The influence of stacks on flow patterns and stratification associated with natural ventilation

We investigate the steady state natural ventilation of an enclosed space in which vent A, located at height hA above the floor, is connected to a vertical stack with a termination at height H, while the second vent, B, at height hB above the floor, connects directly to the exterior. We first examine the flow regimes which develop with a distributed source of heating at the base of the space. If hBhB>hA, then two different flow regimes may develop. Either (i) there is inflow through vent B and outflow through vent A, or (ii) the flow reverses, with inflow down the stack into vent A and outflow through vent B. With inflow through vent A, the internal temperature and ventilation rate depend on the relative height of the two vents, A and B, while with inflow through vent B, they depend on the height of vent B relative to the height of the termination of the stack H. With a point source of heating, a similar transition occurs, with a unique flow regime when vent B is lower than vent A, and two possible regimes with vent B higher than vent A. In general, with a point source of buoyancy, each steady state is characterised by a two-layer density stratification. Depending on the relative heights of the two vents, in the case of outflow through vent A connected to the stack, the interface between these layers may lie above, at the same level as or below vent A, leading to discharge of either pure upper layer, a mixture of upper and lower layer, or pure lower layer fluid. In the case of inflow through vent A connected to the stack, the interface always lies below the outflow vent B. Also, in this case, if the inflow vent A lies above the interface, then the lower layer becomes of intermediate density between the upper layer and the external fluid, whereas if the interface lies above the inflow vent A, then the lower layer is composed purely of external fluid. We develop expressions to predict the transitions between these flow regimes, in terms of the heights and areas of the two vents and the stack, and we successfully test these with new laboratory experiments. We conclude with a discussion of the implications of our results for real buildings.

On the transition from displacement to mixing ventilation with a localized heat source

We investigate the steady state natural ventilation of a room heated at the base and consisting of two vents at different levels. We explore how the air flow rate and internal temperature relative to the exterior vary as a function of the vent areas, position of the vents and heat load in order to establish appropriate ventilation strategies for a room. When the room is heated by a distributed source, the room becomes well mixed and the steady state ventilation rate depends on the heating rate, the area of the vents and the distance between the lower and upper level vents. However, when the room is heated by a localised source the room becomes stratified. If the effective ventilation area is sufficiently large, then the interface separating the two layers lies above the inlet vent and the lower layer is comprised of ambient fluid. In this case the upper layer is warmer than in the well mixed case and the ventilation rate is smaller. However, if the effective area for ventilation is sufficiently small, then the interface separating the two layers lies below the inlet vent and the lower layer is comprised of warm fluid which originates as the cold incoming fluid mixes during descent from the vent through the upper layer. In this case both the ventilation rate and the upper layer temperature are the same as in the case of a distributed heat load. As the vertical separation between lower and upper level vents decreases, then the temperature difference between the layers falls to zero and the room becomes approximately well mixed. These findings suggest how the appropriate ventilation strategy for a room can be varied depending on the exterior temperature, with mixing ventilation more suitable for winter conditions and displacement ventilation for warmer external temperatures.

The effect of an upstream turbine on a low-aspect ratio vane

The interaction between a high-pressure rotor and a downstream vane is dominated by vortex-blade interaction. Each rotor blade passing period two co-rotating vortex pairs, the tip-leakage and upper passage vortex and the lower passage and trailing shed vortex, impinge on, and are cut by, the vane leading edge. In addition to the streamwise vortex the tip-leakage flow also contains a large velocity deficit. This causes the interaction of the tip-leakage flow with a downstream vane to differ from typical vortex blade interaction. This paper investigates the effect these interaction mechanisms have on a downstream vane. The test geometry considered was a low aspect ratio second stage vane located within a S-shaped diffuser with large radius change mounted downstream of a shroudless high-pressure turbine stage. Experimental measurements were conducted at engine-representative Mach and Reynolds numbers, and data was acquired using a fast-response aerodynamic probe upstream and downstream of the vane. Time-resolved numerical simulations were undertaken with and without a rotor tip gap in order to investigate the relative magnitude of the interaction mechanisms. The presence of the upstream stage is shown to significantly change the structure of the secondary flow in the vane and to cause a small drop in its performance.

The low Reynolds number aerodynamics of leading edge flaps

Within the low Reynolds number regime at which birds and small air vehicles operate (Re=15,000-500,000), flow is beset with laminar separation bubbles and bubble burst which can lead to loss of lift and early onset of stall. Recent video footage of an eagle's wings in flight reveals an inconspicuous wing feature: the sudden deployment of a row of feathers from the lower surface of the wing to create a leading edge flap. An understanding of the aerodynamic function of this flap has been developed through a series of low speed wind tunnel tests performed on an Eppler E423 aerofoil. Experiments took place at Reynolds numbers ranging from 40000 to 140000 and angles of attack up to 30°. In the lower range of tested Reynolds numbers, application of the flap was found to substantially enhance aerofoil performance by augmenting the lift and limiting the drag at certain incidences. The leading edge flap was determined to act as a transition device at low Reynolds numbers, preventing the formation of separation bubbles and consequently decreasing the speed at which stall occurs during landing and manoeuvring.

