Showing papers in "Physics of Fluids in 2007"

Hairpin vortex organization in wall turbulencea)

Abstract: Coherent structures in wall turbulence transport momentum and provide a means of producing turbulent kinetic energy. Above the viscous wall layer, the hairpin vortex paradigm of Theodorsen coupled with the quasistreamwise vortex paradigm have gained considerable support from multidimensional visualization using particle image velocimetry and direct numerical simulation experiments. Hairpins can autogenerate to form packets that populate a significant fraction of the boundary layer, even at very high Reynolds numbers. The dynamics of packet formation and the ramifications of organization of coherent structures (hairpins or packets) into larger-scale structures are discussed. Evidence for a large-scale mechanism in the outer layer suggests that further organization of packets may occur on scales equal to and larger than the boundary layer thickness.

Achieving large slip with superhydrophobic surfaces: Scaling laws for generic geometries

Abstract: We investigate the hydrodynamic friction properties of superhydrophobic surfaces and quantify their superlubricating potential. On such surfaces, the contact of the liquid with the solid roughness is minimal, while most of the interface is a liquid-gas one, resulting in strongly reduced friction. We obtain scaling laws for the effective slip length at the surface in terms of the generic surface characteristics (roughness length scale, depth, solid fraction of the interface, etc.). These predictions are successfully compared to numerical results in various geometries (grooves, posts or holes). This approach provides a versatile framework for the description of slip on these composite surfaces. Slip lengths up to 100μm are predicted for an optimized patterned surface.

Propulsion in a viscoelastic fluid

Abstract: Flagella beating in complex fluids are significantly influenced by viscoelastic stresses. Relevant examples include the ciliary transport of respiratory airway mucus and the motion of spermatozoa in the mucus-filled female reproductive tract. We consider the simplest model of such propulsion and transport in a complex fluid, a waving sheet of small amplitude free to move in a polymeric fluid with a single relaxation time. We show that, compared to self-propulsion in a Newtonian fluid occurring at a velocity UN, the sheet swims (or transports fluid) with velocity U∕UN=(1+De2ηs∕η)∕(1+De2), where ηs is the viscosity of the Newtonian solvent, η is the zero-shear-rate viscosity of the polymeric fluid, and De is the Deborah number for the wave motion, product of the wave frequency by the fluid relaxation time. Similar expressions are derived for the rate of work of the sheet and the mechanical efficiency of the motion. These results are shown to be independent of the particular nonlinear constitutive equations ...

Analysis of the Casson and Carreau-Yasuda non-Newtonian blood models in steady and oscillatory flows using the lattice Boltzmann method

Abstract: The lattice Boltzmann method is modified to allow the simulation of non-Newtonian shear-dependent viscosity models. Casson and Carreau-Yasuda non-Newtonian blood viscosity models are implemented and are used to compare two-dimensional Newtonian and non-Newtonian flows in the context of simple steady flow and oscillatory flow in straight and curved pipe geometries. It is found that compared to analogous Newtonian flows, both the Casson and Carreau-Yasuda flows exhibit significant differences in the steady flow situation. In the straight pipe oscillatory flows, both models exhibit differences in velocity and shear, with the largest differences occurring at low Reynolds and Womersley numbers. Larger differences occur for the Casson model. In the curved pipe Carreau-Yasuda model, moderate differences are observed in the velocities in the central regions of the geometries, and the largest shear rate differences are observed near the geometry walls. These differences may be important for the study of atherosclerotic progression.

