An analysis of flow distortion in a multipath sonic anemometer

<p>Sonic anemometers provide point observations of the three-dimensional velocity field at high sampling rates and are crucial instruments for understanding and quantifying the fluxes of momentum, energy and scalars between the atmosphere and Earth’s surface. Since the beginning of sonic anemometry 50 years ago, the characterization of flow distortion, i.e. how the instrument structure alters the flow, has been an ongoing research topic. Multi-path sonic anemometry provides a new opportunity to research and understand flow distortion on the vertical velocity component, since several positions in the small measurement volume can be measured simultaneously. In this work, we use data from a flat terrain measurement campaign in 2020, in which several sonic anemometers were mounted on 4m towers placed 4m apart. The analysis is focused on the Multipath Class-A sonic anemometer (Metek GmbH, Germany), which provides vertical velocity observations from three vertical paths 120 degrees and 0.1m apart. Vertical velocities are also calculated from several combinations of the tilted paths. We investigate how the vertical velocity component is altered depending on wind direction relative to different parts of the instrument structure. We demonstrate that by an optimal combination of the different paths, the vertical velocity variance and fluxes can be significantly enhanced. We also show spectra, and especially look at the high frequency end of the spectrum, where the relative behaviour of the velocity components is known from fundamental turbulence theory. Further, the relative importance of transducer shadowing and pressure-induced blockage effects is discussed.</p>

On the generation and segregation of helicity in geodynamo simulations

SUMMARY Helicity, the inner product of velocity and vorticity, is considered an important ingredient for the maintenance of a dipolar magnetic field in the geodynamo. Outside the tangent cylinder—an imaginary cylinder which circumscribes the inner core—a spatial segregation of helicity has been observed in several simulations, being negative in the north and positive in the south. Such a segregation pattern is important for a dynamo that relies on the α-effect. However, the origin of this pattern in these simulations is poorly understood. In this paper, we use three strongly forced numerical dynamo solutions to study the various sources of helicity, including those due to buoyancy $({H_T})$, Coriolis, Lorentz and viscous forces. We find a strong spatial correlation between the segregation pattern of helicity and ${H_T}$ both in the instantaneous and the time-averaged results. Our results show that, outside the tangent cylinder, ${H_T}$ is dominated by the product $- {u_z}\partial T/\partial \varphi $, where ${u_z}$ is the vertical velocity component and T is the temperature perturbation. It is known that when inertial waves are launched from a localized buoyant anomaly, ${H_T}$ takes the same sign as the local helicity. We conjecture that this is the reason for the spatial correlation between ${H_T}$ and helicity in our simulation results. The flow in our simulations being strongly turbulent, this effect seems to be a statistical one and manifests itself most clearly in the averaged quantities.

CFD Analysis of 2D Unsteady Flow Past a Square Cylinder at Low Reynolds Numbers

A study of the behaviour of flow past a square cylinder for Reynolds numbers 10 and 20 is presented. Open source software Navier2d in Matlab is used in this study. The investigation starts from a uniform initial mesh and then refine the initial mesh using a mesh refinement method which was proposed based on both qualitative theory of differential equations and the finite volume method implemented in Navier2d. The horizontal and vertical velocity component profiles and pressures are shown on the once refined meshes. The comparisons between the profiles and pressures are conducted to show the variations from Reynolds number 10 to 20. The twice refined meshes are also presented and these refined meshes provide the information where the behaviour of flow is complex.

Addressing Spatial Variability of Surface-Layer Wind with Long-Range WindScanners

This paper presents an analysis of mean wind measurements from a coordinated system of long-range WindScanners. From individual scan patterns the mean wind field was reconstructed over a large area, and hence it highlights the spatial variability. From comparison with sonic anemometers, the quality of the WindScanner data is high, although the fidelity of the estimated vertical velocity component is significantly limited by the elevation angles of the scanner heads. The system of long-range WindScanners presented in this paper is close to being fully operational, with the pilot study herein serving not only as a proof of concept but also verifying expectations of reliable wind measurements over arbitrary three-dimensional volumes, in future sustained meteorological campaigns.

Improving Photoresist Spray Coating on 3D Structures for Microfluidic Devices

ABSTRACTSpray coating of a photoresist onto three-dimensional (3D) structure was investigated. To improve the uniformity of photoresist deposition onto the 3D structure, a shield plate with an aperture was used. The shield plate set over a sample permitted the resist deposition on the sample surface located in the aperture area while the plate blocked the deposition for the other area. The spray flow which is suitable for the resist deposition can be used effectively. Numerical analysis revealed that the vertical velocity component of gas flow was enhanced in the aperture area. In the experiments of the spray coating, the difference between the resist film thicknesses deposited on top and bottom trench surfaces was decreased. On the trench sidewall, resist bump formation, which was frequently observed in spray coating, was suppressed. The uniform resist deposition is necessary to realize 3D microdevices by lithography. In the microfluidic devices of dielectrophoresis, aside from the top and botttom trench surfaces, the trench sidewall can be used to fabricate device structures such as electrode for dielectrophoresis.

No relation between the vertical velocity component and the absolute magnitude among globular clusters

The globular cluster luminosity function distribution shows a peak at MV ≈ −7.5 mag. There are some indications that the kinematic parameters are correlated with luminosity. In particular, Alfaro et al. (2001) have studied the properties of the Galactic globular cluster system and they found a correlation between spatial-velocity component and globular cluster absolute magnitude. The authors assumed that the globular clusters can be separated into two groups. The first is composed of globular clusters with MV < −7.5 mag and moving preferentially towards the north Galactic pole, while the faintest globular clusters, composing the second group, move towards the Galactic disk. We have selected a sample of globular clusters using the same criteria as Alfaro et al. (2001) and have checked that this apparent relation indeed exists. Nevertheless, we decided to investigate whether it could be a fortuitous relation or an intrinsic property by checking its validity for eight different epochs at past and future times. The orbital parameters for the globular clusters at these eight epochs were found by orbital integration using a typical Galactic potential. We show that this relation between the vertical velocity component and the absolute magnitude among globular clusters is not coherent with time and the velocity distribution does not support the hypothesis of Alfaro et al. for the existence of two dynamical groups of globular clusters.