Increase In Amplitude Of Oscillations Research Articles

Numerical simulation of transport phenomena and its effect on the weld profile and solute distribution during laser welding of dissimilar aluminium alloys with and without beam oscillation

Remote Laser Welding (RLW) of Aluminium alloys has significant importance in lightweight manufacturing to decrease the weight of the body in white. It is critical to understand the physical process of transport phenomena during welding which is highly related to the mechanical performance of the joints. To investigate the underlying physics during welding and to understand the influence of beam oscillation on heat transfer, fluid flow and material mixing a transient three-dimensional Finite Element (FE) based Multiphysics model has been developed and validated from the experiments. The effect of welding speed, oscillation amplitude and oscillation frequency on the fusion zone dimensions, flow profile, vorticity profile, cooling rate and thermal gradient during the butt welding of Al-5754 to Al-6005, with sinusoidal beam oscillation, is analysed. It was found that one additional vortex is formed during beam oscillation welding due to the churning action of the oscillating beam. With the increase in oscillation amplitude, welds become wider and the depth of penetration decreases. An increase in oscillation frequency leads to an increase in the flow rate of the molten metal suggesting that the beam oscillation introduces a churning action that leads to an increase in mixing. It was highlighted that the material mixing depends on both diffusion and convection.

The effect of temperature and external force on the thermal behavior of oil-based refrigerant inside a nanochannel using molecular dynamics simulation

Heat transfer (HT) has always been one of the major challenges in engineering. Many approaches have been offered to enhance or control HT. One of the common techniques to develop the thermal performance of devices is to use nano-sized channels. This study is aimed to examine the influence of temperature and external force (EF) on nanochannel simulated samples by molecular dynamics simulation (MDS). The present study examined the effect of initial temperature (300–400 K) and EF (with 0.001–0.01 eV/Å) on oil inside the nanochannel. The results show that increasing temperatures up to 350 K increases the heat flux (HF) and thermal conductivity (TC) increase and decreases due to the increased number and intensity of atomic collisions in the structures. The HF and TC at 350 K are 946 W/m2 and 0.66 W/m.K. Also, by changing the EF from 0.001 to 0.01 eV/Å, the HF increases from 911 to 942 W/m2 and the TC by 0.52 to 0.63 W/m.K. So, with anincrement in EF, oscillation amplitude increases, and thermal performance improves. Since the thermal performance of refrigerant is noticeable in various industrial applications, it’s hopeful this study proposed an optimal mechanism in practical samples.

Numerical Investigation of Flow Characteristics in a Centrifugal Pump Impeller with Sinusoidal Flow Rates of Different Oscillation Amplitudes

This study describes the large eddy simulations of a centrifugal pump impeller considering a sinusoidal flow rate and a constant rotation speed. Five different oscillation amplitudes of flow rate (A = 0.1Qd, 0.15Qd, 0.2Qd, 0.25Qd, and 0.3Qd, Qd indicates the design flow rate) are selected to determine the influence of oscillation amplitude on the internal flow characteristics. The simulation results show that, with increasing oscillation amplitude, the alternating stall phenomenon weakens or even disappears during the dropping stage, whereas the opposite trend is observed during the rising stage. The total mean normal vorticity is insensitive to changes in the oscillation amplitude. Moreover, the difference in pressure fluctuations between adjacent passages decreases with increasing oscillation amplitude. The first and second dominant frequencies of the pressure fluctuations are mainly affected by the oscillation amplitude in the non-stall passage. The internal flow exhibits a clear hysteresis effect, and the lag time of the head increases with the oscillation amplitude. Additionally, the average head is approximately 2.38 m, regardless of the oscillation amplitude.

