A population balance model coupled with large eddy simulations (LES) is adapted and applied to study the evolution of oil droplets in an axisymmetric turbulent jet including the effects of droplet breakup. A key unknown in simulating secondary breakup in turbulent multiphase jets is the inflow size distribution generated within the primary breakup zone near the nozzle exit. A monodisperse injection inflow condition is commonly used for simplicity, but this choice is often unrealistic. In order to provide more realistic inlet conditions for LES, we develop a one-dimensional (1D) parcel model to predict the evolution of the dispersed phase along the jet centerline due to the combined effects of advection, radial turbulent transport, and droplet breakup due to turbulence in the regions closer to the jet nozzle that cannot be resolved using coarse LES. The model is validated with experimental data measured far from the nozzle. The 1D model is also used to generate an initial size distribution for use in a coarse-resolution LES of a turbulent jet. Number density fields for each bin of the discretized droplet size distribution are modeled using an Eulerian LES approach and scalar transport equations are solved for each bin. LES results are compared to published experimental data, with good agreement, and we examine the statistics of the velocity field and the concentration of the polydisperse oil droplet plumes for two droplet Weber numbers. We find that the centerline decay rate of the concentration for different droplet sizes is modified in the breakup dominated zone. Unlike Reynolds averaged approaches, LES also allows us to quantify size distribution variability due to turbulence. We quantify the radial and axial distributions and the variability of key quantities such as the Sauter mean diameter, total surface area, and droplet breakup timescale and explore their sensitivity to the Weber number.

The flow of segregated bidisperse assemblies of particles is of major importance for geophysical flows and bedload transport in particular. In the present paper, the mobility of strictly bidisperse segregated particle beds is studied with a coupled fluid discrete element method (DEM). Large particles are initially placed above small ones in a three-dimensional domain inclined at a slope of $10\%$. A gravity-driven water free surface flow induces a downslope shear-driven granular flow of the erodible bed. It is observed that, for the same water flow conditions, the bedload transport rate is higher in the bidisperse configuration than in the monodisperse one. Depending on the Shields number and on the depth of the interface between small and large particles, different transport phenomenologies are observed, ranging from no influence of the small particles to small particles reaching the bed surface due to diffusive remixing. In cases where the small particles hardly mix with the overlying large particles and for the range of studied size ratios $(r<4)$, it is shown that the increased mobility is not a bottom roughness effect, that would be due to the reduction of roughness of the underlying small particles, but a granular flow effect. This effect is analyzed within the framework of the $\mu \left(I\right)$ rheology, modeling the stress to strain rate relation for dense granular flows. It is demonstrated that the buried small particles are more mobile than larger particles and play the role of a “conveyor belt” for the large particles at the surface. Based on rheological arguments, a simple predictive model is proposed for the additional transport in the bidisperse case. It reproduces quantitatively the DEM results for a large range of Shields numbers and for size ratios smaller than 4. The results of the model are used to identify four different transport regimes of bidisperse mixtures, depending on the mechanism responsible for the mobility of the small particles. A phenomenological map is proposed for bidisperse bedload transport and, more generally, for any granular flow on an erodible bed.

We have studied the transport of a passive scalar for passive scalar with Prandtl numbers near unity in a plane channel with rough walls. The study was carried out by direct numerical simulations of the Navier-Stokes equations; an immersed boundary method was used to model the roughness. The well-known departure from the Reynolds analogy, which postulates similarity between the statistics of the scalar and the velocity, is verified. Townsend's similarity, suggesting the smooth-wall and rough-wall statistics collapse away from the wall is confirmed. The role of the form-induced production was a focus of this work. Additional form-induced contributions appear in the Reynolds-stress and scalar-variance budgets due to the roughness; they were quantified and compared between passive scalar and momentum. The form-induced production is more significant for the scalar variance than for the streamwise Reynolds stress and could be the cause of the reduced damping of scalar fluctuations by the roughness. The sheltering caused by tall roughness elements decreases the mean gradients significantly but, in the case of the scalar, is countered by diffusion, so that the form-induced production is larger for the scalar variance. The implications of this finding are discussed.

