# Latest papers in fluid mechanics

### How evaporation and condensation lead to self-oscillations in the single-branch pulsating heat pipe

Author(s): Albert Tessier-Poirier, Thomas Monin, Étienne Léveillé, Stéphane Monfray, Fabien Formosa, and Luc G. Fréchette

The physics behind the instability at the source of oscillations in a single-branch pulsating heat pipe is investigated. Oscillations produced by the resonator increase in amplitude when pressure induced by evaporation-condensation (positive feedback mechanism) overcomes the viscous losses (dissipation).

[Phys. Rev. Fluids 4, 103901] Published Wed Oct 09, 2019

### Flow field evolution and entrainment in a free surface plunging jet

Author(s): Syed Harris Hassan, Tianqi Guo, and Pavlos P. Vlachos

An investigation finds that in free-surface plunging jets, shear layer vortices form right below the free surface, then develop and disintegrate faster than vortices in canonical free jets. This leads to shorter potential cores, faster decay of the mean centerline velocity, and enhanced liquid entrainment from the ambient.

[Phys. Rev. Fluids 4, 104603] Published Wed Oct 09, 2019

### Effect of electrostatic forces on the distribution of drops in turbulent channel flows

The effect of electrostatic forces on the distribution of drops in turbulent channel flows is examined by direct numerical simulations. The droplets and suspending fluid are assumed to be leaky dielectric fluids. We set the electrical conductivity ratio (R = σi/σo) smaller than the dielectric permittivity ratio (S−1 = εi/εo) to drive the flow from the drop poles to their equators. The results show that an applied external electric field has a significant effect on the microstructure and the flow properties. For flows without an electric field, where the Mason (Mn) number is infinity, the drops aggregated in the core of the channel and the liquid streamwise velocity are similar to those in single-phase flow. For Mn = 0.1, a low electric intensity, most of the drops are driven to the walls due to the unbalanced electric force on the drop interface. For Mn = 0.05, drops are more likely to stick together because of the stronger combination of electrohydrodynamic effect and dielectrophoretic force between drops. Therefore, the number of drops in the middle of the channel increases while still many drops are in the wall layer. For Mn = 0.007, the electric intensity is very strong and all the drops in the channel tend to line up and form columns spanning the channel width. These columns become unstable when the flow drives them close to each other. It is also found that an increase of the electric intensity can lead to an increase in the average wall shear stress. In addition, the liquid streamwise velocity will become more uniform, which means the effective viscosity of the system is increased, when Mn = 0.007.

### Intermittent fluid connectivity during two-phase flow in a heterogeneous carbonate rock

Author(s): Catherine Spurin, Tom Bultreys, Branko Bijeljic, Martin J. Blunt, and Samuel Krevor

Subsurface fluid flow is ubiquitous in nature, and understanding the interaction of multiple fluids as they flow within a porous medium is central to many geological, environmental, and industrial processes. It is assumed that the flow pathways of each phase are invariant when modeling subsurface fl...

[Phys. Rev. E 100, 043103] Published Tue Oct 08, 2019

### Thermoconvective instabilities in a horizontal porous annulus with an external wavy wall

Author(s): Jabrane Belabid

A numerical analysis reveals that the heat transfer inside a horizontal porous annulus filled with a saturated porous medium depends strongly on the waviness of the cold wall. Particular attention is paid to the influence of the waviness parameters on the thermoconvective instabilities.

[Phys. Rev. Fluids 4, 103501] Published Tue Oct 08, 2019

### Experimental investigation of vertical turbulent transport of a passive scalar in a boundary layer: Statistics and visibility graph analysis

Author(s): G. Iacobello, M. Marro, L. Ridolfi, P. Salizzoni, and S. Scarsoglio

The dynamics of a passive scalar plume is experimentally studied in a rough-wall turbulent boundary layer. Besides classical statistics, complex networks are exploited to highlight the temporal structure of vertical turbulent transport series in terms of occurrence and intensity of extreme events.

[Phys. Rev. Fluids 4, 104501] Published Tue Oct 08, 2019

### Energy-based analysis and anisotropic spectral distribution of internal gravity waves in strongly stratified turbulence

Author(s): Naoto Yokoyama and Masanori Takaoka

The weak turbulence of internal gravity waves and the strong turbulence of eddies coexist in stratified turbulence. The wave-number range of the anisotropic weak-wave turbulence is identified based on energy decomposition and characteristic time scales.

