# Latest papers in fluid mechanics

### Experimental investigation of a nonspherical water droplet falling in air

We experimentally investigate the shape oscillations of an initially nonspherical water droplet falling in air using high-speed imaging. We design a customized experimental setup that allows us to study the freely falling droplets of initially oblate/prolate/tilted configurations. The setup uses a pneumatic piston-cylinder arrangement and a superhydrophobically coated plate to propel a droplet upwards in air whose motion is then recorded using a high-speed camera. Due to the propulsive force imparted to the droplet, it undergoes oblate–prolate oscillations and eventually comes to rest at a maximum height, at which time the droplet has a zero vertical velocity and a nonspherical shape with an inclination to the horizontal. We study the effect of the initial aspect ratio and size of the droplet on its shape oscillations during its downward motion.

### On the three-dimensional flow development around circular finned cylinders

The three-dimensional flow development around the circular finned cylinders is investigated numerically. Three finned cylinders with constant fin pitch (p), fin thickness (t), and effective diameter (Deff) and a range of diameter ratios (Df/Dr) within 1.25 ≤ Df/Dr ≤ 2.5 are considered in this study. One bare cylinder with a diameter equivalent to the effective diameter of the finned cylinders is also considered. The numerical simulations are performed using the large eddy simulation turbulence model for a free-stream velocity corresponding to the Reynolds number Re = 3900, defined based on the effective diameter (Deff). This study provides novel insights into the flow physics in the channel between the fins and its effect on the three-dimensional flow development. The results accentuate that the three-dimensional flow development around the finned cylinders fundamentally differs from that of the bare cylinder. In particular, the flow separation topology at the surface of the finned cylinders differs significantly from that of the bare cylinder due to flow entrainment between the fins. The distinct flow separation leads to the formation of streamwise edge vortices, which induces downwash flow in the near wake of the finned cylinders. The combined effect of the entrainment between the fins and downwash flow affects the vortex formation length and mean pressure distribution around the cylinder. As a result, the structural loading on the surface of the finned cylinders is distinctively different from that of the bare cylinder with profound dependence on the fin parameters.

### Effects of fuel injection angle on mixing performance of scramjet pylon-cavity flameholder

A numerical investigation on the effects of fuel injection angle on various mixing parameters within a pylon-cavity aided supersonic combustor flameholder under non-reactive flow conditions is performed. The computational model based on Reynolds-averaged Navier–Stokes equations for compressed real gas is solved by a coupled, implicit, second-order upwind solver with a two-equation Menter’s shear stress transport turbulence model. The steady simulations are experimentally validated using wall pressure data, two-dimensional (2D) velocity field, and fuel mass fraction. Three distinct fuel injection locations at the cavity floor are used for sonic hydrogen fuel injection at 90° and 45° injection angles, with a crossflow Mach number of 2.2. The results show deeper fuel jet penetration capability for the transverse injection when compared to an angled injection, whereas better mixing capability is observed for the latter. The fuel jet vortex pairs formed due to the interaction of the surrounding cavity flow with the barrel shock play a vital role in the mixing mechanisms. The lower pressure regions due to the barrel shock result in the formation of a secondary fuel jet vortex pair. The Kelvin–Helmholtz instability observed between the counter-rotating vortex pairs results in the formation of smaller eddies, which enhance the fuel dispersion and transport.

### Cold atmospheric plasma for SARS-CoV-2 inactivation

Syndrome coronavirus 2 (SARS-CoV-2) infectious virions are viable on various surfaces (e.g., plastic, metals, and cardboard) for several hours. This presents a transmission cycle for human infection that can be broken by developing new inactivation approaches. We employed an efficient cold atmospheric plasma (CAP) with argon feed gas to inactivate SARS-CoV-2 on various surfaces including plastic, metal, cardboard, basketball composite leather, football leather, and baseball leather. These results demonstrate the great potential of CAP as a safe and effective means to prevent virus transmission and infections for a wide range of surfaces that experience frequent human contact. Since this is the first-ever demonstration of cold plasma inactivation of SARS-CoV-2, it is a significant milestone in the prevention and treatment of coronavirus disease 2019 (COVID-19) and presents a new opportunity for the scientific, engineering, and medical communities.

