Motion In Turbulent Flow Research Articles

Coherent structures, uniform momentum zones and the streamwise energy spectrum in wall-bounded turbulent flows

Large-scale motions (LSMs) in wall-bounded turbulent flows have well-characterised instantaneous structural features (Kovasznay et al., J. Fluid Mech., vol. 41 (2), 1970, pp. 283–325; Meinhart & Adrian, Phys. Fluids, vol. 7 (4), 1995, pp. 694–696) and a known spectral signature (Monty et al., J. Fluid Mech., vol. 632, 2009, pp. 431–442). This work aims to connect these previous observations through the development and analysis of a representative model for LSMs. The model is constructed to be consistent with the streamwise energy spectrum (Monty et al. 2009) and amplification characteristics of the Navier–Stokes equations (McKeon & Sharma, J. Fluid Mech., vol. 658, 2010, pp. 336–382), and is found to naturally recreate characteristics of instantaneous turbulent structures, including a bulge shape (Kovasznay et al. 1970) and the presence of uniform momentum zones (Meinhart & Adrian 1995) in the streamwise velocity field. The observed structural similarity between the LSM representative model and instantaneous experimental data supports the use of travelling wave models to connect statistical and instantaneous descriptions of coherent structures, and clarifies a simple general equivalency between symmetry in a Reynolds-decomposed velocity field and asymmetry in the laboratory frame.

Self-sustaining processes at all scales in wall-bounded turbulent shear flows

We collect and discuss the results of our recent studies which show evidence of the existence of a whole family of self-sustaining motions in wall-bounded turbulent shear flows with scales ranging from those of buffer-layer streaks to those of large-scale and very-large-scale motions in the outer layer. The statistical and dynamical features of this family of self-sustaining motions, which are associated with streaks and quasi-streamwise vortices, are consistent with those of Townsend's attached eddies. Motions at each relevant scale are able to sustain themselves in the absence of forcing from larger- or smaller-scale motions by extracting energy from the mean flow via a coherent lift-up effect. The coherent self-sustaining process is embedded in a set of invariant solutions of the filtered Navier-Stokes equations which take into full account the Reynolds stresses associated with the residual smaller-scale motions.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.

On the genesis and evolution of barchan dunes: morphodynamics

Barchan dunes are crescent-shaped formations of sand that can dominate both desert and subaqueous landscapes when the supply of sand is scarce. Because of the complexity and scale of the underlying phenomena, the mechanisms governing the entire process from the genesis to the long-term evolution of barchan fields still remain to be better understood. Herein, we attempt to present a description of this process in a subaqueous environment by employing a large-eddy simulation approach that couples turbulent flow and sand-bed morphodynamics. We show that the seeds of the emergent structure in barchan fields are random turbulent flow motions near the initially flat bed. We also provide high-resolution insights into phenomena such as barchan migration and merging, and show how transverse sand waves are formed and migrate over the barchan horns. Furthermore, the transverse sand waves over the barchan horns are shown to be the seeds of the newly born barchans at the end points of the two horns of a barchan through the process known as calving. To show this, we examine the celerity, wavelength and amplitude of the transverse sand waves over the barchan as they approach the end of its horn. The celerity and wavelength of these transverse sand waves are shown to be the defining factors in determining the frequency of the calving process. The amplitude of the newly born barchans (through calving) is also shown to be associated with the amplitude of the transverse waves near the end of the horn. The simulation data also show that the wavelength of the newly born barchans (the distance between individual dunes) is closely related to that of the transverse sand waves over their maternal barchan. Finally, we use the simulation results to discuss past conclusions derived from theory, conventional models and field observations.

Space-Time Correlations and Dynamic Coupling in Turbulent Flows

Space-time correlation is a staple method for investigating the dynamic coupling of spatial and temporal scales of motion in turbulent flows. In this article, we review the space-time correlation models in both the Eulerian and Lagrangian frames of reference, which include the random sweeping and local straining models for isotropic and homogeneous turbulence, Taylor's frozen-flow model and the elliptic approximation model for turbulent shear flows, and the linear-wave propagation model and swept-wave model for compressible turbulence. We then focus on how space-time correlations are used to develop time-accurate turbulence models for the large-eddy simulation of turbulence-generated noise and particle-laden turbulence. We briefly discuss their applications to two-point closures for Kolmogorov's universal scaling of energy spectra and to the reconstruction of space-time energy spectra from a subset of spatial and temporal signals in experimental measurements. Finally, we summarize the current understanding of space-time correlations and conclude with future issues for the field.

