# Latest papers in fluid mechanics

### Heat transfer in rough-wall turbulent thermal convection in the ultimate regime

Author(s): Michael MacDonald, Nicholas Hutchins, Detlef Lohse, and Daniel Chung

An effective scaling exponent between the Nusselt and Rayleigh numbers of 0.42 is predicted for fully rough turbulent thermal convection in the ultimate regime. This is distinct from the value of 0.38 for smooth walls and from 0.5 for the asymptotic (infinite Rayleigh number) ultimate regime.

[Phys. Rev. Fluids 4, 071501(R)] Published Mon Jul 22, 2019

### The age of a wake

This study attempts to quantify the decay rates of stratified wakes in active oceanic environments, characterized by the presence of intermittent turbulence and double-diffusive convection. Of particular interest is the possibility of utilizing standard oceanographic microstructure measurements as a means of wake identification and analysis. The investigation is based on a series of direct numerical simulations of wakes produced by a sphere uniformly propagating in stratified two-component fluids. We examine and compare the evolution of wakes in fluid systems that are (i) initially quiescent, (ii) double-diffusively unstable, and (iii) contain preexisting turbulence. The model diagnostics are focused primarily on the dissipation of turbulent kinetic energy (ε) and thermal variance (χ). The analysis of decay patterns of ε and χ indicates that microstructure generated by an object of D = 0.6 m in diameter moving at the speed of U = 0.02 m/s could be detected, using modern high-resolution profiling instruments, for 0.5–0.7 h. The detection period depends on environmental conditions; convective overturns are shown to be particularly effective in terms of dispersion of microscale wake signatures. The extrapolation of model results to objects of ∼10 m in diameter propagating with speeds of ∼10 m/s suggests that the microstructure-based wake detection is feasible for at least 4 h after the object’s passage through the monitored areas. The overall conclusion from our study is that the measurement of microscale signatures of turbulent wakes could represent a viable method for hydrodynamic detection of propagating submersibles.

### Effect of vortex-induced vibration of finned cylinders on heat transfer enhancement

Two-degree-of-freedom vortex-induced vibration (VIV) of a finned cylinder with heat transfer is studied numerically at the Reynolds number Re = 150. The governing equations in the Arbitrary Lagrangian-Eulerian frame are solved by the finite volume method. The dynamics of the oscillating cylinder (with or without fins) in the fluid flow was approximated as a mass-spring system. The effects of the number and arrangement of the fins (14 different cases) on the vortex shedding pattern, vibration amplitude, and frequency and heat transfer of the cylinder are investigated and discussed. The results indicate that in comparison with the stationary state, the effects of the number and arrangement of the fins on the wake pattern and the heat transfer enhancement in the VIV state are significant. Different vortex shedding pattern like 2S, P, 2P, S + P and combination of them with stable or unstable interactions between vortices and cylinders are observed in an oscillating cylinder. In the vibration state of finned cylinders, the heat transfer enhances up to 50.4% with respect to the stationary state and increases up to 64% with respect to the stationary smooth cylinder.

### Numerical investigation of wake structures of an atmospheric entry capsule by modal analysis

This study investigates the flow structures behind an atmospheric entry capsule at Mach number 0.4 through an improved detached eddy simulation and a modal analysis. The simulated flowfields reveal relatively low-frequency peaks of St ≈ 0.016 and St = 0.17–0.2 in the aerodynamic coefficient variation, where St is the nondimensional frequency. Then, the dominant fluid structures that cause the frequency peaks are identified through dynamic mode decomposition and the compressive-sensing-based mode selection method. Many of the dominant fluid phenomena have a frequency of St ≈ 0.2. In this frequency range, the fluid phenomena are mainly characterized with a large-scale vortex shedding separated from the capsule’s shoulder part and with a helical fluid structure in the wake. Moreover, the variation in the lift coefficient of the capsule is mainly attributed to the large-scale vortex shedding phenomenon. Furthermore, a fluid phenomenon at a frequency of St = O(0.01) is found, which describes the pulsation, or periodic growth or shrinkage, of the recirculation bubble, accompanied by pressure fluctuation behind the capsule that exerts a large drag fluctuation of the capsule. Additionally, this phenomenon seems related to the dynamic instability phenomena of the capsule, as indicated by its time scale, which is close to that of the capsule’s attitude motion.