### Flow visualization of an N95 respirator with and without an exhalation valve using schlieren imaging and light scattering

This work demonstrates the qualitative fluid flow characteristics of a standard N95 respirator with and without an exhalation valve. Schlieren imaging was used to compare an adult male breathing through an N95 respirator with and without a valve. The schlieren imaging technique showed the flow of warm air passing through these respirators but did not provide information about droplet penetration. For this, strategic lighting of fog droplets was used with a mannequin head to visualize the penetration of droplets through both masks. The mannequin exhaled with a realistic flow rate and velocity that matched an adult male. The penetration of fog droplets was also visualized with a custom system that seals each respirator onto the end of a flow tube. Results of these qualitative experiments show that an N95 respirator without an exhalation valve is effective at blocking most droplets from penetrating through the mask material. Results also suggest that N95 respirators with exhalation valves are not appropriate as a source control strategy for reducing the proliferation of infectious diseases that spread via respiratory droplets.

### Attenuation of pulsation and oscillation using a disk at mid-section of spiked blunt body

The two most important modes of flow instability are observed on a flat-faced cylinder fitted with a spike, namely, pulsation and oscillation. A new technique is proposed in this article to attenuate both pressure and aerothermal fluctuations over the cylinder by using a circular disk at the mid-length of the sharp-tipped spike, which splits the separation region in front of the cylinder. Shock tunnel experiments are carried out at a free stream Mach number of M∞ = 5.7 and a Reynolds number of RD = 0.5 × 106 (based on the cylinder diameter, D). Aeroheating is measured using platinum thin film sensors painted over the front face of the cylinder. It is shown that the flow pulsation can be totally mitigated for the ratio of spike length to diameter (L/D = 1.0) by using a disk of diameter d/D = 28% at the mid-section of the spike, and the oscillation flow is suppressed using the same disk diameter with L/D = 1.5. This phenomenon is well evident from the measured heat transfer and schlieren data. Besides, a similar analysis showed that the flow fluctuation over an aerospike mounted on a hemispherical blunt body can be diminished with the use of double-aerospike, which, in turn, decreases the heat flux peak in the proximity of the reattachment. This is an effective means of flow fluctuation mitigation and presents to be a good thermal protection for the fore body.

### Microscopic fluid dynamics of a wire screen bound to a slit resonator excited by acoustic waves

The microscopic fluid dynamics of a wire screen bound to a slit resonator excited by incident sound waves of different intensity are investigated numerically. The microscopic flow features help in understanding the acoustic behavior. A normal impedance-tube model is used in this investigation, and the wire mesh is modeled as an array of eight identical tiny circular cylinders arranged in parallel. Tonal waves of different sound pressure levels and frequencies are introduced from the termination of the tube through a non-reflecting boundary condition. Direct numerical simulations are carried out to solve for the flow and acoustic fields simultaneously, and the velocity and vorticity fields around the resonator are resolved. Upon closer inspection, the tiny cylinders suppress the vortex shedding from the slit excited by high-intensity incident sound waves, thereby retarding the nonlinear acoustic behavior of the slit. Furthermore, the wire mesh contributes greatly to the absorption of acoustic energy through scrubbing loss and flow separation. The acoustic impedance and absorption coefficient are derived using a two-microphone method. The numerical results show that the wire mesh increases the resistance of the resonator significantly while hardly affecting its reactance.

### Inner and outer scales in turbulent Rayleigh–Bénard convection

As in other kinds of wall-bounded turbulence, flow and heat transport in turbulent Rayleigh–Bénard convection (RBC) can be divided into an inner layer and an outer layer. This paper refines the traditional inner scales, the Townsend inner scales, by determining the Prandtl number Pr effect, and proposes new scales for the outer layer. Major findings for the inner layer include (i) the mean modified pressure peaks in the inner layer, and the peak location scales with the Townsend inner length scale lν = ν/uinner, where ν is the kinematic viscosity and uinner is the Townsend inner velocity. (ii) The peak value of the mean modified pressure Pmax scales as [math], where ρref is the fluid density and the coefficient ΨP is largely independent of the Reynolds number but is strongly influenced by the Prandtl number. (iii) The thickness of the thermal inner layer scales with a thermal diffusional length scale lα = Ψα α/uinner, where α is the thermal diffusivity and the coefficient Ψα is largely independent of the Reynolds number but is strongly influenced by the Prandtl number. Like passive scalar transport in a pressure-driven turbulent plane Poiseuille flow, the Prandtl number dependence of Ψα (and ΨP) can be approximated by a power law Ψα ∼ ΨP ∼ Prm, where m is a constant of about 0.5. In the outer layer, the vertical component of velocity fluctuation variance at the RBC midplane ⟨[math]⟩mp is introduced as a new governing parameter in the scaling of flow and heat transfer. The new outer velocity and temperature scales for turbulent RBC are different from the Deardorff scales, which were developed for convective atmospheric boundary layers. The new outer scales are compared with direct numerical simulation data and experimental measurements.

