Unsteady Acceleration Research Articles

Insight into Darcy flow of ternary‐hybrid nanofluid on horizontal surfaces: Exploration of the effects of convective and unsteady acceleration

AbstractThe transport phenomenon of water conveying spherical carbon nanotubes, cylindrical graphene, and platelet aluminum oxide nanoparticles exhibits a linear relationship between the pressure gradient causing the dynamics of ternary‐hybrid nanofluid and the velocity. However, it is essential to note that little is known about the computation of roughness at the surface transition zone during the early stages of accretion. The Darcy flow of ternary‐hybrid nanofluid was investigated and reported in this paper with the intention to provide insight into the convective and unstable acceleration related to leading‐edge accretion. The governing equations roughly represented that the time‐dependent fluid flow were nondimensionalized using the Blasius‐Rayleigh Stokes variable, and the three‐stage Lobatto IIIa integration formula for a finite difference (MATLAB package bvp4c) was used to solve numerically. Based on the analysis of results, it is worth concluding that the velocity decreases significantly due to a growth in the leading‐edge accretion (γ) because the convective acceleration increases while the unsteady acceleration decreases for . As , convective acceleration increases while the unsteady acceleration decreases. As γ enlarges from 90° to 180°, both forms of the acceleration are decreasing properties, but unsteady deceleration is bound to manifest.

Flat plate boundary layer accelerated by shock wave propagation

The flat plate transitional boundary layer response to the acceleration induced by the shock wave propagation is studied using large-eddy simulations. The steady boundary layer global behaviour is first investigated before focusing on the transient response of a turbulent region following the shock wave propagation. It is shown that the transient response of the turbulent region exhibits strong similarities with the spatial transition process to turbulence induced by free-stream turbulence, the so-called bypass transition. The boundary layer does not evolve gradually from the initial turbulence intensity to the final turbulence intensity but undergoes a temporal transition process composed of three distinct phases. These three different phases are comparable with the three stages of a bypass transition (i.e. buffeted laminar flow, transition and fully turbulent) because they are governed by the same physical processes. On the other hand, it is highlighted that this temporal response is identical to that described by He & Seddighi (J. Fluid Mech., vol. 715, 2013, pp. 60–102) during the study of an incompressible boundary layer undergoing an increase of mass flow rate. The boundary layer compression by the shock propagation does not contribute to any significant change in the turbulence dynamic after an unsteady acceleration.

Unsteady and inertial dynamics of a small active particle in a fluid

It is well known that the reversibility of Stokes flow makes it difficult for small microorganisms to swim. Inertial effects break this reversibility, allowing new mechanisms of propulsion and feeding. Therefore it is important to understand the effects of unsteady and fluid inertia on the dynamics of microorganisms in flow. In this work, we show how to translate known inertial effects for non-motile organisms to motile ones, from passive to active particles. The method relies on a principle used earlier by Legendre and Magnaudet (1997) to deduce inertial corrections to the lift force on a bubble from the inertial drag on a solid sphere, using the fact that small inertial effects are determined by the far field of the disturbance flow. The method allows for example to compute the inertial effect of unsteady fluid accelerations on motile organisms, and the inertial forces such organisms experience in steady shear flow. We explain why the method fails to describe the effect of convective fluid inertia.

Numerical Solution for Unsteady Acceleration MHD Third-Grade Fluid Flow in a Rotating Frame Through Porous Medium Over Semi-Infinite Boundary Condition with a Presence of Heat Transfer

