Bibliography - M. I. Radulescu

1. Radulescu, M. I., and Chung K Law, 2007: The transient start of supersonic jets. Journal of Fluid Mechanics, 578, doi:10.1017/S0022112007004715 331-369
   [ Abstract ]

   This study investigates the initial transient hydrodynamic evolution of highly underexpanded slit and round jets. A closed-form analytic similarity solution is derived for the temporal evolution of temperature, pressure and density at the jet head for vanishing diffusive fluxes, generalizing a previous model of Chekmarev using Chernyi’s boundary-layer method for hypersonic flows. Two-dimensional numerical simulations were also performed to investigate the flow field during the initial stages over distances of ∼1000 orifice radii. The parameters used in the simulations correspond to the release of pressurized hydrogen gas into ambient air, with pressure ratios varying between approximately 100 and 1000. The simulations confirm the similarity laws derived theoretically and indicate that the head of the jet is laminar at early stages, while complex acoustic instabilities are established at the sides of the jet, involving shock interactions within the vortex rings, in good agreement with previous experimental findings. Very good agreement is found between the present model, the numerical simulations and previous experimental results obtained for both slit and round jets during the transient establishment of the jet. Criteria for Rayleigh–Taylor instability of the decelerating density gradients at the jet head are also derived, as well as the formulation of a model addressing the ignition of unsteady expanding diffusive layers formed during the sudden release of reactive gases.

2. Radulescu, M. I., G. J. Sharpe, Chung K Law, and J.H.S. Lee, 2007: The hydrodynamic structure of unstable cellular detonations. Journal of Fluid Mechanics, 580, doi:10.1017/S0022112007005046 31-81
   [ Abstract ]

   The study analyses the cellular reaction zone structure of unstable methane–oxygen detonations, which are characterized by large hydrodynamic fluctuations and unreacted pockets with a fine structure. Complementary series of experiments and numerical simulations are presented, which illustrate the important role of hydrodynamic instabilities and diffusive phenomena in dictating the global reaction rate in detonations. The quantitative comparison between experiment and numerics also permits identification of the current limitations of numerical simulations in capturing these effects. Simulations are also performed for parameters corresponding to weakly unstable cellular detonations, which are used for comparison and validation. The numerical and experimental results are used to guide the formulation of a stochastic steady one-dimensional representation for such detonation waves. The numerically obtained flow fields were Favre-averaged in time and space. The resulting onedimensional profiles for the mean values and fluctuations reveal the two important length scales, the first being associated with the chemical exothermicity and the second (the ‘hydrodynamic thickness’) with the slower dissipation of the hydrodynamic fluctuations, which govern the location of the average sonic surface. This second length scale is found to be much longer than that predicted by one-dimensional reaction zone calculations.

3. Radulescu, M. I., Chung K Law, and G. J. Sharpe, 2005: Structure of unstable gaseous detonations waves. Physics of Fluids, 17(091105), doi:10.1063/1.1942517
   [ Abstract ]

   Detonation waves are supersonic combustion waves. The figures illustrate their typical unstable structure and the hydrodynamic compressible turbulence generated via instabilities and self-sustained by the chemical energy release. The grayscale photographs are schlieren records of the vertical density gradients in a methane–oxygen detonation wave, illustrating the turbulent structure comprised primarily of transverse shocks, shear layers, and density interfaces separating light reacted gases and heavier unreacted gas. The detonation propagates to the right at an average Mach number of ˜ 6. The color figures illustrate the structure of the wave (pressure and temperature) obtained numerically. The front is organized in a characteristic cellular structure and substructure, consisting of interacting triple shock Mach intersections (frontal Mach stems, incident shocks, transversely propagating reflected shocks, and convected shear layers). The triple points are driven by the chemical exothermicity behind the strong Mach stems. Due to the exponential dependence of the reaction rates on local temperature, gases shocked by the weaker incident shocks have ignition delay times several orders of magnitude longer, hence accumulate as unreacted volumes behind the front. These unreacted gases react mainly through turbulent mixing with the hot reacted gases. Shear layers at the triple shock interactions are Kelvin–Helmholtz unstable and promote gas ignition by turbulent mixing of mass and heat. The transverse shocks, which sweep perpendicularly to the main front, further disrupt these density interfaces by the Richtmyer–Meshkov instability involving the baroclinic torque. Unstable detonations thus rely on compressible turbulence interactions to promote the local reaction rates of gases which escape ignition due to the unsteadiness of the leading front. The detonation wave structure thus provides an excellent setting to study exothermicity-driven compressible turbulence, manifested primarily by the interaction of shocks, density interfaces, and vortical flows.

Direct link to page: http://cmi.princeton.edu/bibliography/results.php?author=3664