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CONTENTS
Volume 8, Number 3, May 2021
 


Abstract
Flutter is a dangerous phenomenon encountered in flexible structures subjected to aerodynamic forces. This includes aircraft, helicopter blades, engine rotors, buildings and bridges. Flutter occurs as a result of interactions between aerodynamic, stiffness, and inertia forces on a structure. The conventional method for designing a rotor blade to be free from flutter instability throughout the helicopter's flight regime is to design the blade so that the aerodynamic center (AC), elastic axis (EA) and center of gravity (CG) are coincident and located at the quarter-chord. While this assures freedom from flutter, it adds constraints on rotor blade design which are not usually followed in fixed wing design. Periodic Structures have been in the focus of research for their useful characteristics and ability to attenuate vibration in frequency bands called "stop-bands". A periodic structure consists of cells which differ in material or geometry. As vibration waves travel along the structure and face the cell boundaries, some waves pass and some are reflected back, which may cause destructive interference with the succeeding waves. In this work, we analyze the flutter characteristics of helicopter blades with a periodic change in their sandwich material using a finite element structural model. Results shows great improvements in the flutter rotation speed of the rotating blade obtained by using periodic design and increasing the number of periodic cells.

Key Words
aeroelastic; finite element; flutter; helicopter; hovering; periodic structure; rotor, vibration

Address
Hossam T. Badran: 1.)Arab Organization for Industrialization, Cairo 11421, Egypt
2.) Aerospace Engineering Department, Cairo University, Giza 12613, Egypt

Mohammad Tawfik : Academy of Knowledge, Cairo 11765, Egypt

Hani M. Negm: Aerospace Engineering Department, Cairo University, Giza 12613, Egypt

Abstract
The perspective of hydrogen (H₂ ) aviation is discussed. While production of carbon dioxide (CO₂ ) free renewable H₂ is progressing towards costs comparable to those of today's steam reforming of methane, at about 1-1.5 $ per kg H₂ , the development of specific aviation infrastructure, as well as aircraft, is still in its infancy. Over the 21 years of this century, the most important manufacturers have only proposed preliminary studies, artist impressions more than detailed engineering studies. The major technical challenge is the fueling and safe and efficient storage of the H₂ , requiring a complete redesign of infrastructure and aircraft. From a political perspective, negative is the speculation on the global warming potential of contrails, and the covid19 pandemic which has largely disrupted the aviation sector, with the future of mass transport at risk of drastic downsizing. Especially the "great reset" agenda, limiting mass transport also for other goals than the simple control of viral spreading during the pandemic, may harm the deployment of H₂ aviation, as elite aviation does not motivate huge investments in the use of a non-exhaustible fuel such as renewable H₂ , leaving favored alternatives such as hydrocarbon jet fuels, and in the longer term electric aviation.

Key Words
aviation; CcH₂; CO₂ emission; H₂O emission hydrogen; LH₂

Address
Alberto Boretti: Deanship of Research, Prince Mohammad Bin Fahd University P.O. Box 1664.,Al Khobar 31952. Kingdom of Saudi Arabia


Abstract
The paper presents the analytical solutions for thick orthotropic laminated plates using trignometric shear deformation theory. The effects transverse shear and transverse normal strains are included with linear and nonlinear thermal loads. The displacement field of the theory includes the trigonometric functions in thickness coordinate of plate to account for these effects. The displacement field enforces to give the realistic variation of shear stresses across the thickness of plate and thus obviates the need of shear correction factor. The main novelty of the present study is the inclusion of thickness stretching effect in the theory. Another novelty is the application of nonlinear thermal profile consistent with the displacement field of the theory. The principle of virtual work is used to obtain the governing equations and boundary conditions. Simply supported laminated square plates are considered for numerical study to evaluate thermoelastic response. The results obtained by present theory with thickness stretching effect are compared with other refined theories disregarding this effect. It is observed that the results of present theory deviate significantly from the results of other higher order shear deformation theories for antisymmetric crossply laminated plates. The results of symmetric cross-ply laminated plates subjected to linear sinusoidal thermal load are in close agreement with those of exact theory, which validates the accuracy of present shear and normal deformation theory.

Key Words
orthotropic plates; principle of virtual work; shear correction factor; thermoelastic analysis

Address
Sandhya K. Swami: Department of Civil Engineering, Marathwada Institute of Technology,Aurangabad - 431005, Maharashtra, India

Yuwaraj M. Ghugal: Department of Applied Mechanics, Government College of Engineering, Karad-415124, Maharashtra, India


Abstract
A helmet is a kind of shielding equipment used to shield the head from fatal injuries. The helmet experiences drag while moving at a certain velocity. The total drag on the helmet increases with an increase in velocity. The drag force at high velocity has a significant effect on the rider's neck and may result in cervical spondylosis. Now a day's neck pain, neck sprain, spondylosis have become significant issues related to the human body. The reduction of drag on the helmet will be a boon for society, which will reduce the force on the neck. The decrease in drag is an essential field of study. The drag force can be reduced by various methods like coating on the surface, modifying the helmet's shape, etc. The study's purpose is to decrease drag on the helmet by improving the helmet's shape. The CFD analysis is carried out to find the best profile of the helmet and fineness ratio of the boat-tailed helmet to minimize drag. The CFD results are validated with the wind tunnel laboratory outcomes. Based on the findings, a considerable reduction in the drag is accomplished at the velocity of 32.5 m/s.

Key Words
boat-tail; CFD; drag; helmet

Address
Khizar A. Pathan, Aadil N. Shaikh and Arsalan A. Pathan: Department of Mechanical Engineering, Trinity College of Engineering and Research, Pune, Maharashtra, 411048, India

Sher A. Khan: Department of Mechanical Engineering, Faculty of Engineering, International Islamic University Malaysia, Kuala Lumpur, Selangor, Malaysia

Shahnawaz A. Khan: Oil and Gas Division, Siemens Energy, Pune, Maharashtra, 411013, India

Abstract
This study presents an efficient computational methodology to perform ablative thermal response analysis of carbon/phenolic composites by introducing a novel dual-domain technique for heat transfer and gas diffusion physics. Phenomena such as in-depth heat transfer, material decomposition (i.e. pyrolysis), in-depth gas diffusion, and surface recession required for ablation analysis of carbon/phenolic composites are simulated. The proposed method is verified with reference simulation test data from Ablation Workshop for a one-dimensional model under four different combinations with surface heat flux, temperature, pressure boundary conditions, and surface recession conditions verified. A two-dimensional ablation problem was also solved, showing its scalability. Temperatures, recession depth, depth of boundaries between layers, the mass flux of char, and pyrolysis gas are obtained and compared with the reference for all cases.

Key Words
charring ablation; carbon/phenolic composite; finite element analysis; thermal protection system

Address
Taehoon Park and Kang-Hyun Lee: Department of Aerospace Engineering, Seoul National University, Gwanak-gu Gwanak-ro 1 Seoul 08826, South Korea

Gun Jin Yun: 1.) Department of Aerospace Engineering, Seoul National University, Gwanak-gu Gwanak-ro 1 Seoul 08826, South Korea
2.) Institute of Advanced Aerospace Technology, Seoul National University, Gwanak-gu Gwanak-ro 1, Seoul 08826, South Korea



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