### Effects of flexibility and entanglement of sodium hyaluronate in solutions on the entry flow in micro abrupt contraction-expansion channels

In this study, the effects of polymer flexibility and entanglement on elastic instability were investigated by observing sodium hyaluronate (hyaluronic acid sodium salt, Na-HA) solution in planar abrupt contraction-expansion microchannels. As the rigidity of Na-HA depends on the ionic strength of a solvent, Na-HA was dissolved in water and phosphate buffered saline with concentrations from 0.15 wt. % to 0.45 wt. %. The rheological properties were measured and analyzed to detect the Na-HA overlap and entanglement concentrations. The flow regimes of the Na-HA solutions in several planar abrupt contraction-expansion channels were characterized in the Reynolds number and Weissenberg number space. The effects of the solvent, solution concentration, and channel geometry on the elastic corner vortex growth curve and flow regimes characterized by the Weissenberg number were analyzed. It was found that the entanglement of Na-HA in the solution is a more dominant factor affecting the flow regimes than the solution relaxation time and polymer rigidity.

### Modeling pattern formation in soft flowing crystals

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

A multicomponent lattice Boltzmann method with a force term for the effects of the near-contact interactions is applied to two droplets colliding and to emulsion production in microchannels.

[Phys. Rev. Fluids 4, 072201(R)] Published Fri Jul 19, 2019

### Flow of a model shear-thickening micellar fluid past a falling sphere

Author(s): Shijian Wu and Hadi Mohammadigoushki

Formation of an unusually extended wake behind a falling sphere in a shear thickening dilute micellar solution (CTAB/5mS) is reported. It is suggested that this behavior is linked to flow-induced micellar structure formation where micelles transition from rodlike to wormlike micelles around the falling sphere.

[Phys. Rev. Fluids 4, 073303] Published Fri Jul 19, 2019

### Rotating convection with centrifugal buoyancy: Numerical predictions for laboratory experiments

Author(s): Susanne Horn and Jonathan M. Aurnou

Centrifugal buoyancy affects the flow structures and the heat transport in all rotating convective experiments. Numerical simulations are used to provide predictions of the centrifugal effects expected to arise in laboratory studies of the dynamically rich system of Coriolis-centrifugal convection.

[Phys. Rev. Fluids 4, 073501] Published Fri Jul 19, 2019

### Emulsification with rectangular tubes

Author(s): Erwan Crestel, Ladislav Derzsi, Hugo Bartolomei, Jérôme Bibette, and Nicolas Bremond

The flow of two immiscible liquids or fluids in bounded systems where confinement geometry varies can lead to drop or bubble formation. A comprehensive experimental investigation on such an emulsification, or foaming process, occurring at the end of a glass rectangular tube is presented.

[Phys. Rev. Fluids 4, 073602] Published Fri Jul 19, 2019

### Compaction of liquid immersed granular packings by small upward flows

Author(s): Georges Gauthier and Philippe Gondret

We conduct experiments to examine the compaction dynamics of a liquid immersed granular packing under an upward flow which is either continuous or made up of repeated short bursts. We find that compaction is possible in the continuous case and compaction efficiency can be enhanced for short bursts.

[Phys. Rev. Fluids 4, 074308] Published Fri Jul 19, 2019

### Active flow control for drag reduction of a plunging airfoil under deep dynamic stall

Author(s): Brener L. O. Ramos, William R. Wolf, Chi-An Yeh, and Kunihiko Taira

Large-eddy simulations are performed to study active flow control of deep dynamic stall for an SD7003 airfoil in plunging motion. For some frequencies, flow actuation disrupts the formation of the dynamic stall vortex, leading to drag reduction, while lift is almost unaffected.

