Rong Fung Huang

Vortex shedding and shear-layer instability of wing at low-Reynolds numbers

Flow patterns and characteristics of vortex shedding and shear-layer instability of a NACA 0012 cantilever wing are experimentally studied. Smoke-wire and surface oil-flow techniques are employed to visualize the flow patterns and evolution of vortex shedding. Hot-wire anemometers are used to characterize the frequency domain of the unsteady flow structures. Several characteristic flow modes are classified in the domain of chord Reynolds number and root angle of attack. Effects of the juncture and wing tip are discussed. Vortex shedding can be classified into four characteristic modes. Vortex shedding at low and high angles of attack are found to have different dominant mechanisms. Effects of the juncture and wing tip on the vortex shedding are discussed. Shear-layer instabilities are found to be closely related to the behaviors of the vortex shedding. Behaviors of the shear-layer instabilities can be traced back to the characteristics of the boundary layer on the suction surface of the airfoil.

The stability and visualized flame and flow structures of a combusting jet in cross flow

Abstract The blowoff stability and flame behavior of a combusting propane gas jet issuing from a well-contoured burner perpendicularly to a cross air stream in a wind tunnel test section is studied experimentally. A category of never-lift flames was found to have different stability characteristics and behavior from the conventionally reported liftable flames. The stability domain of the never-lift flames covers higher cross-flow velocities and lower fuel jet velocities compared with the liftable flames. The flame configurations in the stability domain are indentified by characteristic modes: down-washed flame, flashing flame, developing flame, dual-flame, flickering flame, and pre-blowoff flame. The schlieren photographs are presented in order to discuss the effects of the flow structures on the general behavior of the flames in each characteristic mode and on the flame stability characteristics. The bisector of the eddy travelling avenue reasonably depicts the trajectory of the combusting jet in cross flow. Correlations for the trajectories of cold and combusting jets in cross flow are obtained.

Airborne Nanoparticle Exposures while Using Constant-Flow, Constant-Velocity, and Air-Curtain-Isolated Fume Hoods

Tsai et al. (Airborne nanoparticle exposures associated with the manual handling of nanoalumina and nanosilver in fume hoods. J Nanopart Res 2009; 11: 147-61) found that the handling of dry nanoalumina and nanosilver inside laboratory fume hoods can cause a significant release of airborne nanoparticles from the hood. Hood design affects the magnitude of release. With traditionally designed fume hoods, the airflow moves horizontally toward the hood cupboard; the turbulent airflow formed in the worker wake region interacts with the vortex in the constant-flow fume hood and this can cause nanoparticles to be carried out with the circulating airflow. Airborne particle concentrations were measured for three hood designs (constant-flow, constant-velocity, and air-curtain hoods) using manual handling of nanoalumina particles. The hood operators airborne nanoparticle breathing zone exposure was measured over the size range from 5 nm to 20 mum. Experiments showed that the exposure magnitude for a constant-flow hood had high variability. The results for the constant-velocity hood varied by operating conditions, but were usually very low. The performance of the air-curtain hood, a new design with significantly different airflow pattern from traditional hoods, was consistent under all operating conditions and release was barely detected. Fog tests showed more intense turbulent airflow in traditional hoods and that the downward airflow from the double-layered sash to the suction slot of the air-curtain hood did not cause turbulence seen in other hoods.

Surface flow and vortex shedding of an impulsively started wing

The particle tracking flow visualization method (PTFV) and particle image velocimetry (PIV) are used to obtain a clear picture of vortex evolution on the suction surface of an impulsively started NACA 0012 wing. The experiments are conducted in a towing water tank. The formation, evolution, and shedding of the vortex system on the suction surface are observed and analysed by streak pictures of particle images. Five characteristic vortex evolution regimes are identified in the parameter domain of angle of attack and chord Reynolds number. The pathline patterns, instantaneous streamlines, and vorticity of various vortex evolution processes are presented. Stable vortex shedding in the wake is eventually established after the initial period of complex vortex evolution on the suction surface of the wing. Various types of instabilities in the wake, e.g. instability wave, surface vortex shedding, and bluff-body vortex shedding, are found to correspond to different evolution processes of the surface flow. The shedding frequency of the vortices is correlated and compared with several conventional results. Topological critical points, separatrices, and alleyways are identified and discussed to elucidate the unsteady structure of the instantaneous streamline patterns. The topological rule for the number of singular points is verified.

Characteristic flow modes of wake-stabilized jet flames in a transverse air stream

Abstract The flow fields of wake-stabilized jet flames in a transverse air stream have been studied experimentally employing reactive Mie scattering flow visualization and laser Doppler velocimetry. Five characteristic flow modes are identified in different ranges of jet-to-wind momentum flux ratio, R: down-wash (R 10) modes. The streamlines, vorticity distributions, as well as flame appearance, which vary significantly in the different modes, are presented and discussed. The down-wash and reverse flow regions are recognized as the primary mechanisms related to the flame stability. The reduction of the down-wash effect with increase of the jet-to-wind momentum flux ratio and the change of vorticity from negative to positive in the near-wake region of the burner tip lead to blow-off of flames. The turbulence intensity in the wake of the jet is stronger than that in the wake of the tube. The values of Reynolds shear stress at the plane of symmetry are positive in the wake of the jet, but negative in the wake of the tube.

