1,720,993 research outputs found
High-speed turbulent gas jets: an LES investigation of Mach and Reynolds number effects on the velocity decay and spreading rate
Full text linkThe 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. 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
Advanced model for the interaction of a Ti plume produced by a ns-pulsed laser in a nitrogen environment
No full textThe expansion of a titanium plume evaporated by a ns laser pulse and expanding in nitrogen environment has been investigated by means of a numerical model. The 2D fluid dynamic equations have been coupled with a chemical model implementing the state-to-state kinetics of nitrogen molecules, focusing on the role of molecular vibration in the plasma evolution. The calculated vibrational distributions considerably depart from the Boltzmann one, showing overpopulated tails. Large differences with results obtained considering macroscopic models have been predicted
Large Eddy Simulation of High-Density Ratio Jets
No full textThe aim of this project is the analysis of hydrogen jets by using a Large Eddy
Simulation model in order to gain new insights into the fluid dynamic process of
turbulent mixing. The investigation of such jets is a relevant subject for the
development of new propulsion systems, to improve the performance and the
efficiency of direct-injection engines and also for safety issues. The computations
of hydrogen jets require accurate and robust computational tools. Hence, the use of
a suitable code and high performance computing is mandatory to successfully
carry out the simulations.
The present work is carried out by means of an in-house code, named FLEDS. A
significant issue when dealing with compressible free jets, especially with hydrogen
jets, is the presence of sharp density gradients in the flow field, that cause
non-physical spurious oscillations. In order to avoid these instabilities, an
innovative localized Artificial Diffusivity Scheme has been implemented. The flow
model has been assessed by comparing the numerical results with both theoretical
considerations and experimental measurements.
The influence of both high air/hydrogen density ratios and jet Mach numbers on the
turbulent mixing and the spreading rate has been analyzed by means of FLEDS
code. This is a major issue dealing with direct injection in combustion systems,
since the efficiency of combustion and pollutant emissions are strictly related to the
mixing of air and fuel in diffusive flames
Assessment of a consistent multi-internal-temperature kinetic model for hypersonic neutral air flows using a finite volume solver
No full textA multi-internal-temperature approach for hypersonic air kinetics has been consistently derived from the state-specific vibrational kinetics. Vibrational levels have been grouped in a limited number of subsets (one to five), each one characterized by its own concentration and temperature, approximating the entire distribution as a piecewise Boltzmann. The capability of the reduced-order model in terms of accuracy and computational savings has been tested comparing the results with those obtained using the state-to-state approach. Firstly, a 0D heat bath evolution in thermochemical non-equilibrium is considered. Then, the proposed model has been implemented in a finite volume solver for the solution of the Euler equations, employing a Flux Vector Splitting scheme with MUSCL reconstruction, and used to solve an axisymmetric hypersonic flow past a sphere
On the influence of non equilibrium in the free stream conditions of high enthalpy oxygen flows around a double-cone
No full textThis work presents a detailed analysis of the shock wave/boundary layer interaction in hypersonic flows around a double-cone. Numerical simulations have been carried out by solving the axisymmetric Navier- Stokes equations for an oxygen mixture. Thermochemical non-equilibrium is taken into account by employing the multi-temperature model (mT) proposed by Park and the State-to-State model (StS). The occurrence of surface processes is also analyzed considering both non-catalytic and fully-catalytic wall models. It is shown that the evaluation of correct free stream conditions is fundamental for the prediction of the separation extent and the corresponding heat transfer in high enthalpy conditions
A Numerical Analysis of Hydrogen Underexpanded Jets
No full textThe aim of this work is the study of the fluid dynamic structure
of underexpanded hydrogen jets by using a High Performance
Computing (HPC) methodology. An axial symmetric
two-dimensional turbulent flow model, which solves the Favre-averaged
Navier-Stokes equations for a multicomponent gas
mixture, has been implemented and validated. In order to predict
the decrease in spreading rate with increasing Mach number, a
compressibility correction has been added to the turbulence closure
model.
The flow model has been assessed by comparing spreading
and centerline property decay rates of subsonic jets at different
Mach numbers with those obtained both by theoretical considerations
and experimental measurements. Besides, the Mach disk
structure of an underexpanded jet has been analysed, thus confirming
the suitability of the computational model.
To take into account the effects of real gases, both van der
Waals and Redlich-Kwong equations of state have been implemented.
The computations performed under ICEs conditions
show that the values of Mach number and pressure just behind
the Mach disk are affected by the use of real gas equations
Simulation of High-Enthalpy Turbulent Shock Wave/Boundary Layer Interaction Using a RANS Approach
No full textA Numerical Analysis of Hydrogen Underexpanded Jets under Real Gas Assumption
No full textThis work examines the fluid dynamic structure of underexpanded gas jets by using a high-performance computing (HPC) methodology in order to untangle the question of whether it is necessary to include the real gas assumption dealing with hydrogen jets. The answer to this question is needed in order to guarantee accurate numerical simulations of such jets in practical engineering applications, such as direct-injection hydrogen engines. An axial symmetric turbulent flow model, which solves the Favre-averaged Navier–Stokes equations for a multicomponent gas mixture, has been implemented and validated. The flow model has been assessed by comparing spreading and centerline property decay rates of subsonic jets at different Mach numbers with those obtained by both theoretical considerations and experimental measurements. Besides, the Mach disk structure of underexpanded jets has been recovered, thus confirming the suitability and reliability of the computational model. To take into account the effects of real gases, both van der Waals and Redlich–Kwong equations of state have been implemented. The analysis of a highly underexpanded hydrogen jet with total pressure equal to 75 MPa, issuing into nitrogen at 5 MPa, shows that the use of real gas equations of state affects significantly the jet structure in terms of temperature, pressure, and Mach number profiles along the jet centerline and also in terms of jet exit conditions, with differences up to 38%
Large Eddy Simulation of High-Density Ratio Hydrogen Jets
No full textThe aim of this work is the analysis of a subsonic high-density ratio hydrogen jet by using a Large Eddy Simulation
(LES) model in order to gain new insights into the fluid dynamic process of turbulent mixing of compressible jets. FLEDS
(Flow - Large Eddy and Direct Simulation) code has been employed, where a sixth-order compact finite difference scheme
is used to discretize the governing equations. The simulation of compressible free jets is challenging in the presence of
large density gradients. The injection of hydrogen in air implies high velocities, large diffusion and even higher density
gradients, that can cause non-physical spurious oscillations. Therefore, a Localized Artificial Diffusivity (LAD) scheme has
been implemented in order to remove the numerical instabilities, that are not dumped by the high-order non-dissipative
numerical scheme employed in this work. A hydrogen jet with Mach number equal to 0.8 issuing into still air has been
considered. The LES results show that the normalized centerline velocity decay rate is in good agreement with the expected
theoretical steady state profile. Besides the spreading rate of the jet accounts for the effects of both the jet Mach number and
the high air/hydrogen density ratio
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