Dimensionless Oscillation Amplitude Research Articles

Influence of low air pressure on flame instability of methane buoyant laminar diffusion flames

This work experimentally investigated the buoyancy induced flame instability of laminar diffuison flames under different sub-atmospheric pressures (20kPa–100 kPa) in a reduced pressure chamber. Circular nozzle with inner diameter of 8mm was used to produced the laminar diffuison flames, and methane as one of the most commonly used renewable energies, was selected as the studied fuel gas. In this work, flame oscillation regime, flame oscillation amplitude, flame oscillation intensity and flame oscillation frequency were measured and analyzed. The experimental results show that first, for a given mass flow rate, a stabe laminar diffusion flame would transform into a Tip-flickering flame and then even a Bulk-flickering flame with increasing the air pressure. Such evolution was then interpreted physically and a dimensionless parameter was proposed for the purpose of determining flame oscillation regime. Second, The dimensionless flame oscillation amplitude can be non-dimentionlly correlated with a proposed dimensionless parameter, and a model for predicting the flame oscillation amplitude has been developed. Third, the flame oscillation frequency increased with increasing air pressure for a fixed mass flow rate. A global model for predicting the flame oscillation frequency under different sub-atmospheric pressures has been established.

Dynamic Surface Tension of a Charged Spherical Water Drop

In the asymptotic calculations of the first order of smallness by the dimensionless amplitude of oscillations of the water drop, the effect of the dynamic surface tension on the parameters of oscillations was investigated on the model of an ideal uncompressible liquid. It was shown that the effect of the dynamic surface tension is significantly manifested at the frequencies of the external influence of the order inverse to the water relaxation time. At such frequencies, under the influence of the external influences, the double electric layer is destroyed (there is a violation of the order of water molecules in the near-surface layer), the free energy of the surface increases, and, with it, the value of the surface tension of the liquid. The dynamic surface tension affects the acoustic radiation from the droplet through a change in the value of the surface tension coefficient. The contribution to electromagnetic radiation of the oscillating droplet is associated with the destruction of the ordering of the near-surface water molecules in the double electric layer.

Effect of the density ratio on the oscillatory Saffman–Taylor instability in vertical conical Hele–Shaw cell

The stability of an oscillating interface between two immiscible liquids with a high viscosity contrast in an axisymmetric conical Hele–Shaw cell in dependence on the density ratio is studied experimentally. The symmetry axis of the cell is vertical, while the tangential component of the gravy acts on the axisymmetric interface. The contact line is almost motionless, while the low-viscosity liquid penetrates a high-viscosity one in the form of an axisymmetric “tongue” in the course of an oscillating cycle. The increase in the oscillation amplitude leads to the development of azimuthal patterns (fingers) at the interface. Fingers of a low-viscosity liquid appear when a viscous liquid is being squeezed out of the cell and reach their maximum length at the maximum displacement of the interface. Then, the fingers decrease and are replaced by small depressions penetrating into the low-viscosity liquid in the phase of maximum boundary displacement toward the low-viscosity liquid. The increase in the density ratio of liquids has a stabilizing effect on the interface: The instability threshold is shifted to the higher oscillation amplitudes. Also, the stability threshold is independent of whether the high-viscosity or low-viscosity liquid is denser than the other one. We propose a new dimensionless parameter that controls the stability of the interface—the multiplication of the square root of the capillary number and the dimensionless amplitude of interface oscillations. It is revealed that the critical value of the stability parameter increases linearly with an increase in the density ratio of liquids.

Experimental study on energy harvesting maximization from the flow-induced vibration of a highly confined bluff body

Flow-induced vibrations of rigid prisms supported elastically were studied experimentally in a free-surface water channel with a high blockage (2/5). The study focused on finding the prism cross-sectional shape that maximizes the efficiency of energy harvesting. Seven cross-sectional shapes were tested: square, circular, 45° tilted square, equilateral triangle, isosceles 120° triangle, D-section, and C-section. All other dimensionless parameters of the problem, mass ratio, damping, blockage ratio, reduced velocity range, and the Reynolds (Re) number (characteristic velocity times characteristic length divided by kinematic viscosity) range (400–1070), were kept unchanged. By doing so, the effect of the cross-sectional shape was isolated. D-section proved to be the geometry with the highest values of energy transfer efficiency. A hysteresis loop was present in its oscillatory response (dimensionless oscillation amplitude vs reduced velocity). This loop was characterized by two branches, (+) and (−), meaning a bi-valued amplitude response for each reduced velocity. Regarding temporal patterns of wake topology and body motion, it was found that synchronization occurs in the (+) branch, but not in the (−). Regarding vortex shedding modes, particle image velocimetry was used for identification purposes, and it was found that the 2P mode is the dominant mode in the (+) branch, while the 2S mode pervades the (−). Finally, a new relative reduced velocity definition was introduced, and, when re-plotting the experimental results, it was found that the hysteresis loop disappears, thereby providing a more compact mathematical description of the observed phenomena.