The aim of this work is to present a suitable numerical solution for unsteady non-Newtonian third-grade fluid which rotates at z -axis and pass through a porous medium. The fluid flows in magnetic field with constant acceleration and the semi-infinite boundary condition are highlighted. The fluid problem is also deal with heat transfer. The nonlinear partial differential equation is discretised using the finite difference method (FDM). The linear system obtained for three different domains (lengths). Consequently, the asymptotic interpolation method is merged to solve problems of large sizes. This hybrid method yielded results that satisfied the boundary condition that reaches zero as length grows to infinite length. For velocity profile and temperature distribution, a comparison of FDM and hybrid method is shown. It is discovered that the hybrid method produces better results than FDM for this infinitely large problem. Several analyses have been carried out to investigate the effect of various fluid parameter values. The findings reveal that as the porosity parameter increases, the velocity decreases. The Grashof and Prandtl numbers demonstrate the relationship to the temperature distributions. The effects of the magnetic field and the non-Newtonian parameters were also illustrated, as these parameters influence the velocity distribution of the fluid flow.

Experimental investigation of heat pipe performance under translational acceleration

Heat pipes provide highly efficient heat transfer and are powerful tools in thermal management systems. In contrast to most established systems, heat pipes operating within moving systems such as electric vehicles or machine tools are exposed to unsteady acceleration forces. In order to guarantee proper heat pipe function in scenarios involving accelerated movement, it is necessary to examine the effect of acceleration forces on the heat pipe behavior. This paper presents experimental investigations on the thermal performance of translationally moving heat pipes. The designed experimental set-up consists of a measurement platform mounted to a linear direct drive. By obtaining the temperature gradient between evaporator and condenser section, the thermal resistance of heat pipes is determined at standstill and under linear motion. The experiments were conducted using heat pipes with sintered, mesh and grooved wicks. The results demonstrate the motion influence on the thermal resistances depending on the heat pipe orientation and heat input.

Effects of Varying Inhalation Duration and Respiratory Rate on Human Airway Flow

Studies of flow through the human airway have shown that inhalation time (IT) and secondary flow structures can play important roles in particle deposition. However, the effects of varying IT in conjunction with the respiratory rate (RR) on airway flow remain unknown. Using three-dimensional numerical simulations of oscillatory flow through an idealized airway model (consisting of a mouth, glottis, trachea, and symmetric double bifurcation) at a trachea Reynolds number (Re) of 4200, we investigated how varying the ratio of IT to breathing time (BT) from 25% to 50% and RR from 10 breaths per minute (bpm) corresponding to a Womersley number (Wo) of 2.41 to 1000 bpm (Wo = 24.1) impacts airway flow characteristics. Irrespective of IT/BT, axial flow during inhalation at tracheal cross-sections was non-uniform for Wo = 2.41, as compared to centrally concentrated distribution for Wo = 24.1. For a given Wo and IT/BT, both axial and secondary (lateral) flow components unevenly split between left and right branches of a bifurcation. Irrespective of Wo, IT/BT and airway generation, lateral dispersion was a stronger transport mechanism than axial flow streaming. Discrepancy in the oscillatory flow relation Re/Wo2 = 2 L/D (L = stroke length; D = trachea diameter) was observed for IT/BT ≠ 50%, as L changed with IT/BT. We developed a modified dimensionless stroke length term including IT/BT. While viscous forces and convective acceleration were dominant for lower Wo, unsteady acceleration was dominant for higher Wo.

CFD Prediction of Ship-Bank Interaction

Ships operating in different shallow-water areas are at risk of grounding due to squat, heel and wave-induced motions. The paper aims not only at assessing the ship squat in any bathymetry, but also to reveal the waves influence on the environment. A 3D numerical model for predicting ship resistance in confined waters by taking the ship sinkage into account is proposed. A time-domain model is used to predict unsteady squat and dynamic acceleration effects for a vessel travelling in non-uniform water depth. A quasi-steady approach where the prediction at each time step is based on steady-state heave force and pitch moment in uniform water depth is used. Resistance, sinkage and trim, the free-surface elevation as well as the wake flow structure are considered in the ship performance analysis. The results show the wave pattern in the subcritical, critical and supercritical regions. The total resistance/drag is calculated at various ship speeds in shallow water using CFD and compared with the calculated deep water resistance. Attention is paid to the verification and validation based on a grid convergence study, as to the exploration of the modelling error in RANS computations to enable more accurate and reliable predictions of the bank effects.

