To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Dimensionless numbers are important in biomechanics because their constancy can imply dynamic similarity between systems, despite possible differences in medium or scale. A dimensionless parameter that describes the tail or wing kinematics of swimming and flying animals is the Strouhal number, St = fA/U, which divides stroke frequency (f) and amplitude (A) by forward speed (U). St is known to govern a well-defined series of vortex growth and shedding regimes for airfoils undergoing pitching and heaving motions. Propulsive efficiency is high over a narrow range of St and usually peaks within the interval 0.2 < St < 0.4 (refs 3-8). Because natural selection is likely to tune animals for high propulsive efficiency, we expect it to constrain the range of St that animals use. This seems to be true for dolphins, sharks and bony fish, which swim at 0.2 < St < 0.4. Here we show that birds, bats and insects also converge on the same narrow range of St, but only when cruising. Tuning cruise kinematics to optimize St therefore seems to be a general principle of oscillatory lift-based propulsion.

Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency

1. Introduction 2. Fixed, rigid wing aerodynamics 3. Flexible wing aerodynamics 4. Flapping wing aerodynamics.

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Aerodynamics of Low Reynolds Number Flyers

WE found, on experimental grounds in Article I, that the field of air‐flow past a short body of low resistance shape, such as an aerofoil, comprises two dissimilar parts: (a) a thin boundary layer enveloping the body and dominated by viscous effects, and (b) a motion outside the boundary layer in which viscosity is much less important. It will be remembered that in the external motion occur the large pressure changes, which, transmitted through the boundary layer, account for nearly all the lift and for part of the drag. These pressures we observed to be calculable from the velocities without appreciable error by Bernoulli's equation. In the present Article we confine attention to this external flow, assuming it to be steady, incompressible, and inviscid. Its dependence upon (a), already discussed to some extent, we ignore; the boundary layer is conceived to be everywhere very thin, so that the only role it plays is to allow of relative velocity at the surface of the body. The assumptions made, excepting that of incompressibility, will appear drastic, and it will not be surprising if some of our deductions prove discordant with experimental fact. Nevertheless, they lead to a theory which finds many applications and uses in real fluid motion, and, in particular, gives an intimate view of aerofoil flow that is very close to the truth. It is convenient to develop our reasoning in analytical terms and for simplicity to restrict the flow to two dimensions (Article 1, §5). But the engineer will find special scope in this part of aerodynamics for graphical methods in the solution of particular problems.

Aerodynamics for Engineers

Transverse jets and their control

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.

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

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.

Surface flow and vortex shedding of an impulsively started wing

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

Thee owpatternsofaswirlingjetsubjecttotheine uencesofvariouscentralblockagesarestudiedexperimentally via the smoke-wire e ow visualization method. Details of the observed e ow structures are described and discussed. Thee owstructuresdisplay multiplemodes,depending on theblockageratio and swirlnumber. When theblockage ratio is less than about 0.1, swirling wake, vortex breakdown, and three-dimensional vortex shedding are observed for different ranges of swirl number. When the blockage ratio is greater than about 0.1, the bluff-body effect becomes signie cant. In this case, a pair of counter-rotating vortex rings and a central swirling jet characterize the e owe eld. Axisymmetric vortex breakdown and three-dimensional vortex shedding may appear on the central swirling jetwhen theswirlnumberorblockageratio islargerthan critical values.Somecharacteristicquantities of thee owe eld aremeasuredandcorrelated,includingthefreeseparationlines,recirculationlengths,andfrequencies of the vortex shedding.

Observations of Swirling Flows Behind Circular Disks

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 operator's 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.

Rong Fung Huang

Papers

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

Surface flow and vortex shedding of an impulsively started wing

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

Observations of Swirling Flows Behind Circular Disks

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