ACCELERATING OF THE ELECTRONS OF ELECTRON BUNCHES IN A WAVEGUIDE LOADED DIELECTRIC STRUCTURE

The paper presents some results of experimental studies of the excitation of wake fields and the acceleration of electrons in waveguide-dielectric structures (DS) upon injection of a sequence of electron bunches into them. Exper-iments have shown an increase in the amplitude of the wake wave and the acceleration of a small fraction of elec-trons when the wavelength of the excited field is equal to the doubled bunch length. A simple physical model of the observed phenomenon is given. Also, the paper proposes a method for accelerating a part of each electron bunch in the steady-state mode of the resonator dielectric structure. Some of the electrons are “cut out” by the collimator and enter the accelerating phase of the previously excited wake wave. The wave is displaced due to the difference in the distances traveled by the wave and the accelerated part of the electrons.

Wind turbine load validation in wakes using wind 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 of 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 to 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, i.e., the target fields. The lidar-reconstructed wake fields are then input into aeroelastic simulations of the DTU 10 MW wind turbine for carrying out the load validation analysis. The power and load time series, predicted with lidar-reconstructed fields, exhibit a high correlation with the corresponding target simulations, thus reducing the statistical uncertainty (realization-to-realization) inherent to engineering wake models such as the DWM model. We quantify a reduction in power and loads' statistical uncertainty by a factor of between 1.2 and 5, depending on the wind turbine component, when using lidar-reconstructed fields compared to the DWM model results. Finally, we show that the number of lidar-scanned points in the inflow and the size of the lidar probe volume are critical aspects for the accuracy of the reconstructed wake fields, power, and load predictions.

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.

An Approach for the Accurate Investigation of Full-Scale Ship Boundary Layers and Wakes

Existing recommended practices in the literature do not provide clear and concise guidance for the selection of the most suitable numerical modelling strategy for investigating the boundary layer around a ship at full scale. For example, the International Towing Tank Conference procedure for calculating the nominal wake fields of full-scale ships does not clearly specify which turbulence modelling approach should be used to accurately represent the near-wall flow in the ship’s aft region.This paper presents a numerical approach that can accurately represent the boundary layer of full-scale ships. Three turbulence modelling strategies, suitable for the simulation of ship flows, have been assessed: k-ε, k-ω SST RANS and an IDDES formulation. Results from each method have been compared against the full-scale ship propeller torque data of the MV Regal, a 138m long general cargo vessel. Additionally, the capability of each turbulence strategy to resolve time-dependent features of the flow, such as the bilge vortex and its effect on the boundary layer velocity fields, has been evaluated.The results from this investigation show that the IDDES based numerical model replicated the sea trials measurements with the highest degree of accuracy. Furthermore, this study confirmed that the choice of turbulence strategy has a major impact on the full-scale velocity fields in the aft region of a ship.

Coherent optical transition radiation imaging for compact accelerator electron-beam diagnostics

Application of coherent optical transition radiation (COTR) diagnostics to compact accelerators has been demonstrated for the laser-driven plasma accelerator case recently. It is proposed that such diagnostics for beam size, beam divergence, microbunching fraction, spectral content, and bunch length would be useful before and after any subsequent acceleration in crystals or nanostructures. In addition, there are indications that under some scenarios a microbunched beam could resonantly excite wake fields in nanostructures that might lead to an increased acceleration gradient.

Interference analysis and operability envelopes design for deepwater parallel strings

Dual derrick operations are widely used for field development during offshore drilling. During this process, complex operations and complicated hydrodynamic interaction may contribute to the interference collisions of parallel strings. In this context, this study addresses interference collisions for parallel strings during the deepwater dual derrick operation. To analyze the response of parallel strings a mechanical model for deepwater parallel strings is established. Moreover, Huse wake model and strip model are used for calculating the hydrodynamic influence in different wake fields. The research results validate that the collision will occur during the operation considering the hydrodynamic wake shielding effects and interference evaluation criterion. The increasing platform offsets and surface current are the leading causes of parallel strings collisions. To avoid the risk of interference collisions an innovative procedure for operability envelopes is developed by synthesizing the platform offsets and surface currents. The proposed operability envelopes method for parallel strings is automatically completed which can save much workforce and resources. A case study on deepwater drilling in the South China Sea has been applied to verify the effectiveness of these methods. Besides, the proposed methodology can effectively reduce collision accidents and provide technical support for the offshore oil and gas exploration.