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Lakshmi K.,Center for Excellence in Computational Engineering and Networking | Muralikrishna P.,Naval Physical and Oceanographic Laboratory DRDO | Soman K.P.,Center for Excellence in Computational Engineering and Networking
Proceedings - 2013 IEEE International Multi Conference on Automation, Computing, Control, Communication and Compressed Sensing, iMac4s 2013 | Year: 2013

Channel estimation is an important aspect in wireless communication, in which an estimate of the interference caused to the normal transmission is found, which is then cancelled to retrieve the original signal. In UnderWater Acoustic transmission, two main effects are delay spread and Doppler shift. It has been found[10] that while sampling in the delay - Doppler domain, the effect of the channel can be treated as sparse. Thus framing the estimation problem as an optimization problem of the form of a Basis Pursuit De Noising (BPDN)[21] and solving it using sparse reconstruction methods could be a good technique. In addition to giving good sparse solution, the technique also assures low computational complexity, (due to iterative nature of solution methodology) when compared to traditional estimation methods like Least Square Estimation (LSE) and Minimum Mean Square Error Estimation(MMSE). © 2013 IEEE. Source


Sasikumar K.,Naval Physical and Oceanographic Laboratory DRDO | Manoj N.R.,Naval Physical and Oceanographic Laboratory DRDO | Mukundan T.,Naval Physical and Oceanographic Laboratory DRDO | Khastgir D.,Indian Institute of Technology Kharagpur
Journal of Applied Polymer Science | Year: 2014

In this work, application of rubber-MWNT nanocomposite for underwater acoustic sensors is explored. The nanocomposite is developed by incorporating multiwalled carbon nanotubes (MWNT) into carboxylated nitrile rubber by mechanical mixing. The addition of MWNT up to 10 phr is found to result in about 330% increase in tensile strength, 140% increase in modulus, and 160% increase in tear strength. Transmission electron microscopy and scanning electron microscopy analyses indicate uniform dispersion of nanotubes in the rubber matrix. Dynamic mechanical analysis shows that damping at ambient temperature gradually increases with increasing filler content. This is attributed to the augmented frictional energy loss at the interface. The damping peak position shifts upward with increase in MWNT concentration, which may be gainfully used to tune to the operational frequency range of underwater acoustic sensors. Payne effect is observed at higher filler concentration due to the breakage of aggregates formed by filler-filler interaction. The nanocomposite may find application for damping structural vibrations and thus to improve the performance of underwater acoustic sensors. © 2014 Wiley Periodicals, Inc. Source


Sasikumar K.,Naval Physical and Oceanographic Laboratory DRDO | Sasikumar K.,Indian Institute of Technology Kharagpur | Manoj N.R.,Naval Physical and Oceanographic Laboratory DRDO | Mukundan T.,Naval Physical and Oceanographic Laboratory DRDO | Khastgir D.,Indian Institute of Technology Kharagpur
Composites Part B: Engineering | Year: 2016

In this study, compressive hysteretic damping properties of carboxylated nitrile rubber (XNBR) and its composites containing multiwalled carbon nanotubes (MWNT) were investigated at high strains by Universal Testing Machine (UTM) and at low strains by Dynamic Mechanical Analyzer (DMA). Degree of damping is less than the neat XNBR at low MWNT loading and it increases significantly at higher loading. At constant MWNT loading, the extent of damping depends on applied strain. Mullins effect is observed in both neat XNBR and composites due to the presence of ionic clusters. Differential Scanning Calorimetry (DSC) and X-ray diffraction analysis (XRD) confirm the reversible nature of the ionic cluster formation/deformation. Deformation of ionic clusters and nanotube agglomerates contribute to the overall increase in damping. Morphology characterization by Transmission Electron Microscope (TEM) reveals agglomeration of nanotubes at higher loading levels. A schematic diagram about the structural changes due to application of heat and stress is proposed. The results would be of great assistance in the design of application specific composites for various engineering applications. © 2015 Published by Elsevier Ltd. Source


Sasikumar K.,Naval Physical and Oceanographic Laboratory DRDO | Manoj N.R.,Naval Physical and Oceanographic Laboratory DRDO | Ramesh R.,Naval Physical and Oceanographic Laboratory DRDO | Mukundan T.,Naval Physical and Oceanographic Laboratory DRDO
International Journal of Nanotechnology | Year: 2012

Nanotechnology is an area which has vast potentials for turning fundamental research into successful innovations. In the present paper, damping in Polyurethane (PU)-Multiwalled carbon nanotube (MWNT) nanocomposites is studied by Dynamic Mechanical Analysis (DMA) and Free Layer Damping (FLD) methods. MWNT is functionalised using HNO 3/H 2SO 4 mixture, thionyl chloride and ethylene diamine to introduce amide groups on the nanotube surface. Composites are prepared from a two-part PU resin system and the functionalised MWNT. The work addresses the concept of damping from molecular level interactions using DMA to an actual vibration damping technique, namely FLD. DMA study clearly shows a 70% increase of loss tangent of the nanocomposites as compared to neat PU, indicative of the high damping efficacy. The effectiveness of the developed nanocomposite is established by FLD measurements wherein a good damping of the order of 5-15 dB is observed in the frequency range of 10-1000 Hz, over that of neat PU. The added advantage of the system is the damping capability in the low frequency region also. The study is expected to be helpful in finding damping applications in aircraft and missile substructures, ships and submarines, machinery supports, mounting platforms of electronic equipment, bridges, buildings, etc. Copyright © 2012 Inderscience Enterprises Ltd. Source

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