Structure Of Wall Turbulence Research Articles

Extended self-similar scaling law of multi-scale eddy structure in wall turbulence

The longitudinal fluctuating velocity of a turbulent boundary layer was measured in a water channel at a moderate Reynolds number. The extended self-similar scaling law of structure function proposed by Benzi was verified. The longitudinal fluctuating velocity, in the turbulent boundary layer was decomposed into many multi-scale eddy structures by wavelet transform. The extended self-similar scaling law of structure function for each scale eddy velocity was investigated. The conclusions are I) The statistical properties of turbulence could be self-similar not only at high Reynolds number, but also at moderate and low Reynolds number, and they could be characterized by the same set of scaling exponents xi (1)(n) = n/3 and xi (2)(n) = n/3 of the fully developed regime. 2) The range of scales where the extended self-similarity valid is much larger than the inertial range and extends far deep into the dissipation range,vith the same set of scaling exponents. 3) The extended selfsimilarity is applicable not only for homogeneous turbulence, but also for shear turbulence such as turbulent boundary layers.

The role of near wall turbulent structures on sediment transport

In this paper, we present the results of an experimental investigation designed to analyze the interaction between near wall coherent structures (or burst) and bedload transport in an open channel flow. Laser doppler velocimetry (LDV) measurements of the instantaneous velocity near the wall were coupled with real time measurements of sand particle trajectories on the bottom of an hydraulic flume. We will first give a description of the bursting phenomenon and we will present the experimental design. Then, we will give some details about the specific signal processing techniques we used in this work to detect coherent structures on the velocity signal. The third part of this paper will present our results. We will show that the period between two consecutive displacements of a solid particle can be compared to the mean period between two consecutive ejections. This would suggest that these structures are involved in bedload transport. For small displacements, the time distribution between two consecutive movements shows two modes. One would be associated to particles whose mean deposit time corresponds to the ejection period, the second would be associated to particles whose mean deposit time is close to the sweep period. We made the assumption that there exist two transport modes at the wall: the dominant one was transport by ejections, but sweeps can be involved in the removal of particles whose friction coefficient with the wall is high.

Low-speed streaks in drag-reduced turbulent flow

The effect of a surfactant drag-reducing additive (530 ppm Habon G solution) on the structure of wall turbulence, both in a flume and in pipe flow, was investigated experimentally. Real-time infrared thermography was used for flow visualization and measurements of the spanwise spacing between the thermal streaks. The experiments were carried out over a broad range of friction velocities, i.e., up*=0.51–3.27 cm/s. With wall shear velocities 1.54⩽up*⩽3.27 cm/s drag reduction of 82%–85% was achieved in a tube flow, well below the predictions of the Virk maximum drag reduction asymptote proposed for high polymers. The results of spanwise streak spacing indicate that wall shear velocity may be an appropriate parameter for describing nondimensional streak spacing behavior in drag reducing flows. A hypothesis, based on the average spanwise streak spacing λ+, can be applied to describe the mean velocity profiles of Habon G solutions. The ratio (λp+−100)/100 was applied to describe mean velocity profiles with 530 ppm Habon G solution.

Structure of near wall turbulence downstream of a wall mounted protrusion: an interesting Reynolds stress suppression phenomena

A local suppression in the generation of near wall Reynolds stress is achieved by modifying the buffer region and sublayer (y+ <30) of a turbulent pipe flow with a 16.4 wall unit high wall mounted protrusion. Multi-component, multi-point, time resolved laser Doppler velocimetry measurements are made in the undisturbed and modified ARL/PSU glycerin tunnel pipe flow at a Reynolds number of approximately 10000. A downstream converging flow field is produced by the divergence of the approaching mean flow around the protrusion. A pair of counter-rotating vortices, ≈ 15 wall units in diameter with common flow down, are generated by the protrusion and also contribute to the wall directed flow convergence. The convergence region is 15 wall units high and more than 100 wall units long and appears to decouple the near wall region from the outer turbulent wall layer. Locally, turbulent velocity fluctuations in the form of Reynolds stress producing events, sweeps and ejections, are retarded within this region. This results in a reduction in near wall uv Reynolds stress and local wall shear. Interestingly, the counter-rotating vortices act to increase turbulent diffusion in a manner which is uncorrelated with Reynolds stress generation.

An experimental study of the effects of three-dimensionality on the near wall turbulence structures using flow visualization

The effects of three-dimensionality on the turbulence producing near-wall structures over a rotating disk are examined using hydrogen bubble visualization. Laser Doppler anemometry measurements of the tangential and radial velocities indicate the flow to resemble that of an “infinite” disk. It was found that the crossflow acts to reduce vertical mixing during the turbulence ejection events, thus reducing the efficiency of the boundary layer in producing shear stress.

