Longitudinal Turbulence Research Articles

3-D wind-induced effects on bridges during balanced cantilever erection stages

Nowadays balanced cantilever construction plays an essential role as a sophisticated erection technique of bridges due to its economical and ecological advantages. Experience teaches that wind has a great importance with regard to this construction technique, but methods proposed by codes to take wind effects into account are still rather crude and, in most cases, completely lacking. Also research in this field is quite limited and aimed at studying only the longitudinal shear and the torque at the pier base, caused by the mean wind velocity and by the longitudinal turbulence actions over the deck. This paper advances the present solutions by developing a new procedure that takes into account all wind effects both on the deck and on the pier. The proposed model assumes the mean wind velocity as orthogonal to the bridge plane and considers the effects produced by all the three turbulence components and by the vortex shedding. The applications point out the role of each loading component on different bridge configurations and show that disregarding the presence of some effects may imply oversimplified results and relevant underestimations.

Observation on wind turbulence characteristics and velocity spectra near the ground at the coastal region

In this study, we carried out in situ observations and measured the wind characteristics and velocity spectra near the ground at the northeastern coast of Taiwan. Wind turbulence characteristics and three components of wind velocity spectra are obtained from analysis of data measured from July 1998 to June 1999 by using the ultrasonic anemometer mounted on a tower with a height of 26 m. The quadrant analysis technique is employed to investigate the contributions of stress components to the total Reynolds stress. Analysis of the observation data shows that the ratio of averaged values of longitudinal and vertical turbulence intensity, u 2 / U ̄ : w 2 / U ̄ is approximately 1:0.45. This is approximately close to the ratio 1:0.5, which was the averaged mean value calculated from many measurements of field data as indicated by Counihan. Observational results show that properly scaled longitudinal wind velocity spectrum is found to be in agreement with the Von Karman's form of spectrum equation. It also exhibited that properly scaled measured results of the lateral and vertical wind velocity spectra are approximately close to the predictions of the isotropic turbulence spectra equations.

A turbulence model based on principal components

Literature on turbulence is very wide, providing several models for the power spectral density function of the single-point turbulence components, for the two-point coherence function of the same turbulence component and for the single-point coherence function of different turbulence components. On the other hand, no suitable and simple model seems to be available for representing the two-point coherence function of different turbulence components, in particular of the longitudinal and vertical turbulence components, which would be useful for modelling buffeting actions on bridges. The Proper Orthogonal Decomposition provides efficient tools to formulate a new model of the turbulence field based on principal components. Furthermore, it suggests physical principles and defines mathematical rules to establish an appropriate model of the two-point coherence function of the longitudinal and vertical components, completing the statistical model of turbulence. Embedded in a Monte Carlo framework, this new representation can be used to simulate multi-dimensional and multi-variate random turbulence fields.

A large stratified shear flow water channel facility

A large multilayer, recirculating, salt-stratified water channel is described. The channel was similar to, but much larger than that used by Stillinger et al. (J Fluid Mech 131:73–87, 1983), and with a more compact and simpler recirculating system. Each layer was driven independently by a pump and had an adjustable velocity range from 0 to 120 mm/s. The density stratification can be readily obtained with a buoyancy frequency of 0 to 1.7 s–1. Hot-film anemometry, a newly developed salinity probe, and a thermistor together with a PC were used to measure, collect, and process the data. The facility can also be used as a towing tank. The facility has also been used for grid-turbulence experiments in uniform and shear flows with stratification. These experiments were carried out to test the facility for changes in stratification. The experiments did not change the density gradient substantially, especially in the middle portion of the vertical profile. These experiments had gradient Richardson numbers typically larger than one and are not meant to cover an extensive range of experimental conditions. The main findings showed that the stratification hardly changes the rate of decay of longitudinal turbulence velocity fluctuation, but enhances the decay of vertical turbulence fluctuation; the decay of vertical velocity fluctuation in shear stratified experiments is even larger than for corresponding cases without shear; and the normalized buoyancy flux initially increases behind the grid, then falls off rapidly at the onset of buoyancy effects and goes to zero at about Nt=2, and then tends to oscillate about zero.

Wind tunnel simulation of atmospheric boundary layer flows

The present work shows how thick boundary layers can be produced in a short wind tunnel with a view to simulate atmospheric flows. Several types of thickening devices are analysed. The experimental assessment of the devices was conducted by considering integral properties of the flow and the spectra: skin-friction, mean velocity profiles in inner and outer co-ordinates and longitudinal turbulence. Designs based on screens, elliptic wedge generators, and cylindrical rod generators are analysed. The paper describes in detail the experimental arrangement, including the features of the wind tunnel and of the instrumentation. The results are compared with experimental data published by other authors and with naturally developed flows.

