Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session HJ: Convection and Buoyancy Driven Flows V |
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Chair: Sam Paolucci, University of Notre Dame Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 6 |
Monday, November 20, 2006 2:00PM - 2:13PM |
HJ.00001: Transient effects of sudden changes of heat load in a naturally ventilated room C.P. Caulfield, D.J. Bower, S. Fitzgerald, A.W. Woods Using reduced numerical models and small-scale laboratory experiments, we investigate the transient effects of changing isolated heat loads discontinuously within a large, ventilated space. We consider the emptying filling box (with high and low openings) driven by a single isolated source of buoyancy. The original steady state consists of a buoyant layer, whose depth (for the simplest case of a point source plume) is determined by the geometric properties of the room alone. When the buoyancy flux of the source is increased, a new layer `fills' the room from the top with a more buoyant layer. The original layer disappears due to entrainment by the rising plume. The behaviour is qualitatively different when the source buoyancy flux is decreased. In this case, the rising plume fluid is now relatively dense, and so it inevitably collapses back to `intrude' below the original layer. In this case, the original layer disappears due to both draining through the upper opening, and penetrative entrainment by the dense plume. We compare the predictions of three numerical models using different penetrative entrainment parametrizations to a sequence of laboratory experiments. This entrainment reduces the density of the intruding layer, and so the rising plume eventually stalls, and no longer reaches the (draining) original layer. We demonstrate that it is necessary to consider the transient effects of penetrative entrainment when the reduction in source buoyancy flux is sufficiently small. [Preview Abstract] |
Monday, November 20, 2006 2:13PM - 2:26PM |
HJ.00002: Transients in Natural Ventilation Diogo Bolster, Paul Linden We examine the natural ventilation flow which occurs when a source of buoyancy is confined within a room with vents at upper and lower levels. We use the classic single room flow considered by Linden et al. (1990). The steady state flow in the single room case is well understood. A well-mixed buoyant layer develops, whose depth is determined purely by the geometry of the vents. However, understanding the transients that occur when there are changes in the source of buoyancy is critical in modelling real life phenomena. Previous work has modelled transient natural ventilation by dividing the room into two well mixed spaces. This could be considered by some as an oversimplification since, in reality, the upper part of the room will have vertical density gradients. By taking this stratification into account in our models, we address the influence that it has in contrast to the well mixed case. In presenting a variety of cases, we consider when it might be necessary, if at all, to include this stratification in models. Finally we compare numerical and analytical solutions with the results of analogue laboratory experiments. [Preview Abstract] |
Monday, November 20, 2006 2:26PM - 2:39PM |
HJ.00003: Similarity Solutions of a Heated Vertical Wall Immersed in a Stratified Environment Zachary Zikoski, Samuel Paolucci Free convective flow along a heated vertical wall immersed in a thermally stratified environment is studied using Lie group theory. The symmetries admitted by the governing system of boundary layer equations are identified along with the admitted classes of temperature stratifications. The cases of both prescribed wall temperature and prescribed wall heat flux are considered. The resulting similarity variables, the reduced systems of equations and their corresponding solutions are presented for each class of temperature stratification. [Preview Abstract] |
Monday, November 20, 2006 2:39PM - 2:52PM |
HJ.00004: An MRI study of Laminar Entrainment in an Autocatalytic Chemical Plume M.C. Rogers, A.J. Sederman, M.D. Mantle, S.W. Morris, S.B. Dalziel, L.F. Gladden Plumes are formed when a continuous source of buoyancy is supplied at a localized source. Buoyancy can be created by either a heat flux, a compositional difference, or a combination of both. Here we study laminar plumes due to an autocatalytic chemical reaction - the iodate-arsenous acid (IAA) reaction. In the absence of buoyancy effects, the nonlinear kinetics of the IAA reaction produces a sharp propagating front, rather like a weak flame front. However, the reaction produces buoyancy both by exothermicity, and by the compositional difference between the reactant and product, causing a plume to form. Velocity measurements were made on horizontal cross sections of IAA chemical plumes using an MRI technique known as the Gradient Echo Rapid Velocity and Acceleration Imaging Sequence (GERVAIS). Ordinary laminar plumes made by the injection of compositionally buoyant fluid exhibit Gaussian velocity distributions across the plume conduit. In contrast, we show that entrainment of fresh reactant into the conduit of an IAA chemical plume creates a buoyancy flux at the interface between reactant and product solution. This gives IAA chemical plumes unique, non-Gaussian velocity profiles. [Preview Abstract] |
Monday, November 20, 2006 2:52PM - 3:05PM |
HJ.00005: Progress in PDPA Measurements of Buoyancy Driven Flow in the Indoor Environment Ian Spitzer, David Marr, Mark Glauser A great deal of research has been conducted in order to model particle motion in indoor spaces near the human body. The latest set of experiments conducted in the Indoor Flow Laboratory at Syracuse University focus on PDPA measurements in the breathing zone of a breathing thermal manikin. Phase Doppler Particle Anemometry (PDPA) measurements provide particle size, concentration, and velocity data. A closed loop ventilation system introduces seed particles into the 288 ft$^{3}$ (8$\times$6$\times$6) chamber at a velocity comparable to that of a displacement ventilation system in order to isolate the thermal effects of the heated body. The ability to relate the pollutant concentrations near the floor to those found within the breathing zone is critical in order to assess the capacity of the human thermal plume to act as a pump, thereby transporting PM into the breathing zone. [Preview Abstract] |
Monday, November 20, 2006 3:05PM - 3:18PM |
HJ.00006: The effects of harmonic and stochastic gravity modulation on fluid mixing V.K. Siddavaram, G.M. Homsy We study the effects of zero-mean harmonic and stochastic vertical gravity modulations on the mixing characteristics of two miscible Boussinesq fluids initially separated by a thin horizontal diffusion layer. For harmonic modulation, the main parameter governing the flow is Grashof number, $Gr$, based on the viscous length scale, $l_{\nu}=\sqrt{\frac{\nu}{\omega}}$, where $\omega$ is the frequency of the modulation. Contrary to the case of constant gravity, for which the arrangement is unstable, we observe a critical $Gr$ for the occurrence of Rayleigh-Taylor instability. This is explained on the basis of earlier work by Gresho \& Sani (1970). As $Gr$ is increased, we observe that the flow-field becomes chaotic. We investigate the route to chaos and compute various metrics to characterize it. The stochastic modulation is characterized by an exponentially damped cosine auto-correlation, $\langle g(t) g(t+\tau)\rangle/\langle g^{2}(t)\rangle=e^{-\lambda\tau}cos(\omega\tau)$, and has a power spectrum which is a Lorentzian with width $\lambda$ and peak at $\omega$, on which the Grashof number is based. We find that stochastic modulation leads to Rayleigh-Taylor instabilies at smaller equivalent Gr than harmonic modulation. [Preview Abstract] |
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