Guest Editorial

J. Fluids Eng. 2016;138(7):070301-070301-1. doi:10.1115/1.4033022.

The theoretical, numerical, and experimental study of compressible, variable-density, and incompressible turbulent mixing associated with Richtmyer–Meshkov (RM), Rayleigh–Taylor (RT), and Kelvin–Helmholtz (KH) instabilities is motivated by diverse applications in science and engineering including combustion and other chemically reacting flows, stratified geophysical flows, inertial confinement fusion (ICF), and astrophysical flows (supernovae, molecular clouds, and stellar interiors, for example). The study of these instabilities and associated mixing is particularly challenging due to the fact that they involve multiple fluids (or materials), rather than single fluids. The Reynolds number becomes very large in many of these applications, and the instabilities rapidly lead to turbulent mixing. In the case of ICF, which is currently an intensively studied approach to controlled thermonuclear fusion (and a potential alternative to magnetic fusion): (1) these instabilities lead to growth of perturbations on the interfaces within the fuel capsules; (2) the perturbations grow into the nonlinear regime by mode-coupling, eventually resulting in the mixing of materials; and (3) the material mixing inhibits or otherwise reduces the efficiency of thermonuclear burning of the fuel.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2016;138(7):070901-070901-9. doi:10.1115/1.4032529.

Mach number and initial conditions effects on Richtmyer–Meshkov (RM) mixing are studied by the vertical shock tube (VST) at Los Alamos National Laboratory (LANL). At the VST, a perturbed stable light-to-heavy (air–SF6, A = 0.64) interface is impulsively accelerated with a shock wave to induce RM mixing. We investigate changes to both large and small scales of mixing caused by changing the incident Mach number (Ma = 1.3 and 1.45) and the three-dimensional (3D) perturbations on the interface. Simultaneous density (quantitative planar laser-induced fluorescence (PLIF)) and velocity (particle image velocimetry (PIV)) measurements are used to characterize preshock initial conditions and the dynamic shocked interface. Initial conditions and fluid properties are characterized before shock. Using two types of dynamic measurements, time series (N = 5 realizations at ten locations) and statistics (N = 100 realizations at a single location) of the density and velocity fields, we calculate several mixing quantities. Mix width, density-specific volume correlations, density–vorticity correlations, vorticity, enstrophy, strain, and instantaneous dissipation rate are examined at one downstream location. Results indicate that large-scale mixing, such as the mix width, is strongly dependent on Mach number, whereas small scales are strongly influenced by initial conditions. The enstrophy and strain show focused mixing activity in the spike regions.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(7):070902-070902-7. doi:10.1115/1.4032530.

This paper brings new insights on an experiment, measuring the Kelvin–Helmholtz (KH) instability evolution, performed on the OMEGA-60 laser facility. Experimental radiographs show that the initial seed perturbations in the experiment are of multimode spectrum with a dominant single-mode of 16 μm wavelength. In single-mode-dominated KH instability flows, the mixing zone (MZ) width saturates to a constant value comparable to the wavelength. However, the experimental MZ width at late times has exceeded 100 μm, an order of magnitude larger. In this work, we use numerical simulations and a statistical model in order to investigate the vortex dynamics of the KH instability for the experimental initial spectrum. We conclude that the KH instability evolution in the experiment is dominated by multimode, vortex-merger dynamics, overcoming the dominant initial mode.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(7):070903-070903-9. doi:10.1115/1.4030954.

Recent experiments have shown that when a dense layer of solid particles surrounding a high-energy reactive material is explosively dispersed, the particles cluster locally leading to jetlike patterns. The formation of these coherent structures has yet to be fully understood and is believed to have its origin in the early moments of the explosive dispersal. This paper focuses on the early moments of an explosive dispersal of particles. In particular, the effect of initial perturbations on both the gas and particulate phase is investigated, considering heavy particles with a low initial particle volume fraction. Two-dimensional simulations are carried out, and results suggest that a distinctive heterogeneity in the form of a single wavelength perturbation in the rapidly expanding detonation products does not have a significant impact on the early evolution of neither the gas phase nor the cloud of particles. In contrast, the equivalent distinctive heterogeneity in the initial particle volume fraction distribution lingers for the duration of our simulations. Developing instabilities in the gas phase and at the inner- and outer-most front of the particle bed display a dominant wavelength equal to the wavelength of the initial perturbation in the particle volume fraction.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(7):070904-070904-12. doi:10.1115/1.4032533.

