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CONTENTS
Volume 7, Number 4, July 2020
 

Abstract
In this paper, a numerical method is utilized to study the effect of a new vibration absorber on vibration response of the stiffened functionally graded (SFG) cylindrical shell under a couple of axial and transverse compressions. The material composition of the stiffeners and shell is continuously changed through the thickness. The vibration absorber consists of a mass-spring-damper system which is connected to the ground utilizing a linear local damper. To simplify, the spring element of the vibration absorber is called global potential. The von Kármán strain-displacement kinematic nonlinearity is employed in the constitutive laws of the shell and stiffeners. To consider the stiffeners in the model, the smeared stiffener technique is used. After obtaining the governing equations, the Galerkin method is applied to discretize the nonlinear dynamic equation of system. In order to find the nonlinear vibration responses, the fourth order Runge-Kutta method is utilized. The influence of the stiffeners, the dynamic absorber parameters on the vibration behavior of the SFG cylindrical shell is investigated. Also, the influences of material parameters of the system on the vibration response are examined.

Key Words
SFG cylindrical shell; dynamic absorber; nonlinear vibration response; compression loading

Address
Kamran Foroutan and Habib Ahmadi: Faculty of Mechanical Engineering, Shahrood University of Technology, Shahrood, Iran

Abstract
The proposed study aims to numerically investigate the performance of hydrofoils in the context of amphibious aircraft application. In particular, we also study the effectiveness of a slotted hydrofoil in minimizing the cavitation phenomenon, to improve the overall water take-off performance of an amphibious aircraft. We use the ICON A5 as a base model for this study. First, we propose an approach to estimate the required hydrofoil surface area and to select the most suitable airfoil shape that can minimize cavitation, thus improving the hydrodynamic efficiency. Once the hydrofoil is selected, we perform 2D numerical studies of the hydrodynamic and cavitating characteristics of a non-slotted hydrofoil on ANSYS Fluent. In this work, we also propose to use a slotted hydrofoil to be a passive method to control the cavitation performance through the boundary layer control. Numerical results of several slotted configurations demonstrate notable improvement on the cavitation performance. We then perform a multiobjective optimization with a response surface model to simultaneously minimize the cavitation and maximize the hydrodynamic efficiency of the hydrofoil. The optimization takes the slot geometry, including the slot angle and lengths, as the design variables. In addition, a global sensitivity study has been carried and it shows that the slot widths are the more dominant factors.

Key Words
slotted hydrofoil; cavitation inhibition; passive flow control; RSM optimization

Address
Fernando Roca Conesa: Universitat Politècnica de València, Camino de Vera s/n, Valencia 46022, Spain

Rhea Patricia Liem: Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong


Abstract
Thermal buckling of functionally graded sandwich cylindrical shells is presented in this study. Material properties and thermal expansion coefficient of FGM layers are assumed to vary continuously through the thickness according to a sigmoid function and simple power-law distribution in terms of the volume fractions of the constituents. Equilibrium and stability equations of FGM sandwich cylindrical shells with simply supported boundary conditions are derived according to the Donnell theory. The influences of cylindrical shell geometry and the gradient index on the critical buckling temperature of several kinds of FGM sandwich cylindrical shells are investigated. The thermal loads are assumed to be uniform, linear and nonlinear distribution across the thickness direction. An exact simple form of nonlinear temperature rise through its thickness taking into account the thermal conductivity and the inhomogeneity parameter is presented.

Key Words
FGM sandwich cylindrical shells; thermal buckling; nonlinear temperature rise; Donnell theory

Address
Ahmed Amine Daikh: 1.) Structural Engineering and Mechanics of Materials Laboratory, Department of Civil Engineering, Mascara, Algeria
2.) Mechanics of Structures and Solids Laboratory, Faculty of Technology, University of Sidi Bel Abbes, Algeria


Abstract
This paper presents the flutter analysis and optimum design of axially functionally graded box beam cantilever wing section by considering various geometric and material parameters. The coupled dynamic equations of the continuous model of wing system in terms of material and cross-sectional properties are formulated based on extended Hamilton\'s principle. By expressing the lift and pitching moment in terms of plunge and pitch displacements, the resultant two continuous equations are simplified using Galerkin\'s reduced order model. The flutter velocity is predicted from the solution of resultant damped eigenvalue problem. Parametric studies are conducted to know the effects of geometric factors such as taper ratio, thickness, sweep angle as well as material volume fractions and functional grading index on the flutter velocity. A generalized surrogate model is constructed by training the radial basis function network with the parametric data. The optimized material and geometric parameters of the section are predicted by solving the constrained optimal problem using firefly metaheuristics algorithm that employs the developed surrogate model for the function evaluations. The trapezoidal hollow box beam section design with axial functional grading concept is illustrated with combination of aluminium alloy and aluminium with silicon carbide particulates. A good improvement in flutter velocity is noticed by the optimization.

Key Words
aeroelastic tailoring; axial functional grading; cantilever wing; flutter boundary; firefly algorithm

Address
L. Prabhu : Department of Aerospace Engineering, Lakireddy Bali Reddy College of Engineering, Mylavaram, Krishna District, Andhra Pradesh – 521230, India

J. Srinivas: Department of Mechanical Engineering, National Institute of Technology, Rourkela, Odisha – 769008, India


Abstract
A forward-facing aerospike attached to a payload fairing of a satellite launch vehicle significantly alters its flowfield and decreases the aerodynamic drag in transonic and low supersonic speeds. The present payload fairing is an axisymmetric configuration and consists of a blunt-nosed body along with a conical section, payload shroud, boat tail and followed by a booster. The main purpose of the present numerical simulations is to evaluate flowfield and assess the performance of aerodynamic drag coefficient with and without aerospike attached to a payload fairing of a typical satellite launch vehicle in freestream Mach number range 0.8 ≤ M∞ ≤ 3.0 and freestream Reynolds number range 33.35 x 10⁶/m ≤ Re∞ ≤ 46.75 x 10⁶/m which includes the maximum aerodynamic drag and maximum dynamic conditions during ascent flight trajectory of the satellite launch vehicle. A numerical simulation has been carried out to solve time-dependent compressible turbulent axisymmetric Reynolds-averaged Navier-Stokes equations. The closure of the system of equations is achieved using the Baldwin-Lomax turbulence model. The aerodynamic drag reduction mechanism is analysed employing numerical results such as velocity vector plots, density and Mach contours in conjunction with the experimental flow visualization pictures. The variations of wall pressure coefficient over the payload fairing with and without aerospike are exhibiting different kind of flowfield characteristics in the transonic and low supersonic speeds. The numerically computed results are compared with schlieren pictures, oil flow patterns and measured wall pressure distributions and exhibit good agreement between them.

Key Words
aerodynamics drag coefficient; aerospike; blunt-nose body; payload fairing; CFD simulation; fluid dynamics; transonic flow; supersonic flow; shock wave; satellite launch vehicle

Address
R. C. Mehta: Department of Aeronautical Engineering, Noorul Islam Centre for Higher Education, Kumaracoil 629180, India


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