Travelling waves in pipe flow

A family of three-dimensional travelling waves for flow through a pipe of circular cross section is identified. The travelling waves are dominated by pairs of downstream vortices and streaks. They originate in saddle-node bifurcations at Reynolds numbers as low as 1250. All states are immediately unstable. Their dynamical significance is that they provide a skeleton for the formation of a chaotic saddle that can explain the intermittent transition to turbulence and the sensitive dependence on initial conditions in this shear flow.Comment: 4 pages, 5 figure

Boundary-layer turbulence as a kangaroo process

A nonlocal mixing-length theory of turbulence transport by finite size eddies is developed by means of a novel evaluation of the Reynolds stress. The analysis involves the contruct of a sample path space and a stochastic closure hypothesis. The simplifying property of exhange (strong eddies) is satisfied by an analytical sampling rate model. A nonlinear scaling relation maps the path space onto the semi-infinite boundary layer. The underlying near-wall behavior of fluctuating velocities perfectly agrees with recent direct numerical simulations. The resulting integro-differential equation for the mixing of scalar densities represents fully developed boundary-layer turbulence as a nondiffusive (Kubo-Anderson or kangaroo) type of stochastic process. The model involves a scaling exponent (with → in the diffusion limit). For the (partly analytical) solution for the mean velocity profile, excellent agreement with the experimental data yields 0.58. © 1995 The American Physical Society

Towards Unsteady Simulation of Combustor-Turbine Interaction Using an Integrated Approach

In this paper a CFD solver with the ability of dealing with both reacting and compressible flows is developed, so that an integrated simulation of the whole. system “combustor and turbine” can be performed. To its validation, the combustor turbine interaction in a jet engine consisting of a Rolls-Royce combustor together with the first high-pressure turbine stage NGV (Nozzle-Guide-Vane) is studied. The unstructured CFD solver follows a pressure-based approach, using a PISO algorithm (Pressure Implicit with Splitting of Operator) recently extended for compressible flows. In order to allow acoustic waves to leave the computational domain, nonreflecting boundary conditions based on the NSCBC method (Navier-Stokes Characteristic Boundary Conditions) have been implemented. The numerical methods have been coupled with the Flamelet Generated Manifold combustion model (FGM) extended for compressible flows. After successfully verifying the NSCBC implementation, various numerical results describing the combustor-turbine interactions of the jet engine are analyzed and discussed in terms of temperature and total pressure fields with and without NGV: It could be shown that the influence of the NGV on the combustor flow is relatively limited. Differences in the combustor flow field are mainly due to spatial and temporal averaging used for the simulation without NGV to calculate the pressure field at combustor outlet. These numerical results demonstrate the ability of the developed numerical model in its steady computation mode to well capture the evolving flow properties in both combustor and turbine components