Exact Invariant Solutions for Coherent Turbulent Motions in Couette and Poiseuille Flows

The dynamical systems approach recently applied to understand subcritical transitions in wall-bounded shear flows is combined with the use of large-eddy simulations to investigate the nature of large-scale coherent motions in turbulent Couette and Poiseuille flows. Exact invariant solutions of the filtered Navier-Stokes (LES) equations are computed by using the Smagorinsky model to parametrize small-scale motions. These solutions can be continued into exact solutions of the Navier-Stokes equations, therefore providing a bridge between coherent large-scale motions in wall-bounded fully developed turbulent flows and invariant solutions appearing in transitional flows.

Interaction between inner and outer layer in drag‐reduced turbulent flows

AbstractEffects of wall‐based skin‐friction drag reduction strategies on the statistical properties of large‐scale motions in moderate Reynolds number turbulent flows have been investigated by exploiting Direct Numerical Simulation of turbulent channels. To educe large scales, a new efficient parallel distributed memory algorithm has been implemented which delivers data‐driven modes of increasing characteristic lengthscales: the Fast and Adaptive Bidimensional Empirical Mode Decomposition (FABEMD). The influence of wall‐based skin friction reduction on large scales is studied by comparing single point statistics, such as r.m.s. fluctuations, and two‐point statistics, as cross‐correlation functions in controlled and uncontrolled channel flow fields at constant friction Reynolds number. The traditional way of observing large‐scale footprinting at the wall, as cross‐correlation of the streamwise velocity components at different wall distances, has been found to be unreliable when comparing drag‐reduced flows, due to the arbitrary choice of a reference plane in the logarithmic layer. A more sound way of observing the footprinting via the correlation of the streamwise velocity with the friction velocity is addressed and shows an increase of the footprinting in drag‐reduced flows. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Computational bridge aeroelasticity using turbulence models

AbstractLong‐span bridges may collapse due to the interaction of turbulent wind flow and structural motion. The investigation of the fluid‐structure interaction requires appropriate models for the structure, the wind flow and the coupling between them. Modeling and simulation of the wind flow is still a challenging aspect. The aim of this paper is to show the application of Reynolds Averaged Navier‐Stokes (RANS) equations with a turbulence model in the context of bridge aeroelasticity. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-sustaining process of minimal attached eddies in turbulent channel flow

It has been recently shown that the energy-containing motions (i.e. coherent structures) in turbulent channel flow exist in the form of Townsend’s attached eddies by a numerical experiment which simulates the energy-containing motions only at a prescribed spanwise length scale using their self-sustaining nature (Hwang, J. Fluid Mech., vol. 767, 2015, pp. 254–289). In the present study, a detailed investigation of the self-sustaining process of the energy-containing motions at each spanwise length scale (i.e. the attached eddies) in the logarithmic and outer regions is carried out with an emphasis on its relevance to ‘bursting’, which refers to an energetic temporal oscillation of the motions (Flores & Jiménez, Phys. Fluids, vol. 22, 2010, 071704). It is shown that the attached eddies in the logarithmic and outer regions, composed of streaks and quasi-streamwise vortical structures, bear the self-sustaining process remarkably similar to that in the near-wall region: i.e. the streaks are significantly amplified by the quasi-streamwise vortices via the lift-up effect; the amplified streaks subsequently undergo a ‘rapid streamwise meandering motion’, reminiscent of streak instability or transient growth, which eventually results in breakdown of the streaks and regeneration of new quasi-streamwise vortices. For the attached eddies at a given spanwise length scale ${\it\lambda}_{z}$ between ${\it\lambda}_{z}^{+}\simeq 100$ and ${\it\lambda}_{z}\simeq 1.5h$, the single turnover time period of the self-sustaining process is found to be $Tu_{{\it\tau}}/{\it\lambda}_{z}\simeq 2$ ($u_{{\it\tau}}$ is the friction velocity), which corresponds well to the time scale of the bursting. Two additional numerical experiments, designed to artificially suppress the lift-up effect and the streak meandering motions, respectively, reveal that these processes are essential ingredients of the self-sustaining process of the attached eddies in the logarithmic and outer regions, consistent with several previous theoretical studies. It is also shown that the artificial suppression of the lift-up effect of the attached eddies in the logarithmic and outer regions leads to substantial amounts of turbulent skin-friction reduction.

