Pasquale Cinnella

Flux-split algorithms for flows with non-equilibrium chemistry and vibrational relaxation

Abstract The numerical computation of gas flows with non-equilibrium thermodynamics and chemistry is considered. Several thermodynamic models are discussed, including an equilibrium model, a general non-equilibrium model, and a simplified model based upon vibrational relaxation. Flux-splitting procedures are developed for the fully-coupled inviscid equations involving fluid dynamics, chemical production, and internal energy relaxation processes. New forms of flux-vector split and flux-difference split algorithms valid for non-equilibrium flow, are embodied in a fully coupled, implicit, large-block structure. Several numerical examples in one space dimension are presented, including high-temperature shock tube and nozzle flows.

Characteristic-based algorithms for flows in thermochemical nonequilibrium

Novel numerical techniques based upon Steger-Warming, Van Leer, and Roe-type flux splittings are presented in three-dimensional generalized coordinates for the Navier-Stokes equations governing flows out of chemical and thermal equilibrium. Attention is placed on convergence to steady-state solutions with fully coupled chemistry. Time integration schemes including explicit m-stage Runge-Kutta, implicit approximate-factorization, relaxation, and LU decomposition are investigated and compared in terms of residual reduction per unit of CPU time

Coupling Heat Transfer and Fluid Flow Solvers for Multidisciplinary Simulations

The feasibility of multidisciplinary simulations for realistic geometries involving detailed physical models is demonstrated. Specifically, a three-dimensional chemically reacting fluid flow solver is coupled with a solid-phase heat transfer solver that includes cooling channels. Both fluid- and solid-phase models employ the integral, conservative form of the governing equations and are discretized by means of two finite volume numerical schemes. To keep the heat flux consistent, a special algorithm is developed at the interface between the solid and fluid regions. Physical and thermal properties of the solid materials can be temperature dependent, and different materials can be used in different parts of the domains due to a multiblock gridding strategy. The cooling channel model is developed by using conservation laws of mass, momentum, and energy, taking into account the effects of heat transfer and friction. The coupling of the models (solid and fluid, solid and cooling channels) is detailed. A hot-air nozzle test case is examined

Numerical simulations of reactive flows

Reactive flows can be defined as fluid flows that are significantly affected by chemical reactions (e.g., combustion, dissociation, and biochemical processes) and/or thermodynamic nonequilibrium (e.g., vibrational excitation). Practical applications of reactive fluid flows can be found easily in everyday life, from car engines to heating systems to blood circulation in living beings. Less obvious but equally important areas where reacting and/or nonequilibrium fluid flows play a vital role include aerospace propulsion, manufacturing processes in the electronic industry, and lasers. Common to all these examples is the intimate coupling between fluid dynamics, chemistry, and physics, which renders a detailed understanding of the flow elusive in most cases. The “computer revolution” has profoundly affected the way scientists and engineers tackle these problems (and many others), allowing a third investigative tool, computer simulation, in addition to the traditional means of theoretical and experimental studies. The dramatic increase in computational capabilities of recent years has translated into significant progress towards the goal of accurately simulating reactive flows. This task is particularly complicated because it requires major advances in two areas, computational fluid dynamics (CFD) and physical modeling. Each area is challenging on its own and must be closely coupled to reproduce the physical reality. Using very simple physics (incompressible fluids, or ideal gases), CFD researchers have been able to simulate fluid flows over or inside extremely complicated geometries: for example, full maneuvering aircraft and submarines, or a human heart. By using very simple geometries (spheres, cones, rectangular domains, or smoothly varying channels), many complex physical problems have been investigated: for example, hypersonic dissociating and ionizing flows, lasers, and combustion mixtures involving hundreds of chemical species. The present challenge, still largely unfulfilled, is to combine geometrical and physical complexity to achieve realistic simulations that can improve the basic understanding of reactive, nonequilibrium fluid flows. The accurate simulation of reactive flows has benefited from significant advances in the quality of CFD simulations. Finite-volume techniques, upwind algorithms for the discretization of the convective fluxes, and implicit time-integration schemes have been instrumental in the achievement of three-dimensional simulations (see Hirsh [1990]). Implicit techniques in particular are essential for reactive flow calculations, because the wide range in characteristic time and space scales that can be encountered in the same application renders other approaches extremely inefficient. Other useful developments in the quest for better algorithms include: preconditioning schemes for the efficient simulation of low-speed reactive flows; multidimensional techniques for capturing discontinuities or rapid variations in the flow (shock waves, slip lines, strong expansions); and adaptive gridding, which allows the major flow features to be accurately detected and tracked with significantly smaller computational resources than more traditional approaches.

