The creation and evolution of coherent structures in plant canopy flows and their role in turbulent transport

2016 ◽  
Vol 789 ◽  
pp. 425-460 ◽  
Author(s):  
Brian N. Bailey ◽  
R. Stoll
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Conceptual Model ◽  
Coherent Structures ◽  
Mixing Layer ◽  
Three Dimensional ◽  
Large Reynolds Number ◽  
Plant Canopy ◽  
Two Dimensional ◽  
The Creation

In this paper we used simulation tools to study turbulent boundary-layer structures in the roughness sublayer. Of particular interest is the case of a neutrally-stratified atmospheric boundary layer in which the lower boundary is covered by a homogeneous plant canopy. The goal of this study was to formulate a consistent conceptual model for the creation and evolution of the dominant coherent structures associated with canopy roughness and how they link with features observed in the overlying inertial sublayer. First, coherent structures were examined using temporally developing flow where the full range of turbulent scales had not yet developed, which allowed for instantaneous visualizations. These visualizations were used to formulate a conceptual model, which was then further tested using composite-averaged structure realizations from fully-developed flow with a very large Reynolds number. This study concluded that quasi two-dimensional mixing-layer-like roller structures exist in the developed flow and give the largest contributions to mean Reynolds stresses near the canopy. This work fully acknowledges the presence of highly three-dimensional and localized vortex pairing processes. The primary argument is that, as in a mixing layer, the smaller three-dimensional vortex interactions do not destroy the larger two-dimensional structure. Because the flow has a very large Reynolds number, the roller-like structures are not well-defined vortices but rather are a conglomerate of a large range of smaller-scale vortex structures that create irregularities. Because of this, the larger-scale structure is more difficult to detect in correlation or conditional sampling analyses. The frequently reported ‘scalar microfronts’ and associated spikes in pressure occur in the slip-like region between adjacent rollers. As smaller vortices within roller structures stretch, they evolve to form arch- and hairpin-shaped structures. Blocking by the low-flux canopy creates vertical asymmetry, and tends to impede the vertical progression of head-down structures. Head-up hairpins are allowed to continually stretch upward into the overlying inertial sublayer, where they evolve into the hairpin structures commonly reported to populate wall-bounded flows. This process is thought to be modulated by boundary-layer-scale secondary instability, which enhances head-up hairpin formation along quasi-streamwise transects.

The Three-Dimensional Turbulent Boundary Layer on an Infinite Yawed Wing

1971 ◽  
Vol 22 (4) ◽  
pp. 346-362 ◽  
Author(s):  
J. F. Nash ◽  
R. R. Tseng
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Lift Coefficient ◽  
Three Dimensional ◽  
Numerical Representation ◽  
Two Dimensional ◽  
Incompressible Turbulent Boundary Layer ◽  
Displacement Thickness ◽  
Attachment Line

SummaryThis paper presents the results of some calculations of the incompressible turbulent boundary layer on an infinite yawed wing. A discussion is made of the effects of increasing lift coefficient, and increasing Reynolds number, on the displacement thickness, and on the magnitude and direction of the skin friction. The effects of the state of the boundary layer (laminar or turbulent) along the attachment line are also considered.A study is made to determine whether the behaviour of the boundary layer can adequately be predicted by a two-dimensional calculation. It is concluded that there is no simple way to do this (as is provided, in the laminar case, by the principle of independence). However, with some modification, a two-dimensional calculation can be made to give an acceptable numerical representation of the chordwise components of the flow.

On the stability of three-dimensional boundary layers with application to the flow due to a rotating disk

1955 ◽  
Vol 248 (943) ◽  
pp. 155-199 ◽  
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Wave Velocity ◽  
Critical Points ◽  
Three Dimensional ◽  
Rotating Disk ◽  
Velocity Profiles ◽  
Wave Number ◽  
Two Dimensional ◽  
China Clay

