Latest papers in fluid mechanics

Author(s): Hao Wu, Zhenyu Zhang, Fujun Zhang, Kun Wu, and William L. Roberts

Turbulent flows laden with liquid droplets are known as a member of turbulent dispersed multiphase flows (TDMF) that occurs in many natural, chemical, and industrial processes. In this research, we utilize a phase/Doppler particle analyzer to experimentally evaluate the impact of droplets generated from air-assisted atomization on the turbulent modulation of continuous gas-phase flow. The presence of liquid droplets affects gas-phase turbulence in three distinct manners: increasing fluctuation velocity, expanding the region with steep fluctuation velocity gradients, and causing axial stretching within the shear region.

[Phys. Rev. Fluids 8, 094301] Published Tue Sep 05, 2023

Author(s): Andrew J. Fox, C. Ricardo Constante-Amores, and Michael D. Graham

Dynamical systems with extreme events are difficult to capture with data-driven modeling, due to the relative scarcity of data within extreme events. A recently developed technique called CANDyMan decomposes the system into separate charts containing extreme and non-extreme states, learning dynamical models in each chart via time-mapping neural networks, then stitching the charts into a global dynamical model. We apply CANDyMan to a low-dimensional model of turbulent shear flow which undergoes extreme intermittent quasi-laminarization events. We show that the multi-chart model better forecasts the dynamical system evolution than either a standard single-chart model or Koopman-based model.

[Phys. Rev. Fluids 8, 094401] Published Tue Sep 05, 2023

Author(s): Dhawal Buaria and Katepalli R. Sreenivasan

Recently, Shete et al. [Phys. Rev. Fluids 7, 024601 (2022)] explored the characteristics of passive scalars in the presence of a uniform mean gradient, mixed by stationary isotropic turbulence. They concluded that at high Reynolds and Schmidt numbers, the presence of both inertial-convective and vis…

[Phys. Rev. Fluids 8, 097601] Published Tue Sep 05, 2023

Author(s): Hsu Chew Lee, B. Wu, Peng Dai, Minping Wan, and Andrei N. Lipatnikov

Reported in the paper are results of unsteady three-dimensional direct numerical simulations of laminar and turbulent, lean hydrogen-air, complex-chemistry flames propagating in forced turbulence in a box. To explore the eventual influence of thermodiffusive instability of laminar flames on turbulen…

[Phys. Rev. E 108, 035101] Published Fri Sep 01, 2023

Author(s): Daniël P. Faasen, Farzan Sepahi, Dominik Krug, Roberto Verzicco, Pablo Peñas, Detlef Lohse, and Devaraj van der Meer

Dissolution and mass transfer of carbon dioxide gas into liquid barriers plays a vital role in many environmental and industrial applications. We study the downward dissolution and propagation dynamics of CO2 into a vertical water barrier confined to a narrow vertical glass cylinder, using both experiments and direct numerical simulations (DNS). Tracking the CO2 front propagation in time leads to discovery of two distinct transport regimes: A purely diffusive and an enhanced diffusive regime. Using DNS, we explain the propagation dynamics of these two transport regimes, namely by disentangling the contributions of diffusion and convection to the propagation of the CO2 front.

[Phys. Rev. Fluids 8, 093501] Published Fri Sep 01, 2023

Author(s): A. Chahine, J. Sebilleau, R. Mathis, and D. Legendre

A special caterpillar like motion is reported for glycerin droplets sliding on a horizontal hydrophobic substrate under the influence of a shear flow. The droplet elongates in the flow direction adopting a rivulet shape with the development of waves resulting in a caterpillar like motion.

[Phys. Rev. Fluids 8, 093601] Published Fri Sep 01, 2023

Author(s): Gabriel Hadjerci and Basile Gallet

We derive a scaling prediction for the typical core radius of the isolated vortices that span baroclinic turbulence. This refinement of the vortex-gas theory does not impact the scaling behavior of the eddy diffusivity for linear bottom drag. By contrast, it induces a new scaling-law for the eddy diffusivity in the ultra-low-quadratic-drag regime. We validate these predictions numerically.

