0

### Guest Editorial

J. Fluids Eng. 2017;140(5):050301-050301-1. doi:10.1115/1.4038402.
FREE TO VIEW

The study of compressible turbulent mixing associated with Richtmyer–Meshkov (RM), Rayleigh–Taylor, and Kelvin–Helmholtz instabilities is motivated by applications in science and engineering including inertial confinement fusion, supersonic combustion, detonation, instability of collapsing gas bubbles, stratified flows in geophysical applications, chemical engineering, core-collapse supernovae, and molecular clouds. Further, the interaction of shock waves with materials is also of interest in biomedical applications such as fragmentation of cancer cells during shock-wave chemotherapy and cavitation-damage to human tissues during lithotripsy. In many of these applications the Reynolds number is very high and the instabilities rapidly lead to turbulent mixing. In the case of inertial confinement fusion, which is regarded as a promising approach to controlled thermonuclear fusion: (1) these instabilities lead to the growth of perturbations on the interfaces within the capsules; (2) the perturbations become nonlinear, transition to turbulence, enhancing material mixing; and (3) material mixing inhibits thermonuclear burning of the fuel.

Commentary by Dr. Valentin Fuster

### SPECIAL SECTION PAPERS

J. Fluids Eng. 2017;140(5):050901-050901-10. 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 (RTI). 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 antiparallel to the interface that transport vorticity and cause the growth rate to oscillate in time with nearly a zero mean value.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(5):050902-050902-10. doi:10.1115/1.4038397.

A laser ignition system suitable for a hypersonic scramjet engine is considered. Wall-modeled large eddy simulation (LES) is used to study a scramjet-like geometry with a single hydrogen injector on the inlet, at a Mach 8 flight condition with a total enthalpy of 2.5 MJ. Detailed chemical kinetics and high fidelity turbulence modeling are used. 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.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(5):050903-050903-8. 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 mesomechanical hydrocode modeling under different conditions of the interstitial gaseous phase. A two-phase analytical approach focusing on the role of thermal nonequilibrium between the phases and an advanced two-phase model complement the mesomechanical analysis by demonstrating a similar trend due to the effect of pressure in the interstitial air.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(5):050904-050904-9. doi:10.1115/1.4038399.

We develop a new high-order numerical method for continuum simulations of multimaterial 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 tenth-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 multimaterial 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 (eighth-order) and numerical resolution is demonstrated using a suite of test problems containing smooth solutions. Finally, the Richtmyer–Meshkov (RM) instability between copper and aluminum is simulated in two-dimensional (2D) and a parametric study is performed to assess the effect of initial perturbation amplitude and yield stress.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(5):050905-050905-8. doi:10.1115/1.4038401.

We describe the behavior of a multimode interface that degenerates into a turbulent mixing layer when subjected to a spherical implosion. Results are presented from three-dimensional (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 (RM) instability, the Rayleigh–Taylor (RT) instability, as well as the Bell–Plesset (BP) effects. We report on several quantities of interest to the turbulence modeling community, including the turbulent kinetic energy (TKE), components of the anisotropy tensor, density self-correlation, and atomic mixing, among others.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(5):050906-050906-8. 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 (RTI). 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, α, 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 (two-dimensional (2D)/three-dimensional (3D)) and to numerical miscibility.

Commentary by Dr. Valentin Fuster

### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;140(5):051101-051101-8. doi:10.1115/1.4038531.

We analyze the use of water solutions of Xanthan Gum (XG) 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 pipes than tubes. We believe this is due to the relative increase of the buffer zone in an annular geometry.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(5):051102-051102-11. 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.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(5):051103-051103-11. 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 two 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 the numerical results are compared with experimental results. The simulations of the SC have been performed with different inlet boundary conditions. Comparison between an analytical formulation with the computational fluid dynamics (CFD) results shows that the flow inside an SC is highly three-dimensional (3D). 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 toward the bottom from $θ=30deg$ to $45deg$ after the curve of the SC begins, and on the top between two stay vanes.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(5):051104-051104-11. doi:10.1115/1.4038667.

