Accepted Manuscripts

Abdullah A. Kendoush
J. Fluids Eng   doi: 10.1115/1.4038663
Analytical solutions were obtained for the virtual mass of a Taylor bubble rising in a liquid confined by a circular pipe under transient conditions. The solution of the virtual mass coefficient was based on potential inviscid flow. The present solution is applicable to low viscosity liquids and to Capillary number (Ca)<0.005. The virtual mass solution showed dependence on bubble geometry. The present solution was validated by comparison with the available numerical solutions and experimental data of other investigators.
TOPICS: Bubbles, Pipes, Geometry, Inviscid flow, Viscosity, Transients (Dynamics)
Yash Joshi and B R Vinoth
J. Fluids Eng   doi: 10.1115/1.4038668
Numerical simulations of laminar pipe and channel flows were carried out (i) to understand the effect of inlet conditions, viz. flat inlet and streamtube inlet, on entry lengths, and (ii) to investigate the flow development in radial/transverse locations. Results show that hydrodynamic entry lengths from the streamtube inlet simulations are significantly lower compared to the entry lengths from the flat inlet simulations for low Reynolds numbers. Moreover, results from the current study (Newtonian flow with no-slip) as well as the results from the literature (non-Newtonian flow with no-slip) showed that for many flow situations, the slowest development of axial velocity in the transverse location happens to be very near to the wall. For the above cases, the existing entry length criteria (centerline as well as global entry length) is not appropriate to define the entry length. We have proposed a new entry length criterion based on the displacement thickness which is an integral measure of the velocity profile. A new entry length correlation using the displacement thickness criterion is proposed for Newtonian flows in pipe and channel based on simulations with the streamtube inlet condition.
TOPICS: Channel flow, Pipes, Flow (Dynamics), Engineering simulation, Simulation, Displacement, Non-Newtonian flow, Computer simulation, Reynolds number
Marios Kapsis and Li He
J. Fluids Eng   doi: 10.1115/1.4038667
Recent advances in manufacturing technologies, such as additive manufacturing, have raised the potential of choosing surface finish pattern as a design parameter. Hence, understanding and prediction of aerothermal effects of machined micro-structures ('machined roughness') would be of great interest. So far, however, roughness has been largely considered as a stochastic attribute and empirically modelled. A relevant question is: if and how would shape of the machined roughness elements matter at such fine scales? In this paper, a systematic computational study has been carried out on the aerothermal impact of some discrete micro-structures. Two shapes of configurations are considered: hemispherical and rectangular elements for a Reynolds number range typical for such structures (Re<5000). Several validation cases are studied as well as the turbulence modelling and grid sensitivities are examined to ensure the consistence of the results. Furthermore, LES analyses are performed to contrast the behaviour in a well-established turbulent to a transitional flow regime. The results reveal a distinctive common flow pattern change (from an 'open separation' to a 'reattached separation') associated with a drastic change of drag correlation from a low to a high loss regime. The results indicate a clear dependence of drag and heat transfer characteristics on the element pattern and orientation relative to the flow. The distinctive performance correlations with Reynolds number can be affected considerably by the element shape, for both a transitional and a turbulent flow regime. The results also consistently illustrate that conventional empirical stochastic roughness parameters would be unable to predict these trends.
TOPICS: Flow (Dynamics), Heat transfer, Separation (Technology), Matter, Turbulence, Drag (Fluid dynamics), Reynolds number, Surface roughness, Finishes, Design, Manufacturing technology, Modeling, Shapes, Additive manufacturing
Technical Brief  
Xueying Yan, Rupp Carriveau and David S.-K. Ting
J. Fluids Eng   doi: 10.1115/1.4038661
When buoyant vortex rings form, azimuthal disturbances occur on their surface. When the magnitude of the disturbance is sufficiently high, the ring will become turbulent. This paper establishes conditions for categorization of a buoyant vortex ring as laminar, transitional, or turbulent. The transition regime of enclosed-air buoyant vortex rings rising in still water were examined experimentally via two high-speed cameras. Sequences of the recorded pictures were analyzed using MATLAB. Key observations were summarized as follows: for Reynolds number lower than 14000, Bond number below 30, and Weber number below 50, the vortex ring could not be produced. A transition regime was observed for Reynolds numbers between 40000 and 70000, Bond numbers between 120 and 280 and Weber number between 400 and 800. Below this range, only laminar vortex rings were observed, and above, only turbulent vortex rings.
