Vertical Component Of Turbulence Research Articles

Turbulence coherence in complex wind field: Focusing on its on-site characteristics based on multi-point clustering

Turbulence coherence in mountainous areas necessitates meticulous evaluation due to the complex terrain blocking. This study focuses on the turbulence coherence of the non-uniform wind field in a canyon via the anemometers positioned along a bridge span. Based on a proposed multi-point clustering, two categories of strong winds impacted by a nearby ridge are identified. The turbulence coherence of the two strong winds is then investigated with an extra focus on the phase spectrum. Several complex coherence functions are extended from commonly employed real forms, including the simplified Krenk's formula proposed. The fitting results of them all suggest that parameters such as the decay factor of the two strong winds show a negative linear correlation with separation but their relationship with wind speed is not significant. For the phase spectrum, in addition to skew winds, the contribution of horizontal wind shear is first emphasized in the two strong winds with piecewise fittings. This paper also discusses single-point coherence between the along-wind and vertical turbulence components. It is found to be unusually positive in sign due to the influence of the local topography and a formula with a constant term is suggested that substantially improves the fitting compared to Solari's formula.

The probabilistic turbulence profiles of tropical cyclones in open and flat terrain

The fluctuating winds are usually characterized in the frequency domain by the power spectral density (PSD) function and the coherence function, in which the turbulence intensity and turbulence integral scale are key parameters to describe the gust characteristics of turbulent winds. These parameters estimated from tropical cyclone (TC) observations show significant scattering. Starting from a critical review of the state of the art, the Von Karman spectra are chosen for the TC winds. The turbulence intensity and the turbulence integral scale used in wind spectral functions are discussed in detail. By introducing randomness into the empirical models, the probabilistic longitudinal turbulence profiles of TCs are derived from a wide collection of field measurements with open and flat terrain in the literature. Moreover, the probabilistic models of the lateral and vertical turbulence components are obtained by assuming proportionality between these two components and the longitudinal one. The mean and standard deviation of the developed models for TCs are compared with those defined in the seven major wind loading codes worldwide as well as the model given by Solari to testify these models and illustrate their differences to synoptic winds.

Acceleration-Based In Situ Eddy Dissipation Rate Estimation with Flight Data

Inducing civil aviation aircraft to bumpiness, atmospheric turbulence is a typical risk that seriously threatens flight safety. The Eddy Dissipation Rate (EDR) value, as an aircraft-independent turbulence severity indicator, is estimated by a vertical wind-based or aircraft vertical acceleration-based algorithm. Based on the flight data of civil aviation aircraft, the vertical turbulence component is obtained as the input of both algorithms. A new method of computing vertical acceleration response in turbulence is put forward through the Unsteady Vortex Lattice Method (UVLM). The lifting surface of the target aircraft is assumed to be a combination of wing and horizontal tail in a turbulent flight scenario. Vortex rings are assigned on the mean camber surface, forming a non-planar UVLM, to further improve the accuracy. Moreover, the neighboring vortex lattices are placed as close as possible to the structural edge of control surfaces. Thereby, a complete algorithm for estimating vertical acceleration and in situ EDR value from Quick Access Recorder (QAR) flight data is proposed. Experiments show that the aerodynamic performance is computed accurately by non-planar UVLM. The acceleration response by non-planar UVLM is able to track the recorded acceleration data with higher accuracy than that of the linear model. Different acceleration responses at different locations are also obtained. Furthermore, because the adverse effects of aircraft maneuvers are separated from turbulence-induced aircraft bumpiness, the new acceleration-based EDR algorithm shows better accuracy and stability.

Strategies for modeling homogeneous isotropic turbulence and investigation of spatially correlated aerodynamic forces on a stationary model

A numerical simulation method is proposed for the generation of homogeneous isotropic turbulence and is applied to the investigation of the spanwise coherence aerodynamic force on a stationary airfoil. First, the inflow turbulence is generated by modeling a grid at the inlet of the computational domain. To verify the accuracy of the generated turbulence field, a series of relevant parameters are compared with the results of wind tunnel tests and previous studies. A model with NACA 0015 profile is then placed in the downstream turbulence. The relationship between the spanwise correlation width of the vertical turbulence component and the lift force is determined. Possible mechanisms for the result are discussed. This study demonstrates that the proposed turbulence generation method is an effective tool for investigating the spatial distribution of unsteady aerodynamic loads on elongated bodies.