Reprint of: Temperature Uniformity Enhancement and Flow Characteristics of Embedded Gradient Distribution Micro Pin Fin Arrays Using Dielectric Coolant for Direct Intra-Chip Cooling

Targeting at reducing temperature gradient in the direct intra-chip cooling, embedded gradient distribution micro pin fin arrays are fabricated. The flow characteristics and temperature uniformity performance are experimentally investigated using dielectric coolant (HFE7100). It is found the dependence of friction factor on Re is weak after reaching a critical Re (293) with a correlation of f~Re−0.14. Obvious flow fluctuations appear with steady stagnant symmetric vortices and unsymmetrical vortices by the experimentally validated numerical results, attributing to the abrupt rise of pressure drop. Moreover, the present gradient micro-pin fin arrays show great strengths in improving thermal performance and lowering temperature gradient. The thermal resistance and maximum temperature difference reduce by 34.6% and 8 K at the Qv of 100 ml/min and qw of 40 W/cm2 compared to the uniform micro pin fin arrays for a same pumping power. Notably, the local temperature uniformity is also improved by gradient distribution micro pin fin arrays with absolute temperature deviations less than 2 K from x=1 mm to x=10 mm. The local Nu based on experimental data show a rapid increase (29%) in the second junction zone where unsteady wakes and flow acceleration occur, bringing considerable enhancement in mixing and local heat transfer.

관내에서 가속 운동하는 구의 벽면효과 전산해석

The study of the flow past a accelerating body within a tube is mostly under the condition of the terminal velocity of the steady state rather than the unsteady state, and thus hydrodynamic analysis of around accelerating body is required. Therefore, in this study, the force acting on a 3D sphere with an unsteady constant acceleration of 9.81 m/s2 in a cylindrical tube of various diameters is analyzed using computational fluid dynamics. A relative moving frame based on the finite volume method is employed with SST k-ω turbulence model. The comparison with the results of previous studies shows that the computational method of this study is valid. Also, the drag change is explained through the wake analysis, and the drags on bodies in the accelerating flow are quantitatively evaluated to assess the effect of the wall.

Temperature Uniformity Enhancement and Flow Characteristics of Embedded Gradient Distribution Micro Pin Fin Arrays Using Dielectric Coolant for Direct Intra-Chip Cooling

Targeting at reducing temperature gradient in the direct intra-chip cooling, embedded gradient distribution micro pin fin arrays are fabricated. The flow characteristics and temperature uniformity performance are experimentally investigated using dielectric coolant (HFE7100). It is found the dependence of friction factor on Re is weak after reaching a critical Re (293) with a correlation of f~Re−0.14. Obvious flow fluctuations appear with steady stagnant symmetric vortices and unsymmetrical vortices by the experimentally validated numerical results, attributing to the abrupt rise of pressure drop. Moreover, the present gradient micro-pin fin arrays show great strengths in improving thermal performance and lowering temperature gradient. The thermal resistance and maximum temperature difference reduce by 34.6% and 8 K at the Qv of 100 ml/min and qw of 40 W/cm2 compared to the uniform micro pin fin arrays for a same pumping power. Notably, the local temperature uniformity is also improved by gradient distribution micro pin fin arrays with absolute temperature deviations less than 2 K from x=1 mm to x=10 mm. The local Nu based on experimental data show a rapid increase (29%) in the second junction zone where unsteady wakes and flow acceleration occur, bringing considerable enhancement in mixing and local heat transfer.