The present work aims at expanding the set of buoyancy-driven unstable reference flows—a critical ingredient in the development of turbulence models—by considering the recently introduced “Unstably Stratified Homogeneous Turbulence” (USHT) in both its self-similar and transient regimes. The previously established accuracy of an anisotropic Eddy-Damped Quasi-Normal Markovian Model (EDQNM) on the USHT has allowed us to: (i) build a data set of well defined transient flows from Homogeneous Isotropic Turbulence (HIT) to late-time self-similar USHT and (ii) on this basis, calibrate, validate, and compare three common Reynolds-Averaged Navier–Stokes (RANS) mixing models (two-equation, Reynolds stress, and two-fluid). The model calibrations were performed on the self-similar flows constrained by predefined long range correlations (Saffman or Batchelor type). Then, with fixed constants, validations were carried out over the various transients defined by the initial Froude number and mixing intensity. Significant differences between the models are observed, but none of them can accurately capture all of the transient regimes at once. Closer inspection of the various model responses hints at possible routes for their improvement.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(7):070905-070905-7. doi:10.1115/1.4032532.

The implicit large-eddy simulation (ILES) has been utilized as an effective approach for calculating many complex flows at high Reynolds number flows. Richtmyer–Meshkov instability (RMI) induced flow can be viewed as a homogeneous decaying turbulence (HDT) after the passage of the shock. In this article, a critical evaluation of three methods for estimating the effective Reynolds number and the effective kinematic viscosity is undertaken utilizing high-resolution ILES data. Effective Reynolds numbers based on the vorticity and dissipation rate, or the integral and inner-viscous length scales, are found to be the most self-consistent when compared to the expected phenomenology and wind tunnel experiments.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(7):070906-070906-16. doi:10.1115/1.4032600.

Experimental data are presented for large arrays of rotating, finite-height cylinders to study the dependence of the three-dimensional (3D) mean flows on the geometric and rotational configurations of the array. Two geometric configurations, each with two rotational configurations, were examined at a nominal Reynolds number of 600 and nominal tip-speed ratios of 0, 2, and 4. It was found that the rotation of the cylinders drives the formation of streamwise and transverse flow patterns between cylinders and that net time–space averaged transverse and vertical flows exist within the developed flow region of the array. This net vertical mean flow provides an additional mechanism for the exchange of momentum between the flow within the array and the flow above it, independent from the turbulent exchange mechanisms which are also observed to increase by almost a factor of three in a rotating array. As an array of rotating cylinders may provide insight into the flow kinematics of an array of vertical axis wind turbines (VAWTs), this planform momentum flux (both mean and turbulent) is of particular interest, as it has the potential to increase the energy resource available to turbines far downstream of the leading edge of the array. In the present study, the streamwise momentum flux into the array could be increased for the rotating-element arrays by up to a factor of 5.7 compared to the stationary-element arrays, while the streamwise flow frontally averaged over the elements could be increased by up to a factor of four in the rotating-element arrays compared to stationary-element arrays.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(7):070907-070907-11. doi:10.1115/1.4032750.

In the present paper, a dual jet consisting of a wall jet and an offset jet has been numerically simulated using two-dimensional unsteady Reynolds-Averaged Navier–Stokes (RANS) equations to examine the effects of jet width (w) variation on the near flow field region. The Reynolds number based on the separation distance between the two jets (d) has been considered to be Re = 10,000. According to the computational results, three distinct flow regimes have been identified as a function of w/d. For w/d ≤ 0.5, the flow field remains to be always steady with two counter-rotating stable vortices in between the two jets. On the contrary, within the range of 0.6 ≤ w/d < 1.6, the flow field reveals a periodic vortex shedding phenomenon similar to what would be observed in the wake of a two-dimensional bluff body. In this flow regime, the Strouhal number of vortex shedding frequency decreases monotonically with the progressive increase in the jet width. For w/d ≥ 1.6, the periodic vortex shedding is still evident, but the Strouhal number becomes insensitive to the variation of jet width.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(7):070908-070908-8. doi:10.1115/1.4032754.