Scalings and decay of fractal-generated turbulence

Abstract: A total of 21 planar fractal grids pertaining to three different fractal families have been used in two different wind tunnels to generate turbulence The resulting turbulent flows have been studied using hot wire anemometry Irrespective of fractal family, the fractal-generated turbulent flows and their homogeneity, isotropy, and decay properties are strongly dependent on the fractal dimension Df≤2 of the grid, its effective mesh size Meff (which we introduce and define) and its ratio tr of largest to smallest bar thicknesses, tr=tmax∕tmin With relatively small blockage ratios, as low as σ=25%, the fractal grids generate turbulent flows with higher turbulence intensities and Reynolds numbers than can be achieved with higher blockage ratio classical grids in similar wind tunnels and wind speeds U The scalings and decay of the turbulence intensity u′∕U in the x direction along the tunnel’s center line are as follows (in terms of the normalized pressure drop CΔP and with similar results for v′∕U and w′∕U)

Dissipation and decay of fractal-generated turbulence

Abstract: Space-filling fractal square grids fitted at the entrance of a wind tunnel’s test section generate unusually high Reynolds number homogeneous isotropic turbulence which decays locked into a single length-scale l. Specifically, during turbulence decay along the streamwise coordinate x, E11(k1,x)=u′2lf(k1l) over the entire range of wavenumbers, where l and the function f are about the same for all the grids tried here. As a result, this fractal-generated turbulence has the following properties which we have also observed in the decaying region: L∕λ is constant, independent of the x grid and Reλ; ϵ∼Reλ−1u′3∕Lu; and E11(k1)∼(u′3∕Lu)2∕3k1−5∕3 instead of E11(k1)∼ϵ2∕3k1−5∕3 in the observed range of wavenumbers where f(k1l)∼(k1l)−5∕3.

Boundary Conditions for the Moving Contact Line Problem

Abstract: The physical processes near a moving contact line are investigated systematically using molecular dynamics and continuum mechanics. Constitutive relations for the friction force in the contact line region, the fluid-fluid interfacial force, and the stresses in the fluid-solid interfacial region are studied. Verification of force balance demonstrates the importance of the normal stress jump across the contact line region. Effective boundary conditions are derived using force balance. It is found that in the flow regime studied, the deviation of the wall contact angle from the equilibrium contact angle is proportional to the velocity of the contact line. The effective continuum model is solved numerically and the behavior of the apparent contact angle and the wall contact angle is studied. It is found that the fluid-fluid interface near the wall exhibits a universal behavior. The onset of the nonlinear response for the contact line motion is studied within the framework of Blake’s molecular kinetic theory.

Penalty immersed boundary method for an elastic boundary with mass

Abstract: The immersed boundary (IB) method has been widely applied to problems involving a moving elastic boundary that is immersed in fluid and interacting with it. Most of the previous applications of the IB method have involved a massless elastic boundary and used efficient Fourier transform methods for the numerical solutions. Extending the method to cover the case of a massive boundary has required spreading the boundary mass out onto the fluid grid and then solving the Navier-Stokes equations with a variable mass density. The variable mass density of this previous approach makes Fourier transform methods inapplicable, and requires a multigrid solver. Here we propose a new and simple way to give mass to the elastic boundary and show that the method can be applied to many problems for which the boundary mass is important. The method does not spread mass to the fluid grid, retains the use of Fourier transform methodology, and is easy to implement in the context of an existing IB method code for the massless case. Two verifications of the method are given. One is a numerical convergence study that shows that our numerical scheme is second-order accurate for a particular test problem. The other is direct comparison with experimental data of vortex-induced vibrations of a massive cylinder, which shows that the results obtained by the present method are quite comparable to the experimental data.

Artificial Fluid Properties for Large-Eddy Simulation of Compressible Turbulent Mixing

Abstract: An alternative methodology is described for Large-Eddy Simulation of flows involving shocks, turbulence and mixing. In lieu of filtering the governing equations, it is postulated that the large-scale behavior of an ''LES'' fluid, i.e., a fluid with artificial properties, will be similar to that of a real fluid, provided the artificial properties obey certain constraints. The artificial properties consist of modifications to the shear viscosity, bulk viscosity, thermal conductivity and species diffusivity of a fluid. The modified transport coefficients are designed to damp out high wavenumber modes, close to the resolution limit, without corrupting lower modes. Requisite behavior of the artificial properties is discussed and results are shown for a variety of test problems, each designed to exercise different aspects of the models. When combined with a 10th-order compact scheme, the overall method exhibits excellent resolution characteristics for turbulent mixing, while capturing shocks and material interfaces in crisp fashion.

