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On the domain of validity of the near-parallel combined stability analysis for the 2D intermediate and far bluff body wake
No full textAn original Pitot-static probe design for subsonic boundary layer investigation
Full text linkThis work presents a Pitot-static probe designed and developed to help in some boundary layer (BL) measurements, as for instance when average velocity profiles are required around models already available for wind tunnel tests but not provided with static pressure taps. The probe reads both total and static pressures at the desired location and by traversing it through the BL leads to the desired velocity measurements on the body under test, without having to rebuild the model and without using more expensive and complicated techniques. The probe is presented by describing its geometry in detail and completing the information by the relevant operating instructions. The probe response is then characterized by reporting its directional sensitivity and a validation for a standard test case. The specific probe presented here is suitable for a certain range of BL thicknesses, however the design can be easily scaled to different ranges
Discharge Stability Enhancement in Surface Corona Actuators
Full text linkA surface corona actuator for flow control, already tested in wind tunnel experiments, is here characterized at the bench in still air. The anodic electrode is provided with periodic triangular tips and is operated with a pulsed waveform. The operating stability and voltage range turn out to be wider than for dc operation. The actuator is characterized by means of several measurements as a function of the frequency, including power absorption and ionic wind measurements in different locations. The ionic wind is measured first by a micropitot probe obtaining time-averaged values, then by a hot wire anemometer obtaining instantaneous velocities and turbulence spectra. The behavior of the actuator is interpreted accounting for the creation and removal of charges in the gap subject to a periodic excitation
Convective instability in wake intermediate asymptotics
Get PDFThe paper presents a multiscale analysis of the intermediate region of the twodimensional convectively unstable wake past a bluff body. A recent asymptotic expansion solution was used as basic flow (Tordella & Belan, Phys. Fluids, vol. 15, 2003, 1897). This solution was obtained by matching an inner to an outer flow, both of which are Navier–Stokes solutions. By introducing a spatio-temporal multiscaling into the instability problem, an inhomogeneous Orr–Sommerfeld equation and an associated
modulation equation are obtained. The streamwise variation of the instability characteristics can then be deduced from the wave modulation, by considering the system to be perturbed by waves with a complex wavenumber that corresponds of the dominant saddle point of the local dispersion relation, taken at different positions downstream of the wake, and at different Reynolds numbers. The corrections of no parallelism are remarkable in the intermediate wake. When the disturbance is related to an early intermediate station, the corrections lead to absolute instability in the upstream portion of the intermediate wake, where, in addition, the spatial growth rate decreases. When the disturbance is related to a section in the far field, conditions of minimal temporal stability are reached about 20 body scales downstream. In the far field the temporal damping increases with the Reynolds number
Highly underexpanded jets in the presence of a density jump between an ambient gas and a jet
Full text linkAn experimental research concerning highly underexpanded jets made of different gases from the surrounding ambient is here described. By selecting different species of gases, it was possible to vary the jet-to-ambient density ratio in the 0.04–12 range and observe its effect on the jet morphology. By adjusting the stagnation and ambient pressures, it has been possible to select the Mach number of the jets, independently from the density ratio. Each jet is therefore characterized by its maximum Mach number, ranging from 10 to 50. The Reynolds number range of the nozzle is 103–5×104. The spatial evolution of the jets was observed over a much larger scale than the nozzle diameter. The gas densities were evaluated from the light emission induced by an electron beam and the gas concentrations were obtained by analyzing the color of the emitted light. The results have shown that the morphology of the jets depends to a greater extent on the density ratio. Jets that are lighter than the ambient exhibit a more intense jet-ambient mixing than jets that are heavier than the ambient, while the effects of changing the jet Mach number do not seem to be too large in the explored range. These results can be expressed by means of two simple scaling laws relevant to the near field (pre-Mach-disk) and the mid-long term field (post-Mach-disk), respectively.
PRE Kaleidoscope Images: August 2010
http://pre.aps.org/kaleidoscope/August2010/pr
A system of fast acceleration of a mass of gas for the laboratory partial simulation of stellar jets
No full textNew developments in laboratory hypersonic jets. Work Shop of the Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas (CMSO), Alba, September 29, 2007.
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