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Showing papers in "Physics of Fluids in 2009"


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate that periodic, micropatterned superhydrophobic surfaces, previously noted for their ability to provide laminar flowdrag reduction, are capable of reducing drag in the turbulent flow regime.
Abstract: In this paper, we demonstrate that periodic, micropatterned superhydrophobic surfaces, previously noted for their ability to provide laminar flowdrag reduction, are capable of reducing drag in the turbulent flow regime. Superhydrophobic surfaces contain micro- or nanoscale hydrophobic features which can support a shear-free air-water interface between peaks in the surface topology. Particle image velocimetry and pressure drop measurements were used to observe significant slip velocities, shear stress, and pressure drop reductions corresponding to drag reductions approaching 50%. At a given Reynolds number,drag reduction is found to increase with increasing feature size and spacing, as in laminar flows. No observable drag reduction was noted in the laminar regime, consistent with previous experimental results for the channel geometry considered. The onset of drag reduction occurs at a critical Reynolds number where the viscous sublayer thickness approaches the scale of the superhydrophobic microfeatures and performance is seen to increase with further reduction in viscous sublayer height. These results indicate superhydrophobic surfaces may provide a significant drag reducing mechanism for marine vessels.

550 citations


Journal ArticleDOI
TL;DR: In this paper, an analytical self-similar solution for the viscous flow in the spreading drop is obtained which satisfies the full Navier-Stokes equations, and an expression for the thickness of the boundary layer is used for the estimation of the residual film thickness formed by normal drop impact and the maximum spreading diameter.
Abstract: This study is devoted to a theoretical description of an unsteady laminar viscous flow in a spreading film of a Newtonian fluid. Such flow is generated by normal drop impact onto a dry substrate with high Weber and Reynolds numbers. An analytical self-similar solution for the viscous flow in the spreading drop is obtained which satisfies the full Navier–Stokes equations. The characteristic thickness of a boundary layer developed near the wall uniformly increases as a square root of time. An expression for the thickness of the boundary layer is used for the estimation of the residual film thickness formed by normal drop impact and the maximum spreading diameter. The theoretical predictions agree well with the existing experimental data. A possible explanation of the mechanism of formation of an uprising liquid sheet leading to splash is also proposed.

323 citations


Journal ArticleDOI
TL;DR: In this paper, the authors developed the theory of laminar dispersion in finite-length channel flows at high Peclet numbers, completing the classical Taylor-Aris theory which applies for long-term, long-distance properties.
Abstract: This article develops the theory of laminar dispersion in finite-length channel flows at high Peclet numbers, completing the classical Taylor–Aris theory which applies for long-term, long-distance properties. It is shown, by means of scaling analysis and invariant reformulation of the moment equations, that solute dispersion in finite length channels is characterized by the occurrence of a new regime, referred to as the convection-dominated transport. In this regime, the properties of the dispersion boundary layer and the values of the scaling exponents controlling the dependence of the moment hierarchy on the Peclet number are determined by the local near-wall behavior of the axial velocity. Specifically, different scaling laws in the behavior of the moment hierarchy occur, depending whether the cross-sectional boundary is smooth or nonsmooth (e.g., presenting corner points or cusps). This phenomenon marks the difference between the dispersion boundary layer and the thermal boundary layer in the classical Leveque problem. Analytical and numerical results are presented for typical channel cross sections in the Stokes regime.

276 citations


Journal ArticleDOI
TL;DR: In this paper, direct numerical simulations (DNSs) and experiments of a spatially developing zero-pressure gradient turbulent boundary layer are presented up to Reynolds number Re-theta=2500, based on momentum of the boundary layer.
Abstract: Direct numerical simulations (DNSs) and experiments of a spatially developing zero-pressure-gradient turbulent boundary layer are presented up to Reynolds number Re-theta=2500, based on momentum th ...

