In this essay, we recall the specificities of the transition to turbulence in wall-bounded flows and present recent achievements in the understanding of this problem. The transition is abrupt with laminar-turbulent coexistence over a finite range of Reynolds numbers, the transitional range. The archetypical cases of Poiseuille pipe flow and plane Couette flow are first reviewed at the phenomenological level, together with a few other flow configurations. Theoretical approaches are then examined with particular emphasis on the existence of special nontrivial solutions to the Navier-Stokes equations at finite distance from laminar flow. Dynamical systems theory is most appropriate to analyze their role, in particular with respect to the transient character of turbulence in the lower transitional range. The extensions needed to deal with the prominent spatiotemporal features of the transition are then discussed. Turbulence growth/decay in terms of statistical physics of many-body systems and the relevance of directed percolation as a stochastic process able to account for it are next scrutinized. To conclude, we advocate the recourse to well-designed modeling able to provide us with a conceptually coherent picture of the full transitional range and put forward some open issues.
Recent experimental approaches were reviewed to the residual stress/strain in bone tissue using X-ray diffraction (XRD) techniques. After a brief introduction of the experimental methods and obtained results, we discussed the generation mechanisms and biomechanical implications of the residual stress/strain in bone tissue. Strain gauge approaches provided the existence of residual stresses in the bone at the whole bone level. XRD approaches have also provided the existence of residual stresses at the tissue and mineral phase levels. The residual strains at the mineral phase related to the degree of orientation of the HAp crystals. The distributions of residual stress were obtained around the surface and along the radial depth of the diaphysis of quadrupedal extremities. The correlation between the residual stress and the osteon structures was indicated and the difference of residual stress with growth was revealed. It would appear that the residual stress state might be generated by the indeterminate structure in the hierarchical structures of the bone tissue relating to bone adaptation with the bone formation and reconstruction process. A long-term study is needed to better understand the generation and biomechanical implications of residual stress in bone tissue throughout the hierarchical structure during maturation and aging.
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