Flows in a corrugated channel are investigated by a high-speed camera and a particle image velocimetry (PIV) system. The bottom wall of the rectangular channel was corrugated with periodic grooves while the top wall and two sidewalls were flat plates made of Plexiglas. Flow visualization data from the high-speed camera determine the critical Reynolds number to be around 1500 by examining the stability of the vortex in the groove as well as fluid ejection from the groove. The visualization data for turbulent flow also show how a vortex evolves within the groove and triggers another vortex formation in the subsequent groove, and how fluid ejected from the groove triggers another ejection from the subsequent groove. Thus, strong hydrodynamic interactions are observed between successive corrugations. In addition, PIV data provide the profiles of velocities and Reynolds stresses as a function of Reynolds number. Time-averaged streamlines show that a large, stable vortex exists in the groove for laminar flow. On the other hand, for turbulent flow, the vortex is unstable inside the groove, often prompting fluid ejection which interacts with the bulk flow. Especially the Reynolds stress of the square of velocity fluctuation in the direction normal to the bulk flow significantly increases as the fluid ejection from the groove intensifies with increasing Reynolds number.

Commentary by Dr. Valentin Fuster

Research Papers: Flows in Complex Systems

J. Fluids Eng. 2016;138(7):071101-071101-12. doi:10.1115/1.4032571.

Jet flow control is important for mixing enhancement and noise mitigation. In previous efforts, we have used validated simulations to examine the effect of localized arc filament plasma actuators (LAFPA) on perfectly expanded Mach 1.3 jets. Here, we extend the analysis to an underexpanded jet at the same Mach number to examine the effect of shocks and expansions on control authority. After validation of the baseline flow, it is shown that the downstream evolution is relatively independent of Reynolds number. Simulations performed at different values of upstream pressure indicate that the higher stagnation pressure yields shock cells that are quantitatively stronger but qualitatively similar to those observed for the lower upstream stagnation pressure condition. For control simulations, axisymmetric mode pulsing is considered at two different Strouhal numbers of St = 0.3 and St = 0.9. These simulations show that the response of the jet to flow control is a strong function of the actuation frequency. Relative to the no-control case, actuating at the column-mode instability frequency (St = 0.3) results in an increase in the rate of spreading of the shear layer. Phase-averaged results indicate the formation of large toroidal vortices formed as a result of amplification of the column-mode instabilities that are excited at this frequency. On the other hand, the higher frequency actuation affects the initial shear-layer instability and interferes with the formation of the large-scale structures. Detailed integral azimuthal length scale analyses reveal that despite the absence of the axisymmetric toroids, the St = 0.9 case shows the dominance of the axisymmetric mode even at large distances from the nozzle exit. This indicates that flow control methods need not always have a visual signature of their influence on the system.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(7):071102-071102-12. doi:10.1115/1.4032751.

Tight porous materials are used as pneumatic components in a wide range of industrial applications. Such porous materials contain thousands of interconnected irregular micropores, which produce a large pressure drop (ΔP) between the upstream and downstream sides of the porous material when a fluid flows through it. The relationship between the pressure drop and flow rate (i.e., ΔP-G characteristics) is a very important basic characteristic. Temperature is one of the factors that affect the ΔP-G characteristics because variations in temperature change the viscosity and density of the fluid. In this study, we experimentally analyzed the ΔP-G characteristics of tight porous materials by heating them using an electromagnetic system. First, we experimentally investigated the change in the ΔP-G curve under the condition of constant heating power. Then, based on the Darcy–Forchheimer theory, we introduced an experimental method to determine the average temperature of the fluid. The results show that the temperature reaches approximately 500 K in the small flow rate range, which produces considerable changes in the ΔP-G curve. As the flow rate increases, the temperature decreases, and thus, the ΔP-G curve at constant heating power converges to the curve for the room temperature. Furthermore, we compared three porous materials with different permeability coefficients and porosities and analyzed the effect of these parameters on the ΔP-G characteristics. We also performed experiments at different downstream pressures to study the effect of the average density on the ΔP-G characteristics.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(7):071103-071103-8. doi:10.1115/1.4032752.

