Shear Flow Phenomena Research Articles

Analysis of wall-pressure fluctuation sources from direct numerical simulation of turbulent channel flow

The sources of wall-pressure fluctuations in turbulent channel flow are studied using a novel framework. The wall-pressure power spectral density (PSD) $(\phi_{pp}(\omega))$ is expressed as an integrated contribution from all wall-parallel plane pairs, $\phi_{pp}(\omega)=\int_{-\delta}^{+\delta}\int_{-\delta}^{+\delta}\Gamma(r,s,\omega)\,\mathrm{dr}\,\mathrm{ds}$, using the Green's function. Here, $\Gamma(r,s,\omega)$ is termed the net source cross spectral density (CSD) between two wall-parallel planes, $y=r$ and $y=s$ and $\delta$ is the half channel height. Direct Numerical Simulation (DNS) data at friction Reynolds number of $180$ and $400$ are used to compute $\Gamma(r,s,\omega)$. Analysis of the net source CSD, $\Gamma(r,s,\omega)$ reveals that the location of dominant sources responsible for the premultiplied peak in the power spectra at $\omega^+\approx 0.35$ and the wavenumber spectra at $\lambda^+\approx 200$ is in the buffer layer at $y^+\approx 16.5$ and $18.4$ for $Re_{\tau}=180$ and $400$, respectively. The contribution from a wall-parallel plane (located at distance $y^+$ from the wall) to wall-pressure PSD is log-normal in $y^+$ for $\omega^+>0.35$. A dominant inner-overlap region interaction of the sources is observed at low frequencies. Further, the decorrelated features of the wall-pressure fluctuation sources are analyzed using spectral Proper Orthogonal Decomposition (POD). We require the modes to be orthogonal in an inner product with a symmetric positive definite kernel. Spectral POD supports the case that the net source is composed of two decorrelated components - active (dominant mode) and inactive (remaining modes). The structure represented by the dominant POD mode at the premultiplied wall-pressure PSD peak inclines in the downstream direction. At the low-frequency linear PSD peak, the dominant mode resembles a large scale vertical pattern.

Instability by localized disturbances in critical region in a precessing sphere

The flow state in a precessing sphere of radius a is characterized by the spin angular velocity Ωs, the precession angular velocity Ωp and the kinematic viscosity ν of fluid. The spin and precession axes are assumed to be orthogonal to each other. When two non-dimensional parameters, e.g. the Reynolds number Re = a2Ωs/ν and the Poincaré number Po = Ωp/Ωs are in some range of values, the flow is believed to approach a steady state even if it starts from an arbitrary initial condition. Such a stable region for the steady flow in the whole parameter space (Re, Po) is, however, not known yet. Here, we investigate, by the linear stability analysis of disturbances localized in a critical region, the boundary of the stable region of the steady flow in the strong spin and weak precession limit. It is found that the boundary curve takes the power form, asymptotically for . This asymptotic form agrees well with the corresponding experimental observation (Goto et al 2011 Proc. 7th Int. Symp. on Turbulence and Shear Flow Phenomena) as well as the results by numerical simulation (Lin et al 2015 Phys. Fluids 27 046611). The critical mode is composed of two types of sinusoidal waves along the critical circle: one travels around the spin axis with phase velocity faster than Ωs and the other slower.

Non-reciprocal scattering in shear flow.

This work presents a study of scattering phenomena in shear flows and its application to impedance walls. These flows are described by a dimensionless shear layer thickness and a mean Mach number. Both transmission through a given shear layer and reflection on an acoustic treatment are studied. This paper shows that the dimensionless thickness of the shear layer may be an interesting tool to reach perfect absorption or large lateral displacement of a Gaussian beam.