Turbulence Intensities of Shallow Rain-Impacted Flow over Rough Bed

A thorough understanding of the effect of raindrop impacts on the characteristics of a shallow flow is very helpful to properly predict and control water erosion. The turbulence properties of a shallow rain-impacted flow over a rough bed were studied using a two-dimensional fiber-optic laser Doppler velocimeter and an oscillating type raindrop generator. It was found that the value of momentum correction factor β for a rough boundary was much smaller than that for a smooth boundary at a given value of Reynolds number because of the vertical mixing effect of the boundary roughness. For the condition without rainfall, the vertical distribution of the longitudinal turbulence intensity became more uniform as the boundary roughness increased. For the fully rough condition with rainfall and constant roughness size, the effect of roughness on the distributions of turbulence intensities was small. Similar to the condition with smooth boundary, the rainfall-generated turbulence was greatest near the water surface and decreased downward at an exponential rate for the flow over rough bed. Two types of statistical correlations were developed to predict the distributions of both the longitudinal and the vertical turbulence intensities depending on whether the flow is completely penetrated by the raindrops.

Turbulent Flow over Step with Rounded Edges: Experimental and Numerical Study

The study of turbulent flow over man-made obstacles and natural bed forms is an area of active research. This investigation was conducted to explore the differences between flow over a backward facing step and flow over a step with gradual transitions. Detailed measurements of longitudinal mean velocity and turbulence intensity were obtained using a one component LDV system. A turbulence-based numerical model was implemented to reproduce the measured flow variables and to explore additional flow characteristics, including Reynolds stresses and eddy viscosity. The separation that takes place in the rounded expansion plays a significant role in the flow pattern. In contrast to flow over a backward facing step, flow over a step with rounded edges shows a relatively shorter separation bubble and a slower return to the developed flow condition.

Investigation of wind speeds over multiple two-dimensional hills

Abstract A wind tunnel investigation of the wind flow over two-dimensional, 1 : 1000 scale hills in a simulated atmospheric boundary layer has been performed. The mean speed and longitudinal turbulence have been measured over a variety of hill geometries which included shallow sinusoidal hills, steep sinusoidal hills, consecutive hills and an irregularly shaped hill. Results from this investigation are presented and are briefly compared with wind speeds calculated using CFD (computational fluid dynamics).

Nonlinear aeroelastic analysis of airfoils: bifurcation and chaos

Different types of structural and aerodynamic nonlinearities commonly encountered in aeronautical engineering are discussed. The equations of motion of a two-dimensional airfoil oscillating in pitch and plunge are derived for a structural nonlinearity using subsonic aerodynamics theory. Three classical nonlinearities, namely, cubic, freeplay and hysteresis are investigated in some detail. The governing equations are reduced to a set of ordinary differential equations suitable for numerical simulations and analytical investigation of the system stability. The onset of Hopf-bifurcation, and amplitudes and frequencies of limit cycle oscillations are investigated, with examples given for a cubic hardening spring. For various geometries of the freeplay, bifurcations and chaos are discussed via the phase plane, Poincaré maps, and Lyapunov spectrum. The route to chaos is investigated from bifurcation diagrams, and for the freeplay nonlinearity it is shown that frequency doubling is the most commonly observed route. Examples of aerodynamic nonlinearities arising from transonic flow and dynamic stall are discussed, and special attention is paid to numerical simulation results for dynamic stall using a time-synthesized method for the unsteady aerodynamics. The assumption of uniform flow is usually not met in practice since perturbations in velocities are encountered in flight. Longitudinal atmospheric turbulence is introduced to show its effect on both the flutter boundary and the onset of Hopf-bifurcation for a cubic restoring force.

Variation of turbulence intensities and integral scales during the passage of a hurricane

Abstract A high-resolution wind speed data set acquired at the Field Research Facility in Duck, NC during the passage of Hurricane Bob (1991) is evaluated in terms of longitudinal and lateral turbulence intensities, and longitudinal integral scales. The calculated parameters are analyzed with regard to mean wind speeds and direction and are separated into stationary and nonstationary segments.

