# Latest papers in fluid mechanics

### Discontinuity in the sedimentation system with two particles having different densities in a vertical channel

Author(s): Deming Nie and Jianzhong Lin

The two-dimensional lattice Boltzmann method was used to numerically study a sedimentation system with two particles having different densities in a vertical channel for Galileo numbers in the range of 5≤Ga≤15 (resulting in a Reynolds number, based on the settling velocity, approximately ranging bet...

[Phys. Rev. E 99, 053112] Published Mon May 20, 2019

### Invariant manifolds in stratified turbulence

Author(s): N. E. Sujovolsky, G. B. Mindlin, and P. D. Mininni

A low dimensional model for stratified turbulence predicts the existence of invariant manifolds for the evolution of temperature and velocity gradients. Fluid elements evolve preferentially along these manifolds, associated with stable regions and with regions prone to develop local convection.

[Phys. Rev. Fluids 4, 052402(R)] Published Mon May 20, 2019

### Stabilization of unsteady flows by reduced-order control with optimally time-dependent modes

Author(s): Antoine Blanchard and Themistoklis P. Sapsis

The Optimally Time-Dependent (OTD) modes, a set of deformable orthonormal modes that track transient instabilities, are incorporated into a robust, inexpensive control algorithm that can steer any trajectory of a high-dimensional nonlinear system toward a fixed point of the governing equations.

[Phys. Rev. Fluids 4, 053902] Published Mon May 20, 2019

### Numerical investigation of highly unsteady accelerated/decelerated flows for blunt bodies experiencing impulsive motion

Scale-resolving simulations (SRSs) such as large-eddy simulation (LES) and hybrid LES/Reynolds-averaged Navier–Stokes are applied to analyze the unsteady characteristics of the drag and flow fields for blunt bodies subject to impulsive motion. These simulations reveal highly transient behaviors of the primary and secondary vortices triggered by inertial forces during impulsive motion. A distinctive characteristic of the transient drag, namely, the existence of a plateau region, induced by a circular cylinder during impulsive acceleration is also revealed. It is found that among the SRS methods, the LES one-equation eddy method is able to more precisely capture the intrinsic local pressure gradient arising from unsteadiness of the vortices together with the instantaneous vorticity magnitude. The secondary vortices created by inertial forces and shear stresses due to the impulsive motion and the interactions of these vortices with the primary vortices turn out to play a key role in the transient behaviors of the flow field and the drag. The mechanism causing the highly unsteady flow field and its relationship to the bluntness of the shape configuration are also explored using the LES one-equation eddy method. It is found from the range of retained local Cp values and the transient vorticity distribution along the cylinder surface that the mobility of the unsteady flow separation point depends on the bluntness of the configuration.

### Phase difference effect on collective locomotion of two tandem autopropelled flapping foils

Author(s): Xingjian Lin, Jie Wu, Tongwei Zhang, and Liming Yang

The collective locomotion of two tandem autopropelled flapping foils is greatly affected by the phase difference. Two distinct vortex interactions are observed—merging interaction and broken interaction—which respectively result in the highest efficiency for the follower and the leader.

[Phys. Rev. Fluids 4, 054101] Published Fri May 17, 2019

### Numerical simulation of flow over a parallel cantilevered flag in the vicinity of a rigid wall

Author(s): Li Wang and Fang-bao Tian

Flow over a parallel cantilevered flag in the vicinity of a rigid wall is numerically studied using an immersed boundary–lattice Boltzmann method (IB–LBM) in two-dimensional domain, where the dynamics of the fluid and structure are, respectively, solved by the LBM and a finite-element method (FEM), ...

[Phys. Rev. E 99, 053111] Published Thu May 16, 2019

### Numerical investigation of the flow characteristics of underexpanded methane jets

In the present work, numerical simulations are carried out to investigate underexpanded methane jets with phase separation effects. In order to predict the fuel injection and the mixture formation in the constant volume chamber, a hybrid, pressure-based solver is combined with a vapor-liquid equilibrium model and a moving mesh methodology. The thermodynamic models are based on the cubic equation of state of Soave, Redlich, and Kwong. A compressibility correction for the widely known kωSST turbulence model is implemented additionally. Application-relevant simulations with a total fuel pressure of 300 bars and five different chamber pressures ranging from 12 to 60 bars were defined. Furthermore, the influence of two fuel and chamber temperatures, 294 and 363 K, is analyzed. Depending on the chamber pressure, two different flow structures of the potential core can be distinguished: (1) A series of typical shock barrels for small pressure ratios and moderately underexpanded jets and (2) a shear layer consisting of a two-phase mixture which enfolds the potential core for high pressure ratios and highly underexpanded jets. Increasing the fuel temperature leads to less significant phase separations, while an increase in the chamber pressure does not affect the structure of the potential core. A comparison with experimental measurements shows a very good agreement of the simulated structure of the potential core, providing evidence that the underlying phenomena are predicted correctly and suggesting that a moving mesh strategy and consistent two-phase thermodynamics implementation are necessary for a physical representation of high-pressure injections.