[Phys. Rev. Fluids 8, 094501] Published Fri Sep 01, 2023

Author(s): Pratyaksh Karan, Uddipta Ghosh, Fabian Brau, Yves Méheust, and Tanguy Le Borgne

Reaction fronts are regions in a flow field where two solutes, brought together by advection and dispersion, mix and react. Often, injection of one solute into a porous reservoir of the other manifests in a spherical reaction front, the dynamics of which is addressed here. In a porous medium, mixing between the two solutes is greatly enhanced because of hydrodynamic dispersion, resulting in a more advanced position of the front, while the rate of reaction also gets augmented significantly during the initial transient regimes. We show that when dispersion is sufficiently strong, these early enhancements are also maintained at larger times, when the front inevitably reaches a stationary state.

[Phys. Rev. Fluids 8, 084502] Published Thu Aug 31, 2023

Author(s): Orr Avni and Yuval Dagan

Intricate connections involving vortical flow patterns, droplet relaxation times, drag forces, and transport processes are examined using a Lagrangian particle tracking approach. We focus on mass transport and phase transition in droplets within Burgers vortices, capturing the essence of small-scale structures within turbulent flows. Thermodynamic gradients generate complex dynamics; droplets may stabilize around the vortex, revealing a periodic solution only when a high pressure drop generates a condensation core. This dynamical periodicity may suggest droplet clustering induced by phase change, while the emergence of the condensation region reveals a distinct bifurcation point.

[Phys. Rev. Fluids 8, 083604] Published Wed Aug 30, 2023

Author(s): Orr Avni and Yuval Dagan

This second part of a paper series extends the previous model to account for the role of heat transfer in the equilibrium and transient response of droplet-vortex systems, serving as a case model for a broader range of interactions between droplets and vortical flows. The presented scaling analysis elucidates the coupling between heat and mass transport mechanism and uncovers the timescale characteristic of each within vortical structures. We analyze the extent to which nonlinear transport processes play a significant role in the droplet’s dynamic response. This analysis may serve as an estimation for the thermodynamic conditions in which the incorporation of nonlinear effects is substantial.

[Phys. Rev. Fluids 8, 083605] Published Wed Aug 30, 2023

Author(s): Zhao-Bo Wang, Long Chen, Yan-Wu Cao, Chun-Xu Ke, Juan-Cheng Yang, and Ming-Jiu Ni

Numerical verification is presented of Davidson’s theoretical proposal that the angular momentum parallel to the magnetic field of a single vortex is conserved whereas the perpendicular angular momentum decays exponentially during a free-decay evolution. Theoretical scaling laws for the linear process and the nonlinear process of a single MHD vortex decay are given. It is found that the initial linear phase of a single MHD vortex decay characterized by the dominance of the Lorentz force over the inertial force exists locally. However, the whole evolution is closer to the global nonlinear behavior of a single vortex decay, for which the viscous dissipation is almost equal to the Joule dissipation.

[Phys. Rev. Fluids 8, 083703] Published Wed Aug 30, 2023

Author(s): Hiroya Mamori, Yusuke Nabae, Shingo Fukuda, and Hiroshi Gotoda

We numerically study the dynamic state of a low-Reynolds-number turbulent channel flow from the viewpoints of symbolic dynamics and nonlinear forecasting. A low-dimensionally (high-dimensionally) chaotic state of the streamwise velocity fluctuations emerges at a viscous sublayer (logarithmic layer).…

[Phys. Rev. E 108, 025105] Published Tue Aug 29, 2023

Author(s): J. Galen Wang, Daniel R. Ladiges, Ishan Srivastava, Sean P. Carney, Andy J. Nonaka, Alejandro L. Garcia, and John B. Bell

We investigate the role of steric effects on induced-charge electro-osmosis, focusing on flow field and charge distribution. Steric effects are crucial mesoscale phenomena, but they are either neglected or overly simplified in previous theoretical and computational studies. Using a hybrid Eulerian-Lagrangian method, our simulations highlight the importance of the steric effects, which lead to intriguing characteristics at strong electric fields that have not been captured by the traditional continuum method, including a suppression of velocity scaling and the overcharging of co-ions to the surface charge.