Single-Particle Motion and Vortex Stretching in Three-Dimensional Turbulent Flows.

Three-dimensional turbulent flows are characterized by a flux of energy from large to small scales, which breaks the time reversal symmetry. The motion of tracer particles, which tend to lose energy faster than they gain it, is also irreversible. Here, we connect the time irreversibility in the motion of single tracers with vortex stretching and thus with the generation of the smallest scales.

Self-similarity of the large-scale motions in turbulent pipe flow

Townsend’s attached eddy hypothesis assumes the existence of a set of energetic and geometrically self-similar eddies in the logarithmic layer in wall-bounded turbulent flows, which can be scaled with their distance to the wall. To examine the possible self-similarity of the energetic eddies in fully developed turbulent pipe flow, we performed stereo particle image velocimetry measurements together with a proper orthogonal decomposition analysis. For two Reynolds numbers, $Re_{{\it\tau}}=1330$ and 2460, the resulting modes/eddies were shown to exhibit self-similar behaviour for eddies with wall-normal length scales spanning a decade. This single length scale provides a complete description of the cross-sectional shape of the self-similar eddies.

On the self-sustained nature of large-scale motions in turbulent Couette flow

Large-scale motions in wall-bounded turbulent flows are frequently interpreted as resulting from an aggregation process of smaller-scale structures. Here, we explore the alternative possibility that such large-scale motions are themselves self-sustained and do not draw their energy from smaller-scale turbulent motions activated in buffer layers. To this end, it is first shown that large-scale motions in turbulent Couette flow at $Re=2150$ self-sustain, even when active processes at smaller scales are artificially quenched by increasing the Smagorinsky constant $C_{s}$ in large-eddy simulations (LES). These results are in agreement with earlier results on pressure-driven turbulent channel flows. We further investigate the nature of the large-scale coherent motions by computing upper- and lower-branch nonlinear steady solutions of the filtered (LES) equations with a Newton–Krylov solver, and find that they are connected by a saddle–node bifurcation at large values of $C_{s}$. Upper-branch solutions for the filtered large-scale motions are computed for Reynolds numbers up to $Re=2187$ using specific paths in the $Re{-}C_{s}$ parameter plane and compared to large-scale coherent motions. Continuation to $C_{s}=0$ reveals that these large-scale steady solutions of the filtered equations are connected to the Nagata–Clever–Busse–Waleffe branch of steady solutions of the Navier–Stokes equations. In contrast, we find it impossible to connect the latter to buffer-layer motions through a continuation to higher Reynolds numbers in minimal flow units.

The evolution of large-scale motions in turbulent pipe flow

A dual-plane snapshot proper orthogonal decomposition (POD) analysis of turbulent pipe flow at a Reynolds number of 104 000 is presented. The high-speed particle image velocimetry data were simultaneously acquired in two planes, a cross-stream plane (2D–3C) and a streamwise plane (2D–2C) on the pipe centreline. The cross-stream plane analysis revealed large structures with a spatio-temporal extent of $1{-}2R$, where $R$ is the pipe radius. The temporal evolution of these large-scale structures is examined using the time-shifted correlation of the cross-stream snapshot POD coefficients, identifying the low-energy intermediate modes responsible for the transition between the large-scale modes. By conditionally averaging based on the occurrence/intensity of a given cross-stream snapshot POD mode, a complex structure consisting of wall-attached and -detached large-scale structures is shown to be associated with the most energetic modes. There is a pseudo-alignment of these large structures, which together create structures with a spatio-temporal extent of approximately $6R$, which appears to explain the formation of the very-large-scale motions previously observed in pipe flow.

