1,721,068 research outputs found
Theoretical Considerations on Shock Reflections and Their Implications on the Evaluations of Air Intake Performance
No full textMany theoretical studies have shown the existence of a hysteresis effect in the solution of oblique shock reflections. In fact, a wide domain of free-stream Mach number and shock angle values exists where regular reflection and Mach reflection are both possible solutions for the same flow conditions. Part of this domain overlaps the typical operating conditions of supersonic air intakes, and therefore it is of practical interest to obtain a deeper understanding of the theoretical problem. Indeed, although both solutions are theoretically possible, they yield very different flowfields and consequently large discrepancies in the evaluation of the air intake performance. Numerical solutions for steady configurations have been carried out and compared with the flow evolution obtained for time-dependent cases. The results have confirmed numerically the existence of the multiple solution domain where hysteresis takes place in time-dependent simulations. The analysis of the physical and numerical problems encountered has provided indications for a correct simulation in practical applications
Methodology to solve flowfields of plug nozzles for future launchers
No full textIn the development of future launchers, in particular the single stage to orbit launcher, the use of plug nozzles looks promising because of the possibility of improving performance; therefore the capability to achieve accurate evaluations of the relevant flowfields is required. Nevertheless, because of typical flow features, analyses performed by the common computational fluid dynamics methods lead to uncertain and time-consuming results, To circumvent some of these problems an approach based on a simple simulation model that replaces the mixing layer between the nozzle jet and external flow by a contact discontinuity was recently proposed by the authors. Following this approach the complex calculation of the turbulent mixing of flows having different thermodynamic characteristics can be avoided, while correctly accounting for the main features of the flowfield. In this paper this simple model was implemented to compute linear plug nozzle flowfields at different ambient conditions and for different geometries of the plug. The expected flow behavior has been well reproduced and good agreement with the theoretical nozzle performance has been achieved, allowing a performance analysis for some of the main characteristic parameters to be carried out
Analysis of Unsteady Supersonic Viscous Flows by a Shock Fitting Technique
No full textAn extension of Moretti's classical shock-fitting technique is proposed to solve complicated unsteady viscous flows. This version allows the automatic treatment of flow structures featuring triple points and shock interactions. A fitting of contact discontinuities has also been introduced for a number of problems. The fitting procedure is used with a Navier-Stokes solver based on the A scheme. Validation tests for selected cases are presented
Numerical Analysis of the Slipstream Effect in External Expansion Nozzles
No full textFuture high-speed jet propulsion engines need sysstems to expand the exhaust gas, which must work efficiently along the whole vehicle flight path. Among the solution proposed, external expansion systems show the property of adapting themselves to the ambient pressure, allowing better performance than conventional nozzles. Unfortunately, for adaptation they have to interact with the external air stream, and this interaction can signifiantly reduce performance. This paper shows how large this effect can be for a generic external expansion nozzle configuration. The analysis is carried out ny solving numerically the Euler equations by a methodology based on the lambda-scheme and on the fitting of the flow discontinuities
Numerical Aspects of the Solution of the Non-Conservative Navier-Stokes Equations for High Speed Flows
No full textThe numerical solution of the laminar Navier-Stokes equations for high-speed flows is considered. The equations are written in non-conservative form, and the shocks treated by a shock-fitting technique which allows accurate predictions of the shock location and of its propagation velocity. Since a critical point of the numerical calculations of viscous high speed flows is the prediction of the interaction between shock wave and boundary layer, numerical tests are performed to assess the capability of the present technique to handle these phenomena in both weak and strong interaction regimes
Shock structure in separated nozzle flows
No full textIn the case of high overexpansion, the exhaust jet of the supersonic nozzle of rocket engines separates from nozzle wall because of the large adverse pressure gradient. Correspondingly, to match the pressure of the separated flow region, an oblique shock is generated which evolves through the supersonic jet starting approximately at the separation point. This shock reflects on the nozzle axis with a Mach reflection. Thus, a peculiar Mach reflection takes place whose features depend on the upstream flow conditions, which are usually not uniform. The expected features of Mach reflection may become much difficult to predict, depending on the nozzle shape and the position of the separation point along the divergent section of the nozzle
The Use of Shock-Fitting Techniques to Simulate Discontinuities in Transonic and Supersonic Flows
No full textThe study illustrates the possible advantages of using shock fitting techniques to solve transonic and supersonic flowfields. The governing equations are written in nonconservative form and solved by the Lambda scheme in the smooth regions of the flowfield. Shocks and contact discontinuities are solved explicitly, by an advanced version of a fitting technique, which allows accurate predictions of the location and propagation velocity of the discontinuities. Numerical tests are presented to show the capability of the technique to handle complex flow structures in both steady and unsteady regimes
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