Impact of Longitudinal Acceleration and Deceleration on Bluff Body Wakes

This study investigated the unsteady acceleration aerodynamics of bluff bodies through the study of a channel mounted square cylinder undergoing free-stream acceleration of ± 20 m / s 2 with Reynolds numbers spanning 3.2e4 to 3.6e5. To achieve this, a numerical simulation was created with a commercial finite volume unstructured computational fluid dynamics code, which was first validated using Improved Delayed Detached Eddy Simulation against experimental and direct numerical simulated results. Then, the free stream conditions were subjected to a periodic velocity signal where data were recorded and ensemble averaged over at least 30 distinct acceleration and deceleration data points. This enabled the comparison of body forces and flow field variations among accelerating, steady and decelerating free-stream conditions. Body force analysis determined that decelerating and accelerating drag forces varied −47% and 44%, respectively, in comparison to steady free-stream conditions. In addition, several differences were also observed and explored such as near-body flow structures, wake dynamics, Kármán vortices and vorticity production during the aforementioned conditions. The primary interest of this study was for the future application towards road vehicles for predictive dynamic modeling and aerodynamic development.

A new method to calculate unsteady particle kinematics and drag coefficient in a subsonic post-shock flow

We introduce a new method (piecewise integrated dynamics equation fit, PIDEF) that uses the particle dynamics equation to determine unsteady kinematics and drag coefficient (CD) for a particle in subsonic post-shock flow. The uncertainty of this method is assessed based on simulated trajectories for both quasi-steady and unsteady flow conditions. Traditional piecewise polynomial fitting (PPF) shows high sensitivity to measurement error and the function used to describe CD, creating high levels of relative error (1) when applied to unsteady shock-accelerated flows. The PIDEF method provides reduced uncertainty in calculations of unsteady acceleration and drag coefficient for both quasi-steady and unsteady flows. This makes PIDEF a preferable method over PPF for complex flows where the temporal response of CD is unknown. We apply PIDEF to experimental measurements of particle trajectories from 8-pulse particle tracking and determine the effect of incident Mach number on relaxation kinematics and drag coefficient of micron-sized particles.

Universal Rim Thickness in Unsteady Sheet Fragmentation.

Unsteady fragmentation of a fluid bulk into droplets is important for epidemiology as it governs the transport of pathogens from sneezes and coughs, or from contaminated crops in agriculture. It is also ubiquitous in industrial processes such as paint, coating, and combustion. Unsteady fragmentation is distinct from steady fragmentation on which most theoretical efforts have been focused thus far. We address this gap by studying a canonical unsteady fragmentation process: the breakup from a drop impact on a finite surface where the drop fluid is transferred to a free expanding sheet of time-varying properties and bounded by a rim of time-varying thickness. The continuous rim destabilization selects the final spray droplets, yet this process remains poorly understood. We combine theory with advanced image analysis to study the unsteady rim destabilization. We show that, at all times, the rim thickness is governed by a local instantaneous Bond number equal to unity, defined with the instantaneous, local, unsteady rim acceleration. This criterion is found to be robust and universal for a family of unsteady inviscid fluid sheet fragmentation phenomena, from impacts of drops on various surface geometries to impacts on films. We discuss under which viscous and viscoelastic conditions the criterion continues to govern the unsteady rim thickness.

Numerical simulation of liquid droplet breakup in supersonic flows

A five-equation model based on finite-difference frame was utilized to simulate liquid droplet breakup in supersonic flows. To enhance the interface-capturing quality, an anti-diffusion method was introduced as a correction of volume-fraction after each step of calculation to sharpen the interface. The robustness was guaranteed by the hybrid variable reconstruction in which the second-order and high-order method were respectively employed in discontinuous and continuous flow fields. According to the recent classification of droplet breakup regimes, the simulations lay in the shear induced entrainment regime. Comparing to the momentum of the high-speed air flows, surface tension and viscid force were negligible in both two-dimensional and three-dimensional simulations. The inflow conditions were set as Mach 1.2, 1.5 and 1.8 to reach different dynamic pressure with the liquid to gas density ratio being 1000 initially. According to the results of simulations, the breakup process was divided into three stages which were analyzed in details with the consideration of interactions between gas and liquid. The shear between the high-speed gas flow and the liquid droplet was found to be the sources of surface instabilities on windward, while the instabilities on the leeward side were originated by vortices. Movement of the liquid mass center was studied, and the unsteady acceleration was observed. In addition, the characteristic breakup time was around 1.0 based on the criterion of either droplet thickness or liquid volume fraction.

