JOURNAL OF SOLID AND FLUID MECHANICS
Year:2016 | Volume:6 | Issue:1|Papers Count:25

Many energy-harvesting devices use the piezoelectric elements in order to convert mechanical vibrations into usable electrical energy. The input excitation is usually assumed to be a deterministic harmonic wave, while, in practical situations, the mechanical excitation of the media is a random signal. The objective of this work is to study energy harvesting in the piezoelectric devices using the random vibration theory. In the first step, a lumped parameter physical model of the device is presented. A mathematical model is then developed by acquiring the normalized differential equations governing the voltage induced in the energy-harvesting circuit as well as the length of the piezoelectric materials. The random vibration theory is then utilized to derive the analytical expressions for the statistical properties voltage, power, and length of the piezoelectric materials in terms of the statistical properties of the excitation, which is assumed to be a band limited white noise. It is shown that with a proper selection of the system parameters, the expected value for the harvested power can be effectively maximized. The qualitative and quantitative knowledge resulting from this effort is expected to enable the analysis, optimization, and synthesis of the energy-harvesting piezoelectric devices.

In this work, the homogenous and intra-ply hybrid composites of brittle/ductile fibers (basalt as brittle fiber, and nylon as ductile fiber) are produced in order to study the effects of bending deflection on the damages put to the composite structures. The composite samples used in this study are of different volume percentages of basalt fiber (0, 50, 66, 75, and 100%). All the samples including six layers are prepared by the hand lay-up method. The three-point bending test with different deflections (10 mm, 20 mm, 30 mm) are performed on the prepared samples. The residual tensile strength is employed to study the amount of bending damages put to the specimens. The results obtained indicate that in samples comprising a high content of basalt fiber, by increasing the deflection, the damage put to the composite structure increases. With increase in the ductile (nylon) fiber content, increasing the bending deflection does not have a significant effect on the damages.

In this work, measurement and analysis of the dimensionless frequency (Strouhal number) of vortex shedding of an axial compressor cascade in moderate Reynolds numbers are carried out. Assessment of these measurements can help a more precise prediction of the wake-induced transition on the downstream blades. To measure the flow field in the wake, a hot film anemometry is used. The measurements are conducted at three different incidence angles and the Reynolds numbers ranging from 240000 to 530000 based on the blade chord length and flow velocity. Based upon these measurements, there is linear correlation between the vortex shedding and the Reynolds number, and by increasing the Reynolds number, the vortex shedding frequency increases. The results obtained show that the Strouhal number for the Reynolds number equal or below 360000 has a lower scattering compared with the Reynolds number above 360000. Also decreasing the attack angle increases the wake region. Moreover, the results obtained show that the vortex shedding frequency at moderate and low Reynolds numbers displays different behaviors, which could result in different boundary layer formations at the trailing edge of blades.

A high speed and the absence of a precise control over pressure distribution have confined the sheet EMF in a die to simple shapes having shallow depths. It is possible to reach a higher depth by applying a convex punch instead of a concave die. In this work, sheet EMF on a punch and sheet EMF in a die are investigated. The electromagnetic part of the work is analytically investigated, and its mechanical part is numerically studied using FEM in the Abaqus software. In order to couple the electromagnetic and mechanical parts, the no-coupling method is used. The results obtained are verified by comparing them with the earlier experimental ones in the literature. Al 1050 is used in this work. The rate-dependent and rate-independent hardenings are taken into consideration for the mechanical behavior of the material. Using an appropriate hardening model for the material yields acceptable results. Furthermore, a convex punch is used instead of a concave die to reach a higher depth in sheet EMF.