270 citations


Journal ArticleDOI
TL;DR: In this article, the authors numerically investigated liquid droplet impact behavior onto a dry and flat surface using a coupled level set and volume-of-fluid framework, volume/surface integrated average based multimoment method, and a continuum surface force model.
Abstract: We numerically investigated liquid droplet impact behavior onto a dry and flat surface. The numerical method consists of a coupled level set and volume-of-fluid framework, volume/surface integrated average based multimoment method, and a continuum surface force model. The numerical simulation reproduces the experimentally observed droplet behavior quantitatively, in both the spreading and receding phases, only when we use a dynamic contact angle model based on experimental observations. If we use a sensible simplified dynamic contact angle model, the predicted time dependence of droplet behavior is poorly reproduced. The result shows that precise dynamic contact angle modeling plays an important role in the modeling of droplet impact behavior.

265 citations


Journal ArticleDOI
TL;DR: In this paper, scaling arguments are formulated to define dimensionless variables which capture all the parameters that control the droplet breakup process, including the flow rates and the viscosities of the two immiscible fluids, the interfacial tension between the fluids and the numerous dimensions in the flow focusing device.
Abstract: Droplet formation processes in microfluidic flow focusing devices have been examined previously and some of the key physical mechanisms for droplet formation revealed. However, the underlying physical behavior is still too poorly understood to utilize it for generating droplets of precise size. In this work, we formulate scaling arguments to define dimensionless variables which capture all the parameters that control the droplet breakup process, including the flow rates and the viscosities of the two immiscible fluids, the interfacial tension between the fluids and the numerous dimensions in the flow focusing device. To test these arguments, we perform flow focusing experiments and systematically vary the dimensional parameters. Through these experiments, we confirm the validity of the scaling arguments and find a power law relationship between the normalized droplet size and the capillary number. We demonstrate that droplet formation can be separated into an upstream process for primary droplet formation...

224 citations


Journal ArticleDOI
TL;DR: In this article, the influence of bending rigidity of a flexible heaving wing on its propulsive performance in a two-dimensional imposed parallel flow is investigated in the inviscid limit.
Abstract: The influence of the bending rigidity of a flexible heaving wing on its propulsive performance in a two-dimensional imposed parallel flow is investigated in the inviscid limit. Potential flow theory is used to describe the flow over the flapping wing. The vortical wake of the wing is accounted for by the shedding of point vortices with unsteady intensity from the wing’s trailing edge. The trailing-edge flapping amplitude is shown to be maximal for a discrete set of values of the rigidity, at which a resonance occurs between the forcing frequency and a natural frequency of the system. A quantitative comparison of the position of these resonances with linear stability analysis results is presented. Such resonances induce maximum values of the mean developed thrust and power input. The flapping efficiency is also shown to be greatly enhanced by flexibility.

221 citations


Journal ArticleDOI
TL;DR: In this article, the optimal transient growth of perturbations sustained by a turbulent channel flow following the same approach recently used by del Alamo and Jimenez [J. 559, 205] was computed using generalized Orr-Sommerfeld and Squire operators.
Abstract: We compute the optimal transient growth of perturbations sustained by a turbulent channel flow following the same approach recently used by del Alamo and Jimenez [J. Fluid Mech. 559, 205 (2006)]. Contrary to this previous analysis, we use generalized Orr–Sommerfeld and Squire operators consistent with previous investigations of mean flows with variable viscosity. The optimal perturbations are streamwise vortices evolving into streamwise streaks. In accordance with del Alamo and Jimenez, it is found that for very elongated structures and for sufficiently large Reynolds numbers, the optimal energy growth presents a primary peak in the spanwise wavelength, scaling in outer units, and a secondary peak scaling in inner units and corresponding to λz+≈100. Contrary to the previous results, however, it is found that the maximum energy growth associated with the primary peak increases with the Reynolds number. This growth, in a first approximation, scales linearly with an effective Reynolds number based on the cen...

