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The unsteady three-dimensional flow of nanofluid over a stretching surface in the presence of a magnetic field and thermal radiation are investigated. Zero nanoparticle flux at the wall has been considered. Buongiorno's model for nanofluid has been used in this study. Employing similarity transformations, the governing partial differential equations are transformed to ordinary ones and the transformed equations are then solved numerically by using the shooting technique with the help of Runge–Kutta method. The computed results are compared with previously reported work and found in excellent agreement. Fluid velocity initially decreases with the increase of the unsteadiness parameter. Temperature and nanoparticle volume fraction both increase with the increase of unsteadiness parameter. Due to the increase of thermal radiation temperature increases, nanoparticle volume fraction decreases initially but increases away from the surface. The effect of the other governing parameters on velocity, temperature and concentration fields are discussed through their graphical representations. Besides, the local skin friction coefficient, local Nusselt number and local Sherwood number are presented through graphs. [ABSTRACT FROM AUTHOR]

A three-dimensional, multi-passage unsteady numerical study was conducted to enhance the understanding of unsteady flow phenomena in the tip region of highly loaded compressors. The first-stage rotor of a three-stage transonic low-reaction compressor was chosen as the computational model. Three different tip clearance sizes were calculated to demonstrate the effect of the tip clearance dimension on the unsteadiness in the rotor tip region. It was found that the unsteadiness existed at the vicinity of the stall point when the tip clearance size was larger than the design value. The unsteadiness in the tip region appeared as a "multi-passage structure" in the nine-passage unsteady simulation and it propagated along the circumferential direction. Tip leakage vortex breakdown was the source of unsteady flow behavior. Besides, special attention was paid to the difference between the conventional transonic rotor and the low-reaction rotor. The scale of the flow separation downstream of the shock wave was controllable for the low-reaction rotor even at near-stall conditions. The boundary layer would reattach to the blade surface due to local axial acceleration. Finally, attempts were made to study the stall mechanism of the low-reaction rotor. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics (CFD) was used to investigate how residence time analysis based on passive tracer transport in environmental and chemical engineering fluid systems can be significantly affected by flow or tracer release unsteadiness. The unsteadiness may be caused by intermittent influent flow, time-varying operations, and nonuniform tracer release profiles. Oftentimes these unsteady conditions are considered negligible and their impact on residence time analysis is not considered. Systems analyzed included a full-scale stabilization pond, a full-scale oxidation ditch, and full-scale and lab-scale baffled tanks. In the case of the stabilization pond, it was found that time-dependent inflow based on realistic water consumption pattern does not significantly affect mean residence time (MRT) but does have a nonnegligible effect on baffling factor θ10 relative to constant inflow. In the case of a surface aerated oxidation ditch, aerator speed was found to have a nonnegligible impact on MRT, and thus it is recommended that CFD tracer studies should consider changes in aerator speed during daily cycles of operation. The slug and step methods were considered for tracer analysis in baffled contactors. It was found that the tracer release time in the slug method should be kept at less than 5% of the theoretical residence time in order to obtain accurate measurements of residence time characteristics. Furthermore, residence time distribution obtained from tracer analysis based on the step method can be affected by the initial transient and subsequent fluctuations in the time series of injected tracer concentration that often characterizes physical experiments. [ABSTRACT FROM AUTHOR]

Abstract In this article, the flow of an incompressible nanoliquid induced due to unsteady contracting cylinder is investigated. Analysis of heat transport phenomenon is reported under uniform heat absorption/generation and convective type boundary condition. A set of transformations is implemented to reduce PDEs to ODEs. The obtained coupled nonlinear equations are treated numerically through the help of RKF 45 technique. Physical evaluation of convective condition and heat absorption/generation on thermal analysis is discussed through graphs. Output shows that both the parameters of convective condition and heat absorption/generation give an increment in liquid temperature and its related thickness of boundary layer. [ABSTRACT FROM AUTHOR]

A pulsating turbulent pipe flow has been investigated experimentally using hot-film anemometry and particle image velocimetry. Particularly, a ‘paradoxical phenomenon’ that is known to occur for a range of forcing frequencies and time-averaged Reynolds numbers has been investigated in detail. The paradoxical phenomenon is that the oscillating component of the wall shear stress exhibits a smaller amplitude in a turbulent flow compared to in a laminar flow exposed to the same oscillation in the pressure gradient. In here, the phenomenon is explained by splitting the response of the wall shear stress into one contribution resulting from the imposed pressure gradient τ ∼ p , and a second contribution resulting from the oscillating Reynolds shear stress, τ ∼ t . At the conditions of maximum reduction of the wall shear stress amplitude, τ ∼ p and τ ∼ t are nearly 136° out of phase. The contributions are thus interfering destructively, this being the ultimate reason for the reduced amplitude. It is also shown that the level of reduction is dependent on the imposed forcing amplitude, this in turn residing from a dependence of the time-development of the oscillating Reynolds shear stress on the forcing amplitude. [ABSTRACT FROM AUTHOR]