Supports and joints play a basic and important role in the engineering structures. It is necessary to identify the various parameters of supports. The stiffness and damping parameters are the most important parameters of a support. In this work, an inverse method based on the dynamic acceleration measurement data is used to identify and study the stiffness and damping coefficients of the supports of the cantilever and doubly clamped beams. To this end, an optimization problem is defined and subsequently solved using the least squares method. In the cantilever beam, the effects of various parameters such as the magnitude of measurement errors, number of measured data, number of sensors, time duration of the applied load, magnitude of the stiffness and damping parameters, and time interval of data collection are studied on the inverse solutions. For the doubly-clamped beam, the effects of magnitude of measurement errors, number of measured data, data type, and number of sensors on the results are studied. The results obtained show that the doubly-clamped beam problem is much more difficult than the cantilever beam one. It is appropriate to use two sensors for problems with the doubly-clamped beam. By a careful investigation of the numerical results obtained, an attempt is made to answer the questions and difficulties that may occur during the practical tests.

Nowadays sandwich sheets with a polymer core have replaced many applications of single-layer sheets. The complex behavior of sandwich sheets during deformation shows the need to review their formability. In this work, we study the effects of the geometric parameters of sandwich sheet layers on the formability of the aluminum/polyurethane/aluminum sandwich sheets. To evaluate the formability of the sandwich sheets, the punch stretching test with a hemispherical punch is performed. After verifying the finite element model, it was observed that the difference between the output of the strain on the finite element method and the experimental results was 7-15%, which seems to be appropriate. The results of the experimental method and finite element simulation show that with increase in the core thickness of the sandwich sheet, the formability decreases. It is also proved that arrangement of the layers plays an important role in the formability of sandwich sheets.

The anisotropy of mechanical property in sheet metals, due to the rolling process, causes useful effects such as increasing drawability and harmful effects like the earing phenomenon. In this work, the variation in the anisotropic coefficient during the uniaxial tension of the AA6061-O sheet is considered using the digital image correlation technique by the two marking methods of regular arrays of circles and irregular accidental pattern. The meshed samples of this alloy are cut in the three directions 0, 45, and 90 degrees with respect to the rolling direction, and stretched by an Instron test machine. During the test, the transverse and longitudinal strains are measured, and then, using these two parameters, the anisotropic parameters in different directions, as more as the planar and normal anisotropic coefficients, are computed. It is demonstrated that during the straining on the samples, the anisotropic parameters in the 0 and 45 directions show a decreasing trend, and in the 90 direction, it has a wavy manner. The results obtained indicate that after 10 percent elongation, the normal and planar anisotropies of the sheet decrease by 51% and 63%, respectively. Finally, the plastic-hardening behavior of the sheet in different stages of the tension is studied using the Hill’s quadratic yield criterion, and it appears that the yield surface grows totally as an anisotropic manner.

In this work, the automatic fault diagnosis of rotating machines is discussed using the vibrating data measured from different points of machines and a smart fuzzy knowledge-based system. To this end, a new vibrations’ identification chart, recently published, is used. This chart, containing frequency characteristics and phase angle, is represented for some usual defects such as unbalancy, misalignment, bent shaft, and mechanical looseness. Designed fuzzy knowledge-based system has a very simple structure. It does not require any complicated training such as those used for neural network training. In order to evaluate the performance of the designed fuzzy system in actual applications, it is used for the fault diagnosis of some rotating machines in Isfahan Steel Company such as fans. The effects of different membership functions such as the non-linear Gaussian, bell-shaped, sigmoid, s-shape, and z-shape function for inputs and outputs of fuzzy rules database are studied and the results obtained are compared with those for some neural network-based fault diagnosis systems and experimental results. These results show that the designed smart fuzzy system has an acceptable performance in detecting faults.

The optimization of the design of systems containing rotating shafts, so that the system has appropriate vibration characteristics, is one of the most important factors involved in the performance and efficiency of the rotor system operations. Vibration of rotary equipment is usually caused by factors such as unbalanced mis-alignment, abrasion parts, and abrasion moving parts. The frequency of these vibration forces is usually an integer multiple of rotating speed (spin rate) so that if the initial critical speed is as far away as possible from the rotational speed of the rotor, the rotor works in a safer operational area, and other parts of the system need a few time for repair or revision. In this work, the Myklestadt-Prohl method is used to compute the initial critical whirling speed of a multi-stepped rotor and genetic algorithm to optimize the location of the bearings in the rotor so that the first critical speed of the system has a maximum value for the least number of bearing.