A note on the effective slip properties for microchannel flows with ultrahydrophobic surfaces

Abstract: A type of superhydrophobic surface consists of a solid plane boundary with an array of grooves which, due to the effect of surface tension, prevent a complete wetting of the wall. The effect is greatest when the grooves are aligned with the flow. The pressure difference between the liquid and the gas in the grooves causes a curvature of the liquid surface resisted by surface tension. The effects of this surface deformation are studied in this paper. The corrections to the effective slip length produced by the curvature are analyzed theoretically and a comparison with available data and related mathematical models is presented.

Examination of a critical roughness height for outer layer similarity

Abstract: The existence of a critical roughness height for outer layer similarity between smooth and rough wall turbulent boundary layers is investigated. Results are presented for boundary layer measurements on flat plates covered with sandgrain and woven mesh with the ratio of the boundary layer thickness to roughness height (δ∕k) varying from 16 to 110 at Reθ=7.3×103–13×103. In all cases tested, the layer directly modified by the roughness (the roughness sublayer) is confined to a region <5k or <3ks from the wall (where ks is the equivalent sandgrain roughness height). In the larger roughness cases, this region of turbulence modification extends into the outer flow. However, beyond 5k or 3ks from the wall, similarity in the turbulence quantities is observed between the smooth and rough wall boundary layers. These results indicate that a critical roughness height, where the roughness begins to affect most or all of the boundary layer, does not exist. Instead, the outer flow is only gradually modified with increas...

Simulation of hydrodynamically interacting particles near a no-slip boundary

Abstract: The dynamics of spherical particles near a single plane wall are computed using an extension of the Stokesian dynamics method that includes long-range many-body and pairwise lubrication interactions between the spheres and the wall in Stokes flow. Extra care is taken to ensure that the mobility and resistance tensors are symmetric, positive, and definite—something which is ineluctable for particles in low-Reynolds-number flows. We discuss why two previous simulation methods for particles near a plane wall, one using multipole expansions and the other using the Rotne-Prager tensor, fail to produce symmetric resistance and mobility tensors. Additionally, we offer some insight on how the Stokesian dynamics paradigm might be extended to study the dynamics of particles in any confining geometry.

Laminar flow in a microchannel with hydrophobic surface patterned microribs oriented parallel to the flow direction

Abstract: This paper reports results of an analytical and experimental investigation of the laminar flow in a parallel-plate microchannel with ultrahydrophobic top and bottom walls. The walls are fabricated with microribs and cavities that are oriented parallel to the flow direction. The channel walls are modeled in an idealized fashion, with the shape of the liquid-vapor meniscus approximated as flat. An analytical model of the vapor cavity flow is employed and coupled with a numerical model of the liquid flow by matching the local liquid and vapor phase velocity and shear stress at the interface. The numerical predictions show that the effective slip length and the reduction in the classical friction factor-Reynolds number product increase with increasing relative cavity width, increasing relative cavity depth, and decreasing relative microrib/cavity module length. Comparisons were also made between the zero shear interface model and the liquid-vapor cavity coupled model. The results illustrate that the zero shea...

Self-consistent high-Reynolds-number asymptotics for zero-pressure-gradient turbulent boundary layers

Abstract: The asymptotic behavior of mean velocity and integral parameters in flat plate turbulent boundary layers under zero pressure gradient are studied for Reynolds numbers approaching infinity. Using the classical two-layer approach of Millikan, Rotta, and Clauser with a logarithmic velocity profile in the overlap region between “inner” and “outer” layers, a fully self-consistent leading-order description of the mean velocity profile and all integral parameters is developed. It is shown that this description fits most high Reynolds number data, and in particular their Reynolds number dependence, exceedingly well; i.e., within experimental errors.

