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Browsing by Author Ghassemi, Hassan: 51 Scientific Journals of the Maritime University of Szczecin

  • Nourghasemi, Hadi; Bakhtiari, Mohammad; Ghassemi, Hassan (Scientific Journals Maritime University of Szczecin, Zeszyty Naukowe Akademia Morska w Szczecinie, )
    One of the most effective methods to diminish the drag of a planing craft is to use a step at the bottom of the hull. A stepped hull causes a reduction of the wetted area and, as a result, a decrease in the drag. The step may be designed as a straight line through the entire width of the hull or may be V-shaped with a forward or backward swept angle. In this paper, the effects of the step forward swept angle on the hydrodynamic performance of a hard chine planing vessel are investigated by finite volume method (FVM). Reynolds-Averaged Navier Stokes (RANS) equations with a standard k-ε turbulence model coupled with volume of fluid (VOF) equations are solved in order to simulate a transient turbulent free surface flow around the hull with the help of Ansys CFX software. In order to predict hull motions, equations of rigid body motions for two degrees of freedom (2-DOF) are coupled with fluid flow governing equations. To validate the presented numerical model, first the numerical results are compared with available experimental data, and then the obtained numerical results of the drag, dynamic trim, sinkage, wetted keel length, wetted chine length, pressure distribution on the hull, wetted surface and wake profile at different Froude numbers and step angles are presented and discussed.

