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A new procedure of determining the time resolved capacitance of a plasma actuator during operation is introduced, representing a simple diagnostic tool that provides insight into the phenomenological behavior of plasma actuators. The procedure is demonstrated by presenting example correlations between consumed electrical energy, size of the plasma region, and the operating voltage. It is shown that the capacitance of a plasma actuator is considerably increased by the presence of the plasma; hence a system that has previously been impedance matched can be considerably de-tuned when varying the operating voltage of the actuator. Such information is fundamental for any attempts to increase the energy efficiency of plasma-actuator systems. A combined analysis of the capacitance, light emission, size of the plasma region, force production, and power consumption is presented.

Active control of laminar boundary layers with dielectric barrier discharge (DBD) plasma actuators (PAs) has made considerable progress in the last 15 years. First pioneering experiments have motivated numerous researchers to gain a deeper insight into the underlying working principles and corresponding quantification of the actuator performance. These investigations clearly show the strengths but also the weaknesses of the PA as a flow control device. Presently, the boundary-layer control (BLC) with PAs experiences the transition from lab studies to real flight applications. However, the PA community still struggles with the poor fluid mechanic efficiency and the limited momentum flux of the actuator. This review therefore addresses the question how applicable the actuator is as an energy efficient flow control device for future in-flight applications. Since any successful flow control requires detailed knowledge of the actuator’s control authority, this discussion is built upon a careful and comprehensive summary of performance evaluation measures and the interplay with various changes of thermodynamic and kinematic environmental conditions. Consequently, this review for the first time provides a comprehensive discussion of all required steps for successful DBD-based in-flight flow control spanning from the power supply to the achieved flow-control success in one coherent document.

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Magnetic resonance velocimetry (MRV) measurements are performed in a 1:1 scale model of a singlecylinder optical engine to investigate the volumetric flow within the intake and cylinder geometry during flow induction. The model is a steady flow water analogue of the optical IC-engine with a fixed valve lift of 9:21 mm to simulate the induction flow at crank-angle 270 bTDC. This setup resembles a steady flow engine test bench configuration. MRV measurements are validated with phaseaveraged particle image velocimetry (PIV) measurements performed within the symmetry plane of the optical engine. Differences in experimental operating parameters between MRV and PIV measurements are well addressed. Comparison of MRV and PIV measurements is demonstrated using normalized mean velocity component profiles and showed excellent agreement in the upper portion of the cylinder chamber (i.e., y ! À 20 mm). MRV measurements are further used to analyze the ensemble average volumetric flow within the 3D engine domain. Measurements are used to describe the 3D overflow and underflow behavior as the annular flow enters the cylinder chamber. Flow features such as the annular jet-like flows extending into the cylinder, their influence on large-scale in-cylinder flow motion, as well as flow recirculation zones are identified in 3D space. Inlet flow velocities are analyzed around the entire valve curtain perimeter to quantify percent mass flow rate entering the cylinder. Recirculation zones associated with the underflow are shown to reduce local mass flow rates up to 50 %. Recirculation zones are further analyzed in 3D space within the intake manifold and cylinder chamber. It is suggested that such recirculation zones can have large implications on cylinder charge filling and variations of the in-cylinder flow pattern. MRV is revealed to be an important diagnostic tool used to understand the volumetric induction flow within engine geometries and is potentially suited to evaluate flow changes due to intake geometry modifications.

The piezo XAFS technique in combination with an in situ cell has been used for the investigation of fast solid-solid transformations with millisecond time resolution. The technique records X-ray absorption spectra (XAFS) in a continuous mode by making use of piezo tilt tables in the X-ray monochromator, which are actuated by an oscillatory high voltage. The application of this technique in the field of solid-state chemistry and catalysis is illustrated by investigations of the X-ray absorption near-edge structure (XANES) of the Cu K edge during the autocatalytic reduction of a Cu/ZnO methanol catalyst and the spontaneous decomposition of (NH 4 ) 2 Cr 2 O 7 to Cr 2 O 3 . The reduction of Cu(II) to Cu(0) occurs within a few seconds. The existence of an intermediate Cu(I) phase could be revealed by the analysis of the preedge position and the white line intensity, but the Cu(I) intermediate is very unstable. The spontaneous decomposition of (NH 4 ) 2 Cr 2 O 7 occurs within about 30 s and shows after a short acceleration period a rather constant reaction rate and a quite long decay period.