Laboratory hypersonic jets

Context. The free hypersonic jets can be found in several technological applications and even in astrophysical observations. The astrophysical context is that of the jets issuing from Young Stellar Objects (YSOs). In the present dissertation the propagation of hypersonic jets in a laboratory vessel has been mainly studied. Aims.In order to have a further insight into the jets from YSOs, a set of experiments is performed in the range of Mach numbers from 7 to 20 and for jetto-ambient density ratios from 0.85 up to more than 100, using different gas species and observing jet lengths in the order of 100 initial radii or more. Exploiting the scalability of the ydrodynamic equations, we intend to reproduce the YSO jet behaviour as far as the diagnostic quantities as head velocities and elapsed times are concerned. In some cases, we have made comparisons between our experimental results and the existing numerical ones and also made comparisons with the observed morphologies obtained by other research groups. Also the jet morphology after the impact at the end of the vessel and the interaction with the backward flow has been investigated. Methods.In the experiments the gas pressure and temperature are increased by a fast, quasi isoentropic compression by means of a piston system coupled with de Laval nozzles; it is performed by means of special facilities where the jet Mach numbers (in the range of 10 to 20) and the jet to ambient density ratios (in the range of 0.1 up to values exceeding 100) can be set independently of each other, what permits the investigation of a wide parameter range in the relevant physics. The visualizations and measurements are based on the electron beam technique: the jets are weakly ionized, and then a fast camera captures fluorescent images. Indications about the jets propagation and their resulting morphologies are obtained by means of several image processing techniques carried out on fluorescent images in order to denoising and measurement. Results.Our experimental results in long scale jets, owing to their originality, could be served as a benchmark for existing and future numerical simulations and also a reference for future experimental investigation on the long scale jets. These results, showing the collimated jet flow, has given an answer to the famous issue of the jet morphology: " the jet shape, including the pumping phenomena, is just due to fluid dynamics and does not require magnetic confinement, periodic energy pumping or any other complex causes". In some cases the results are compared with the existing numerical simulations, and their agreements, included the jet head advance speed, the formation of knots, the travelling structures after the jet impact with the vessel end, and also their density distribution, are fairly good in the majority of comparisons. Conclusions. Pressure-matched hypersonic jets maintain their collimation for long distances in terms of the initial jet radii, without including magnetic effects,leading to a qualitative good agreement with the observed YSO jets morphologies. Also their evolution with regard to the effective variables, the combination of the Mach number and density ratio, has allowed the global morphologies.

Contesto.