TOPICS: Turbulence, Reynolds number, Vortices, Matlab, Water
Technical Brief  
Fnu Rituraj and Andrea Vacca
J. Fluids Eng   doi: 10.1115/1.4038659
This article proposes a novel orifice flow model for non-Newtonian fluids. The orifice model is developed for sharp orifices with small apertures (orifice to pipe diameter ratio: 0.04 < ß < 0.16) for which predictive models are not present in literature. The orifice flow experiment is conducted with three different orifices and three different fluids. From the experimental data, a correlation is developed that relates Euler number to Reynolds number and orifice diameter ratio. It also accounts for elastic effects of the fluid on orifice flow by including Weissenberg number in the model. The developed model predicts the experimental data within reasonable accuracy.
TOPICS: Flow (Dynamics), Non-Newtonian fluids, Modeling, Orifices, Fluids, Reynolds number, Pipes
Ryota Kobayashi, Koichi Nishibe, Yusuke Watabe, Kotaro Sato and Kazuhiko Yokota
J. Fluids Eng   doi: 10.1115/1.4038660
This paper presents a fundamental study on jet vectoring control by adjusting the dimensionless frequency of synthetic jets over time without changing the injection nozzle shape in actuators. This work involves the introduction of asymmetric slots with various sharp projection lengths in free synthetic jets for various actuator frequencies. The influences of the dimensionless parameters, sharp projection length C, and actuator frequency f* on the behavior of free synthetic jets are experimentally investigated under the same slot width b and Reynolds number Re = 990, and numerical simulations are performed to supplement these experiments. Furthermore, the behavior of synthetic jets is compared with that of continuous jets. The measurements of the velocities for both jet types are performed for the flow visualizations to observe the jet behaviors obtained using the smoke-wire method. The typical flow patterns and the time-averaged velocity distributions of the synthetic jets for various sharp projection lengths and dimensionless frequencies are demonstrated through the experiment. The influence of the dimensionless frequency on the stagnation point near a rigid wall when the inclined synthetic jets form a recirculation flow is also investigated. Furthermore, the degree of the bend of the jets is evaluated based on the change in the jet center's position at a reference downstream cross-section. The results show that the jet direction of the synthetic jets induced by the asymmetric slots is related to both the dimensionless sharp projection length and the dimensionless frequency.
TOPICS: Jets, Actuators, Flow (Dynamics), Computer simulation, Reynolds number, Wire, Flow visualization, Nozzles, Shapes, Smoke
Binaya Baidar, Jonathan Nicolle, Chirag Trivedi and Michel J. Cervantes
J. Fluids Eng   doi: 10.1115/1.4038662
The Winter-Kennedy (WK) method is commonly used in relative discharge measurement and to quantify efficiency step-up in hydropower refurbishment projects. The method utilizes the differential pressure between 2 taps located at a radial section of a spiral case, which is related to the discharge with the help of a coefficient and an exponent. Nearly a century old and widely used, the method has shown some discrepancies when the same coefficient is used after a plant upgrade. The reasons are often attributed to local flow changes. To study the change in flow behavior and its impact on the coefficient, a numerical model of a semi-spiral case (SC) has been developed and validated. The simulations of the SC have been performed with different inlet boundary conditions. Comparison between an analytical formulation with the CFD results shows that the flow inside an SC is highly three-dimensional. The magnitude of the secondary flow is a function of the inlet boundary conditions. The secondary flow affects the vortex flow distribution and hence the coefficients. For the SC considered in this study, the most stable WK configurations are located towards the bottom from ?=30° to 45° after the curve of the SC begins, and on the top between two stay vanes.