Iterative PIV interrogation for complex wall-bounded flows

The strategy of the iterative PIV algorithm is discussed with the aim of investigating transitional wall-bounded flows. It is proposed to start the algorithm with a coarse prediction of the large-scale tangential velocity gradient to eliminate its negative effect. A window correlation with image deformation technique is then iteratively conducted to capture the fluctuating flow structures. Synthetic particles are created based on the DNS result of a typical hypersonic transitional boundary layers to evaluate the performance of the algorithm. Effects of several parameters, including the mean tangential displacement gradient, particle diameter, particle density and background noise on the level on the evaluation, are systematically investigated. Four methods to initially predict the near-wall flow are compared, including the directly-used window correlation with image deformation (WC) (Scarano 2002 Meas. Sci. Technol. 13 R1–19), the WC method with static particles added in the solid region (Zhu et al 2013 Meas. Sci. Technol. 24 125302), the image gradiometry method (Nguyen et al 2009 Proc. 6th Int. Symp. on Turbulence and Shear Flow Phenomena (Seoul, Korea, 2009)) and the newly-proposed joint-translation-shear correlation method method. The results show that, compared with the other three approaches, the performance of the new method is greatly enhanced for evaluating the dynamical flow properties, even if the displacement gradient is very high. Finally, experimental data on artificially disturbed boundary layers from a low-speed and a high-Mach-number wind tunnel are used to evaluate the new method, which is able to evaluate the mean and instantaneous velocity fields in both cases.

Ninth International Symposium on Turbulence and Shear Flow Phenomena

The International Biennial Symposia on Turbulence and Shear Flow Phenomena are devoted to fundamental and applied research in turbulence, as well as in complex laminar and transitional shear flows....

Transient elasticity and polymeric fluids: Small-amplitude deformations.

Transient elasticity (TE) is a concept useful for a systematic generalization of viscoelasticity. Due to its thermodynamic consistency, it naturally leads to a simple description of non-Newtonian effects displayed by polymeric fluids, granular media, and other soft matter. We employ a continuum-mechanical theory that is derived from TE and tailored to polymeric fluids, showing how it captures a surprisingly large number of phenomena in shear and elongational flows, including stationary, oscillatory, and transient ones, as well as the flow down an inclined channel. Even the Weissenberg effect is well accounted for. This theory is applicable for small- as well as large-amplitude deformations. We concentrate on the former in the present article, leaving the latter to a companion article.

Foreword to special issue: Eighth International Symposium on Turbulence and Shear Flow Phenomena (TSFP-8)

Click to increase image sizeClick to decrease image size AcknowledgementsWe gratefully acknowledge the efforts of all authors in preparing high-quality papers. Special thanks also go to the referees for their valuable review work which makes it possible that this document of current research in the field of turbulence and shear flow phenomena appears before the review process for TSFP-9 (http://tsfp9.org/) will be completed.Disclosure statementNo potential conflict of interest was reported by the authors.

Effect of Hydrophobic Residues on Interfacial Fibrillization Kinetics

The presence of amyloid fibril plaques in the brain is strongly associated with several neurological disorders. Prediction of fibril formation is a daunting challenge because of the large number of routes to formation, the sensitivity of protein structures to various stresses, differences in fibrillization propensities between proteins, and the vast number of possible amyloid structures. Bovine insulin differs from human by replacement of just three amino acids, yet this difference is sufficient to result in vastly altered fibrillization kinetics. One possibility for this large deviation may be the hydrophobicity of the three substituted residues. Quantification of these differences can provide clues into how and why fluid flows and interfaces influence protein stability and induce aggregation. The exact roles of shear flow, mixing, and interfacial phenomena on fibrillization are difficult to distinguish because these effects have been coupled in previous experiments. In this work, precision flow devices are used to decouple such effects. Mixing and shear flow have been separated using a combination of a rotating Couette flow and solid body rotation. Interfacial effects are determined by kinetic differences between flows with and without the air-water interface and by using a knife-edge viscometer to investigate the effects of interfacial shear. Together, these flow geometries and boundary conditions can be used to distinguish the individual effects of different fluid phenomena. Comparing flow and interfacial effects on fibrillization time between human and bovine insulin facilitates determination of the roles amino residues play in destabilization and aggregation. These findings have important implications for protein stability at interfaces and within biophysically relevant flow fields.