199 citations


Journal ArticleDOI
TL;DR: In this article, a novel flow energy harvesting device consisting of a flapping foil mounted on a damper and a rotational spring was examined, where self-induced and self-sustained flapping motions, including a heaving motion h(t) and a pitching motion α(t), are excited by an incoming flow and power extraction is achieved from the heaving response.
Abstract: By using a Navier–Stokes model, we examine a novel flow energy harvesting device consisting of a flapping foil mounted on a damper (representing the power generator) and a rotational spring. Self-induced and self-sustained flapping motions, including a heaving motion h(t) and a pitching motion α(t), are excited by an incoming flow and power extraction is achieved from the heaving response. Depending upon the configuration of the system and the mechanical parameters (e.g., the location of the pitching axis and the stiffness of the rotational spring), four different responses are recorded: (i) the foil remains stable in its initial position (α=0 and h=0); (ii) periodic pitching (around α=0) and heaving motions are excited; (iii) the foil undergoes irregular motions characterized by switching between oscillations around two pitching angles; and (iv) the foil rotates to a position with an angle to the incoming flow and oscillates around it. The existence of response (ii) suggests the feasibility of controllab...

191 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed a mechanism of droplet breakup in a symmetric microfluidic T junction driven by pressure decrement in a narrow gap between the droplet and the channel wall.
Abstract: We propose a mechanism of droplet breakup in a symmetric microfluidic T junction driven by pressure decrement in a narrow gap between the droplet and the channel wall. This mechanism works in a two-dimensional setting where the capillary (Rayleigh–Plateau) instability of a cylindrical liquid thread, suggested earlier [D. Link, S. Anna, D. Weitz, and H. Stone, Phys. Rev. Lett. 92, 054503 (2004)] as the cause of breakup, is not operative, but it is likely to be responsible for the breakup also in three dimensions. We derive a dependence of the critical droplet extension on the capillary number Ca by combining a simple geometric construction for the interface shape with lubrication analysis in a narrow gap where the surface tension competes with the viscous drag. The theory, formally valid for Ca1/5⪡1, shows a very good agreement with numerical results when it is extrapolated to moderate values of Ca.

186 citations


Journal ArticleDOI
TL;DR: In this paper, a set of direct numerical simulations of isotropic turbulence passing through a nominally normal shock wave is presented, and the instantaneous structure of the shock/turbulence interaction is examined using averages conditioned on the instantaneous shock strength.
Abstract: A set of direct numerical simulations of isotropic turbulence passing through a nominally normal shock wave is presented. Upstream of the shock, the microscale Reynolds number is 40, the mean Mach number is 1.3–6.0, and the turbulence Mach number is 0.16–0.38. It is shown that the Kolmogorov scale decreases during the shock interaction, which implies that the grid resolution needed to resolve the viscous dissipation is finer than that used in previous studies. This leads to some qualitative differences with previous work, e.g., a rapid increase in the streamwise vorticity variance behind the shock and large anisotropy of the postshock Reynolds stresses. The instantaneous structure of the shock/turbulence interaction is examined using averages conditioned on the instantaneous shock strength. For locally strong compressions, the flow is characterized by overcompression, followed by an expansion. At points where the shock is locally weak, the profiles differ qualitatively depending on the strength of the inc...

Journal ArticleDOI
TL;DR: In this paper, the authors compared existing and new numerical simulations for the shape of the lamella generated at the early times of drop impact for various impact conditions, and showed that if the Reynolds and Weber numbers are high enough, the evolution of lamella thickness almost does not depend on the viscosity and surface tension.
Abstract: This study is devoted to the analysis of inertia dominated axisymmetric drop collisions with a dry substrate or with another liquid drop. All the previous theoretical and semiempirical models of drop collisions are based on the assumption that the flow in the lamella and its thickness are determined by the impact conditions, mainly by the Reynolds and Weber numbers. In this study the existing experimental data are compared to existing and new numerical simulations for the shape of the lamella generated at the early times of drop impact for various impact conditions. The results show that if the Reynolds and Weber numbers are high enough, the evolution of the lamella thickness almost does not depend on the viscosity and surface tension. Therefore these results completely change our understanding of the flow generated by drop collisions. Moreover, we demonstrate that the theoretical models based on the approximation of the shape of the deforming drop by a disk and the models based on the energy balance appr...