TOPICS: Pressure, Flow (Dynamics), Computer simulation, Simulation, Computational fluid dynamics, Engineering simulation, Boundary-value problems, Sensitivity analysis, Vortex flow, Hydropower
Ping Wang, Kumar Raman, Stephan A. MacLaren, Channing M. Huntington, Sabrina R. Nagel, Kirk A Flippo and Shon T Prisbrey
J. Fluids Eng   doi: 10.1115/1.4038532
We present simulations of a new experimental platform at the National Ignition Facility for studying the hydrodynamic instability growth of a high energy density fluid interface that undergoes multiple shocks, i.e. is "re-shocked". In these experiments, indirect-drive laser cavities drive strong shocks through an initially solid, planar interface between a high-density plastic and low-density foam, in either one or both directions. The first shock turns the system into an unstable fluid interface with the pre-machined initial condition that then grows via the Richtmyer-Meshkov and Rayleigh-Taylor instabilities. Backlit x-ray imaging is used to visualize the instability growth at different times. Our main result is that this new high energy density re-shock platform is established, and that the initial data confirm the experiment operates in a hydrodynamic regime similar to what simulations predict. The simulations also reveal new types of edge effects that can disturb the experiment at late times and suggest ways to mitigate them.
TOPICS: Simulation, Shock (Mechanics), Design, Engineering simulation, Density, Ignition, Fluids, Lasers, X-ray imaging, Hydrodynamic stability, Cavities
Zhongxin Gao, Wenruo Zhu, Long Meng, Jianguan Zhang, Fei Zhang, Luoping Pan and Li Lu
J. Fluids Eng   doi: 10.1115/1.4038535
The pressure fluctuations in both the rotating runner and the other fixed components in a model Francis turbine under various loads were experimentally measured by means of onboard measuring equipment in the runner and data storage device on the shaft in this study. Large pressure fluctuation were observed under both small guide vane opening and large guide vane opening conditions. Flow separation at the blade suction surface led to large pressure fluctuations for small guide vane openings, the unsteady flow around the inlet on the blade pressure side led to large pressure fluctuations for large openings. The pressure fluctuations correlation between the runner and other components of the turbine, mainly the draft tube, was analyzed in detail for both small guide vane opening (12°) and large guide vane opening (30°). The results show that the pressure fluctuations in the runner space increased by the superposition of draft tube vortex rope pressure fluctuations and runner inter blade vortices pressure fluctuations, resulting in much larger pressure fluctuations in the runner space than in other components.
TOPICS: Pressure, Fluctuations (Physics), Francis turbines, Guide vanes, Blades, Vortices, Flow separation, Ropes, Unsteady flow, Data storage systems, Turbines, Suction, Stress
Michell Luiz Costalonga, Bruno V. Loureiro and Edson J. Soares
J. Fluids Eng   doi: 10.1115/1.4038531
We analyze the use of water solutions of Xantham Gum for drag reduction (DR) in annular spaces. We provide a direct quantitative comparison between the DR in an annulus and that in straight tubes. We can fairly compare the data from the two geometries by using the general definition of the Reynolds number, which is independent of the geometry. With such a definition, the product of the friction factor by Re is a constant in laminar flows. Moreover, the friction factor for a turbulent flow of Newtonian fluids in an annulus fits Colebrook's correlation. Our main results show that the DR is more pronounced in annular spaces than tubes. We believe this is due to the relative increase of the buffer zone in an annular geometry.