Two-scale nonlocal shear rate formulation of Bingham plastic fluid

This paper deals with a phenomenological model based on a nonlocal Bingham constitutive law. This nonlocal viscoplastic law accounts for possible microstructured effects of a heterogeneous fluid. Thermodynamic arguments are presented for the justification of this two-scale nonlocal shear rate law, which can be specialized either as a purely nonlocal or a gradient-type Bingham law. Plane shearing and uniaxial flow assumptions between parallel walls are basic and tractable hypotheses for a first investigation of nonlocal and gradient shear rate law. Analytical solutions are presented for the plane Poiseuille stationary flow inside a straight smooth channel of infinite width. Two higher boundary conditions at the walls are assumed for this so-called micromorphic medium, a static higher-order boundary condition and a kinematic one. A boundary layer phenomenon can be observed for such nonlocal problems. Parametric studies show the key role of the characteristic lengths of the nonlocal model on the shear flow phenomena. Consequently, the heterogeneous nature of microstructured yield-stress fluids may significantly affect velocity profiles, maximum velocity and total flow rate, which have been recently reported in literature.

Surfactant destabilization and non-linear phenomena in two-fluid shear flows at small Reynolds numbers

The flow of two superposed fluids in a channel in the presence of an insoluble surfactant is studied. Asymptotic analysis when one of the layers is thin yields a system of coupled weakly non-linear evolution equations for the film thickness and the local surface surfactant concentration. Film and main flow dynamics are coupled through a non-local term, and in the absence of surfactants the model is non-linearly stable with trivial large time solutions. Instability arises due to the presence of surfactants and the pseudo-differential non-linear system is solved numerically by implementing accurate linearly implicit methods. Extensive numerical experiments reveal that the dynamics are mostly organized into travelling or time-periodic travelling wave pulses, but spatiotemporal chaos is also supported when the length of the system is sufficiently large.

FRACTAL GEOMETRY OF THE WAKE SHED BY A FLAPPING FILAMENT IN FLOWING SOAP-FILM

In this study, we illustrate the fractal nature of the wake shed by a periodically flapping filament. Such wake structure is a combination of primary vortex shedding resulting in the von Kármán vortex street, a series of concentrated vortex dipoles formed when the trailing edges of filaments reach their maximum amplitudes and small eddies form along the shear layer connected with the concentrated vortices due to the shear layer instability. The vortex dynamics of the flapping filament are visualized and imaged experimentally using a soap-film flow tunnel with a high-speed camera and a low pressure sodium lamp as a light source. The wake fractal geometry is measured using the standard box-counting method and it is shown that the fractal dimension of the soap pattern boundaries in the wake is D = 1.38 ± 0.05, which agrees well with those measured for fully developed turbulences and other shear flow phenomena. The invariant of the fractality in the wake induced by the flapping filament thus provides another illustration of the geometrical self-similarity and nonlinear dynamics of chaotic fluid flows.

Sixth International Symposium on Turbulence and Shear Flow Phenomena

Click to increase image sizeClick to decrease image size Acknowledgments Guest Editors, Journal of Turbulence Special Issue, Members of the TSFP-6 Organizing Committee

On generation of Alfvénic-like fluctuations by drift wave–zonal flow system in large plasma device experiments