Turbulent smoke flow in evacuation staircases during a high-rise residential building fire

Purpose – The purpose of this paper is to study the behavior of smoke flow in a typical high-rise residential building fire in six common smoke control systems. Design/methodology/approach – The pressure, temperature and CO2 concentration were used to trace the motion of turbulent smoke flow through CFD. Findings – It is found that the hot smoke could rise up and spread into the indoor space on the upper floors through the staircase. When the pressure in the evacuation staircase is higher, it would be more difficult for the smoke to enter the staircase and transport vertically. On the other hand, the smoke would soon transport to the indoor space on the upper floors horizontally. During this process, the smoke shows a more disorder horizontal transport under the sole effect of thermal buoyancy than the co-existence of thermal buoyancy and the air inlet. Research limitations/implications – Because of the chosen research approach, the research results may need to be tested by further experiments. Practical implications – The paper includes implications for the design of smoke control systems and evacuation in a building fire. Originality/value – This paper fulfils an identified need to study the behavior of smoke in a fire and optimize the design of smoke control systems.

Breaking bore: Physical observations of roller characteristics

In an estuary, a tidal bore is a hydraulic jump in translation generated at the leading edge of the flood tide during the early flood tide under spring macrotidal conditions in a narrow funnelled channel. After formation, the bore is traditionally analysed as a hydraulic jump in translation and its leading edge is characterised by a breaking roller for Fr1>1.3–1.5. Herein new unsteady experiments were conducted to investigate in details the upstream propagation of breaking bore roller. The toe perimeter shape fluctuated rapidly with transverse distance and time. A characteristic transverse wave length of the toe perimeter was observed. Both the standard deviation of toe perimeter location and characteristic transverse wave length were comparable to field observations. The celerity of the roller toe fluctuated rapidly with time and space. The instantaneous longitudinal profile of the roller free-surface showed significant temporal and spatial fluctuations. Although the bore propagation may be analysed in an integral form in first approximation, the rapid fluctuations in roller toe perimeter and free-surface profiles indicated a strongly three-dimensional turbulent flow motion.

Comparison of large- and very-large-scale motions in turbulent pipe and channel flows

Statistical measures of turbulence intensities in turbulent pipe and channel flows at a friction Reynolds number of Reτ ≈ 930 were explored by a population of large-scale motions (LSMs) and very-large-scale motions (VLSMs). Although the statistical measures characterizing these internal turbulent flows were similar in the near-wall region, the extents of the mean streamwise velocities and cross-stream components of the turbulence intensities differed in the core region. The population density of VLSMs/LSMs decreased/increased significantly in the core region of the pipe flow. The survival time of VLSMs of the pipe flow was shorter than that of the channel flow. The area fractions of the VLSMs displayed similar trends to the population density. The wall-normal and spanwise turbulence intensities in the pipe flow increased in the core regions due to the high-speed large-scale structures and associated motions above the structures. The large-scale structures increased the streamwise intensity and the Reynolds shear stress in the pipe and channel flows, whereas the effective streamwise intensities and the Reynolds shear stress were equivalent in both flows.

Application of modified eddy dissipation concept with large eddy simulation for numerical investigation of internal combustion engines

Computational methodology of large eddy simulation (LES) is tested in a real internal combustion engine geometry. Premixed turbulent combustion and the behavior of flame propagation are studied and compared with different LES and combustion process models. KIVA-3V, an open source Fortran code, is modified to implement large-eddy simulation technique based on one-equation sub-grid scale (SGS) turbulent kinetic energy and Smagorinsky models. For combustion process, improved turbulent-mixing combustion models based on eddy-dissipation model (EDM) and modified eddy-dissipation concept (EDC) are implemented and studied in case of using LES. The models with modified combustion processes of EDM and EDC are validated using experimental data of premixed combustion of laboratory burner with application of converging optimum and desired combustion model constants. Also experimental in-cylinder pressure data are used to validate the numerical CFD results of engine geometry. The physics of flow structure, flame propagation, heat energy released and fuel mass consumption during combustion are studied and compared by LES models. It is observed that large-scale of unsteady turbulent flow motions are developed before spark ignition that makes different behavior of flame front shape in combustion process with different turbulent models. Also numerical simulations indicate that different LES turbulence models may not capture the same flame propagation structure, whereas LES dynamic SGS viscosity models in comparison with standard constant models, almost capture the same flame front shape and propagation structure in the combustion chamber.

