Velocity Node Research Articles

A hybrid fennec fox and sand cat optimization algorithm for clustering scheme in VANETs

The popularity of intelligent vehicles with cutting-edge vehicular applications has fueled the rapid expansion of Vehicular Ad hoc Networks (VANETs) in recent years. VANETs are a network of vehicles designed to exchange and explore real-time data using a well-developed and effectively organized data transport technology. However, the major issue of dynamic topology and cluster stability always has an impact on choosing an optimal path between the cars. At this point, an intelligent clustering technique in VANETs that handles dynamic topology and cluster stability is critical for efficient route selection between vehicular nodes. This is an NP-hard issue that can be effectively solved using an intelligent nature-inspired algorithm that can discover near-optimal solutions in the search space. An Intelligent Hybrid Fennec Fox and Sand Cat Optimization Algorithm (HFFSCOA) -Based Clustering Scheme is proposed in this paper as a novel route clustering optimization strategy that takes grid size, orientation, velocity node density, and communication range into account while achieving its goal. This HFFSCOA contributed to the route clustering process, which determines dependable and optimal routes between vehicular nodes for the purpose of building and evaluating ideal Cluster Heads (CHs) in the network. HFFSCOA's findings clearly demonstrated its usefulness and efficacy in terms of the number of vehicles, network size, changeable communication ranges, and number of clusters built in the network. The statistical results of HFFSCOA also confirmed an enhanced cluster Optimization rate of 56.21% and an increased cluster stability of 92.34.

Experimental Investigation of High Frequency Flame Response on Injector Coupling in a Perfectly Premixed Multi-Jet Combustor

Abstract High frequency injector-coupled thermoacoustic instabilities are a major threat to multi-jet combustors in rocket and gas turbine engines. The complex three-dimensional acoustic coupling between the combustion chamber and injector acoustics cause local fluctuations in heat release. In turn, multiple thermoacoustic feedback mechanisms close the thermoacoustic loop and serve as a source of the thermoacoustic instability. Except for the flame deformation and flame displacement mechanism, the underlying feedback mechanisms for high frequency instabilities are to a large extent unknown. The paper at hand gives new insights into the injector-coupled convective driving mechanisms that are present in multi-jet combustors at perfectly premixed conditions. The forced flame response to the first transverse combustor mode is investigated for two distinct injector tube lengths: one with an axial acoustic velocity node and one with a velocity antinode coupling at the injector–combustor interface. Phase locked OH* images reveal convectively transported coherent vortex structures as the main source of the flame response. The origin of the flame response can be linked to the axial acoustic velocity at the injector–combustor interface using numerical simulations. Both configurations show a clear oscillation of the heat release fluctuations in-phase with the acoustic pressure fluctuations. In similarity to time delay models in low frequency thermoacoustics, a wave number model is proposed to estimate the local flame response due to feed flow modulations and validated with the experimental results.

AXIAL ACOUSTIC VELOCITY INTERACTION WITH HOLLOW CONE LIQUID SHEET IN A SWIRLING FLOW FIELD

This experimental investigation concerns the effect of imposed axial acoustic velocity fluctuations on the primary atomization of a hollow cone liquid sheet in the presence of a strong air swirl. The atomization dynamics are elucidated by positioning the spray at an acoustic velocity node, antinode, and a mixed point in the standing wave field generated due to the imposed axial acoustic excitation. High-speed shadowgraph images acquired in-sync with dynamic pressure measurements are processed to clarify the unstable behavior observed in the spray dynamics; this was achieved by extracting key parameters such as breakup length, spatial growth rates, phase differences, and by employing Proper Orthogonal Decomposition (POD). A novel method to obtain the breakup length of a hollow cone spray from the position of maximum wave amplitude is presented. The breakup length is the smallest for the mixed point. The phase difference between the left and right half-angle fluctuations shows that the flapping motion of the spray is predominantly observed at the mixed point for different air-to-liquid ratios. Another novel approach is adopted to identify the physical mechanisms corresponding to each POD spatial mode by comparing POD spatial modes obtained from experiments to those generated artificially.

Effect of an external sound superimposed on the self-excited oscillation in a loop-tube thermoacoustic system

The influence of an external sound applied to a loop-tube type thermoacoustic system on the energy conversion efficiency is experimentally examined. The investigation is carried out by studying the effect of a loudspeaker (SP) set as an external sound source. As a result, it is found that the location of the SP affects the sound field in the system and that the amount of energy generated increases or decreases. The increasing or decreasing effect differs depending on the location of the SP. Furthermore, it is confirmed that, provided the SP is located near the particle velocity node, the sound energy can be increased by more than the input power to the SP without changing the sound field in the tube. From these results it can be confirmed that, similar to a straight-tube type thermoacoustic system, the energy conversion efficiency can be enhanced by locating the SP at a suitable position even in a loop-tube type system without end surfaces.