Experimental determination of aerodynamic admittance functions of a bridge deck considering oscillation effect

The effective aerodynamic shape of a bridge deck during a buffeting response can be modified relative to its original section in the stationary state, and the identification method for aerodynamic admittance functions (AAFs) of a bridge deck considering the oscillation effect is established based on wind tunnel tests in conjunction with theoretical analyses. The aerodynamic forces on a bridge deck strip and surrounding fluctuating wind speeds, as well as the response of the model in a turbulent flow, are measured simultaneously on a stochastic vibrating spring-suspended sectional model (SSSM). A new method is proposed to separate the aerodynamic forces on the bridge deck strip into self-excited and buffeting force components. The AAFs with respect to longitudinal and vertical turbulence components are then identified according to the extracted buffeting forces and measured fluctuating wind speeds via a colligated least square method (CLSM). The approach is validated experimentally on a closed-box bridge deck strip using force-balance measurements, but it can be applied to bridge decks of any cross-section. The bridge deck AAF is confirmed to depend on its oscillation state, and the wind tunnel test for the identification of bridge deck AAFs was carried out in the oscillating state for high accuracy.

Assessment of overturning risk of high-speed trains in strong crosswinds using spectral analysis approach

This study introduces a spectral analysis framework for assessing overturning risk of high-speed trains in strong crosswinds. The wind turbulence relative to moving vehicles is used to model stochastic wind excitation, whose spectral characteristics are determined by a newly introduced general method from the spectrum and coherence function of turbulence relative to ground. The unsteady aerodynamic forces on vehicles are modeled with consideration of longitudinal, lateral and vertical turbulence components. Based on the wind tunnel experiments of a typical China railway high-speed train model, the side and lift force coefficients and aerodynamic admittance functions associated with different turbulence components are extracted, where the effects of spatial coherence of turbulence are explicitly accounted for. The probabilistic overturning risk is then evaluated through unloading rate of wheel-rail contact force, which leads to the determination of probabilistic characteristic wind curve. The results demonstrated that the dynamic wheel-rail contact force induced by track irregularities is lower than that by wind turbulence. In addition to the traditionally considered longitudinal turbulence, the lateral and vertical turbulence components also have great contribution to vehicle response. Adequate modeling of aerodynamic admittance functions is also important for better quantifications of vehicle response and overturning risk.

Effects of co-spectral densities of atmospheric turbulence on the dynamic response of cable-supported bridges: A case study

This study deals with the importance of the co-spectral density of the horizontal and vertical turbulence components when predicting the wind-induced dynamic response of long-span bridges. The simultaneous measurements of turbulence and response accelerations of the Sotra Bridge are investigated. A turbulence model including the cross-spectral density of the horizontal and vertical components is fitted to the turbulence measurements. This turbulence model is used to predict the standard deviations of the wind-induced accelerations of the Sotra Bridge. The predicted response is compared to the measurements, and it is concluded that the predicted horizontal response is closer to the measured response values if the co-spectral density of the turbulence components are included. The vertical response was not affected by the co-spectral density of the horizontal and vertical turbulence components. It is concluded that the influence on the predicted response is observable, but the contribution is low compared to the overall uncertainty in the modelling of the wind field.

Effect of depth on flow over a fixed dune

Laser Doppler velocimeter (LDV) measurements were carried out to study the effect of depth on the flow over a train of fixed two-dimensional dunes. Conventionally averaged velocity and turbulence parameters reveal large peaks in the streamwise and vertical components of turbulent intensities and shear stress, along the shear layer emanating from the dune crest. A secondary peak in the streamwise turbulence profiles some distance beyond the shear layer indicates maintenance of turbulence generated on the previous dune and convection of the flow history from one dune to the next. Analyses based on triple products and quadrant decomposition of velocity fluctuations reveals the central role of the shear layer in dictating the flow properties over the entire depth. The depth influences the flow in the near-bed region and the length of the separation zone is longer at a shallower depth. The streamwise mean profiles collapse onto a single curve in the outer region beyond the shear layer, indicating a degree of similarity and independence from the near-bed flow. The profiles of the vertical component of turbulence reveal a systematic dependence on flow depth, with lower turbulence intensity at larger depths. The quantitative effect of flow depth is evident in the measurements at all levels, including triple products and quadrant decomposition.