A dynamic global-coefficient subgrid-scale eddy-viscosity model for large-eddy simulation in complex geometries

Abstract: An improvement of the dynamic procedure of Park et al. [Phys. Fluids 18, 125109 (2006)] for closure of the subgrid-scale eddy-viscosity model developed by Vreman [Phys. Fluids 16, 3670 (2004)] is proposed. The model coefficient which is globally constant in space but varies in time is dynamically determined assuming the “global equilibrium” between the subgrid-scale dissipation and the viscous dissipation of which utilization was proposed by Park et al. Like the Vreman model with a fixed coefficient and the dynamic-coefficient model of Park et al., the present model predicts zero eddy-viscosity in regions where the vanishing eddy viscosity is theoretically expected. The present dynamic model is especially suitable for large-eddy simulation in complex geometries since it does not require any ad hoc spatial and temporal averaging or clipping of the model coefficient for numerical stabilization and more importantly, requires only a single-level test filter in contrast to the dynamic model of Park et al., whi...

Influence of the Damköhler number on turbulence-scalar interaction in premixed flames. I. Physical insight

Abstract: Scalar dissipation rate is a central quantity in turbulent flame modeling as it is closely related to the reaction rate. It is well known that turbulence-scalar interaction plays a vital role in turbulent flows with scalar mixing and thus on the scalar dissipation rate. This interaction process is characterized by the tensor inner product between the scalar gradient vector and the turbulence strain rate tensor and it is found to depend strongly on the Damkohler number, Da. Two direct numerical simulation data sets are analyzed in detail in order to understand the physics of Da dependence. The well known alignment of scalar gradient with the most compressive principal strain rate resulting in production of the scalar gradient by turbulence is observed for low (Da<1) Damkohler number flame, whereas the turbulence dissipates the scalar gradient in high Da flame. This dissipation of the scalar gradient in the high Da flame is because of its preferential alignment with the most extensive principal strain rate....

On the bimodal dynamics of the turbulent horseshoe vortex system in a wing-body junction

Abstract: The turbulent boundary layer approaching a wall-mounted obstacle experiences a strong adverse pressure gradient and undergoes three-dimensional separation leading to the formation of a dynamically rich horseshoe vortex (HSV) system. In a pioneering experimental study, Devenport and Simpson [J. Fluid Mech. 210, 23 (1990)] showed that the HSV system forming at the leading edge region of a wing mounted on a flat plate at Re=1.15×105 exhibits bimodal, low-frequency oscillations, which away from the wall produce turbulent energy and stresses one order of magnitude higher than those produced by the conventional shear mechanism in the approaching turbulent boundary layer. We carry out numerical simulations for the experimental configuration of Devenport and Simpson using the detached-eddy-simulation (DES) approach. The DES length scale is adjusted for this flow to alleviate the well known shortcoming of DES; namely that of premature, laminar-like flow separation. The numerical simulations reproduce with good acc...

Experimental study of the formation and scaling of a round synthetic jet

Abstract: The flow field from a piston-cylinder synthetic jet actuator was investigated in detail over a range of dimensionless stroke values, L∘∕D∘, and Reynolds numbers, ReU∘. In each of the test flows examined, only one of these dimensionless groups was varied. The flow fields were examined using particle image velocimetry measurements in a plane bisecting the jet. A slug flow model was used to determine scaling parameters for the jet flow field. In the near-field of the orifice, the flow was dominated by the vortex ring formed during the expulsion phase of the actuator cycle, and the flow field scaled exclusively on the actuator stroke, L∘. For distances from the orifice greater than L∘, the flow field resembled a conventional, round turbulent jet. The resemblance was not complete as the synthetic jet had a faster spreading rate with a correspondingly more rapid decline in the mean centerline velocity. The dimensionless jet momentum was comparable at the higher stroke values for the same Reynolds numbers, and t...

