Turbulence statistics from three different nacelle lidars

Atmospheric turbulence can be characterized by the Reynolds stress tensor, which consists of the second-order moments of the wind field components. Most of the commercial nacelle lidars cannot estimate all components of the Reynolds stress tensor due to their limited number of beams; most can estimate the along-wind velocity variance relatively well. Other components are however also important to understand the behavior of, e.g., the vertical wind profile and meandering of wakes. The SpinnerLidar, a research lidar with multiple beams and a very high sampling frequency, was deployed together with two commercial lidars in a forward-looking mode on the nacelle of a Vestas V52 turbine to scan the inflow. Here, we compare the lidar-derived turbulence estimates with those from a sonic anemometer using both numerical simulations and measurements from a nearby mast. We show that from these lidars, the SpinnerLidar is the only one able to retrieve all Reynolds stress components. For the two- and four-beam lidars, we study different methods to compute the along-wind velocity variance. By using the SpinnerLidar's Doppler spectra of the radial velocity, we can partly compensate for the lidar's probe volume averaging effect and thus reduce the systematic error of turbulence estimates. We find that the variances of the radial velocities estimated from the maximum of the Doppler spectrum are less affected by the lidar probe volume compared to those estimated from the median or the centroid of the Doppler spectrum.

Measurements of the local temperature correlation time in a turbulent flame using coherent anti-Stokes Raman spectroscopy

The feasibility to determine the timescale of pulsations of “instantaneous” local temperatures in a turbulent flame at a microsecond time scale by using coherent anti-Stokes Raman scattering (CARS) spectroscopy is demonstrated for the first time to our knowledge. A laboratory laser measurement complex was utilized, based on two CARS-spectrometers employing synchronized pulse-repetitive lasers with 10 ns pulse duration. The system enabled to record, with high temporal resolution (in one single laser shot) and at a variable delay between two sequential shots following each other in pairs at a repetition rate of 10 Hz, series of CARS spectra of N2 molecules from a probe volume as small as 0.03×0.03×2 mm3. From the spectra, “instantaneous” temperatures at a given delay were derived. The obtained values enabled calculation of the correlation coefficient of temperature pulsations vs the delay. The results are presented for the series of 500 single-shot coupled measurements, at the delays in the range 1 μs – 0 ms, of local gas temperatures in a few points of an open turbulent partially premixed methane-air flame of a model burner with visually distinguishable stability of combustion. The average temperatures were between 1500 K and 1800 K. The measurements allowed temperature correlation times in the selected points of the flame to be derived.

Employing a shadowgraph imaging technique for cloud microphysical measurements on a mountain observatory

<p>Microphysical properties of cloud droplets, such as droplet size distribution and droplet<br>number concentration have been studied after performing a series of field experiments in<br>summer 2019 at Umweltforschungsstation Schneefernerhaus (UFS), an environmental<br>research station located just below the peak of Zugspitze in the German Alps.<br>“VisiSize D30” manufactured by Oxford Laser Ltd. is a shadowgraph imaging instrument<br>utilized for the first time to measure the size and velocity of cloud droplets during this<br>campaign. It applies a method called “Particle/Droplet Image Analysis” (PDIA) which<br>involves illuminating the region of interest from behind with an infrared pulse laser whilst<br>collecting shadow images of droplets passing through the measurement volume with a<br>high-resolution camera. Droplets detected inside the depth of field are then measured<br>based on their shadow images, and size distribution is built by analyzing a series of<br>images. Furthermore, while turbulent orographic clouds passing our measurement site<br>at UFS observatory during the campaign, a Phase Doppler Interferometer (PDI) device,<br>manufactured by Artium Tech. Inc., was also constantly measuring droplets passing<br>through its probe volume.<br>Analysis of simultaneously collected data from the two instruments, and applying<br>modifications to the original algorithms illustrate a reasonable agreement regarding the<br>droplet sizing and velocimetry between VisiSize D30 and PDI, at least for diameters<br>larger than 13 μm. Moreover, discrepancies have been observed concerning the<br>droplet number concentration results, especially in smaller sizes. Further investigation<br>by applying appropriate filters on data has allowed the attribution of discrepancies to<br>the different optical performance of the sensors regarding small droplets, and to high<br>turbulent velocity fluctuations relative to the mean flow resulting in an uncertain estimate<br>of the volume of air passing through the PDI probe volume.</p>