TOPICS: Flow (Dynamics), Drag (Fluid dynamics), Space, Polymer solutions, Annulus, Geometry, Friction, Fluids, Turbulence, Laminar flow, Reynolds number, Polishing equipment, Drag reduction, Water
Christopher Stephen, Shouqi Yuan, Ji Pei and Xing Cheng G
J. Fluids Eng   doi: 10.1115/1.4038533
For a pump, the inlet condition of flow determines the outlet conditions of fluid (i.e. energy). As a rule to minimize the losses at the entry of pump, the bends should be avoided as one of the methods. But for the case of vertical inline pump it is unavoidable, in order to save the space for installation. For the purpose of investigation in inlet pipe of vertical inline pump, the Unsteady Reynolds Averaged Navier Stokes equations are solved using the computational fluid dynamics code. The results have been shown that there is a good agreement between the performance characteristics obtained from the simulation and experiments. The velocity coefficient from the simulation along the inlet pipe sections is well matched with the theoretical values and found to have variation near the exit of inlet pipe. The pressure and velocity coefficients studies depict the flow physics at each section along with the study of helicity at the exit of inlet pipe to determine the recirculation effects. It is observed that the vortices associated to the motion of the particles are moved towards the surfaces and are more intense than the mean flow. The trends of pressure coefficient at the exit of inlet pipe were addressed with reference to the various flow rates for eight set of radial lines. Hence, this work concludes that for inlet pipe, the generation of circulation was due to the stream path and the reverse flow from the impeller and was reconfirmed with the literature.
TOPICS: Flow (Dynamics), Pipes, Pumps, Pressure, Simulation, Impellers, Computational fluid dynamics, Physics, Fluids, Particulate matter, Vortices, Reynolds-averaged Navier–Stokes equations, Performance characterization
Abeer Bakhsh and Ravindra Samtaney
J. Fluids Eng   doi: 10.1115/1.4038487
We investigate the linear stability of both positive and negative Atwood ratio interfaces accelerated either by a fast magnetosonic or hydrodynamic shock in cylindrical geometry. For the magnetohydrodynamic (MHD) case, we examine the role of an initial seed azimuthal magnetic field on the growth rate of the perturbation. In the absence of a magnetic field, the Richtmyer-Meshkov growth is followed by an exponentially increasing growth associated with the Rayleigh-Taylor instability. In the MHD case, the growth rate of the instability reduces in proportion to the strength of the applied magnetic field. The suppression mechanism is associated with the interference of two waves running parallel and anti-parallel to the interface that transport of vorticity and cause the growth rate to oscillate in time with nearly a zero mean value.
TOPICS: Magnetic fields, Magnetohydrodynamics, Stability, Waves, Shock (Mechanics), Vorticity, Geometry, Rayleigh-Taylor instability
Nicholas Gibbons, Rolf Gehre, Stefan Brieschenk and Vincent Wheatley
J. Fluids Eng   doi: 10.1115/1.4038397
A laser ignition system suitable for a hypersonic scramjet engine is considered. Wall-Modelled Large Eddy Simulation is used to study a scramjet-like geometry with a single hydrogen injector on the inlet and an air crossflow at Mach 8 air, using detailed chemical kinetics and high fidelity turbulence modelling. The laser forms a kernel of high temperature plasma inside the fuel plume that briefly ignites the flow and leads to massive disruption of the flow structures around the jet, due to the expanding plasma kernel driving a blast wave that collides with the surrounding flow. The blast wave produces vorticity as it passes through the fuel-air interface, but comparably less than that produced by the jetting of the hot gas affected by the laser as it expands outward into the crossflow. The remnant of the plasma rolls up into a powerful vortex ring and noticeably increases the fuel plume area and the volume of well mixed reactants present in the simulation. These results indicate that the laser ignition system does more than just supply the energy to ignite the flow, it also substantially alters the flow structure and the mixing process.