[Phys. Rev. Fluids 4, 104602] Published Tue Oct 08, 2019

### Inertial effects in triple-layer core-annular pipeline flow

Triple-layer core-annular flow is a novel methodology for efficient heavy oil transportation. As usual, high shear rates concentrating in a lubricating fluid layer reduce the pressure drop significantly. Novel is the use of a viscoplastic fluid bounding the lubricant and protecting the transported core. For sufficiently large yield stress, the skin remains unyielded, preventing any interfacial instabilities. By shaping the skin, we generate lubrication forces to counterbalance buoyancy of the core fluid, i.e., an eccentric position of the core is the result of buoyancy and lubrication forces balancing. Here, we extend the feasibility of this method to large pipes and higher flow rates by considering the effects of inertia and turbulence in the lubrication layer. We show that the method can generate enough lubrication force to balance the buoyancy force for a wide range of density differences and pipe sizes if the proper shape is imposed on the unyielded skin.

### Notable effect of the subgrid-scale stress anisotropy on mean-velocity prediction through budget of the grid-scale Reynolds-shear stress

In large eddy simulation (LES), the mean-velocity distribution in wall turbulence depends strongly on the distribution of the ensemble-averaged Reynolds (Re) shear stress, which consists of two parts: the resolved grid-scale (GS) and unresolved subgrid-scale (SGS) components. As the grid resolution becomes coarser, the GS component decreases and thus the SGS component must increase to compensate for this. The GS decrease is originally caused by filtering, through which the power spectrum is cut off mainly in the high-wavenumber region. Therefore, the SGS model has been discussed mostly in terms of the energy transfer between the GS and SGS components. Recently, however, some studies have found that the SGS-stress anisotropy directly influences instantaneous GS vortex motions. This also means that the SGS stress may have a large effect on the ensemble-averaged GS Re stress because the instantaneous fluctuation of the SGS stress correlates with that of the velocity gradient in the GS budget. In this study, we investigate in detail the effect of the SGS stress on predicting the resolved GS Re shear stress through its budget. For this purpose, we perform a priori tests with highly resolved LES data of a plane channel flow. The knowledge obtained is then confirmed by a posteriori tests for various grid resolutions and Reynolds numbers. It is found that the SGS-stress anisotropy is very important for providing a reasonable trend of the GS Re shear stress, leading to more accurate prediction of the mean velocity for coarse-grid resolutions.

### Application of two-branch deep neural network to predict bubble migration near elastic boundaries

Compared to the drawbacks of traditional experimental and numerical methods for predicting bubble migration, such as high experimental costs and complex simulation operations, the data-driven approach of using deep neural network algorithms can provide an alternative method. The objective of this paper is to construct a two-branch deep neural network (TBDNN) model in order to improve the high-fidelity bubble migration results and further reduce dependence on the quantity of experimental data. A TBDNN model is obtained by embedding the features of the Kelvin impulse into a basic deep neural network (BDNN) system. The results show that compared to the original BDNN model, TBDNN performs much better in accurately predicting bubble migration based on the same amount of training data. Using the TBDNN model, the critical condition of bubble oscillation at a fixed location can be detected under the influence of boundary properties (normalized stiffness and mass) and bubble standoff. Furthermore, the initial position of the bubble and normalized stiffness of boundaries have a positive correlation with bubble migration, whereas normalized mass has a negative impact. It was found that the normalized mass of boundaries plays the most important role in affecting bubble migration compared to the standoff and stiffness when using the method of variable sensitivity analysis.

### Solutal convection induced by dissolution

Author(s): Julien Philippi, Michael Berhanu, Julien Derr, and Sylvain Courrech du Pont

The buoyancy instability occurring at the interface of a soluble body suddenly immersed in a quiescent solvent is explored. Numerical simulations are used to derive and confirm scaling laws based on a constant solutal Rayleigh number. Dissolution erosion rates are predicted in some geological situations.

[Phys. Rev. Fluids 4, 103801] Published Mon Oct 07, 2019

### Sedimentation of a small sphere in stratified fluid

Author(s): Hojun Lee, Itzhak Fouxon, and Changhoon Lee

An extensive theoretical and numerical study of gravitational settling of small particles in a stratified fluid was performed. Adopting the integral equation on surface traction, we derived the drag enhancement due to stratification at low Reynolds and Peclet numbers, and confirmed with simulations.

[Phys. Rev. Fluids 4, 104101] Published Mon Oct 07, 2019

### Erratum: “The stabilization of a hypersonic boundary layer using local sections of porous coating” [Phys. Fluids 24, 034105 (2012)]

### Erratum: “Analysis of the flow of a thin liquid film on the surface of a rotating, curved, axisymmetric substrate” [Phys. Fluids 30, 082110 (2018)]

### Flow boiling heat transfer in silicon microgaps with multiple hotspots and variable pin fin clustering

Microfluidic interlayer cooling has been demonstrated as a practical solution for the vertical stacking of high power microelectronics. Although a considerable amount of studies has been presented for single phase cooling with this approach, the flow boiling features in more complex arrangements have not been as thoroughly studied. The embedded cooling of microelectronics is feasible with the use of dielectric refrigerants, which are ideally used in two-phase conditions in order to exploit the latent heat of vaporization. In the present investigation, the two-phase cooling in silicon microgaps is assessed under variable power and heat transfer surface area densities. The dielectric refrigerant HFE-7200 is used as the working fluid under flow boiling conditions, analyzing useful characteristics such as the two-phase flow regime, heat transfer, and pressure drop. The present investigation uses a numerical model that is capable of predicting the relevant features of flow boiling phenomena through a mechanistic phase-change model. The results from this study demonstrate that multiple hotspots with variable pin densities can be effectively controlled, with relatively uniform temperatures, under flow boiling conditions with dielectric fluids.