Bifurcation characteristics of a two-dimensional structurally non-linear airfoil in turbulent flow

The dynamics of a structurally non-linear two-dimensional airfoil in turbulent flow is investigated numerically using a Monte Carlo approach. Both the longitudinal and vertical components of turbulence, corresponding to parametric (multiplicative) and external (additive) excitation, respectively, are modelled. The properties of the airfoil are chosen such that the underlying non-excited, deterministic system exhibits binary flutter; the loss of stability of the equilibrium point due to flutter then leads to a limit cycle oscillation (LCO) via a supercritical Hopf bifurcation. For the random system, the results are examined in terms of the probability structure of the response and the largest Lyapunov exponent. The airfoil response is interpreted from the point of view of the concepts of D- and P-bifurcations, as defined in random bifurcation theory. It is found that the bifurcation is characterized by a change in shape of the response probability structure, while no discontinuity in the variation of the largest Lyapunov exponent with airspeed is observed. In this sense, the trivial bifurcation obtained for the deterministic airfoil, where the D- and P-bifurcations coincide, appears only as a P-bifurcation for the random case. At low levels of turbulence intensity, the Gaussian-like bell-shaped bi-dimensional PDF bifurcates into a crater shape; this is interpreted as a random fixed point bifurcating into a random LCO. At higher levels of turbulence intensity, the post-bifurcation PDF loses its underlying deterministic LCO structure. The crater is transformed into a two-peaked shape, with a saddle at the origin. From a more universal point of view, the robustness of the random bifurcation scenario is critiqued in light of the relative importance of the two components of turbulent excitation.

A Comparison of Standard Coherence Models for Inflow Turbulence With Estimates from Field Measurements

The Long-term Inflow and Structural Test (LIST) program, managed by Sandia National Laboratories, Albuquerque, NM, is gathering inflow and structural response data on a modified version of the Micon 65/13 wind turbine at a site near Bushland, Texas. With the objective of establishing correlations between structural response and inflow, previous studies have employed regression and other dependency analyses to attempt to relate loads to various inflow parameters. With these inflow parameters that may be thought of as single-point-in-space statistics that ignore the spatial nature of the inflow, no significant correlation was identified between load levels and any single inflow parameter or even any set of such parameters, beyond the mean and standard deviation of the hub-height horizontal wind speed. Accordingly, here, we examine spatial statistics in the measured inflow of the LIST turbine by estimating the coherence for the three turbulence components (along-wind, across-wind, and vertical). We examine coherence spectra for both lateral and vertical separations and use the available ten-minute time series of the three components at several locations. The data obtained from spatial arrays on three main towers located upwind from the test turbine as well as on two additional towers on either side of the main towers consist of 291 ten-minute records. Details regarding estimation of the coherence functions from limited data are discussed. Comparisons with standard coherence models available in the literature and provided in the International Electrotechnical Commission (IEC) guidelines are also discussed. It is found that the Davenport exponential coherence model may not be appropriate especially for modeling the coherence of the vertical turbulence component since it fails to account for reductions in coherence at low frequencies and over large separations. Results also show that the Mann uniform shear turbulence model predicts coherence spectra for all turbulence components and for different lateral separations better than the isotropic von Ka´rma´n model. Finally, on studying the cross-coherence among pairs of turbulence components based on field data, it is found that the coherence observed between along-wind and vertical turbulence components is not predicted by the isotropic von Ka´rma´n model while the Mann model appears to overestimate this cross-coherence.

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.

Structurally Nonlinear Fluttering Airfoil in Turbulent Flow

The dynamics and response of a structurally nonlinear two-dimensional airfoil in turbulent flow are investigated numerically. The solution is analyzed in terms of probability density function (PDF), response mean square, power spectral density, and Lyapunov exponent. Both the longitudinal and vertical components of turbulence are considered. It is shown that turbulent flow, due to its longitudinal component, diminishes the stability of the aeroelastic system by advancing the flutter point and decreasing the damping, thus confirming results from previous work where only the longitudinal component was modeled. Furthermore, the shift in flutter airspeed is strongly dependent on the random stiffness terms. It is also observed that turbulence may advance the airspeed at which the pitch angle marginal PDF changes from uni- to bimodal compared with the nonexcited deterministic case, whereas the heave transition appears to be postponed. This observation is attributed to the nonlinearity considered, a cubic stiffness in torsion. Finally, in terms of response mean square, the system appears to he more sensitive to the presence of the longitudinal excitation component at pre- rather than at postinstability airspeeds.