Outer Acoustic Streaming Flow Driven by Asymmetric Acoustic Resonances.

While boundary-driven acoustic streaming resulting from the interaction of sound, fluids and walls in symmetric acoustic resonances have been intensively studied in the literature, the acoustic streaming fields driven by asymmetric acoustic resonances remain largely unexplored. Here, we present a theoretical and numerical analysis of outer acoustic streaming flows generated over a fluid–solid interface above which a symmetric or asymmetric acoustic standing wave is established. The asymmetric standing wave is defined by a shift of acoustic pressure in its magnitude, i.e., , and the resulting outer acoustic streaming is analyzed using the limiting velocity method. We show that, in symmetric acoustic resonances (), on a slip-velocity boundary, the limiting velocities always drive fluids from the acoustic pressure node towards adjacent antinodes. In confined geometry where a slip-velocity condition is applied to two parallel walls, the characteristics of the obtained outer acoustic streaming replicates that of Rayleigh streaming. In an asymmetric standing wave where , however, it is found that the resulting limiting velocity node (i.e., the dividing point of limiting velocities) on the slip-velocity boundary locates at a different position to acoustic pressure node and, more importantly, is shown to be independent of , enabling spatial separation of acoustic radiation force and acoustic streaming flows. The results show the richness of boundary-driven acoustic streaming pattern variations that arise in standing wave fields and have potentials in many microfluidics applications such as acoustic streaming flow control and particle manipulation.

Nonlinear dynamics of a solid particle in an acoustically excited viscoelastic fluid. II. Acoustic radiation force.

An analytical formula is derived for acoustic radiation force exerted by an axisymmetric acoustic wave on an isotropic solid spherical particle in a compressible viscoelastic fluid. The particle is assumed to undergo pulsation, translation, and shape deformations of all orders. The fluid motion is described by the compressible Oldroyd-B model. No restrictions are imposed on the particle size with respect to the acoustic wavelength and the viscous penetration depth. The obtained analytical formula is used in numerical simulations. The results show that the acoustic radiation force can be increased by more than a factor of 10 due to the viscoelastic nature of the fluid, depending on the parameters. It is also observed that the sign of the force can change so that the force becomes directed to the velocity node instead of the velocity antinode if the driving frequency, the particle size, and the relaxation time of the polymer solution are relatively small.

Performance Analysis of TCP Newreno Over Mobility Models Using Routing Protocols in MANETs

A MANET is a collection of nodes connected wirelessly that try to converse with each other with no need for any central control or infrastructure establishment. The model of mobility depicts the poignant nature of every node which is mobile in MANETs that is considered to be realistic. It plays a vital role in measuring the performance of MANETs. Mobility is considered to be the prime motive in simulation, because it is a huge influence over the design and network's performance due to limitation in resources and it lead to packet delivery ratio (PDR), varying velocity node energy (NE). Lots of work has been done to improve the above problems. Therefore, there is a requirement of more improvement in this area to enhance overall performance of mobility models. This paper presents a comparative simulation-based analysis of Gauss Markov, Manhattan, and random waypoint mobility models over TCP Newreno that uses a DSDV and AODV routing protocols. Moreover, experiment results and performance analysis have been performed with PDR and NE of the varying number of mobile nodes.

The response of an axisymmetric jet placed at various positions in a standing wave

Abstract

An improved LTI algorithm for multiphase raytracing in undulating surface model

Abstract Based on the linear traveltime interpolation (LTI) algorithm, we propose an improved LTI (referred as ILTI) algorithm for multiphase seismic ray tracing, which uses a velocity node in model parameterization and introduces secondary nodes between adjacent velocity nodes. To better fit the undulating surface model, an irregular velocity node is used in near the irregular interface, and regular velocity nodes are still used in the region far away from the irregular interface. We derive an iterative fixed-point formula for calculating traveltime. By combining multistage computational technology and wavefront narrowband expansion technology, the proposed ILTI algorithm can efficiently trace the multiphase seismic raypath and compute the corresponding traveltime field. Through comparison and analysis with the traditional LTI algorithm, its computational accuracy can be highlighted by at least one order of magnitude. Compared with the popular fast marching method (FMM) and irregular shortest-path (ISPM) algorithms, it also has the advantages in terms of computational accuracy and efficiency. Numerical simulations in the Marmousi model show that the algorithm is also suitable for tracking multiphase seismic rays in the complex velocity model.

