# Latest papers in fluid mechanics

### Passive feedback control of transonic buffet flow

The activated structural degree of freedom often causes undesirable large-amplitude structural vibrations in unstable flows, such as the frequency lock-in phenomenon in the transonic buffet flow. The underlying mechanism is the instability characteristic of the system that becomes more complex, namely, further instability on the structural mode because of the effect of fluid-structure interaction. In this paper, we study the feasibility of improving the stability of the transonic buffet flow by activating the structural degree of freedom based on numerical simulations and a low-dimensional model. Results reveal that the activated structural degree of freedom can stabilize the buffet flow, eliminating the source of the oscillatory flow. The essence of this strategy is to efficiently utilize the coupling effect between the structural mode and the fluid mode to improve the stability of the fluid mode by properly decreasing the stability margin of the structural mode. From another perspective, this approach is a new passive-feedback flow control strategy, which does not change the shape of the subject compared with the classical passive control and which does not need extra actuation and power compared with the active control.

### Bubble interactions and bursting behaviors near a free surface

The strong bubble interactions and bursting behaviors near a free surface are studied numerically with a compressible two-phase flow solver. The interface is captured by the volume of fluid method. We investigate the effects of the dimensionless distance between the bubble and the free surface γf (scaled by the maximum bubble radius) ranging from 0.25 to 1.5. For the nonbursting cases, the essential evolution of the toroidal bubble is well captured, including the splitting, coalescence, and recollapse. Generally, a relatively thin spike is generated at the free surface during the first cycle of the bubble. Subsequently, a wider secondary spike at the base of the first spike is formed during the second bubble cycle, which leads to the formation of the crown-shaped spike. When γf is sufficiently small, the bubble bursts at the free surface and an opening cavity is generated. The pressure inside the cavity and the atmospheric pressure are not balanced immediately, and the pressure difference lasts for a while, causing the inward gas flow and the final closure of the cavity. The gas flow is found to play a vital role in the bubble bursting behaviors, which has not been well understood. By comparing with previous studies, three distinct bursting patterns are identified and discussed.

### A three-dimensional smoothed particle hydrodynamics dispersion simulation of polydispersed sediment on the seafloor using a message passing interface algorithm

Technical activities on seafloor for harvesting polymetallic nodules result in a displacement of a large amount of sediment, which is convected away from the site by the underlying currents and turbulent diffusion, with a possible impact on the benthic communities living in the neighborhood. To better understand the dispersion mechanism of the resuspended sediment, a smoothed particle hydrodynamics technique augmented by a message passing interface parallel algorithm to address the intensive demand on the three-dimensional simulations is developed. Our numerical results show that the resuspended sediment would occupy a downstream area extending to about 5 km, for a nominal current speed of 5 cm/s. The evolution of the sediment plume occurs mainly along the current direction, while the turbulent diffusion disperses the sediment laterally. Coarse sediment particles are found to return to the seafloor fairly quickly after being resuspended, while fine particles are more persistent in the suspended state and travel much further downstream. In 900 tons of sediment resuspended for 18 h, 318 tons have returned to the bottom at the end of the simulation period. The majority of the deposited sediment is composed of coarse sediment particles (d > 60 μm), and almost half of the deposited sediment is distributed within the harvesting region. The sediment deposition rate reaches up to 48% of the resuspension rate and is still rising after 18 h. The horizontal turbulent diffusivity, which is supposed to be weak at the ocean bottom, does not have any obvious influence on the dispersion of the resuspended sediment; it only slightly reduces the deposition rate.

### Intense sound radiation by high-speed flow: Turbulence structure, gas properties, and near-field gas dynamics

Author(s): David A. Buchta and Jonathan B. Freund

Weak near-field nonlinearities explain key features of the intense sound radiated by high-speed turbulence. The sound-field wave structure, pressure skewness, and pressure intensity are also shown to be relatively insensitive to the large-scale turbulence structure.

[Phys. Rev. Fluids 4, 044605] Published Thu Apr 11, 2019

### A new formulation for two-wave Riemann solver accurate at contact interfaces

This study proposes a new formulation for the Harten, Lax, and van Leer (HLL) type Riemann solver which is capable of solving contact discontinuities accurately but with robustness for strong shock. It is well known that the original HLL, which has incomplete wave structures, is too dissipative to capture contact discontinuities accurately. On the other side, contact-capturing approximate Riemann solvers such as HLL with Contact (HLLC) usually suffer from spurious solutions, also called carbuncle phenomenon, for strong shock. In this work, a new accurate and robust HLL-type formulation, the so-called HLL-BVD (HLL Riemann solver with boundary variation diminishing) is proposed by modifying the original HLL with the BVD algorithm. Instead of explicitly recovering the complete wave structures like the way of HLLC, the proposed method restores the missing contact with a jump-like function. The capability of solving contact discontinuities is further improved by minimizing the inherent dissipation term in HLL. Without modifying the original incomplete wave structures of HLL, the robustness for strong shock has been reserved. Thus, the proposed method is free from the shock instability problem. The accuracy and robustness of the new method are demonstrated through solving several one- and two-dimensional tests. Results indicate that the new formulation based on the two-wave HLL-type Riemann solver is not only capable of capturing contact waves more accurately than the original HLL or HLLC but, most importantly, is free form carbuncle instability for strong shock.