### Flow boiling heat transfer in silicon microgaps with multiple hotspots and variable pin fin clustering

Microfluidic interlayer cooling has been demonstrated as a practical solution for the vertical stacking of high power microelectronics. Although a considerable amount of studies has been presented for single phase cooling with this approach, the flow boiling features in more complex arrangements have not been as thoroughly studied. The embedded cooling of microelectronics is feasible with the use of dielectric refrigerants, which are ideally used in two-phase conditions in order to exploit the latent heat of vaporization. In the present investigation, the two-phase cooling in silicon microgaps is assessed under variable power and heat transfer surface area densities. The dielectric refrigerant HFE-7200 is used as the working fluid under flow boiling conditions, analyzing useful characteristics such as the two-phase flow regime, heat transfer, and pressure drop. The present investigation uses a numerical model that is capable of predicting the relevant features of flow boiling phenomena through a mechanistic phase-change model. The results from this study demonstrate that multiple hotspots with variable pin densities can be effectively controlled, with relatively uniform temperatures, under flow boiling conditions with dielectric fluids.

### On the peripheral intensification of two-dimensional vortices in smaller-scale randomly forcing flow

The evolution of a monopolar vortex embedded in the field of smaller-scale randomly forced vorticity is examined using fully nonlinear two-dimensional simulations at large Reynolds numbers. The vortex is considered to be compact if its angular momentum decreases with the radius on the scale comparable to the radius of maximum azimuthal velocity. The energy decays without forcing, while the vortex remains compact despite its viscous spreading. This scenario dramatically changes in the strong forcing regime, characterized by the substantial growth of the vortex energy due to the increase in velocity and angular momentum at the vortex periphery so that ultimately, the vortex transforms into a noncompact structure. The maximum of angular momentum redistribution is found to be proportional to the enstrophy of smaller-scale vorticity field. The results have important implications for better understanding the fate of vortices and physical mechanisms of energy transfer.

### Study on flow separation and transition of the airfoil in low Reynolds number

As typical flow characteristics in a low Reynolds number, laminar separation bubbles (LSBs) and transition to turbulence over airfoils have been extensively studied in recent years. In order to analyze their flow mechanism, numerical investigation using the finite volume method to solve the Reynolds averaged Navier-Stokes equations with a transition Shear Stress Transport (SST) four-equation transition model is performed in this work, combined with the experimental study facilitated by the oil film interferometry technique. Specifically, the transition SST four-equation transition model is solved to simulate the separation location and LSB structure at low Reynolds numbers on a Wortmann FX63-137 airfoil. Good agreement is obtained between the numerical simulation and experimental measurements regarding the separation, transition and reattachment location, aerodynamic coefficients, and overall flow structures. At higher Reynolds numbers of 200 000 and 300 000, similar bubble structures on the airfoil surface are observed, and the location of the bubble moves toward the leading edge of the airfoil by increasing the angle of attack. However, in Reynolds numbers ranging from 300 000 to 500 000, significant changes of the laminar flow separation structures emerge. The flow structure changes from the classical laminar separation bubble to the nonclassical separation flow structure that is composed of a major vortex 1(V1) and a minor vortex 2(V2). Due to the small distance between V1 and V2, it is difficult to distinguish the delicate structure of the two separation bubbles from the classical laminar separation bubble by the experimental method.

### Statistical behavior of turbulent kinetic energy transport in boundary layer flashback of hydrogen-rich premixed combustion

Author(s): Umair Ahmed, Abhishek L. Pillai, Nilanjan Chakraborty, and Ryoichi Kurose

Statistical behavior of turbulent kinetic energy (TKE) transport in boundary layer flashback of hydrogen-rich premixed combustion is analyzed for the first time. Flame alters the structure of the turbulent boundary layer, which is evident from the changes in the budget of the TKE transport equation.

[Phys. Rev. Fluids 4, 103201] Published Fri Oct 04, 2019

### Energy cascades in active-grid-generated turbulent flows

Author(s): D. O. Mora, E. Muñiz Pladellorens, P. Riera Turró, M. Lagauzere, and M. Obligado

An experimental study on active-grid-generated turbulence finds that different operating protocols can produce different energy cascades. Also, independently of the protocol, the turbulence dissipation constant can be modeled from the zero crossings of velocity fluctuations.

[Phys. Rev. Fluids 4, 104601] Published Fri Oct 04, 2019