The aim of this paper is to investigate two dimensional unsteady flow of a viscous incompressible fluid about stagnation point on stretching sheet in the presence of time dependent free stream velocity. It is assumed that fluid viscosity and thermal diffusivity are assumed to vary as linear function of temperature. Fluid is considered in the presence of radiation effect. Using time-dependent stream function, partial differential equations corresponding to the momentum and energy equations are converted into non-linear differential equations. Numerical solutions of these equations are obtained by using Runge-Kutta Fehlberg method with the help of shooting technique. In the present work the effect of unsteadiness parameter, stretching sheet parameter, temperature-dependent fluid viscosity parameter and thermal diffusivity parameter on flow and heat transfer characteristics are discussed. Skin-friction and Nusselt number at the sheet are computed and discussed. [ABSTRACT FROM AUTHOR]

This work aims to study the interaction of a shock wave with a cloud of particles to quantify flow unsteadiness and velocity fluctuations using particle-resolved direct numerical simulation (PR-DNS). Three cases are studied, with each case revealing one aspect of the intricate flow phenomena involved in this interaction. The unsteady interaction of a shock wave with a transverse array of particles reveals the origin of unsteadiness under the effect of mutual wave-wave and wave-wake interactions between the particles. In the second case, the interaction of a shock with a particle cloud is studied, with a focus on the interaction of the complex wave system with the vortical structure. A budget analysis of the vorticity equation reveals the sources of strong unsteadiness in the particle cloud. A detailed analysis of the velocity fluctuation and kinetic energy in the fluctuating motion is performed to ascertain the importance of the velocity fluctuations that arise from the strong unsteadiness. An analogous analysis is presented, in the third case, for a gradually-induced flow on the same particle cloud along with a comparison to the shock induced case to assess the impulsive effect of shock on intensity of the fluctuating field statistics. [ABSTRACT FROM AUTHOR]

Experimental investigations of unsteady cavitating flows in a hydrofoil tip leakage region with different gap sizes are conducted to highlight the development of gap cavitation. The experiments were taken in a closed cavitation tunnel, during which high-speed camera had been used to capture the cavitation patterns. A new visualization method based on image processing was developed to capture time-dependent cavitation patterns. The results show that the visualization method can effectively capture the cavitation patterns in the tip region, including both the attached cavity in the gap and the tip leakage vortex (TLV) cavity near the trailing edge. Moreover, with the decrease of cavitation number, the TLV cavity develops from a rapid onset-growth-collapse process to a continuous process, and extends both upstream and downstream. The attached cavity in the gap develops gradually stretching beyond the gap and combines with the vortex cavity to form the triangle cavitating region. Furthermore, the influences of gap size on the cavitation are also discussed. The gap size has a great influence on the loss across the gap, and hence the locations of the inception attached cavity. Besides, inception locations and extending direction of the TLV cavity with different gap sizes also differ. The TLV in the case with = 0.061 is more likely to be jet-like compared with that in the case with = 0.024, and the gap size has a great influence on the TLV strength. [ABSTRACT FROM AUTHOR]

The critical decay rate theory (C.A. Eckett, J.J. Quirk, J.E. Shepherd, Journal of Fluid Mechanics 421 (2000) 147–183) on detonation initiation in unsteady flow was extended from ideal to Noble-Abel and van der Waals gas, in which the finite molecular volume effect (or repulsion force) and the inter-molecular attraction effect (or attraction force) were included. Considering the reactive Euler equations, the reaction zone temperature-gradient equation was established independently of the equation of state and wave geometry, and then solved asymptotically by assuming large activation energy, planar wave front and a one-step irreversible reaction model. A critical decay time was derived as the critical condition for detonation initiation. It was found that the inter-molecular attraction effect makes initiation more difficult by increasing the critical decay time, whereas the finite molecular volume effect promotes initiation. The real gas effects are enhanced when increasing the reduced activation energy and/or decreasing the heat capacity ratio of perfect gas. The applicability and generality of the asymptotic solutions were evaluated by comparison with quasi-unsteady simulation employing detailed reaction models for hydrogen-air and ethylene-air mixtures, and with the further simplified, fully analytical solutions based on strong-shock assumption. The critical conditions of the asymptotic solutions are qualitatively consistent with those obtained with the detailed mechanisms. The asymptotic solutions can also be well reproduced by the simple analytical solutions in most conditions. This study thus establishes a simple, yet reliable, method to evaluate the real gas effect on detonation initiation in high-pressure conditions. [ABSTRACT FROM AUTHOR]