[Phys. Rev. Fluids 8, 083702] Published Tue Aug 29, 2023

Author(s): Ryan M. McMullen, Michael C. Krygier, John R. Torczynski, and Michael A. Gallis

Turbulent boundary layers on reentry vehicles may be modified by roughness, permeability, and noncontinuum effects induced by the fibrous thermal-protection-system (TPS) material coating its surface. To investigate these effects, we perform molecular-gas-dynamics (MGD) simulations of turbulent Couette flow over a substrate inspired by TPS materials. We find that rarefaction within the substrate can significantly increase its permeability, while in the free portion of the channel, we observe surprisingly good agreement between our MGD simulations and the Navier-Stokes equations, even when the local Knudsen number approaches the value typically associated with breakdown of the continuum assumption.

[Phys. Rev. Fluids 8, 083401] Published Mon Aug 28, 2023

Author(s): Robert Hartmann, Guru S. Yerragolam, Roberto Verzicco, Detlef Lohse, and Richard J. A. M. Stevens

Moderate rotation is able to enhance the heat transport in (rotating) Rayleigh-Bénard convection. While the heat transport can significantly exceed the nonrotating value for Rayleigh numbers Ra≲5×108, no or only little enhancement remains at larger Ra. Here, we aim to identify the reason for the vanishing heat transport enhancement and its different behavior towards large Ra: How is the loss of enhancement related to the onset of geostrophic turbulence? What role does rotation-induced shearing at the plates play in this context?

[Phys. Rev. Fluids 8, 083501] Published Mon Aug 28, 2023

Author(s): Calum S. Skene and Steven M. Tobias

Our paper investigates the modification of the Lagrangian statistics of a fluid flow by a magnetic field undergoing dynamo action, for a family of flows generated by spatially periodic oscillating-in-time forcings. A Floquet stability analysis shows that an exact periodic solution to these flows becomes unstable, leading to a turbulent fluid flow regime. By calculating the finite-time-Lyapunov-exponents we can then characterize the amount of stretching and chaos present in these flows. In all cases considered, we show that the amount of chaos in the saturated dynamo flow is reduced from that of the underlying turbulent flow, providing insight into how these dynamos saturate.

[Phys. Rev. Fluids 8, 083701] Published Mon Aug 28, 2023

Author(s): Aysan Razzaghi and Arun Ramachandran

We present a strategy to use hydrodynamic force in a six-port microfluidic channel to steer two drops towards collision in an extensional flow. By implementing an analytical solution in the control loop, the flow rates that are required to steer the drops toward their respective target points can be determined using a single control parameter. This parameter is a dimensionless time scale that can manipulate the drops either by engaging all six ports to create a flow field with two stagnation points, or by deactivating some of the ports and creating a linear extensional flow.

[Phys. Rev. Fluids 8, 084201] Published Mon Aug 28, 2023

Author(s): Sugan Durai Murugan and Samriddhi Sankar Ray

The long-time solutions of the Galerkin-truncated, three-dimensional, incompressible Euler equations relax to an absolute equilibrium state. We uncover the mechanism that triggers thermalization in physical space and conclude with a numerical prescription to suppress the oscillations that precede onset of thermalization in numerical simulations of the Euler equation.

[Phys. Rev. Fluids 8, 084605] Published Mon Aug 28, 2023

Author(s): Yoshiki Sakurai and Takashi Ishihara

Direct numerical simulations (DNS) of compressible isothermal turbulence (CIT) at turbulent Mach number around 0.3 in a periodic box with the number of grid points and the Taylor microscale Reynolds number up to 4096^3 and 853 are conducted using an 8th-order compact finite difference scheme. The DNS results show that the solenoidal component of the energy spectrum and the solenoidal dissipation rate obtained from CIT agree well with those from incompressible turbulence when normalized by solenoidal quantities. Results with high spatial resolution show that the dilatational component of the energy spectrum for CIT exhibits a power law at high wavenumbers due to the appearance of shocklets.

[Phys. Rev. Fluids 8, 084606] Published Mon Aug 28, 2023

Author(s): Hamidreza Anbarlooei, Fabio Ramos, and Daniel O. A. Cruz

This study offers a detailed description of momentum exchange in turbulent boundary layer flows at extreme Reynolds numbers. We introduce a new power-law formula for skin-friction, revealing insights into the boundary layer’s asymptotic thickness. Our findings align with experimental data, suggesting a universal transition in wall-bounded flows at high Reynolds numbers.

[Phys. Rev. Fluids 8, 084607] Published Mon Aug 28, 2023