Journal ArticleDOI
TL;DR: In this paper, a numerical model based on the Navier-Stokes equations is presented to investigate the performance of a bioinspired flapping foil system in low Reynolds numbers.
Abstract: As demonstrated in recent studies, the bioinspired flapping foils are capable of harvesting kinetic energy from incoming wind or current. A practical measure to achieve this is via the coupling between different modes in a system with multiple degrees of freedom. A typical scenario includes external activation of one motion mode and extracting the mechanical energy from other modes that follow. In this study we create a numerical model based upon the Navier–Stokes equations to investigate the performance of such a system in low Reynolds numbers. The effects of both the mechanical design and the operational parameters are examined. Specifically, we concentrate on the vorticity control mechanisms involved in the process, and demonstrate that through vortex-body interactions energy of the leading-edge vortices can be partially recovered to enhance the energy harvesting capacity.

Journal ArticleDOI
TL;DR: In this paper, a general method for the derivation of asymptotic nonlinear shallow water and deep water models is presented, starting from a general dimensionless version of the water-wave equations, and reducing the problem to a system of two equations on the surface elevation and the velocity potential at the free surface.
Abstract: A general method for the derivation of asymptotic nonlinear shallow water and deep water models is presented. Starting from a general dimensionless version of the water-wave equations, we reduce the problem to a system of two equations on the surface elevation and the velocity potential at the free surface. These equations involve a Dirichlet-Neumann operator and we show that all the asymptotic models can be recovered by a simple asymptotic expansion of this operator, in function of the shallowness parameter (shallow water limit) or the steepness parameter (deep water limit). Based on this method, a new two-dimensional fully dispersive model for small wave steepness is also derived, which extends to uneven bottom the approach developed by Matsuno \cite{matsuno3} and Choi \cite{choi}. This model is still valid in shallow water but with less precision than what can be achieved with Green-Naghdi model, when fully nonlinear waves are considered. The combination, or the coupling, of the new fully dispersive equations with the fully nonlinear shallow water Green-Naghdi equations represents a relevant model for describing ocean wave propagation from deep to shallow waters.

Journal ArticleDOI
TL;DR: In this article, a method for generating inflow conditions for large eddy simulations (LESs) of spatially developing turbulent boundary layers is presented, which uses the Cholesky decomposition of the Reynolds stress tensor to enforce secondorder moments starting from a normalized stochastic velocity signal.
Abstract: A method for generating inflow conditions for large eddy simulations (LESs) of spatially developing turbulent boundary layers is presented. It is an adaptation of the synthetic eddy method (SEM) of Jarrin et al. [Int. J. Heat Fluid Flow 27, 585 (2006)], which uses the Cholesky decomposition of the Reynolds stress tensor to enforce second-order moments starting from a normalized stochastic velocity signal, the latter being constructed with a superimposition of turbulent structures with prescribed geometrical shape and random signs and position. The present method modifies the definition of the stochastic signal so that it can be split into several modes, with different time, length and velocity scales and also with different vorticity contents. The idea is to reproduce more realistically the distribution of scales in the wall-normal direction of a turbulent boundary layer flow. The novelty of the proposed modified SEM is that physical information concerning the coherent vortical structures of such flows ar...

Journal ArticleDOI
TL;DR: Zeng et al. as mentioned in this paper characterized the structure of the wake for a stationary particle in a linear shear flow and compared with those for a particle moving parallel to a wall in a quiescent ambient flow.
Abstract: To understand and better model the hydrodynamic force acting on a finite-sized particle moving in a wall-bounded linear shear flow, here we consider the two limiting cases of (a) a rigid stationary spherical particle in a linear wall-bounded shear flow and (b) a rigid spherical particle in rectilinear motion parallel to a wall in a quiescent ambient flow. In the present computations, the particle Reynolds number ranges from 2 to 250 at separation distances to the wall from nearly sitting on the wall to far away from the wall. First we characterize the structure of the wake for a stationary particle in a linear shear flow and compare with those for a particle moving parallel to a wall in a quiescent ambient [see L. Zeng, S. Balachandar, and P. Fischer, J. Fluid Mech. 536, 1 (2005)]. For both these cases we present drag and lift results and obtain composite drag and lift correlations that are valid for a wide range of Re and distance from the wall. These correlations have been developed to be consistent with all available low Reynolds number theories and approach the appropriate uniform flow results at large distance from the wall. Particular attention is paid to the case of particle in contact with the wall and the computational results are compared with those from experiments.

