Fluid Mechanics and Thermodynamics of Turbomachinery

For this new edition, author S. Larry Dixon is joined by Cesare Hall from the University of Cambridge, whose diverse background of teaching, research and work ...

Fluid mechanics of pulse pressure-gain combustors

The fluid dynamic operation of a valveless pulse combustor has been studied experimentally and numerically. Through phase-locked chemiluminescence and pressure measurements it is shown that mechanical energy is created periodically in the flame surface, with an efficiency of 1.6%. This mechanical energy leaves the pulse combustor through unsteady jets at the aerovalve inlet and the tailpipe exit stations. Two thermodynamically distinct flows are identified: a flow that is transported from inlet to exit and participates in combustion along the way, and a flow that is ingested and then ejected from the combustor without undergoing combustion. It is the latter of these two flows which has the greatest quantity of net work done on it. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.

The fluid mechanics of a two-dimensional Aaberg exhaust hood

In this paper a study of the air flow pattern created by a two-dimensional Aaberg exhaust hood local ventilation system is presented. A mathematical model of the flow, in terms of the stream function ψ, is derived analytically for both laminar and turbulent injections of fluid. Streamlines and lines of constant speed deduced from the model are examined for various values of the governing dimensionless operating parameter and predictions are given as to the area in front of the hood from which the air can be sampled. The effect of the injection of fluid on the centre-line velocity of the flow is examined and a comparison of the results with the available experimental data is given. © 1992.

Nature-inspired microfluidic propulsion using magnetic actuation.

In this work we mimic the efficient propulsion mechanism of natural cilia by magnetically actuating thin films in a cyclic but non-reciprocating manner. By simultaneously solving the elastodynamic, magnetostatic, and fluid mechanics equations, we show that the amount of fluid propelled is proportional to the area swept by the cilia. By using the intricate interplay between film magnetization and applied field we are able to generate a pronounced asymmetry and associated flow. We delineate the functional response of the system in terms of three dimensionless parameters that capture the relative contribution of elastic, inertial, viscous, and magnetic forces.

Influence of insoluble surfactant on the deformation and breakup of a bubble or thread in a viscous fluid

The influence of surfactant on the breakup of a prestretched bubble in a quiescent viscous surrounding is studied by a combination of direct numerical simulation and the solution of a long-wave asymptotic model. The direct numerical simulations describe the evolution toward breakup of an inviscid bubble, while the effects of small but non-zero interior viscosity are readily included in the long-wave model for a fluid thread in the Stokes flow limit. The direct numerical simulations use a specific but realizable and representative initial bubble shape to compare the evolution toward breakup of a clean or surfactant-free bubble and a bubble that is coated with insoluble surfactant. A distinguishing feature of the evolution in the presence of surfactant is the interruption of bubble breakup by formation of a slender quasi-steady thread of the interior fluid. This forms because the decrease in surface area causes a decrease in the surface tension and capillary pressure, until at a small but non-zero radius, equilibrium occurs between the capillary pressure and interior fluid pressure. The long-wave asymptotic model, for a thread with periodic boundary conditions, explains the principal mechanism of the slender thread's formation and confirms, for example, the relatively minor role played by the Marangoni stress. The large-time evolution of the slender thread and the precise location of its breakup are, however, influenced by effects such as the Marangoni stress and surface diffusion of surfactant. © 2008 Cambridge University Press.

A comparison of winter pre-heating requirements for natural displacement and natural mixing ventilation

In winter, natural ventilation can be achieved either through mixing ventilation or upward displacement ventilation (P.F. Linden, The fluid mechanics of natural ventilation, Annual Review of Fluid Mechanics 31 (1999) pp. 201-238). We show there is a significant energy saving possible by using mixing ventilation, in the case that the internal heat gains are significant, and illustrate these savings using an idealized model, which predicts that with internal heat gains of order 0.1 kW per person, mixing ventilation uses of a fraction of order 0.2-0.4 of the heat load of displacement ventilation assuming a well-insulated building. We then describe a strategy for such mixing natural ventilation in an atrium style building in which the rooms surrounding the atrium are able to vent directly to the exterior and also through the atrium to the exterior. The results are motivated by the desire to reduce the energy burden in large public buildings such as hospitals, schools or office buildings centred on atria. We illustrate a strategy for the natural mixing ventilation in order that the rooms surrounding the atrium receive both pre-heated but also sufficiently fresh air, while the central atrium zone remains warm. We test the principles with some laboratory experiments in which a model air chamber is ventilated using both mixing and displacement ventilation, and compare the energy loads in each case. We conclude with a discussion of the potential applications of the approach within the context of open plan atria type office buildings.