### Effect of finite amplitude of bottom corrugations on Fabry-Perot resonance of water waves

Author(s): Jie Zhang and Michel Benoit

Recently, the mechanism of Fabry-Perot (F-P) resonance in optics was extended to monochromatic water waves propagating in a domain with two patches of sinusoidal corrugations on an otherwise flat bottom. Assuming small-amplitude surface waves, an asymptotic linear analytical solution (ALAS) was deri...

[Phys. Rev. E 99, 053109] Published Wed May 15, 2019

### Faraday waves over a permeable rough substrate

Author(s): Diego Barba Maggi, Alejandro Boschan, Roman Martino, Marcelo Piva, and Jean-Christophe Géminard

We report on an experimental study of the Faraday instability in a vibrated fluid layer situated over a permeable and rough substrate, consisting either of a flat solid plate or of woven meshes having different openings and wire diameters, open or closed (by a sealing paint). We measure the critical...

[Phys. Rev. E 99, 053110] Published Wed May 15, 2019

### Predicting the maximum spreading of a liquid drop impacting on a solid surface: Effect of surface tension and entrapped air layer

Author(s): Thijs C. de Goede, Karla G. de Bruin, Noushine Shahidzadeh, and Daniel Bonn

At low impact velocities, a droplet does not immediately make contact with a surface because of an entrapped air layer, leading to delayed surface wetting. High-speed images are used to investigate the influence of this entrapped air layer and the surface wettability on droplet spreading.

[Phys. Rev. Fluids 4, 053602] Published Wed May 15, 2019

### Why capillary flows in slender triangular grooves are so stable against disturbances

Author(s): Nicholas C. White and Sandra M. Troian

Lyapunov and non-modal stability analysis reveal why stationary and self-similar capillary flows in slender open triangular channels are so stable to disturbances. This robust feature allows superior flow management for applications ranging from space micropropulsion to microfluidic diagnostics.

[Phys. Rev. Fluids 4, 054003] Published Wed May 15, 2019

### Reynolds-average Navier-Stokes study of steady and pulsed gaseous jets with different periods for the shock-induced combustion ramjet engine

The mixing process is very important for the shock-induced combustion ramjet engine. In the current study, the steady jet, as well as pulsed jets with different periods, is investigated in order to achieve adequate fuel/air mixing in the supersonic flow. Flow field properties are studied numerically based on grid independency analysis and code validation. The influence of the hydrogen distribution, as well as the flow field parameters such as mixing efficiency, total pressure recovery coefficient, and fuel penetration depth, is deeply analyzed for different jet-to-crossflow pressure ratios, namely, 10.29 and 25.15. The obtained results predicted by the three-dimensional Reynolds-averaged Navier-Stokes equations coupled with the two equation shear stress transport k-ω turbulence model show that the grid scale makes only a slight difference to wall pressure profiles. The pulsed jets with different periods are beneficial for the mixing process, especially when the jet-to-crossflow pressure ratio is high, and it has special advantages on reducing the total pressure loss and improving the fuel penetration depth. Among the pulsed jets considered in the current study, the T1 pulsed jet with higher frequency has the best performance, and its mixing augmentation mechanism is predicted. Its mixing enhancement mechanism is focusing on merging a mass of air around into the fuel core by the intermittent injection.

### Hysteresis behavior in spanwise rotating plane Couette flow with varying rotation rates

Author(s): Yuhan Huang, Zhenhua Xia, Minping Wan, Yipeng Shi, and Shiyi Chen

In direct numerical simulations of spanwise rotating plane Couette flow, hysteresis is found between a 2-pair-roll-cell as the rotation increases to a 3-pair-roll-cell when the rotation decreases.

