Aerospace Science and Technology 

Abstract: Empirical alterations of eddy-viscosity turbulence models to account for system rotation and streamline curvature are discussed. Except in a narrow class of flows, the streamline curvature itself is an inadequate entry into a model, because it is not Galilean-invariant. We propose a measure of the extra influence on the turbulence which is invariant, fully defined in three dimensions, and unifies rotation and curvature effects. This is at the expense of involving higher derivatives than the “traditional” (non-invariant) terms do. It is closely related to an idea of Knight & Saffman [1]. Its interpretation is straightforward, at least in two dimensions, and coincides with the classical measures in a few “building-block” flows such as rotating shear flow or a curved boundary layer. It can be used in one-equation, two-equation, and similar models in conjunction with empirical functions which depend on the model; an example is given.

Abstract: Ceramic thermal barrier coatings (TBCs) offer the potential to significantly improve efficiencies of aero engines as well as stationary gas turbines for power generation. On internally cooled turbine parts temperature gradients of the order of 100 to 150 °C can be achieved. Today, state-of-the-art TBCs, typically consisting of an yttria-stabilised zirconia top coat and a metallic bond coat deposited onto a superalloy substrate, are mainly used to extend lifetime. Further efficiency improvements require TBCs being an integral part of the component which, in turn, requires reliable and predictable TBC performance. Presently, TBCs produced by electron beam physical vapour deposition are favoured for high performance applications. The paper highlights critical R&D needs for advanced TBC systems with a special focus on reduced thermal conductivity and life prediction needs.

Abstract: A refined trigonometric shear deformation theory (RTSDT) taking into account transverse shear deformation effects is presented for the thermoelastic bending analysis of functionally graded sandwich plates. Unlike any other theory, the number of unknown functions involved is only four, as against five in case of other shear deformation theories. The theory presented is variationally consistent, does not require shear correction factor, the displacement components are expressed by trigonometric series representation through the plate thickness to develop a two-dimensional theory and gives rise to transverse shear stress variation such that the transverse shear stresses vary parabolically across the thickness satisfying shear stress free surface conditions. The sandwich with homogeneous facesheet and FGM core is considered. Material properties of the present FGM core are assumed to vary according to a power law distribution in terms of the volume fractions of the constituents. The influences played by the transverse shear deformation, thermal load, plate aspect ratio, and volume fraction distribution are studied. Numerical results for deflections and stresses of functionally graded metal–ceramic plates are investigated. It can be concluded that the proposed theory is accurate and simple in solving the thermoelastic bending behavior of functionally graded plates.

Abstract: The chemistry of the Ni-based superalloys designed for single crystal gas turbine blades has significantly evolved since the development of the first generation of alloys derived from columnar grained materials. The overall performance of the second and third generations has been significantly improved by the addition of increasing amounts of rhenium. However, the problems of increased density, grain defects and microstructural stability have also become more and more acute and render necessary to carefully control the level of the various alloying elements in order to effectively benefit from the high potential of the most recently developed third generation alloys.

Abstract: Shock/turbulent boundary-layer interactions produce generally unsteadiness of the shock system. This phenomenon is reviewed and analyzed, by evaluating the results of various experiments; in particular results in the case of a shock reflection are presented. It appears that the frequency of the fluctuations produced by the shock motion are much lower than the characteristic frequencies of turbulence in the incoming boundary layers. It is checked that in the supersonic case, a previously proposed scaling provides the correct order of magnitude of the shock motion frequencies. It is suggested from experimental results that the three-dimensional structure of the separated bubble may be at the origin of such unsteadiness.