Finite element analysis of stress-strain state of the deformation zone of a UFG TI Grade 4 workpiece subjected to abrasive-free ultrasonic finishing

An effective approach to increasing the fatigue resistance of metal products is to create compressive residual stresses on the surface of the product using surface plastic deformation (SPD) processing. One of the effective SPD methods is the process of abrasive-free ultrasonic finishing (AFUF). Another well-known approach to improving mechanical properties including fatigue resistance is to create an ultrafinegrained (UFG) structural state in the product. This research focuses on the finite-element study of the stress-strain state of a UFG workpiece subjected to SPD by the AFUF method. Commercially pure Grade 4 titanium in the UFG state obtained by the ECAP-Conform method was chosen as a workpiece material. In the course of the study, the stress-strain state of the deformation zone was analyzed after a single indentation with subsequent unloading under the elastic-plastic scenario. The effect of the indenter oscillation amplitude and its geometry on radial residual stresses including their depth of occurrence, average normal stress and strain intensity was analyzed. It was found that as the indenter radius increases, the strain intensity (e) value decreases. The e parameter distribution has a gradient nature with a decrease in values from the surface to the center of the workpiece. An analysis of simulation results shows that radial residual stresses in the deformation zone are predominantly compressive, and, accordingly, they will increase the fatigue resistance of the finished product. It was established that as the indenter oscillation amplitude increases, the values of compressive radial residual stresses also increase. Their maximum values reach 540 MPa at an amplitude of 75 μm with the depth of these stresses up to 0.3 mm. An increase in the indenter radius, i.e. in fact the contact area, leads to an increase in the magnitude of compressive radial residual stresses with an almost linear behavior.

Numerical investigation on stall flutter of an airfoil with split drag rudder

This paper investigates the stall flutter characteristic of a National Advisory Committee for Aeronautics (NACA) 0012 airfoil with a split drag rudder (SDR). The effect of split angle and initial disturbance on stall flutter properties are studied by the computational fluid dynamics (CFD)/computational structure dynamics (CSD) coupling method. The inherent flow mechanism is illustrated with the vorticity and pressure distribution contours. The results show that stall flutter occurs when the pitch motion amplitude of the response exceeds the pitch instability branches of the energy maps. The increase in the split angle leads to an increase in the additional stiffness of the aerodynamic forces so that the amplitude of the pitching motion induced by the initial disturbance decreases and the bifurcation speed increases with the split angle. The trailing edge vortex and the standing vortex between the rudders form affected by the SDR, which reduces the nose-up moment of the airfoil at small angles of attack and increases the nose-down moment after stall occurs. It enables airfoil to obtain more energy from the flow field so that the LCO oscillation amplitude increases. Moreover, it is found that the SDR-induced trailing-edge vortices will excite a large-amplitude stall flutter of an airfoil with a zero initial angle of attack and disturbance.

Measurement and analysis of the dynamic response of a double-liquid lens driven by sinusoidal signals

The dynamic response of a double-liquid lens driven by sinusoidal signals with different frequencies and amplitudes is researched experimentally. The results show that the oscillation frequency of the liquid lens interface is twice that of the driving signal at a certain oscillation position. When the driving voltage amplitude is unchanged, and the oscillating position of the liquid interface is stable, the oscillation amplitude increases first and then decreases with the increase of frequency. When the driving voltage frequency is constant, with the increase of the driving voltage amplitude, the deviation of the oscillation position of the liquid interface from the initial position and oscillation amplitude increases. According to the force analysis of the liquid interface, the reasonable explanation of the above experimental results is presented.

Transient electrohydrodynamics of a liquid drop: The interplay of fluid flow evolution and dynamic response, and the density ratio effects

Computer simulations are performed to study some of the less explored aspects of the transient electrohydrodynamics of a liquid drop in uniform DC electric fields. The governing equationsof the problem are solved using a parallelized front tracking/finite difference method in the framework of Taylor-Melcher's leaky dielectric theory. For density- and viscosity-matched fluid systems, the evolution of the flow field at a high Ohnesorge-squared number Oh^{2}=μ^{2}/γρa is studied. It is shown that the instantaneous flow pattern is the result of superposition of deformation- and hydrodynamic shear-driven vortices, and that depending on the placement of the fluid systems on the deformation-circulation map, there exists two different paths for the development of the velocity field toward steady state. Examination of the steady-state flow patterns shows that the location of the maximum velocity can shift from the (classically known) drop surface to inside the drop along the poles. The effect of Oh^{2} on the dynamic response of the drop and the kinetic energy of the fluid is studied. For high Oh^{2} number flows, the dynamic response is monotonic while the kinetic energy evolves in a nonmonotonic way, achieving a distinct peak before settling to steady state. However, for low Oh^{2} number flows, both the dynamic response and kinetic energy are oscillatory. Inspection of the results show a two-way coupling between the deformation rate and the fluid flow. The effect of the density ratio ρ[over ̃]=ρ_{i}/ρ_{o} (drop to ambient) on the dynamic response and fluid flow strength shows that for high Oh^{2} number flows both parameters remain essentially intact at steady state, while their evolution modes transition from a monotonic response to an oscillatory one at high density ratio. However, for low Oh^{2} number flows, with an increase in ρ[over ̃], the oscillation frequency of both parameters remain intact, while their oscillation amplitudes increase.

