Concluding Remarks

The topic of laminar-turbulent transition was given much room in this chap­ter. This was deemed to be necessary because of its large importance for hypersonic vehicle design.

A definitive improvement of the capabilities to predict laminar-turbulent transition, but also to optimize surface shapes in order to, for instance, delay transition, is mandatory for CAV’s as well as for ARV’s. Vehicles of these classes are viscous-effect dominated, which—as was discussed and shown in several of the preceding chapters—regards the thermal state of the surface, thermal surface effects, the drag of the vehicle, the thermal loads in view of the structure and materials concept of the vehicle, and issues of aerothermo – dynamic propulsion integration.

For RV’s the improvement of transition prediction is highly desirable, too, because the effectiveness of these vehicles must be improved by minimizing the mass of the thermal protection system. In this regard laminar-turbulent
transition on the lower branch of the re-entry trajectory, as well as on alter­native lower altitude trajectories than preferred today, including contingency trajectories, is of great importance and demands a more accurate and reliable prediction than is possible today.

The general knowledge about transition phenomena in high-speed flows is already rather good, and the development of new prediction methods is en­couraging. Here non-local and non-linear theory appears to have the necessary potential. In the not too far future the use of such methods in aerothermody – namic numerical simulations and optimizations also in industrial design work will be no problem in view of the still strongly growing computer capabilities.

Necessary to achieve the improvements of transition prediction is con­tinuous concerted research, extension and use of “quiet” ground-simulation facilities, and in-flight measurements on ad-hoc experimental vehicles or in passenger experiments on other vehicles. Ground and flight measurements need also the careful recording of the thermal state of the surface by means of a suitable hot experimental technique. A combination of analytical work, computational simulation, ground-facility simulation, and in-flight simula­tion (unified approach [143]), in a transfer-model ansatz, which takes also into account—where necessary—the coupling of the flow to the vehicle sur­face, is considered to be necessary, in order to advance this scientifically and technically fascinating and challenging field [3].