MOTION-RELATED BODY-FORCE FUNCTIONS IN TWO-DIMENSIONAL LOW-SPEED FLOW

The paper examines two-dimensional force functions useful in the wind-response analysis of bluff bodies, such as bridges, that are elongated in the across-wind direction. Airfoil-type theoretical 2-D indicial, admittance, and oscillatory force functions and their interrelationships are first recalled for their analogical value. Following this, a spectral force expression is developed for the lift on a bluff section due to a cross-wind with a vertical turbulence component. The expression proposed involves an aerodynamic admittance that is based upon measured flutter derivatives for the bluff section plus information on the coherence of vertical turbulence in the atmospheric boundary layer.

Wettability Effects on Liquid Flow Characteristics in a Bubbling Wall Jet along a Vertical Flat Plate.

A bubbling wall jet was generated along a vertical flat plate with different wettability by using a single-hole nozzle. Water flow characteristics in the jet were measured with a laser Doppler velocimeter. Effects of the wettability of the plate emerged on the water flow characteristics primarily in the momentum region near the nozzle tip where the inertia force of injected gas governed the flow field. In the buoyancy region located above the momentum region, the horizontal distributions of the vertical mean velocity and the root-mean-square value of the vertical turbulence component of water flow were hardly dependent on the wettability of the plate and followed their respective similar distributions.

Comparison between the atmospheric boundary layer in Paris and its rural suburbs during the ECLAP experiment

The ECLAP experiment has been performed during the winter of 1995 in order to study the influence of the urban area of Paris on the vertical structure and diurnal evolution of the atmospheric boundary layer, in situations favourable to intense urban heat island and pollution increase. One urban site and one rural site have been instrumented with sodars, lidars and surface measurements. Additional radiosondes, 100 m masts and Eiffel Tower data were also collected. This paper gives a general overview of this experiment, and presents results of the analysis of four selected days, characterized by various wind directions and temperature inversion strengths. This analysis, which consists in a comparison between data obtained in the two sites, has been focused on three parameters of importance to the ABL dynamics: the standard deviation of vertical velocity, the surface sensible heat flux, and the boundary layer height. The vertical component of turbulence is shown to be enhanced by the urban area, the amplitude of this effect strongly depending on the meteorological situation. The sensible heat flux in Paris is generally found larger than in the rural suburbs. The most frequent differences range from 25–65 W m -2, corresponding to relative differences of 20–60%. The difference of unstable boundary layer height between both sites are most of the time less than 100 m. However, sodar and temperature data show that the urban influence is enhanced during night-time and transitions between stable and unstable regimes.

Wettability Effects on Bubble Characteristics in a Bubbling Wall Jet along a Vertical Flat Plate.

A bubbling wall jet was generated along a vertical flat plate with different wettability by using a single-hole nozzle. Water flow characteristics in the jet were measured with a laser Doppler velocimeter. Effects of the wettability of the plate emerged on the water flow characteristics primarily in the momentum region near the nozzle tip where the inertia force of injected gas governed the flow field. In the buoyancy region located above the momentum region, the horizontal distributions of the vertical mean velocity and the root-mean-square value of the vertical turbulence component of water flow were hardly dependent on the wettability of the plate and followed their respective similar distributions.

Volcanic plume simulation on large scales

The plume model ATHAM (Active Tracer High Resolution Atmospheric Model) is designed to simulate explosive volcanic eruptions for a given mass flux of pyroclastic material under realistic atmospheric background conditions. Based on the assumption that all particles are small the model's equations are simplified such that, besides equations for gaseous, liquid and solid constituents of arbitrary concentrations, only the volume means of momentum and heat are predicted. The exchange of momentum and heat between the fluid's constituents are treated diagnostically. A prognostic turbulence closure scheme describing the entrainment of ambient air into the plume takes into account the anisotropy of the horizontal and vertical components of turbulence. Its length scale is assumed to be isotropic. Microphysical processes such as the exchange of heat and momentum between dry air, water vapor, cloud water, precipitable water, ice crystals and graupel are parameterized. Ash and lapilli represent the spectrum of silicate particles. A diagnostic sedimentation velocity allows for the separation of gas and particles. The model is formulated with an implicit time stepping scheme. The equations of motion and the transport equations for tracers are formulated in flux form in order to guarantee the conservation of momentum and all tracer masses. The heat transport equation is in advective form. The wave equation and the equations for the transport of momentum, heat and tracers are solved using a combined line-relaxation successive overrelaxation scheme. Two-dimensional experiments for symmetric cases with cylindrical coordinates yield qualitatively similar results to other dynamic–thermodynamic models. However, entrainment processes are now computed quantitatively through the turbulence closure and condensed matter has a sophisticated description. In order to study the transferability of results from computationally cheap two-dimensional experiments to costly three-dimensional simulations of a realistic plume, comparisons between calculations with and without cylindrical coordinates are performed. Finally, experiments for different atmospheric background conditions allow investigation of plume development on the influence of cross wind effects, and temperature and humidity profiles.

