Where are rainfall drops falling in a turbulent wind field ?

<p>Weather radars measure rainfall in altitude whereas hydro-meteorologists are mainly interested in rainfall at ground level. During their fall, drops are advected by the wind which affects the location of the measured field. In this study, we investigate the fall of rain drops in a turbulent wind field between an height of 1500m and the ground.</p><p>The equation governing a rain drop motion relates the acceleration to the forces of gravity and buoyancy along with the drag force. The latter depends non-linearly on the instantaneous relative velocity between the drop and the local wind; which yields to complex behaviour. In this work, the drag force is expressed in a standard way with the help of a drag coefficient, which is itself determined according to a Reynolds number. Corrections accounting for the oblateness of drops greater than 1-2 mm are implemented. Such corrections are validated through comparison of retrieved “terminal fall velocity” (i.e. without wind) with commonly used relationships in the literature.</p><p>An explicit numerical scheme is implemented to solve this equation for 3+1D turbulent wind field, and hence analyse the temporal evolution of the velocities and trajectories of rain drops during their fall. Two types of wind inputs are used : (i) Four months of 100 Hz 3D sonic anemometers data. (ii) Numerical simulations of space-time varying wind carried out with the help of Universal Multifractals which are a framework that has been widely used to characterize and simulate geophysical fields extremely variable over a wide range of scales such as wind.</p><p>The behaviour of drop velocities is then characterized through temporal multifractal analysis. It notably enables to highlight a scale, depending on the drop size, below which turbulent eddies have a limited impact on their motion. Finally the dispersion on the ground of drops all starting at the same location is quantified and consequences on rainfall remote sensing with radars discussed.</p><p> </p><p>Authors acknowledge the RW-Turb project (supported by the French National Research Agency - ANR-19-CE05-0022), for partial financial support.</p><p> </p>

A CFD Study for Floating Offshore Wind Turbine Aerodynamics in Turbulent Wind Field

The present study is aimed at investigating the turbulent wind effect on FOWT through the usage of a high-fidelity computational fluid dynamics (CFD) method. This method is believed to resolve the wind field, giving us a more in-depth examination into the aerodynamics of FOWT. The work is built upon our previous studies on the modelling of a coupled aero-hydro-mooring FOWT system under regular wave and uniform wind. In the present study, we replaced the previously uniform wind with a temporal and spatial variable turbulent wind field using a time-varying spectrum. The turbulent wind is generated with Mann’s wind turbulence model while the Von Karman wind spectrum is used to represent wind turbulence. The present study shows that when turbulent wind is present, there may be fluctuations of the rotor thrust and power outputs, causing the non-uniform wake region. Despite this, both the dynamic motions and the mooring tensions of the floater are not significantly influenced by the wind turbulence under the present inflow wind conditions.

Temporal evolution of rain drops’ velocities in a turbulent wind field

<p>It is commonly assumed that a rain drop falls vertically at a speed equal to its so called “terminal fall velocity” which has been determined both empirically and theoretically by equating the net gravity force with the drag force due to the fact the drop is moving in the atmosphere. This velocity depends on the size of the drop, usually characterized by its equivolumic diameter.</p><p>In this investigation we study the temporal evolution of the velocity of a rain drop falling through turbulent wind field. The equation governing a rain drop motion relates the acceleration to the forces of gravity and buoyancy along with the drag force. The latter depends non-linearly on the instantaneous relative velocity between the drop and the local wind. The whole complexity of the resulting behaviour arises from this feature. In this work, the drag force is expressed in a standard way with the help of a drag coefficient, which is itself determined according to a Reynolds number. It should be mentioned that in this initial work, the strong assumption that the drops remain spherical in their fall is made. It is well known that its not true for drops greater than typically 1-2 mm which tend to become oblate, and potential effects on the results will be discussed.</p><p>An explicit numerical scheme is implemented to solve this equation for 3+1D turbulent wind field to study the temporal evolution of the velocities as well as the trajectories of rain drops over few hundreds of meters. The variations in both space and time of the wind field are simulated with the help of a Universal Multifractals which are a framework that has been widely used to characterize and simulate geophysical fields extremely variable over a wide range of scales such as wind.</p><p>Temporal multifractal analysis are then carried out on the simulated drop velocity, which enables to characterize the behaviour of drops according to their size, and notably a scale below which turbulent eddies have a limited impact on their motion. Finally the consequences of these findings on rainfall remote sensing with radars are briefly discussed.</p>

Modelling and Comparison of Different Control Techniques for 1-MW Wind Turbine to Extract Maximum Power Through Pitch Angle Control

Wind energy is the most clean and attractive source in addition to it abundant in nature. The main challenge in extracting the energy through wind is the uneven and unfamiliar turbulent wind field. Wind turbine pitch system plays pivotal role in achieving required blade angle along with it to run generator at its rated speed. This paper focuses blade pitch control to improve power and to keep the system working in stable and safe manner. Blade pitch angle control has an important role in achieving maximum power, so the proposed controller is presented to maximize the power and protect the system in case of uncertain conditions. Proposed controller is compared with conventional controller to investigate in addition to validate the technique as well as working. Different case studies over variable wind speed has been discussed to get the improved power and to achieve safe and normal operation. Simulation is implemented on 1-MW wind turbine through MATLAB/Simulink and achieved the improved results from proposed controller.

Characterisation of Small-Scale Atmospheric Wind-Field Structures Using Coherent Wind Lidar With Short Pulses

A lidar design has been developed at ONERA that uses short square pulses (75 ns) to have a small spatial resolution (22.5 m) and be able to measure small-scale atmospheric wind-field structures. Results show that the system is able to resolve the small-scale structures of vortices and to measure wind field structures of a turbulent wind field down to ~20 m.

Non-Stationary Turbulent Wind Field Simulation of Long-Span Bridges Using the Updated Non-Negative Matrix Factorization-Based Spectral Representation Method

Numerical simulation of the turbulent wind field on long-span bridges is an important task in structural buffeting analysis when it comes to the system non-linearity. As for non-stationary extreme wind events, some efforts have been paid to update the classic spectral representation method (SRM) and the fast Fourier transform (FFT) has been introduced to improve the computational efficiency. Here, the non-negative matrix factorization-based FFT-aided SRM has been updated to generate not only the horizontal non-stationary turbulent wind field, but also the vertical one. Specifically, the evolutionary power spectral density (EPSD) is estimated to characterize the non-stationary feature of the field-measured wind data during Typhoon Wipha at the Runyang Suspension Bridge (RSB) site. The coherence function considering the phase angles is utilized to generate the turbulent wind fields for towers. The simulation accuracy is validated by comparing the simulated and target auto-/cross-correlation functions. Results show that the updated method performs well in generating the non-stationary turbulent wind field. The obtained wind fields will provide the research basis for analyzing the non-stationary buffeting behavior of the RSB and other wind-sensitive structures in adjacent regions.