ON THE EFFECT OF VISCOSITY RELAXATION ON ELECTROMAGNETIC RADIATION INTENSITY OF AN OSCILLATING CHARGED DROPLET

Theoretical asymptotic calculations linear with respect to small dimensionless amplitude of oscillations have been employed to study the influence of the viscoelastic properties of a charged droplet of an electrically conducting viscous liquid on the intensity of its electromagnetic radiation. It has been shown that allowance for the effect of viscosity relaxation leads to a decrease in the damping decrement value, which is determined by energy losses for electromagnetic wave emission, and the intensity of high-frequency electromagnetic radiation; a substantial reduction in the viscous damping decrement of small cloud droplets; and an essential dependence of the viscous damping decrement on the characteristic relaxation time. It has been found that the viscosity relaxation of a liquid has no significant effect on damped capillary oscillations and electromagnetic radiation of rain droplets. Viscoelasticity and viscosity of the liquid do not influence the conditions critical for the realization of the electrostatic instability of a droplet with respect to its own charge.

SOME FEATURES OF CAPILLARY WAVE ENERGY CONVERSION ON LIQUID SURFACE IN THE PRESENCE OF DYNAMIC SURFACE TENSION

The effect of dynamic surface tension on capillary oscillations of a droplet is studied in the theoretical asymptotic calculations of the first order of smallness with respect to the dimensionless amplitude of oscillations of charged droplets of a polar liquid. The calculations are carried out within the framework of a model of an ideal incompressible electrically conducting liquid. It has been shown that allowance for the effect of dynamic surface tension increases the order of the dispersion equation, which acquires one more damping root relevant to the destruction of the near-surface electrical double layer (disordering of molecules in the near-surface layer). What is interesting about the revealed damping is that it takes place in an ideal liquid, while the characteristic damping time coincides with that measured experimentally. Free energy transformations occur between mechanical, thermal, electromagnetic, and mechanical again forms of energy, with all of the transformations being caused by the effect of the dynamic surface tension. It has been shown that the dynamic surface tension has a weak effect on the low-frequency oscillations of the droplets, while it essentially affects the high-frequency oscillations causing their rapid damping.

Drag force on an oscillatory spherical bubble in shear‐thinning fluid

Power-law shear-thinning fluid motions induced by a translating spherical bubble with sinusoidal oscillation at a high frequency are numerically studied. We focus on reducing the time-averaged drag force $D$ on the bubble owing to the oscillation-enhanced shear-thinning effect. Under the assumption of negligible convection, the unsteady Stokes equation is directly solved in a finite-difference manner over a wide parameter space of the dimensionless oscillation amplitude $A$ (corresponding to the oscillatory-to-translational velocity ratio) and the power-law index $n$ of the viscosity. The results show that, for small amplitude ( $A\ll 1$ ), the drag reduction ratio $1-D/D_0$ (here, $D_0$ is $D$ with no oscillation) is proportional to $A^2$ . In contrast, for large amplitude ( $A \gg 1$ ), the drag ratio $D/D_0$ is proportional to $A^{n-1}$ , revealing a power-law behaviour. In the case of $A \gg 1$ for a strong shear-thinning fluid with small $n$ , the square of the vorticity over the entire domain is much smaller than that of the shear rate, and thus the bulk flow may be regarded as irrotational. To provide a fundamental perspective on the drag reduction mechanism, a theoretical model is proposed based on potential theory, and demonstrated to well capture the power-law relation between $D/D_0$ and $A$ .