Acoustophoretic agglomeration patterns of particulate phase in a host fluid

Ultrasound-assisted processing of particulate phase in a host fluid relies on the induced acoustic force field. Understanding the agglomeration phenomenon in the particulate phase under acoustic forces will provide better insight about the acoustophoresis quality and a way to design a well-controlled process. In this work, a dynamic model consisting of acoustic and hydrodynamic forces is proposed for tracking the motion of micro-spheres under ultrasound fields with planar and non-planar wave fronts. The agglomeration of particles at the nodal plane was simulated taking into account the contact and collisions between spheres. The numerical simulations were conducted for both sound hard and compressible spheres to investigate the behaviors of single and multiple-phase particle populations. For the case of a plane standing-wave, the interaction between solid-bubble allows the solid particles to stay at the velocity node which is their unstable equilibrium location. With a Bessel standing wave as a non-planar pressure field, the agglomeration patterns of particles are generally different from the case of plane standing wave, which implies the significance of the particle tracking simulations for predicting the agglomeration patterns and locations under ultrasound fields with arbitrary wave fronts.

Entropy and flame transfer function analysis of a hydrogen-fueled diffusion flame in a longitudinal combustor

In this work, entropy generation and flame transfer function investigations are conducted on a hydrogen-burnt diffusion flame in a longitudinal combustor with acoustic waves present. For this, a time-domain 2D numerical model of a jet diffusion flame is developed to gain insights on its dynamic response to acoustic disturbances at either resonant or non-resonant frequencies. The model is validated first by comparing the numerical results such as turbulence intensities, pressure and velocity mode shape and flame shapes with the experimental data available in the literature. The model is then applied to evaluate the effects of the frequencies and amplitudes of the forcing acoustic waves, and the flame-holder/nozzle axial positions on entropy generation of both hydrogen- and propane-fueled flames. It is found that the entropy generation rate is sensitive to acoustic forcing frequencies, amplitudes and the nozzle axial positons. Furthermore, entropy produced from the heat conduction and the chemical reaction processes is shown to be dominant and secondary respectively. However, the mass diffusion is found to play a negligible role on the entropy generation. As the acoustic forcing frequency is set to 385 Hz near resonance, the total entropy generation rates are minimized, and the mass diffusion contribution is maximized with the flame being placed at velocity node locations in comparison with other flame-holding locations. Finally, flame transfer function (FTF) analysis is performed by using two different methods. It is shown that the flame responds strongly to low-frequency acoustic disturbances, acting like a band-pass filter. Increasing the acoustic intensity leads to the flame being more sensitive to the acoustic disturbances over more frequency bands.

Acoustic radiation force acting on a heavy particle in a standing wave can be dominated by the acoustic microstreaming.

We numerically investigate the contribution of the microstreaming to the acoustic radiation force acting on a small elastic spherical particle placed into an ultrasonic standing wave. When an acoustic wave scatters on a particle the acoustic radiation force and the microstreaming appear as nonlinear time-averaged effects. The compressible Navier-Stokes equations are solved up to second order in terms of the small Mach number using a finite element method. We show that when the viscous boundary layer thickness to particle radius ratio is sufficiently large and the particle is sufficiently dense, the acoustic microstreaming dominates the acoustic radiation force. In this case, our theory predicts migration of the particle to the velocity node (pressure antinode).

Periodic self-modulation of an electrodynamically driven heated wire near resonance.

A thin nichrome wire driven near resonance by the Lorentz force and heated by an alternating electrical current is a popular lecture demonstration. Due to the convective cooling of the portions of the wire moving with the greatest amplitude, only glowing regions near a velocity node will be visible in a darkened room. Nonlinear effects and the thermal expansion coefficient of the wire displace the wire's tensioning mass. By adiabatic invariance, the work done on or by the vibrating wire, due to the changes in the mass's elevation, causes the natural frequency of the standing wave resonance to be shifted. Competition between the thermal inertia of the wire and the convective heat transfer coefficient introduces an exponential thermal relaxation time so that the amplitude of vibration is dependent on the ratio of the drive frequency to the changing resonance frequency at an earlier (retarded) time. These thermal and kinetic effects are incorporated into three coupled nonlinear ordinary differential equations that are separated by the method of multiple time scales and are solved numerically, reproducing both the spontaneous appearance of stable periodic amplitude modulation and the hysteretic behavior observed with increasing or decreasing of the drive frequency.