### Comparative study of the dynamics of laser and acoustically generated bubbles in viscoelastic media

Author(s): Chad T. Wilson, Timothy L. Hall, Eric Johnsen, Lauren Mancia, Mauro Rodriguez, Jonathan E. Lundt, Tim Colonius, David L. Henann, Christian Franck, Zhen Xu, and Jonathan R. Sukovich

Experimental observations of the growth and collapse of acoustically and laser-nucleated single bubbles in water and agarose gels of varying stiffness are presented. The maximum radii of generated bubbles decreased as the stiffness of the media increased for both nucleation modalities, but the maxim...

[Phys. Rev. E 99, 043103] Published Wed Apr 10, 2019

### Generation of high-intensity ultrasound through shock propagation in liquid jets

Author(s): Gabriel Blaj, Mengning Liang, Andrew L. Aquila, Philip R. Willmott, Jason E. Koglin, Raymond G. Sierra, Joseph S. Robinson, Sébastien Boutet, and Claudiu A. Stan

An investigation shows how single shock waves, produced in water jets by x-ray laser pulses, propagate and become ultrasonic shock trains through oblique shock reflections and cavitation. These trains are one of the loudest sounds ever made in liquid water and may damage samples studied with x-ray lasers.

[Phys. Rev. Fluids 4, 043401] Published Wed Apr 10, 2019

### Faraday instability in double-interface fluid layers

Author(s): Kevin Ward, Farzam Zoueshtiagh, and Ranga Narayanan

Three-fluid systems subjected to periodic mechanical oscillations are theoretically and experimentally examined. The addition of the third fluid can trigger both stabilizing or destabilizing effects on the interfaces, and additional codimension points are obtainable.

[Phys. Rev. Fluids 4, 043903] Published Wed Apr 10, 2019

### Molecular dynamics study of the translation and rotation of amphiphilic Janus nanoparticles at a vapor-liquid surface

Author(s): Joel Koplik and Charles Maldarelli

The equilibrium contact angle of a heterogeneous colloidal particle at a liquid/vapor interface varies with position and may have distinct local equilibria at different orientations. We show that the free energy determines a particle’s orientation as a function of orientation and immersion depth.

[Phys. Rev. Fluids 4, 044201] Published Wed Apr 10, 2019

### Effect of a bottom gap on the mean flow and turbulence structure past vertical solid and porous plates situated in the vicinity of a horizontal channel bed

Author(s): K. Basnet and G. Constantinescu

A study of the wake past a porous barrier situated some distance above the ground shows that the presence of the bottom surface modifies the wake structure and breaks the antisymmetry of the von Karman billows. The bleeding flow increases the distance at which the wake billows form.

[Phys. Rev. Fluids 4, 044604] Published Wed Apr 10, 2019

### Flow instability in weakly eccentric annuli

A temporal linear stability analysis of laminar flow in weakly eccentric annular channels has been performed. It has been shown that, even for eccentricities ε and Reynolds numbers that were much smaller than those considered in previous studies, flow instability occurred in the form of travelling waves having characteristics that are very different from those of Tollmien-Schlichting waves and which were triggered at mid-gap by an inviscid mechanism that is associated with the presence of inflection points in azimuthal profiles of the base velocity. The critical stability conditions have been determined for 0 ≤ ε ≤ 0.1 and for diameter ratios 0 < γ < 1. The critical Reynolds number Rec decreased with increasing γ for 0 < γ ≲ 0.13, reached a minimum at γ ≈ 0.13, and increased with further increase in γ. The lowest observed Rec was 529 and occurred for ε = 0.1 and γ ≈ 0.13. As ε → 0, Rec ∝ ε−2. The critical wave number and the critical frequency of the disturbances decreased with increasing γ and approached zero as γ → 1, whilst their ratio was nearly constant in the range of parameters considered in this study. The most unstable regions were found to be at roughly mid-gap on the two flanks of the annulus, and the phase speed of the disturbances was close to the base flow velocity at these regions.

### Eigenvector perturbation methodology for uncertainty quantification of turbulence models

Author(s): Roney L. Thompson, Aashwin Ananda Mishra, Gianluca Iaccarino, Wouter Edeling, and Luiz Sampaio

Establishing turbulence models as reliable tools for aerospace design requires quantification of uncertainties in model predictions. A methodology for turbulence model uncertainty estimation in complex flows using physics-based perturbation is introduce and validated.

[Phys. Rev. Fluids 4, 044603] Published Tue Apr 09, 2019

### Magnetic quenching of the inverse cascade in rapidly rotating convective turbulence

Author(s): Stefano Maffei, Michael A. Calkins, Keith Julien, and Philippe Marti

Numerical simulations of a simplified asymptotic model find that sufficiently strong magnetic fields prevent the formation of large-scale vortices and saturate the inverse cascade at a finite length-scale.