Vorticity dynamics of a bileaflet mechanical heart valve in an axisymmetric aorta

Abstract: We present comprehensive particle image velocimetry measurements and direct numerical simulation (DNS) of physiological, pulsatile flow through a clinical quality bileaflet mechanical heart valve mounted in an idealized axisymmetric aorta geometry with a sudden expansion modeling the aortic sinus region. Instantaneous and ensemble-averaged velocity measurements as well as the associated statistics of leaflet kinematics are reported and analyzed in tandem to elucidate the structure of the velocity and vorticity fields of the ensuing flow-structure interaction. The measurements reveal that during the first half of the acceleration phase, the flow is laminar and repeatable from cycle to cycle. The valve housing shear layer rolls up into the sinus and begins to extract vorticity of opposite sign from the sinus wall. A start-up vortical structure is shed from the leaflets and is advected downstream as the leaflet shear layers become wavy and oscillatory. In the second half of flow acceleration the leaflet shea...

An energy balance approach of the dynamics of drop impact on a solid surface

Abstract: The description of physical mechanisms involved in the impact of a drop upon a dry, partially wettable substrate is still a matter of debate. One way to analyze the balance of these mechanisms is the development of an analytical one-dimensional (1D) model based upon the energy equation. The assimilation of the drop to a cylinder allows a reduction of the energy equation to a second-order differential equation. This paper proposes a semi-empirical description of viscous dissipation taking into account the rolling motion near the contact line. The dissipation due to the rolling motion is added to the calculated dissipation in the core of the droplet. We compare our model to previous ones using a large set of literature data covering a wide range of viscosity, velocity impact, and equilibrium contact angle values. The new dissipation description proposed is shown to supersede those described in previous 1D models. Our model closely predicts the maximum spread factor and the time at which it is obtained on the whole range of Ohnesorge and Weber numbers considered. It also distinguishes between deposition with a steady variation in the wetted area from deposition with advancing and receding phases. The main limitations of the model lie in its inability to reproduce the spread factor at the very beginning of the impact and the rebounding observed after a receding phase for very high values of the equilibrium contact angle.

Investigations on the impact of a drop onto a small spherical target

Abstract: This paper reports on experimental and theoretical investigations of the impact of a droplet onto a spherical target. Spatial and temporal variation of film thickness on the target surface is measured. Three distinct temporal phases of the film dynamics are clearly visible from the experimental results, namely the initial drop deformation phase, the inertia dominated phase, and the viscosity dominated phase. Experiments are also conducted to study the effect of droplet Reynolds number and target-to-drop size ratio on the dynamics of the film flow on the surface of the target. It is observed that for a given target-to-drop size ratio, the nondimensional temporal variation of film thickness collapses onto a single curve in the first and second phases. The transition to the viscosity dominated regime occurs earlier for the low Reynolds number cases and residual thickness is also larger. A simplified quasi-one-dimensional approach has been used to model the flow on the spherical target. The theory accounts fo...

Unsteadiness of an axisymmetric separating-reattaching flow : Numerical investigation

Abstract: The separated flow over a cylinder elongated by another cylinder of a smaller diameter is investigated numerically at the high subsonic regime using zonal detached eddy simulation (ZDES) and compared with the experimental data of Depres, Reijasse, and Dussauge [AIAA J. 42, 2541 (2004)]. First, it is shown that this axisymmetric step flow has much in common with the two-dimensional facing step flows as regards the shear layer instability process. Second, the statistical and spectral properties of the pressure fluctuations are scrutinized. Close to the step, the surface pressure signature is characterized by low frequencies f.Lr∕U∞=O(0.08) (where Lr and U∞ denote, respectively, the mean reattachment length and free-stream velocity) and an upstream velocity of 0.26U∞ while in the second half-part of the recirculation higher frequencies fluctuations at f.Lr∕U∞≈0.6 and a downstream convection velocity 0.6U∞ are the dominant features. The current calculation shows that the separated bubble dynamics depends on very complex interactions of large eddies formed in the upstream free shear layer with the wall in the reattachment region. These structures are shed with a nondimensional frequency of about 0.2. Besides, it has been observed that the secondary corner vortex experiences a cycle of growth and decay. The correspondence between the frequencies of this secondary corner vortex dynamics and the flapping motion (f.Lr∕U∞≈0.08) suggests that there should be different aspects of the same motion. These results show that there is an ordered structure in this axisymmetric separating/reattaching flow which is dominated by large scale coherent motion. This is confirmed by a two-point correlation analysis of the pressure signals showing that the flow is dominated by highly coherent antisymmetric modes at the flapping and vortex shedding frequencies whose signatures are evidenced in the spectrum of the computed buffet loads. Possible onsets of a large-scale self-sustained motion of the separated area are finally discussed and the existence of an absolute instability of the axisymmetric recirculation bubble originating from a region located near the middle of the recirculating zone is conjectured.