### Non-isothermal mixing characteristics in the extreme near-field of turbulent jets in hot crossflow: Effects of jet exit turbulence and velocity profile

The non-reacting and reacting jets-in-crossflow (JICF) are important flow configurations for effective mixing and combustion in practical applications. Many studies in the literature examine the overall mixing characteristics of isothermal, unconfined, non-reacting JICF. This experimental study expands on our recently published work that examined mixing characteristics in the near-field of a non-reacting jet in a hot vitiated crossflow (1500 K) for the jet-to-crossflow density ratio between 3.2 and 7.8 issuing from a round jet with a fully developed turbulent pipe flow exit profile. In this study, effects of the changing jet exit velocity profile to top-hat as well as exit turbulence levels (28% and 40%) with parabolic profiles are examined. Temperature measurements were made using laser Rayleigh scattering. The jet trajectory, centerline concentration decay based on adiabatic mixing assumption, Favre-averaged scalar dissipation, and mixing time scales were compared with the previous study results. Center-plane mixing metrics indicated that top-hat and pipe flow jets behave similarly, with better near-field mixing at lower momentum flux ratios and higher density ratios. The elevated turbulence cases have a higher near-field mixing efficiency with rates that are nearly independent of momentum flux ratios above 9.3 at a constant density ratio. Scalar dissipation analysis showed that the elevated turbulence jets differ from the nominal turbulence top-hat and pipe exit jet cases with a lack of strong peaks and slightly higher upstream crossflow magnitudes. Reducing the density ratio resulted in a decrease in the windward and leeward dissipation region size and magnitude.

### On the connection between Lagrangian and Eulerian metrics for recurrent particulate flows

Data analysis and recently developed data-assisted simulations of particulate flows often require assessing the similarity of their spatial structure at different times. To that end, various metrics have been defined in the literature, either of Eulerian, field-based or of Lagrangian, particle-position-based nature. We demonstrate the equivalence of a broad class of these distance functions for sufficiently recurrent states analytically and numerically on simulation data of a small-scale, flat fluidized bed consisting of Np = 50 000 grains. The investigated Eulerian and Lagrangian metrics led to consistent dynamic properties. Both identified a correlation dimension of about Dcorr ≲ 20, which is orders of magnitude smaller than the number of microscopic degrees of freedom. Similarly, the prediction time massively exceeded the mean free duration between particle collisions. Both observations were caused by the formation of mesoscopic structures. Complementary simulations of a fully 3D bed showed that the agreement of the metrics also holds for more complex motion. To calculate Lagrangian distances, we employed the Hungarian algorithm with complexity [math] for which we investigated different approximations leading to significant speed ups. In particular, we tracked randomly selected subsets of grains down to 10% of their total number and evaluated their mutual distance with only minor deviations from results of the full system. Our study clearly demonstrates that fundamental dynamic properties of granular matter are widely independent from the type of distance function used to investigate them. The final choice may be made based upon performance considerations or any specific information provided by different kinds of metrics.

### Influence of operating parameters in particle spreading, separation, and capturing in a hybrid free flow magnetophoretic bio-separator

In clinical applications, magnetic bead-based analyte separation has attracted interest over other types of separation techniques in the microfluidic protocol. The objective of the present study is to separate two different types of magnetic and one type of nonmagnetic particles from each other simultaneously with minimum cross-contamination in a microchannel. A numerical study is carried out for characterizing one hybrid microfluidic device. The device works on the principle of split-flow thin fractionation, field-flow fractionation, and free flow magnetophoresis. The geometry of the microfluidic bioreactor had been established by Samanta et al. in 2017, whereas the present research emphasized the impact of operating parameters in particle spreading, separation, and capture in the hybrid free flow magnetophoretic device. The impact of magnetic and fluidic forces on transport, separation, and capture of the three different types of particles is analyzed. The performance of the microfluidic device is checked by capture efficiency and separation indices for different operating conditions. Transport of the three different types of microspheres in the microchannel is prescribed following an Eulerian–Lagrangian model by using an in-house code. Two types of magnetic particles of diameters 2 µm and 1 µm and one nonmagnetic particle of 0.5 µm diameter are used. Some group variables comprising of magnetic and fluidic parameters are found as an exclusive function of capture efficiency and separation index. In addition, from curve fitting, the universal dependence of capture efficiency and separation index on the various group variables is recognized for different curves with a reasonably high degree of compliance.