An experimental study of the flow structure within a dense gas plume

A wind tunnel study was conducted to examine how a dense gas plume could affect the mean flow and turbulence structure of the boundary layer containing the plume. For this, a neutral atmospheric boundary layer developing over an aerodynamically rough surface was simulated in the US EPA's Meteorological Wind Tunnel. The dense gas plume was created by releasing CO 2 through a small circular source at ground level. A procedure was developed to make reasonably accurate mean velocity and turbulence measurements within the dense gas plumes by using hot-film anemometry in a range where the probe response was insensitive to the concentration of CO 2. Both the flow visualization and quantitative measurements of concentration and velocity fields indicated that, at low wind speeds, the dense gas plumes exhibited significant buoyancy effects on the flow structure. Within the dense plumes, mean velocity profiles were observed to have changed significantly in shape, with reduced speeds near the surface and increased velocities farther away from the surface. Consistent with these changes in mean velocity profiles, significant reductions in the roughness length and friction velocity were observed. Both the longitudinal and vertical turbulence intensities were also found to be greatly reduced within the dense plumes at low wind speeds. These changes in mean flow and turbulence structure were not only related to the dense-gas concentrations, but also to the local velocity gradients and the growth of the dense plumes with distance from the source. The local gradient Richardson number is found to be the most appropriate parameter for describing the changes in the mean flow and turbulence structure. Significant dense gas effects were observed when the Richardson number increased beyond its critical value (0.25) for the dynamic stability of a stratified flow. Our experimental results show that, in an existing turbulent flow, turbulence is not completely suppressed even when the gradient Richardson number exceeds one.

Combined effect of wall suction and riblets on a low Rθ turbulent boundary layer

AbstractA low Reynolds number turbulent boundary layer is subjected to a relatively strong rate of suction, which is applied through a short porous strip. Downstream of the strip, the effectiveness of a riblet surface is assessed by comparison,to that of a smooth wall. Measured mean velocity and longitudinal turbulence intensity profiles indicate that the riblets do not hinder the action the suction can have on the flow.

A wake model for two-dimensional (sharp-edged) bluff bodies

A two-dimensional wake model for turbulence profiles is developed for (sharp-edged) bluff bodies based on similarity (self-preservation) and universality (different shapes). The method employs spectral filtering around the shedding peak and normalization by the local maximum longitudinal turbulence ( I um ) and by the corresponding lateral coordinate ( y m).

Post-Instability Behavior of a Structurally Nonlinear Airfoil in Longitudinal Turbulence

The effect of longitudinal atmospheric turbulence on the dynamics of an airfoil with a hardening cubic structural nonlinearity in pitch is investigated. The aerodynamics is taken as linear. For the so-called nonexcited case, corresponding to zero turbulence, two distinct regions of dynamic response are obtained. For velocities below the e utter speed, the airfoil response is characterized as a stable equilibrium position; whereas after the onset of e utter, limit cycle oscillations occur. However, three different regions of dynamic behavior are observed when the airfoil is excited by longitudinal turbulence. For the lowest and highest ranges of velocity, the dynamic response is the same as that obtained for the nonexcited case. However, in the middle range of velocities, a new form of dynamic behavior is obtained, where the airfoil response is concentrated about the equilibrium position. The existence of this new region of dynamic behavior is attributed to the parametric nature of the excitation. In addition, for the excited case, e utter occurs at a lower velocity than for the nonexcited case; whereas the onset of limit cycle oscillations occurs at a higher velocity.

Similarity law and turbulence intensity profiles in a bubbly boundary layer at low void fractions

This paper follows on from the research recently published by Moursali et al. (1995). While the first data reported were relative to the void migration, the wall shear stress and in a very limited extent to the liquid velocity, the present results provide new information on the kinematic and turbulent structure of a bubbly boundary layer at low air concentrations. The mean liquid velocity is shown experimentally to obey a modified logarithmic law of the wall in the presence of millimetric bubbles. An expression for this modified law is derived by simple analytical considerations and non-dimensional scaling. The skin friction calculated on this theoretical basis fits the measurements satisfactorily. Longitudinal turbulence intensity profiles are also obtained. They show that the turbulence is increased by two relatively uncoupled mechanisms: a modification of the wall production and the creation of pseudo-turbulence in the external layer. Finally, the mixing length inferred from the data is given and compared with some of the models which are proposed in the literature.