Leukocyte margination in a model microvessel

Abstract: The physiological inflammation response depends upon the multibody interactions of blood cells in the microcirculation that bring leukocytes (white blood cells) to the vessel walls. We investigate the fluid mechanics of this using numerical simulations of 29 red blood cells and one leukocyte flowing in a two-dimensional microvessel, with the cells modeled as linearly elastic shell membranes. Despite its obvious simplifications, this model successfully reproduces the increasingly blunted velocity profiles and increased leukocyte margination observed at lower shear rates in actual microvessels. Red cell aggregation is shown to be unnecessary for margination. The relative stiffness of the red cells in our simulations is varied by over a factor of 10, but the margination is found to be much less correlated with this than it is to changes associated with the blunting of the mean velocity profile at lower shear rates. While velocity around the leukocyte when it is near the wall depends upon the red cell properties, it changes little for strongly versus weakly marginating cases. In the more strongly marginating cases, however, a red cell is frequently observed to be leaning on the upstream side of the leukocyte and appears to stabilize it, preventing other red cells from coming between it and the wall. A well-known feature of the microcirculation is a near-wall cell-free layer. In our simulations, it is observed that the leukocyte’s most probable position is at the edge of this layer. This wall stand-off distance increases with velocity following a scaling that would be expected for a lubrication mechanism, assuming that there were a nearly constant force pushing the cells toward the wall. The leukocyte’s near-wall position is observed to be less stable with increasing mean stand-off distance, but this distance would have potentially greater effect on adhesion since the range of the molecular binding is so short.

Outer-layer similarity in the presence of a practical rough-wall topography

Abstract: High-resolution particle image velocimetry measurements are made in the streamwise–wall-normal plane of a zero-pressure-gradient turbulent boundary layer over smooth and rough walls at Reθ≈13000. The roughness considered herein is replicated from a surface scan of a turbine blade damaged by deposition of foreign materials and its topography is highly irregular and contains a broad range of topographical scales. Two physical scalings of the same roughness topography are considered, yielding two different rough surfaces: RF1 with k=4.2mm and RF2 with k=2.1mm, where k is the average peak-to-valley roughness height. At Reθ≈13000, these roughness conditions yield k+≡k∕y*=207, δ∕k=28, ks+=115, and δ∕ks=48 for RF1 and k+=91, δ∕k=50, ks+=29, and δ∕ks=162 for RF2 (where δ is the boundary-layer thickness, ks is the equivalent sand-grain height, and y* is the viscous length scale). The mean velocity deficits along with the Reynolds normal and shear stress profiles for both roughness conditions collapse on the smooth...

Simulation of the motion of flexible fibers in viscous fluid flow

Abstract: A model for flexible fibers in viscous fluid flow is proposed, and its predictions compared with experiments found in the literature. The incompressible three-dimensional Navier–Stokes equations are employed to describe the fluid motion, while fibers are modeled as chains of fiber segments, interacting with the fluid through viscous and dynamic drag forces. Fiber segments, from the same or from different fibers, interact with each other through normal, frictional, and lubrication forces. Momentum conservation is enforced on the system to capture the two-way coupling between phases. Quantitative predictions could be made, and showed good agreement with experimental data, for the period of Jeffery orbits in shear flow, as well as for the amount of bending of flexible fibers in shear flow. Simulations, using the proposed model, also successfully reproduced the different regimes of motion for threadlike particles, ranging from rigid fiber motion to complicated orbiting behavior, including coiling and self-entanglement.