Comprehensive Numerical Study of Jet-Flow Impingement over Flat Plates

Thepresentstudyattemptsanumericalinvestigationofthecomplexe owe eldthatoccurswhenanunderexpanded jet collides against a solid surface. Numerous examples of this problem can be found in the aerospace industry (e.g., rocket test stands, multistage separation ). A simplie ed geometry, already employed in previous experimental inquiries, was chosen as a test case: an underexpanded, axisymmetric, air jet impinging on a e at plate at varied angles. The three-dimensional Navier ‐Stokes equations were solved by means of a second-order-accurate Roetype algorithm with a generalized grid formulation. The computational domain includes theconvergent ‐divergent nozzle and the external e eld. The numerical results show various jet-shock and shock-shock interactions and compare very well with experimental data, including shadowgraph pictures and both location and values of the peak pressures on the inclined plate. This investigation focused on performing a thorough comparison between experiments and simulations, thereby establishing some level of cone dence in the accuracy and reliability of the numerical tool developed, CHEM. CHEM can accommodate more complicated and realistic geometries and physical conditions than those encountered in this study: with further ree nement and validation it can be used for rocket plume and plume/solid surface interaction simulations, both on the ground and in e ight.

General solution procedure for flows in local chemical equilibrium

This study details the derivation and application of an approximate Riemann solver of the Roe type specifically designed for the numerical simulation of inviscid and viscous flow problems involving general and virtually arbitrary mixtures of thermally perfect gases in local chemical equilibrium. The solution procedure is by no means limited to airflows and will be applied to an oxygen/hydrogen mixture as well. A «black box» solver for the local equilibrium composition of a gas mixture of known density and internal energy is coupled with the flow solver, which is based on the newly derived flux-difference-split technique. A few test cases, including both external and internal flows, illustrate the capabilities and the overall efficiency of the flow solver

Numerical Simulations of Fluids with a General Equation of State

A numerical technique suitable for the simulation of two-phase, chemically recting flows is introduced. Mixtures of thermally imperfect gases can be utilized, and preconditioning techniques ensure accuracy and robustness of the present approach for virtually any Mach number. Preliminary results are presented, and a few avenues for future work are briefly discussed.

Truly two-dimensional algorithms for radiative heat transfer calculations in reactive flows

Abstract The present study details the inclusion of radiative heat transfer phenomena in the numerical simulation of reactive hypersonic and atmospheric reentry flows. Truly two-dimensional algorithms are developed for the radiative source term in the governing equations, whereby the determination of the specific intensity field is obtained by means of a numerical integration over directions of propagation of radiation. The one-dimensional Slab approximation is lifted, and the analysis presented allows the determination of the radiative characteristics of the entire flowfield, rather than being limited to the stagnation streamline, thereby providing the potential for an accurate assessment of two-dimensional relieving effects throughout the flowfield. A few preliminary results are presented for the Mach 47 flow over a cylinder and a sphere, including a comparison of the two-dimensional algorithms with the one-dimensional approximation and an emission-dominated-case. The effects of improving the modeling of radiative heat transfer are demonstrated. The present approach can be easily extended to three-dimensional problems.

A Thermodynamic Model for Chemically Reacting, Two-Phase Fluids

A review of some of the issues related to the modeling of fluid mixtures is presented. A general thermodynamics model that can accommodate chemically reacting, two-phase fluids is introduced. The goal is to develop a model that is accurate and amenable to implementation in state-of-the-art flow solvers. Some preliminary results are presented, and a few avenues for future research are briefly discussed.

Two-dimensional radiative heat-transfer calculations for nonequilibrium flows

The present study details the inclusion of radiative heat-transfer phenomena in the numerical simulation of reactive hypersonic and atmospheric re-entry flows. Truly two-dimensional algorithms are developed for the radiative source term in the governing equations, whereby the determination of the specific intensity field is obtained by means of a numerical integration over directions of propagation of radiation. The one-dimensional slab approximation is lifted, and the analysis presented allows the determination of the radiative characteristics of the entire flowfield, rather than being limited to the stagnation streamline, thereby providing an accurate assessment of two-dimensional relieving effects in the stagnation region. A few preliminary results are presented for the Mach 47 flow over a cylinder, including a comparison of the two-dimensional algorithm with the one-dimensional approximation and an emission-dominated case. The effects of improving the modeling of radiative heat transfer are demonstrated. 31 refs.