Low Reynolds number hydrodynamics and mesoscale simulations

Hydrodynamics and hydrodynamic interactions are fundamental for the motility of microswimmers. This includes the propulsion mechanism itself, the synchronized motion of flagella in flagellar bundles and beating cilia of cilia arrays, and even extends to collective behaviors. The general importance of hydrodynamics has stimulated the development of mesoscale simulation approaches to efficiently study dynamical properties of objects embedded in a fluid. In this minireview, the properties of flows at low Reynolds numbers are discussed, thereby the unsteady acceleration term is typically taken into account (Landau-Lifshitz Navier-Stokes equations). Specifically, the synchronization of microrotors by time-dependent hydrodynamic interactions is discussed and the propulsion of a rotating helix. Moreover, the multiparticle collisions dynamics method (MPC), a mesoscale simulation approach for fluids, is outlined. Simulation results for the flow field of a model E. Coli bacterium and its swimming behavior next to a surface are presented.

Dynamic Stall Analysis of S809 Pitching Airfoil in Unsteady Free Stream Velocity

AbstractIn this paper, the dynamic stall of S809 airfoil that widely used in horizontal axis wind turbine, in different reduced frequencies is investigated. The simulation was carried out numerically handling Navier-Stokes equations. For this purpose, the segregated solver with SIMPLE algorithm was chosen to solve the momentum equations. The effect of turbulence on the flow field is taken into account using Shear Stress Transport (SST)K-ωturbulence model. The obtained numerical results are compared with experimental and a few numerical results. The results indicate that theK-ωSST model is in good agreement with experimental results for both steady and unsteady conditions. Furthermore, a non-dimensional parameter, relating the acceleration of unsteady free stream velocity and acceleration of pitch motion (known as reduced frequency), is also investigated. In addition, the results show that any increase in the reduced frequency increases the instantaneous aerodynamic characteristics of oscillating airfoil.

Effects of thermal fluctuations and fluid compressibility on hydrodynamic synchronization of microrotors at finite oscillatory Reynolds number: a multiparticle collision dynamics simulation study.

We investigate the emergent dynamical behavior of hydrodynamically coupled microrotors by means of multiparticle collision dynamics (MPC) simulations. The two rotors are confined in a plane and move along circles driven by active forces. Comparing simulations to theoretical results based on linearized hydrodynamics, we demonstrate that time-dependent hydrodynamic interactions lead to synchronization of the rotational motion. Thermal noise implies large fluctuations of the phase-angle difference between the rotors, but synchronization prevails and the ensemble-averaged time dependence of the phase-angle difference agrees well with analytical predictions. Moreover, we demonstrate that compressibility effects lead to longer synchronization times. In addition, the relevance of the inertia terms of the Navier-Stokes equation are discussed, specifically the linear unsteady acceleration term characterized by the oscillatory Reynolds number ReT. We illustrate the continuous breakdown of synchronization with the Reynolds number ReT, in analogy to the continuous breakdown of the scallop theorem with decreasing Reynolds number.

