Showing papers on "Reynolds number published in 2011"

High–Reynolds Number Wall Turbulence

Abstract: We review wall-bounded turbulent flows, particularly high–Reynolds number, zero–pressure gradient boundary layers, and fully developed pipe and channel flows. It is apparent that the approach to an asymptotically high–Reynolds number state is slow, but at a sufficiently high Reynolds number the log law remains a fundamental part of the mean flow description. With regard to the coherent motions, very-large-scale motions or superstructures exist at all Reynolds numbers, but they become increasingly important with Reynolds number in terms of their energy content and their interaction with the smaller scales near the wall. There is accumulating evidence that certain features are flow specific, such as the constants in the log law and the behavior of the very large scales and their interaction with the large scales (consisting of vortex packets). Moreover, the refined attached-eddy hypothesis continues to provide an important theoretical framework for the structure of wall-bounded turbulent flows.

The Structure of Isotropic Turbulence at Very High Reynolds Numbers

Abstract: A recapitulation is first given of a recent theory of homogeneous turbulence based on the condition that the Fourier amplitudes of the velocity field be as randomly distributed as the dynamical equations permit. This theory involves the average infinitesimal-impulse-response functions of the Fourier amplitudes and employs a new kind of perturbation method which yields what are belived to be exact expansions of third- and higher-order statistical moments of the Fourier amplitudes in terms of second-order moments and these response functions.In the present paper the theory is applied in lowest approximation (called the direct-interaction approximation) to stationary isotropic turbulence of very high Reynolds number. The characteristic wave-number where An is a universal constant.The theory is compared with experiment and is found to be slightly better supported than the Kolmogorov theory. However, it is stressed that extreme caution must be exercised in interpreting the experimental evidence as support for either theory.An analysis is given of the relations between the Kolmogorov theory, Heisen-berg's heuristic theory, the analytical theories of Heisenberg and Chandrasekhar, the theories of Proudman & Reid and Tatsumi, and the present theory.

Direct Numerical Simulations of Gas-Liquid Multiphase Flows

Abstract: Direct numerical simulations of bubbly flows are reviewed and recent progress is discussed. Simulations, of homogeneous bubble distribution in fully periodic domains at relatively low Reynolds numbers have already yielded considerable insight into the dynamics of such flows. Many aspects of the evolution converge rapidly with the size of the systems and results for the rise velocity, the velocity fluctuations, as well as the average relative orientation of bubble pairs have been obtained. The challenge now is to examine bubbles at higher Reynolds numbers, bubbles in channels and confined geometry, and bubble interactions with turbulent flows. We briefly review numerical methods used for direct numerical simulations of multiphase flows, with a particular emphasis on methods that use the so-called "one-field" formulation of the governing equations, and then discuss studies of bubbles in periodic domains, along with recent work on wobbly bubbles, bubbles in laminar and turbulent channel flows, and bubble formation in boiling.

Wake transition of a rolling sphere

Abstract: We present dye visualizations of the flow generated by a sphere rolling along a solid surface in a quiescent fluid at low Reynolds numbers. Recent experimental and numerical studies of this configuration (Stewart et al. 2008, 2010) have shown a transition from steady to periodic flow at a Reynolds number Re (based on the sphere diameter and rolling speed) near 100. The unsteady flow involves the shedding of hairpin vortices, and the flow remained symmetric in the Reynolds number range considered, up to Re = 200. The present visualizations show that for higher Re the spatial symmetry of the unsteady wake is lost.

Plasmon-assisted optofluidics.

Abstract: We study the ability of a plasmonic structure under illumination to release heat and induce fluid convection at the nanoscale. We first introduce the unified formalism associated with this multidisciplinary problem combining optics, thermodynamics, and hydrodynamics. On this basis, numerical simulations were performed to compute the temperature field and velocity field evolutions of the surrounding fluid for a gold disk on glass while illuminated at its plasmon resonance. We show that the velocity amplitude of the surrounding fluid has a linear dependence on the structure temperature and a quadratic dependence on the structure size (for a given temperature). The fluid velocity remains negligible for single nanometer-sized plasmonic structures (<1 nm/s) due to a very low Reynolds number. However thermal-induced fluid convection can play a significant role when considering either micrometer-size structures or an assembly of nanostructures.