A phenomenon of boundary-layer instability is discussed from the theoretical and experimental points of view. The china-clay evaporation technique shows streaks on the surface, denoting a vortex system generated in the region of flow upstream of transition. Experiments on a swept wing are described briefly, while experiments on the flow due to a rotating disk receive much greater attention. In the latter case, the axes of the disturbance vortices take the form of equi-angular spirals, bounded by radii of instability and of transition. A frequency analysis of the disturbances shows that there is a narrow band of disturbance components of high amplitude, some frequencies within this band corresponding to disturbances fixed relative to the surface and others corresponding to moving waves. Furthermore, the determination of velocity profiles for the rotating-disk flow is described, the agreement with the theoretical solution for laminar flow being quite satisfactory; for turbulent flow, however, the empirical theories are not very satisfactory. In order to explain the vortex phenomenon just discussed, the general equations of motion in orthogonal curvilinear co-ordinates are examined by superimposing an infinitesimal disturbance periodic in space and time on the main flow, and linearizing for small disturbances. An important result is that, within the range of certain approximations, the velocity component in the direction of propagation of the disturbance may be regarded as a two-dimensional flow for stability purposes; then the problem of stability formally resembles the well-known two dimensional problem. However, it is important to emphasize that this result—namely, that the flow curvature has little influence on stability—is applicable only to the possible modes of instability in a local region. The nature of three-dimensional flows is discussed, and the importance of co-ordinates along and normal to the stream-lines outside the boundary layer is examined. In accord with the formal two-dimensional nature of the instability, there is a whole class of velocity distributions, corresponding to different directions, which may exhibit instability. The question of stability at infinite Reynolds number is examined in detail for these profiles. As for ordinary two-dimensional flows, the wave velocity of the disturbance must lie somewhere between the maximum and minimum of the velocity profile considered. The points where the wave velocity equals the fluid velocity are called critical points, of which most of the profiles considered have two. Then Tollmien’s criterion that velocity profiles with a point of inflexion are unstable at infinite Reynolds number is extended to the case of profiles with two critical points. One particular profile—namely, that for which the point of inflexion lies at the point of zero velocity—may generate neutral disturbances of zero phase velocity, corresponding to the disturbances visualized by the china-clay technique. A variational method for the solution of certain of the eigenvalue problems associated with stability at infinite Reynolds number is derived, found by comparison with an exact solution to be very accurate, and applied to the rotating disk. The fixed vortices predicted by the theory have as their axes equi-angular spirals of angle 103°, in good agreement with experiment, but the agreement between theoretical and experimental wave number is not good, the discrepancy being attributed to viscosity. Finally, the correlation between the experimentally observed and theoretically possible disturbances is discussed and certain conclusions drawn therefrom. The streamlines of the disturbed boundary layer show the existence of a double row of vortices, one row of which produces the streaks in the china clay. Application of the theory to other physical phenomena is described.

scholarly journals Exact coherent structures at extreme Reynolds number

2016 ◽  
Vol 794 ◽  
pp. 1-4 ◽  
Author(s):  
G. P. Chini
Keyword(s):  
Reynolds Number ◽  
Coherent Structures ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Navier Stokes ◽  
Large Reynolds Number ◽  
Navier Stokes Equations ◽  
Wall Flows ◽  
Tollmien Schlichting ◽  
Turbulent Wall

Exact coherent structures (ECS), unstable three-dimensional solutions of the Navier–Stokes equations, play a fundamental role in transitional and turbulent wall flows. Dempsey et al. (J. Fluid Mech., vol. 791, 2016, pp. 97–121) demonstrate that at large Reynolds number reduced equations can be derived that simplify the computation and facilitate mechanistic understanding of these solutions. Their analysis shows that ECS in plane Poiseuille flow can be sustained by a novel inner–outer interaction between oblique near-wall Tollmien–Schlichting waves and interior streamwise vortices.

Free-stream coherent structures in growing boundary layers: a link to near-wall streaks

2015 ◽  
Vol 778 ◽  
pp. 451-484 ◽  
Author(s):  
Kengo Deguchi ◽  
Philip Hall
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Coherent Structures ◽  
Free Stream ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Base Flow ◽  
Interaction Problem ◽  
Near Wall

In a recent paper, Deguchi & Hall (J. Fluid Mech., vol. 752, 2014a, pp. 602–625) described a new kind of exact coherent structure which sits at the edge of an asymptotic suction boundary layer at high values of the Reynolds number $Re$. At a distance $\ln Re$ from the wall, the structure is driven by the fully nonlinear interaction of tiny rolls, waves and streaks convected downstream at almost the free-stream speed. The interaction problem satisfies the unit-Reynolds-number three-dimensional Navier–Stokes equations and is localized in a layer of the same depth as the unperturbed boundary layer. Here, we show that the interaction problem is generic to any boundary layer that approaches its free-stream form through an exponentially small correction. It is shown that away from the layer where it is generated the induced roll–streak flow is dominated by non-parallel effects which now play a major role in the streamwise evolution of the structure. The similarity with the parallel boundary layer case is restricted only to the layer where it is generated. It is shown that non-parallel effects cause the structure to persist only over intervals of finite length in any growing boundary layer and lead to a flow structure reminiscent of turbulent boundary layer simulations. The results found shed light on a possible mechanism to couple near-wall streaks with coherent structures located towards the edge of a turbulent boundary layer. Some discussion of how the mechanism adapts to a three-dimensional base flow is given.

