Effects Of Velocity Ratio Research Articles

Study on the motion characteristics of Janus based on the squirmer model in the flow

The motion characteristics of Janus in the flow are studied numerically using the lattice Boltzmann method based on the squirmer model. The effects of velocity ratio J on the right and left hemisphere surface of Janus, particle Reynolds number Rep, flow Reynolds number Rec, initial orientation angle φ0 on Janus trajectory, and lateral equilibrium position yeq/H are analyzed. The results showed that, for the motion of Janus in stationary power-law fluids in a channel, Janus moves randomly in a small space in shear-thickening fluids when Rep is low and exhibits three motion modes at Rep = 5. The larger the J value, the easier it is for Janus to reach yeq/H. The higher the Rep, the closer the yeq/H is to the lower wall. In shear-thinning fluids, the motion of Janus exhibits significant randomness at Rep = 0.5 and 1, reaches the same yeq/H at Rep = 2 and 3, and tends toward yeq/H near the centerline and along the upper wall, respectively, at Rep = 4 and 5. For the motion of Janus particles in a channel flow of power-law fluids, in shear-thinning fluids, no matter what value J is, Janus reaches yeq/H on the centerline. The lower the Rep, the closer the yeq/H is to the wall. Two particles move toward yeq/H when Rep ≥ 1. The higher the Rep, the closer the yeq/H is to the centerline. The two particles will exhibit the upstream mode at Rep = 2. Two particles eventually reach yeq/H at different Rec. When φ0 > 0°, the two particles first eventually tend toward yeq/H = 0.2 and 0.8. When the value of φ0 is negative, the larger the absolute value of φ0 and higher the Rep, the more likely particles are to exhibit upstream mode.

Effect of velocity ratio and Mach number on thin lip coaxial jet

Abstract The effect of nozzle lip thickness and velocity ratio on coaxial subsonic jet mixing, at different Mach numbers, has been studied experimentally and numerically. Decay of coaxial subsonic jets emanating from coaxial nozzles of lip thickness 0.7, 1.7 and 2.65 mm with velocity ratio (VR) from 0.2 to 1.0 at primary jet exit Mach numbers of 0.6, 0.8 and 1.0 has been studied. Free jet without co-flow (VR0) was also studied for comparison. Jet centerline Mach number decay, turbulence and velocity variation in the radial direction are analyzed. The results show that mixing the coaxial jet at a low-velocity ratio is better than a high-velocity ratio, at all Mach numbers of the present study. The nozzle lip thickness has a significant influence on the secondary jet. Mixing of the jet in the presence of VR0.2 coaxial jet is found to be the highest. Characteristic decay of Mach 0.8 and 1.0 jet for lip thickness 1.7 and 2.65 mm is faster than lip thickness 0.7 mm. For a given lip thickness, increasing of velocity ratio is found to retard the mixing between primary and secondary jets.

Effect of Velocity Ratio, Viscosity Ratio, Contact Angle, and Channel Size Ratio on Droplet Formation

Effect of Velocity Ratio, Viscosity Ratio, Contact Angle, and Channel Size Ratio on Droplet Formation

Combined effects of velocity ratio and radii sizes of internal cylinder on the flow instability in a two-sided lid-driven cubical cavity

This paper computes the laminar flow in a three-dimensional (2D) two-sided lid-driven cubical cavity induced by a centrally located circular cylinder by Finite Volume Method and multigrid acceleration. The cavity has left and right parallel lid-driven walls and all the other walls completing the domain are motionless. Furthermore, different wake parameters are considered throughout this study; velocity ratios [[Formula: see text] and 0.75] and radius size of the inner cylinder [[Formula: see text] and 0.2]. Thereafter, the study has been extended to the unsteady state by further increasing Reynolds number till reaching up to 2000. It is noticed that the highest critical Reynolds number corresponds to the lowest radius size and conversely. By further increasing the velocity ratio till reaching 0.25, 0.5 and 0.75, where the left moving lid began to have an effect, the opposite phenomenon occurs and we note that the highest critical Reynolds number corresponds to the highest radius. This can be explained by the fact that when the cavity is two-sided lid-driven, the velocity of the fluid particles accelerates. Besides, it has been proved that both parameters [Formula: see text] and a radius size [Formula: see text] promote faster transition to the unsteadiness rather than the remaining velocity ratios and radii sizes.