In this study, we exploit the Renard-Deck identity [J. Fluid Mech. 790, 339 (2016)] to decompose the mean friction drag in adverse-pressure-gradient turbulent boundary layers (APG-TBLs) into three components, associated with viscous dissipation, turbulence kinetic energy production, and spatial growth of the flow, respectively. We consider adverse-pressure-gradient turbulent boundary layers developing on flat plates and airfoils, with friction Reynolds numbers in the range $200<{\text{Re}}_{\tau }<2000$, and with Rotta-Clauser pressure-gradient parameters ($\beta$) ranging from 0 to 50. The effects of Reynolds number, adverse pressure gradient, and the pressure-gradient history on the contributing components are individually investigated, and special attention is paid to the comparisons with zero-pressure-gradient turbulent boundary layers (ZPG-TBLs). Our results indicate that the inner peaks of the dissipation and production terms are located at $y^{+}\approx 6$ and $y^{+}\approx 16.5$, respectively, and their outer peaks scale with the $99\%$ boundary-layer thickness (${\delta }_{99}$), i.e., $y/{\delta }_{99}\approx 0.7$ and $0.53$, respectively. These results are independent of the friction Reynolds number, the magnitude of $\beta$, and its development history. Moreover, the spatial-growth component is negative in the investigated APG-TBLs, and its magnitude increases with $\beta$.

Flame-flame interactions are analyzed in twin hydrocarbon and hydrogen turbulent premixed flames. The interactions are identified with the help of Morse theory for critical points. Flame topology in the vicinity of critical points is analyzed and is categorized into four main groups, namely tunnel formation, tunnel closure, reactant pockets, and product pockets. The number of flame-flame interactions is presented in the form of histograms for the different flame cases. The relative frequency of occurrence of each type of topology changes with the turbulence intensity. In hydrocarbon flames, the fraction of product pockets and tunnel formation events increases with turbulence intensity, whereas the fraction of reactant pockets and tunnel closure events decreases. The results for hydrocarbon flames are compared with those for hydrogen flames and the differences are explained both qualitatively and quantitatively. An additional comparison is made between the number of flame-flame interactions observed for a single hydrocarbon flame against the number of interactions for twin hydrocarbon flames. It is found that having two flames in the domain does not significantly alter the number of interactions per flame, therefore indicating that the observed interactions are mainly self-interactions within individual flames.

The classical transition from laminar to turbulent flow is affected if solid particles are added. The transition behavior is a function of particle size $d$ and solid volume fraction $\varphi$ and the flow undergoes a smooth transition, as opposed to intermittent, if $\varphi$ exceeds a certain threshold. In this work we show that, for particle-laden pipe flows with large particle-to-pipe diameter ratios $d/D$, the $\varphi$ threshold for altering the transition is much lower than previously reported for smaller particles. Magnetic resonance velocimetry reveals that particles introduce turbulent-like fluid velocity fluctuations in laminar flow. Factors that might control the limits between “classical” and “smooth” transition in the state space spanned by $d/D$ and $\varphi$ are discussed based on scaling analyses.

Heat transport in natural convection commonly encountered in natural and engineering flows is affected by buoyancy forces and shear. The effect of rotational shear on heat transport is examined here. Direct numerical simulation of Rayleigh-Bénard convection (RBC) with a rotating lid is performed at various rotation Reynolds numbers (Re) and Rayleigh numbers (Ra). The simulations are done for a range $\text{Ra}\in [2\times {10}^3,2\times {10}^7]$ and $\text{Re}\in [300,3000]$. The rotating lid induces a shear into the flow resulting in two prominent flow regimes: a rotation-dominated (RD) regime and a convection-dominated (CD) regime. The RD and CD regimes are identified based on the heat transport scaling exponent of Ra. The flow topology in these two regimes is distinct, with the CD flow showing more RBC-like flow structures while the RD regime shows a central axial vortex flow. The variations of boundary layer thickness with Ra and Re also demonstrate the regimes. A parameter $\chi \left(\text{Ra,Re}\right)$ is developed to demarcate the two regimes.

The thermal convection of dielectric fluid in an alternating electric field is investigated by the linear stability theory. We consider fluid layers confined in parallel plate capacitors without any externally imposed temperature difference. Only the internal heating by dielectric loss generates temperature gradients. The thermal variation of fluid permittivity induces electrical heterogeneity in the fluid and results in the dielectrophoretic (DEP) force, which can drive the convective motion of fluid. Assuming electric fields of high frequency, we develop a theoretical model to describe the flow dynamics under dielectric heating. For simplicity, the capacitor is placed either in microgravity environments or in a horizontal configuration on the earth. We determine the critical conditions for the DEP force to overcome stabilizing diffusion effects for convection generation. All the analyses are performed in the light of the similarity between the DEP force and the thermal Archimedean buoyancy, introducing an effective electric gravity. Examining energy transfer processes to convection flow, we confirm that the driving mechanism of convection in microgravity is similar to the ordinary thermal convection but in an electric effective gravity except for a stabilizing thermoelectric feedback effect. In the horizontal configuration, we show that the competition of the electric gravity with the earth's gravity affects the critical conditions and enriches the flow patterns of the resulting convection.