Effect of large bulk viscosity on large-Reynolds-number flows

2014 ◽  
Vol 751 ◽  
pp. 142-163 ◽  
Author(s):  
M. S. Cramer ◽  
F. Bahmani
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Bulk Viscosity ◽  
Three Dimensional ◽  
Rigid Bodies ◽  
Large Reynolds Number ◽  
Steady Flows ◽  
Outer Flow ◽  
First Order ◽  
Large Bulk

AbstractWe examine the inviscid and boundary-layer approximations in fluids having bulk viscosities which are large compared with their shear viscosities for three-dimensional steady flows over rigid bodies. We examine the first-order corrections to the classical lowest-order inviscid and laminar boundary-layer flows using the method of matched asymptotic expansions. It is shown that the effects of large bulk viscosity are non-negligible when the ratio of bulk to shear viscosity is of the order of the square root of the Reynolds number. The first-order outer flow is seen to be rotational, non-isentropic and viscous but nevertheless slips at the inner boundary. First-order corrections to the boundary-layer flow include a variation of the thermodynamic pressure across the boundary layer and terms interpreted as heat sources in the energy equation. The latter results are a generalization and verification of the predictions of Emanuel (Phys. Fluids A, vol. 4, 1992, pp. 491–495).

The stability of a two-dimensional stagnation flow to three-dimensional disturbances

1978 ◽  
Vol 84 (3) ◽  
pp. 517-527 ◽  
Author(s):  
S. D. R. Wilson ◽  
I. Gladwell
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Boundary Layer Thickness ◽  
Three Dimensional ◽  
Two Dimensional ◽  
Stagnation Flow ◽  
Stagnation Line ◽  
Two Dimensional Flow ◽  
The Stability ◽  
Secondary Vortices

Experiments have shown that the two-dimensional flow near a forward stagnation line may be unstable to three-dimensional disturbances. The growing disturbance takes the form of secondary vortices, i.e. vortices more or less parallel to the original streamlines. The instability is usually confined to the boundary layer and the spacing of the secondary vortices is of the order of the boundary-layer thickness. This situation is analysed theoretically for the case of infinitesimal disturbances of the type first studied by Görtler and Hämmerlin. These are disturbances periodic in the direction perpendicular to the plane of the flow, in the limit of infinite Reynolds number. It is shown that the flow is always stable to these disturbances.

scholarly journals Wave interactions in a three-dimensional attachment-line boundary layer

1990 ◽  
Vol 217 ◽  
pp. 367-390 ◽  
Author(s):  
Philip Hall ◽  
Sharon O. Seddougui
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Stokes Equations ◽  
Hydrodynamic Instability ◽  
Three Dimensional ◽  
Low Frequency ◽  
Leading Edge ◽  
Two Dimensional ◽  
Oblique Wave ◽  
Attachment Line

The three-dimensional boundary layer on a swept wing can support different types of hydrodynamic instability. Here attention is focused on the so-called ‘spanwise instability’ problem which occurs when the attachment-line boundary layer on the leading edge becomes unstable to Tollmien–Schlichting waves. In order to gain insight into the interactions that are important in that problem a simplified basic state is considered. This simplified flow corresponds to the swept attachment-line boundary layer on an infinite flat plate. The basic flow here is an exact solution of the Navier–Stokes equations and its stability to two-dimensional waves propagating along the attachment line can be considered exactly at finite Reynolds number. This has been done in the linear and weakly nonlinear regimes by Hall, Malik & Poll (1984) and Hall & Malik (1986). Here the corresponding problem is studied for oblique waves and their interaction with two-dimensional waves is investigated. In fact oblique modes cannot be described exactly at finite Reynolds number so it is necessary to make a high-Reynolds-number approximation and use triple-deck theory. It is shown that there are two types of oblique wave which, if excited, cause the destabilization of the two-dimensional mode and the breakdown of the disturbed flow at a finite distance from the leading edge. First a low-frequency mode closely related to the viscous stationary crossflow mode discussed by Hall (1986) and MacKerrell (1987) is a possible cause of breakdown. Secondly a class of oblique wave with frequency comparable with that of the two-dimensional mode is another cause of breakdown. It is shown that the relative importance of the modes depends on the distance from the attachment line.