[Phys. Rev. Fluids 4, 074603] Published Fri Jul 19, 2019

### Contributions of hydrodynamic features of a swirling flow to thermoacoustic instabilities in a lean premixed swirl stabilized combustor

A comprehensive study on influences of hydrodynamic features of a swirling flow on thermoacoustic instabilities in a swirl stabilized combustor is performed by using large eddy simulations along with the dynamically thickened flame combustion model. The governing equations in full compressible form are solved by an in-house developed high-order numerical solver. The combustor is simulated in six different equivalence ratios to assess effects of equivalence ratio on the contributions of hydrodynamic features in inducing thermoacoustic instabilities. The obtained results show that the combustor suffers from combustion instabilities at equivalence ratios of 0.55, 0.6, 0.75, and 0.8, while it is stable at the midrange equivalence ratios (0.65 and 0.7). The results indicate that the instabilities are the result of the lock-in mechanism between heat release fluctuations induced by hydrodynamic features and the mixed first tangential and quarter wave longitudinal mode of the combustor. Investigations are carried out to evaluate contributions of central and side recirculation zones, precessing vortex core, and coherent structures in heat release fluctuations. The results show that contributions of hydrodynamic features highly depend on the combustor operating condition. At low equivalence ratios (0.55 and 0.6), coherent structures and side and central recirculation zones are the key features to induce heat release fluctuations in phase with the acoustic perturbations, while at equivalence ratios of 0.75 and 0.8, coherent structures and precessing vortex core play the main role in inducing combustion instabilities.

### Sound waves in polarized fluids

The speed of sound is known to depend only on the properties of the medium through which it travels. In this paper, we show that polarizing a dielectric fluid reduces the speed of sound waves in it. We also show that the reduction depends on the magnitude of the field. The striction force causing the slowing of sound in dielectric fluids is also present in a polarized ferrofluid. However, it is far too feeble to cause an observable effect.

### Impact of tab location relative to the nozzle exit on the shock structure of a supersonic jet

This paper presents an experimental study of a Mach 2.0 jet manipulated using rectangular tabs to understand the mixing enhancement at the overexpanded and perfectly expanded state of the jet. This paper also compares the mixing effectiveness of the tabs in comparison with the fluidic injection reported in our previous work [Kumar et al., “Empirical scaling analysis of supersonic jet control using steady fluidic injection,” Phys. Fluids 31(5), 056107 (2018)]. Tabs used in this investigation were rectangular strips of aspect ratio, AR, 2 (AR = length of the tab/width of the tab) and are positioned at 0De, 0.25De, 0.5De, and 0.95De (De is the nozzle exit diameter) downstream of the nozzle exit. Pitot pressure measurements were carried out along the jet centerline and in the radial directions to examine the supersonic core length ([math]) and jet spread, respectively. The jet stream has been visualized using the shadowgraph technique in the orthogonal planes of the manipulated jet. The mixing capability of the manipulated jet quantified based on the reduction in supersonic core length [math] strongly depends on the control technique and its location along the downstream direction. Three types of flow categories are identified, i.e., the “jet bifurcation,” “complex and strong shock-cell structure,” and “weak shock structure,” which depend on the tab location ([math]) and account for the jet mixing. The present study reveals that the tabs should be positioned downstream of the first shock crossover point which results in shorter core length and, hence, higher jet mixing. A conceptual model of the flow structure under control is proposed.

### Semi-empirical pressure loss model for viscous flow through high aspect ratio rectangular orifices

A predictive model is developed for the pressure loss coefficient for a viscous flow through a rectangular orifice on a pipe-installed thick plate. The model is developed based on the 1-dimensional Navier-Stokes equation and an asymptotic increase in velocity modeled to have a direct relation with the flow convergence in the near-inlet region. Here, the flow velocity increases asymptotically from the steady mean upstream value to the orifice velocity. This phenomenon is represented by a convergence parameter, [math], used in the velocity transition model to quantify the length of the convergence zone. The static pressure drop is measured experimentally for varying orifice aspect ratio, AR, at creeping Reynolds numbers (0.01 ≤ Re ≤ 0.1). A significantly wider range of AR is covered (1 ≤ AR ≤ 250), compared to related works in the literature. Results show that the relative dominance of the convergence phenomenon is affected by AR. The maximum length of convergence is for the square orifice (AR = 1), as the flow experiences comparable convergence from all directions, whereas for higher AR, convergence becomes less dominant in one of the two midplanes of investigation. The loss coefficient thus decreases as AR increases. At constant Re, higher AR generally leads to higher pressure drop but lower values of the loss coefficient. The velocity gradient in the convergence zone is also determined as a function of AR and Re which verifies that lower AR takes a longer distance for the velocity transition due to increased convergence.