Recent advances in manufacturing technologies, such as additive manufacturing (AM), have raised the potential of choosing surface finish pattern as a design parameter. Hence, understanding and prediction of aerothermal effects of machined microstructures (machined roughness) would be of great interest. So far, however, roughness has been largely considered as a stochastic attribute and empirically modeled. 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 microstructures. 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 modeling and grid sensitivities are examined to ensure the consistency of the results. Furthermore, large eddy simulation (LES) analyses are performed to contrast the behavior 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.

Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;140(5):051201-051201-10. 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 (CFD) 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 with the motion of the particles are moved toward 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.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(5):051202-051202-6. 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
Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(5):051203-051203-9. doi:10.1115/1.4038760.

High-speed flows with shock waves impinging on turbulent boundary layers pose severe challenge to current computational methods and models. Specifically, the peak wall heat flux is grossly overpredicted by Reynolds-averaged Navier–Stokes (RANS) simulations using conventional turbulence models. This is because of the constant Prandtl number assumption, which fails in the presence of strong adverse pressure gradient (APG) of the shock waves. Experimental data suggest a reduction of the turbulent Prandtl number in boundary layers subjected to APG. We use a phenomenological approach to develop an algebraic model based on the available data and cast it in a form that can be used in high-speed flows with shock-induced flow separation. The shock-unsteadiness (SU) k–ω model is used as the baseline, since it gives good prediction of flow separation and the regions of APG. The new model gives marked improvement in the peak heat flux prediction near the reattachment point. The formulation is applicable to both attached and separated flows. Additionally, the simplicity of the formulation makes it easily implementable in existing numerical codes.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(5):051204-051204-6. doi:10.1115/1.4038757.

Flow drag reduction induced by chemical additives, more commonly called drag-reducing agents (DRAs), has been studied for many years, but few studies can manifest the mechanism of this phenomenon. In this paper, a new mathematical model is proposed to predict the upper limit of drag reduction with polymer DRAs in a turbulent pipe flow. The model is based on the classic finitely extensible nonlinear elastic-Peterlin (FENE-P) theory, with the assumption that all vortex structures disappear in the turbulent flow, i.e., complete laminarization is achieved. With this model, the maximum drag reduction by a DRA at a given concentration can be predicted directly with several parameters, i.e., bulk velocity of the fluid, pipe size, and relaxation time of the DRA. Besides, this model indicates that both viscosity and elasticity contribute to the drag reduction: before a critical concentration, both viscosity and elasticity affect the drag reduction positively; after this critical concentration, elasticity still works as before but viscosity affects drag reduction negatively. This study also proposes a correlation format between drag reduction measured in a rheometer and that estimated in a pipeline. This provides a convenient way of pipeline drag reduction estimation with viscosity and modulus of the fluids that can be easily measured in a rheometer.

Commentary by Dr. Valentin Fuster

### Research Papers: Multiphase Flows

J. Fluids Eng. 2018;140(5):051301-051301-9. doi:10.1115/1.4038759.

The present study focuses on experimental characterization of interfacial instability pertinent to liquid jet and liquid sheet in the first wind-induced zone. To accomplish this objective, the interfacial wave growth rate, critical wave number, and breakup frequency associated with air-assisted atomizer systems were extracted by utilizing high-speed flow visualization techniques. For a range of liquid to gas velocities tested, nondimensionalization with appropriate variables generates the corresponding correlation functions. These functions enable to make an effective comparison between interfacial wave developments for liquid jet and sheet configurations. It exhibits liquid sheets superiority over liquid jets in the breakup processes leading to efficient atomization.

Commentary by Dr. Valentin Fuster

### Technical Brief: Technical Briefs

J. Fluids Eng. 2018;140(5):054501-054501-6. doi:10.1115/1.4038659.

This paper 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 ≤ β ≤ 0.16) for which predictive models are not present in the 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.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(5):054502-054502-5. 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 was 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 14,000, 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 40,000 and 70,000, 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.

Commentary by Dr. Valentin Fuster