Sharif Digital Repository

In this study, natural super-cavitating flow around three different conical cavitators with wedge angles of 30°, 45° and 60° is investigated. We apply the k−ϵ turbulence model and the volume of fluid (VOF) technique to numerically study the three-dimensional cavitating flow around the cavitators. The turbulence approach is coupled with a mass transfer model which is implemented into the finite-volume package. Simulations are performed for different cavitation numbers. Finally, the effects of some important parameters such as the cavitation index, inlet velocity, Froude number and wedge angle of cavitators on the geometrical characteristics of the super-cavities are discussed. Our numerical... 

Two types of conventional rubber modifiers and a series of hollow plastic micro-spheres were employed as toughening agents in a diglycidyl ether of bisphenol A (DGEBA) epoxy in Part I (Bagheri R, Pearson RA. Polymer 1996;37:4529) of this study. It was found that the rubber modifiers with different cavitation resistance and hollow plastic micro-spheres which act as pre-existing microvoids toughen epoxies in the same manner. The current study is composed to further examine the previous results in terms of the role inter-particle distance in rubber/microvoid toughened epoxies. It is shown that the fracture toughness in toughened blends goes through a ductile-to-brittle transition with... 

This paper introduces a new idea of controlling cavitation around a hydrofoil through a passive cavitation controller called artificial cavitation bubble generator (ACG). Cyclic processes, namely, growth and implosion of bubbles around an immersed body, are the main reasons for the destruction and erosion of the said body. This paper aims to create a condition in which the cavitation bubbles reach a steady-state situation and prevent the occurrence of the cyclic processes. For this purpose, the ACG is placed on the surface of an immersed body, in particular, the suction surface of a 2D hydrofoil. A simulation was performed with an implicit finite volume scheme based on a SIMPLE algorithm... 

Unsteady cloud cavitation phenomenon is an important subject due to its undesirable effects in various applications such as ship propeller, rudder and hydraulic machinery systems. We present an efficient passive control method to control the cavitation instabilities which may be caused by the shedding of cavity structures in the vicinity of the solid surface of an immersible body. We proposed a passive control method so called Cylindrical Cavitating-bubble Generators (CCGs) on the surface of a benchmark hydrofoil and analyzed the effects of this passive controller on the dynamics of the unsteady cloud cavitation. First we modeled the unsteady cavitating flow around the hydrofoil without CCGs... 

In this work, numerical simulation is used to study the stability enhancement of high speed supercavitating hydrofoils.Although supercavitation is known as one of the most effective methods for drag reduction, producing the cavity, either by ventilation or by cavitator at front of the body, may cause some instabilities on cavity surface and thus on the projectile's motion. Therefore removing these instabilities comes as an important point of discussion. First of all, we calculate the sources of instabilities and measure respective forces and then present some approaches that significantly reduce these instabilities. One of these methods that could produce more stable supercavities is... 

In this paper, the hydrodynamic performance, cavitation inception, and acoustic performance of a 5-bladed B-series propeller are experimentally investigated in the Sharif University of Technology cavitation tunnel. For hydrodynamic performance, thrust, torque, and efficiency of the propeller model were measured in open water tests. In the cavitation study, in addition to blade cavitation, tip vortex cavitation was also observed, and the cavitation bucket diagram was extracted in different advance coefficients. Finally, for the acoustic analysis of the propeller, the emitted noise in the cavitation tunnel was recorded using two hydrophones according to the recommended procedures of the... 

We present an efficient method to control the evolution of unsteady cloud cavitation around the CAV2003 benchmark hydrofoil using passive cavitation controllers so called cavitation-bubble generators (CGs). Cavitation control may be used in many engineering applications, particularly in the marine and turbo machinery field. We first simulated the unsteady cavitating flow around the hydrofoil without CGs using a Partially-averaged Navier–Stokes (PANS) method, and validated the acquired results against experimental data. We coupled the turbulence model with a mass transfer model and successfully implemented it in the open source toolbox OpenFOAM. Next, we studied the effect of different CGs on... 

In this study, the nodal discontinuous Galerkin method is formulated in three-dimensions and applied to simulate three-dimensional non-cavitating/cavitating flows. For this aim, the three-dimensional preconditioned Navier-Stokes equations based on the artificial compressibility approach considering appropriate source terms to model cavitating phenomena are used. The spatial derivative terms in the resulting equations are discretized by utilizing the nodal discontinuous Galerkin method on tetrahedral elements and the derivative of the solution vector with respect to the artificial time is discretized by applying an explicit time integration method. An artificial viscosity method is formulated... 

In this study, the hydrodynamics and noise prediction of a five blade marine propeller were analyzed through numerical and experimental methods under variety operational conditions. The hydrodynamics of the propeller was studied and the characteristic curves were presented in both numerical and experimental methods. Inception and development of sheet cavitation conditions are obtained in both numerical and experimental methods. The cavitation was started and developed by either increasing the propeller rotational speed in constant pressure or decreasing pressure, while the velocity was kept constant. Good agreements are observed between numerical and experimental results, qualitatively and... 

