Current Issue : January - March Volume : 2018 Issue Number : 1 Articles : 5 Articles
In this study, we proposed an empirical algorithm for significant wave height\n(SWH) retrieval from TerraSAR-X/TanDEM (TS-X/TD-X) X-band synthetic aperture radar (SAR)\nco-polarization (vertical-vertical (VV) and horizontal-horizontal (HH)) images. As the existing\nempirical algorithm at X-band, i.e., XWAVE, is applied for wave retrieval from HH-polarization\nTS-X/TD-X image, polarization ratio (PR) has to be used for inverting wind speed, which is treated as\nan input in XWAVE.Wind speed encounters saturation in tropical cyclone. In our work, wind speed is\nreplaced by normalized radar cross section (NRCS) to avoiding using SAR-derived wind speed, which\ndoes not work in high winds, and the empirical algorithm can be conveniently implemented without\nconverting NRCS in HH-polarization to NRCS in VV-polarization by using X-band PR. A total of\n120 TS-X/TD-X images, 60 in VV-polarization and 60 in HH-polarization, with homogenous wave\npatterns, and the coincide significant wave height data from European Centre for Medium-Range\nWeather Forecasts (ECMWF) reanalysis field at a 0.125ââ??¦ grid were collected as a dataset for tuning\nthe algorithm. The range of SWH is from 0 to 7 m. We then applied the algorithm to 24 VV and 21\nHH additional SAR images to extract SWH at locations of 30 National Oceanic and Atmospheric\nAdministration (NOAA) National Data Buoy Center (NDBC) buoys. It is found that the algorithm\nperforms well with a SWH stander deviation (STD) of about 0.5 m for both VV and HH polarization\nTS-X/TD-X images. For large wave validation (SWH 6ââ?¬â??7 m), we applied the empirical algorithm\nto a tropical cyclone Sandy TD-X image acquired in 2012, and obtained good result with a SWH\nSTD of 0.3 m. We concluded that the proposed empirical algorithm works for wave retrieval from\nTS-X/TD-X image in co-polarization without external sea surface wind information....
Airborne Wind Energy (AWE) is an emerging technology in the field of renewable\nenergy that uses kites to harvest wind energy. However, unlike for conventional wind turbines,\nthe wind environment in AWE systems has not yet been studied in much detail. We propose\na simulation framework using Large Eddy Simulation to model the wakes of such kite systems\nand offer a comparison with turbine-like wakes. In order to model the kite effects on the flow,\na lifting line technique is used. We investigate different wake configurations related to the\noperation modes of wind turbines and airborne systems in drag mode. In the turbine mode,\nthe aerodynamic torque of the blades is directly added to the flow. In the kite drag mode, the\naerodynamic torque of the wings is directly balanced by an opposite torque induced by on-board\ngenerators; this results in a total torque on the flow that is zero. We present the main differences\nin wake characteristics, especially flow induction and vorticity fields, for the depicted operation\nmodes both with laminar and turbulent inflows....
The Chinese Gaofen-3 (GF-3) synthetic aperture radar (SAR) launched by the China\nAcademy of Space Technology (CAST) has operated at C-band since September 2016. To date,\nwe have collected 16/42 images in vertical-vertical (VV)/horizontal-horizontal (HH) polarization,\ncovering the National Data Buoy Center (NDBC) buoy measurements of the National Oceanic and\nAtmospheric Administration (NOAA) around U.S. western coastal waters. Wind speeds from NDBC\nin situ buoys are up to 15 m/s and buoy-measured significant wave height (SWH) has ranged\nfrom 0.5 m to 3 m. In this study, winds were retrieved using the geophysical model function\n(GMF) together with the polarization ratio (PR) model and waves were retrieved using a new\nempirical algorithm based on SAR cutoff wavelength in satellite flight direction, herein called\nCSAR_WAVE. Validation against buoy measurements shows a 1.4/1.9 m/s root mean square error\n(RMSE) of wind speed and a 24/23% scatter index (SI) of SWH for VV/HH polarization. In addition,\nwind and wave retrieval results from 166 GF-3 images were compared with the European Centre\nfor Medium-Range Weather Forecasts (ECMWF) re-analysis winds, as well as the SWH from the\nWaveWatch-III model, respectively. Comparisons show a 2.0 m/s RMSE for wind speed with a\n36% SI of SWH for VV-polarization and a 2.2 m/s RMSE for wind speed with a 37% SI of SWH\nfor HH-polarization. Our work gives a preliminary assessment of the wind and wave retrieval\nresults from GF-3 SAR images for the first time and will provide guidance for marine applications of\nGF-3 SAR....
The current trend of the wind energy industry aims for large scale turbines installed\nin wind farms. This brings a renewed interest in vertical axis wind turbines (VAWTs) since\nthey have several advantages over the traditional Horizontal Axis Wind Tubines (HAWTs)\nfor mitigating the new challenges. However, operating VAWTs are characterized by complex\naerodynamics phenomena, presenting considerable challenges for modeling tools. An accurate\nand reliable simulation tool for predicting the interaction between the obtained wake of an\noperating VAWT and the flow in atmospheric open sites is fundamental for optimizing the design\nand location of wind energy facility projects. The present work studies the wake produced by\na VAWT and how it is affected by the surface roughness of the terrain, without considering\nthe effects of the ambient turbulence intensity. This study was carried out using an actuator\nline model (ALM), and it was implemented using the open-source CFD library OpenFOAM to\nsolve the governing equations and to compute the resulting flow fields. An operational H-shaped\nVAWT model was tested, for which experimental activity has been performed at an open site\nnorth of Uppsala-Sweden. Different terrains with similar inflow velocities have been evaluated.\nSimulated velocity and vorticity of representative sections have been analyzed. Numerical results\nwere validated using normal forces measurements, showing reasonable agreement....
Building upon a recent study that showed windbreaks to be effective in increasing the\npower output of a wind turbine, the potential of windbreaks in a large wind farm is explored\nusing simplified formulations. A top-down boundary layer approach is combined with methods\nof estimating both the roughness effects of windbreaks and the induced inviscid speed-up for\nnearby turbines to investigate power production impact for several layouts of infinite wind farms.\nResults suggest that the negative impact of windbreak wakes for an infinite wind farm will outweigh\nthe local inviscid speed-up for realistic inter-turbine spacings, with the break-even point expected\nat a spacing of âË?¼25 rotor diameters. However, the possibility that windbreaks may be applicable\nin finite and other wind farm configurations remains open. Inspection of the windbreak porosity\nreveals an impact on the magnitude of power perturbation, but not whether the change is positive or\nnegative. Predictions from the boundary-layer approach are validated with power measurements\nfrom large-eddy simulations....
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