RETRACTED ARTICLE: Grain refinement of AA5754 aluminum alloy by ultrasonic cavitation: Experimental study and numerical simulation

In this work, an experimental investigation was carried out on the grain refinement of molten AA5754 Aluminum alloy through ultrasonic treatment. The cavitation induced heterogeneous nucleation was suggested as the major mechanism for grain refinement in the AA5754 aluminum alloy. A numerical simulation was performed to predict the formation, growth and collapse of cavitation bubbles in the molten AA5754 Aluminum alloy. Moreover, the acoustic pressure distribution and the induced acoustic streaming by ultrasonic horn reactor were investigated. It is suggested that the streaming by ultrasonic could transport the small bubbles formed in the ultrasonic cavitation zone into the bulk of melt rapidly. These micro-bubbles are collapsed due to acoustic vibrations where the resulting micro-jets are strong enough to break the oxide layer and to wet the impurities. These exogenous particles, intermetallics and oxides could contribute to the formation of fine, uniform and equiaxed microstructure across the treated melt. The experimental results confirmed the simulation predictions.     

Link reliability for IS-54/136 handsets with different receiverstructures

We estimate link reliabilities for IS-54/136 digital cellular handsets operating with or without an equalizer in urban, suburban, rural, and mountainous environments. We define the reliability of a user's receiver as the probability that the bit error rate (BER) is less than some specified value. The probability is taken over all mobile positions in a cell area and the BER is averaged over multipath fading. Using a range of tools for modeling and simulation of the digital cellular link (transmitter, channel, and receiver), we present an extensive set of results showing the influence of: (1) receiver structures (differential detection with no equalizer, differential detection with selection diversity, or coherent detection with a medium-complexity equalizer); (2) joint distribution of the channel's RMS delay spread and average signal-to-noise ratio (SNR) (this distribution is based on an environment-specific model reported previously); and (3) vehicle speed (0-200 km/h). In all simulations, we assumed a two-path Rayleigh fading channel characterized by: (1) the delay between paths and (2) the ratio of power received from the first path to that from the second path (the RMS delay spread relates to these two parameters). For typical cell sizes, we find that imposing an equalization requirement in IS-54/136 handsets is overly stringent in all environments, except mountainous areas. For these environments, achieving high reliability requires either equalization or other measures, such as smaller cells, directional base-station antennas, or dual-diversity handsets     

A new method for finding the first‐ and second‐order eigenderivatives of asymmetric non‐conservative systems with application to an FGM plate actively controlled by piezoelectric sensor/actuators

In this paper, a new method for computing eigenvalue and eigenvector derivatives of asymmetric non‐conservative systems with distinct eigenvalues is presented. Several approaches have been proposed for eigenderivative analysis of systems with asymmetric and non‐positive‐definite mass, damping and stiffness matrices. The proposed formulation that is developed by combining the modal and algebraic methods neither have the complications of modal methods in calculating the complex left and right eigenvector derivatives nor suffer from numerical instability problems usually associated with algebraic methods. The method is applied to a functionally graded material (FGM) plate actively controlled by piezoelectric sensor/actuators. In this system, the feedback signal applied to each actuator patch is implemented as a function of the electric potential in its corresponding sensor patch. The use of this closed‐loop controlling system leads to a non‐self‐adjoint system with complex eigenvalues and eigenvectors. A finite element model is developed for static and dynamic analysis of closed‐loop controlled FGM plate. The first‐ and second‐order approximations of Taylor expansion are used to estimate the corresponding changes in the plate modal properties due to change in design parameters (the displacement feedback gains and the piezoelectric layer thickness in each S/A pair). Copyright © 2008 John Wiley & Sons, Ltd.     

Modification of the LSMPS method for the conservation of the thermal energy in laser irradiation processes

In this work, new least-square moving particle semi-implicit (LSMPS) formulations for the modeling of the heat conduction in laser irradiation processes for both thick blocks and thin plates are developed. These new LSMPS formulations guarantee the conservation of the total thermal energy during the heat exchange between particles. The conservation of the thermal energy in the LSMPS method was implemented together with multiresolution techniques for the discretization of the domain with particles of different sizes so that a better characterization of the thermal gradients in the vicinity of the laser beam can be obtained. The simulation of laser irradiation processes for thin plates is still very challenging for particle methods with spherical particles and this is essentially because it is difficult to accommodate a minimum number of particles along the thickness direction without increasing considerably the resolution or the number of particles in the entire plate. In order to overcome this difficulty, a new multiresolution method based on particles with ellipsoidal shapes was also developed for a more efficient modeling of the laser irradiation in thin plates. By conducting the heat conduction simulations, in which the standard LSMPS method can provide accurate temperature distribution and by comparing the results with an analytical solution, it was confirmed that the proposed method is as accurate as the standard LSMPS method. Moreover, the heat conduction with an external heat source, in which the total thermal energy is not conserved by using the standard LSMPS method, was successfully simulated by using the proposed method. The simulations of laser irradiations were also conducted, and the validity of the proposed method has been confirmed by comparing numerical results with experimental data.     