Effects of velocity ratio on dynamics of sand-water coaxial jets

Detailed laboratory experiments were performed to study the dynamics and mixing properties of vertically discharged sand-water coaxial jets in stagnant water. The effects of velocity ratio on the fluid dynamics of two-phase coaxial jets were studied. The axial and radial distributions of sand concentration and velocity were measured with an advanced optical fiber probe (PV6), and sand concentration and velocity measurements were formulated based on flow characteristics. The experimental results demonstrated that the axial velocity and concentration decay rate along the mixing zone, a length approximately six times greater than the nozzle diameter, were independent of the velocity ratio, Ru. Beyond the initial mixing zone, the axial decay rate of sand concentration increased with increasing velocity ratio and became similar to the concentration decay rate of slurry jets. The axial velocity decay of sand-water coaxial jets indicated an inverse relationship with velocity ratio: velocity decay rates decreased with increasing Ru. Experimental observations of sand-water coaxial jets showed that the spreading rate of coaxial jets decreased with increasing velocity ratio; for the coaxial jet with the highest Ru, the spreading rate reached the spreading rate of slurry jets. The integral quantities of jets such as mass and momentum fluxes were calculated using sand concentration and velocity profiles. Average drag coefficients were calculated using momentum balance and proposed equations for sand velocity and concentration. The variations of drag coefficient and interparticle collision parameter indicated that the particle grouping effect reduced the drag coefficient in sand-water coaxial jets with lower velocity ratios. The axial variations of the interparticle collision parameter indicated a strong correlation with velocity ratio. The strong correlation between velocity ratio and the mixing properties of sand-water coaxial jets suggests the velocity ratio is an effective design parameter to control and optimize particle mixing in stagnant water.

Rebound Dynamics of Two Droplets Successively Impacting an Inclined Surface

The dynamic behaviors of two droplets successively impacting inclined surfaces are simulated by a three-dimensional pseudopotential lattice Boltzmann model based on multi-relaxation-time. The effect of velocity ratio of two successive droplets on the contact time is investigated and two rebounding regimes are identified depending on whether the coalesced droplet retouches the surface or not. Increasing the velocity ratio leads to a stronger interaction between the two droplets and the phenomenon of coalesced droplet retouching the surface is observed when the velocity ratio exceeds a threshold, resulting in a longer contact time. An outcome map of droplet rebounding is obtained at various velocity ratios and contact angles of surface. It is found that the coalesced droplet cannot rebound from the surface at a larger velocity ratio and a lower contact angle of surface. Furthermore, the effect of the length between impact points on the contact time is exhibited, and a longer length is beneficial to coalesced droplet rebounding.

Qualitative effect of velocity ratio on the behaviour of a circular synthetic jet flush mounted on a torpedo model in quiescent and cross-flow conditions

A synthetic jet on a curved surface and at high velocity ratios is of immense interest for its possible usage in a torpedo or other naval applications. However, despite exhaustive research on synthetic jets on a flat surface in cross-flow, very few studies have been conducted on synthetic jets on a torpedo-like surface. This study experimentally explores a synthetic jet mounted on a torpedo shaped model in quiescent and cross-flow conditions. The synthetic jet actuation mechanism consists of a motor driven eccentric cam, which oscillates the diaphragm and generates synthetic jet. Initially, the synthetic jet is characterized for two different diaphragm displacements and at four distinct actuation frequencies in the range of 1 Hz – 6 Hz in a quiescent flow environment. Subsequently, similar study is carried out in cross-flow for three different cross-flow velocities ranging from 7.2 – 32 cm/s, at a fixed amplitude of diaphragm oscillations. The flow visualization results qualitatively indicate that the vortex rings in synthetic jet can be classified into two major groups: stretched vortex rings and distorted tilted vortex rings. The flow structures primarily depend on the velocity ratio, which is function of actuation frequency and cross-flow velocity. At a low velocity ratio, the leading vortex is followed by a strong trailing jet. Two distinctly different trajectories are observed for the leading and trailing vortices at higher velocity ratios. At a low cross-flow velocity and higher actuation frequency, synthetic jet leads to increase in the velocity within the boundary layer on the upstream side of the orifice.