Various unsteady or transient models have evolved in order to help engineers achieve economy of analysis, design, construction, operation and maintenance. The specific usage of each model is strongly dependent on the level of unsteadiness in the system and on the accuracy, assumptions and limitations of the applied mathematical model and its numerical solution. Although the research literature is quite clear on these issues, there is often much confusion in practice. In this paper, the key practical differences and advantage for the four transient models — water hammer models, rigid water column analysis, quasi-steady analysis and so-called Joukowski approach — are compared and contrasted with respect to three criteria: their physical attributes, the hydraulic predictions they lead to, and the related numerical considerations of stability and accuracy. A useful guideline for determining the degree of unsteadiness is presented and then linked to an appropriate unsteady model. [ABSTRACT FROM AUTHOR]

The turbocharger is the key enabler for energy conservation in Internal Combustion Engine (ICE). A turbine of a turbocharger is fed by highly pulsating flow due to the reciprocating engine, resulting in different behavior from that at steady conditions. This paper investigates the behavior of a mix-flow turbine under pulsating conditions via an analytical model and an in-house developed 1-D unsteady code ONDAS . Firstly, an analytical model of the ‘unsteadiness’ based on the mass flow imbalance is built and the mechanism of the unsteady behavior is discussed in details. Next, a 1-D unsteady model of the turbine under pulsating conditions is established and validated by experimental results. Finally, influence of the frequency, the magnitude and their product on unsteady behavior of the turbine is investigated by the reduced order model for further exploration of the analytical model. The investigation clearly demonstrates the impact of pulsating flow on the unsteady performance and unveils the mechanism of the ‘unsteadiness’ of turbine under pulsating conditions. [ABSTRACT FROM AUTHOR]

An unsteady three-dimensional stagnation-point flow of a nanofluid past a circular cylinder with sinusoidal radius variation is investigated numerically. By introducing new similarity transformations for the velocity, temperature, and nanoparticle volume fraction, the basic equations governing the flow and heat and mass transfer are reduced to highly nonlinear ordinary differential equations. The resulting nonlinear system is solved numerically by the fourth-order Runge-Kutta method with the shooting technique. The thermophoresis and Brownian motion effects occur in the transport equations. The velocity, temperature, and nanoparticle concentration profiles are analyzed with respect to the involved parameters of interest, namely, unsteadiness parameter, Brownian motion parameter, thermophoresis parameter, Prandtl number, and Lewis number. Numerical values of the friction coefficient, diffusion mass flux, and heat flux are computed. It is found that the friction coefficient and heat transfer rate increase with increasing unsteadiness parameter (the highest heat transfer rate at the surface occurs if the thermophoresis and Brownian motion effects are absent) and decrease with increasing both thermophoresis and Brownian motion parameters. The present results are found to be in good agreement with previously published results. [ABSTRACT FROM AUTHOR]

Abstract: Numerical simulation has been used to investigate a number of significant physical processes for reverse osmosis in a spiral-wound membrane, including fluid-flow regimes, flow unsteadiness, and fluid-flow periodicity. The independent variables included the feed spacer strand angle, feed spacer thickness, inter strand spacing, strand radius, radius of the center cylinder, and Reynolds number. Results extracted from the numerical solutions were post-processed to provide algebraic correlations for both the dimensionless pressure drop and the Sherwood number. For the range of Reynolds numbers relevant to practice, turbulence was found to be nonexistent. Flow unsteadiness was identified for certain ranges of the Reynolds number and the geometrical parameters. The extent of the unsteadiness was sufficiently moderate so as not to have any practical effect on the dimensionless pressure drop and Sherwood number. Fluid-flow periodicity was verified. [Copyright &y& Elsevier]

Abstract: The thermal performance of a gas turbine airfoil with a cooled pressure side and a trailing edge cutback is investigated and discussed in relationship with the unsteady behavior of coolant injection. The focus is on the pressure side coolant injection through discrete holes. The cascade was tested at an exit Mach number of 0.2 and 0.6 for different coolant to mainstream mass flow ratios. Laser Doppler Velocimetry (LDV) and high speed flow visualizations were used to investigate the unsteady mixing process taking place between coolant and main flow downstream of the holes. This behavior was correlated with the film cooling effectiveness distributions over the vane surface. In the first row, hairpin clockwise rotating vortices and counterclockwise shear layer vortices were observed, depending on the coolant to mainstream velocity ratio. The second row turned out to be characterized by a lower velocity ratio and lower turbulent activity, consistently with the higher thermal protection observed also at high injection rates. The increase in mainstream Mach number kept the jets closer to the wall, resulting in lower turbulent characteristics and lower mixing, consistently with higher thermal protection. [Copyright &y& Elsevier]