TOPICS: Waves, Lasers, Hypersonic flow, Flow (Dynamics), Plasmas (Ionized gases), Fuels, Scramjets, Plumes (Fluid dynamics), Ignition systems, Chemical kinetics, Vorticity, Ejectors, Modeling, Vortices, Geometry, Hydrogen, Large eddy simulation, High temperature, Turbulence, Simulation
Anatoly Resnyansky
J. Fluids Eng   doi: 10.1115/1.4038398
Deformation and mixing of solid particles in porous materials are typical consequences under shock compression and are usually considered as the major contributors to energy dissipation during shock compression while a contribution from the interaction between the solid and gaseous phases attracts less attention. The present work illustrates the phase interaction process by meso-mechanical hydrocode modeling under different conditions of the interstitial gaseous phase. A two-phase analytical approach focusing on the role of thermal non-equilibrium between the phases and an advanced two-phase model complement the meso-mechanical analysis by demonstrating a similar trend due to the effect of pressure in the interstitial air.
TOPICS: Porous materials, Shock (Mechanics), Compression, Modeling, Pressure, Deformation, Particulate matter, Energy dissipation, Equilibrium (Physics)
Akshay Subramaniam, Niranjan Ghaisas and Sanjiva Lele
J. Fluids Eng   doi: 10.1115/1.4038399
We develop a new high-order numerical method for continuum simulations of multi-material phenomena in solids exhibiting elastic-plastic behavior using the diffuse interface numerical approximation. This numerical method extends an earlier single material high-order formulation that uses a 10th-order high-resolution compact finite difference scheme in conjunction with a localized artificial diffusivity (LAD) method for shock and contact discontinuity capturing. The LAD method is extended here to the multi-material formulation and is shown to perform well for problems involving shock waves, material interfaces and interactions between the two. Accuracy of the proposed approach in terms of formal order (8th-order) and numerical resolution is demonstrated using a suite of test problems containing smooth solutions. Finally, the Richtmyer-Meshkov instability between copper and aluminum is simulated in 2D and a parametric study is performed to assess the effect of initial perturbation amplitude and yield stress.
TOPICS: Flow (Dynamics), Engineering simulation, Simulation, Resolution (Optics), Numerical analysis, Approximation, Yield stress, Solids, Copper, Aluminum, Shock waves, Shock (Mechanics)
Assaf Shimony, Guy Malamud and Dov Shvarts
J. Fluids Eng   doi: 10.1115/1.4038400
A comprehensive numerical study was performed, in order to examine the effect of density ratio on the mixing process inside the mixing zone formed by Rayleigh-Taylor instability. This effect exhibits itself in the mixing parameters and increase of the density of the bubbles. The motivation of this work is to relate the density of the bubbles to the growth parameter for the self-similar evolution, a, we suggest an effective Atwood formulation, found to be approximately half of the original Atwood number. We also examine the sensitivity of the parameters above to the dimensionality (2D/3D) and to numerical miscibility.
TOPICS: Density, Rayleigh-Taylor instability, Bubbles
Ismael Boureima, Praveen Ramaprabhu and Nitesh Attal
J. Fluids Eng   doi: 10.1115/1.4038401
We describe the behavior of a multimode interface that degenerates in to a turbulent mixing layer when subjected to a spherical implosion. Results are presented from 3D numerical simulations performed using the astrophysical FLASH code, while the underlying problem description is adopted from Youngs and Williams (YW). During the implosion, perturbations at the interface are subjected to growth due to the Richtmyer-Meshkov instability, the Rayleigh-Taylor instability, as well as Bell-Plesset effects. We report on several quantities of interest to the turbulence modeling community, including the turbulent kinetic energy, components of the anisotropy tensor, density self-correlation, and atomic mixing among others.
TOPICS: Turbulence, Computer simulation, Kinetic energy, Anisotropy, Tensors, Modeling, Rayleigh-Taylor instability, Density
Guest Editorial  
Ben Thornber, Vincent Wheatley and Oleg Schilling
J. Fluids Eng   doi: 10.1115/1.4038402
The 15th International Workshop on the Physics of Compressible Turbulent Mixing was held at the University of Sydney in 2016, here we introduce peer-reviewed papers based upon the research presented at the conference.
TOPICS: Physics, Workshops (Work spaces), Turbulence

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