A Spectrum Correction Method Based on Optimizing Turbulence Intensity

Based on the classical spectral representation method of simulating turbulent wind speed fluctuation, a harmonic superposition algorithm was introduced in detail to calculate the homogeneous turbulence wind field simulation in space. From the view of the validity of the numerical simulation results in MATLAB and the simulation efficiency, this paper discussed the reason for the bias existing between three types of turbulence intensity involved in the whole simulation process: simulated turbulence intensity, setting reference turbulence intensity, and theoretical turbulence intensity. Therefore, a novel spectral correction method of a standard deviation compensation coefficient was proposed. The simulation verification of the correction method was carried out based on the Kaimal spectrum recommended by IEC61400-1 by simulating the uniform turbulent wind field in one-dimensional space at the height of the hub of a 15 MW wind turbine and in two-dimensional space in the rotor swept area. The results showed that the spectral correction method proposed in this paper can effectively optimize the turbulence intensity of the simulated wind field, generate more effective simulation points, and significantly improve the simulation efficiency.

Modelling the Spectral Shape of Continuous-Wave Lidar Measurements in a Turbulent Wind Tunnel

This paper describes the development of a model for the turbulence spectrum measured by a short-range, continuous-wave lidar. The lidar performance was assessed by measurements conducted with two WindScanners in an open jet wind tunnel equipped with an active grid, for a range of different turbulent wind conditions. A one-dimensional hot wire anemometer was used as a reference for characterising the lidar turbulence measurement. In addition to addressing the statistics, the correlation between the time series and the mean error on the wind speed, the lidar measurement turbulence spectrum is compared with a theoretical spectrum using Taylor's frozen turbulence hypothesis. A theoretical model for the probe length averaging effect is applied in the frequency domain, using a Lorentzian filter in combination with generated white noise, and evaluated by qualitatively matching the lidar measurement spectrum. High goodness of fit coefficients and low mean absolute errors between hot wire and WindScanner were observed for the measured time series. The correlation showed an inverse relationship with the prevalent turbulence intensity in the flow for cases with a comparable power spectrum shape. Larger flow structures can be captured more accurately by the lidar, whereas small-scale turbulent flow structures are partly filtered out as a result of the lidars' probe volume averaging. It is demonstrated that an accurate way to define the frequency at which the lidar power spectrum starts to deviate from the hot wire reference spectrum is the point at which the coherence drops below 0.5. This coherence-based cut-off frequency increases linearly with the mean wind speed and is generally an order of magnitude lower than the probe length cut-off frequency, estimated according to a simple model based on Taylor's frozen turbulence hypothesis. A convincing match between the modelled and the actual WindScanner power spectrum was found for various different cases, which confirmed that the deviation of the lidar measurement power spectrum in the higher frequency range can be analytically explained and modelled as a combination of a Lorentzian probe length averaging effect and white noise in the lidar measurement.

Review of Evidence of Aerosol Transmission of SARS-CoV-2 Particles

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) causes Coronavirus disease (COVID-19) through multiple transmission routes and understanding the mode of transmission is very important for its containment and prevention. Consequently, inadequate attention has been given to the spread of respiratory droplets in indoor conditions under microclimatologic turbulent wind promoted by aerosol from talking (loud), coughing, sneezing, toilet flushing of an isolation room, and resuspension of the settled virus from the surfaces. To this end, this study is presenting an early review of the process and evidence of aerosol transmission of SARS-CoV-2 particles. There are significant results of many studies including those under peer review that support aerosol and airborne transmission which government agencies should consider for reducing the transmission rate.

Experimental Investigation of the Coupling Between Aero- and Hydrodynamical Loads On A 12 MW Semi-Submersible Floating Wind Turbine

Model tests were performed with a model of the INO WINDMOOR 12 MW floating wind turbine in the Ocean Basin at SINTEF Ocean. The tests were done at a scale of 1:40. RealTime Hybrid Model testing was used for the modelling of the wind turbine rotor and aerodynamic loads. A subset of the results is analysed to study the influence of the wind on the platform motions, the acceleration at tower top, the loads at base of tower and the relative wave elevation. The study is based on the comparison of the quantities of interest for different tests with the same moderate sea-state but with different wind modelling: no wind, constant thrust force, turbulent wind of 11.5 m/s and turbulent wind of 25 m/s. The wind modelling has a minimal influence on the platform surge and pitch response in the wave-frequency range. On the other hand, the aerodynamic loads, including wind turbine controller dynamics and turbulent wind, has an important impact on the low-frequency surge and pitch response. The aerodynamic loads are important for the loads at tower base due to the dominance of the tower-RNA induced gravitational loads at low-frequency. Maximum relative wave elevation was found to be mainly dependent on the thrust induced mean pitch angle.

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>

Active flap control with the trailing edge flap hinge moment as a sensor: using it to estimate local blade inflow conditions and to reduce extreme blade loads and deflections

Active trailing edge flaps are a promising technology that can potentially enable further increases in wind turbine sizes without the disproportionate increase in loads, thus reducing the cost of wind energy even further. Extreme loads and critical deflections of the blade are design-driving issues that can effectively be reduced by flaps. In this paper, we consider the flap hinge moment as a local input sensor for a simple flap controller that reduces extreme loads and critical deflections of the DTU 10 MW Reference Wind Turbine blade. We present a model to calculate the unsteady flap hinge moment that can be used in aeroelastic simulations in the time domain. This model is used to develop an observer that estimates the local angle of attack and relative wind velocity of a blade section based on local sensor information including the flap hinge moment of the blade section. For steady wind conditions that include yawed inflow and wind shear, the observer is able to estimate the local inflow conditions with errors in the mean angle of attack below 0.2∘ and mean relative wind speed errors below 0.4 %. For fully turbulent wind conditions, the observer is able to estimate the low-frequency content of the local angle of attack and relative velocity even when it is lacking information on the incoming turbulent wind. We include this observer as part of a simple flap controller to reduce extreme loads and critical deflections of the blade. The flap controller's performance is tested in load simulations of the reference turbine with active flaps according to the IEC 61400-1 power production with extreme turbulence group. We used the lifting line free vortex wake method to calculate the aerodynamic loads. Results show a reduction of the maximum out-of-plane and resulting blade root bending moments of 8 % and 7.6 %, respectively, when compared to a baseline case without flaps. The critical blade tip deflection is reduced by 7.1 %. Furthermore, a sector load analysis considering extreme loading in all load directions shows a reduction of the extreme resulting bending moment in an angular region covering 30∘ around the positive out-of-plane blade root bending moment. Further analysis reveals that a fast reaction time of the flap system proves to be critical for its performance. This is achieved with the use of local sensors as input for the flap controller. A larger reduction potential of the system is identified but not reached mainly because of a combination of challenging controller objectives and the simple controller architecture.