The strength of adhesively-bonded joints plays a very important role in the behavior of loaded composite structures. In this work, the debonding behavior of the adhesive joint between the sandwich panel and the pultruded profile is investigated by the finite element method. The cohesive zone model and the contact elements are used to simulate the adhesive joint between the stiffener and the panel. Representing the adhesive material, an embedded layer is modeled between the sandwich panel and the profile. The effects of the geometrical parameters including the thickness of the adhesive layer and profile on the behavior of the joint are investigated. Moreover, the effect of the initial defect in the adhesive joint on the debonding results is studied. The results obtained show that in a perfect bonding, increasing the thicknesses of the profile and adhesive layer improves the joint behavior against debonding. However, in a defected joint, increasing the profile thickness decreases the debonding load, and presence of the initial defect decreases the debonding load by 51%. The results obtained also show that the presence of an embedded adhesive layer has a positive effect on the debonding behavior of the joint. An initial debonding load without adhesive layer is decreased by 40% in comparison with the joint containing a 10-mm adhesive layer.

Buckling failure is a quite common occurrence in plates under compression, in particular, when the plates thickness is small with respect to the other plate sizes. Such a mode of crisis can often precede strength failure. Nowadays one of the most common types of repairs is carried out by attaching composite patches on the defeated surfaces. These patches have important advantages such as high strength and corrosion resistance, while their weight is low. In this work, experimental studies are done on the critical buckling load of the notched plates repaired by the fiber metal laminate (FML) patches subjected to axial compression. For this purpose, the rectangular plates made by 2024T4 aluminum with central notches are considered. The desired parameters involved in this work are the layer sequence, thickness, and dimensions of the FML patches. The results obtained show an increase in the buckling strength of the repaired parts. Smart choosing of the parameters results in an increase in the buckling load up to 44.13%.

Among the various ductile fracture models, the Gurson-Tevergard-Needleman (GTN) damage model has been widely used; it includes the three steps nucleation, growth, and coalescence of voids during plastic deformation. An important issue in the GTN model is the exact calculation of the model parameters, which is very time-consuming and costly. Therefore, the finite element simulation method is used for this purpose. The porous metal plasticity model in the Abaqus software does not consider the coalescence of voids step, and analyzes issues on the basis of the two steps of nucleation and growth of voids. This causes an error in the results. In this work, the uniaxial tension test of aluminum alloy 5083-O is simulated using the GTN damage model. The simulation is carried out using the finite element software Abaqus through writing code in the UMAT subroutine. The GTN damage model parameters of AA5083-O are evaluated by matching the experimental engineering stress-strain curve and the simulated curve. The results obtained show that the code written through the UMAT subroutine addition leading to improvement in the simulation of the GTN damage model in Abaqus software provides an acceptable prediction of the GTN model parameters.

The purpose of this work is to perform an experimental study and also modeling the plastic deformation of rectangular plates under a low-rate impact loading by the drop hammer system. In the experimental section, some experiments are conducted on the rectangular plates with different levels of energy to survey the mechanical behavior of the steel and aluminum plates according to the applied load. The modeling section consists of presenting an explicit function for the experimental data by singular value decomposition (SVD) based on dimensionless parameters, and also the multi-objective modeling and design of the adaptive neuro-fuzzy inference system (ANFIS) by genetic algorithm. Generally, the aim of modeling is a reliable and satisfactory prediction of the deflection-thickness ratio of plates under impact loads. A comparison is made between the modeling results and experimental data in order to validate the results. Investigation of the training and prediction data errors, which is based upon the root-mean-square error (RMSE) and coefficient of determination (R2), shows that the results of the optimal design of ANFIS are closer to the experimental results of mathematical modeling by the SVD method, with the exception that a mathematical function based on the experimental data is presented by the SVD method.