Effect of velocity ratio on the interaction between plasma synthetic jets and turbulent cross-flow

Plasma synthetic jet actuators (PSJAs) are particularly suited for high-Reynolds-number, high-speed flow control due to their unique capability of generating supersonic pulsed jets at high frequency (${>}5$ kHz). Different from conventional synthetic jets driven by oscillating piezoelectric diaphragms, the exit-velocity variation of plasma synthetic jets (PSJs) within one period is significantly asymmetric, with ingestion being relatively weaker (less than $20~\text{m}~\text{s}^{-1}$) and longer than ejection. In this study, high-speed phase-locked particle image velocimetry is employed to investigate the interaction between PSJAs (round exit orifice, diameter 2 mm) and a turbulent boundary layer at constant Strouhal number (0.02) and increasing mean velocity ratio ($r$, defined as the ratio of the time-mean velocity over the ejection phase to the free-stream velocity). Two distinct operational regimes are identified for all the tested cases, separated by a transition velocity ratio, lying between $r=0.7$ and $r=1.0$. At large velocity and stroke ratios (first regime, representative case $r=1.6$), vortex rings are followed by a trailing jet column and tilt downstream initially. This downstream tilting is transformed into upstream tilting after the pinch-off of the trailing jet column. The moment of this transformation relative to the discharge advances with decreasing velocity ratio. Shear-layer vortices (SVs) and a hanging vortex pair (HVP) are identified in the windward and leeward sides of the jet body, respectively. The HVP is initially erect and evolves into an inclined primary counter-rotating vortex pair ($p$-CVP) which branches from the middle of the front vortex ring and extends to the near-wall region. The two legs of the $p$-CVP are bridged by SVs, and a secondary counter-rotating vortex pair ($s$-CVP) is induced underneath these two legs. At low velocity and stroke ratios (second regime, representative case $r=0.7$), the trailing jet column and $p$-CVP are absent. Vortex rings always tilt upstream, and the pitching angle increases monotonically with time. An $s$-CVP in the near-wall region is induced directly by the two longitudinal edges of the ring. Inspection of spanwise planes ($yz$-plane) reveals that boundary-layer energization is realized by the downwash effect of either vortex rings or $p$-CVP. In addition, in the streamwise symmetry plane, the increasing wall shear stress is attributed to the removal of low-energy flow by ingestion. The downwash effect of the $s$-CVP does not benefit boundary-layer energization, as the flow swept to the wall is of low energy.

An investigation on convective heat transfer performance around piezoelectric fan vibration envelope in a forced channel flow

An experimental and numerical investigation is performed in the current study to further explore the convective heat transfer performance by a vertically-oriented piezoelectric fan in the presence of channel flow. The effects of velocity ratio and fan tip-to-heated surface clearance are taken into considerations. It is illustrated that the presence of channel flow has an innegligible influence on the vibration amplitudes of the piezoelectric fan under large channel flow velocities. In the presence of channel flow, the vortical structures at the edges of vibrating fan are certainly suppressed, especially under large velocity ratios. On the other hand, the vortical streaming flow mixes with the channel flow to form a long stripe of vortical structure downstream of the fan vibration envelope. Under small velocity ratios, the impingement role of streaming flow along fan tip is still dominated and simultaneously the channel flow passing through the vibration envelope is effectively pulsated. Therefore, combined flows generally produce heat transfer enhancement around the fan vibration envelope related to the pure vibrating fan, especially at a small non-dimensional tip-to-surface gap. While under large velocity ratios, the impingement role of streaming flow induced by a vibrating fan is seriously weakened by the strong channel flow. The convective heat transfer produced by combined flows in the fan vibration envelope is generally reduced in comparison with pure piezoelectric fan. Related to the pure channel flow, the combined flows effectively improve the convective heat transfer, especially downstream of the fan vibration envelope.