Journal ArticleDOI
TL;DR: In this article, the mean velocity profiles for all nine rough surfaces collapse with smooth-wall results when presented in velocity-defect form, supporting the use of similarity methods, indicating that these surfaces might not be thought of as surface roughness in a traditional sense but instead surface “waviness.
Abstract: Results of an experimental investigation of the flow over a model roughness are presented. The series of roughness consists of close-packed pyramids in which both the height and the slope were systematically varied. The aim of this work was to document the mean flow and subsequently gain insight into the physical roughness scales which contribute to drag. The mean velocity profiles for all nine rough surfaces collapse with smooth-wall results when presented in velocity-defect form, supporting the use of similarity methods. The results for the six steepest surfaces indicate that the roughness function ΔU+ scales almost entirely on the roughness height with little dependence on the slope of the pyramids. However, ΔU+ for the three surfaces with the smallest slope does not scale satisfactorily on the roughness height, indicating that these surfaces might not be thought of as surface “roughness” in a traditional sense but instead surface “waviness.”

Journal ArticleDOI
TL;DR: In this article, the authors consider a shear flow at low Reynolds number past a two-dimensional array of bubbles and calculate analytically the effective slip length of the surface as a function of the bubble geometry in the dilute limit.
Abstract: Laminar flow over a bubble mattress is expected to experience a significant reduction in friction since the individual surfaces of the bubbles are shear-free. However, if the bubbles are sufficiently curved, their protrusion into the fluid and along the flow direction can lead to an increase in friction as was recently demonstrated experimentally and computationally. We provide in this paper a simple model for this result. We consider a shear flow at low Reynolds number past a two-dimensional array of bubbles and calculate analytically the effective slip length of the surface as a function of the bubble geometry in the dilute limit. Our model is able to reproduce quantitatively the relationship between effective friction and bubble geometry obtained in numerical computations and, in particular, (a) the asymmetry in friction between convex and concave bubbles and (b) the existence of a geometric transition from reduced to enhanced friction at a critical bubble protrusion angle.

Journal ArticleDOI
TL;DR: In this article, an infinitely long cylinder with arbitrary beating motion in the Oldroyd-B fluid is considered, and it is shown that swimming velocities are diminished by nonlinear viscoelastic effects.
Abstract: Many micro-organisms swim through gels and non-Newtonian fluids in their natural environments. In this paper, we focus on micro-organisms which use flagella for propulsion. We address how swimming velocities are affected in nonlinearly viscoelastic fluids by examining the problem of an infinitely long cylinder with arbitrary beating motion in the Oldroyd-B fluid. We solve for the swimming velocity in the limit in which deflections of the cylinder from its straight configuration are small relative to the radius of the cylinder and the wavelength of the deflections; furthermore, the radius of the cylinder is small compared to the wavelength of deflections. We find that swimming velocities are diminished by nonlinear viscoelastic effects. We apply these results to examine what types of swimming motions can produce net translation in a nonlinear fluid, comparing to the Newtonian case, for which Purcell’s “scallop” theorem describes how time-reversibility constrains which swimming motions are effective. We find that a leading order violation of the scallop theorem occurs for reciprocal motions in which the backward and forward strokes occur at different rates.