Cavitation behavior of fully submerged propellers is an important issue in the marine engineering field. Cavitation tunnel is usually used for studying hydrodynamic performances and cavitation behavior of a propeller. In the present study, the cavitation behavior of the DTMB P4119 propeller was studied both in a uniform flow and in the wake field of a specific vessel. Considering the dimensions of the test section of K23 cavitation tunnel of Sharif University of Technology and related blockage effects, the propeller was scaled so that the model was 0.195 (m) in diameter. A wake screen was used to generate the desired wake field, which itself was primarily determined by CFD simulations around... 

In this work, a high-order nodal discontinuous Galerkin method is applied and assessed for the simulation of compressible non-cavitating and cavitating flows. The one-fluid approach with the thermal effects is used to properly model the cavitation phenomenon. Here, the spatial and temporal derivatives in the system of governing equations are discretized using the nodal discontinuous Galerkin method and the third-order TVD Runge–Kutta method, respectively. Various numerical fluxes such as the Roe, Rusanov, HLL, HLLC and AUSM+-up and two discontinuity capturing methods, namely, the generalized MUSCL limiter and a generalized exponential filter are implemented in the solution algorithm. At... 

The present study focuses on the simulation of two dimensional unsteady cavitating flows. For simulation of unsteady behaviors of cavitation which have practical applications, the development of unsteady PISO algorithm based on the non-conservative approach is utilized. For multi-phase simulation, single-fluid Navier-Stokes equations, along with the volume fraction transport equation, are employed. The bubble dynamics model is utilized to simulate phase change between vapor and liquid phases of the water. Unsteady simulation of cavitation around NACA66(MOD) and supercavitation around a flat plate normal to flow direction are performed to clarify accuracy of presented model. Numerical results... 

In this work, deformation behavior of AA2017-T4 at elevated temperatures was studied employing uni-axial tensile and creep experiments. Tensile tests were carried out at temperatures varying 150–500 °C under different strain rates then, a combination of neural network and dynamic material modeling was utilized to construct the processing maps. Furthermore, creep experiments were conducted to assess inelastic deformation behavior of the alloy at temperatures between 150 and 225 °C and stresses in the range of 150 to 230 MPa. Microstructural evaluations were carried out for determination of microstructural changes and formation of voids and cavities within the samples. The results showed that... 

In this study, transient 2D/axisymmetric simulations of cavitating flows are performed using a modified 'Volume-of-Fluid' (VOF) technique. Simulation of the cavitation is based on a homogenous equilibrium flow model. To predict the shape of the cavity, the Navier-Stokes equations in addition to an advection equation for the liquid volume fraction are solved. Mass transfer between the phases is treated as a sink term in the VOF equation. The numerical method is used for different geometries in a wide range of cavitation numbers. Computed shapes of cavities were found to be in good agreement with those of the reported experiments. The simulation results also compared well with those obtained... 

In last decades, rubber-toughened polymer blends have been the object of considerable interest by many investigators, owing to their attractive mechanical as well as physical properties. Polypropylene (PP) is a type of polyolefin which can be toughened using rubber particles. In another paper [J. Appl. Polym. Sci., submitted for publication], the role of ethylene-propylene (EPR) particles on the deformation mechanism during tensile tests and the fracture mechanism under quasi-static loading tests has been investigated. This paper is instead focused on the role of EPR particles on the fracture mechanism under dynamic loading (impact properties) of EPR/PP blends. Blends with different weight... 

Supercavitation is known as the way of viscous drag reduction for the projectiles, moving in the liquid phase. In recent works, there is distinct investigation between cavitation flow and momentum transfer far away from the cavity surface. In fact such methodologies consider cavitation flow statically, rather than taking dynamic effects of overall flow into account. However, it seems that there is strong connection between overall flow and what takes place in the sheet cavity where a constant pressure distribution is assumed. Thereby, in order to configure the system conditions which may be cause of cavity perturbation and so system oscillation, we need to use proper methodologies in which... 

Supercavitation is known as the way of viscous drag reduction for the projectiles, moving in the liquid phase. In recent works, there is distinct investigation between cavitation flow and momentum transfer far away from the cavity surface. In fact such methodologies consider cavitation flow statically, rather than taking dynamic effects of overall flow into account. However, it seems that there is strong connection between overall flow and what takes place in the sheet cavity where a constant pressure distribution is assumed. Thereby, in order to configure the system conditions which may be cause of cavity perturbation and so system oscillation, we need to use proper methodologies in which... 

Supercavitation is known as the way of viscous drag reduction for the projectiles, moving in the liquid phase. In recent works, there is distinct investigation between cavitation flow and momentum transfer far away from the cavity surface. However, it seems that there is strong connection between overall flow and what takes place in the sheet cavity where a constant pressure distribution is assumed. Furthermore as we'll see, pressure distribution on cavity surface caused due to overall conditions, induct nonaxisymetric forces and they may need to be investigated. Primarily we describe how pressure distribution into the cavity can cause separation of the aero boundary layer. Then we present... 

Cavitating flow is investigated around marine propellers, experimentally and numerically. Two different types of conventional model propellers are used for the study. The first one is a four bladed model propeller, so called model A, and the second one is a three bladed propeller, model B. Model A is tested in different cavitation regimes in a K23 cavitation tunnel. The results are presented in characteristic curves and related pictures. Finally, the results are discussed. Model B is investigated based on existing experimental results. In addition, model B is used for validation of the numerical solution prior to the testing of model A. The cavitation phenomenon is predicted numerically on a...