Frequency-Specific Modulation of Slow-Wave Neural Oscillations via Weak Exogeneous Extracellular Fields Reveals a Resonance Pattern.

Single neurons often exhibit endogenous oscillatory activity centered around a specific frequency band. Transcranial alternating current stimulation (tACS) can generate a weak oscillating extracellular field in the brain that causes subthreshold membrane potential shifts that can affect spike timing at the single neuron level. Many studies have now shown that the endogenous oscillation can be entrained when the tACS frequency matches that of the exogenous extracellular field. However, the effect of tACS on the amplitude of the endogenous oscillation has been less well studied. We investigated this by using exogenous extracellular fields to modulate slow-wave neural oscillations in the ketamine anesthetized male Wistar rat. We applied spatially broad extracellular fields of different frequencies while recording spiking activity from single neurons. The effect of the exogenous extracellular field on the slow-wave neural oscillation amplitude (NOA) followed a resonance pattern: large modulations were observed when the extracellular frequency matched the endogenous frequency of the neuron, while extracellular fields with frequencies far away from the endogenous frequency had little effect. No changes in spike-rate were observed for any of the extracellular fields applied. Our results demonstrate that in addition to the previously reported entrainment and Arnold tongue patterns, weak oscillating extracellular fields modulate the amplitude of the endogenous neural oscillation without any changes in spike-rate, and that this modulation follows a frequency-specific resonance pattern.SIGNIFICANCE STATEMENT Neural activity often oscillates around specific endogenous frequencies. Transcranial alternating current stimulation (tACS) is a neuromodulation method which biases spike-times and alter endogenous activity. Most tACS studies focus on entrainment effects which occur when tACS and endogenous neural frequencies are matched. In this study we varied the frequency of the applied tACS and investigated its effect on amplitude of the neural oscillation. Our results revealed a resonance pattern where tACS frequencies close to the endogenous frequency caused an increase in neural oscillation amplitude (NOA) specifically at the applied tACS frequency, while applying tACS frequencies farther away caused little or no change in NOA. Furthermore, applying tACS at differing frequencies caused the amplitude of the neural oscillation at the prestimulation endogenous frequency to decrease.

Numerical study of keyhole dynamics and porosity formation during high‐power oscillating laser welding of medium‐thick aluminum alloy plates

High-power oscillating laser welding (HP-OLW) provides an effective approach for joining medium-thick aluminum alloy plates. However, the effects of laser beam oscillation on keyhole dynamics and porosity formation requires further understanding. In this work, the keyhole dynamics and porosity formation during HP-OLW of medium-thick aluminum alloy plates are investigated using numerical simulation, and effective methods for suppressing porosity defects are proposed. Results indicate that the molten pool behind keyhole shows a downward flow trend during HP-OLW, which in fact is adverse to the escaping of bubbles. Nevertheless, the generation probability of bubble is decreased because the keyhole can re-act on bubbles and eliminate them. The probability of bubble generation is highest at t = [kT, T/8 + kT] because keyhole tends to collapse at this moment. The bubbles have highest probability for escaping at t = [3T/8 + kT, 5T/8 + kT] because laser beam can re-act on bubbles. The porosity defects can be suppressed by reducing the number of bubbles generated or by enhancing the escape probability of bubbles. This can be achieved by increasing oscillation amplitude or decreasing welding speed. Experimental verification shows that the porosity decreases significantly from 6.4% to 1.4% when v decreases from 30 mm/s to 15 mm/s. This work can provide guidance for suppression of porosity defects in HP-OLW of aluminum alloy, which has important theoretical significance and application value.