Stacked Heterogeneous Ensemble Learning Model in Mixed Convection Heat Transfer from a Vertically Oscillating Flat Plate

In this study, the effects of mixed convection heat transfer from a moving vertical flat plate with an experimental and stacked heterogeneous ensemble learning approach are analyzed. In the experimental work, the effects on both natural and forced convection of dimensionless oscillation amplitude (Ao), dimensionless oscillation frequency (Wo) and Rayleigh number (Ra) are investigated. In the experiments, the vertical movement of the plate is provided by a flywheel-motor assembly. The average Nusselt numbers (Nu) on the fixed plate and the moving plate surface were obtained. Additionally, this study is focused on the prediction of heat transfer of a moving flat plate using single-based algorithms (Gradient Boosting, AdaBoost, Multilayer Per-ceptron) and a stacked heterogeneous ensemble learning model. The statistical per-formance of the single-based algorithms and the stacked ensemble model is meas-ured in the prediction of mixed convection heat transfer. The results show that the stacked-based ensemble learning model yielded the MSE = 2.01, RMSE = 1.42, MAE = 1.1 and R2 = 0.99 values. Overall, this study reveals that the proposed stacked en-semble machine learning model can be used successfully for modeling convection heat transfer of a moving plate.

ACOUSTIC RADIATION OF CHARGED DROPLETS OSCILLATING IN THE SUPERPOSITION OF GRAVITATIONAL AND ELECTROSTATIC FIELDS

The intensity of acoustic radiation and the characteristic widths (depending on the radii and charges of the drops) of the frequency ranges in which the radiation falls have been calculated for charged droplets of convective clouds and near-ground fogs, which are immobile in the superposition of gravitational and electric fields. The calculations have been carried out using the methods of classical mathematical physics taking into account two small parameters: the dimensionless oscillation amplitude and the equilibrium spheroidal deformation of a charged droplet in an external electrostatic field. All calculations have been performed using the model of an ideal incompressible electroconducting liquid. It has turned out that the acoustic radiation from clouds and fogs occurs in the ultrasonic frequency range, while the acoustic radiation of large rain-drops is in the acoustic range audible to the human ear. The boundary between the ultrasonic and sonic radiations is determined by droplet sizes, electric field strengths (in a small vicinity of the droplets), and the values of interfacial tension coefficient.

Mixed convection heat transfer from a vertical flat plate subjected to periodic oscillations

In this study, effects on mixed convection heat transfer of oscillation parameters on a vertical flat plate surface subjected to constant heat flux are experimentally and numerically investigated. The experimental setup includes a hanger–pulley system installed above a transparent enclosure contain a moving experimental model, flywheel-motor assembly generating the oscillating movement of the experimental model, power supply, and datalogger. The experimental model comprises two copper plates with attached thermocouples and Kapton heaters placed between the plates. In the study, heat flux applied to surface of the plates (q″), the Womersley number (Wo) and dimensionless oscillation amplitude (Ao) are varied. The effects of these parameters on the heat transfer performance are analyzed. This study is numerically solved using a control-volume based Computational Fluid Dynamics solver based on experimental data. The numerical results are compared with the experimental results and open literature. Instantaneous velocity and temperature profiles on the plate are obtained to explain the heat transfer mechanism. According to the numerical and experimental results, heat transfer performance is significantly affected by oscillation parameters and heat flux applied to the plate surface. The mixed convection heat transfer increases with the increase in oscillation parameters for all tested heat fluxes. The obtained results are presented as a function of dimensionless numbers.

Asymptotic Analysis of Nonlinear Resonance Interaction of Capillary Waves of Arbitrary Symmetry on Moving Charged Jet at Multimode Initial Deformation

The problem of research of a nonlinear resonance between capillary waves on a surface of the charged jet at multimode initial deformation moving regarding the material environment is considered. It is shown in analytical asymptotic calculations of the second order on the dimensionless amplitude of oscillations that on a surface of a jet an internal nonlinear resonant interaction of capillary waves of any symmetry, both degenerate and secondary combinational, takes place. Positions of resonances depend on physical parameters of the system: the values of the coefficient of a surface tension and of the radial electric field at a surface of a jet, the velocity of its movement regarding the material environment, the values of the wave and azimuthal numbers of the interacting waves, a range of the waves defining initial deformation.

Forced Convection Heat Transfer from Oscillating Horizontal Cylinder.(Dept.M)

An experimental investigation studies the behavior of heat transferred from a horizontal Oscillated cylinder, in case of cross air flow. The investigation studied the effect of dimensionless oscillation frequency (SC), dimensionless Oscillation amplitude (AID), and Reynolds number on the rate of hear transfer. The oscillated cylinder has 53 mm O.D, the dimensionless oscillation frequency (Sc) of the cylinder is varied from 0.08 to 0.45, while the dimensionless oscillation amplitude (AID) ranged between 0.037 and 0.27. The Reynolds number is ranged between 3x104 and 7.8x104 and the rate of heal fur is varied from 480 to 3000W/m2. The test rig of the present work has been examined in the case of stationary cylinder and the obtained results are in a good agreement with the available data in the literature of about ±5%, while in the case of oscillating cylinder the rate of hear transfer data indicate a maximum enhancement of about 70% comparing with stationary case at Sc = 0.2 und A/D = 0.27. The present data has been correlated in a form of Nusselt number as a function of dimensionless frequency (Sc), dimensionless amplitude (A/D) and Reynolds number (Re) with average deviation of about ±ɹ% relative to the experimental results.

