SINGULAR PERTURBATION APPROACH TO MOVING MASS CONTROL OF BUOYANCY-DRIVEN AIRSHIP IN 3-D SPACE

The attitude control problem and the guidance problem are solved in 3-D for a buoyancy-driven airship actuated by the combined effects of an internal air bladder which modulates the airshiprs net weight and of two moving masses which modulate its center of mass. A simple and clear modeling is introduced to derive the 8 degree of freedom （DOF） mathematical model. Nonlinear control loops are derived through maximal feedback linearization with internal stability for both dynamics in the longitudinal plane and in the lateral plane. Based on a singular perturbation approach, the superposition of these two control actions in the longitudinal plane and in the lateral plane is shown to achieve the control of the dynamics in 3-D space. The simulations of the airship tracking specified attitude, moving direction and speed in 3-D space are presented.

MATLAB-Based Simulation of Buoyancy-Driven Underwater Glider Motion

The mass configuration of the buoyancy-driven underwater glider is decomposed and defined. The coupling between the glider body and its internal masses is addressed using the energy law. A glider motion model is established, and the corresponding simulation program is derived using MATLAB. The characteristics of the glider motion are explored using this program. The simula- tion results show that the basic characteristic of a buoyancy-driven underwater glider is the periodic alternation of downward and upward motions. The glider's spiral motion can be applied to missions in restricted regions. The glider's horizontal velocity, gliding depth and its motion radius in spiral motion can be changed to meet different application purposes by using different glider parameter designs. The simulation also shows that the model is appropriate and the program has strong simulation functions.

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.

A novel method for quantitatively identifying driving forces and evaluating their contributions to oil and gas accumulation

Different driving forces govern the formation of distinct types of oil and gas accumulation and yield diverse oil and gas distributions.Complex oil and gas reservoirs in basins are commonly formed by the combination of multiple forces.It is very difficult but essential to identify driving forces and evaluate their contributions in predicting the type and distribution of oil and gas reservoirs.In this study,a novel method is proposed to identify driving forces and evaluate their contribution based on the critical conditions of porosity and permeability corresponding to buoyancy-driven hydrocarbon accumulation depth(BHAD).The application of this method to the Nanpu Sag of the Bohai Bay Basin shows that all oil and gas accumulations in the reservoirs are jointly formed by four driving forces:buoyance(Ⅰ),non-buoyance(Ⅱ),tectonic stress(Ⅲ1)and geofluid activity(Ⅲ2).Their contributions to all proven reserves are approxi-mately 63.8%,16.2%,2.9%,and 17.0%,respectively.The contribution of the driving forces is related to the depth,distance to faults and unconformity surfaces.Buoyancy dominates the formation of conven-tional reservoirs above BHAD,non-buoyant dominate the formation of unconventional reservoirs below BHAD,tectonic stress dominates the formation of fractured reservoirs within 300 m of a fault,and geoflu-ids activity dominates the formation of vuggy reservoirs within 100 m of an unconformity surface.

The mathematical model of kitchen smoke exhaust system in high-rise residential buildings considering the Influence of Stack effect

In high-rise buildings with large indoor and outdoor temperature difference,neglecting the effect of stack effect in smoke exhaust shafts may cause calculation error of the fluid network model.In this paper,the mathematical model of kitchen smoke exhaust system considering the influence of stack effect was put forward and it can be inserted different range hood sub-models.Compared with the results of six working conditions of the model without considering the stack effect,the error of the proposed model were reduced by 7.6%,4.3%,4.1%,2.8%,2.4%,and 2.1%.While the indoor and outdoor temperature difference varies from−5℃ to 49℃,the effect of stack effect on the pressure in the flue and the flow rate for each user was studied for six operating rates s.The results show that under the combined effect of stack effect and flue resistance,the static pressure of the kitchen smoke exhaust system showed a low-high-low distribution,and the maximum static pressure in the flue moved toward the bottom with the increase of temperature difference.User flow rates exhibit a low-high-low-high distribution,with an increased flow rate in the bottom users and the largest flow rate in the top users.