According to recent experiments, magnetically confined fusion plasmas with “drift wave–zonal flow turbulence” (DW-ZF) give rise to broadband electromagnetic waves. Sharapov et al. [Europhysics Conference Abstracts, 35th EPS Conference on Plasma Physics, Hersonissos, 2008, edited by P. Lalousis and S. Moustaizis (European Physical Society, Switzerland, 2008), Vol. 32D, p. 4–071] reported an abrupt change in the magnetic turbulence during L-H transitions in Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] plasmas. A broad spectrum of Alfvénic-like (electromagnetic) fluctuations appears from E×B flow driven turbulence in experiments on the large plasma device (LAPD) [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] facility at UCLA. Evidence of the existence of magnetic fluctuations in the shear flow region in the experiments is shown. We present one possible theoretical explanation of the generation of electromagnetic fluctuations in DW-ZF systems for an example of LAPD experiments. The method used is based on generalizing results on shear flow phenomena from the hydrodynamics community. In the 1990s, it was realized that fluctuation modes of spectrally stable nonuniform (sheared) flows are non-normal. That is, the linear operators of the flows modal analysis are non-normal and the corresponding eigenmodes are not orthogonal. The non-normality results in linear transient growth with bursts of the perturbations and the mode coupling, which causes the generation of electromagnetic waves from the drift wave–shear flow system. We consider shear flow that mimics tokamak zonal flow. We show that the transient growth substantially exceeds the growth of the classical dissipative trapped-particle instability of the system.

Turbulence and Shear Flow Phenomena-5 Symposium

The International Biennial Symposia on Turbulence and Shear Flow Phenomena (TSFP) is aimed at promoting research and communication of important results in turbulence research and various associated...

On the dynamics and frequency response of fully-pulsed turbulent round jets: Computations using two-time-scale/strain-sensitized eddy viscosity models

The present paper reports simulations of ‘fully-pulsed’ (or on/off) turbulent round jets. Unsteady forcing of the jet is imposed over a range of frequencies from 2 to 10 Hz and, for a jet pulsed at 10 Hz, comparison is made with the experimental data of Bremhorst and Gehrke (2000) [Bremhorst, K., Gehrke, P.J., 2000. Measured Reynolds stress distributions and energy budgets of a fully-pulsed round air jet. Exp. Fluids 28, 519–531]. Calculations are performed using the two-time-scale, or split-spectrum, eddy viscosity turbulence model of Hanjalić et al. (1980) [Hanjalić, K., Launder, B.E., Schiestel, R., 1980. Multiple-time-scale concepts in turbulent transport modelling. In: Bradbury, L.J.S., Durst, F., Launder, B.E., Schmidt, F.W., Whitelaw, J.H. (Eds.), Turbulent Shear Flows 2. Springer-Verlag, Berlin, pp. 36–49]. The model, in addition to its split-spectrum features, places an increased emphasis on the role of irrotational straining in the energy cascade process. In seeking to distinguish between the effects of spectral partitioning and strain sensitivity in the model, a modified form is tested in which the turbulence response to shear and normal straining is treated more conventionally, while the split-spectrum approach is retained [Bremhorst, K., Craft, T.J., Launder, B.E., 2003. Two-time-scale turbulence modelling of a fully-pulsed axisymmetric air jet. In: Proc. 3rd Int. Symp. on Turbulence and Shear Flow Phenomena, Sendai, pp. 711–716]. In confirmation of the results of Bremhorst et al., it is found that, in general, the original Hanjalić et al. scheme is in better agreement with the measurements of Bremhorst and Gehrke than is the modified version. The critical role played by the starting vortex in influencing centreline flow dynamics is identified, and it is shown that it is the behaviour of the Hanjalić et al. model in a highly rotational mean velocity field that largely accounts for its generally superior performance in the computation of fully-pulsed jets.

A Buoyancy–Vorticity Wave Interaction Approach to Stratified Shear Flow

Abstract Motivated by the success of potential vorticity (PV) thinking for Rossby waves and related shear flow phenomena, this work develops a buoyancy–vorticity formulation of gravity waves in stratified shear flow, for which the nonlocality enters in the same way as it does for barotropic/baroclinic shear flows. This formulation provides a time integration scheme that is analogous to the time integration of the quasigeostrophic equations with two, rather than one, prognostic equations, and a diagnostic equation for streamfunction through a vorticity inversion. The invertibility of vorticity allows the development of a gravity wave kernel view, which provides a mechanistic rationalization of many aspects of the linear dynamics of stratified shear flow. The resulting kernel formulation is similar to the Rossby-based one obtained for barotropic and baroclinic instability; however, since there are two independent variables—vorticity and buoyancy—there are also two independent kernels at each level. Though having two kernels complicates the picture, the kernels are constructed so that they do not interact with each other at a given level.