Investigation of heat transfer performance using an oscillating vertical flat plate

In this study, the heat transfer performance of an oscillating vertical flat plate is experimentally and numerically investigated. Rayleigh number (Ra), Womersley number (Wo), and dimensionless oscillation amplitude (Ao) are changed, and the effects of these parameters on heat transfer performance are analysed. The numerical simulations are performed using a finite volume method-based computational fluid dynamics procedure, and the numerical results are validated by comparisons with the experiments. To explain the heat transfer mechanism, the instantaneous velocity and temperature profiles on the plate surface are obtained. It has been observed that the periodic oscillations continuously renewed the velocity and thermal boundary layer, increased the convection effects, and improved the heat transfer. Consequently, the heat transfer performance was significantly affected by the oscillation parameters and increased as both Wo and Ao increased for all tested Rayleigh numbers (Ra). The maximum heat transfer improvement achieved was about 45% at a high amplitude (Ao = 1.4) and frequency (Wo = 146) at Rayleigh number (Ra = 1.17 × 107). The results are presented as a function of dimensionless numbers.

On the Stability of Capillary Waves on the Surface of a Layer of Viscous Conductive Liquid on Metal Wire

The dispersion equation is derived analytically and in a linear approximation using the dimensionless amplitude of oscillations for capillary waves on the charged surface of a layer of viscous conductive liquid on a metal wire. The metal wire is shown to stabilize the capillary waves. The wire effect becomes substantial only at high values of the wire radius. The appearance of the electric charge on the liquid layer complicates the spectrum of the realizing motions of the liquid and quantitatively changes the values of the frequencies, increments, and decrements.

Experimental investigation of mixed convection on an oscillating vertical flat plate

Abstract This study experimentally investigates the mixed-convection heat transfer on a moving vertical flat plate. The setup comprises a hanger–pulley system above a transparent volume in which the experimental model moves; a flywheel-motor assembly, which generates the oscillation movement; a power supply; and a datalogger. The experimental model comprises two copper plates on which the thermocouples are placed and Kapton heaters between the plates. For the study, heat flux was applied to the plate surface (q”) and the Womersley number (Wo) and dimensionless oscillation amplitude (Ao) were varied; the effects of these parameters on mixed-convection heat transfer were analyzed. The experiments were performed for four different values of Ao and five different oscillation frequencies with three different heat fluxes. The results show that heat-transfer performance is significantly affected by Ao, Wo, and heat flux applied to the plate surface. We observed that heat-transfer performance increases with the increase of both the Wo number and the dimensionless oscillation amplitude for all heat fluxes tested. The obtained results are presented as a function of dimensionless numbers. We also compare experimental results with other researchers' prior-published studies.

A COMMENT ON UNSTEADY–PERIODIC FLOW FRICTION FACTOR: AN ANALYSIS ON EXPERIMENTAL DATA GATHERED IN PULSATILE PIPE FLOWS

In 1940’s, Schultz- Grunow proposed that time-average value of friction factor, λ_u,ta was similar to its corresponding steady state value, λ for the presence of gradual and slow oscillations in pulsatile flows. A recent approach was available for low frequency pulsatile flows through narrow channels in transitional and turbulent regimes by Zhuang et al, in 2016 and 2017. In this analysis; extensive experimental data of λ_u,ta in fully laminar and turbulent sinusoidal flow are processed in the measured time-average Reynolds number range of 1390 ≤ Re_ta ≤ 60000 disregarding the transitional regime. The ranges of dimensionless frequency-Womersley number, √(ω') and oscillation amplitude, A_1 are 2.72 ≤ √(ω') ≤ 28 and 0.05 ≤ A_1≤ 0.96 respectively. A multiplication element is defined as Mel = Re_ta×√(ω^'). A modified friction multiplier, λ_(Mel ) which is similar to the conceptual parameter of Zhuang et al’s friction factor ratio C ( λ_Mel = λ_(u,ta)/λ ) is also referred. The correlation of λ_Mel = λ_Mel (Mel) is dependent on flow regime and the magnitude of Re_ta for the range of √(ω^') > 1.32. The proposal of Schultz-Grunow is verified irrespective of the oscillations in turbulent regime since the magnitude of λ_Mel = 1 is observed for turbulent flow cases with Re_(ta ) ≥ 35000. In laminar regime the magnitude of Re_(ta ) is governing the fact. The magnitude of λ_Mel varies in 0.589 ≤ λ_Mel ≤ 28.125 for Re_(ta ) ≤ 5000 while λ_Mel = 1 is obtained for Re_(ta ) > 5000. The graphical representation of λ_Mel = λ_Mel (Mel) can be considered as a counterpart of Moody Diagram in pulsatile fields for a significant practice.

