S. Dalilsafaei, N. Montazerin, M.h Fazel Zarandi,
Volume 19, Issue 6 (6-2019)
Abstract
In this paper, a fuzzy clustering system is presented to display the flow field changes in the rotor outlet of centrifugal . What is important in the research done in the field of is the need for all the fields to properly understand the phenomena of flow inside the , which has complexities. For this reason, the most advanced laboratory equipment is used in this regard, which is associated with issues such as time consuming, high costs, and a large number of required tests, and doubles the importance of simulating and observing current phenomena through artificial intelligence algorithms. The present system operates on the basis of fuzzy clustering so that the spatial data (from the PIV measurement system) by the number of specific clusters to the field display in the initial time; then, by applying changes to the cluster related to the time series (from the system measurement of LDA) that contains the recorded changes of the current during the time of the data mining, the new field data are obtained at a new time step and the clustering of the data shows the variation of the flow field in the fuzzy environment. In this paper, the flow field was investigated for 6 successive steps, and the results of the system output showed the variation of the flow field from the rotor at different angles.
K. Maleki Bagherabadi , M. Sani, M.s. Saidi ,
Volume 19, Issue 8 (8-2019)
Abstract
Micro-mixers are vital components of “Lab-on-a-Chip” devices. Their main functionality is the mixing of two streams with desired quality and at minimum mixing time. In this work, numerical modelings of some active and passive micro-mixers with innovative designs are reported. Increasing mixing quality and decreasing mixing time are the design objectives. Our numerical model features solving the set of non-linear and inter-coupled Poisson-Nernst-Planck-Naiver-Stokes equations (PNP-NSE) instead of using simplified models like Poisson-Boltzmann (PB). These equations describe a more realistic model of the physics involved at continuum level by incorporating diffusion, electro-migration, and convection, which are the dominant phenomena in electro-kinetic micro-mixers especially those using AC voltage electrodes. The computations are carried out using Rayan (in-house code). The traditional Poisson-Boltzmann (PB) model relies on simplifying assumptions and is proven to lose its accuracy in complex geometries and near active electrodes. On the other hand, the PB model is much less sophisticated and therefore much less computationally expensive. One of the contributions of this research is to show that in passive micro-mixers making the obstacles smaller but more numerous increases the mixing quality (for the case studied by 13%). The other major contribution of this work is the introduction of the combination of the vertical and horizontal AC electrodes. This new design creates jets normal to the direction of the mainstream which is responsible for enhancing chaotic mixing. This results in a stable mixing quality of 99% at 2.7s.
S. Fathollahi, M.r. Tavakoli, F. Hoseinzadeh,
Volume 19, Issue 9 (9-2019)
Abstract
In the present study, a parametric study has been carried out to investigate the influence of ice accretion on the aerodynamic performance of NACA0012 airfoil through numerical simulations using FENSAP-ICE. The results reveal that at zero angle of attack the ice profile created on the leading edge of the airfoil is symmetric. The most dominant feature in the flow-field of an iced airfoil is a recirculation zone that forms due to concavity regions created on both upper and lower surfaces of the airfoil. The numerical simulations show that the appearance of the recirculation zone alters significantly the aerodynamic coefficients. At the angle of attack 12°, lift coefficient decreases by %20.58 and the drag coefficient increases by %15.92 in comparison with the clean airfoil. The effects of temperature and air flow velocity on the ice accretion created on the NACA0012 were investigated for glaze ice and rime ice. The thickness of ice increases with decreasing temperature, and glaze ice with the sharp horn is created at the temperatures ranging from 0°C to -14°C. Making the transition from glaze ice to rime ice occurs at temperatures varied from -14°C to -16°C and at temperatures below -16°C rime ice is created. In order to eliminate the ice accretion, a thermal de-icing system is simulated. By applying a heat power of 30 watts, the melting of 21.41 gr horn ice starts and the created ice on the airfoil surface is completely melted. It should be noted that with the introduction of thermal de-icing system the runback water flow on the airfoil’s surface occurs.
Me. Reyahipoor, A.r. Shafiei, S.a.m. Salehizadeh,
Volume 19, Issue 11 (11-2019)
Abstract
The material point method (MPM) is a numerical technique to modeling the large deformation and interaction between different phases of materials. MPM combines the best aspects of both Lagrangian and Eulerian formulations while avoiding some shortcomings of them. In MPM a body is modeled with the particles which carry all physical properties of the continuum such as mass, momentum, strains and stresses. The background mesh is used to solving the momentum equation. In the first phase, information is mapped from particles to nodes. In the second phase, momentum equations are solved for the nodes, and then the updated nodes are mapped to the particles to updating their positions and velocities. In the third phase the grid is reset. In numerical simulation of granular flows, large deformations and interactions between grain boundaries and buildings lead to the complexity in the structural behavior of the material and, as a result, the complexity of the simulations. From different numerical techniques, the material point method is a suitable method for simulating such problems. In this study, the problem of the collapse of a column of granular material on a rigid wall was simulated in two dimensions through material point method. The surface profile and displacement of the front were compared with the laboratory results which a good accordance is observed between them. The results show that the ratio of the initial column plays an important role in the development of granular mass.