Interface-resolved DNS of vertical particulate channel flow in the turbulent regime

Abstract: We have conducted a direct numerical simulation (DNS) study of dilute turbulent particulate flow in a vertical plane channel, considering thousands of finite-size rigid particles with resolved phase interfaces. The particle diameter corresponds to approximately 11 wall units and their terminal Reynolds number is set to 136. The fluid flow with bulk Reynolds number 2700 is directed upward, which maintains the particles suspended upon average. Two density ratios were simulated, differing by a factor of 4.5. The corresponding Stokes numbers of the two flow cases were O(10) in the near-wall region and O(1) in the outer flow. We have observed the formation of large-scale elongated streak-like structures with streamwise dimensions of the order of 8 channel half-widths and cross-stream dimensions of the order of one half-width. At the same time, we have found no evidence of significant formation of particle clusters, which suggests that the large structures are due to an intrinsic instability of the flow, triggered by the presence of the particles. It was found that the mean fluid velocity profile tends towards a concave shape, and the turbulence intensity as well as the normal stress anisotropy are strongly increased. The effect of varying the Stokes number while maintaining the buoyancy, particle size and volume fraction constant was relatively weak.

Turbulence in supersonic boundary layers at moderate Reynolds number

Abstract: We study the organization of turbulence in supersonic boundary layers through large-scale direct numerical simulations (DNS) at , and momentum-thickness Reynolds number up to (corresponding to ) which significantly extend the current envelope of DNS in the supersonic regime. The numerical strategy relies on high-order, non-dissipative discretization of the convective terms in the Navier–Stokes equations, and it implements a recycling/rescaling strategy to stimulate the inflow turbulence. Comparison of the velocity statistics up to fourth order shows nearly exact agreement with reference incompressible data, provided the momentum-thickness Reynolds number is matched, and provided the mean velocity and the velocity fluctuations are scaled to incorporate the effects of mean density variation, as postulated by Morkovin’s hypothesis. As also found in the incompressible regime, we observe quite a different behaviour of the second-order flow statistics at sufficiently large Reynolds number, most of which show the onset of a range with logarithmic variation, typical of ‘attached’ variables, whereas the wall-normal velocity exhibits a plateau away from the wall, which is typical of ‘detached’ variables. The modifications of the structure of the flow field that underlie this change of behaviour are highlighted through visualizations of the velocity and temperature fields, which substantiate the formation of large jet-like and wake-like motions in the outer part of the boundary layer. It is found that the typical size of the attached eddies roughly scales with the local mean velocity gradient, rather than being proportional to the wall distance, as happens for the wall-detached variables. The interactions of the large eddies in the outer layer with the near-wall region are quantified through a two-point amplitude modulation covariance, which characterizes the modulating action of energetic outer-layer eddies.

Implicit Large Eddy Simulation of transition to turbulence at low Reynolds numbers using a Discontinuous Galerkin method

Abstract: The present work predicts the formation of laminar separation bubbles at low Reynolds numbers and the related transition to turbulence by means of Implicit Large Eddy Simulations with a high-order Discontinuous Galerkin method. The flow around an SD7003 infinite wing at an angle of attack of 4° is considered at Reynolds numbers of 10 000, 22 000, and 60 000 in order to gain insight into the characteristics of the laminar and turbulent regimes. At the lowest Reynolds number studied, the flow remains laminar and two dimensional over the wing surface, with a periodic vortex shedding. For higher Reynolds numbers, the flow is unsteady over the upper wing surface and exhibits a separation bubble along which the flow transitions to turbulence. Tollmien–Schlichting (TS) waves are observed in the boundary layer, and transition is found to be caused by unstable TS modes as revealed by the growth of the stream-wise amplification factor. Copyright © 2010 John Wiley & Sons, Ltd.

Hydrodynamic nucleation of quantized vortex pairs in a polariton quantum fluid

Abstract: Quantized vortices appear in quantum gases at the breakdown of superfluidity. In liquid helium and cold atomic gases, they have been indentified as the quantum counterpart of turbulence in classical fluids. In the solid state, composite light‐matter bosons known as exciton polaritons have enabled studies of non-equilibrium quantum gases and superfluidity. However, there has been no experimental evidence of hydrodynamic nucleation of polariton vortices so far. Here we report the experimental study of a polariton fluid flowing past an obstacle and the observation of nucleation of quantized vortex pairs in the wake of the obstacle. We image the nucleation mechanism and track the motion of the vortices along the flow. The nucleation conditions are established in terms of local fluid density and velocity measured on the obstacle perimeter. The experimental results are successfully reproduced by numerical simulations based on the resolution of the Gross‐Pitaevskii equation. H ydrodynamic instabilities in classical fluids were studied in the pioneering experiments of BOnard in the 1910’s. Convective BOnardRayleigh flows and BOnardVon KAErmAEn streets are now well known examples in nonlinear and chaos sciences 1 . In conventional fluids, the flow around an obstacle is characterized by the dimensionless Reynolds number ReD vR= , withv and the fluid velocity and dynamical viscosity, respectively, and R the diameter of the obstacle. When increasing the Reynolds number, laminar flow, stationary vortices, BOnardVon KAErmAEn streets of moving vortices and fully turbulent regimes are successively observed in the wake of the obstacle 1 .