Other publications

  • Safaei, Aliakbar; Ghassemi, Hassan; Ghiasi, Mahmoud (Scientific Journals Maritime University of Szczecin, Zeszyty Naukowe Akademia Morska w Szczecinie, )
    In line with the ambition of ship-owners to preserve capital and reduce operational costs, the selection of an optimum, safe and secure route for an envisaged voyage has always been a challenge for ship-owners, masters, and engineers. Due to the many complexities and parameters that affect the selection of an optimal route, the topic has become very interesting to many researchers. Each of the parameters affecting the process of route selection has its own values and weights, and these values change depending on specific situations and objectives. This sensitivity to context increases the difficulty selecting the optimum route. In this research, the optimization of a tanker-sized VLCC voyage for predefined and different routes is addressed in order to identify the optimum route. To reduce the number of variable parameters, major values have been assigned to the ship profile and sea conditions. The time domain analysis and the solution of equations of motion are then performed. The proposed route is designed by using a Bezier curve, and this route is then optimized with the objective of decreasing fuel consumption using the Fletcher-Powell method. The resulting optimized route shows a 3.7% savings in fuel consumption
  • Razaghian, Amir Hossein; Ghassemi, Hassan (Scientific Journals Maritime University of Szczecin, Zeszyty Naukowe Akademia Morska w Szczecinie, )
    This paper investigates the open-water characteristics of the 5-blade propeller with accelerating and deceler- ating ducts using the Reynolds-Averaged Navier-Stokes (RANS) equation code. In the first step, numerical open-water hydrodynamic characteristics of the propeller in the absence of a duct were validated using the available experimental data. The shear stress transport (SST) turbulence model was chosen, which shows less error in thrust and torque coefficients than others. In the second step, two accelerating and decelerating ducts, namely ducts 19A and N32, were modeled. In these simulations, the clearance value was selected at 3 percent of the propeller’s diameter and uniform-flow conditions were assumed. After analysis of the mesh sensitivity for the propeller thrust, the results were compared to the corresponding open-water condition values. In this regard, results of the hydrodynamic coefficients, pressure distribution, and coefficients on the propeller-blade surface and ducts were also analyzed and discussed.
  • Mahmoodi, Kumars; Ghassemi, Hassan; Nowruzi, Hashem (Scientific Journals Maritime University of Szczecin, Zeszyty Naukowe Akademia Morska w Szczecinie, )
    Ocean wave energy is known as a renewable energy resource with high power potential and without negative environmental impacts. Wave energy has a direct relationship with the ocean’s meteorological parameters. The aim of the current study is to investigate the dependency between ocean wave energy flux and meteorological parameters by using data mining methods (DMMs). For this purpose, a feed-forward neural network (FFNN), a cascade-forward neural network (CFNN), and gene expression programming (GEP) are implemented as different DMMs. The modeling is based on historical meteorological and wave data taken from the National Data Buoy Center (NDBC). In all models, wind speed, air temperature, and sea temperature are input parameters. In addition, the output is the wave energy flux which is obtained from the classical wave energy flux equation. It is notable that, initially, outliers in the data sets were removed by the local distribution based outlier detector (LDBOD) method to obtain the best and most accurate results. To evaluate the performance and accuracy of the proposed models, two statistical measures, root mean square error (RMSE) and regression coefficient (R), were used. From the results obtained, it was found that, in general, the FFNN and CFNN models gave a more accurate prediction of wave energy from meteorological parameters in the absence of wave records than the GEP method.
  • Zakerdoost, Hassan; Ghassemi, Hassan (Scientific Journals Maritime University of Szczecin, Zeszyty Naukowe Akademia Morska w Szczecinie, )
    Hydrodynamic shape optimization plays an increasingly important role in the shipping industry. To optimize ship hull and propeller shapes for minimum total (friction+wave) calm-water resistance and maximum open water efficiency, respectively, the main particulars of a hull and propeller model are considered as design variables. The optimization problem is performed by using an integrated hull-propeller system optimization problem (HPSOP) code in a multi-level and multi-point methodology in early-stage ship design. Three numerical methods with variable fidelity are employed to carry out the hydrodynamic performance analysis of a ship’s hull and propeller. A ship and its propeller are selected as initial models to illustrate the effectiveness of the proposed optimization procedure. The numerical results show that the developed technique is efficient and robust for hydrodynamic design problems.
  • Ghassemi, Hassan; Ghafari, Hamidreza; Homayoun, Esmaeil (Scientific Journals Maritime University of Szczecin, Zeszyty Naukowe Akademia Morska w Szczecinie, )
    This current work investigates the effect of duct and number of blades on the hydrodynamic performance of the horizontal axis tidal stream turbine (HATST). The numerical method based on Reynolds averaged Navier- Stokes (RANS) equations is employed to compare the hydrodynamic performance for various cases of this device. For validation of the numerical results, a 3-blade HATST without-duct has been compared against experimental data. The analysis and comparison of the simulation results show that using duct for HATST has increased the power coefficient, the torque coefficient, the trust coefficient, and the force on the blade. In addition, the simulation results of the cases with a greater number of blades shows that the trust coefficient increased and the force on the blade decreased. Therefore, it is recommended to use ducted HATST with a great number of blades to extract more energy from the tidal stream.
  • Ghassemi, Hassan (Scientific Journals Maritime University of Szczecin, Zeszyty Naukowe Akademia Morska w Szczecinie, )
    Propellers usually operate in the ship’s stern, where the inflow of the non-uniform wake generates oscillating loads and changes the hydrodynamic performance. Therefore, determination of the forces on propellers and hydrodynamic performance due to a non-uniform wake field are the challenging problems for naval architects and hydrodynamists. The main objectives of the present study are to assess the hydrodynamic performance for a single blade and all the blades. The propeller is a B-series propeller under non-uniform wake field behind the Seiun-Maru (hereafter SM) ship hull. A practical approach is employed to calculate the hydrodynamic oscillating loads of the ship propeller under a non-uniform wake field. Results of the computations on the propeller behind the SM ship, due to a non-uniform wake field, are presented and analyzed using classical mathematical methods over a single cycle. The results show that a variation of thrust with the discussed parameters is the same as that shown for torque, also the blade-frequency of the total force, thrust and torque is an increasing function of radial sections, whereas these parameters decrease with increasing radial blade sections.
  • Abbasi, Alireza; Ghassemi, Hassan; Molyneux, David (Scientific Journals Maritime University of Szczecin, Zeszyty Naukowe Akademia Morska w Szczecinie, )
    The purpose of this research was to investigate the power and thrust coefficients of a horizontal axis tidal stream turbine (HATST) with different blade geometries, including twist angles, blade numbers, and section profiles. The RANS equations and Star-CCM+ commercial software were used to numerically analyze these variables. Furthermore, the turbulence model used in this study is a Realisable k-ε turbulent model. Nine different models were defined by changing the twist angle, thickness, camber, and blade numbers. The results are presented, and the power and thrust coefficients are compared against TSR for each of the nine different models. The pressure distribution and flow velocity contour are also presented and discussed.
  • Firouzi, Jabbar; Ghassemi, Hassan; Vakilabadi, Karim Akbari (Scientific Journals Maritime University of Szczecin, Zeszyty Naukowe Akademia Morska w Szczecinie, )
    A propeller shaft generally experiences three linear forces and three moments, the most important of which are thrust, torque, and lateral forces (horizontal and vertical). Thus, we consider 4DOFs (degrees of freedom) of propeller shaft vibrations. This paper is presented to obtain the vibration equations of the various coupled vibrations of the propeller shaft at the stern of a ship (including coupled torsional-axial, torsional-lateral, axial- lateral, torsional-axial, and lateral vibrations). We focused on the added hydrodynamic forces (added mass and added damping forces) due to the location of the propeller behind the ship. In this regard, the 4DOFs of the coupled vibration (torsional-longitudinal and lateral vibrations in the horizontal and vertical directions) equations of shaft and propeller systems located behind a ship were extracted with and without added mass and damping forces. Also, the effect of mass eccentricity was considered on vibrations occurring at the rear of the ship.
  • Motallebi, Mohammad; Ghafari, Hamidreza; Ghassemi, Hassan; Shokouhian, Mehdi (Scientific Journals Maritime University of Szczecin, Zeszyty Naukowe Akademia Morska w Szczecinie, )
    In this paper, the second-order hydrodynamic force on fixed and floating tandem cylinders has been calculated and different parameters have been taken into consideration. An incident wave is diffracted by the fixed cylinder, and as a result low-frequency waves radiate toward the floating cylinder and cause low-frequency second-order hydrodynamic forces to act on the surface of the floating cylinder. The interactions between the fixed and floating cylinders have been investigated by changing the distance between them, as well as the draft and radius of the floating cylinder. By employing perturbation series analysis over the wetted surface, the second-order wave excitation force has been calculated. The maximum force applied on the floating cylinder becomes non-dimensional when considering it with and without the fixed cylinder. The results showed the effect that the existence of the fixed cylinder had on the increase in the second-order forces is quite evident where, for a significant parameter of the floating cylinder, the force in the heave direction was enhanced by up to 1.55 times.

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