Winds and buoyancy-driven circulation in the Tampa Bay

The present study is concentrated on the empirical studies on the circulation in the Tampa Bay by analyzing velocity data at the Skyway Bridge Station in the Tampa Bay. Analyses focus on three factors responsible for the circulation: tides, winds and buoyancy gradients. The analysis of the current data obtained at the Skyway Bridge Station shows these three components of the circulation: the tidal currents are nearly uniform with depth; a vigorous and persistent buoyancy-driven mean now is directed into the bay at this location with speed of about 6 -- 8 cm/s; and synoptic scale wind fluctuations result in similarly large current fluctuations with winds blowing into the bay causing currents to flow out of the bay, and the versa.

Modeling and Assessment of Buoyancy-Driven Stratified Airflow in High-Space Industrial Hall

In industrial environment,heat sources often are contaminant sources and health threatening contaminants are mainly passive,so a detailed understanding of airflow mode can assist in work environment hygiene measurement and prevention.This paper presented a numerical investigation of stratified airflow scenario in a high-space industrial hall with validated commercial code and experimentally acquired boundary conditions.Based upon an actually undergoing engineering project,this study investigated the performance of the buoyancy-driven displacement ventilation in a large welding hall with big components manufactured.The results have demonstrated that stratified airflow sustained by thermal buoyancy provides zoning effect in terms of clean and polluted regions except minor stagnant eddy areas.The competition between negative buoyant jets from displacement radial diffusers and positive buoyant plume from bulk object constitutes the complex transport characteristics under and above stratification interface.Entrainment,downdraft and turbulent eddy motion complicate the upper mixing zone,but the exhaust outlet plays a less important role in the whole field flow.And the corresponding suggestions concerning computational stability and convergence,further improvements in modelling and measurements were given.

Ventilation schemes for high-temperature buoyancy-driven plumes from spherical surfaces of a large-scale experimental facility

A large-scale sphere-shaped experimental facility for neutrino detection is designed as a 23-latitudinal layer composite by using organic glass as the major raw material and is assembled via mass polymerization through a top-to-bottom approach.Heating belts at 4200 W/m 2 are used to anneal the bonding joints of external and internal spherical surfaces and produce high-temperature thermal plumes.Buoyancy-driven plumes should be effectively mitigated using ventilation to ensure the near-surface air temperatures above the finished layers can be delicately controlled within 21±1℃to minimize the deformation of the facility.Schemes to control plumes on both surfaces were investigated using Computational Fluid Dynamics(CFD)method by following a performance-based approach.First,an independent field study was conducted to measure surface temperature and heat flux of mass polymerization and provide references for simulations.Second,dynamic buoyancy-driven plumes pro-duced along the external and internal spherical surfaces were simulated under a no-ventilation scenario.After contacting with the plumes,three periods,in which buoyancy,convection,and advection,were dominating,can be observed according to the changes of near-surface air temperature.Moreover,the temperature and Ra num-ber of the surface-attached plumes were used as indicators to assess the intensity of the plumes quantitatively.Third,three major ventilation schemes,i.e.,general,push-pull,and sphere-attached ventilations(with three sub-designs),were compared under the same air change rate level on the basis of the following perspectives:(1)air temperature distributions above the polymerizing layer,(2)overall heat exhaust efficiency,and(3)total spaces where temperature was higher than 22℃.Results indicated that the combination of push-pull and side-supply ventilations,by which the heat exhaust efficiencies were up to 1.87-3.24,was found to be most effective to control thermal plumes,with approximately 0.1%of the total surrounding air exceeding 22℃.

Axisymmetry Breaking to Travelling Waves in the Cylinder with Partially Heated Sidewall

The transition from an axisymmetric stationary flow to three-dimensional time-dependent flows is carefully studied in a vertical cylinder partially heated from the side, with the aspect ratio A = 2 and Prandtl number Pτ=0.021. The flow develops from the steady toroidal pattern beyond the first instability threshold, breaks the axisymmetric state at a Rayleigh number near 2000, and transits to standing or travelling azimuthal waves. A new result is observed that a slightly unstable flow pattern of standing waves exists and will transit to stable travelling waves after a long time evolution. The onset of oscillations is associated with a supercritical Hopf bifurcation in a system with O（2） symmetry.

NUMERICAL STUDY OF BUOUNCY- AND THERMOCAPILLARY-DRIVEN FLOWS IN A CAVITY

Thermocapillary-and buoyancy-driven convection in open cavities with differentially heated endwalls is investigated by numerical solutions of the two- dimensional Navier-Stokes equations coupled with the energy equation. We studied the thermocapillary and buoyancy convection in the cavities, filled with low-Prandtl- number fluids, with two aspect-ratios A=1 and 4, Grashof number up to 10~5 and Reynolds number |Re|≤10~4. Our results show that thermocapillary can have a quite significant effect on the stability of a primarily buoyancy-driven flow, as well as on the flow structures and dynamic behavior for both additive effect (i.e., positive Re) and opposing effect (i.e., negative Re).

Linear global stability of a confined plume

A linear stability analysis is performed for a plume flow inside a cylinder of aspect ratio 1. The configu- ration is identical to that used by Lopez and Marques （2013） for their direct numerical simulation study, It is found that the first bifurcation, which leads to a periodic axisymmetric flow state, is accurately pre- dicted by linear analysis： both the critical Rayleigh number and the global frequency are consistent with the reported DNS results. It is further shown that pressure feedback drives the global mode, rather than absolute instability.

A theoretical study on gaseous pollutant flushing of natural ventilation driven by buoyancy forces in industrial buildings

The acceleration of industrialization worsening indoor environments of industrial buildings has drawn more attention in recent years.Natural ventilation can improve indoor air quality(IAQ)and reduce carbon emissions.To evaluate gaseous pollutant levels in industrial buildings for the development of buoyancy-driven natural ventilation,two theoretical models of pollutant flushing(Model I and Model II)are developed based on the existing thermal stratification theory in combination with the mixing characteristics of lower pollutant.The results show that indoor pollutant flushing is mainly dependent on the pollution source intensity and effective ventilation area.The mixing characteristics of lower pollutant has an important effect on pollutant stratification and evolution during ventilation,but it does not change the prediction results at steady state.When the dimensionless pollution source intensity is larger than 1,the pollution source should be cleaned up or other ventilation methods should be used instead to improve IAQ.In addition,the comparisons between Model I and Model II on instantaneous pollutant concentration are significantly influenced by the pollution source intensity,and the actual pollutant concentration is more likely to be between the predicted values of Model I and Model II.To reduce pollutant concentration to a required level,the pollution source intensity should be in a certain range.The theoretical models as well as the necessary conditions for ventilation effectiveness obtained can be used for the ventilation optimization design of industrial buildings.