Quadrupole Electromagnetic Radiation of a Charged Drop Oscillating in the Superposition of Collinear Gravitational and Electrostatic Fields

The analytical expression for the quadrupole moment of a charged conducting droplet caused by the presence of the self- and induced charges on the droplet surface is obtained. The droplet is assumed to be motionless in the superposition of gravitational and electrostatic fields. The analytical calculations are carried out in the first order in the dimensionless oscillation amplitude of the droplet. The intensity of electromagnetic radiation of the droplet generated by time variation in its quadrupole moment is estimated in the second order of smallness with respect to the square of the ratio of the characteristic linear droplet size to the length of emitted wave.

A numerical investigation of fluid flows induced by the oscillations of thin plates and evaluation of the associated hydrodynamic forces

The paper is devoted to the problem of harmonic oscillations of thin plates in a viscous incompressible fluid. The two-dimensional flows caused by the plate oscillations and their hydrodynamic influence on the plates are studied. The fluid motion is described by the non-stationary Navier–Stokes equations, which are solved numerically on the basis of the finite volume method. The simulation is carried out for plates with different thicknesses and shapes of edges in a wide range of control parameters of the oscillatory process: dimensionless frequency and amplitude of oscillations. For the first time in the framework of one model all two-dimensional flow regimes, which were found earlier in experimental studies, are described. Two new flow regimes emerging along the stability boundaries of symmetric flow regimes are localized. The map of flow regimes in the frequency–amplitude plane is constructed. The analysis of the hydrodynamic influence of flows on the plates allow us to establish new effects associated with the influence of the shape of the plates on the drag and inertia forces. Due to these effects, the values of hydrodynamic forces can differ by 90 % at the same parameters of the oscillation. The lower and upper estimates of hydrodynamic forces obtained in the work have a good agreement with the experimental data presented in the literature.

Stiff and soft active control of friction by vibrations and their energy efficiency

The present paper builds upon the results of two recent theoretical studies on the influence of friction by normal and sideways oscillations. The findings are in part rewritten to a more compact and dimensionless form so as to present the results for both oscillation modes side by side in a consistent manner. Thereby, it is shown that for the considered system the macroscopic coefficient of friction is only a function of a dimensionless sliding velocity and a dimensionless oscillation amplitude. Furthermore, the energy efficiency is characterized for both modes for the first time by comparing the total energy needed for a sliding motion which includes the superimposed oscillations with the energy needed for the same sliding motion without the additional oscillations. It is shown that this ratio is also only a function of the two dimensionless system parameters. We consider a simple one-spring model in a displacement-controlled setting. Any system-dynamical feedback is neglected. The lower end of the spring either slides, sticks or jumps on a rigid plane. In the case of normal oscillations, the macroscopic coefficient of friction can be reduced only when the contact point undergoes a stick-slip motion (“stiff control of friction”) whereas with sideways oscillations the macroscopic coefficient of friction can be reduced also when the contact point is continuously sliding (“soft control of friction”). It is found that the motion with superimposed sideways oscillations requires more energy for any combination of system parameters, than the corresponding motion without the oscillations. For the case of normal oscillations however, there are combinations of system parameters for which the motion with the superimposed oscillations requires less, the same, or more energy than for the reference case without the oscillations.

Active control of friction by transverse oscillations

The present paper is devoted to a theoretical analysis of sliding friction under the influence of in-plane oscillations perpendicular to the sliding direction. Contrary to previous studies of this mode of active control of friction, we consider the influence of the stiffness of the tribological contact in detail and show that the contact stiffness plays a central role for small oscillation amplitudes. In the present paper we consider the case of a displacement-controlled system, where the contact stiffness is small compared to the stiffness of the measuring system. It is shown that in this case the macroscopic coefficient of friction is a function of two dimensionless parameters—a dimensionless sliding velocity and dimensionless oscillation amplitude. In the limit of very large oscillation amplitudes, known solutions previously reported in the literature are reproduced. The region of small amplitudes is described for the first time in this paper.