Boundary-layer transition triggered by hairpin eddies at subcritical Reynolds numbers

Subcritical transition in a flat-plate boundary layer is examined experimentally through observing its nonlinear response to energetic hairpin eddies acoustically excited at the leading edge of the boundary-layer plate. When disturbed by the hairpin eddies convecting from the leading edge, the near-wall flow develops local three-dimensional wall shear layers with streamwise vortices. Such local wall shear layers also evolve into hairpin eddies in succession to lead to the subcritical transition beyond the x-Reynolds number Rx = 3.9 × 104, where the momentum thickness Reynolds number Rθ is 127 for laminar Blasius flow without excitation, and is about 150 under the excitation of energetic hairpin eddies. It is found that in terms of u- and v-fluctuations, the intensity of the near-wall activity at this critical station is of almost the same order as or slightly less than that of the developed wall turbulence. The development of wall turbulence structure in this transition is also examined.

A five-velocity-component laser-Doppler velocimeter for measurements of a three-dimensional turbulent boundary layer

A five-velocity-component LDV system was designed to make five nearly simultaneous velocity component measurements: coincident instantaneous U, V, W components of the velocity at one point and two V velocity components at two nearby locations. The research is aimed at examining the relations among measured velocity components in order to investigate the near wall turbulence structure of 3D turbulent boundary layers (3DTBLs) and aid in the development of new turbulence models for 3D subsonic pressure-driven turbulent boundary layer (TBL). To map the relations between the 'sweeps' and 'ejections' of near wall turbulent fluid, two V measurement points can be traversed within a selected domain. In order to map the velocity field for given V measurement locations, a U, V, W measurement point is traversed in a domain defined by the two V measurement points. At the same time, the system can make measurements throughout the whole boundary layer to investigate other phenomena. Data from this system show that the uncertainties are low and the repeatibility of measurements is excellent. Data presented include the mean and fluctuation velocities, shear stresses, some of the triple velocity fluctuation correlations and some auto and cross correlation coefficients obtained at one measurement station of a high Reynolds number 3DTBL flow.

Some Progress in the Research of the Dynamical Structure in Wall Turbulence.

Four topics concerning the coherent structures of the turbulent boundary are discussed in this brief review. Firstly comments are given on the identification of C. S. (coherent structure), secondly some dynamical models of C. S. are surveyed, thirdly our recent study of the fractal nature of C. S. is summarized and an intimate relationship between the burst and the fractality is clarified, and finally some preliminary discussions are presented concerning the application of Kullback-divergence to turbulent flow.

Funnel-shaped vortical structures in wall turbulence

The structure of the turbulent boundary layer in a horizontal open channel was investigated experimentally by means of laser Doppler anemometry (LDA) and by flow visualization synchronized with the LDA. These experiments indicate that the dominant structures in the wall region are large scale streamwise vortices which originate at the wall and grow and expand into the outer flow region. The shape of the vortices is that of an expanding spiral, wound around a funnel which is laid sideways in the direction of flow. Most of the observations of wall turbulence phenomena made over the years, such as quasistreamwise vortices, ejections, and sweeps seem to be part of these funnel-shaped vortices.

On the structure and control of near wall turbulence

A simplified self-sustaining cycle is proposed for the events in the near wall region of a turbulent boundary layer. An approximate quantitative analysis of the resulting model predicts the right dimensions for the sublayer streaks and for other flow structures. The model is checked further by applying it to a set of numerical simulations in which the longitudinal and transverse no-slip conditions are applied at different positions with respect to the wall, and to the analysis of flow over riblets. Substantial changes in the flow statistics are obtained, including drag reductions, and the resulting trends are also predicted correctly by the model.

Wall pressure fluctuations associated with complex flows.

The relationship between wall pressure fluctuations and turbulent structures in a boundary layer has been the subject of research for well over three decades. In that time, the focus of attention was for the canonical case of an equilibrium boundary layer. These studies have identified the causalities between the near-wall flow structures and the high-frequency pressures, and the outer-flow structures and the low-frequency pressures. For complex flows, the characteristics of the turbulence activities within the boundary layer have unique structures that are distinct from the canonical case. Accordingly, the features of the wall pressure field are quite different. As an example of a complex flow, the fluctuating wall pressures and turbulence statistics measured downstream of a backward facing step will be presented and compared to an equilibrium flow. The data demonstrate that the elevated low-frequency pressures directly track coherent turbulence structures in the flow field that result from passage over the step. In addition, these elevated pressures exhibit a slower spatial decay rate than that for equilibrium flows. These results illustrate the high sensitivity of the wall pressure to organized flow structures associated with complex flows.

Drag reduction caused by the injection of polymer thread into a turbulent pipe flow

Drag reduction caused by the injection of concentrated polymer solutions into a turbulent pipe flow was studied. Measurements were made of the radial distribution of fluctuating velocities by means of video image analysis. The results showed that a higher velocity was observed for injected polymer threads and both the radial fluctuation and the Reynolds stress were significantly suppressed. It was suggested that the wall turbulence structure might be controlled by suppressing the large scale turbulent motion in the turbulent core region.

