The Effect of Boundary Conditions on Simulation of Horizontally Homogeneous Atmospheric Boundary Layer

The Influence of six kinds of combinations of different boundary conditions on the maintenance of homogeneity of atmospheric boundary layer in computational wind engineering was investigated. The inlet condition was applied by either prescribing velocity and turbulent quantities or using a periodic boundary. The top boundary condition includes symmetry, prescribing velocity and turbulent quantities, constant static pressure and applying driving shear stress. Numerical results show that the stream-wise velocity and the turbulent dissipation rate are almost immune to boundary conditions, but the turbulent kinetic energy is affected by boundary conditions dramatically. Best maintenance can be obtained by using periodic boundary condition at inlet and outlet and applying driving shear stress on the top of the domain.

Quantitative formulation of mechanism of sintering process during creep deformation of refractory concretes

This paper is concerned with quantitative formulation of the mechanism of the sintering process during secondary state creep deformation of refractory concretes. Investigated concretes varied in, both, chemical and mineralogical compositions. The sintering process during secondary state creep within refractory concrete has an isothermal character. Thus, an attempt was made to describe the mentioned process quantitatively. Creep was investigated at three different temperatures: 1200, 1300 and 1400?C. Variations of the microstructure of concrete samples, exposed to constant static pressure and constant temperature during certain time-intervals, were investigated using a scanning electron microscope. Obtained results of the investigation proved that creep resistance is an irreplaceable method when the decision about the best possible type of refractory concrete for application in metallurgical furnaces is required.

On an approximate model for the shape of a liquid–air interface receding in a capillary tube

A good approximation to modelling the shape of a liquid–air meniscus advancing or receding in a capillary tube of radius a can be constructed by balancing the curvature of the interface with the sum of a viscous stress valid near the contact line and a constant static pressure. This model has unique solutions for each value of the boundary condition, i.e. the dynamic contact angle. When the meniscus recedes at very small capillary numbers, the model predicts a critical receding velocity beyond which a liquid layer of the receding fluid (a liquid tail) develops along the solid (see figure 4). The length of the layer increases as the receding speed and the contact angle decrease. This layer regime is characterized by menisci whose macroscopic curvature is >1/a.

The Prediction of the Optimum Performance of Ejectors

A simple, incompressible flow, ejector model was formulated and analysed. It was found that the results obtained analytically agreed closely with those of an experimentally conducted optimization carried out by other workers. The model was also used to compare the relative merits of constant static pressure versus constant area mixing: it was shown that the latter was superior to the former when nozzle, mixing tube and diffuser friction losses were taken into account. The work also indicated that the efficiency of an ejector tends to increase as the secondary-to-primary stream density ratio decreases.

Practical Bypass Mixing Systems for Fan Jet Aero Engines

SummaryMixing systems have many applications in gas turbines and aircraft jet propulsion, e.g. mixing zones in combustion chambers, ejectors for jet lift thrust augmentors and supersonic propulsion systems. A further application similar to that of combustion chamber mixing is that of mixing the cold and hot exhausts of a bypass jet engine. These are both characterised by mixing at constant static pressure and approximately constant total pressure as opposed to the more general case of unequal pressures in ejector systems (Fig. 1).The exhaust mixing process as used in Rolls-Royce bypass jet engines, e.g. Spey and Conway, enables the potential of the bypass principle, in terms of minimum weight and fuel consumption, to be exploited by a simple practical device.This is achieved by mixing the two streams in a common duct of fairly short dimensions with a corrugated metal interface on the inlet side. The consideration of these practical systems forms the main topic of this paper.

The Pressure Registered by a Very Small Static Hole

Static pressure in a moving fluid is usually measured by means of a static hole either in a probe or on the wall of a duct. Such a hole creates a disturbance in the flow and thus has some error associated with it. The determination of the magnitude of this error has been attempted by several investigators and the procedure adopted in each case has been as follows :The pressure in a region of sensibly constant static pressure was registered using holes of various diameter and a curve of pressure versus hole size plotted. The true static pressure was then obtained by extrapolating this curve to zero hole size, and the errors for the various holes were found. This procedure involves the assumption that, as the hole size tends to zero, the pressure registered tends to the true value. It is usually argued that this must be so because the disturbance caused by the static hole is progressively reduced as the hole size is reduced. Obviously, the validity, or otherwise, of the argument is of fundamental importance in the estimation of static hole error, and a somewhat more rigorous justification of the use of the extrapolation procedure is attempted in what follows.