Investigation of the fatigue behavior of reinforced polymer composites is necessary to design a reliable and durable structure. Due to the difference between the fatigue properties of reinforced polymers and traditional isotropic engineering materials, introducing a criteria for prediction of the fatigue life of reinforced polymers is very useful in design and analysis. This work is aimed at the life prediction of reinforced plates subjected to bending fatigue loads. The glass-epoxy and graphite-epoxy plates, which are subjected to the distributed and concentrated transverse loads, are studied based upon the strength degradation model. Distribution of the stresses at the plies of the plate subjected to transverse load is acquired using the first-order shear deformation plate theory. Then the strength degradation model is used to predict the strength reduction of each plie of the plates in the loading cycles. The degradation of plies and reduction of the strength in each loading stage are accumulated, and the residual strength of the plies is obtained as a function of the load cycles. This procedure is repeated until the plies fail, and the final catastrophic failure occurs in the plate.

Industrial press units are divided into two primary groups, single-action and double-action. A single-action press unit has a mechanism that guides drawing slide. A double-action press unit has a blank holder mechanism as well. A double-action press die has more weight and is more expensive than a single-action one. For the following reasons, single action die, uses for double action press that blank holder mechanism is not in use in this situation and unit changes to single action. The conclusion of this work is the dynamics and stress analysis of the later state. In this work, for a kinematics analysis, a mathematical model of the mechanism is presented. Later, a dynamics analysis of the double-action mechanism with a standard die and with a small die of a single-action state is provided. This work is concluded by the stress analysis of the two states. Conclusions yield that the use of a single-action die for a double-action press increases the force and stress in the ram mechanism joints. Thus this usage in the double-action press increases the failure chance in the press mechanism.

This work is an experimental study of the temperature and volume fraction effects, which provides a new model for the nanofluid viscosity of MWCNTs-EG. The MWCNT viscosity with a diameter between 1 and 2 nm in EG is calculated at 30 to 60 °C and the volume fractions of 0.025 to 0.3 using a viscometer. The type of viscometer is Brookfield, and its model is DV-I Prime. It was observed that the viscosity decreased, while the temperature increased, and at high volume fractions of the nanofluids, it showed non-Newtonian properties. Since no study has yet been carried out on this nanofluid, it is for the first time that a new equation about viscosity is obtained. The Einstein and Bachelor models are compared with the empirical relationships, and also to observe the similarities, a new Newtonian formula is offered.

In this work, a numerical simulation of the effect of hyperthermia on skin cancer treatment is carried out using the Fe Pt magnetic nanoparticles with variable magnetic field conditions.. The numerical solution is presented for the analysis of the bio-heat transfer and magnetic induction equations in a cylindrical skin tumor situated in a healthy tissue considering the evaporation and convection heat transfers. In order to show the validity of the work, the results obtained are compared with those of the studies already existing in the literature. The bio-heat equation is used to predict the temperature rise in terms of characteristics of the magnetic nanoparticles, applied magnetic field, and tissue. The results obtained reveal that the nanoparticle diameter has a major effect on the temperature rise. These results also show that the temperature field in the axial direction (from surface to depth) of tissue and the effect of hyperthermia decrease with increase in the convection heat transfer coefficient. In other words, hyperthermia is more effective in the presence of environmental natural convection. Moreover, the position of maximum temperature inside the tumor varies by changing the heat transfer coefficient. Also the amount of evaporation has a negligible effect on the hyperthermia treatment.