We consider the well-known problem of the interaction of a vortex filament with a perpendicular plane in a viscous incompressible fluid. In this study, the vortex filament is represented by a semi-infinite rotating needle. Different models are considered: a zero-radius needle and fixed and movable in the axial direction needles of a finite radius. The ranges of the existence of the solution are found, and the correspondence of the flow around a finite-radius needle to that around a zero-radius needle, as the needle radius decreases, is studied. [ABSTRACT FROM AUTHOR]

Abstract: This study numerically investigates the effect of the location of an inner heated cylinder along a horizontal or diagonal line in a cooled enclosure on the fluid flow and heat transfer in the enclosure for a Rayleigh number Ra =107. The flow and thermal fields bifurcate from the steady or unsteady to the unsteady or steady states at critical positions. This phenomenon occurs at both the horizontal and the diagonal lines. The major origin of the unsteady state depends on the position of the cylinder. Thus, the unsteadiness near the upper corners, the center of the enclosure, and the lower corners is governed by a series of cells known as Benard cells, the strong sway of the rising upwelling plume from the inner cylinder, and the inner vortices periodically merging and separating within the enlarged lower primary eddy, respectively. The variation in the local Nusselt number of the cylinder and enclosure is dominated by the gaps between the inner heated cylinder and the cooled enclosure, the upwelling and downwelling thermal plumes, and the upward returning flow. [Copyright &y& Elsevier]

Due to the small size and short propulsion pulses of solid propellant microthrusters, unsteady characteristics and/or fabrication errors may cause appreciable effects on the total thrust. The present work simulates a 2D planar and conical nozzle of 200 μm throat diameter using the traditional momentum equations and turbulence closure model. Unsteady characteristics were found to be insensitive to the inlet mass flux but sensitive to the chamber size. The characteristic time of transient momentum variations in the nozzle was found to be on the order of 0.1 ms, and that for decay was smaller than that for development by about 25%. This work confirmed that the magnitude of thrust was not sensitive to distortions in the nozzle geometry. A mismatch of the nozzle throat changed the position of sonic plane, but its effect on thrust was almost negligible. [ABSTRACT FROM PUBLISHER]

Abstract: Plasma spraying using liquid feedstock allows realizing thin coatings (<100μm). In this process, the plasma jet fluctuations play an important role in the behavior of the liquid fragmentation and then in the final deposit characteristics. In this study, numerical simulations of two different plasma jets (argon or argon hydrogen mixture) issuing into air are investigated. The computations are based on a compressible model and a Large Eddy Simulation (LES) modeling for the turbulence. The aim of this work is to analyze the unsteady effects of the plasma jet regarding those of turbulence. In particular, the characterization of the turbulent zone inside the plasma jet is provided. A turbulence analysis is carried out in order to estimate the levels of turbulence and modeling effects. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we present line fountain behavior at low-Reynolds numbers obtained by experiments. The experiments are conducted over the range of Reynolds number and Froude number . It is observed that the fountain behavior can be categorized broadly into four regimes: the steady; flapping; laminar mixing; and jet-type mixing behavior, at full development. The critical Froude number for transition from a steady to unsteady flow varies with the Reynolds number. For , the transition is independent of Re and is well described by the line. Over the range , the transition can be approximated by a constant line. For , there is a higher dependency on the Reynolds number with a very sharp increase in the critical Froude number and it is hypothesized that the demarcation line follows , where . The fountain exhibits flapping behavior in the range . These observed fountain behaviors are mapped on to a plot. In addition, the observed, non-dimensionalised fountain height is found to be in reasonable agreement with previous results on laminar line fountains. In particular, the experimental results confirm the scaling in the steady regime which was obtained previously by scaling analysis. [Copyright &y& Elsevier]

Energy components in the flow field at high angle of attack were analyzed by the dynamic measurement. The effect of the unsteadiness induced by these components on the flow was analyzed as well. The results showed that the flow itself at high angle of attack is a kind of "vortices behavior" and the effect of unsteadiness on the asymmetry of flow is relatively weak. The key factor that can essentially affect the flow at high angle of attack is the response of the dynamic unsteadiness of the vortices to the unpredictable micro-disturbance coming from near the nose of the model. [ABSTRACT FROM AUTHOR]