Role of induced axial acoustics in transverse acoustic flame response

This paper addresses the mechanisms through which transverse acoustic oscillations excite unsteady heat release. Forced and self-excited transverse acoustic instability studies to date have strong coupling between the transverse and axial acoustic fields near the flame. This is significant, as studies suggest that it is not the transverse disturbances themselves, but rather the induced axial acoustic disturbances, that control the bulk of the heat release response. This paper presents results from an experiment that controls the relative amplitudes of transverse and axial disturbances and measures the flow field and heat release response for an acoustically compact, swirling flame. 5 kHz, simultaneous sPIV and OH-PLIF measured the flow field and flame edge, and OH* chemiluminescence measured the relative heat release. Experiments performed with essentially the same transverse acoustic wave field, but with and without axial acoustics, show that significant heat release oscillations are only excited in the former case. The results show that the axial disturbances are the dominant cause of the heat release oscillations. These observations support the theory that the key role of the transverse motions is to act as the “clock” for the instability, setting the frequency of the oscillations while having a negligible direct effect on the actual heat release fluctuations. They also show that transverse instabilities can be damped by either actively canceling the induced axial acoustics in the nozzle (rather than the much larger energy transverse combustor disturbances), or by passively tuning the nozzle impedance to drive an axial acoustic velocity node at the nozzle outlet.

The Influence of Stack Position and Acoustic Frequency on the Performance of Thermoacoustic Refrigerator with the Standing Wave

Abstract Thermoacoustic refrigerator uses acoustic power to transport heat from a low-temperature source to a high-temperature source. The increasing interest in thermoacoustic technology is caused due to its simplicity, reliability as well as application of environmentally friendly working fluids. A typical thermoacoustic refrigerator consists of a resonator, a stack of parallel plates, two heat exchangers and a source of acoustic wave. The article presents the influence of the stack position in the resonance tube and the acoustic frequency on the performance of thermoacoustic refrigerator with a standing wave driven by a loudspeaker, which is measured in terms of the temperature difference between the stack edges. The results from experiments, conducted for the stack with the plate spacing 0.3 mm and the length 50 mm, acoustic frequencies varying between 100 and 400 Hz and air as a working fluid are consistent with the theory presented in this paper. The experiments confirmed that the temperature difference for the stack with determined plate spacing depends on the acoustic frequency and the stack position. The maximum values were achieved for resonance frequencies and the stack position between the pressure and velocity node.

Numerical Study of a Methane Jet Diffusion Flame in a Longitudinal Tube with a Standing Wave

Abstract Flame stability plays an important role in a combustion/propulsion system, which involves the interaction between flow, acoustics, and flame. Although intensive studies have been carried out to investigate the flame-acoustic interactions, the jet flame excited by standing waves formed in a longitudinal tube has not received much attention. In this work, a methane-burnt (CH 4 ) jet diffusion flame in a longitudinal tube with a standing wave produced from a loudspeaker is studied numerically. 2-D unsteady RANS simulations are performed by using ANSYS FLUENT with the standard k − ɛ turbulence model and a one-step Eddy-Dissipation combustion model. The acoustic fluctuations are generated by using User Defined Functions. The numerical results show that a longitudinal standing wave can be successfully obtained in the tube by the numerical method, and both the acoustic velocity node and antinode can be observed. It is also found that the jet flame characteristics are highly sensitive to its axial location in the tube when the standing wave is present. The flame is unsteady when it is located at the velocity antinode where large velocity fluctuation exists. When the jet is placed at acoustic velocity nodes, however, the flame is relatively steady. Although the minimum velocity fluctuation at the two velocity nodes is almost at the same level, different flame temperatures are observed, due to the different turbulence kinetic energy.

Numerical study of dynamic response of a jet diffusion flame to standing waves in a longitudinal tube

In this work, the dynamic response of a propane-burnt (C3H8) jet diffusion flame in a longitudinal tube to acoustic waves produced from a loudspeaker are studied. For this, 2-D numerical simulations are conducted by using FLUENT to investigate the interaction of acoustics-flow-flame. Unsteady RANS simulations with one-step Eddy-Dissipation (ED) combustion model are used in the model. And acoustic fluctuations are generated by using User Defined Functions (UDF). The numerical model is validated first by comparing the numerical results with the experimental measurements in the absence of a flame. Further validation is performed by comparing with flame-involved experimental results available in the literature. It is shown that the jet and the flame characteristics are highly sensitive to its axial location, especially when standing waves are present in the tube. The jet experiences large velocity fluctuations. Flow reversal is observed, when the jet is placed at acoustic velocity antinodes. However, the jet and flame are much stable in the velocity node region. Due to the large-amplitude acoustic disturbances, an interesting unsteady mushroom-shaped flame is observed. The numerical model is then used to determine the flame transfer function (FTF), which is important to characterize the flame-acoustic coupling behaviors. It is shown that the flame transfer function is nonlinear. Furthermore, it depends strongly on not only the amplitude of the acoustic disturbances but also the frequency. Such strong responses of the flame to acoustic waves are due to the ‘resonance’ effect of the tube. The present work opens up new applicable way to model and characterize the flame-acoustics-flow interaction via flame transfer function.

