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Persistence is the remnant signal that plagues HgCdTe infrared detectors used for astronomy applications after a bright illumination. Briefly, any perturbation on these detectors generates a nonlinear signal with higher amplitude than a dark current and lasts for hours. The traditional hypothesis used to explain this phenomenon is based on trapping/emission processes from deep-level defects in the space charge region (SCR) of the diode. Inspired by deep-level transient spectroscopy formalism, we have developed an analytical model describing the trap emission current from the SCR of the photodiode. We also take into account the intrinsic non-linearity of the source follower per detector ROIC architecture. Compared to data obtained on detectors built in-house at CEA-LETI, the model allows the estimation that a trap density on the order of the residual doping is enough to explain the persistence amplitude. A graded trap density in the SCR is in addition necessary to explain the persistence measurement as a function of the stress amplitude. Limits of the model are also underlined in the case of higher persistence amplitude. In this case, trap density should be close to the doping. This implies that N doping of the diode would be compensated, which is an extreme scenario out of the scope of this model. [ABSTRACT FROM AUTHOR]

Near field liquid structures and penetration of a kerosene jet injected in a Mach 2 crossflow were studied experimentally in the LAPCAT-II Dual Mode Ramjet Combustor at Onera, using high spatial and temporal resolution imaging techniques. The experiments performed in this study provide measurements in high temperature conditions, with kerosene as liquid fuel to bring new data in these conditions. The fuel spray is injected through a single orifice, perpendicularly to the supersonic flow, at temperatures from ambient to 1500 K and jet-to-crossflow momentum flux ratio from 3.1 to 8.6. Five different test cases are defined to show independently the influence of the injection ratio and the supersonic flow temperature on the liquid jet atomization. A high spatial resolution imaging system is used to detail the characteristic spatial scales of this atomization process in the supersonic crossflow. The surface waves and droplets produced in the windward side of the liquid jet are characterized qualitatively. High-resolution visualizations bring a new insight of the droplet trajectories in the leeward region of the jet. Schlieren imaging is used to detail the complex shock wave structures in such a supersonic flow. Penetration of the kerosene jet is measured by shadowgraphy and schlieren imaging in five test cases and compared to other studies of the literature whenever it is possible. High-speed schlieren recorded at 210 kHz is also performed to capture the temporal dynamics of the shocks and measure the liquid jet velocity. [ABSTRACT FROM AUTHOR]

The CATalytic Experiment (CATE) is an in-flight demonstration of catalysis effects at the surface of thermal protection materials. A high-catalytic coating was applied over the baseline ceramic material on the windward side of the intermediate experimental vehicle (IXV). The temperature jump due to different catalytic activities was detected during re-entry through measurements made with near-surface thermocouples on the windward side of the vehicle. The experiment aimed at contributing to the development and validation of gas/surface interaction models for re-entry applications. The present paper summarizes the design of CATE and its integration on the windward side of the IXV. Results of a qualification campaign at the Plasmatron facility of the von Karman Institute for Fluid Dynamics are presented. They provided an experimental evidence of the temperature jump at the low-to-high catalytic interface of the heat shield under aerothermal conditions relevant to the actual IXV flight. These tests also gave confidence so that the high-catalytic patch would not endanger the integrity of the vehicle and the safety of the mission. A preliminary assessment of flight data from the thermocouple measurements shows consistency with results of the qualification tests. [ABSTRACT FROM AUTHOR]

We performed an experimental study of large-scale wakes in a rotating shallow-water layer. Standard particle image velocimetry was used to measure the horizontal velocity field, while a laser-induced fluorescence technique was used to measure the geopotential deviation (i.e., the interface deviation). According to these measurements, we were able to quantify the dynamics in a wide region of parameter space beyond the quasi-geostrophic regime. For obstacles larger than the deformation radius and with small Rossby numbers, a significant asymmetry occurs in the wake between cyclonic and anticyclonic vortices. These parameters correspond to a frontal geostrophic regime with the relative interface deviation being larger than 0.1–0.2. In this case, anticyclones remain coherent and circular, whereas cyclones tend to be elongated and distorted. More surprisingly, for some extreme cases, coherent cyclones do not emerge at all, and only an anticyclonic vortex street appears several diameters behind the obstacle. The transition from a quasi-geostrophic to a frontal geostrophic regime is characterized by a strong increase in the Strouhal number, which can reach a value up to 0.6. Hence, we found that a large-scale wake could differ strongly from the classical Karman street when the relative geopotential deviation becomes larger than the Rossby number. [ABSTRACT FROM AUTHOR]

Clouds often reveal a small-scale vortex shedding in the wake of mountainous islands. Unlike the classical bidimensional Karman street, these observed vortex streets are affected by the earth’s rotation and vertical stratification of the atmosphere. These effects induce a selective destabilization of anticyclonic vorticity regions. It is well known that inertial instability, also called centrifugal instability, induces a three-dimensional destabilization of anticyclonic vortices when the absolute vorticity is larger than the local Coriolis parameter. However, we have shown by means of laboratory experiments, that it is a different type of instability which is mainly responsible for the secondary destabilization of rotating Karman streets. A series of experiments were performed to study the wake of a cylinder in a rotating deep water layer, at a medium Reynolds number and an order one Rossby number. We have shown that the unstable mode is a core-centered perturbation and its vertical wavelength strongly depends on the Rossby number. These dynamical characteristics are the signature of an elliptical instability in rotating flows. We have also checked that an asymptotic instability criterion, valid around an elliptical stagnation point, was satisfied in the early stage of shedding when the anticyclonic boundary layer detaches and rolls up in elliptical structures. Similar asymmetric Karman streets were also found in the shallow-water configuration when the vertical to horizontal aspect ratio is close to unity. [ABSTRACT FROM AUTHOR]

Experiments regarding the modification of the foil geometry and/or active or passive mass injection in the vortex core have been performed to investigate the possibility of inhibiting tip vortex cavitation. The ejection at very low flow rates of drag-reducing polymer solutions at the tip of hydrofoils and propeller blades has demonstrated effectiveness as a tip vortex cavitation inhibitor. This paper reports the results obtained with an elliptical hydrofoil, of 8cm maximum chord and 12cm haif-span, operating at Reynolds numbers, of =10, much larger than those previously reported in the literature. Lift coefficients and critical cavitation numbers were determined for a variety of flow and polymer solution ejection conditions. Tangential and axial components of the mean velocity as well as velocity fluctuations along the vortex path were also measured. At 12.5 m/s free stream velocity and a variety of angles of attack, the ejection of a 500 ppm aqueous solution of a drag-reducing polymer at a flow rate of about 5 cm/s leads to a decrease of up to 30% in the cavitation number. This occurs without modification of the lift coefficient and, hence, of the midspan bound circulation of the foil. Moreover, water injection does not cause any appreciable change in the cavitation numbers. The tangential velocity profiles along the vortex path during polymer ejection indicate that the potential region remains the same, while the viscous core dimension increases, and the maximum tangential velocity decreases substantially as compared to the no ejection or water ejection experiments. Thus, the pressure coefficients at the vortex axis are smaller than for the no ejection or water ejection cases and cause the reduction of the critical cavitation numbers. It is speculated that this inhibition effect is due only to swelling of the polymer solution when exiting the ejection orifice. [ABSTRACT FROM AUTHOR]