A phase-equation approach to boundary–layer instability theory: Tollmien-Schlichting waves

1995 ◽  
Vol 304 ◽  
pp. 185-212 ◽  
Author(s):  
Philip Hall
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Evolution Equation ◽  
Modulational Instability ◽  
Reaction Diffusion Equations ◽  
Large Reynolds Number ◽  
Two Dimensional ◽  
Equation Approach ◽  
Phase Equation ◽  
Tollmien Schlichting

Our concern is with the evolution of large-amplitude Tollmien-Schlichting waves in boundary-layer flows. In fact, the disturbances we consider are of a comparable size to the unperturbed state. We shall describe two-dimensional disturbances which are locally periodic in time and space. This is achieved using a phase equation approach of the type discussed by Howard & Kopell (1977) in the context of reaction-diffusion equations. We shall consider both large and O(1) Reynolds number flows though, in order to keep our asymptotics respectable, our finite-Reynolds-number calculation will be carried out for the asymptotic suction flow. Our large-Reynolds-number analysis, though carried out for Blasius flow, is valid for any steady two-dimensional boundary layer. In both cases the phase-equation approach shows that the wavenumber and frequency will develop shocks or other discontinuities as the disturbance evolves. As a special case we consider the evolution of constant frequency/wavenumber disturbances and show that their modulational instability is controlled by Burgers equation at finite-Reynolds-number and by a new integro-differential evolution equation at large-Reynolds-numbers. For the large Reynolds number case the evolution equation points to the development of a spatially localized singularity at a finite time.

scholarly journals The relationship between free-stream coherent structures and near-wall streaks at high Reynolds numbers

2017 ◽  
Vol 375 (2089) ◽  
pp. 20160078 ◽  
Author(s):  
K. Deguchi ◽  
P. Hall
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Coherent Structures ◽  
Free Stream ◽  
Stokes Equations ◽  
Numerical Approach ◽  
Wall Turbulence ◽  
Large Reynolds Number ◽  
Equilibrium Solutions ◽  
New Family

The present work is based on our recent discovery of a new class of exact coherent structures generated near the edge of quite general boundary layer flows. The structures are referred to as free-stream coherent structures and were found using a large Reynolds number asymptotic approach to describe equilibrium solutions of the Navier–Stokes equations. In this paper, first we present results for a new family of free-stream coherent structures existing at relatively large wavenumbers. The new results are consistent with our earlier theoretical result that such structures can generate larger amplitude wall streaks if and only if the local spanwise wavenumber is sufficiently small. In a Blasius boundary layer, the local wavenumber increases in the streamwise direction so the wall streaks can typically exist only over a finite interval. However, here it is shown that they can interact with wall curvature to produce exponentially growing Görtler vortices through the receptivity process by a novel nonparallel mechanism. The theoretical predictions found are confirmed by a hybrid numerical approach. In contrast with previous receptivity investigations, it is shown that the amplitude of the induced vortex is larger than the structures in the free-stream which generate it. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’.

scholarly journals Three-dimensional vortex organization in a high-Reynolds-number supersonic turbulent boundary layer

2010 ◽  
Vol 644 ◽  
pp. 35-60 ◽  
Author(s):  
G. E. ELSINGA ◽  
R. J. ADRIAN ◽  
B. W. VAN OUDHEUSDEN ◽  
F. SCARANO
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Velocity Field ◽  
Turbulent Boundary Layer ◽  
Coherent Structures ◽  
Large Scale ◽  
Three Dimensional ◽  
Velocity Fields ◽  
Hairpin Structure ◽  
Streamwise Direction

Tomographic particle image velocimetry was used to quantitatively visualize the three-dimensional coherent structures in a supersonic (Mach 2) turbulent boundary layer in the region between y/δ = 0.15 and 0.89. The Reynolds number based on momentum thickness Reθ = 34000. The instantaneous velocity fields give evidence of hairpin vortices aligned in the streamwise direction forming very long zones of low-speed fluid, consistent with Tomkins & Adrian (J. Fluid Mech., vol. 490, 2003, p. 37). The observed hairpin structure is also a statistically relevant structure as is shown by the conditional average flow field associated to spanwise swirling motion. Spatial low-pass filtering of the velocity field reveals streamwise vortices and signatures of large-scale hairpins (height > 0.5δ), which are weaker than the smaller scale hairpins in the unfiltered velocity field. The large-scale hairpin structures in the instantaneous velocity fields are observed to be aligned in the streamwise direction and spanwise organized along diagonal lines. Additionally the autocorrelation function of the wall-normal swirling motion representing the large-scale hairpin structure returns positive correlation peaks in the streamwise direction (at 1.5δ distance from the DC peak) and along the 45° diagonals, which also suggest a periodic arrangement in those directions. This is evidence for the existence of a spanwise–streamwise organization of the coherent structures in a fully turbulent boundary layer.

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