Evaluation of unsteady pressure fields and forces in rotating airfoils from time-resolved PIV

The instantaneous pressure fields and aerodynamic loads are obtained for rotating airfoils from time-resolved particle image velocimetry (TR-PIV) measurements. These allowed evaluating the contribution from the local acceleration (unsteady acceleration) to the instantaneous forces. Traditionally, this term has been neglected for wind turbines with quasi-steady flows, but results show that it is a dominant term in the wake where high temporal variations in the flow field are present due to vortex shedding. Briefly, time-resolved particle image velocimetry TR-PIV measurements are used to calculate flow velocity fields and corresponding spatial and temporal derivatives. These derivatives are then used in the Poisson equation to solve for the pressure field and later used in the integral momentum equation to solve for the instantaneous forces. The robustness of the measurements is analyzed by calculating the PIV uncertainty and the independence of the calculated forces. The experimental mean aerodynamic forces are compared with theoretical predictions from the blade element momentum theory showing good agreement. The instantaneous pressure field showed dependence with time in the wake due to vortex shedding. The contribution to the instantaneous forces from each term in the integral momentum equation is evaluated. The analysis shows that the larger contributions to the normal force coefficient are from the unsteady and the pressure terms, and the larger contribution to the tangential force coefficient is from the convective term.

Experimental study of unsteady turbulent flow coefficients through granular porous media and their contribution to the energy losses

This paper aims to cast light on the coefficients of non-linear equation governing unsteady turbulent flow through coarse porous media commonly known as Forchheimer’s equation. Experimental pilot made use of a physical model consisting of a flume of 13 m length, being controlled by an electro-mechanical device to create different types of flow regimes. A range of granular media with a mean diameter of 7.5 mm for Small crashed (Sc) to 14 mm for Medium crashed (Mc) were packed in the flume at different runs to simulate coarse porous media capable of sustaining turbulent flow, either unsteady or steady. Findings indicate that: coefficient of the linear flow term decreases (47.3% and 91.9% in Sc and Mc aggregate) with increasing mean grain size, respectively. In addition, coefficient of the turbulent term-the effects of inertia forces-increases (87.9% and 27.2% in Sc and Mc aggregate) with increasing mean grain size, respectively. Furthermore, coefficient of the unsteady term-local acceleration term in the Forchheimer’s equationshows a rapid increase (146.9% in Sc and 125.6% in Mc) with increasing velocity. Moreover, the nature of hydraulic gradient (i) variations versus Reynolds number (Re) — which were plotted for all sets of observations-confirms the existence of turbulent conditions in most experiments. Finally, the head losses and contribution of above-mentioned terms to the pressure drop was quite analyzed.

Some investigations on the mean and fluctuating velocities of an oscillating Taylor bubble

The slug flow characterized by large elongated bubbles also called Taylor bubbles is widely encountered in nuclear reactor steam generators, cooling plants, reboilers, etc. The analysis of slug flow is very important as the instability caused by such flows can affect the safety features of nuclear reactors and other two-phase flow equipments. In this paper, we study the motion of a Taylor bubble rising in stagnant fluids in a vertical oscillating pipe. The investigation is restricted to high Reynolds numbers and to an intermediate range of Bond numbers where the effects of surface tension can be considered. The Froude number ranged between 0.22 and 0.33. Firstly, detailed analysis of models proposed in the literature for the motion of a Taylor bubble in an unsteady acceleration field is realized. The velocity differential equation obtained in the case of potential and axisymmetric flow without surface tension given in the literature is first non-dimensionalized to highlight dimensionless numbers. Then, the instantaneous velocity of the bubble is numerically determined. Mean and fluctuating velocities as well as the phase shift (U¯b, Uf and ϕ) are estimated by using a technique based on the nonlinear least squares method. Results enable a discussion on the role played by dimensionless numbers on the dynamics of the bubble. It is found that the two parameters, the relative acceleration and the Bond number (a and Bo) have a governing role on the evolution of mean and fluctuating velocities while the ratio of the oscillation amplitude to the pipe diameter (b/D) has little influence when b/D>10−1. Further, the velocities U¯b and Uf as well as the phase shift ϕ are experimentally investigated. Correlations allowing the prediction of mean and fluctuating velocities as well as the phase shift depending on the Bond number and relative acceleration are proposed. The correlations are found to be in good agreement with our experimental results obtained in a previous work.