This paper is a continuous study of a previously investigated novel winglet-shroud (WS) tip configuration. Two additional sealing fins are fixed on the WS tip to further reduce tip leakage. This configuration is referred to WS with seals (WSS) tip. Secondary flow structures and total pressure loss coefficients on a transverse plane downstream of the blade trailing edge are measured. Flow in a blade cascade is also numerically simulated to obtain more information of flow fields. Compared with the WS tip, both experimental and numerical results show that the WSS tip can further improve the aerodynamic performance as expected. Relative to the plain tip, the WSS and WS tips can reduce total pressure loss on one plane downstream of the blade trailing edge by 50% and 28%, respectively. This is mainly due to reduced intensity of tip leakage vortex (TLV). For the tip leakage mass flow rate, the WS tip decreases it by 33.6%, while the implement of two additional sealing fins contributes to an extremely high reduction of 88.7%. This demonstrates that the use of sealing fins is effective to control the tip leakage flow and improve flow fields. In addition, a deeper analysis by applying a normalized helicity scheme to identify the evolution of different vortices and by tracing trajectories of the fluid near the tip offers credible supports for results.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(7):071104-071104-13. doi:10.1115/1.4032753.

The present study addresses the drag owing to the presence of vegetation and turbulent characteristics in a mobile bed channel, characterized by fully submerged vegetation formed by nonuniform vegetation densities. The influence of seepage on the velocity profiles, Reynolds stress, and turbulence intensities is discussed. Experimental results show that vegetation density is one of the important parameters that affect the flow resistance. It is found that higher vegetation density when placed at the downstream side leads to a reduction in velocity, Reynolds stress, and turbulent intensities. Downward seepage increases the near bed velocity, Reynolds stress, and turbulent intensities. Moment analysis shows that there is an increase in the inrush of flow, and sediment particles are transported more toward the streamwise direction with the application of seepage. The dominance of sweep events over ejection events increases more sediment transport. However, high vegetation density when placed at the downstream portion slightly decreases the dominance of sweep event. Drag coefficient decreases near the vegetation top and increases near the bed. Downward seepage decreases the effect of drag offered by the vegetation stems. The reduction in flow characteristics, viz., velocity, Reynolds stress, turbulent intensities, in the downstream portion of lesser spacing vegetation stems is attributed an increased drag coefficient.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2016;138(7):071301-071301-11. doi:10.1115/1.4032590.

The working performance of the synchronal rotary multiphase pump (SRMP), alike other types of positive-displacement multiphase pumps, is strongly affected by leakage loss. In this paper, the leakage loss in the SRMP with a full range of inlet gas volume fractions (GVFs) was theoretically and experimentally investigated. The leakage flows in the SRMP were modeled as the one-dimensional gas–liquid flows through narrow gaps. Two types of leakage flow models, homogeneous leakage flow model (HLFM) and separated leakage flow model (SLFM), were developed. The experimental work was conducted to measure the volumetric flow rate of the SRMP using the mixtures of air and N32 oil as working fluids under various inlet GVFs and differential pressures. Comparisons between the simulated and experimental pump flow rates showed that both the accuracies of the HLFM and SLFM related to the inlet GVF. In addition to the differential pressure, the leakage loss of the SRMP was affected by the inlet GVF. The leakage flow rate increased with the inlet GVF due to the changes in physical properties of the gas–liquid leakage flow. Parametric analysis showed that leakage loss in the SRMP can be effectively reduced by reducing the rotor radial clearance without much effect on its mechanical efficiency, whereas the optimum geometric parameters of the rotor and cylinder must be calculated by means of the optimization study with consideration of both the leakage loss and friction loss.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Fluids Eng. 2016;138(7):074501-074501-5. doi:10.1115/1.4032596.

When a liquid droplet impacts a superhydrophobic surface with anisotropic surface patterning in the form of alternating ribs and cavities, the rebounding droplet may exhibit a unique two-pronged jet emission. Droplet impact experiments with 11 different fluids of viscosity that varied by more than three orders of magnitude were conducted, and this paper quantifies the Capillary number, Ca, and Ohnesorge number, Oh, ranges over which the two-pronged phenomenon occurs. For Oh > 0.0154, the behavior was never observed, while at lower values of Oh, the behavior occurs for an intermediate range of Ca that depends on Oh.

Commentary by Dr. Valentin Fuster

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