Inventive nesting behaviour in the keyhole wasp Pachodynerus nasidens Latreille (Hymenoptera: Vespidae) in Australia, and the risk to aviation safety

The keyhole wasp (Pachodynerus nasidens Latreille 1812), a mud-nesting wasp native to South and Central America and the Caribbean, is a relatively recent (2010) arrival in Australia. In its native range it is known to use man-made cavities to construct nests. A series of serious safety incidents Brisbane Airport related to the obstruction of vital airspeed measuring pitot probes on aircraft possibly caused by mud-nesting wasps, prompted an assessment of risk. An experiment was designed to determine the species responsible, the types of aircraft most affected, the seasonal pattern of potential risk and the spatial distribution of risk on the airport. A series of replica pitot probes were constructed using 3D-printing technology, representing aircraft with high numbers of movements (landings and take-offs), and mounted at four locations at the airport. Probes were monitored for 39 months. Probes blocked by mud nesting wasps were retrieved and incubated in mesh bags. Emerging wasps were identified to species. Results show that all nests in probes were made by P. nasidens, and peak nesting occurs in the summer months. Nesting success (as proportion of nests with live adult emergents) was optimal between 24 and 31°C and that probes with apertures of more than 3 mm diameter are preferred. Not all areas on the airport are affected equally, with the majority of nests constructed in one area. The proportion of grassed areas within 1000 m of probes was a significant predictor of nesting, and probe volume may determine the sex of emerging wasps.

Wind turbine load validation in wakes using field reconstruction techniques and nacelle lidar wind retrievals

This study proposes two methodologies for improving the accuracy of wind turbine load assessment under wake conditions by combining nacelle-mounted lidar measurements with wake wind field reconstruction techniques. The first approach consists in incorporating wind measurements of the wake flow field, obtained from nacelle lidars, into random, homogeneous Gaussian turbulence fields generated using the Mann spectral tensor model. The second approach imposes wake deficit time-series, which are derived by fitting a bivariate Gaussian shape function on lidar observations of the wake field, on the Mann turbulence fields. The two approaches are numerically evaluated using a virtual lidar simulator, which scans the wake flow fields generated with the Dynamic Wake Meandering (DWM) model. The lidar-reconstructed wake fields are input to aeroelastic simulations of the DTU 10 MW wind turbine and the resulting load predictions are compared with loads obtained with the target (no lidar-based) DWM simulated fields. The accuracy of load predictions is estimated across a variety of lidar beam configurations, probe volume sizes, and atmospheric turbulence conditions. The results indicate that the 10-min power and fatigue load statistics, predicted with lidar-reconstructed fields, are comparable with results obtained with the DWM simulations. Furthermore, the simulated power and load time-series exhibit a high level of correlation with the target observations, thus decreasing the statistical uncertainty (realization-to-realization) by a factor between 1.2 and 5, compared to results obtained with the baseline, which is DWM simulated fields with different random seeds. Finally, we show that the spatial resolutions of the lidar's scanning strategies as well as the size of the probe volume are critical aspects for the accuracy of the reconstructed wake fields and load predictions.

Inventive nesting behaviour in the keyhole wasp Pachodynerus nasidens Latreille (Hymenoptera: Vespidae) in Australia, and the risk to aviation safety