Effects of magnetic field on nanofluid forced convection in a partially heated microchannel

Abstract: This paper numerically examines the laminar forced convection of a water–Al 2 O 3 nanofluid flowing through a horizontal microchannel. The middle section of the microchannel is heated with a constant and uniform heat flux. The middle section is also influenced by a transverse magnetic field with a uniform strength. The effects of pertinent parameters such as the Reynolds number (0≤ Re ≤1000), the solid volume fraction (0≤ ϕ ≤0.04) and the Hartmann number (0≤ Ha ≤100) on the flow and temperature fields and the heat transfer performance of the microchannel are examined against numerical predictions. The results show that the microchannel performs better heat transfers at higher values of the Reynolds and Hartmann numbers. For all values of the Reynolds and Hartmann numbers considered in this study, the average Nusselt number on the middle section surface of the microchannel increases as the solid volume fraction increases. The rate of this increase is considerably more at higher values of the Reynolds number and at lower values of the Hartmann number.

Low-Mach-number turbulence in interstellar gas revealed by radio polarization gradients

Abstract: The interstellar medium of the Milky Way is multiphase1, magnetized2 and turbulent3. Turbulence in the interstellar medium produces a global cascade of random gas motions, spanning scales ranging from 100 parsecs to 1,000 kilometres (ref. 4). Fundamental parameters of interstellar turbulence such as the sonic Mach number (the speed of sound) have been difficult to determine, because observations have lacked the sensitivity and resolution to image the small-scale structure associated with turbulent motion5, 6, 7. Observations of linear polarization and Faraday rotation in radio emission from the Milky Way have identified unusual polarized structures that often have no counterparts in the total radiation intensity or at other wavelengths8, 9, 10, 11, 12, and whose physical significance has been unclear13, 14, 15. Here we report that the gradient of the Stokes vector (Q, U), where Q and U are parameters describing the polarization state of radiation, provides an image of magnetized turbulence in diffuse, ionized gas, manifested as a complex filamentary web of discontinuities in gas density and magnetic field. Through comparison with simulations, we demonstrate that turbulence in the warm, ionized medium has a relatively low sonic Mach number, Ms ≲ 2. The development of statistical tools for the analysis of polarization gradients will allow accurate determinations of the Mach number, Reynolds number and magnetic field strength in interstellar turbulence over a wide range of conditions.

Fluid mixing from viscous fingering.

Abstract: Mixing efficiency at low Reynolds numbers can be enhanced by exploiting hydrodynamic instabilities that induce heterogeneity and disorder in the flow. The unstable displacement of fluids with different viscosities, or viscous fingering, provides a powerful mechanism to increase fluid-fluid interfacial area and enhance mixing. Here we describe the dissipative structure of miscible viscous fingering, and propose a two-equation model for the scalar variance and its dissipation rate. Our analysis predicts the optimum range of viscosity contrasts that, for a given P\'eclet number, maximizes interfacial area and minimizes mixing time. In the spirit of turbulence modeling, the proposed two-equation model permits upscaling dissipation due to fingering at unresolved scales.

Large-eddy simulation of the flow and acoustic fields of a Reynolds number 10 5 subsonic jet with tripped exit boundary layers

Abstract: Large-eddy simulations (LESs) of isothermal round jets at a Mach number of 09 and a diameter-based Reynolds number ReD of 105 originating from a pipe are performed using low-dissipation schemes in combination with relaxation filtering The aim is to carefully examine the capability of LES to compute the flow and acoustic fields of initially nominally turbulent jets As in experiments on laboratory-scale jets, the boundary layers inside the pipe are tripped in order to obtain laminar mean exit velocity profiles with high perturbation levels At the pipe outlet, their momentum thickness is δθ(0)=0018 times the jet radius, yielding a Reynolds number Reθ=900, and peak turbulence intensities are around 9% of the jet velocity Two methods of boundary-layer tripping and five grids are considered The results are found to vary negligibly with the tripping procedure but appreciably with the grid resolution Based on analyses of the LES quality and on comparisons with measurements at high Reynolds numbers, fine d

Experimental measurement of large-scale three-dimensional structures in a turbulent boundary layer. Part 1. Vortex packets

Abstract: Experimental measurements of the three-dimensional (3D) velocity field in a moderate Reynolds number zero pressure-gradient boundary layer are presented. The measurements are analysed to produce 3D correlations and conditional averaging techniques are used to further elucidate the underlying structure. The results show clear evidence of vortex-packet-type structures and shed new light on the detailed 3D structure of such packets in a real zero pressure-gradient boundary layer.