### Symmetry breaking of Worthington jets by gradients in liquid pool depth

We investigate the impact of a vertically falling droplet onto a non-uniform liquid depth having a linear slope of the bottom substrate. Here, we report that the resulting jet direction is inclined to the shallow liquid depth after the droplet impact, which is found to be markedly distinct from a vertically falling droplet onto a uniform liquid bath. From experimental and numerical results, we observe that initially the cavity grows almost axisymmetrically, and then, when it retracts, asymmetric capillary waves exhibit. The asymmetric cavity reversal leads to the inclined jet ejection that is related to pressure distribution and velocity of the interface. For the systematic study, we explore the jet dynamics by varying the surface tension, the droplet size, the droplet impact speed, the inclination angle of the bottom substrate, and the depth of the liquid bath. Finally, we provide a simple scaling model to predict the inclination angle of the resulting jet after the drop impact on the inclined liquid pool.

### Liquid metal slingshot

Author(s): Zhiping Yuan, Xudong Zhang, Huimin Hou, Zhifeng Hu, Xiaomin Wu, and Jing Liu

Theory and experiment are combined to investigate how two liquid metal drops resting on a surface first coalesce and then jump off the surface.

[Phys. Rev. Fluids 5, 111601(R)] Published Mon Nov 09, 2020

### Droplet shape relaxation in a four-channel microfluidic hydrodynamic trap

Author(s): Shweta Narayan, Davis B. Moravec, Andrew J. Dallas, and Cari S. Dutcher

Droplet dynamics in microconfined environments influence the behavior of liquid-liquid emulsions and multiphase flows. Here, a microfluidic hydrodynamic trap is used to study the shape relaxation of confined droplets following a perturbation of their spherical shape, to better understand droplet deformation events with varying viscosity ratios and droplet sizes.

[Phys. Rev. Fluids 5, 113603] Published Mon Nov 09, 2020

### Long-time evolution of interfacial structure of partial wetting

Author(s): Mengfei He

When a plate is swiftly pulled out of a liquid bath, a thin liquid film is dragged out by the moving surface. High-speed imaging reveals surprising structure of such a seemingly mundane process. A new technique allows precise measurement of the structure, and a systematic comparison with the reverse process of plunging a plate is carried out

[Phys. Rev. Fluids 5, 114001] Published Mon Nov 09, 2020

### Splashing during impact on heated granular beds

Author(s): Fangye Lin, Yihua Yang, Jun Zou, and William D. Ristenpart

An experimental study of a solid ball impacting heated granular beds is presented. The result shows that temperature plays a greater role in granular splashing than previously suspected. These observations are interpreted in terms of two physical effects: the influence of the gas viscosity on the ejecta drag force and an enhanced static strength within the bed caused by thermal expansion of the granules.

[Phys. Rev. Fluids 5, 114302] Published Mon Nov 09, 2020

### Phase-suppressed hydrodynamics of solitons on constant-background plane wave

Author(s): A. Chabchoub, T. Waseda, M. Klein, S. Trillo, and M. Onorato

The significance of local phase shifts in the experimental realization of soliton and breather waves in finite water depth as well as in deep water is explored. When suppressing the corresponding phase shift, the coherence of the wave envelope disintegrates by forming distinct soliton patterns. All experimental results are in excellent agreement with the predictions based on the nonlinear Schrödinger equation framework, suggesting the universality of observed dip and extreme wave pattern dynamics.

