Latest papers in fluid mechanics

Effects of body forces on vorticity and enstrophy evolutions in turbulent premixed flames

Physics of Fluids - Mon, 03/01/2021 - 01:52
Physics of Fluids, Volume 33, Issue 3, March 2021.
The effects of body forces (alternatively, the Froude number) on both vorticity and enstrophy evolutions within the flame brush have been analyzed using direct numerical simulation data of freely propagating statistically planar turbulent premixed flames subjected to different turbulence intensities. The turbulence parameters are taken to represent the thin reaction zone regime of premixed turbulent combustion. The enstrophy has been found to decay significantly from the unburned to the burned gas side of the flame brush for high turbulence intensities, and this trend becomes particularly prominent with the strengthening of the body force promoting unstable stratification. However, local instances of enstrophy generation have been observed, and in some cases, the decay of enstrophy is arrested across the flame brush for small turbulence intensities. This trend strengthens with the increasing magnitude of the body force promoting stable stratification. The enstrophy generation due to the baroclinic torque is primarily responsible for this local enstrophy generation for small turbulence intensities especially under the body force promoting stable stratification. This baroclinic torque contribution is also found to be responsible for anisotropic behavior of vorticity components within the flame brush. The vortex stretching and viscous dissipation terms have been found to be the leading order source and sink terms, respectively, in the enstrophy transport for high turbulence intensities especially in the case of body force promoting unstable stratification. However, baroclinic torque and the sink term due to dilatation rate continue to play significant roles even for high turbulence intensity cases considered here, but their relative importance increases with a decreasing turbulence intensity especially under the body force that promotes stable stratification. The surface-weighted entrainment velocity has been found to be mostly unaffected by the body force in this analysis, and a minor influence can be discerned in the case of small turbulence intensities where an unstable stratification tends to promote high negative values of entrainment velocity only toward the unburned gas side of the flame brush.

Base flow decomposition for complex moving objects in linear hydrodynamics: Application to helix-shaped flagellated microswimmers

Physical Review E - Fri, 02/26/2021 - 10:00

Author(s): Ji Zhang, Mauro Chinappi, and Luca Biferale

The motion of microswimmers in complex flows is ruled by the interplay between swimmer propulsion and the dynamics induced by the fluid velocity field. Here we study the motion of a chiral microswimmer whose propulsion is provided by the spinning of a helical tail with respect to its body in a simpl...


[Phys. Rev. E 103, 023109] Published Fri Feb 26, 2021

Nanostructured jumping-droplet thermal rectifier

Physical Review E - Fri, 02/26/2021 - 10:00

Author(s): Ji-Xiang Wang, Patrick Birbarah, Donald Docimo, Tianyu Yang, Andrew G. Alleyne, and Nenad Miljkovic

Analogous to an electrical rectifier, a thermal rectifier (TR) can ensure that heat flows in a preferential direction. In this paper, thermal transport nonlinearity is achieved through the development of a phase-change based TR comprising an enclosed vapor chamber having separated nanostructured cop...


[Phys. Rev. E 103, 023110] Published Fri Feb 26, 2021

Confined turbulent convection driven by a combination of line and distributed sources of buoyancy

Physical Review Fluids - Fri, 02/26/2021 - 10:00

Author(s): Johanna Mader, Maarten van Reeuwijk, and John Craske

What happens if a box is heated and cooled by a combination of localized and distributed heat sources? This question is investigated using direct numerical simulations for a range of heating regimes to produce different steady states. Two conceptual models are developed, and it is shown that the transition from a stratified to a well-mixed environment occurs when the distributed and localized heating are of equal strength.


[Phys. Rev. Fluids 6, 023503] Published Fri Feb 26, 2021

Active chaotic mixing in a channel with rotating arc-walls

Physical Review Fluids - Fri, 02/26/2021 - 10:00

Author(s): Kamal El Omari, Eliane Younes, Teodor Burghelea, Cathy Castelain, Yann Moguen, and Yves Le Guer

An active inline mixer, called Rotating Arc-Wall mixer (RAW), suitable for flows at low Reynolds number and high Péclet number, is studied. The forcing protocol, imposed by three rotating circular arc-walls, is time periodic. We use simple phenomenological arguments to estimate heuristically the mixing efficiency with two nondimensional control parameters: the frequency of the forcing protocol, and the strength of the cross flow relative to the primary flow. The validity and limitations of the proposed mixing conditions are explained by the transport mechanisms in the mixer, and the beneficial role of chaotic advection for mixing is highlighted.


[Phys. Rev. Fluids 6, 024502] Published Fri Feb 26, 2021

Clustering of vector nulls in homogeneous isotropic turbulence

Physical Review Fluids - Fri, 02/26/2021 - 10:00

Author(s): D. O. Mora, M. Bourgoin, P. D. Mininni, and M. Obligado

We analyze the vector nulls of velocity, Lagrangian acceleration, and vorticity, coming from direct numerical simulations of forced homogeneous isotropic turbulence. We study the scaling of clusters of these null points and compare them with datasets of point inertial particles with different Stokes numbers. We find that inertial particles display preferential concentration with a degree of clustering that resembles some properties of the clustering of the Lagrangian acceleration nulls.