R. Hassanzadeh, M. Mohammadnejad,
Volume 19, Issue 11 (11-2019)
Abstract
In the present research, the effects of the internal overlap ratio on the performance of a two-blade vertical axis Savonius wind turbine is investigated using numerical simulation. Considerations are performed on both conventional and Bach-type rotors. For this purpose, the power characteristics of the wind turbine are examined under tip speed ratios ranging from 0.2 to 1.2 and wind speeds of 3, 5, and 7 m/s. In order to capture the turbulence characteristics, SST k-ω model is used and the obtained results are validated against the available data in the open literature. The instantaneous behavior of flow and the time-averaged data are presented for both conventional and Bach-type rotors. The obtained results of the research reveal that for all tip speed ratios and wind speeds, the optimum overlap ratio for conventional and Bach types rotors, respectively. On the other hand, regardless of the wind speed and overlap ratio, the maximum power coefficients are obtained in tip speed ratios of 0.8 and 0.7 for conventional and Bach-type rotors, respectively. Finally, it is found that in all tip speed and overlap ratios, in both rotors, the power coefficient increases with increasing the wind speed.
M. Shakarami, A. Shanehsazzadeh, N. Shabakhty,
Volume 20, Issue 1 (1-2020)
Abstract
The New Wave theory has recently applied for predicting wave forces on marine structures including offshore wind turbines. However, the validation of the theory in determining wave force has not been fully confirmed. However, the validation of the results for predicting stability parameters of marine structures is necessary. In the present article the prediction of the New wave theory of water surface profile, wave kinematics and offshore wind turbine monopile pier responses to the wave, including base shear, overturning moment and maximum displacement are compared to the experimental data and results from linear irregular wave time series generated from the wave spectrum. The comparisons show that the results are promising and in an acceptable level of accuracy for design purposes. Since the New wave theory takes very short time of processing in compare to real irregular time series, the theory is considered as the reliable substitute for prediction of wave forces on offshore wind turbines. The comparison with the results of the conventional 5th Stokes regular waves shows that the new wave theory is significantly more accurate in predicting wave kinematics and wave loads on offshore wind turbine monopiles.
M.h. Enferadi, M.r. Ghasemi, N. Shabakhty,
Volume 20, Issue 1 (1-2020)
Abstract
Service life and safety of a steel jacket platform is influenced by vibrations generated by environmental loads, waves and winds. Vibrations of the structure and deck may cause fatigue in the structural elements and joints. Also may disrupt the operation of the drilling equipment and facilities as well as the operation of the platform. Therefore, the main aim of this research is to control the vibrations of the steel jacket platform through shape memory alloys dampers. Shape memory alloys have two important properties of shape memory as well as superelastic behavior and are quite suitable for damping applications. In these alloys, crystal structures transition from the austenite to the martensite phase, and vice versa are accompanied by the energy dissipation. In this research, a 90m steel jacket structure equipped with SMA dampers installed in 80m water depth has been modeled as a multi-degree-of-freedom system and analyzed under the time history of wave loads. For solving the differential equations of system vibration and modeling the hysteresis behavior of the shape memory alloys elements, the direct integration alpha method and multi-linear idealized constitutive model have been used, respectively. Jacket platform equipped with the shape memory alloys dampers shows the better result with 42% reduction in deck displacement, 62% reduction in deck acceleration and 32% reduction in shear force of platform base.
Behrad Alizadeh Kharkeshi, Rouzbeh Shafaghat, Omid Jahanian, Kourosh Rezanejad, Rezvan Alamian,
Volume 21, Issue 12 (12-2021)
Abstract
Wave conditions have a significant effect on the hydrodynamic behavior of OWC. As the interactions of the WEC and the incident waves are important, therefore, due to the importance of coefficients in evaluating the performance of the OWC, in this paper, the experimental evaluation of dimensionless hydrodynamic coefficients of a MC-OWC is applied. To define the experimental tests, considering the installation location of the converter on the break water, the conditions of the Caspian Sea implemented. Calibration and uncertainty analysis have performed, experimental tests have been carried out in the wave tank of the BNUT. According to the results, assuming a dimensionless water depth, with increasing dimensionless frequency of the wave, the dimensionless coefficient of transmitted wave, the dimensionless coefficient of reflected wave, dimensionless coefficient of discharge and the dimensionless coefficient of pressure increase. The results showed that due to the change of dimensionless wave number from 1.9 to 3.3, discharge coefficients, reflected wave, pressure and transmitted wave are 1.6 times, 2.2 times, 2.8 times, respectively, are 3.5 times, the dimensionless coefficient of the transmitted wave is highly sensitive to the wave conditions; the dimensional coefficient of discharge will have less changes compared to other coefficients. On the other hand, the results showed that the OWC in this study has an efficiency of 41.8% in the best case. This efficiency occurs at the dimensionless natural frequency of 0.88 and the dimensionless water intake depth of 0.032; under these conditions, the amplitude of water fluctuations inside the OWC reaches 9.6 cm.