Experimental and CFD investigation on the solidification process in a co-rotating twin screw melt conditioner

Twin screw melt conditioners are used for mixing purposes and are mainly used for polymer processing. These conditioners (extruders) can be used for liquid metal processing in which liquid metal/slurry is subjected to high shear stress. This process results in grain refinement of structure. In this article, in a co-rotating twin screw melt conditioner, the solidification process of a liquid along with temperature variations of the melt with regard to the complexity of the flow has been examined. With the aid of dynamic mesh scheme, a Computational Fluid Dynamic (CFD) simulation was performed. The achieved results were in good agreement with values based on the experimental measurements. It was concluded that shearing and pouring temperatures play important roles in solidification progression and the main reason of surviving nuclei is heat dissipation from the barrel. Also, the main factor affecting the grain size is the temperature differences between the pouring and the setting temperature. It was observed that, the twin screw melt conditioner can decrease the temperature gradient and with the help of turbulence, providing appropriate conditions for formation of fine and equiaxed grains.     

A new approach for evaluating clipping distortion in multicarriersystems

Multicarrier signals are known to suffer from a high peak-to-average power ratio, caused by the addition of a large number of independently modulated subcarriers in parallel at the transmitter. When subjected to a peak-limiting channel, such as a nonlinear power amplifier, these signals may undergo significant spectral distortion, leading to both in-band and out-of-band interference, and an associated degradation in system performance. This paper characterizes the distortion caused by the clipping of multicarrier signals in a peak-limiting (nonlinear) channel. Rather than modeling the effects of distortion as additive noise, as is widespread in the literature, we identify clipping as a rare event and focus on evaluating system performance based on the conditional probability of bit error given the occurrence of such an event. Our analysis is based on the asymptotic properties of the large excursions of a stationary Gaussian process, and offers important insights into both the true nature of clipping distortion, as well as the consequent design of schemes to alleviate this problem     

Performance Analysis of Slotted Carrier Sense IEEE 802.15.4 Medium Access Layer

Advances in low-power and low-cost sensor networks have led to solutions mature enough for use in a broad range of applications varying from health monitoring to building surveillance. The development of those applications has been stimulated by the finalization of the IEEE 802.15.4 standard, which defines the medium access control (MAC) and physical layer for sensor networks. One of the MAC schemes proposed is slotted Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), and this paper analyzes whether this scheme meets the design constraints of those low-power and low-cost sensor networks. The paper provides a detailed analytical evaluation of its performance in a star topology network, for uplink and acknowledged uplink traffic. Both saturated and unsaturated periodic traffic scenarios are considered. The form of the analysis is similar to that of Bianchi for IEEE 802.11 DCF only in the use of a per user Markov model to capture the state of each user at each moment in time. The key assumptions to enable this important simplification and the coupling of the per user Markov models are however different, as a result of the very different designs of the 802.15.4 and 802.11 carrier sensing mechanisms. The performance predicted by the analytical model is very close to that obtained by simulation. Throughput and energy consumption analysis is then performed by using the model for a range of scenarios. Some design guidelines are derived to set the 802.15.4 parameters as function of the network requirements.     

Numerical and experimental investigation of the grain refinement of liquid metals through cavitation processing

An investigation was carried out on the grain refinement of molten AA5754 Aluminum alloy through intensive shearing. The results show intensive shearing via cavitation decreases the grain size significantly. The above hypothesis for structure refinement was evaluated and an experiment was performed to ensure the creditability of this assumption. Finally, it was simulated by computational fluid dynamics (CFD) modeling. It was understood that shearing is the responsible mechanism for creation of cavitation bubbles and further collapse of them. It was also concluded the pressure which generated from the collapse of the bubble is well enough for braking the oxide layer and wetting them. It was proved that breaking of the oxide layer wets the impurity particles upon collapse of cavitation bubbles and provides additional nuclei and additional grain refinement. The suggested mechanism includes improved wetting by breaking the oxide layer through fatigue via continuous hitting of the micro-jets, local undercooling upon the collapse of cavitation bubbles, and pre-solidification inside fine capillaries.     

Modification of dynamic characteristics of FGM plates with integrated piezoelectric layers using first‐ and second‐order approximations

This paper proposes a method for calculating the required changes in the physical properties of plates made of functionally graded materials (FGM) in order to achieve the desired eigenfrequency shifts in the structure. A finite element formulation based on the classical laminated plate theory (CLPT) is presented for an FGM plate with integrated piezoelectric layers. Using this formulation, an efficient method based on the first‐order and second‐order approximations in Taylor expansion is expressed to calculate the corresponding changes in the plate modal properties due to changes in parameters which characterize the structure's dynamic behaviour. An initial sensitivity analysis is carried out to identify the regions within the structure where modifications are most effective on the structure's dynamic characteristics. The proposed algorithm is applied to a case study with two different boundary conditions and the results are validated against exact solutions. The influence of structural modifications on dynamic response of the plate is also studied using the Newmark‐β method. Copyright © 2006 John Wiley & Sons, Ltd.