Vortex induced vibration of four cylinders configurations at critical spacing in 0° and 45° flow incidence angle

This work presents a numerical study on vortex-induced vibration of four circular cylinders with one and two degrees of freedom subjected to uniform free-stream velocity U at α=0°and α=45°angle of incidence, Reynolds number of Re=UD/ν=150, center-to-center cylinders distance of L=3.5D and a mass ratio of ρc/ρ=2.5and24/π. The structural damping is set to zero for reduced velocity UR=U/(fnD) from 1 to 14. The high-precision code Incompact3d coupled to a moving immersed boundary method is applied to solve the governing equations. The oscillation amplitudes (Ax, Ay), transverse force frequency ratio f/fn, wake Strouhal number St, vorticity fields, and force coefficients statistics are investigated. A wake-stiffness is inferred by f/fn=2 of the lateral cylinders in the 45°rotated configuration. For the one degree of freedom case, when α changes from 0°to 45°, the overall peaks of the force coefficients statistics and oscillation amplitudes increase. In contrast, for the two degrees of freedom case (2dof), only the |C¯L| and CL′ overall peaks increase about 10%, and the Ax peak decreases around 37%. Besides, when L/D is reduced from 5 to 3.5 in 2dof, the overall maximum Ay is slightly affected. However, for α=0°, the |C¯L| , CD′ and Ax peaks respectively are 106%, 284% and 177% higher at L/D=3.5. For α=45°, the CD′ and Ax peaks increase 42% and 33%, respectively.

Optimal Spin Polarization for Spin–Orbit-Torque Memory and Oscillator

Spin–orbit-torque arising from in-plane polarized spin current has attracted considerable attention for its potential in high-performance magnetic random access memories and spin torque oscillators (STOs). Recent studies have shown that an out-of-plane spin polarization can be even more efficient to drive the magnetization dynamics. In this work, we thoroughly examine the effect of the out-of-plane polarized spin current through macro-spin simulations. We propose separately the desirable conditions of spin polarization for spintronic memory and oscillator devices. It is found that for memory applications, a comparable strength of out-of-plane and in-plane spin polarization is necessary to simultaneously achieve a reduced switching current density and a faster switching speed. Besides, in STOs, the presence of a sizable out-of-plane polarization can provide an increase of oscillation amplitude and/or an improvement on frequency tunability. Our results address the importance of deliberate control on the spin polarization in designing efficient spintronic devices.

Temporal Instability of Liquid Jet in Swirling Gas with Axial Velocity Oscillation

In the present study, viscous potential flow theory is employed to investigate the temporal instability of a viscous liquid jet in swirling gas with axial velocity oscillation. The dispersion-relation equation is obtained between dimensionless growth rate and wave number. Because of the axial velocity oscillation of gas, there are more than one instability regions, including Kelvin–Helmholtz (K–H) instability region and parametric instability region. The results suggest that increasing gas swirling strength stabilizes the liquid jet in axisymmetric mode, while it enhances the instability in nonaxisymmetric modes. And the gas viscosity has a destabilizing effect. In addition, increasing the forcing frequency enhances instability in the K–H instability region. However, it has a stabilizing effect in the parametric instability region. Furthermore, increasing oscillation amplitude enhances instability in both the K–H instability region and the parametric instability region. Because of the competition between the parametric and K–H instabilities, the location of dominant wave number is uncertain.

Investigation into steam generator tube life expectancy affected by SFR transients

One of the decay heat removal systems in sodium cooled fast reactors (SFR) is the operation grade decay heat removal system (OGDHRS). During deployment of OGDHRS, steam generator has to operate at off-normal operating conditions which may lead to serious concerns with respect to flow stability in steam generator and the associated thermal fatigue on the tubes. Towards understanding this, tests are performed on a 19-tube once through steam generator simulating two OGDHRS deployment schemes. The consequent fatigue damage assessment of steam generator tubes is carried out using Finite Element Method based commercial software and RCC MR French design code. The fatigue damage fraction is estimated to be∼0.06 suggesting the acceptability of both the schemes for future Indian SFR. The effects of dry-out point temperature oscillation amplitude and its frequency on tube life are also studied. It is found that the oscillation frequency doesn't have much influence on the stress/strain range. It is further found that an increase in oscillation amplitude leads to a near linear increase in the stress range.