Ali Ebrahimi, Rouzbeh Shafaghat, Mahdi Yousefifard, Ali Haji Abadi,
Volume 23, Issue 1 (12-2022)
Abstract
In this study, the effect of transverse steps location on hydrodynamic components and the longitudinal stability of the vessel has been investigated. The vessel studied in this research is a planning catamaran, each demi-hull with two transverse steps. At first, vessel resistance with a weight of 5.3 kg within a range of length Froude number of 0.49 to 2.9 in calm water has been calculated. Then, craft behavior was evaluated at displacements of 5.3, 4.6, and 4 Kg using the numerical method. The numerical simulation results have been validated with similar experimental results. The craft in 4 and 5.3 kg weights, in Froude numbers greater than 2.43 and 2.9, respectively, has a Porpoising instability. In order to improve the longitudinal stability of the vessel, the Taguchi test design has been used to determine the optimal location of the transverse steps. The results showed that by placing the transverse steps in the optimum location, the Porpoising instability in the vessel has been resolved. In planing mode, vessel resistance decreased by 12%, 9.5%, and 6.6% in the optimum state of transverse steps compared to the base state for the mentioned weights. In similar conditions, the lift force on the vessel increased by 15, 10, and 7 percent for the mentioned weights, respectively.
Mohammad Saadatbakhsh, Sadegh Sadeghzadeh,
Volume 24, Issue 5 (4-2024)
Abstract
Superhydrophobic surfaces have gained significant attention as a promising approach for drag reduction of submerged objects. Accurate evaluation and prediction of drag reduction induced by these surfaces require expensive experimental measurements, numerical simulations, or the development of reliable models and correlations. In this paper, a model is proposed for calculating the skin friction coefficient and drag reduction of superhydrophobic flat surfaces. Utilizing previous data on the skin friction coefficient of flat surfaces under no-slip boundary conditions, a model is developed to estimate the skin friction reduction and skin friction coefficient of these surfaces after applying superhydrophobic coatings. The validity of the model is verified by comparing its results with those of computational fluid dynamics (CFD) simulations of flow over a flat plate at different velocities. The results of the model and simulations indicate that for inlet velocities of 1, 5, and 25 m/s and a slip length of 50 μm, drag reductions of 15%, 41%, and 77%, respectively, are expected. Additionally, the skin friction reduction increases with increasing flow Reynolds number. The developed model is validated for flat surfaces and its ability to accurately estimate the skin friction coefficient and drag force of these surfaces is thoroughly examined. However, further investigations are required to assess the model's validity for curved surfaces and variable slip lengths.
Mohammad Ali Jahangiri, Reza Attarnejad, Nima Noei,
Volume 24, Issue 8 (7-2024)
Abstract
This research focuses on topology optimization of fluid-structure interaction (FSI) problems using the level set method. To couple the fluid and structure equations, the Arbitrary Lagrangian-Eulerian (ALE) description is employed within a monolithic formulation. The use of ALE in FSI problems, while eliminating numerical instabilities caused by the convective term, enhances the speed and accuracy of finite element solutions in fluid-structure interaction. Additionally, considering the fluid in the unsteady state allows for the interpretation of optimal topology at any given moment of the analysis. The objective function of the optimal topology design problem is to minimize the structural compliance in the dry state, subject to a fixed volume of the design domain. To determine the normal velocity in the reaction-diffusion equation (RDE), adjoint sensitivity analysis based on pointwise gradients is used. The results obtained from this approach, compared to other topology optimization methods in the literature, demonstrate higher accuracy and clearer definition of structural boundaries.
Saeed Asil Gharebaghi, Mohammad Shirzad,
Volume 24, Issue 9 (8-2024)
Abstract
Vortex-induced vibration is a critical phenomenon that occurs in offshore structures and causes fatigue and damage to these structures. Previous studies show that these structures show complex and sometimes non-linear behavior. This study uses a rigid cylinder with non-linear support to evaluate the hydrodynamic parameters in these systems. The amount of non-linearity of the support has been changed, and its effect on the hydrodynamic parameters of the system has been investigated. The displacement and velocity of the cylinder were obtained by solving the two-dimensional Reynolds averaged Navier-Stokes and cylinder motion equations. The lift, potential, and vortex coefficients were calculated. Finally, the Strouhal number was determined. The results show that the system's behavior consists of two branches. In branch 1, the motion amplitude of the cylinder is small, but in branch 2, its amplitude is multiplied. By increasing the non-linearity of the support, the range of branch 2 becomes smaller, and the velocity of the cylinder oscillation increases. Raising the support non-linearity reduces the lift force and Strouhal number