Phase diagram of vertically shaken granular matter

Abstract: A shallow, vertically shaken granular bed in a quasi-two-dimensional container is explored experimentally yielding a wider variety of phenomena than in any previous study: (1) bouncing bed, (2) undulations, (3) granular Leidenfrost effect, (4) convection rolls, and (5) granular gas. These phenomena and the transitions among them are characterized by dimensionless control parameters and combined in a full experimental phase diagram.

Characteristics of flow over traveling wavy foils in a side-by-side arrangement

Abstract: Flow over traveling wavy foils in a side-by-side arrangement has been numerically investigated using the space-time finite element method to solve the two-dimensional incompressible Navier-Stokes equations The midline of each foil undergoes lateral motion in the form of a streamwise traveling wave, which is similar to the backbone undulation of swimming fish Based on the phase difference between the adjacent undulating foils, two typical cases, ie, in-phase and anti-phase traveling wavy movements, are considered in the present study The effects of lateral interference among the foils on the forces, power consumption, propeller efficiency, and flow structures are analyzed It is revealed that the lateral interference is of benefit to saving the swimming power in the in-phase case and enhancing the forces in the anti-phase case Some typical vortex structures, eg, vortex-pair row, single vortex row, and in-phase and anti-phase synchronized vortex-street, are observed in the wake of the traveling wavy

Experiments on bubble pinch-off

Abstract: A bubble is slowly grown from a vertical nozzle until it becomes unstable and pinches off. We use ultra-high-speed video imaging, at frame-rates up to 1millionfps, to study the dynamics and shape of the pinch-off neck region. For bubbles in water (Bo≃1.0) the radius of the neck reduces with a power law behavior R∼tα, over more than 2 decades, with an exponent in the range α=0.57±0.03, in good agreement with other available studies, but which is slightly larger than 1∕2 predicted by Rayleigh-Plesset theory. The vertical curvature in the neck increases more slowly than the azimuthal curvature, making the neck profiles more slender as pinch-off is approached. Self-similar shapes are recovered by normalizing the axial coordinate by a separate length scale which follows a different power law, Lz∼tγ, where γ=0.49±0.03. Results for air, He, and SF6 gas are identical, suggesting that the gas density plays a minimal role in the dynamics. The pinch-off in water leaves behind a tiny satellite bubble, around 5μm in d...

Properties of d- and k-type roughness in a turbulent channel flow

Abstract: Roughness is classified by the so-called roughness function, which represents the downward shift of the velocity profile relative to a smooth wall. The dependence of the roughness function on the Reynolds number is discussed with the aim of clarifying the difference between d-type and k-type behaviors. This difference has been traditionally associated with the stability of the flow within the roughness elements. The present direct numerical simulation results indicate that the difference more correctly reflects the different contributions from the frictional drag and pressure drag to the total stress.

Stochastic estimation of a separated-flow field using wall-pressure-array measurements

Abstract: Concurrent, surface-pressure and planar, particle image velocimetry (PIV) measurements were obtained in the separating/reattaching flow region downstream of an axisymmetric, backward-facing step at a Reynolds number of 8081, based on step height. The surface-pressure and PIV measurements were used to investigate the evolution of coherent structures in the flow field by employing proper orthogonal decomposition (POD) and multipoint, linear, stochastic estimation (mLSE) analysis techniques. POD was used to determine the dominant modes in the pressure signature, while mLSE was used to estimate the dominant flow structures above the wall from the wall-pressure POD modes over a series of time steps. It was found that a large-scale, coherent structure develops in place (i.e., temporally) at approximately half the reattachment distance. Once this structure reaches a height equivalent to the step, it sheds and accelerates downstream. This growth in place, and then shedding, resembles the evolution of the flow str...