Structure of attached and lifted gas jet flames in hysteresis region

The flame structure and hysteresis chacracteristics of lifting and reattaching propane jet flames over contoured nozzles were studied. High-speed schlieren photography was employed to investigate the flow field in the near-nozzle region of cold jets, attached flames, lifted flames, and their transitions. Fine-wire thermocouples were used in conjunction with a computer data acquisition system to record temperature field in the stabilization region. Gas sampling and analysis were employed to determine the mean concentrations of oxygen and carbon monoxide The mean velocities were determined with a total pressure tube. The flow in the flame stabilization region is observed to be laminar in attached flames and dominated by organized structures in lifted flames. The characteristics of those structures have been measured during liftoff and reattachment transitions and compared with cold jet data. The average dimensionless celerities and Strouhal numbers of the organized structures in the burning parts of the flames were measured as 0.94 and 2.8 during liftoff and 0.62 and 1.30 during reattachment, respectively. The schlieren photographs and radial profiles of temperature and concentrations establish that the flame zone is located away from the shear layer. Further, they show that the molecular diffusion controls the lifting process. The dynamics of the organized structures that result in a wider shear layer govern the reattachment process. This difference between the structures of the flame base in the two configurations accounts for the hysteresis phenomenon. The variations of liftoff and reattachment velocities with nozzle diameter, fuel molecular weight and addition of inert diluents to the jet fluid were measured to substantiate these observations.

Flow field characteristics of swirling double concentric jets

Abstract The flow structures and turbulence properties of double concentric jets with a large separation between the central jet and swirling annular flows are studied using the smoke-wire flow visualization technique and a two-component laser Doppler velocimeter. The smoke-streak flow patterns, characteristic lengths, velocity vectors, streamlines, velocity distributions, and turbulence properties are presented and discussed. The smoke-streak patterns show that a large spatial separation at the exit between the central and swirling annular jets can expedite the formation of a recirculation zone at low swirl and Reynolds numbers. Complex flow structures, single bubble , dual rings , vortex breakdown , and vortex shedding , are found in the recirculation zone. Large central jet velocity induces a large entrainment of the fluids in the recirculation zone and thus reduces the size of the recirculation bubble. The streamline patterns of the dual-ring mode show no stagnation point existing on the central axis, which is different from the non-swirling concentric jets. An off-axis saddle point which fits the topological rule is observed. The turbulence intensities and shear stress around the saddle point are particularly large. A carbon dioxide tracer gas experiment is conducted to examine the mixing capability. Effects of the swirl number and separation between the concentric jets are presented.

Thermal and concentration fields of burner-attached jet flames in cross flow

Abstract The thermal and concentration structures of the burner-attached jet flames in cross flow were experimentally studied. Due to the complex interaction among the cross air flow, issuing fuel jet, and burner tube, the flame behavior in the area around the burner tip was characterized by several modes for different ranges of jet-to-wind momentum flux ratio. The temperature and concentration profiles of the flames of the different characteristics modes presented drastic variations. At low jet-to-wind momentum flux ratios, the downwash flame anchored behind the burner tube. The temperature profiles of the downwash flame near the burner tip were characterized by two side peaks and a center deep. At mid jet-to-wind momentum flux ratios, a center hump presented in the temperature profile. At high jet-to-wind momentum flux ratios, the downwash flame disappeared. The flame existed in the lee side of the issuing fuel jet body and was characterized by two side peaks and a center deep, although the deep was shallow. The temperature variation near the burner tip occurring prior to the blowoff of the flame was discussed. The concentration profiles of CO, CO 2 , and O 2 illustrated the effects of mixing in different characteristics flame modes. The complex characteristics of the temperature and concentration fields suggested that the conventionally used diffusion flame model for the liftable jet flame in cross flow was not adequate for the burner-attached jet flame in cross flow.

Influence of surface flow on aerodynamic loads of a cantilever wing

The properties of the characteristic surface flow modes and their influence on the aerodynamic loads of a cantilever wing at low chord Reynolds numbers were experimentally studied. The chordwise variations of the separation and reattachment locations as well as the length and center position of the separation bubble in the domain of the chord Reynolds number/root angle of attack heavily depend on the characteristic surface flow modes. The aerodynamic loads, e.g., the lift, drag, and moment coefficients, are profoundly influenced by the characteristic behaviors of suction surface flow. The lift coefficient increases with the increase of root angle of attack in the regimes of laminar separation, separation bubble, and transition. The curve of lift coefficient has a largest slope in the laminar separation regime. The increase rate of the lift coefficient decreases when the separation bubble is formed. The stall happens in the turbulent separation regime. The drag coefficient slightly decreases in the laminar separation regime, remains almost a constant in the separation bubble regime, and increases in the transition regime.

Characteristic Modes and Thermal Structure of Nonpremixed Circular-Disc Stabilized Flames

ABSTRACT The characteristic modes and structure of the nonpremixed circular-disc stabilized flamesare experimentally studied. Using direct and schlieren photography techniques, the flames in the stability domain are systematically classified into seven characteristic modes: recirculated, transition, unsteady detached, laminar ring, developing, split flashing, and lifted. The flame behaviorand the combusting and cold flow patterns in each characteristic mode are comprehensively discussed. Two stability limits which are closely related to the flow structure are found. The flame and lift-off heights are correlated to the central-to-annular jet momentum ratio. The structure and transport mechanism in the recirculation bubble dominate the flame behavior for central-to-annular jet momentum ratio lower than 7.5. For the central-to annular jet monentum ratio higher than 7.5, the combusting flow behaves like a turbulent diffusion flame.The temperature profiles are probed by a fine-wire thermocouple. The thermal st...