### Vortex-induced vibrations of dual-step cylinders with different diameter ratios in laminar flows

Vortex-induced vibrations of dual-step cylinders in laminar flows at the Reynolds number of 200 based on a larger diameter are investigated through three-dimensional direct numerical simulations. Four larger-to-smaller diameter ratios (D/d) of 4.0, 2.0, 1.43, and 1.19 are considered for the cylinder with a low mass ratio of 2. Numerical results reveal that D/d significantly influences vibration responses and the vortex shedding process. The case with D/d = 1.43 can effectively reduce vibration amplitudes. The wake vortices are shed in cellular patterns along the cylinder span, with a distinct frequency for each cell. A direct and “X-shaped” connection of wake vortex-shedding modes occurs between different vortex cells, and a loop connection appears between the same cell vortices. A new wake pattern entitled the “out-of-phase vortex shedding” is found downstream of the smaller cylinder at D/d = 2.0. This is manifested by the wake vortices not being shed at a fixed frequency and intensity. The essential reason for this phenomenon lies in a non-lock-in condition between the structural vibration and vortex shedding frequencies of the smaller cylinder. As D/d decreases, a coherence between vortex cells is enhanced, leading to a disappearance of the “out-of-phase vortex shedding.”

### Effect of wing fold angles on the terminal descent velocity of double-winged autorotating seeds, fruits, and other diaspores

Author(s): Richard A. Fauli, Jean Rabault, and Andreas Carlson

Wind dispersal of seeds is an essential mechanism for plants to proliferate and to invade new territories. In this paper we present a methodology used in our recent work [Rabault, Fauli, and Carlson, Phys. Rev. Lett. **122**, 024501 (2019)] that combines 3D printing, a minimal theoretical model, and exp...

[Phys. Rev. E 100, 013108] Published Thu Jul 18, 2019

### Discontinuous shear-thinning in adhesive dispersions

Author(s): Ehsan Irani, Pinaki Chaudhuri, and Claus Heussinger

The flow of a dense dispersion with short-range attractive forces is simulated. Discontinuous shear thinning occurs via rapid loss of particle-tangential viscous dissipation. The phenomenon is closely connected to discontinuous shear thickening, where tangential dissipation is due to solid friction.

[Phys. Rev. Fluids 4, 074307] Published Thu Jul 18, 2019

### Resolving the three-dimensional structure of particles that are aerodynamically clustered by a turbulent flow

We report the first definitive experimental measurement of the four-dimensional (three spatial and one temporal) structure of particles that are aerodynamically clustered. High-speed tomographic imaging of a particle-laden turbulent flow was utilized to detect the temporal evolution of particle clusters at the exit of a long pipe. The measurements confirm that the particle clusters are coherent and ropelike in shape, rather than sheetlike, resolving a question that was not possible to address from previous two-dimensional measurements. These clusters are present right from the exit plane, where they are preferentially located near the jet edge, suggesting that they are generated inside the pipe close to the pipe wall.

### Simulation of Io’s plumes and Jupiter’s plasma torus

Io is a highly volcanic satellite of Jupiter. Its giant plumes rise hundreds of kilometers, creating large targets for incoming ions as Jupiter’s plasma torus overtakes Io in its orbit. Neutral material from Io’s sublimation atmosphere and volcanic plumes supplies the plasma torus, but the details of the interaction between neutral gas at Io and ions in the torus are not well understood. This paper suggests a process by which plume material is energized and ionized so as to supply the torus. We present three-dimensional direct simulation Monte Carlo simulations of giant plumes being bombarded by S+ and O+ ions which are moved based on precomputed electric and magnetic fields. The dependence of the plume/plasma interaction on the plume’s location on Io is investigated. The plume/plasma interaction is seen to be asymmetric even for a plume at the subplasma point because of the electric field that arises in an Io-fixed reference frame. Plasma is found to inflate and heat plume canopies and to give rise to a large, diffuse neutral cloud over the plume’s entire hemisphere. We also find that plasma can explain the thickness of red deposition rings observed on Io.