The keyhole wasp (Pachodynerus nasidens Latreille 1812), a mud-nesting wasp native to South and Central America and the Caribbean, is a relatively recent (2010) arrival in Australia. In its native range it is known to use man-made cavities to construct nests. A series of serious safety incidents Brisbane Airport related to the obstruction of vital airspeed measuring pitot probes on aircraft possibly caused by mud-nesting wasps at prompted an assessment of risk. An experiment was designed to determine the species responsible, the types of aircraft most affected, the seasonal pattern of potential risk and the spatial distribution of risk on the airport. A series of replica pitot probes were constructed using 3D-printing technology, representing aircraft with high numbers of movements (landings and take-offs), and mounted at four locations at the airport. Probes were monitored for 39 months. Probes blocked by mud nesting wasps were retrieved and incubated in mesh bags. Emerging wasps were identified to species. Results show that all nests in probes were made by P. nasidens, and peak nesting occurs in the summer months. Nesting success (as proportion of nests with live adult emergents) was optimal between 24 and 31°C and that probes with apertures of more than 3 mm diameter are preferred. Not all areas on the airport are affected equally, with the majority of nests constructed in one area. The proportion of grassed areas within 1000 m of probes was a significant predictor of nesting, and probe volume may determine the sex of emerging wasps.

Small-angle scattering and scale-dependent heterogeneity

Although small-angle scattering is often discussed qualitatively in terms of material heterogeneity, when it comes to quantitative data analysis this notion becomes somehow hidden behind the concept of correlation function. In the present contribution, a quantitative measure of heterogeneity is defined, it is shown how it can be calculated from scattering data, and its structural significance for the purpose of material characterization is discussed. Conceptually, the procedure consists of using a finite probe volume to define a local average density at any point of the material; the heterogeneity is then quantitatively defined as the fluctuations of the local average density when the probe volume is moved systematically through the sample. Experimentally, it is shown that the so-defined heterogeneity can be estimated by projecting the small-angle scattering intensity onto the form factor of the chosen probe volume. Choosing probe volumes of various sizes and shapes enables one to comprehensively characterize the heterogeneity of a material over all its relevant length scales. General results are derived for asymptotically small and large probes in relation to the material surface area and integral range. It is also shown that the correlation function is equivalent to a heterogeneity calculated with a probe volume consisting of two points only. The interest of scale-dependent heterogeneity for practical data analysis is illustrated with experimental small-angle X-ray scattering patterns measured on a micro- and mesoporous material, on a gel, and on a semi-crystalline polyethylene sample. Using different types of probes to analyse a given scattering pattern enables one to focus on different structural characteristics of the material, which is particularly useful in the case of hierarchical structures.

A review of turbulence measurements using ground-based wind lidars

A review of turbulence measurements using ground-based wind lidars is carried out. Works performed in the last 30 yr, i.e., from 1972–2012 are analyzed. More than 80% of the work has been carried out in the last 15 yr, i.e., from 1997–2012. New algorithms to process the raw lidar data were pioneered in the first 15 yr, i.e., from 1972–1997, when standard techniques could not be used to measure turbulence. Obtaining unfiltered turbulence statistics from the large probe volume of the lidars has been and still remains the most challenging aspect. Until now, most of the processing algorithms that have been developed have shown that by combining an isotropic turbulence model with raw lidar measurements, we can obtain unfiltered statistics. We believe that an anisotropic turbulence model will provide a more realistic measure of turbulence statistics. Future development in algorithms will depend on whether the unfiltered statistics can be obtained without the aid of any turbulence model. With the tremendous growth of the wind energy sector, we expect that lidars will be used for turbulence measurements much more than ever before.

A review of turbulence measurements using ground-based wind lidars

A review of turbulence measurements using ground-based wind lidars is carried out. Works performed in the last 30 yr, i.e. from 1972–2012 are analyzed. More than 80% of the work has been carried out in the last 15 yr, i.e. from 1997–2012. New algorithms to process the raw lidar data were pioneered in the first 15 yr, i.e. from 1972–1997, where standard techniques could not be used to measure turbulence. Obtaining unfiltered turbulence statistics from the large probe volume of the lidars has been and still remains the most challenging aspect. Until now, most of the processing algorithms that have been developed have shown that by combining an isotropic turbulence model with raw lidar measurements, we can obtain unfiltered statistics. We believe that an anisotropic turbulence model will provide a more realistic measure of a turbulence statistic. Future development in algorithms will depend on whether the unfiltered statistics can be obtained without the aid of any turbulence model. With the tremendous growth of the wind energy sector, we expect that lidars will be used for turbulence measurements much more than ever before.