The depinning behavior of a drop on an inclined heterogeneous surface comprising hydrophilic-hydrophobic stripes is investigated by the multiphase lattice Boltzmann method driven by the chemical potential. Different initial states of the drop lead to three types of depinning behavior. When the drop is initially in the moderate state, the front and rear contact angles linearly change as the slope angle continues to increase, and depinning almost simultaneously occurs at the front and rear contact lines. Unilateral depinning occurs at the front contact line for the contracted state and at the rear contact line for the elongated state, with sudden instabilities of the contact angle. It is clearly observed that the unilateral depinning process consists of a slow-moving process and a fast-moving process. The dynamic equilibrium reflects the competition between gravity and the capillary force in the whole drop. At the microscopic scale, based on measurement of the real-time contact angle, it is illustrated that the local force balance in the contact line region is maintained by the unbalanced Young's force and the resistance of the substrate in the slow-moving stage, while the unbalanced Young's force provides the driving force for the motion of the contact line in the fast-moving stage.

The turbulent flow in a three-dimensional asymmetric diffuser was experimentally investigated using time-resolved, three-dimensional particle tracking velocimetry (3D-PTV). The diffuser geometry chosen for this study was a benchmark geometry devised and studied experimentally by Cherry et al. [Intl. J. Heat Fluid Flow 29, 3 (2008)], with a few subsequent numerical simulations at a Reynolds number of 10 000. In the present paper, we applied a state-of-the-art 3D-PTV to measure the 3D structure of the flow field in the entire diffuser for five Reynolds numbers (Re), ranging from 9200 up to 29 400. We found that the mean velocity fields were qualitatively similar across all Re studied. The volumetric fraction of the backflow region, when quantified using an intermittency factor of $\gamma <$ 0.8, was in the range of 11–14%, with a marginal decrease with increasing Re. The maximum values of normal and shear Reynolds stresses were located in the regions close to the edge of the backflow region, and the peak values for the streamwise normal Reynolds stress increased with Re. The corner vortices, which formed in the channel preceding the diffuser, showed the existence of secondary flow well within the diffuser region. The strength of these vortices reduced with increasing Re. A modal decomposition of the turbulent fluctuations using spectral proper orthogonal decomposition (SPOD) showed large-scale structures in the flow. The SPOD analysis revealed that these large-scale structures were associated with low frequency oscillations in the band of St = [0.003 0.03], with two frequency peaks at St = 0.012 and 0.028. The three-dimensional separation along the two diverging walls of the diffuser was characterized by detecting critical points in the near-wall streamlines. These critical points were investigated in relation to three large-scale vortical structures identified within the diffuser. The flow topology in the diffuser showed that two of these vortical structures originated from the wall with the largest diverging angle and were connected by a separation surface. The third vortical structure originated from the neighboring diverging wall and was bound by another large separation surface.

The flux of energy between scales in turbulence can be cast as the result of the interaction between a turbulent stress and a rate of strain. Efficient energy transfer requires that these two tensors be oriented properly relative to each other, but previous work has shown that the instantaneous alignment between them is poor. Here, we consider the temporal dynamics of this alignment in a direct numerical simulation of isotropic turbulence. We show that the orientation of the stress lags behind that of the strain rate, both at a single location and along trajectories. However, the timescale of the reorientation of the stress in these cases does not follow the expected dynamical scaling with length scale. To capture the proper dynamical scaling, we reformulate the energy flux between scales using the right Cauchy-Green strain tensor and the second Piola-Kirchhoff stress tensor. Our results highlight the key role played by the deformation of fluid elements in the physics of the energy cascade, and suggest that their irreversible deformation is a Lagrangian manifestation of the cascade's broken time-reversal symmetry.

This paper is associated with a poster winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.P0020.

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.V0072.

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.V0018.

This paper is associated with a poster winner of a 2019 APS/DFD Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.P0035.

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.V0036.

This paper is associated with a poster winner of a 2019 American Physical Society Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.P0015.

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.V0013.

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.V0040.