[Phys. Rev. Fluids 6, 024609] Published Fri Feb 26, 2021

Large-scale unsteadiness in a compression ramp flow confined by sidewalls

Physical Review Fluids - Fri, 02/26/2021 - 10:00

Author(s): Akshay S. Deshpande and Jonathan Poggie

The complexity of a shock-wave/turbulent boundary layer interaction (SWTBLI) in the presence of sidewalls is investigated both qualitatively and quantitively in this work. Additional spanwise no-slip conditions led to the occurrence of secondary flows, which were not observed in the quasi-two-dimensional cases. Statistical analysis revealed asymmetric back-and-forth motion of the interaction across the midspan, possibly caused by the variation in the instantaneous size of the sidewall separated zones. Coupling between corner and centerline interactions was also observed as a result of higher confinement ratio.


[Phys. Rev. Fluids 6, 024610] Published Fri Feb 26, 2021

Lagrangian analysis for turbulent transport in variable-density turbulence

Physical Review Fluids - Thu, 02/25/2021 - 10:00

Author(s): G. S. Sidharth and J. R. Ristorcelli

It is shown that the basis functions of gradient transport theory are different when there is an additional materially conserved variable. The turbulent fluxes now depend on the mean density gradient indicating the possibility of counter gradient transport from first principles as seen in some laboratory experiments. It is shown that arguments by analogy from constant density transport for the Favre fluxes are not consistent with the Lagrangian results.


[Phys. Rev. Fluids 6, 023202] Published Thu Feb 25, 2021

Modeling the nonlinear aeroacoustic response of a harmonically forced side branch aperture under turbulent grazing flow

Physical Review Fluids - Thu, 02/25/2021 - 10:00

Author(s): Tiemo Pedergnana, Claire Bourquard, Abel Faure-Beaulieu, and Nicolas Noiray

The response of a side branch aperture to harmonic forcing is a key element of the feedback loop describing flow-induced aeroacoustic instability in deep cavities. We derive and validate two physics-based models which, after calibration at a given condition, predict the influence of frequency, mean flow speed, and acoustic pressure amplitude on the response. Notably, these low-order models enable robust analytical amplitude predictions of self-sustained oscillations in deep cavities under turbulent grazing flow


[Phys. Rev. Fluids 6, 023903] Published Thu Feb 25, 2021

Vorticity transfer in a leading-edge vortex due to controlled spanwise bending

Physical Review Fluids - Thu, 02/25/2021 - 10:00

Author(s): Kun Jia, Tyler Scofield, Mingjun Wei, and Samik Bhattacharya

The effect of dynamic spanwise bending on the vortex dynamics of an accelerating flat plate is studied with experiments and numerical simulations. A flat plate, held at an angle of attack of 30 degrees, is accelerated from rest to Reynolds number 2400. It was bent dynamically along the span in a controlled manner with a bending ratio of 0.65. We find that a dynamic spanwise bending induces a change in the effective shear layer velocity along the span’s bent part and creates spanwise vorticity convection. As a result, the growth of circulation in the leading-edge-vortex gets delayed along the bent part, and the final circulation is smaller than the no bending case.


[Phys. Rev. Fluids 6, 024703] Published Thu Feb 25, 2021

Robust propagation of internal coastal Kelvin waves in complex domains

Physical Review Fluids - Thu, 02/25/2021 - 10:00

Author(s): Chenyang Ren, Xianping Fan, Yiling Xia, Tiancheng Chen, Liu Yang, Jin-Qiang Zhong, and H. P. Zhang

Internal coastal Kelvin waves were experimentally generated in a two-layer fluid system on a rotating table. Waves are exponentially localized near the tank boundary and propagate in the same direction as the table rotation along boundaries of complex geometries without being scattered. Our experiments suggest a connection between these unusual wave characteristics and topological properties of the underlying governing equations.


[Phys. Rev. Fluids 6, L022801] Published Thu Feb 25, 2021

Numerical study of the influence of wall roughness on laminar boundary layer flashback

Physical Review Fluids - Wed, 02/24/2021 - 10:00

Author(s): Shuyu Ding, Kai Huang, Yifan Han, and Damir Valiev

Boundary layer flame flashback is a phenomenon that may constitute a key challenge for efficient combustion of novel fuels at gas turbine conditions. In the present work, the effect of wall roughness on the laminar boundary layer flashback is studied systematically using numerical simulation. The results indicate that the wall roughness can attenuate flashback speed due to enhanced heat loss in case of low thermal resistance of the wall. The critical velocity gradient of the oncoming flow is shown to decrease with wall roughness level and increase with gas thermal expansion ratio.


[Phys. Rev. Fluids 6, 023201] Published Wed Feb 24, 2021

Flow in a weakly curved square duct: Assessment and extension of Dean's model

Physical Review Fluids - Wed, 02/24/2021 - 10:00

Author(s): Leonardo Rigo, Damien Biau, and Xavier Gloerfelt

The laminar flow in a weakly bent pipe exhibits very rich dynamical properties. The flow is stationary, periodic or chaotic depending on one control parameter. A very practical simplification inspired by Dean is capable of reproducing this behavior with remarkable accuracy.


