aims at comparing two distinct strategies for coupling energy equations at interface of regions: partitioned and monolithic. While the former enforces strong coupling via iterative procedures, the latter suggests simultaneous solution of energy equation throughout all regions at once. More precisely, the primary objective of this study is to compare computational costs for three finite volume- based CHT solvers which share a same method for handling pressure-velocity coupling in the fluid, i.e. semi-implicit projection method, and are distinguished from the coupling strategy for energy equation at interface of interacting sub-domains. The first method applies simultaneous solution of energy equation across all regions while second and third ones use separate solver for each region with difference in handling iterative loops. The accuracy and convergence rate of three algorithms are assessed via transient solution of conjugate free convection in fluid and conduction in vertical wall. Finally, it is concluded that, in contrast to partitioned solver in OpenFOAM for CHT problems, employing a separate loop to enforce energy-energy coupling per iteration, can be much more beneficial in terms of computational costs.

Bio-fouling as a worldwide marine industries concern is the accumulation of micro and macro-organisms on the submersed surfaces in the sea water. It has destructive effects on the sunk parts of the ships, fish cages and all other marine submersed structures. To reduce bio-fouling on the nylon fibers a dual mix of 2-Hydroxy ethyl methacrylate(HEMA) and methyl acrylate(MA) was grafted on the fiber surface using a pre-irradiation technique in different conditions. Subsequently, degree of grafting and homo and or co-polymer % of the aforementioned monomers at different operative parameters including reaction temperatures (60-90℃), and times (1-4h), pre-irradiation time (10-40min) and also initiator concentration (3-9wt%) were measured and the results were compared and discussed with those calculated values by a traditional design of experiment software (Design expert®). The optimum grafting conditions and the interaction between above mentioned parameters were evaluated by the software using four variables and two surface responses with central point design method. It found that increasing pre-irradiation time and also initiator concentration improved the grafting % of the monomers on the fiber surface. The reason referred to increasing the active sites on the fiber surface after pre-irradiation. With the aid of variance analysis and considering p-values variable with 0.05 confidence interval, it revealed that the important parameters lied down in the order of reaction temperature, reaction time, pre-irradiation time and initiator concentration. Also, the results were confirmed statistically by the characteristic coefficient, adjustment coefficient and fit precision of 96%,93% and 30.302, respectively. The real values for degree of grafting and also homo and co-polymer percent were 47.9% and 41.15% respectively. They were in conformity with predicted values by mathematical model with the values of 38.59 and 43.42%. Also, two equations were proposed by the software for calculation of the aforementioned parameters with studied operative parameters.

To create a relative movement between connected members in a mechanism, the presence of clearance in the joints is inevitable. In addition to raise errors in positioning accuracy, the presence of clearance is one of the most important factors in causing impact, shock, and thus the production of vibrations and sound during the operation of the mechanism. Tolerances and errors due to the process of design and construction, wear and corrosion of joints after a certain period of work and thermal effects are known as the most important sources in creating and increasing clearance. Obviously, if there is a clearance in the revolute joints, one or two degrees of uncontrollable freedom are added to the mechanism, which can be the source of the error. In this paper, the clearance effects of revolute joints on the kinematic behavior of a 3RPR parallel robot are examined. Parallel robots are modeled in the Adams Dynamic Analysis Software, and the simulation results are compared for two ideal (without clearance) and real mode (with clearance). The effects of joint clearance size and its effect on robot behavior are also evaluated. Numerical results show that by growing the size of the clearance, the values of velocity and acceleration increase dramatically.

In this research, effects of various system parameters on nonlinear response of transverse vibrations of beam-like fluid conveying microtube with fixed simply supported boundary conditions under axial magnetic parametric resonance condition is investigated. Reddy’s first order shear deformation theory and Eringen nonlocal elasticity theory are used to derive microtube nonlinear equations of transverse motion considering nonlinear geometric terms of von-Karman. For fluid flow velocities more than flutter critical velocity, behavior of 2 DoF nonlinear system is studied under parametric magnetic resonance condition. By deriving nonlinear response curves, effects of various parameters including magnetic excitation amplitude, parameter of excitation frequency and nonlocal stress parameter on resonance amplitude is investigated and discussed.