Using Flare-Induced Modulation of Three- and Five-Minute Oscillations for Studying Wave Propagation in the Solar Atmosphere

We propose a method for diagnosing the physical conditions in the solar atmosphere using an increase in oscillation amplitudes resulting from minuscule solar flares. As an example, we consider a B2 flare, which caused a sharp short-lived increase in the amplitude of three- and five-minute oscillations in the lower layers of the solar atmosphere. Enhanced three- and five-minute oscillations propagated from the lower layers of the atmosphere into the corona. Such short oscillation trains made it possible to remove the uncertainties arising in the measurements of the phase and group lags between the layers. In addition, the amplification of the oscillations that reach the corona may add to the likelihood of a repeated flare. Studying oscillations in small flare events has the advantage of exploring the atmosphere in its quasi-quiet condition as opposed to powerful flares, which cause substantial and prolonged disturbance of the environment. In addition, small flares are much more common than powerful flares, which allows one to choose from a larger sample of observational material.

Kinematics of sea star legged locomotion.

Sea stars have slower crawling and faster bouncing gaits. Both speed and oscillation amplitude increase during the transition from crawling to oscillating. In the bouncy gait, oscillating vertical velocities precede oscillating horizontal velocities by 90 deg, as reflected by clockwise circular hodographs. Potential energy precedes horizontal kinetic energy by 9.6 deg and so they are nearly in phase. These phase relationships resemble terrestrial running gaits, except that podia are always on the ground. Kinetic and potential energy scale with body mass as Mb 1.1, with the change in kinetic energy consistently two orders of magnitude less, indicating that efficient exchange is not feasible. Frequency of the bouncy gait scales with Mb-0.14, which is similar to continuously running vertebrates and indicates that gravitational forces are important. This scaling differs from the Hill model, in which scaling of muscle forces determine frequency. We propose a simple torque-stabilized inverted pendulum (TS-IP) model to conceptualize the dynamics of this gait. The TS-IP model incorporates mathematics equivalent to an angular spring, but implemented by a nearly constant upward force generated by the podia in each step. That upward force is just larger than the force required to sustain the underwater weight of the sea star. Even though the bouncy gait is the rapid gait for these sea stars, the pace of movement is still very slow. In fact, the observed Froude numbers (10-2 to 10-3) are much lower than those typical of vertebrate locomotion and are as low or lower than those reported for slow-walking fruit flies, which are the lowest values for pedestrian Froude numbers of which we are aware.

Striped patterns in radially driven suspensions with open boundaries

We study the motion of radially driven fluid–immersed particles in a novel Hele–Shaw cell with open boundaries. The initially uniform suspension forms a striped pattern within a specific range of horizontal oscillation frequencies and for sufficiently large amplitudes. We observe that the initial coarsening dynamics of the stripes gradually slows down and the pattern reaches a steady state after a few minutes. The distance between the stripes in the steady state exhibits an exponentially saturating increase with increased oscillation amplitude or frequency. The width of the stripes decreases as a power-law with the frequency, while its amplitude dependence follows a logistic function. We propose a mechanism—based on the interplay between shear stress, hydrodynamic interactions, and frictional forces—to link the structural characteristics of the stripes to the properties of the oscillatory external drive.

The impact of tympanic membrane perforations on middle ear transfer function

PurposeInjury or inflammation of the middle ear often results in the persistent tympanic membrane (TM) perforations, leading to conductive hearing loss (HL). However, in some cases the magnitude of HL exceeds that attributable by the TM perforation alone. The aim of the study is to better understand the effects of location and size of TM perforations on the sound transmission properties of the middle ear.MethodsThe middle ear transfer functions (METF) of six human temporal bones (TB) were compared before and after perforating the TM at different locations (anterior or posterior lower quadrant) and to different degrees (1 mm, ¼ of the TM, ½ of the TM, and full ablation). The sound-induced velocity of the stapes footplate was measured using single-point laser-Doppler-vibrometry (LDV). The METF were correlated with a Finite Element (FE) model of the middle ear, in which similar alterations were simulated.ResultsThe measured and calculated METF showed frequency and perforation size dependent losses at all perforation locations. Starting at low frequencies, the loss expanded to higher frequencies with increased perforation size. In direct comparison, posterior TM perforations affected the transmission properties to a larger degree than anterior perforations. The asymmetry of the TM causes the malleus-incus complex to rotate and results in larger deflections in the posterior TM quadrants than in the anterior TM quadrants. Simulations in the FE model with a sealed cavity show that small perforations lead to a decrease in TM rigidity and thus to an increase in oscillation amplitude of the TM mainly above 1 kHz.ConclusionSize and location of TM perforations have a characteristic influence on the METF. The correlation of the experimental LDV measurements with an FE model contributes to a better understanding of the pathologic mechanisms of middle-ear diseases. If small perforations with significant HL are observed in daily clinical practice, additional middle ear pathologies should be considered. Further investigations on the loss of TM pretension due to perforations may be informative.