Sodar estimates of surface heat flux and mixed layer depth compared with direct measurements

The ECLAP experiment has been performed during the winter of 1995 in order to study the influence of the urban area of Paris on the vertical structure and diurnal evolution of the atmospheric boundary layer, in situations favourable to intense urban heat island and pollution increase. One urban site and one rural site have been instrumented with sodars, lidars and surface measurements. Additional radiosondes, 100 m masts and Eiffel Tower data were also collected. This paper gives a general overview of this experiment, and presents results of the analysis of four selected days, characterized by various wind directions and temperature inversion strengths. This analysis, which consists in a comparison between data obtained in the two sites, has been focused on three parameters of importance to the ABL dynamics: the standard deviation of vertical velocity, the surface sensible heat flux, and the boundary layer height. The vertical component of turbulence is shown to be enhanced by the urban area, the amplitude of this effect strongly depending on the meteorological situation. The sensible heat flux in Paris is generally found larger than in the rural suburbs. The most frequent differences range from 25–65 W m-2, corresponding to relative differences of 20–60%. The difference of unstable boundary layer height between both sites are most of the time less than 100 m. However, sodar and temperature data show that the urban influence is enhanced during night-time and transitions between stable and unstable regimes.

On the Spectra of Turbulent Velocity Field in a Stably Stratified Flow

Abstract This paper describes a method to solve the spectral equation for the balance of turbulent kinetic energy in a stably stratified turbulent shear flow. The cospectra of vertical momentum and heat flux arc modelled with the aid of a basic eddy-viscosity (or turbulent exchange coefficient) function. For the term representing the inertial transfer of turbulent kinetic energy, Pao's [Phys. Fluids 8 (1965)] form is assumed. Analytical expressions for the three-dimensional kinetic energy spectrum as well as the cospectra of momentum and heat flux are obtained over the range of wave numbers k≥kb, which includes the inertial subrange kb≪k≪ks and the viscous subrange k>ks (kb and ks are the buoyancy and Kolmogorov wavenumbers, respectively). The two one-dimensional spectra, e.g., the kinetic energy spectra of the horizontal and vertical components of turbulence are derived from the three-dimensional kinetic energy spectrum. These one-dimensional spectra are compared with the measured data of Gargett et al. [J. Fluid Mech. 144 (1984)] for the case I ( = ks/kb) = 630. Finally, we compute the basic eddy-viscosity function and discuss its behavio

A wind tunnel study of the particle collection efficiency of an inverted frisbee used as a dust deposition gauge

The paper describes wind tunnel measurements of the performance of an inverted Frisbee used as a dust deposition gauge. Tests were conducted using glass spheres to simulate dust up to 183 μm in aerodynamic diameter over a range of wind speeds up to 10 m s −1. An overall figure for the collection efficiency of the Frisbee is around 50%. This is significantly better than for other gauges which are commonly in use for dustfall measurement. The collection efficiency fell with increasing wind speed and also with decreasing particle diameter down to a minimum at around 80–100 μm before rising again. This behaviour seems to be connected with a change in the main particle collection mechanism from deposition to turbulent diffusion. Some tests were performed to verify this finding by increasing the vertical components of turbulence over the gauge; this increased the collection efficiency and may be a useful method of improving performance in the field. A thin coating of liquid paraffin gives greater collection efficiencies at higher wind speeds due to a reduction in the blow-out of collected dust. The interference from the bottle mounted under the Frisbee for collecting excess rainfall was also investigated and found to be significant. Another gauge design was also investigated alongside the Frisbee. This was a snow collector designed originally for acid precipitation studies but which should also be a good dust collector. It proved to be so, though its collection efficiency was less than for a Frisbee.