[Phys. Rev. Fluids 4, 041801(R)] Published Mon Apr 08, 2019

### Global linear instability of rotating-cone boundary layers in a quiescent medium

Author(s): Christian Thomas and Christopher Davies

Numerical simulations of disturbance development in the family of rotating-cone boundary layers illustrates that for sufficiently large azimuthal mode numbers, a form of global linear instability ensues that is characterized by a faster than exponential temporal growth.

[Phys. Rev. Fluids 4, 043902] Published Mon Apr 08, 2019

### Hydrodynamic interactions in polar liquid crystals on evolving surfaces

Author(s): Ingo Nitschke, Sebastian Reuther, and Axel Voigt

The flow of passive and active polar liquid crystals on evolving surfaces is considered. The models are derived as a thin-film limit, and a finite element method is used to study the effect of hydrodynamics on the interplay of topology, geometric properties, and defect dynamics.

[Phys. Rev. Fluids 4, 044002] Published Mon Apr 08, 2019

### Transition to chaos in an acoustically driven cavity flow

Author(s): Gaby Launay, Tristan Cambonie, Daniel Henry, Alban Pothérat, and Valéry Botton

We show that a cavity flow driven by acoustic streaming can sustain low-dimensional chaos. Frequency and nonlinear times series analyses reveal a novel path to chaos with a sequence of two-way transitions between nonchaotic and chaotic states with steps where the dynamics drastically simplifies.

[Phys. Rev. Fluids 4, 044401] Published Mon Apr 08, 2019

### Velocity fluctuations in a dilute suspension of viscous vortex rings

Author(s): Thomas A. Morrell, Saverio E. Spagnolie, and Jean-Luc Thiffeault

Velocity fluctuations in a fluid due to a dilute suspension of vortex rings, such as those generated by a population of small jellyfish, are explored. The slow diffusion of momentum leads to a different stable distribution than is found for velocity fluctuations caused by microswimmers.

[Phys. Rev. Fluids 4, 044501] Published Mon Apr 08, 2019

### Direct numerical simulation of a supersonic turbulent boundary layer subjected to a concave surface

Author(s): Xiaoshuai Wu, Jianhan Liang, and Yuxin Zhao

An investigation finds that a supersonic turbulent boundary layer is highly destabilized by a longitudinal concave surface. The intensification of vortical structures reflects the effect of turbulence amplification. Streaky turbulent motions suggest enhanced inner-outer interactions.

[Phys. Rev. Fluids 4, 044602] Published Mon Apr 08, 2019

### Analysis of rheological behaviors of two-dimensional emulsion globules with asymmetric internal structures in modest extensional flows

The rheological behaviors of complex emulsion globules (CEGs) and its three asymmetric daughter droplets (DDs) have been studied numerically in this paper. Unlike simple eccentric emulsion globules (SEEGs), two more DDs are added into the globules and the three DDs are located initially in an asymmetric distribution with a triangular shape. Through this investigation, an oriented shift and an inverse of CEGs are observed. Especially, the movement of CEGs under more conditions is still caused by the interaction of the asymmetric inner pressure distribution and the total outer drag force. Due to the asymmetric internal structure, the deformation of CEGs caused by the outer flow is asymmetric and so is the interfacial curvature which results in the oriented inner circulation. Compared to SEEGs, the addition of two extra DDs leads to the bigger deformation of CEGs, and more CEGs will shift to the left in the parameter range of our investigation. The increases of the parameters in the discussed ranges will promote CEGs to move to the right. In addition, DDs will move away from the original position and move away from or close to the interface of the mother droplet (MD), which may result in interface contact between DDs and MD. The results investigated in this paper further prove the mechanical mechanism of the oriented shift of the complex emulsions globules and are helpful to the controllable movement of soft globules driven by the asymmetric curvatures.

### Dynamic mode decomposition for the inspection of three-regime separated transitional boundary layers using a least squares method

Transitional boundary layers undergoing separated flow transition for different free stream turbulence intensity levels and Reynolds numbers have been inspected by applying dynamic mode decomposition (DMD) to time-resolved particle image velocimetry data. The identification of the unstable modes responsible for transition suffers from nonlinear effects if the whole dataset is considered for the construction of the snapshot matrix underlying the flow evolution. To overcome this limit, piecewise linear models aimed at the identification of the different regimes in the entire transition process are proposed. In particular, the flow is initially laminar (i.e., stable), it becomes unstable due to transition, and once transition is completed, the fully turbulent condition leads the boundary layer to a stable regime. The norm of the residuals resulting from the application of DMD on a variable streamwise extension of the dataset shows a trend that is associated with the variation of regime. This trend is analyzed by means of the least squares method, which allows identifying the change in the regime with stable, unstable, and turbulent behaviors. The validity of this procedure is proved by comparing it with previously published results. Moreover, since the DMD is applied to limited temporal snapshots, it provides a temporal resolution of growth rate and positions of switch between the boundary layer states. Such information is used to extract from the big dataset under analysis the time sequences characterized by the largest growth rate, hence quickly highlighting the flow physics driving transition.