Flow structure of the solids in a 3‐D gas–liquid–solid fluidized bed

AbstractLocal and macroscopic solids flow structure and kinematics in a 3‐D gas–liquid–solid fluidized bed were studied using a noninvasive radioactive‐particle tracking (RPT) technique. Based on the multisite detection of γ radiations emitted from a single radiolabeled tracer particle freely moving in the fluidized bed, RPT permitted to obtain fast sampling of 3‐D trajectories of the tracer, whose physical properties were similar to those of the solids inventory. These trajectories showed the detailed motion sequences of the solid particles as entrained in the bubble wakes, fluctuating randomly or sinking deterministically in the liquid–solid emulsion. Based on measurements done in the vortical‐spiral flow regime, the dynamic solids flow structure inside a three‐phase fluidized bed can be viewed as a three‐zone core–annulus–annulus structure: (1) a central fast‐bubble flow region with the particles swirling upward; (2) a vortical flow region around the velocity inversion point with the particles momentarily captured in emulsion vortices; and (3) a relatively bubble‐free descending flow region where the particles spiral down between the velocity inversion point and vessel walls. Our solids flow structure of dense fluidized beds are similar to the flow structure of liquid and/or solid in lean fluidized beds (observed through laser sheeting imaging). Measured distributions of local ensemble‐averaged particle velocities and turbulence intensities were consistent with the existence of a toroidal recirculatory solids flow pattern in the bed. Measured mean circumferential ensemble‐averaged radial velocity was essentially zero throughout most of the bed. The solids flow turbulence field was nonisotropic, as radial turbulence intensities were generally lower than longitudinal turbulence intensities.

Velocity and spanwise vorticity measurements in an excited mixing layer of a plane jet

The mixing layer of a plane jet was subjected to periodic weak excitation at two different frequencies corresponding to shear layer mode (Stθ=0.012) and preferred mode (St D =0.36). The nozzle exit boundary layer was identical for the unexcited and excited flows. Measurements of mean velocity, longitudinal and lateral velocity fluctuations, Reynolds shear stress and spanwise component of fluctuating vorticity were made over a longitudinal distance x/D of 6 for both the unexcited and the excited flows. Even weak excitation was observed to influence the development of the mixing layer. Under shear layer mode of excitation, the width of the layer and longitudinal turbulence level decrease compared to the naturally developing (unexcited) flow whereas preferred mode of excitation results in increase in the width and turbulence levels. The rms spanwise vorticity showed an increase for shear layer mode of excitation whereas the preferred mode of excitation resulted in a decrease compared to the values in an unexcited flow. Spectra of velocity and vorticity fluctuations exhibited subharmonic peaks, suggesting the possible occurrence of vortex pairing in both unexpected and excited flows. The influence of excitation is found to decrease as x/D increases and is not significant at x/D=6.

Effect of concentrated wall suction on a turbulent boundary layer

The effect of suction, applied through a short porous wall strip, on a low Reynolds number self-preserving turbulent boundary layer has been quantified by measuring the local wall shear stress and the main Reynolds stresses downstream of the strip. When the suction rate is sufficiently high, pseudo-relaminarization occurs almost immediately downstream of the strip. Farther downstream, transition occurs followed by a slow return to a fully turbulent self-preserving state. During relaminarization, the measured skin friction coefficient cf falls below the level corresponding to the no suction value, reaching a minimum where transition begins. An empirical cf distribution is proposed that groups together results obtained at different streamwise stations and different suction rates. Of all the measured Reynolds stresses, the longitudinal turbulence intensity recovers relatively quickly from the change in boundary conditions while the wall-normal turbulence intensity and the Reynolds shear stress are significantly affected by the suction. The Reynolds shear stress, which is negligible during relaminarization, has the slowest recovery.

The distribution and correlation of fluctuating pressures on the Texas Tech Building

Full scale pressure measurements at the Texas Tech Field Research Laboratory have been used to investigate the distribution and correlation of fluctuating pressures on a low rise building. Eigenvector analysis of the covariances indicate that longitudinal and lateral turbulence are responsible for the majority of pressure fluctuations. However, a significant second eigenvector for flow normal to the roof edges corresponds with the rollup and discrete shedding of the separation bubble as a whole. Conditional sampling of peak pressures shows that they are better correlated spatially than the time averaged fluctuations.

Snowdrifting simulation around Davis Station workshop, Antarctica

This paper describes the simulation of turbulent boundary layer flow in a wind tunnel for the study of snowdrifting around Antarctic buildings. Profiles of mean wind speed and turbulence intensity, and the longitudinal turbulence spectrum, are presented. Similarity parameters proposed by other researchers are applied to the simulation. Snowdrifting around the workshop building of Australian Davis Station was investigated. Two attachments to the building were tested to find a way of alleviating a known snowdrifting problem.