J. van Beeck

Nonintrusive measurements of temperature and size of single falling raindrops

A nonintrusive laser technique, based on the detection of a rainbow, is presented that permits one to determine simultaneously the temperature and size of droplets. Therefore the Airy theory for a rainbow and a calibration rainbow pattern at isothermal conditions are applied. Rainbow patterns coming from droplets in the millimeter range have been recorded on a linear CCD array. It has been found that the sphericity of the droplets plays an important role for this rainbow-based technique.

Rainbow phenomena applied to the measurement of droplet size and velocity and to the detection of nonsphericity

An experimental method is presented that detects whether a droplet is spherical. The method is based on a comparison between two droplet diameters deduced from two different optical interference patterns observed in a rainbow that is created by a droplet scattering laser light. Experimental validation has been carried out with a CCD camera. Once a rainbow pattern has been identified as coming from a spherical droplet, we can derive a reliable droplet velocity and diameter from the same interference patterns, using theories for the rainbow that are valid only for spherical droplets. Preliminary experiments have been carried out with a laser beam and a photomultiplier.

Global rainbow thermometry for droplet-temperature measurement

Standard rainbow thermometry connects the scattering angle of the main rainbow maximum, generated by a single droplet, to the droplets refractive index and thus to its temperature. Droplet nonsphericity influences the rainbow position and therefore degrades the quality of the droplet-temperature measurement. We propose global rainbow thermometry, which measures the average rainbow position that is created by multiple droplets and from which a mean temperature can be derived. The new technique aims at eliminating the nonsphericity effect. The principle of this method is presented, and a typical recorded image is discussed.

A stochastic perturbation method to generate inflow turbulence in large-eddy simulation models: Application to neutrally stratified atmospheric boundary layers

Despite the variety of existing methods, efficient generation of turbulent inflow conditions for large-eddy simulation (LES) models remains a challenging and active research area. Herein, we extend our previous research on the cell perturbation method, which uses a novel stochastic approach based upon finite amplitude perturbations of the potential temperature field applied within a region near the inflow boundaries of the LES domain [Munoz-Esparza et al., “Bridging the transition from mesoscale to microscale turbulence in numerical weather prediction models,” Boundary-Layer Meteorol., 153, 409–440 (2014)]. The objective was twofold: (i) to identify the governing parameters of the method and their optimum values and (ii) to generalize the results over a broad range of atmospheric large-scale forcing conditions, Ug = 5 − 25 m s−1, where Ug is the geostrophic wind. We identified the perturbation Eckert number, Ec=Ug2/ρcpθpm, to be the parameter governing the flow transition to turbulence in neutrally strat...

Turbulent fluxes, stability and shear in the offshore environment: Mesoscale modelling and field observations at FINO1

This paper is focused on the evaluation of five planetary boundary layer (PBL) schemes in the Weather Research and Forecasting model for offshore wind energy purposes. One first order scheme: Yonsey University and four one-and-a-half order schemes: Mellor-Yamada-Janic, Quasi-Normal Scale Elimination, Mellor-Yamada-Nakanishi-Niino, and Bougeault-Lacarrere, are considered. Turbulent flux measurements from the FINO1 platform in the North Sea are used to estimate the Obukhov length, allowing the sorting of the data into different stability classes. In addition, wind LiDAR measurements are used to analyze wind profiles up to 251.5 m, encompassing the heights where todays wind turbines operate. The ability of the different PBL schemes to forecast turbulent fluxes of heat and momentum and surface stability is evaluated. Obukhov length results show that in general, PBL schemes forecast more moderated stable stratifications and a reinforcement of the instability for neutral and convective conditions, compared to ...

