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ItemConstraints on Recoil Leader Properties Estimated from X-ray Emissions in Aircraft-Triggered Discharges(AGU, 2020)During Airbus aircraft campaigns flying into thunderstorms in 2014–2016, X-rays were observed during two stages of aircraft-triggered lightning: nanosecond pulses of X-rays associated with negative leader steps and bursts of X-rays during recoil events. This work will focus on the observations of X-ray bursts associated with recoils. Recoils are observed as microsecond-fast changes in the local electric field, associated with large currents passing through the aircraft, and are found to sometimes be associated with bursts of X-rays. From over 200 aircraft-triggered lightning strikes, 54 recoil events were found to be associated with microsecond bursts of X-rays. The majority of the bursts consist of 1–3 X-ray pulses, with some bursts containing as many as 29 X-ray pulses. We compare the observed superposed X-ray spectrum with modeled spectra using a GEANT4 model of the detector and aircraft, to determine the source potential needed to accelerate the electrons that produce the observed X-rays. A model of the recoil leader was made to determine the gap distance and gap potential between the recoil leader and the aircraft. From the modeling, we determine a solution space for the gap and leader lengths where the gap length is constrained by the observed minimum and maximum times between the onset of the X-ray pulses and the onset of the current pulses detected at the aircraft (1 to 93 m). We also find two constraints from the fitting of the modeled spectra to the superposed spectrum, limiting the leader length to between 1 and ∼240 m.
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ItemEvaluating Sensor Performance for Impact Identification in Composites: a Comprehensive Comparison of FBGs with PZTs(SAGE, 2024)Aerospace composite components require effective monitoring techniques to detect possible internal damage from impact events. To ensure reliable impact identification, sensor measurements can provide valuable information about impact energy and identify potential issues that may require further investigation. However, selecting the most appropriate sensor technology to measure impact force and energy is a challenge. In this article, a systematic and structured approach is presented to compare the expected performance of sensors and their metrological parameters in terms of their ability for impact identification in aerospace composites. The proposed methodology is demonstrated using an application example where fibre Bragg grating (FBG) are compared with piezoelectric (PZT) sensors through comprehensive tests. These tests include the correlation test, the sensitivity test, and the factor test. The correlation test showed a high agreement between FBG and PZT sensors in the time and frequency domain. The sensitivity test indicated a significant correlation between the signal features and the impact energy levels in the energy profiling diagrams, revealing nonlinearities and energy losses indicative of damage. Furthermore, these results emphasise the superior resolution of the FBG sensors and the comparable repeatability of the two sensor types. Finally, the factor test showed that FBG sensors are sensitive to different angles of incidence, while PZT sensors have a more stable directivity. Further analysis also showed that the signal strength of both sensor types decreases with increasing distance from the impact source. Overall, the proposed approach enables a thorough evaluation of the capabilities and limitations of both sensor types. Consequently, it provides information to make an informed decision on the most suitable sensor for impact monitoring systems.
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ItemAutomatic Transition Prediction in a Navier–Stokes Solver Using Linear Stability Theory(AIAA, 2021)A structured Reynolds-averaged Navier–Stokes solver is directly coupled to a linear stability theory (LST) solver to include the effect of laminar–turbulent transition in the flow simulations. The flowfield variables of the flow solver are used to both find streamlines along which transition can be predicted and to provide the LST code with the required boundary-layer profiles. Instabilities included in the analysis are of the Tollmien–Schlichting and crossflow nature relevant to high-Reynolds-number flows in low turbulence environments. The coupling is fully automated and can therefore be used efficiently in the analysis and design of geometries with external flows. The Technical University of Braunschweig’s sickle wing with spanwise-varying crossflow and the natural laminar flow version of the Common Research Model are simulated under various conditions. Applications to these relevant three-dimensional test cases showcase the capability of the method to model the real flow physics. Advantages and challenges of the approach with regard to future design endeavors are discussed.
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ItemAerodynamic Interactions of Side-by-Side Rotors in Ground Proximity(AIAA, 2022)An experimental investigation is conducted to study the aerodynamic behavior of a two-rotor system in ground proximity. The counter-rotating rotors are placed side-by-side in the hovering condition. The time-averaged and unsteady flow behavior is studied when the rotor-to-rotor lateral distance and the distance between the rotors and the ground are varied. The experiments are performed using three-dimensional large-scale volumetric velocimetry with helium-filled soap bubbles as tracers, tracked by the particle motion analysis technique “Shake-The-Box.” The mean velocity field reveals the wake deflection due to the ground plane and the formation of toroidal-shape regions of separated flow below each rotor. The interaction of the wall jets formed by slipstream deflection results in a separation line with the flow emerging from the wall in a fountain-like pattern. Regimes of flow re-ingestion occur when the rotors are sufficiently far apart. The flowfield exhibits the tendency toward asymmetric states, during which the fountain flow column and the domain of re-ingestion shift closer to one of the rotors. A generic classification of flow regimes is proposed in relation to the behavior of two rotors in ground effect.
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ItemThree-Dimensional Quantitative Flow Visualization Around a Thrust Reverser(AIAA, 2023)Volumetric particle tracking velocimetry measurements are performed in a low-speed wind tunnel to study the flowaround a 1:12-scale aircraft model with jet engines operating with thrust reversers. The engine jet and freestream flowvelocity are varied to yield a jet to freestream velocity ratio of Vjet∕V∞ ranging from 1.5 to 6. Measurements at suchscale (0.5 m3) require the use of strongly scattering helium-filled soap bubbles as flow tracers, which are introduced inboth the jet and the freestream flow. The tracer’s three-dimensional motion is determined using an array of camerasand a Lagrangian particle tracking algorithm. The mean velocity field reveals the jet inner structure as well as itsinteraction with the freestream, the ground board, the nacelle, the fuselage, and the horizontal and vertical tails. Theexperiments allow detection of exhaust reingestion as well as the aerodynamic interference with control surfaces at thetail segments in a single measurement volume. The results are in good agreement with conventional temperature rakemeasurements while adding details of the flow topology and of the large-scale unsteady flow fluctuations. Finally, thejet reversal characteristics with varying freestreams and nozzle pressure ratios are assessed, demonstrating thefeasibility and versatility of volumetric velocimetry measurements for industrial aerodynamics.