STABILITY ANALYSIS OF VISCOELASTIC LIQUID DROPLETS EXCITED BY RADIAL OSCILLATIONS

When a liquid drop is periodically excited by an external radial oscillation force, the instability of standing wave mode will be formed on its surface, which is known as the spherical Faraday instability problem. The oscillation frequency of the instability surface wave will render as a harmonic or sub-harmonic mode according to the different fluid physical parameters and the forced excitation conditions. Based on the linear small perturbation theory, this paper studies the instability behavior of the viscoelastic droplet surface wave subjected to the radial oscillating force. The oscillating radial force causes the momentum equations to be Mathieu equations which included time period coefficients. Therefore, the system becomes a parametric instability problem, which can be solved by Floquet theory. In this model, the characteristics of viscoelasticity are treated as an effective viscosity which related to the rheological model of the fluid, which simplifies the solving process of the problem. Based on the analysis of the neutral stability curve and growth rate of the surface wave, the influence of viscoelastic parameters on the stability of droplets were studied. The results showed that the increase of zero-shear viscosity (μ0) as well as deformation retardation time (λ2) can inhibit the growth of droplet surface wave, therefore increased the excitation amplitude which made the droplet unstable at a harmonic mode.With the increase of oscillation amplitude, the regions of unstable growth rate decrease, and as the oscillation frequency increase, the value of droplet surface wave growth rate decrease. Through the analysis of the growth rate, it can be concluded that the increase of the stress relaxation time (λ1) increases the growth rate, thereby promoting the growth of surface wave on the droplet.

Numerical Thermal Analysis of a Hot Noncircular Rotating Cylinder in the Presence of a Magnetic Field

Abstract In this paper for the first time, a Lattice Boltzmann Simulation is performed to analyze the simultaneous effects of a hot rotating elliptic cylinder and the magnetic field on the mixed convection flow in a square enclosure. Complicated flow patterns and isotherm plots are found and analyzed in the concentric annulus between the internal elliptic cylinder and the outer square enclosure. Results indicate that increasing the Reynolds number, instantaneous averaged Nusselt number of the enclosure and its oscillation amplitude increase, while decrease with increasing the Hartmann number especially at its lower values. Furthermore, response surface method is adopted to find the optimal location of the elliptic cylinder. Response surface optimization results reveal that the average Nusselt number shows a decreasing–increasing trend with increasing both nondimensional parameters of cylinder center (Xc, Yc). Finally, the optimal location of the elliptic cylinder for the maximum heat transfer rate is obtained as Xc = 0.65 and Yc = 0.35. Moreover, a comparative study is performed to evaluate the heat transfer effects of the elliptical cylinder rotation as compared to circular cylinder. It was found that the elliptical cylinder rotation has a significant effect on the heat transfer enhancement, especially at high values of Re and Ha. As an example, the heat transfer rate for the elliptical cylinder at Re = 200 is increased by 13% and 34% as compared to the circular cylinder at Ha = 50 and 100, respectively.

Prediction of temperature field and residual stress of oscillation laser welding of 316LN stainless steel

The finite element (FE) analysis in this study is used to predict the effect of oscillation parameters on the temperature field and residual stress and as-welded distortion of 316 LN stainless steel weld joint. The simulated results indicate the simulation temperature field is good in agreement with the geometry morphology of actual weld metal (WM), and with the increase of oscillation frequency, the simulation temperature of WM gradually decreases. However, the temperature first decreases and then increases with the increase of amplitude. The residual stress concentrates on the WM and the surrounding heat-affected zone (HAZ). With the increase of oscillation frequency and amplitude of laser beam, the compressive stress converts to tensile stress, and the residual stress and the as-welded distortion gradually present a decreasing trend. The oscillation laser welding can alleviate the stress concentration phenomenon.