The soft impact of composite sandwich beams with a square-honeycomb core

The dynamic response of end-clamped monolithic beams and sandwich beams of equal areal mass have been measured by loading the beams at mid-span with metal foam projectiles to simulate localised blast loading. The sandwich beams were made from carbon fibre laminate and comprised identical face sheets and a square-honeycomb core. The transient deflection of the beams was determined as a function of projectile momentum, and the measured response was compared with finite element simulations based upon a damage mechanics approach. A range of failure modes were observed in the sandwich beams including core fracture, plug-type shear failure of the core, debonding of the face sheets from the core and tensile tearing of the face sheets at the supports. In contrast, the monolithic beams failed by a combination of delamination of the plies and tensile failure at the supports. The finite element simulations of the beam response were accurate provided the carbon fibre properties were endowed with rate sensitivity of damage growth. The relative performance of monolithic and sandwich beams were quantified by the maximum transverse deflection at mid-span for a given projectile momentum. It was found that the sandwich beams outperformed both monolithic composite beams and steel sandwich beams with a square-honeycomb core. However, the composite beams failed catastrophically at a lower projectile impulse than the steel beams due to the lower ductility of the composite material. © 2011 Elsevier Ltd. All rights reserved.

The parameter space of graphene chemical vapor deposition on polycrystalline Cu

A systematic study of the parameter space of graphene chemical vapor deposition (CVD) on polycrystalline Cu foils is presented, aiming at a more fundamental process rationale in particular regarding the choice of carbon precursor and mitigation of Cu sublimation. CH 4 as precursor requires H 2 dilution and temperatures ≥1000 °C to keep the Cu surface reduced and yield a high-quality, complete monolayer graphene coverage. The H 2 atmosphere etches as-grown graphene; hence, maintaining a balanced CH 4/H 2 ratio is critical. Such balance is more easily achieved at low-pressure conditions, at which however Cu sublimation reaches deleterious levels. In contrast, C 6H 6 as precursor requires no reactive diluent and consistently gives similar graphene quality at 100-150 °C lower temperatures. The lower process temperature and more robust processing conditions allow the problem of Cu sublimation to be effectively addressed. Graphene formation is not inherently self-limited to a monolayer for any of the precursors. Rather, the higher the supplied carbon chemical potential, the higher the likelihood of film inhomogeneity and primary and secondary multilayer graphene nucleation. For the latter, domain boundaries of the inherently polycrystalline CVD graphene offer pathways for a continued carbon supply to the catalyst. Graphene formation is significantly affected by the Cu crystallography; i.e., the evolution of microstructure and texture of the catalyst template form an integral part of the CVD process. © 2012 American Chemical Society.

Closely correlating lower and upper bound plastic analysis of real slabs

An engineer assessing the load-carrying capacity of an existing reinforced concrete slab is likely to use elastic analysis to check the load at which the structure might be expected to fail in flexure or in shear. In practice, many reinforced concrete slabs are highly ductile in flexure, so an elastic analysis greatly underestimates the loads at which they fail in this mode. The use of conservative elastic analysis has led engineers to incorrectly condemn many slabs and therefore to specify unnecessary and wasteful flexural strengthening or replacement. The lower bound theorem is based on the same principles as the upper bound theorem used in yield line analysis, but any solution that rigorously satisfies the lower bound theorem is guaranteed to be a safe underestimate of the collapse load. Jackson presented a rigorous lower bound method that obtains very accurate results for complex real slabs.

The rhythm of fountains: The length and time scales of rise height fluctuations at low and high Froude numbers

The magnitude and frequency of vertical fluctuations of the top of an axisymmetric miscible Boussinesq fountain forms the focus of this work. We present measurements of these quantities for saline-aqueous fountains in uniform quiescent surroundings. Our results span source Froude numbers 0.3 ≤ Fr 0 ≤ 40 and, thereby, encompass very weak, weak, intermediate and forced classes of fountain. We identify distinct scalings, based on known quantities at the fountain source, for the frequency of fountain height fluctuations which collapse our data within bands of Fr0. Notably, our scalings reveal that the (dimensionless) frequency takes a constant value within each band. These results highlight characteristic time scales for the fluctuations which we decompose into a single, physically apparent, length scale and velocity scale within each band. Moreover, within one particular band, spanning source Froude numbers towards the lower end of the full range considered, we identify unexpectedly long-period fluctuations indicating a near balance of inertia and (opposing) buoyancy at the source. Our analysis identifies four distinct classes of fluctuation behaviour (four bands of Fr 0) and this classification matches well with existing classifications of fountains based on rise heights. As such, we show that an analysis of the behaviour of the fountain top alone, rather than the entire fountain, provides an alternative approach to classifying fountains. The similarity of classifications based on the two different methods confirms that the boundaries between classes mark tangible changes in the physics of fountains. For high Fr0 we show that the dominant fluctuations occur at the scale of the largest eddies which can be contained within the fountain near its top. Extending this, we develop a Strouhal number, Strtop, based on experimental measures of the fountain top, defined such that Strtop = 1 would suggest the dominant fluctuations are caused by a continual cycle of eddies forming and collapsing at this largest physical scale. For high- Fr 0 fountains we find Strtop ≈ 0. 9. © 2013 Cambridge University Press.