[Phys. Rev. Fluids 4, 052401(R)] Published Tue May 14, 2019

### On the optimal morphology and performance of a modeled dragonfly airfoil in gliding mode

Numerical investigation on the effect of wing morphology of the dragonfly Aeshna cyanea is carried out to understand its influence on the aerodynamic performance. The two-dimensional wing section has corrugation all over the surface along the chord length on both upper (suction side) and lower (pressure side) surfaces. By considering each corrugation separately on different airfoils at their different positions, 10 single corrugated airfoils were generated. Simulations are performed on these different airfoils to determine the effect of each corrugation on aerodynamic performance. The flow is modeled as incompressible, Newtonian, homogeneous, and unsteady. The angle of attack was varied from 0° to 20°, and the Reynolds number (Re) was varied from 150 to 10 000. The optimum morphology and angle of attack were predicted by using the surrogate-based optimization technique for a maximum gliding ratio at different Re. A fully corrugated pressure side gives the best performance at angles of attack of 9.79° and 14.83° at low Re. At high Re, corrugations on the pressure side which are in the middle and those near the trailing edge give a maximum gliding ratio at angles of attack 9.22° and 5.276°. The spatiotemporal dynamics indicate that corrugations near the leading edge on the upper surface and corrugations near the trailing edge for the lower surface and which are in the middle are beneficial. It is also found that shear drag due to corrugation decreases but pressure drag increases; therefore, the overall drag coefficient for a fully corrugated airfoil increases. Corrugations on the suction side have little influence, while those on the pressure side causes lift enhancement.

### Numerical study of natural convection in a differentially heated square cavity filled with nanofluid in the presence of fins attached to walls in different locations

The phenomenon of natural convection in a square cavity filled with a copper-water nanofluid is investigated numerically. The studied domain is a square cavity with hot and cold isothermal walls at x = 0 and x = L, respectively, while the other walls are adiabatic. The fins are considered perfectly conductive with different lengths (Lf) and positioned at different locations. We examined the situation for Rayleigh numbers ranging between 104 and 106. The governing equations are expressed in the vorticity, stream function, and temperature formulation. The system of equations was solved by the finite difference method, using the upwind scheme. The computation code thus developed was used to analyze the effect of the different locations of the fins on the thermal performances. The obtained results were validated by comparing with those of a previously published work and with those obtained using COMSOL Multiphysics. It has been found that adding fins on the cold and adiabatic walls results in an increase in the average Nusselt number, while it decreases when the fin is located on the hot wall. That is to say, placing the fins on the cold and adiabatic walls increases the thermal performances of the transfer.

### Propulsion by stiff elastic filaments in viscous fluids

Author(s): Panayiota Katsamba and Eric Lauga

Flexible filaments moving in viscous fluids are ubiquitous in the natural microscopic world. For example, the swimming of bacteria and spermatozoa as well as important physiological functions at organ level, such as the cilia-induced motion of mucus in the lungs, or individual cell level, such as ac...

[Phys. Rev. E 99, 053107] Published Mon May 13, 2019

### Normal and oblique droplet impingement dynamics on moving dry walls

Author(s): K. Ashoke Raman

Industrial applications that depend on jetting-based technology, such as painting or additive layered manufacturing, involve sequential deposition of droplets onto a moving surface. Spreading and receding dynamics of these impinging drops depend on the momentum transferred by the moving wall to the ...

[Phys. Rev. E 99, 053108] Published Mon May 13, 2019

### Characterization of blood velocity in arteries using a combined analytical and Doppler imaging approach

Author(s): Bchara Sidnawi, Zhen Chen, Chandra Sehgal, Sridhar Santhanam, and Qianhong Wu

Using ultrasound Doppler imaging we experimentally and analytically reconstruct the blood flow field in arteries and provide an in−vivo-validated, noninvasive, and reliable Wall Shear Stress (WSS) estimation. WSS is a major mechanical modulator of many functions of the cardiovascular system.

[Phys. Rev. Fluids 4, 053101] Published Mon May 13, 2019

### Maximum entropy modeling of oxygen vibrational excitation and dissociation

Author(s): Jiaao Hao and Chih-Yung Wen

An accurate and efficient model based on the maximum entropy principle is established for the vibrational excitation and dissociation of oxygen. Good agreement with state-specific calculations and recent experimental data is obtained.

[Phys. Rev. Fluids 4, 053401] Published Mon May 13, 2019

### Flow field around a confined active droplet

Author(s): Charlotte de Blois, Mathilde Reyssat, Sébastien Michelin, and Olivier Dauchot

Experimental measurements of the velocity field around a droplet swimming close to a wall demonstrate the critical impact of confinement.

[Phys. Rev. Fluids 4, 054001] Published Mon May 13, 2019