Journal ArticleDOI
TL;DR: In this paper, a high speed video camera was used to follow in time the drop spreading, and it allows us to determine precisely the expansion of the drop and the profile of the free surface at the contact line.
Abstract: This paper reports experimental investigations of drop impacts onto chemically treated surfaces with wettability from 5° to 160°. To follow in time the drop spreading, a high speed video camera was used, and it allows us to determine precisely the expansion of the drop and the profile of the free surface at the contact line. By changing the impact velocity, between less than 0.5 and 5 m/s, and the viscosity, from 1 to 100 mPa s, at constant surface tension, a broad range of Reynolds and Weber numbers is explored. This paper is divided into two parts. In the first part, the experimental drop evolution during spreading is directly reported and compared with previous works. Secondly, the emphasis is on the importance of the apparent dynamic contact angle for the prediction of the maximum spreading diameter. This achievement is manifested at low Reynolds numbers at which the matching between the experiment and the model is improved greatly.

Journal ArticleDOI
TL;DR: The AGARD conference proceedings as discussed by the authors give an ample and detailed review of the range of possible applications of the jet interaction flow field, which is the name given to the fluid dynamics phenomenon produced by a jet exhausting in a cross flow.
Abstract: The jet interaction flow field is the name given to the fluid dynamics phenomenon produced by a jet exhausting in a cross flow. This flow field can be found in several technological applications and, due to the presence of separated flows, vortical motions, turbulence, and, if the flow is supersonic shocks and expansion fans, is a formidable fluid dynamics problem. The AGARD conference proceedings 1 give an ample and detailed review of the range of possible applications. Examples range from the low-speed regimes of a chimney plume in a cross flow to the very high-speed regimes of scramjet combustion and missile control systems, from the low mass flow cases of boundary layer control systems and gas-turbine blade cooling to the high mass flow cases of a landing V/STOL vehicle. The basic problem of a fluid injected into a cross flow has several variables depending on its intended application: injector yaw and pitch angle, jet flow conditions subsonic, sonic, and supersonic, freestream conditions subsonic, supersonic, laminar, and turbulent, not to mention the phase and the chemical composition of the injectant single or multiphase, nonreacting or reacting mixture, etc..

Journal ArticleDOI
TL;DR: In this article, a scale decomposition of the fluid kinetic energy (or other quadratic integrals) into band-pass contributions from a series of length scales is proposed.
Abstract: We introduce a novel approach to scale decomposition of the fluid kinetic energy (or other quadratic integrals) into band-pass contributions from a series of length scales. Our decomposition is based on a multiscale generalization of the “Germano identity” for smooth, graded filter kernels. We employ this method to derive a budget equation that describes the transfers of turbulent kinetic energy both in space and in scale. It is shown that the interscale energy transfer is dominated by local triadic interactions, assuming only the scaling properties expected in a turbulent inertial range. We derive rigorous upper bounds on the contributions of nonlocal triads, extending the work of Eyink [Physica D 207, 91 (2005)] for low-pass filtering. We also propose a physical explanation of the differing exponents for our rigorous upper bounds and for the scaling predictions of Kraichnan [Phys. Fluids 9, 1728 (1966); J. Fluid Mech. 47, 525 (1971)]. The faster decay predicted by Kraichnan is argued to be the consequen...

Journal ArticleDOI
TL;DR: In this paper, the authors performed experimental studies of droplet breakup in microfluidic T-junctions in a range of capillary numbers lying between 4×10−4 and 2 ×10−1 and for two viscosity ratios of the fluids forming the dispersed and continuous phases.
Abstract: We perform experimental studies of droplet breakup in microfluidic T-junctions in a range of capillary numbers lying between 4×10−4 and 2×10−1 and for two viscosity ratios of the fluids forming the dispersed and continuous phases. The present paper extends the range of capillary numbers explored by previous investigators by two orders of magnitude. We single out two different regimes of breakup. In a first regime, a gap exists between the droplet and the wall before breakup occurs. In this case, the breakup process agrees well with the analytical theory of Leshansky and Pismen [Phys. Fluids 21, 023303 (2009)]. In a second regime, droplets keep obstructing the T-junction before breakup. Using physical arguments, we introduce a critical droplet extension for describing the breakup process in this case.

