Numerical experiments on the aerodynamics of waveriders.

摘要

Because of a high level of activity in manned space missions and hypersonic transport the ideas on waveriders are currently of great interest. Waveriders have been regarded as the best shapes for space planes. This derives from their high lift capability which will enable the vehicle to slow down at high altitude thus helping it alleviate the kinetic heating problem. The present study reports on the advantages of waveriders for their application to space plane shapes. The advantages of selecting waveriders as lifting shapes is attributed to their flow simplicity by using shapes defined inversely from a two dimensional flow as a basis of their construction. For these deceptively simple shapes initial estimates of the aerodynamic properties can be made through inviscid flow calculations. A historical preview of waveriders suggests that viscous effects are very important for accurate prediction of flowfield around these shapes. However, these effects were not included in the course of development of these shapes. In this study along with the classical theory of waveriders viscosity effects on the waverider design are highlighted. Also emphasised are the important relevant factors in hypersonic flow and the advantages of applying computational fluid dynamics (CFD) for simulation of the flowfield as compared to analytic and experimentation method. In the past, work has been reported on the inclusion of viscous effects by using the boundary layer for the viscous correction. The present study shows that, in the presence of strong viscous-inviscid interaction, viscous effects from these applications can only be reliably predicted using solutions of the Navier-Stokes equations. Based on this strategy numerical solutions of the Navier-Stokes Equations were applied to different waverider shapes to highlight the importance of viscous effects. Since the flow on typical waverider shapes is near conical, then a locally conical approximation was used for two reasons: it simplifies the problem from a 3-D to a 2-D one without compromising significantly accuracy ; it reduces the requirement of computing resources in terms of processor time and storage. Application of the Navier Stokes equations in locally conical form (LCNS) to simulate the flow around idealised waverider shapes revealed interesting off-design flow behaviour for on-design flow conditions. Sensitive effects on performance due to off-design behaviour are observed for caret wings. Results are obtained for 4 cases of caret wings optimised for free stream Mach numbers of 1.44, 1.74, 2.51 and 4.93 and 3 cases of a cone-wing configuration at Mach 10 with angles of attack of 5°, 10° and 15°. For caret wings results show how viscous effects have significant influence even at low Mach numbers. Flow simulation of these cases illustrates the advantage of using CFD on these shapes and shows how incorporating the NS equations provides a powerful tool to explore in detail waverider aerodynamics in on-design and off-design operation. Results also show how suitably it can deal with shock-shock, shock-boundary layer and shock vortex interactions, simultaneously. Also predicted was the effect on heat transfer due to the change in angle of attack of the shape. As caret wing and wing-cone combinations are thought to have limited applicability for practical aircraft shape the studies were extended to more general shapes. This study is the first to deal simultaneously with general shapes derived from both conical and wedge flowfields. General conical-derived shapes were constructed through a numerical approach based on flow around a cone using the Taylor Maccoll theory. For a general wedge-derived shape the base flow was the flow behind wedge induced oblique shock waves. Comparisons were made to evaluate the advantages and disadvantages of each type of configuration generated. Results were discussed in the light of numerical prediction and experimental results published in the literature. It was recognized that if volume constraints are relaxed, then, there are some considerable advantages in using wedge flow as a basis, instead of conical flow. Also it was shown that a change in only the leading edge shape can considerably improve the performance characteristics of waveriders. Furthermore a wedge-derived waverider provides a higher lift than an equivalent cone-derived one and also at off-design conditions a wedge-derived shape shows less sensitivity than its conical counterpart.