Numerical Simulation of Delft-Jet-in-Hot-Coflow (DJHC) Flames Using the Eddy Dissipation Concept Model for Turbulence–Chemistry Interaction

Abstract: In this paper, we report results of a numerical investigation of turbulent natural gas combustion for a jet in a coflow of lean combustion products in the Delft-Jet-in-Hot-Coflow (DJHC) burner which emulates MILD (Moderate and Intense Low Oxygen Dilution) combustion behavior. The focus is on assessing the performance of the Eddy Dissipation Concept (EDC) model in combination with two-equation turbulence models and chemical kinetic schemes for about 20 species (Correa mechanism and DRM19 mechanism) by comparing predictions with experimental measurements.We study two different flame conditions corresponding to two different oxygen levels (7.6% and 10.9% by mass) in the hot coflow, and for two jet Reynolds number (Re = 4,100 and Re = 8,800). The mean velocity and turbulent kinetic energy predicted by different turbulence models are in good agreement with data without exhibiting large differences among the model predictions. The realizable k-? model exhibits better performance in the prediction of entrainment. The EDC combustion model predicts too early ignition leading to a peak in the radial mean temperature profile at too low axial distance. However the model correctly predicts the experimentally observed decreasing trend of lift-off height with jet Reynolds number. A detailed analysis of the mean reaction rate of the EDC model is made and as possible cause for the deviations between model predictions and experiments a low turbulent Reynolds number effect is identified. Using modified EDC model constants prediction of too early ignition can be avoided. The results are weakly sensitive to the sub-model for laminar viscosity and laminar diffusion fluxes.

Parametric numerical study of flow and heat transfer in microchannels with wavy walls

Abstract: Wavy channels were investigated in this paper as a passive scheme to improve the heat transfer performance of laminar fluid flow as applied to microchannel heat sinks. Parametric study of three-dimensional laminar fluid flow and heat transfer characteristics in microsized wavy channels was performed by varying the wavy feature amplitude, wavelength, and aspect ratio for different Reynolds numbers between 50 and 150. Two different types of wavy channels were considered and their thermal performance for a constant heat flux of 47 W/cm 2 was compared. Based on the comparison with straight channels, it was found that wavy channels can provide improved overall thermal performance. In addition, it was observed that wavy channels with a configuration in which crests and troughs face each other alternately (serpentine channels) were found to show an edge in thermal performance over the configuration where crests and troughs directly face each other. The best configuration considered in this paper was found to provide an improvement of up to 55% in the overall performance compared to microchannels with straight walls and hence are attractive candidates for cooling of future high heat flux electronics.

Drag reduction in turbulent boundary layers by in-plane wall motion.

Abstract: Drag-reduction techniques capable of reducing the level of turbulent friction through wall-parallel movement of the wall are described, with special emphasis placed on spanwise movement. The discussion is confined to active open-loop control strategies, although feedback control is briefly mentioned with regard to peculiarities of spanwise sensing and/or actuation. Theoretical considerations are first given to explain why spanwise motion is expected to be particularly effective in skin-friction drag reduction. A review of the spanwise oscillating-wall technique is given next, with discussion of recent results and prospects. Last, waves of spanwise velocity are addressed, either spanwise- or streamwise-travelling. The latter include the oscillating wall as a special case. The generalized Stokes layer--i.e. the laminar, transverse oscillating boundary layer that develops under the action of the streamwise-travelling waves--is described, and its importance in determining turbulent drag reduction discussed. Finally, open issues like energetic efficiency and its dependence on Reynolds number are addressed.

Torque Scaling in Turbulent Taylor-Couette Flow with Co- and Counterrotating Cylinders

Abstract: We analyze the global transport properties of turbulent Taylor-Couette flow in the strongly turbulent regime for independently rotating outer and inner cylinders, reaching Reynolds numbers of the inner and outer cylinders of Rei=2×106 and Reo=±1.4×106, respectively. For all Rei, Reo, the dimensionless torque G scales as a function of the Taylor number Ta (which is proportional to the square of the difference between the angular velocities of the inner and outer cylinders) with a universal effective scaling law G∝Ta0.88, corresponding to Nuω∝Ta0.38 for the Nusselt number characterizing the angular velocity transport between the inner and outer cylinders. The exponent 0.38 corresponds to the ultimate regime scaling for the analogous Rayleigh-Benard system. The transport is most efficient for the counterrotating case along the diagonal in phase space with ωo≈-0.4ωi.