Journal ArticleDOI
TL;DR: In this paper, the authors employ microparticle image velocimetry to investigate laminar microflows in hydrophobic microstructured channels, in particular the slip length.
Abstract: We employ microparticle image velocimetry to investigate laminar microflows in hydrophobic microstructured channels, in particular the slip length. These microchannels consist of longitudinal microgrooves, which can trap air and prompt a shear-free boundary condition and thus slippage enhancement. Our measurements reveal an increase in the slip length when the width of the microgrooves is enlarged. The result of the slip length is smaller than the analytical prediction by Philip [Z. Angew. Math. Phys. 23, 353 (1972)] for an infinitely large and textured channel comprised of alternating shear-free and no-slip boundary conditions. The smaller slip length (as compared with the prediction) can be attributed to the confinement of the microchannel and the bending of the meniscus (liquid-gas interface). Our experimental studies suggest that the curvature of the meniscus plays an important role in microflows over hydrophobic microridges.

Journal ArticleDOI
TL;DR: In this paper, an experimental and theoretical study of the effect of the atmosphere on the evaporation of pinned sessile droplets of water is described, and a mathematical model that takes into account both the atmospheric pressure and the nature of the ambient gas on the diffusion of water vapor in the atmosphere and the thermal conductivity of the substrate is developed.
Abstract: An experimental and theoretical study of the effect of the atmosphere on the evaporation of pinned sessile droplets of water is described. The experimental work investigated the evaporation rates of sessile droplets in atmospheres of three different ambient gases (namely, helium, nitrogen, and carbon dioxide) at reduced pressure (from 40 to 1000 mbars) using four different substrates (namely, aluminum, titanium, Macor, and polytetrafluoroethylene) with a wide range of thermal conductivities. Reducing the atmospheric pressure increases the diffusion coefficient of water vapor in the atmosphere and hence increases the evaporation rate. Changing the ambient gas also alters the diffusion coefficient and hence also affects the evaporation rate. A mathematical model that takes into account the effect of the atmospheric pressure and the nature of the ambient gas on the diffusion of water vapor in the atmosphere and the thermal conductivity of the substrate is developed, and its predictions are found to be in encouraging agreement with the experimental results.

Journal ArticleDOI
TL;DR: In this article, the effect of rotation on shell-to-shell energy transfer in flows with moderate Rossby numbers (down to Ro≈0.03) was studied, where the authors used coherent forcing at intermediate scales, leaving enough room in the spectral space for an inverse cascade of energy to also develop.
Abstract: The effect of rotation is considered to become important when the Rossby number is sufficiently small, as is the case in many geophysical and astrophysical flows. Here we present direct numerical simulations to study the effect of rotation in flows with moderate Rossby numbers (down to Ro≈0.03) but at Reynolds numbers large enough to observe the beginning of a turbulent scaling at scales smaller than the energy injection scale. We use coherent forcing at intermediate scales, leaving enough room in the spectral space for an inverse cascade of energy to also develop. We analyze the spectral behavior of the simulations, the shell-to-shell energy transfer, scaling laws and intermittency, as well as the geometry and the anisotropy of the structures in the flow. At late times, the direct transfer of energy at small scales is mediated by interactions with the largest scale in the system, the energy containing eddies with k⊥≈1, where ⊥ refers to wavevectors perpendicular the axis of rotation. The transfer between...

Journal ArticleDOI
TL;DR: In this article, particle image velocimetry (PIV) measurements characterizing turbulent flow in a channel with superhydrophobic surfaces, structured and wetting surfaces, and smooth bottom surfaces were obtained.
Abstract: This paper reports particle image velocimetry (PIV) measurements characterizing turbulent flow in a channel with superhydrophobic surfaces, structured and wetting surfaces, and smooth bottom surfaces. The superhydrophobic and structured surfaces are fabricated with alternating ribs and cavities. Both longitudinal and transverse rib/cavity orientations were considered and the surfaces were made superhydrophobic by application of a Teflon coating. The widths of the ribs and cavities were 8 and 32μm, respectively, and the depth of the cavities was 15μm. PIV measurements were acquired for all surfaces considered over the Reynolds numbers range from 4800 to 10 000. Results from the smooth bottom wall measurements were used as a basis for comparison. The hydraulic diameter of the channel was nominally 8.2mm with an aspect ratio of 8.9. The PIV data captured aggregate velocities over multiple rib/cavity modules, such that a spanwise-averaged (over the width of the laser beam) velocity profile was obtained at the...

