Hypersonic speed | EPFL Graph Search

In aerodynamics, a hypersonic speed is one that exceeds five times the speed of sound, often stated as starting at speeds of Mach 5 and above. The precise Mach number at which a craft can be said to be flying at hypersonic speed varies, since individual physical changes in the airflow (like molecular dissociation and ionization) occur at different speeds; these effects collectively become important around Mach 5-10. The hypersonic regime can also be alternatively defined as speeds where specific heat capacity changes with the temperature of the flow as kinetic energy of the moving object is converted into heat. Characteristics of flow While the definition of hypersonic flow can be quite vague and is generally debatable (especially due to the absence of discontinuity between supersonic and hypersonic flows), a hypersonic flow may be characterized by certain physical phenomena that can no longer be analytically discounted as in supersonic flow. The peculiarities in hypersoni

Simulation of benchmark and industrial unsteady compressible turbulent fluid flows

Michele Mossi

Large-eddy simulation (LES) is a very promising technique for the numerical computation of unsteady turbulent flows, and seems to be the perfect tool to simulate the compressible air flow around a high-speed train in a tunnel, providing unsteady results for aerodynamic and aeroacoustic analysis. To look into this possible future application of LES, two major lines of investigation are pursued in this thesis: first, the study of the effective ability of shock-capturing schemes to predict fundamental turbulent phenomena; second, the analysis of the aerodynamic phenomena induced by a high-speed train in a tunnel. The numerical simulation of compressible flows requires the use of shock-capturing schemes. These schemes can be relatively dissipative and mask the subgrid-scale contribution introduced in a large-eddy simulation to account for the unresolved turbulence scales. To estimate their effective dissipation and their ability to resolve turbulence phenomena, shock-capturing schemes widely used for aeronautical applications, from second- to fifth-order space accuracy, are employed here for simulating well-known fundamental flows in subsonic and supersonic regimes. Direct and large-eddy numerical simulations are undertaken for the inviscid and viscous Taylor-Green vortex decay problem, the freely decaying homogeneous and isotropic turbulence, and the fully developed channel flow. For all the turbulent flows investigated, several turbulence statistics are computed and the numerical dissipation of the schemes tested is interpreted in terms of subgrid-scale dissipation in a LES context, yielding an equivalent Smagorinsky or dynamic coefficient. This coefficient is for all schemes of the same order of magnitude as the commonly accepted values in LES for the subgrid-scale term. On the grounds of this analysis and of the comparisons of the turbulence statistics with accurate data obtained in the literature, the addition of explicit subgrid-scale models to the shock-capturing schemes tested is not recommended. It is thus concluded that the use of the LES technique for compressible turbulent flows is not yet suitable for industrial applications. The aerodynamic phenomena generated by a high-speed train travelling in a tunnel are also discussed, their importance on the design of high-speed lies is pointed out, and the analysis tools commonly employed for their study are reviewed. To study numerically the three-dimensional, compressible and turbulent air flow around a high-speed train accelerating in a tunnel, by accounting for the unsteady effects at inlet and outlet boundaries due to the propagation of pressure waves generated at the train departure, new coupling conditions between one-dimensional and three-dimensional domains are developed. These conditions are applied successfully to the numerical simulation of the unsteady wake developing behind two- and three-dimensional vehicles, where the averaged Navier-Stokes equations are solved with the turbulence modelling approach. The influence on the wake of the length of the vehicle tail is also discussed and results of multi-dimensional simulations are compared with one-dimensional data.

EPFL1999

Plasma Diagnostics and Modelling of Nanosecond Pulsed Surface Dielectric Barrier Discharge Actuators

Sami Goekce

During the past years, an increasing number of studies have been conducted on the use of electrical discharges for the stabilization of airflows (plasma flow control). Electrical gas discharges transfer energy and momentum to the gas through collisions of free electrons with atoms and molecules. Chemically active species such as ions, radicals and excited species are produced due to these collisions. The use of plasma actuators, notably surface dielectric barrier discharges (SDBD), for flow control applications has been largely investigated, and it has been demonstrated to effectively control the flow at low flow speed (below 30 m/s). Nowadays, the research in this area focuses on ways to improve the plasma actuators for flow speeds relevant to real flight conditions. One promising device for plasma flow control at high flow speed is the nanosecond pulsed surface dielectric barrier discharge. Nanosecond pulsed plasmas in general have also drawn attention from other fields, such as plasma assisted combustion, due to their ability to produce a large amount of active species and producing substantial overheating in a very short time. In this thesis an investigation of nanosecond pulsed SDBD is presented, with a focus on flow-control applications, but also on the production of active species, which are of great interest for plasma-assisted combustion and for other fields. The experimental characterization of the plasma created for different conditions, such as the operating pressure, the polarity and the amplitude of the applied voltage, is conducted. Important plasma parameters such as the gas temperature, the excited species produced, the reduced electric field or the electron density are either directly measured or inferred from emission spectroscopy using existing and novel diagnostic methods. The validity of the diagnostic methods is demonstrated using a numerical model of the plasma. The numerical modelling of the plasma also allows determining the influence of the plasma on the flow for several conditions. The experimentally studied conditions are simulated and compared with experimental results to show the strengths and limitations of the numerical model.

EPFL2014