Abstract
The aim of this work is the investigation of Mach and Reynolds numbers effects on the behaviour of turbulent gas jets in order to gain new insights into the fluid dynamic process of turbulent jet mixing and spreading. An in-house solver (Flow-Large Eddy and Direct Simulation, FLEDS) of the Favre-filtered Navier Stokes equations has been used. Compressibility has been analyzed by considering gas jets with Mach number equal to 0.8, 1.4, 2.0 and 2.6, and Re equal to 10,000. As concerns the influence of Re on gas jets, four cases have been investigated, i.e. mathrm{Re} = 2500, 5000, 10,000 and 20,000, with Mach number equal to 1.4. The results show that, in accordance with previous experimental and numerical studies, the potential core length increases with Mach number. As regards the velocity decay and the spreading rate downstream of the potential core, compressibility effects are not relevant except for the jet with Mach number of 2.6. The normalized turbulent kinetic energy along the centerline as a function of the normalized streamwise distance shows a similar peak at the end of the potential core for all jets, except for the case with Mach number of 2.6. By increasing Re, the length of the potential core decreases up to the same value for all Re higher than 10,000. In the region downstream of the potential core, the velocity decay decreases as Re number increases from 10,000 to 20,000, whereas, for lower values of Re, the influence is almost negligible.
Highlights
The physics of gas jets is a relevant topic for both fundamental fluid dynamics (Pope 2000; Wilcox 1994) and engineering applications (Hamzehloo and Aleiferis 2016)
In Bonelli et al (2016) the authors have studied the behaviour of gas jets with large injection density ratios at high Reynolds number, whereas the present study aims to investigate the effects of compressibility at intermediate and high Mach numbers in the absence of shock and expansion waves, and the influence of a wide range of Re (2 500 < Re < 20,000 ) on the turbulence structure of such jets
As far as the time signal is concerned, a location is chosen for all jets to be enough downstream of the potential core, such that the turbulence spectrum is fully developed, homogeneous and isotropic. Such a location is chosen in a way that for all jets the normalized Turbulent Kinetic Energy (TKE) is taken equal to 0.0065, which corresponds to x∕D = 17.5, 18.9, 22.2 and 33.6 for Mach numbers equal to 0.8, 1.4, 2.0 and 2.6 respectively
Summary
The physics of gas jets is a relevant topic for both fundamental fluid dynamics (Pope 2000; Wilcox 1994) and engineering applications (Hamzehloo and Aleiferis 2016). A comprehensive review can be found in the work of Ball et al (2012) The structure of such “simple” jet is described by the mean streamwise velocity field, ūx(x, r) , that is a function of the streamwise, x, and radial, r, coordinates. The mean streamwise centerline velocity, ūx(x, 0) , and the mean half-width, r1∕2 , (defined as the radial coordinate where the streamwise velocity is half of the velocity at the centerline, i.e. ūx(x, r1∕2) = ūx(x, 0)∕2 ) are used as characteristic variables to verify selfsimilarity.
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