The effect of spatial restriction on the inner-layer structure of wall turbulence

Hot-film-anemometer measurements were carried out in a shear flow between a flat plate and a moving plate fitted with an array of tall fences. The effect of spatial restriction by the fences on the inner-layer structure of the boundary layer developing on the flat-plate side was investigated. It was revealed that the inner-layer structure was maintained even when the tips of the fences were passing at a distance y+ = 45 from the flat plate; the flow did not become laminar-like until the tips reached y+ = 25. These results suggested the physical view that the inner layer of wall turbulence has a tough, self-sustaining structure, which is uniquely determined under a given mean wall shear stress and is hardly influenced by outer-layer disturbances provided that its own spatial extent of about 45 ν/u* from the wall is maintained.

Effects of the subgrid scale models on the structure of wall turbulence.

A large eddy simulation technique with one equation model and a direct numerical simulation by third order upwind differencing are applied to the fully developed turbulent channel flow. It is shown that the suppressing effect of high order numerical diffusion on the near wall structure makes the accuracy of the flow prediction unacceptable, as the resolution by an available number of grid points is comparable with the spacing of streaks.

The effect of spatial restriction on the inner layer structure of wall turbulence. (2nd report The influence of streamwise pitch of the interfering plates)

Spatial restrictions were imposed on the inner layer structure of the turbulent boundary layer which was developing on a moving flat-plate, by means of putting a fixed array of vertical plates close to the moving plate. The nondimensional gap h+ between the flat-plate and the tips of the interfering vertical plates as well as the nondimensional pitch p+ of the interfering plates were systematically changed. From hot-film anemometer measurements, states of flows were distinguished into three regions in the h+ vs. p+ plane, i. e., laminar, transitional, and fully turbulent regions. With increase in the pitch p+, the gap h+ needed to maintain turbulence decreased mildly from 40. This was explained with a view of the distribution of sizes of the coherent structures in the inner layer.

The effect of spatial restriction on the inner layer structure of wall turbulence.

Hot-film anemometer measurements were carried out in a shear flow between a flat plate and a moving plate fitted with an array of tall fences. The effect of spatial restriction by the fences on the inner layer structure of the boundary layer developed on the flat-plate side was investigated. It was revealed that the inner layer structure of the wall turbulence was maintained even when the tips of the fences were passing at a distance y+=50 from the flat plate. The flow did not become laminar-like until the tips came up to y+=2030. There results provoked the physical determined under a given mean wall shear stress and is hardly influenced by outer layer disturbances provided its own spatial extent of about 50v/u* from the wall is ensured.

Turbulence measurements and mass transfer in fully developed flow in a triangular duct with a narrow apex angle

The electrochemical method is applied to the mass transfer measurements in a fully developed turbulent flow in an Isosceles triangular duct with a narrow apex angle. The analysis of the measured fluctuating mass transfer coefficient on a point electrode results in the distribution of turbulent intensity and space correlation coefficient on the side wall of a triangle. In the laminar or transitional region near the apex, a remarkable change in turbulent intensity and space correlation is detected. In addition, even at the fully turbulent region near the base of a triangle, the turbulent characteristics very near the side wall are considerably changed from those of a turbulent round tube flow at the same Reynolds number. The decrease in overall mass transfer coefficient in a triangular duct is qualitatively explained by the change of wall turbulence structure.

Experimental Study on the Structure of Wall Turbulence in Open Channel Flow

Experimental Study on the Structure of Wall Turbulence in Open Channel Flow

Structure of the turbulent boundary in drag-reducing pipe flow

The effect of a drag-reducing additive on the structure of wall turbulence in pipe flow was investigated experimentally. Real-time hologram interferometry was used for flow visualization and turbulence measurements. The real-time modulation of interference fringes by a refractive-index enhancer infused into the near-wall flow was recorded by medium-speed motion photography. The spanwise direction and the direction normal to the wall were studied to investigate the ‘streaks’ and ‘bursts’ that originate in the sublayer. A region of the flow was sampled for spatial and temporal correlations of concentration fluctuations to detect the scales of eddy interaction.The addition of 50 p.p.m. by weight of Separan AP30 to water significantly altered the Newtonian wall-flow structure. The drag-reducing additive suppressed the formation of streaks and the eruption of bursts. When compared at the same wall shear, the sublayer period increased over the Newtonian value by a factor almost equal to the ratio of the corresponding non-dimensional streak spacings.These results suggest a stabilized wall layer in the drag-reducing solution as compared with that of the Newtonian solvent, resulting in less turbulence production and reduced frictional drag. The role of the extensional viscosity of the dilute polymer solution is discussed as a possible mechanism for explaining the visualized and measured phenomena.

Wall turbulence structure and convection heat transfer

Recently accumulated evidence emphasizes the role of coherent, quasi-ordered structures in transport processes of a turbulent shear flow. Statistically, the presence of these structures is manifested as intermittency of the signals of fluctuating quantities leading to departures from gaussianity. This paper presents the results of a conventional statistical analysis of velocity and temperature fluctuations in the wall layers of a channel flow. The results indicate the presence of at least two intermittent phases—inrushes towards the wall and ejections outwards—superimposed on the background turbulence. A conditional sampling and averaging technique, aiming at the detection of the two intermittent phases, is presented. The results demonstrate that the evolution of the probability density distributions in the wall layers could well be explained by the statistical behavior of the intermittent phases.