1,721,010 research outputs found
Undulators and free-electron lasers
No full textThis book is a reference text for all those working in free-electron laser research as well as being a learning aid for physicists and graduate students who wish an introduction to this field. Only a basic understanding of relativistic mechanics and electromagnetism is presupposed. After an overview of early developments and general principles of operation, the different models that can be used to describe free-electron lasers are presented, organized according to their range of applicability. The relevent conceptual and mathematical constructs are built up from first principles with attention to obtaining the practically important results in a simple but rigorous way. Interaction of the undulator with the driving electron accelerator and the laser cavity and design of undulator magnets are treated and an overview is given of some typical experiments
Optimal control by blowing and suction at the wall of algebraically growing boundary layer disturbances
No full textThe upstream perturbations that maximize the spatial energy growth in a boundary layer are called optimal perturbations. The aim of the present paper is to find an optimal control by blowing and suction at the wall that minimizes the energy of perturbation, when the initial disturbance is itself optimal. The problem is examined by a method of receptivity analysis based on the numerical solution of the adjoint of the linearized boundary layer equations. We shall investigate both cases of a flat plate and a concave wall
Erratum to: Error sensitivity to refinement: a criterion for optimal grid adaptation (Theoretical and Computational Fluid Dynamics, (2017), 31, 5-6, (595-605), 10.1007/s00162-016-0413-x)
No full text
Turbulent drag reduction over curved walls
No full textThis work studies the effects of skin-friction drag reduction in a turbulent flow over a curved wall, with a view to understanding the relationship between the reduction of friction and changes to the total aerodynamic drag. Direct numerical simulations are carried out for an incompressible turbulent flow in a channel where one wall has a small bump; two bump geometries are considered, that produce mildly separated and attached flows. Friction drag reduction is achieved by applying streamwise-Travelling waves of spanwise velocity (StTW). The local friction reduction produced by the StTW is found to vary along the curved wall, leading to a global friction reduction that, for the cases studied, is up to 10Â % larger than that obtained in the plane wall case. Moreover, the modified skin friction induces non-negligible changes of pressure drag, which is favourably affected by StTW and globally reduces by up to 10Â %. The net power saving, accounting for the power required to create the StTW, is positive and, for the cases studied, is one half larger than the net saving of the planar case. The study suggests that reducing friction at the surface of a body of complex shape induces further effects, a simplistic evaluation of which might lead to underestimating the total drag reduction
Ray theory of flow instability and the formation of caustics in boundary layers
No full textUnpublished report
- …