Flow-Excited Acoustic Resonance of Isolated Cylinders in Cross-Flow

The flow-excited acoustic resonance of isolated cylinders in cross-flow is investigated experimentally where the effect of the cylinder(s) proximity to the acoustic particle velocity nodes of the cross-modes is presented in this paper. For the case of a single cylinder, the cylinder's location does not significantly affect the vortex shedding process; however, it affects the excitation level of each acoustic cross-mode. When the cylinder is moved away from the acoustic particle velocity antinode of a specific acoustic cross-mode, a combination of the cross-modes is excited with intensities that seem to be proportional to the ratio of the acoustic particle velocities of these modes at the cylinder's location. For the cases of two and three hydrodynamically uncoupled cylinders positioned simultaneously side-by-side in the duct, it is observed that the first three acoustic cross-modes are excited. When one cylinder is positioned at the acoustic particle velocity antinode of a specific cross-mode and another cylinder is positioned at its acoustic particle velocity node, i.e., a cylinder that should excite the resonance and another one that should not excite it, respectively; the excitation always takes over and the resonance occurs at a further elevated levels. It is also observed that the acoustic pressure levels in the cases of multiple cylinders are not resulting from a linear superposition of the excited level obtained from each individual cylinder which indicates that the removal of cylinders at certain locations may not be a viable technique to eliminate the acoustic resonance in the case of tube bundles.

Steady streaming around a pulsating bubble located at the velocity node of a standing wave

We have examined the effect of the no-slip boundary condition on the steady streaming around a radially pulsating bubble, located at the velocity node of a standing sound wave. The no-slip condition can take place on the bubble surface under many circumstances, especially if the surface is contaminated or otherwise coated. We applied the singular perturbation analysis with the oscillation amplitude, ε=U0/(aω)<<1, as a small parameter and took the frequency parameter, M2=iωa2/ν, to be large. Here, a, U0, ω and ν are length scale, velocity scale, frequency, and kinematic viscosity, respectively. We further assumed that the bubble stays spherical in shape, the wavelength is significantly larger than the bubble radius, and no fluid transport takes place from the bubble to the surroundings. Additionally, the lateral and the radial oscillations, while at the same frequency, have a phase difference φ, and the amplitudes of both the oscillations are small compared to the bubble radius. We found that the streaming happened at a lower order than the non-pulsating case and it is more intense than the case of the shear free boundary condition. Furthermore, the phase difference was found to play a significant role in the streaming flow direction and its intensity.

AEROACOUSTIC RESPONSE OF ISOLATED CYLINDER(S) IN CROSS-FLOW AT DIFFERENT LOCATIONS INSIDE A RECTANGULAR DUCT

ABSTRACT: The flow-excited acoustic resonance of single and multiple isolated cylinders in cross-flow is investigated experimentally. The effect of the cylinder(s) proximity to the acoustic particle velocity nodes of the first three acoustic cross-modes is presented. During the experiments, the acoustic cross-modes of the duct hosing the cylinders are self-excited. For the case of single cylinder, it is observed that although the cylinder′s location doesn′t significantly affect the process of vortex shedding, it affects the mechanism of the flow-excited acoustic resonance and the generated acoustic pressure. When the cylinder is shifted away from the acoustic particle velocity anti-node of a certain acoustic cross-mode, a combination of cross-modes is excited with intensities that seem to be proportional to the ratio of the acoustic particles velocities of these modes at the cylinder′s location. For the case of two and three isolated cylinders positioned simultaneously side-by-side in the duct, it is observed that when the cylinders are positioned at different acoustic particle velocity anti-nodes of different cross-modes, the intensities of the excited acoustic resonance of these cross-modes are amplified compared to those with single cylinder. Nevertheless, when one cylinder is positioned at the acoustic particle velocity anti-node of certain cross-modes and another cylinder is positioned at it′s acoustic particle velocity node, i.e a cylinder that should excite the resonance and another one that should suppress it, respectively; the excitation always takes over and the resonance occurs. Moreover, as the cylinder moves closer to the duct′s wall, the Strouhal number value decrease due to the interference between the wake of the cylinder and the duct′s wall. Therefore, the acoustic resonance for this case occurs at slightly higher flow velosities.