In this work, the flow and temperature fields affected by an electric field in the presence of a wire-collecting electrode are numerically studied for the two-dimensional, incompressible, turbulent, and steady-flow conditions by the finite volume approach. This computational methodology includes the use of a structured non-uniform quadrilateral grid, and the standard K- model is adopted as the turbulence model. The computed results are compared with the experimental data; they agree very well. Then the effects of different parameters such as the collecting electrode radius, applied voltage, Reynolds number, and distance between emitting and collecting electrodes on the flow pattern and heat transfer coefficient are evaluated. The numerical results obtained show that the influence of the electro-hydrodynamic phenomenon on the heat transfer enhancement increases with the radius of the grounded electrode and applied voltage but decreases when the Reynolds number and the distance between electrodes are augmented. The results obtained indicate that the electro-hydrodynamic actuator acts as a generator of secondary flow, and these vortices are used to enhance the forced convection heat transfer.

Ground source heat pump, due to high coefficient of performance (COP) and use of low-temperature thermal energy source, is one of the best technologies to use renewable energy resources. In this work, at first, a geothermal heat pump for heating with economizer is simulated, and then the effects of the variations in different parameters such as pressure in different parts of cycle, super-heating at evaporator outlet, sub-cooling at condenser outlet, and soil temperature on heat pump and total COP and exergy efficiency are analyzed. At the end, the ground source heat pump system is optimized in two manners: total COP relative to objective function and total exergy efficiency relative to objective function. The total COP and exergy efficiency values for the basic input mode were 3.674 and 0.488, respectively, and these values increased to 5.323 and 0.72, respectively, after optimization.

In the recent years, achievement of a more accurate numerical method appropriate for different flow regimes to capture discontinuities with less oscillation and numerical errors has been of interest by many researchers. The specific comment in this paper is the comparison of the performance of artificial dissipation and upwind methods in solving the Euler equations for internal compressible flows in a wide range of inlet Mach numbers. In this work, we examine the ability of the AUSM+ upwind method, and the Scalar and Cusp artificial dissipation methods for flows with very low Mach number up to ultrasound and non-viscous flows in a convergent-divergent nozzle. The ability of the AUSM+ and Scalar methods in a 2D inviscid transonic flow between the turbine stator blades at both the supersonic and subsonic outlets is also studied. An excellent performance was observed for the AUSM+ method with more convergence speed and low numerical error in all flow regimes at a converging-diverging nozzle. Further, for the second case, the AUSM+ method coincides with the experimental results very well with lower numerical errors, and satisfies the mass conservation better than Scalar. It should be mentioned that the AUSM+ method is highly recommended for higher Mach numbers.

The electro-chemical machining process (ECM) is one of the non-traditional machining processes used in various industries for the certain advantages it has. This machining process is based upon the anodic dissolution. Having no contact between work piece and tool that leads to a less tool wear and eliminating the cutting forces are some of the most important benefits of electro-chemical machining. There is still no acceptable method available to predict the workpiece shape for a specific tool and also a desired tool for a given cavity because of the complexity of this process, while using the conventional trial and error method to extract the shape of the workpiece and the tool is time-consuming and costly. Simulation of the electro-chemical machining process is a useful method to overcome these problems, and it lets us design and predict the noted parameters, while cost reduction and time improvement are achievable. In this work, anodic dissolution in each time step is simulated using the finite element method to simulate the electro-chemical machining process. By using the results of the simulated model and the sensitivity algorithm, extraction of an improved tool shape and a desired workpiece are possible. The results obtained demonstrate the ability of the method proposed for simulation of the electro-chemical machining process and design tool.