A Modeling Study on Particle Dispersion in Wall-Bounded Turbulent Flows

AbstractThree physical mechanisms which may affect dispersion of particle’s motion in wall-bounded turbulent flows, including the effects of turbulence, wall roughness in particle-wall collisions, and inter-particle collisions, are numerically investigated in this study. Parametric studies with different wall roughness extents and with different mass loading ratios of particles are performed in fully developed channel flows with the Eulerian-Lagrangian approach. A low-Reynolds-numberk–εturbulence model is applied for the solution of the carrier-flow field, while the deterministic Lagrangian method together with binary-collision hard-sphere model is applied for the solution of particle motion. It is shown that the mechanism of inter-particle collisions should be taken into account in the modeling except for the flows laden with sufficiently low mass loading ratios of particles. Influences of wall roughness on particle dispersion due to particle-wall collisions are found to be considerable in the bounded particle–laden flow. Since the investigated particles are associated with large Stokes numbers, i.e., larger thanO(1), in the test problem, the effects of turbulence on particle dispersion are much less considerable, as expected, in comparison with another two physical mechanisms investigated in the study.

The energetic motions in turbulent pipe flow

Snapshot and classical proper orthogonal decomposition (POD) are used to examine the large-scale, energetic motions in fully developed turbulent pipe flow at ReD = 47,000 and 93,000. The snapshot POD modes come in pairs, representing the same azimuthal mode number but with a simple phase shift. The first 10 snapshot POD modes, associated with the very large scale motions (VLSMs), contribute 43% of the average Reynolds shear stress, and for first 80 modes u′ and v′ are anti-correlated so that they all contribute to positive shear stress events. The attached motions are contained in the lower order modes, and detached motions do not appear until snapshot POD mode numbers ≥15. We find that snapshot POD can introduce mode mixing, which is avoided in classical POD. Classical POD also gives frequency information, confirming that the low order modes capture well the behavior of the very large scale motions.

A scanning PIV method for fine-scale turbulence measurements

A hybrid technique is presented that combines scanning PIV with tomographic reconstruction to make spatially and temporally resolved measurements of the fine-scale motions in turbulent flows. The technique uses one or two high-speed cameras to record particle images as a laser sheet is rapidly traversed across a measurement volume. This is combined with a fast method for tomographic reconstruction of the particle field for use in conjunction with PIV cross-correlation. The method was tested numerically using DNS data and with experiments in a large mixing tank that produces axisymmetric homogeneous turbulence at \(R_\lambda \simeq 219\). A parametric investigation identifies the important parameters for a scanning PIV set-up and provides guidance to the interested experimentalist in achieving the best accuracy. Optimal sheet spacings and thicknesses are reported, and it was found that accurate results could be obtained at quite low scanning speeds. The two-camera method is the most robust to noise, permitting accurate measurements of the velocity gradients and direct determination of the dissipation rate.

Influence of flow and ignition fluctuations on cycle-to-cycle variations in early flame kernel growth

Engine Cycle-to-Cycle Variations (CCVs) constitute a restriction for numerous concepts aimed at improving fuel consumption in spark-ignited engines. In consequence, engine CCV modeling has become an active research topic. Nevertheless, few fundamental studies have thus far examined the individual influences of the key parameters. In the present study, we performed 2D Direct Numerical Simulations (DNSs) of early flame kernel growth with semi-detailed chemistry solving to identify the controlling parameters of heat release fluctuations that are relevant to engine CCVs. Our objective was to assess how certain turbulence and flame parameters – namely turbulent flow motion pattern, turbulence intensity, integral length scale, mixture strength and initial kernel size – influence combustion variability. Two extreme mixture conditions were selected (stable operation at stoichiometry and unstable operation at the lean flammability limit), and computation conditions were derived from actual low load engine operating points. Results provide a classification of the key parameters, indicating initial kernel size and turbulent flow motion pattern as the main contributors. Additionally, DNSs highlighted the crucial role of mixture strength in the occurrence of local flame quenching and in the related amplification of heat release fluctuations.