Polygonal hydraulic jump is among the subjects widely studied by scientists in recent years. Although this phenomenon was discovered nearly two decades ago, many of the probable reasons behind it remain unknown. Accordingly, the main goal of the present study is to conduct a laboratory-scale phenomenological investigation on polygonal hydraulic jumps. The results indicated that the main cause of polygonal hydraulic jumps is the presence of disturbances and instabilities in flows, systems, or environments. Given the Plateau–Rayleigh instability, in the presence of surface tension and viscosity effects, the disturbances and instabilities create stable circular jumps unstable and then turn them into polygonal jumps. A stable circular jump was created with the elimination of instabilities, and the jump, unlike what observed in previous studies, was stable at a low to high range of Reynolds number. In addition, the behavior of polygonal hydraulic jumps formed in previous studies in the presence of instabilities was investigated. In a constant flow rate, the area inside the jump was equal for all the possible jumps at an error level of less than 10%. Among the polygons with an equal number of sides, which are possibly observed in a particular flow rate, the jump changes into a regular polygon, as the surface tension naturally tends to create the minimum possible surface area for the jump.

In this paper, the presented correlations in scientific literature for friction factor of turbulent flow in the base fluid and nanofluid has been analyzed statistically. In order to perform the statistical analysis and select the best probability distribution function for the friction factor, from 48 probability distribution functions and 3 statistic of Kolmogorov-Smirnov, Anderson-Darling and chi squared has been used. The results show that, based on all three statistics, the probability distribution function of the friction factor in the base fluid and nanofluid is the Johnson SB probability distribution function. Furthermore, it was observed that the probability distribution function of the friction factor of the base fluid and nanofluid is wider than the normal distribution function. On the other side, the probability distribution function of Johnson SB has a right skewness.

Optimal control appears in some of the well-known areas of science in recent decades. In optimal control field, a classical subject is the switching optimal control problems which arise in some well-known application areas, such as aerospace engineering, cranes and industrial robots. In these problems, the input control jumps from one boundary to another and appears linearly in the objective function and the dynamical equations. What’s important in solving these problems, is finding the switching points with high accuracy. In this paper, an improved shooting method for solving these problems is investigated. For this purpose, the well-known indirect shooting method for numerical solution of this class of optimal control problems is first reviewed and it’s deficiency in obtaining the switching points accurately, is expressed. Then, with redesigning the method by applying a control parameterization, we transform the problem to the solution of the shooting equation, in which, the values of the switching points and the initial values of the costate variables are unknown parameters. This leads the switching points are captured accurately and thereby accurate solution of the problem is obtained. Numerical results of five benchmark examples are presented and efficiency of the method is reported.

In this study, effects of the use of dual transverse protuberances (DTP) as a new, low cost and simple method in controlling the thrust vector in a C-D  nozzle, whose nominal Mach number is 2, was investigated. The protuberances used are two cylindrical shapes that are placed in front of the flow in the divergence part of  nozzle. Protuberances are installed in 80% of the length of the nozzle divergence section from nozzle throat, transversely and at a 45° angle to each other. The flow field was investigated by schlieren imaging, along with measuring the pressure distributions on the nozzle walls. The results show that using the DTP in the nozzle can increase the angle of the thrust vector to 3.5 degrees in the examined conditions. Also,it can be achived in compare to using a single protobrance with lower thrust loss up to 5.5%.

In this paper, a gas pipeline model with leakage is presented. This model does not need to evaluate the characteristics of downstream equipment and does not rely on sophisticated numerical methods. Changes in the flow variables can be accurately predicted by the model. In this model, a new similarity-based approach is proposed between the pipeline and the electrical circuit, in which a circuit equivalent to a specific structure is used to simulate the leakage pipeline. This new approach can be used to assess leakage losses and guide pipeline operation as well as to help detect leaks. We know that many mathematical expressions have been formulated to describe the flow in a pipeline by leakage, but it is not easy to solve. However, the mathematical model of a leaky pipeline can also be developed with this method. This model is validated using data from a real gas pipeline. The simulation results are in good agreement with the experimental results which confirm the validity of the model and the validity and efficiency of the proposed method.

This paper aims to introduce an efficient and passive method in order to reduce the amount of drag exerted on the surfaces of the objects moving in the water. The flow regime is considered as laminar. The desired method is to use rectangular riblets being perpendicular to the flow direction. Two-dimensional computational fluid dynamics is utilized to resolve the flow field around the ribbed surfaces. The effects of geometrical parameters of riblets including width and height as well as free-stream velocity on the amount of drag reduction are numerically calculated. The numerical results certify the efficiency of rectangular riblets as a drag reduction tool in the laminar flow regime. Maximum amount of the drag reduction is about 8.7% achieved for the riblets width of 0.1 millimeter and at free-stream velocity of 10 m/s.