Investigation of Velocity ratios on Mechanical and Microstructural Characterization of Friction Stir Welded Dissimilar 2024 and 7075 Aluminium Alloy

The effect of velocity ratios on tensile properties and microhardness of dissimilar Aluminum alloys 2024 and 7075 welds fabricated via friction stir welding (FSW) have been analyzed in this current study. Microstructure analysis of the cross-section of the joints revealed distinct lamellar bands and various degrees of intermixing that were correlated with tool rotational speeds. Due to the distinct mechanical properties of the two alloys, microhardness measurements showed a consistent asymmetric hardness distribution profile across the weld nugget, regardless of tool rotational speed. Under monotonic tensile loading, an increase in the joint strength was observed with the increase in the velocity ratio. On the other hand, the failure observed through the heat-affected zone at high rotational speeds was produced due to the material softening as confirmed by the microhardness measurements. The observed tensile properties and hardness are correlated with microstructure and fracture features.

Deformation and recrystallization microtextures of an austenitic steel during asymmetrical hot rolling process

Deformation and recrystallization microtextures during the asymmetrical hot rolling (ASHR) process were investigated for a high-Mn austenitic steel by means of electron backscattered diffraction. The effects of velocity ratio and rolling reduction on the texture evolution were also analyzed. It was found that with the increase in velocity ratio and rolling reduction, the surface layer of 60% rolled ASHR plate develops a shear texture containing the γ-fiber, rotated cube and rotated copper components, but in the central layer is still a fcc rolling texture with a minor rotation around the transverse direction. The formation of surface shear texture could be understood as a consequence of the increased shear strain level, and it followed some certain crystallographic rotational relationships with the fcc rolling texture components. Additionally, the change of deformation texture under ASHR condition had an influence on the recrystallization texture, in which the proportion of {112} and {113} orientations was slightly improved, due to their rapid growth advantage and the orientation heredity of rotated copper shear texture.

Comparative Study of Mixing Characteristics of Particle-Laden Shear and Swirl Coaxial Jets

The effect of velocity ratio and recess on the mixing characteristics of gas-particle swirl and shear coaxial injectors was investigated. The concentration and mixing efficiencies of the particles and gas were measured by planar nephelometry based on Mie scattering and acetone planar laser-induced fluorescence methods, respectively. The mixing mechanism and mixing efficiency of the shear and the swirl coaxial injectors were compared with and without recess. The results showed that the mixing efficiency of the former was higher than that of the latter in the case without a recess because the swirling momentum of the coaxial gas did not change the particle concentration half-width. The mixing efficiencies of both injectors in the case without a recess were sensitive to changes in the velocity ratio. However, in the case of injectors with a recess, the mixing efficiency of the swirl coaxial injector was higher than that of the shear coaxial injector because the internal interaction of two propellants led to an increase in the particle concentration half-width, and the mixing efficiency of the swirl coaxial injector was less sensitive to changes in the velocity ratio than that of the shear coaxial injector.

Insights into the growth rate of spatially evolving plane turbulent free-shear layers from 2D vortex-gas simulations

Although the free-shear or mixing layer has been a subject of extensive research over nearly a century, there are certain fundamental issues that remain controversial. These include the influence of initial and downstream conditions on the flow, the effect of velocity ratio across the layer, and the nature of any possible coupling between small scale dynamics and the large scale evolution of layer thickness. In the spirit of the temporal vortex-gas simulations of Suryanarayanan et al. [“Free turbulent shear layer in a point vortex gas as a problem in nonequilibrium statistical mechanics,” Phys. Rev. E 89, 013009 (2014)], we revisit the simple 2D inviscid vortex-gas model with extensive computations and detailed analysis, in order to gain insights into some of the above issues. Simulations of the spatially evolving vortex-gas shear layer are carried out at different velocity ratios using a computational model based on the work of Basu et al. [“Vortex sheet simulation of a plane canonical mixing layer,” Comput. Fluids 21, 1–30 (1992) and “Modelling plane mixing layers using vortex points and sheets,” Appl. Math. Modell. 19, 66–75 (1995)], but with a crucial improvement that ensures conservation of global circulation. The simulations show that the conditions imposed at the origin of the free shear layer and at the exit to the computational domain can affect flow evolution in their respective downstream and upstream neighbourhoods, the latter being particularly strong in the single stream limit. In between these neighbourhoods at the ends is a regime of universal self-preserving growth rate given by a universal function of velocity ratio. The computed growth rates are generally located within the scatter of experimental data on plane mixing layers and closely agree with recent high Reynolds number experiments and 3D large eddy simulation studies. These findings support the view that observed free-shear layer growth can be largely explained by the 2D vortex dynamics of the quasi-2D large-scale structures known to be a characteristic of plane mixing layers.

