Vertical cross-spectra of horizontal velocity components at the Boulder observatory

Two years of continuous sonic anemometer measurements conducted in 2007 and 2008 at the FINO1 platform are used to investigate the characteristics of the single-and two-point velocity spectra in relation to the atmospheric stability in the marine atmospheric boundary layer. The goals are to reveal the limits of current turbulence models for the estimation of wind loads on offshore structures, and to propose a refined description of turbulence at altitudes where Monin-Obukhov similarity theory may be limited. Using local similarity theory, a composite spectrum model, combining a pointed and a blunt model, is proposed to describe the turbulence spectrum for both unstable, neutral and stable conditions. Such a model captures the −1 power law followed by the velocity spectra at an intermediate frequency range in the marine atmospheric boundary layer. For a Monin-Obukhov similarity parameter ζ < 0.3, the Davenport coherence model captures the vertical coherence of the horizontal velocity components well. A two-parameter exponential decay function is found more appropriate for modelling the coherence of the vertical velocity component. Under increasingly stable conditions, the size of the eddies in the vertical coordinate reduces, such that smaller separation distances than that covered in the present dataset may be required to study the coherence with sufficient accuracy.

Section: Case Of a Near-neutral Stabilitymentioning

confidence: 62%

Velocity Spectra and Coherence Estimates in the Marine Atmospheric Boundary Layer

Cheynet

Jakobsen

Reuder

2018

“…An estimation of such coefficients has been proposed by Solari and Piccardo (2001) in neutral conditions, and by Pielke and Panofsky (1970), Ropelewsky et al (1973) and Soucy et al (1982) in non-neutral conditions. More sophisticated coherence models are reported for instance in Deaves and Harris (1978) and Mann (1994).…”

Section: The Turbulence Fieldmentioning

confidence: 99%

A simple and efficient procedure for the numerical simulation of wind fields in complex terrain

Burlando

Carassale

Georgieva

et al. 2007

In spite of recent progress in the prognostic numerical simulation of the atmospheric boundary layer, the explicit simulation of turbulent flows in actual complex terrain is generally still very complicated and time consuming for many environmental applications.In an attempt to develop simpler and more efficient application oriented techniques, although less refined, we propose a multi-step procedure for simulating wind fields. Once obtained the necessary meteorological input, the mass-consistent modelling technique is used to perform high-resolution mean wind flow simulations taking into account recent developments in the atmospheric boundary-layer theory. Besides, a procedure based on a generalisation of the local logarithmic law-of-the-wall over complex terrain is used to estimate the effective parameters characterising the simulated wind profiles. Turbulence intensities and spectral properties are then calculated through the estimated effective parameters, in particular through the effective friction velocity parameter. Finally, time series of the instantaneous velocity field are simulated by the Monte Carlo technique. Two applications of the proposed approach are discussed briefly: the first one is related to a coastal area in southern Italy (the Messina Straits), where the construction of the world's longest central span bridge is being planned; the second one corresponds to the flow in a mountainous area in northern Italy (the Albenga Airport).

“…2013). We refer to these papers for details beyond the short overview provided coherence of various velocity components in tower micrometeorological data (Davenport, 1961a;Panofsky and Singer, 1965;Pielke and Panofsky, 1971;Naito and Kondo, 1974;Panofsky et al, 1974;Brook, 1975;Seginer and Mulhearn, 1978;Kanda and Royles, 1978;Soucy et al, 1982;Bowen et al, 1983;Saranyasoontorn et al, 2004), investigations including the lateral/spanwise coherence (Kristensen and Jensen, 1979;Ropelewski et al, 1973;Panofsky and Mizuno, 1975;Perry et al, 1978;Kristensen, 1979;Kristensen et al, 1981;Schlez and Infield, 1998), the coherence of temperature fluctuations (Davison, 1976) and even meso-scale applications (typically in the horizontal directions) (Hanna and Chang, 1992;Woods et al, 2011;Vincent et al, 2013;Larsén et al, 2013;Mehrens et al, 2016). Together, these measurements cover a great variety of terrain and topography.…”

Section: Comparing Davenport's Hypothesis Tomentioning

confidence: 99%

Vertical Coherence of Turbulence in the Atmospheric Surface Layer: Connecting the Hypotheses of Townsend and Davenport

Krug

Baars

Hutchins

et al. 2019

Statistical descriptions of coherent flow motions in the atmospheric boundary layer have many applications in the wind engineering community. For instance, the dynamical characteristics of large-scale motions in wall-turbulence play an important role in predicting the dynamical loads on buildings, or the fluctuations in the power distribution across wind farms. Davenport (Quarterly a seminal study on the subject and proposed a hypothesis that is still widely used to date. Here, we demonstrate how the empirical formulation of Davenport is consistent with the analysis of Baars et al. (Journal of Fluid Mechanics, 2017, Vol. 823, R2) in the spirit of Townsend's attached-eddy hypothesis in wall turbulence. We further study stratification effects based on two-point measurements of atmospheric boundary-layer flow over the Utah salt flats. No self-similar scaling is observed in stable conditions, putting the application of Davenport's framework, as well as the attached eddy hypothesis, in question for this case. Data obtained under unstable conditions exhibit clear self-similar scaling and our analysis reveals a strong sensitivity of the statistical aspect ratio of coherent structures (defined as the ratio of streamwise and wall-normal extent) to the magnitude of the stability parameter.