[Phys. Rev. Fluids 6, 024101] Published Wed Feb 24, 2021

Effect of stratification on the mixing and reaction yield in a T-shaped micro-mixer

Physical Review Fluids - Wed, 02/24/2021 - 10:00

Author(s): A. Mariotti, C. Galletti, R. Mauri, M. V. Salvetti, and E. Brunazzi

Experiments, i.e. micro-PIV and flow visualization, and direct numerical simulations, are used jointly to investigate how stratification affects mixing and chemical reaction in a T-shaped microreactor fed with two miscible liquids exhibiting a small density difference. The work analyzes the dependence of the degree of mixing on the Reynolds number and correlates the reaction yield with the Damköhler number to help to devise strategies for the practical operation of microreactors with fluids of practical interest.


[Phys. Rev. Fluids 6, 024202] Published Wed Feb 24, 2021

General method for determining the boundary layer thickness in nonequilibrium flows

Physical Review Fluids - Wed, 02/24/2021 - 10:00

Author(s): Kevin Patrick Griffin, Lin Fu, and Parviz Moin

In this work, a new method for computing the boundary layer thickness is proposed by reconstructing an approximate inviscid solution based on the Bernoulli equation. The viscous streamwise velocity profile U[y] agrees with this inviscid reconstruction UI[y] outside the boundary layer, and the solutions diverge from each other at the boundary layer edge. The boundary layer thickness is readily determined by examining the discrepancy between these profiles. Extensive validation suggests that the present method is more robust and more widely applicable than existing methods.


[Phys. Rev. Fluids 6, 024608] Published Wed Feb 24, 2021

Dynamics of bubbles under stochastic pressure forcing

Physical Review E - Tue, 02/23/2021 - 10:00

Author(s): Riccardo Vesipa, Eleonora Paissoni, Costantino Manes, and Luca Ridolfi

Several studies have investigated the dynamics of a single spherical bubble at rest under a nonstationary pressure forcing. However, attention has almost always been focused on periodic pressure oscillations, neglecting the case of stochastic forcing. This fact is quite surprising, as random pressur...


[Phys. Rev. E 103, 023108] Published Tue Feb 23, 2021

Weakly nonlinear Holmboe waves

Physical Review Fluids - Tue, 02/23/2021 - 10:00

Author(s): Joshua Cudby and Adrien Lefauve

Holmboe waves are long-lived traveling waves commonly found in environmental stratified shear flows. Here we study their finite-amplitude properties in the nonlinear but nonturbulent regime, with a weakly nonlinear temporal stability analysis. Using a versatile amplitude expansion method, we analyze supercritical bifurcation diagrams both in Reynolds number and Richardson number, transient phase portraits, and the vertical structures of all components modes up to third order. We believe these results provide a basis for a future fully nonlinear analysis of the Holmboe dynamical system.


[Phys. Rev. Fluids 6, 024803] Published Tue Feb 23, 2021

Stability analysis of a thinning electrified jet under nonisothermal conditions

Physical Review E - Mon, 02/22/2021 - 10:00

Author(s): Dharmansh Deshawar and Paresh Chokshi

The linear stability of a jet propagating under an electric field is analyzed under nonisothermal conditions. The electrified jet of a Newtonian fluid is modeled as a slender filament, and the leaky dielectric model is used to account for the Maxwell stresses within the fluid. The convective heat tr...


[Phys. Rev. E 103, 023107] Published Mon Feb 22, 2021

Wet to dry self-transitions in dense emulsions: From order to disorder and back

Physical Review Fluids - Mon, 02/22/2021 - 10:00

Author(s): Andrea Montessori, Adriano Tiribocchi, Marco Lauricella, Fabio Bonaccorso, and Sauro Succi

Computer simulations show the self-transition between ordered and disordered emulsions in divergent microfluidic channels. The transition is driven by the nonlinear competition between viscous dissipation and surface tension forces as controlled by the device geometry. An unexpected route back to order is observed in the regime of large opening angles where a trend towards increasing disorder would be intuitively expected.


[Phys. Rev. Fluids 6, 023606] Published Mon Feb 22, 2021

Physical invariance in neural networks for subgrid-scale scalar flux modeling

Physical Review Fluids - Mon, 02/22/2021 - 10:00

Author(s): Hugo Frezat, Guillaume Balarac, Julien Le Sommer, Ronan Fablet, and Redouane Lguensat

A physics informed approach is applied to neural networks for subgrid-scale scalar flux modeling. We show that several invariances of the scalar transport equation are not enforced by existing parametric models, which reduce their interpretability and question their application. A new architecture embedding these invariances as hard and soft constraints is proposed. Through different flow configurations, we show that the proposed constraints increase both the performances and the generalization capabilities of the model.


[Phys. Rev. Fluids 6, 024607] Published Mon Feb 22, 2021

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