A novel application of sideways extrusion to produce curved aluminium profiles: Feasibility study

Abstract Conventional curved aluminium alloy profile forming methods which depend on apparatus, additional to extrusion tooling, to bend extruded, straight sections inevitably decrease manufacturing efficiency and have to deal with springback and cross-sectional distortion. In this article, a novel energy-efficient forming method based on internal differential material flow, differential velocity sideways extrusion (DVSE), is proposed, which can directly form billets into curved profiles/sections by extrusion alone, using two opposing punches. A detailed study has been conducted and cold extrusion/bending of round bars and tubes was examined. It was found that curved profiles/sections with no distortion are formed, and adjustable profile curvature can be achieved by changing the velocity ratio of a slower lower punch to a faster upper punch and also extrusion ratio. Lower velocity ratio and larger extrusion ratio tend to increase the material flow velocity gradient at the die exit and lead to greater curvature. Effect of velocity ratio is greater than that of extrusion ratio, especially when velocity ratio is less than 1/3. Effect of the extrusion ratio is small when velocity ratio is greater than 0.5, after which the effect of extrusion ratio becomes more significant as velocity ratio decreases.

The investigation of ship maneuvering with hydrodynamic effects between ships in curved narrow channel

The hydrodynamic interaction between two large vessels can't be neglected when two large vessels are closed to each other in restricted waterways such as in a harbor or narrow channel. This paper is mainly concerned with the ship maneuvering motion based on the hydrodynamic interaction effects between two large vessels moving each other in curved narrow channel. In this research, the characteristic features of the hydrodynamic interaction forces between two large vessels are described and illustrated, and the effects of velocity ratio and the spacing between two vessels are summarized and discussed. Also, the Inchon outer harbor area through the PALMI island channel in Korea was selected, and the ship maneuvering simulation was carried out to propose an appropriate safe speed and distance between two ships, which is required to avoid sea accident in confined waters. From the inspection of this investigation, it indicates the following result. Under the condition of SP12≤0.5L, it may encounter a dangerous tendency of grounding or collision due to the combined effect of the interaction between ships and external forces. Also considering the interaction and wind effect as a parameter, an overtaken and overtaking vessel in narrow channel can navigate while keeping its own original course under the following conditions; the lateral separation between two ships is about kept at 0.6 times of ship length and 15 degrees of range in maximum rudder angle. On the other hand, two ships while overtaking in curved narrow channel such as Inchon outer harbor in Korea should be navigated under the following conditions; SP12 is about kept at 1.0 times of ship length and the wind velocity should not be stronger than 10 m/s.

Effects of rolling parameters of snake hot rolling on strain distribution of aluminum alloy 7075

The realization way of snake rolling was introduced. Flow velocity, strain and stress distribution of 7075 aluminum alloy plate during snake rolling and symmetrical rolling were analyzed in Deform 3D. Effects of velocity ratio, offset distance between two rolls and pass reduction on the distribution of equivalent strain and shear strain were analyzed. The results show that flow velocity and equivalent strain on the lower layer of the plate are larger than those of the upper layer because of the larger velocity of the lower roll and the gap is increased with the increase of velocity ratio and pass reduction. The shear strain of rolling direction in the center point is almost zero during symmetrical rolling, while it is much larger during snake rolling because of the existence of rub zone. The shear strain is increased with the increase of velocity ratio, offset distance and pass reduction. This additional shear strain is beneficial to improve the inhomogeneous strain distribution.

