Revisiting the Local Scaling Hypothesis in Stably Stratified Atmospheric Boundary Layer Turbulence: an Integration of Field and Laboratory Measurements with Large-eddy Simulations

The `local scaling' hypothesis, first introduced by Nieuwstadt two decades ago, describes the turbulence structure of stable boundary layers in a very succinct way and is an integral part of numerous local closure-based numerical weather prediction models. However, the validity of this hypothesis under very stable conditions is a subject of on-going debate. In this work, we attempt to address this controversial issue by performing extensive analyses of turbulence data from several field campaigns, wind-tunnel experiments and large-eddy simulations. Wide range of stabilities, diverse field conditions and a comprehensive set of turbulence statistics make this study distinct

The calibration of (multi-)hot-wire probes. 2. Velocity-calibration

We review a set of velocity calibration methods for one popular configuration of a four-hot-wire probe with the aim of finding a method of estimating with 10% accuracy (based on full scale) the mean flow vector, the rms of the turbulent velocity component and the associated linearized anisotropy invariants. We performed full 3D velocity calibrations of a classic and a sub-miniature probe. With a calibrated King/Jørgensen model and with an interpolation scheme in the real calibrations, we simulated calibrations and (anisotropic) turbulence measurements. Analysis of the idealized and of the realistic simulation data showed that the lookup-table method is the only good tool for processing hot-wire measurements. Polynomial models give large systematic errors. The King/Jørgensen model can only be used for qualitative ends, but it forms a good basis for a simulation-based assessment of the quality of a measured calibration set. Our results indicate that the experimental results reported in the literature, and which were based on hot-wire measurements, may require a reassessment of the calibration method

The calibration of (multi-) hot-wire probes. 1. Temperature calibration

We study the performance of the classical relation for the correction for ambient temperature drift of the signal of a hot-wire anemometer and the influence of practical assumptions. It is shown that most methods to estimate the operational temperature via the temperature/resistance coefficient lead to underestimation of the operational temperature and thus to overcorrection of signals for temperature drift. We found that, in the presence of a sensible heat flow, temperature fluctuations cannot be sufficiently removed from the hot-wire signal when one relies on temperature/resistance coefficients from literature. When only slow temperature drift is involved, most literature values give a satisfactory temperature correction, but this depends on the specific combination of a probe and a literature reference. Therefore it is generally advisable to calibrate the value. A method that uses a ratio of (measured) resistances as a function of temperature, which does not require estimation of the operational temperature of the wire, is shown to depend crucially on a parasitic resistance of a few tenths of an ohm. This parameter can be found by optimizing its value using data from a collection of velocity calibrations at different temperatures. This additional calibration alone suffices to estimate the operational temperature of the wire via optimization. A quick calibration procedure (15 min) is proposed and tested

The calibration of (multi-)hot-wire probes. 2. Velocity-calibration

We review a set of velocity calibration methods for one popular configuration of a four-hot-wire probe with the aim of finding a method of estimating with 10% accuracy (based on full scale) the mean flow vector, the rms of the turbulent velocity component and the associated linearized anisotropy invariants. We performed full 3D velocity calibrations of a classic and a sub-miniature probe. With a calibrated King/Jørgensen model and with an interpolation scheme in the real calibrations, we simulated calibrations and (anisotropic) turbulence measurements. Analysis of the idealized and of the realistic simulation data showed that the lookup-table method is the only good tool for processing hot-wire measurements. Polynomial models give large systematic errors. The King/Jørgensen model can only be used for qualitative ends, but it forms a good basis for a simulation-based assessment of the quality of a measured calibration set. Our results indicate that the experimental results reported in the literature, and which were based on hot-wire measurements, may require a reassessment of the calibration method

Image analysis as measuring technique in flows: Invited Lectures and Extended Abstracts of the Euromech Colloquium 279

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