Journal ArticleDOI
TL;DR: In this paper, a single pressure pulse is applied to bubbles trapped in cylindrical nanoscopic pits (artificial crevices) with radii down to 50 nm, and the threshold for which the bubbles start to nucleate is determined experimentally.
Abstract: The acoustic nucleation threshold for bubbles trapped in cavities has theoretically been predicted within the crevice theory by Atchley and Prosperetti [“The crevice model of bubble nucleation,” J. Acoust. Soc. Am. 86, 1065 (1989)]. Here, we determine this threshold experimentally, by applying a single pressure pulse to bubbles trapped in cylindrical nanoscopic pits (“artificial crevices”) with radii down to 50 nm. By decreasing the minimum pressure stepwise, we observe the threshold for which the bubbles start to nucleate. The experimental results are quantitatively in good agreement with the theoretical predictions of Atchley and Prosperetti. In addition, we provide the mechanism which explains the deactivation of cavitation nuclei: gas diffusion together with an aspherical bubble collapse. Finally, we present superhydrophobic nuclei which cannot be deactivated, unless with a high-speed liquid jet directed into the pit.

Journal ArticleDOI
TL;DR: The effects of intra- and intersubject variabilities in airway geometry on airflow in the human lungs are investigated by large eddy simulation and two morphological factors are found to significantly affect the flows between subjects.
Abstract: The effects of intra- and intersubject variabilities in airway geometry on airflow in the human lungs are investigated by large eddy simulation. The airway models of two human subjects consisting of extra- and intrathoracic airways are reconstructed from CT images. For intrasubject study, airflows at two inspiratory flow rates are simulated on the airway geometries of the same subject with four different levels of truncation. These airway models are the original complete geometry and three geometries obtained by truncating the original one at the subglottis, the supraglottis, and the laryngopharynx, respectively. A comparison of the airflows in the complete geometry model shows that the characteristics of the turbulent laryngeal jet in the trachea are similar regardless of Reynolds number in terms of mean velocities, turbulence statistics, coherent structures, and pressure distribution. The truncated airway models, however, do not produce the similar flow structures observed in the complete geometry. An improved inlet boundary condition is then proposed for the airway model truncated at the laryngopharynx to improve the accuracy of solution. The new boundary condition significantly improves the mean flow. The spectral analysis shows that turbulent characteristics are captured downstream away from the glottis. For intersubject study, although the overall flow characteristics are similar, two morphological factors are found to significantly affect the flows between subjects. These are the constriction ratio of the glottis with respect to the trachea and the curvature and shape of the airways.

Journal ArticleDOI
TL;DR: In this paper, the viscous coupling effects for immiscible two-phase (gas-liquid) flow in porous media were studied using the Shan-Chen type single-component multiphase lattice Boltzmann model.
Abstract: In this paper, the viscous coupling effects for immiscible two-phase (gas-liquid) flow in porous media were studied using the Shan–Chen-type single-component multiphase lattice Boltzmann model. Using the model, the two-phase flows in porous media with density ratio as high as 56 could be simulated and the contact angle of the gas-liquid interface at a solid wall is adjustable. To investigate viscous coupling effects, the co- and countercurrent steady-state two-phase flow patterns and relative permeabilities as a function of wetting saturation were obtained for different capillary numbers, wettabilities, and viscosity ratios. The cocurrent relative permeabilities seem usually larger than the countercurrent ones. The opposing drag-force effect and different pore-level saturation distributions in co- and countercurrent flows may contribute to this difference. It is found that for both co- and countercurrent flows, for strongly wet cases and viscosity ratio M>1, knw increase with the driving force and the vis...