[Phys. Rev. Fluids 5, 114801] Published Mon Nov 09, 2020

### An experimental study of respiratory aerosol transport in phantom lung bronchioles

The transport and deposition of micrometer-sized particles in the lung is the primary mechanism for the spread of aerosol borne diseases such as corona virus disease-19 (COVID-19). Considering the current situation, modeling the transport and deposition of drops in human lung bronchioles is of utmost importance to determine their consequences on human health. The current study reports experimental observations on deposition in micro-capillaries, representing distal lung bronchioles, over a wide range of Re that imitates the particle dynamics in the entire lung. The experiment investigated deposition in tubes of diameter ranging from 0.3 mm to 2 mm and over a wide range of Reynolds number (10−2 ⩽ Re ⩽ 103). The range of the tube diameter and Re used in this study is motivated by the dimensions of lung airways and typical breathing flow rates. The aerosol fluid was loaded with boron doped carbon quantum dots as fluorophores. An aerosol plume was generated from this mixture fluid using an ultrasonic nebulizer, producing droplets with 6.5 µm as a mean diameter and over a narrow distribution of sizes. The amount of aerosol deposited on the tube walls was measured using a spectrofluorometer. The experimental results show that dimensionless deposition (δ) varies inversely with the bronchiole aspect ratio ([math]), with the effect of the Reynolds number (Re) being significant only at low [math]. δ also increased with increasing dimensionless bronchiole diameter ([math]), but it is invariant with the particle size based Reynolds number. We show that [math] for 10−2 ⩽ Re ⩽ 1, which is typical of a diffusion dominated regime. For Re ⩾ 1, in the impaction dominated regime, [math] is shown to be independent of Re. We also show a crossover regime where sedimentation becomes important. The experimental results conclude that lower breathing frequency and higher breath hold time could significantly increase the chances of getting infected with COVID-19 in crowded places.

### Nonequilibrium molecular dynamics study on energy accommodation coefficient on condensing liquid surface—Molecular boundary conditions for heat and mass transfer

Nonequilibrium molecular dynamics (NEMD) studies have been conducted to determine molecular boundary conditions at vapor–liquid interfaces for the kinetic theory of condensation and evaporation. In previous studies, a microscopic formulation of the condensation coefficient was defined as the condensation probability of vapor molecules based on equilibrium molecular dynamics simulations and transition state theory. The condensation coefficient was presented as a function of the translation energy of incoming molecules and surface temperature. Based on this, the velocity distributions of evaporating and reflecting molecules were theoretically expressed under equilibrium conditions. In a practical nonequilibrium situation, the energy transfer by the reflecting molecules is important along with the condensation/evaporation probability. However, it is unclear whether the results obtained under equilibrium conditions can be applied under nonequilibrium conditions. This study, therefore, defines the energy accommodation coefficient of reflecting molecules by comparing the energy transfer due to reflection with that under equilibrium conditions. NEMD simulations are conducted using two surfaces facing each other, an evaporating surface and a condensing surface, for argon molecules under different nonequilibrium conditions. The results show that the velocity distribution of reflecting molecules deviates from those under equilibrium conditions, and the energy accommodation coefficient decreases as nonequilibrium conditions increase. Additionally, an inverted temperature profile is observed. Reflecting molecules play an important role in the sensible heat transfer on the condensing surface, and they are not accommodated on the condensing surface. Thus, they raise the temperature in the vicinity of the condensing surface under nonequilibrium conditions.

### A lower-dimensional approximation model of turbulent flame stretch and its related quantities with machine learning approaches

Flame stretch and its related quantities are three-dimensional (3D), while most planar imaging techniques, widely used in turbulent combustion, can only provide lower-dimensional information of these quantities. In the present work, based on a direct numerical simulation (DNS) database, artificial neural network (ANN) and random forest (RF) models were developed to predict the 3D flame stretch and its related quantities such as the tangential strain rate, displacement velocity, and curvature from lower-dimensional information that can be accessed experimentally. It was found that the performance of the RF model is better than that of the ANN model. In the RF model, the correlation coefficients between the modeled and actual values are more than 0.97, and the determination coefficients are over 0.95. The model performance deteriorates with increasing turbulent intensity. The probability density functions of various quantities predicted by the RF model are in good agreement with those of the DNS. Compromising the model performance and the computational cost, a simplified RF model was proposed by using a few optimal input features. It was found that the discrepancies between the modeled and actual values mainly occur in highly curved regions, which explains the observation that the prediction errors increase with increasing turbulent intensity. Overall, the predictions of the simplified RF model agree well with the actual values.