Foreword to Special Issue 4th International Symposium on Turbulence and Shear Flow Phenomena

Foreword to Special Issue 4th International Symposium on Turbulence and Shear Flow Phenomena

Dynamics of fine scale eddy clusters in turbulent channel flows*

To investigate dynamics of vortex clusters and large-scale structures in the outer layer of wall turbulence, direct numerical simulations of turbulent channel flows have been conducted up to Re τ = 1270. In the outer layer, the vortex clusters are composed of coherent fine-scale eddies (CFSEs) of which diameter and maximum azimuthal velocity are scaled by the Kolmogorov length and velocity. The large-scale structure in the outer layer is composed of these clusters of the CFSEs, which contributes to the streamwise velocity deficit (i.e.low-momentum region). The CFSE clusters are observed in the low-momentum regions of the outer layer, and the scale of those clusters tends to be enlarged with the increase of a distance from the wall. The dynamics of large-scale structures reveals that the cluster structure generated in the bottom of the logarithmic region moves downstream and its scale increases with the increase of the low-momentum region. The CFSE clusters in the low-momentum regions of u′ ≤ −u rms consist of the relatively strong CFSEs, which play an important role in the production of the Reynolds shear stress and the dissipation rate of the turbulent kinetic energy. The process of destruction of the CFSE cluster is also clarified in the outer layer. *This article was chosen from Selected Proceedings of the Fourth International Symposium on Turbulent and Shear Flow Phenomena (Williamsburg, VA USA, 27–29 June 2005) ed J. A. C. Humphrey et al.

Large-eddy simulation and deduced scaling analysis of Rayleigh–Bénard convection up to Ra = 109**

Large-eddy simulation of turbulent Rayleigh–Bénard (RB) convection has been performed for a 6:1:6 open-ended domain for Rayleigh numbers ranging from 6.3 × 105 to 109 at Prandtl number of Pr = 0.71. The scaling analysis based on the LES data shows that the heat transfer follows a single relation of Nu = 0.162 Ra 0.286, which is consistent with the scaling law for the hard turbulence regime reported in several earlier experimental and DNS studies. The present LES also supports some earlier experimental and DNS findings that most of characteristic parameters can be scaled reasonably well with Ra number in the considered Ra number range using a single relation. Nonetheless, it is found that the scaling of several quantities shows a sensible offset from a single relation, and could be fitted better with the separate scaling relations for the soft and hard convective turbulence transitioned at about Ra = 4 × 107. It has been argued that the transition, reflected in the scaling relation, may be attributed to the increasing ‘containing effect’ of the plume leaving the horizontal wall on the plume approaching the wall at large Ra numbers in the near-wall region. **This paper is a modified version from the paper presented at the Forth International Symposium of Turbulence and Shear Flow Phenomena (Williamsburg, Virginia, 27–29 June 2005).

Assessment of phenomenological models for viscosity of liquids based on nonequilibrium atomistic simulations of copper

The shear viscosity of liquid copper is studied using nonequilibrium molecular-dynamics simulations under planar shear flow conditions. We examined variation of viscosity as function of shear rate at a range of pressures (ca. 0 - 40 GPa). We analyzed these results using eight different phenomenological models and find that the observed non-Newtonian behavior is best described by the Powell-Eyring (PE) model: eta(gamma) = (eta(0)-eta(infinity))sinh(-1)(taugamma)(taugamma) + eta(infinity), where gamma is the shear rate. Here eta(0) (the zero-shear-rate viscosity) extracted from the PE fit is in excellent agreement with available experimental data. The relaxation time tau from the PE fit describes the shear response to an applied stress. This provides the framework for interpreting the shear flow phenomena in complex systems, such as liquid metal and amorphous metal alloys.