In this work, the diameter ratio of a geo-thermal borehole heat exchanger is determined in a manner that the entropy generation is minimized. For this purpose, the heat transfer and fluid flow in the heat exchanger are simulated. The SST k-w model is used to model the turbulent flow. The pressure loss calculation for different diameter ratios of the heat exchanger shows that for a diameter ratio of 0.7, the total pressure loss is minimum. The results of the bulk flow temperature show that the outlet temperature increases at a higher heat resistance of the internal wall. The total entropy generation for different diameter ratios at different heat resistance of the internal wall are also presented, and the diameter ratios for minimum entropy generation are determined. The results obtained show that when the heat resistance of the internal wall increases, a minimum entropy generation occurs at higher diameter ratios. On the other hand, reduction in the heat transfer coefficient of the outer wall and the increment in the heat transfer coefficient of the internal wall are not favorable. The increment in heat resistance of the internal wall at higher diameter ratios of the heat exchanger reduces both of these unfavorable effects.

With the development of microelectronics and micromachining technology, micro-heater has found plenty of applications in micro-sensor. Heating electrode material is one of the key factors that affect the power loss, response time, and sensitivity of a micro-heater. In this work, using two various metals, two micro-heaters with the same geometry are designed, fabricated, and characterized on silicon substrates based on the micro-electro-mechanical-system (MEMS) fabrication process. In the first micro-heater, gold, and in the second one, platinum are used as the heating electrode, and the effect of heating electrode material on the performance of the micro-heater is evaluated. Moreover, to improve the micro-heater efficiencies, their design is investigated, and the micro-heater with an optimum design is chosen. The analytical results obtained exhibit that the gold micro-hater has a lower response time and a higher power loss than the platinum micro-heater. The experimental results are in good agreement with the results obtained from the analytical analysis, and show that the fabricated micro-heaters with an optimum design have a high performance; as the power consumption and response time are 36 mW and 1.75 ms, respectively, in the gold micro-heater, and 30 mW and 2.1 ms, respectively, in the platinum micro-heater for a temperature variation from 30 to 450 oC. These results demonstrate that with fabrication of the gold micro-heater, the response time improves by 16.6% in comparison with the platinum micro-heater.

An emulsion is a mixture of two immiscible liquids in which one liquid is dispersed in the form of small drops in the other one. The liquid in the form of droplets is called the discrete phase, whereas the other liquid is termed as a continuous phase. One of the common methods for emulsification is based upon using microfluidic devices. Currently, the study of flow in microfluidic devices is highly regarded due to having many applications in various fields including droplet generation in relation with the standard and quality of human life. In this work, the process of droplet generation in microfluidic flow-focusing devices with orthogonal and intersecting flows is simulated. The effects of viscosity and flow rate of the discreted and continuous phases on the droplet generation process are also investigated. Based on the simulation results, increasing the flow rate of the discreted phase (Qd) at a fixed continuous phase leads to the increment of droplet size, whereas an increase in the flow rate of the continuous phase (Qc) at a fixed discreted phase results in a smaller droplet size. Moreover, for a constant Qc/Qd ratio, regardless of whether the discrete or continuous phase flow rate is fixed, the equal droplet size is achieved. A higher viscosity of the discrete phase provides a larger droplet size, whereas an increase in the viscosity of the continuous phase leads to a smaller droplet size. The simulation results, in comparison with the experimental results, show a good agreement, confirming the accuracy of the proposed method.

In this work, an efficient procedure is presented based on the density-based algorithm with explicit solver to solve the compressible Euler equations on a non-orthogonal mesh with a finite volume formulation. The fluxes of the convected quantities are approximated using the characteristic-based TVD, ACM, and Jameson methods. The aim of this work is to introduce a method based on the characteristic variables and control of the diffusion term in the classic methods in order to capture the shock waves. Hereby, an inviscid supersonic flow is solved, and the results obtained are compared together in view of the resolution and accuracy of the shock wave capturing and solution convergence. The convergence target for the mass and momentum equations is decrement of residuals from 10-6. The results obtained show that in the density-based algorithm, the characteristic velocities are better converged due to the augmentation of the limiter, ACM, and TVD methods, capturing the shock waves with higher resolution relative to the Jameson method. Shock waves include simple waves reflected waves and waves interactions. Also the ACM method is a useful technique, which prevents the smearing of discontinuities and improves the resolution of shocks.