Roughness Effects on Wind-Turbine Wake Dynamics in a Boundary-Layer Wind Tunnel

Increasing demand in wind energy has resulted in increasingly clustered wind farms, and raised the interest in wake research dramatically in the last couple of years. To this end, the present work employs an experimental approach with scaled three-bladed wind-turbine models in a large boundary-layer wind-tunnel. Time-resolved measurements are carried out with a three-component hot-wire anemometer in the mid-vertical plane of the wake up to a downstream distance of eleven turbine diameters. The major issue addressed is the wake dynamics i.e. the flow and turbulence characteristics as well as spectral content under two different neutral boundary-layer inflow conditions. The wind tunnel is arranged with and without roughened surfaces in order to mimic moderately rough and smooth conditions. The inflow characterization is carried out by using all three velocity components, while the rest of the study is focused on the streamwise component’s evolution. The results show an earlier wake recovery, i.e. the velocity deficit due to the turbine is less persistent for the rough case due to higher incoming turbulence levels. This paves the way for enhanced mixing from higher momentum regions of the boundary layer towards the centre of the wake. The investigation on the turbulent shear stresses is in line with this observation as well. Moreover, common as well as distinguishing features of the turbulent-scales evolution are detected for rough and smooth inflow boundary-layer conditions. Wake meandering disappears for rough inflow conditions but persists for smooth case with a Strouhal number similar to that of a solid disk wake.

Simultaneous Determination of Temperature and Size of Droplets from the Rainbow using Airy Theory

The anisotropy of laser light scattered by a droplet exhibits a strong dependence on droplet size and temperature. Around the geometrical rainbow angle this dependence is such that it can be used to determine these two parameters. This determination is done with the help of the Airy theory for the rainbow to avoid an extensive calibration of the angular scattered light intensity. A comparison between the Airy and the Mie theories shows that one has to be careful in applying the Airy theory for this purpose. The Airy theory has also been compared with the experimental first order “monochromatic rainbow” created by single falling droplets crossing a laser beam. There is a qualitative good agreement between experiment and theory after the measured rainbow pattern has been properly smoothed.

Adaptation of mesoscale turbulence parameterisation schemes as RANS closures for ABL simulations

ABSTRACT The present study presents different k-ε turbulence closures for atmospheric boundary layer flows using computational fluid dynamics (CFD) simulations that are consistent with inflow conditions from numerical weather prediction (NWP) simulations. Eight different mesoscale turbulence parameterisation schemes of the Weather Research and Forecasting (WRF) model are covered. To ensure consistency between the NWP and CFD simulations, different closure coefficients of the k − ε turbulence model for each NWP scheme are proposed. This is achieved by combining production–dissipation closure coefficient relationships based on the Monin–Obukhov similarity theory and the formulation based on the Coriolis parameter proposed by Detering and Etling. The proposed methodology has been implemented in the open source CFD toolbox OpenFOAM and is demonstrated at near-neutral stability conditions for the classical Askervein Hill case.

Simultaneous determination of bubble diameter and relative refractive index using glare circles

In the shadow image of a spherical gas bubble, high intensity rings are visible, i.e., glare circles. These can be used to obtain a more precise estimate of the bubble diameter than can be obtained from the shadow contour. The glare circle diameter can also be used to determine the relative refractive index by comparing it with the shadow diameter. The precision of this refractive index measurement reaches the third decimal, which is demonstrated experimentally. Thus, one can simultaneously determine the bubble diameter (from the shadow contour) and the relative refractive index (from the glare circle).

Processing droplet temperature measurement data obtained with rainbow thermometry

The state of the art in Rainbow Thermometry is presented. Rainbow Thermometry is a technique for measuring size and temperature of transparent droplets. For data inversion a rainbow pattern is employed, which is formed by a single droplet or by constructive interference of laser light scattered by an ensemble of spherical droplets. In the first case, one speaks about Standard Rainbow Thermometry (SRT), investigated since 1988. In the second case, the technique is called Global Rainbow Thermometry (GRT), studied since 1999; here, the non-spherical droplets and liquid ligaments results in a uniform background and thus do not influence the interference pattern, formed by the spherical droplets, from which average size and temperature are derived. This is a large improvement with respect to Standard Rainbow Thermometry, which is strongly influenced by particle shape. Moreover, GRT is applicable for smaller droplets than the standard technique because the global pattern is not spoiled by a ripple structure. Data inversion schemes based on inflection points, minima and maxima are discussed for SRT and GRT. The standard technique is applied to a monodisperse burning droplet stream, where the problems with particle shape do not exist. Global Rainbow Thermometry is applied to a heated water spray, where the standard technique fails. For both applications the accuracy in the temperature measurement was a few degrees Celsius.