덕티드 로켓의 이차 연소기 내에서 입자의 크기와 속도비가 유동 혼합에 미치는 영향

본 연구에서는 유동의 속도비와 입자의 크기가 덕티드 로켓의 2차 연소기 내의 유동 혼합 특성에 미치는 영향을 고찰하였다. 연구의 실험적 기법으로는 PIV(Particle Image Velocimetry)가 적용되고 수치적 방법으로는 LES(Large Eddy Simulation)가 사용되었다. PIV 입자로는 직경 <TEX>$5{\mu}m$</TEX>와 <TEX>$50{\mu}m$</TEX>의 폴리스틸렌이 각각 사용되었고, 속도비는 각각 5, 3, 1.5로 수행되어졌다. 고속유로를 통한 유동이 저속유로의 유동과 혼합되어 재부착점과 재순환 영역을 형성하였고, 속도비가 작을수록 재순환 영역이 커졌다. 큰 입자를 적용한 경우가 증가된 모멘텀에 의해 재순환 영역이 크게 나타났고, 유동 특성에 대한 속도비의 영향은 작은 입자를 적용한 경우에 비해 크지 않게 나타났다. In this study, the effect of velocity ratio and particle size on the flow mixing characteristics in the secondary combustor was investigated. Both PIV(Particle Image Velocimetry) technique and LES(Large Eddy Simulation) were applied. Two sizes of Polystyrene PIV seeding particle of 5 and <TEX>$50{\mu}m$</TEX>, and three velocity ratios of 5, 3, and 1.5 were considered. Results showed that the mixing of two air streams created reattachment and recirculation regions. The size of the recirculation region was decreased as the velocity ratio increased. For the larger particle cases, due to the increased momentum by the larger particles, the size of the recirculating regions were larger than that of the smaller particle cases and the effect of the velocity ratio was not as significant as in the smaller particle case.

3D scour below pipelines under waves and combined waves and currents

This paper presents the results of an experimental investigation on three-dimensional local scour below a rigid pipeline subjected to wave only and combined wave and current conditions. The tests were conducted in a conventional wave flume. The major emphasis of the investigation was on the scour propagation speed (free span expansion rate) along the pipeline after local scour was initiated at a controlled location. The effects of flow ratio (steady current velocity vs. combined waves/current velocity), flow incidence angle and pipeline initial embedment depth on free span expansion rate were investigated. It was observed that the scour along the pipeline propagated at a constant rate under wave only conditions. The scour propagation rate decreased with increasing embedment depth, however, increased with the increasing Keuglegan–Carpenter (KC) number. Under combined wave and current conditions, the effect of velocity ratio on scour propagation velocity along the pipeline was quantified. Empirical relationships between the scour propagation rate (Vh) and key parameters such as the KC number and embedment depth (e/D) were established based on the testing results.

Effects of velocity ratio and inflow pulsations on the flow in a T-junction by Large Eddy Simulation

Large Eddy Simulations (LES) of the flow in a T-junction are performed and analyzed in terms of mixing quality, secondary structures and flow modes. Different mass flow ratios between the two inlet branches are applied and strong pulsating inflow conditions are considered in order to simulate engine-like conditions. The mixing quality is assessed by using several mixing parameters. For steady inflow conditions, it is found that the mixing quality depends on the mass flow ratio. For cases where the branch jet impinges on the opposite pipe wall, the largest turbulence intensities and mixing qualities are observed. Furthermore, the distribution of the mean concentration is dependent on the evolution of secondary structures, which also depend on the mass flow ratio. Flow pulsations are found to affect the mixing quality depending on the pulsation frequency. The spatial mixing quality is not necessarily enhanced by the pulsations. The flow structures are attributed to a Kelvin–Helmholtz instability due to the strong shear layer and vortex shedding past the branch jet. The normalized frequency of the vortex shedding is dependent on the mass flow ratio. Dynamical Mode Decomposition (DMD) is applied in order to investigate the spatial shapes of the flow modes. It is found that DMD is able to capture the vortex shedding mechanism even for the pulsating cases.

Comparison of Turbulent Intensity Component of the Stratified Flow in the Braided River with Different Upstream Flowrates

The physical model experiment was carried out to study the effect of velocity ratio of two layers on the turbulence intensity of stratified flow in the typical braided rivers with two symmetrical anabranches. Two velocity ratios were selected, and the distributions of depth-averaged turbulent intensity component in the left anabranch were analyzed. When velocity ratio became smaller, the turbulent intensity increased and peak turbulent intensity zone transferred from the left area of centerline to the right area of centerline at the section before the bend apex of the anabranch. In the outlet section of the anabranch, turbulent intensity increased at the inner bank. The distributions of lateral-averaged turbulence intensity components in the vertical sections of left anabranch were also analyzed. Turbulence intensity component in the interface of hot and cold water was both high in two velocity ratios. As water flowed downstream, the difference of turbulent intensity component in the vertical direction decreased obviously.