Aerodynamics of spherical balloon wind sensors

Scoggins, J. R.

doi:10.1029/jz069i004p00591

Cited by 29 publications

(14 citation statements)

References 1 publication

(1 reference statement)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This phenomenon is known to affect drag as well as heat and mass transfer, which have important implications for sedimentation and other multiphase flows (e.g. Richardson & Zaki 1954;Stringham, Simons & Guy 1969;Hartman & Yates 1993), while the oscillatory motion itself can influence atmospheric measurements using weather balloons (Scoggins 1964;Murrow & Henry 1965). The earliest observation of the vibration of a rising or falling sphere was reported by Newton (1726), who studied lead and wax spheres dropped in water, as well as glass spheres and inflated hog bladders falling through air.…”

Section: Introduction and Preliminary Resultsmentioning

confidence: 98%

The effect of Reynolds number on the dynamics and wakes of freely rising and falling spheres

2010

View full text Add to dashboard Cite

In this paper, we study the effect of the Reynolds number (Re) on the dynamics and vortex formation modes of spheres rising or falling freely through a fluid (where Re = 100-15 000). Since the oscillation of freely falling spheres was first reported by Newton (University of California Press, 3rd edn, 1726, translated in 1999, the fundamental question of whether a sphere will vibrate, as it rises or falls, has been the subject of a number of investigations, and it is clear that the mass ratio m * (defined as the relative density of the sphere compared to the fluid) is an important parameter to define when vibration occurs. Although all rising spheres (m * < 1) were previously found to oscillate, either chaotically or in a periodic zigzag motion or even to follow helical trajectories, there is no consensus regarding precise values of the mass ratio (m * crit ) separating vibrating and rectilinear regimes. There is also a large scatter in measurements of sphere drag in both the vibrating and rectilinear regimes.In our experiments, we employ spheres with 133 combinations of m * and Re, to provide a comprehensive study of the sphere dynamics and vortex wakes occurring over a wide range of Reynolds numbers. We find that falling spheres (m * > 1) always move without vibration. However, in contrast with previous studies, we discover that a whole regime of buoyant spheres rise through a fluid without vibration. It is only when one passes below a critical value of the mass ratio, that the sphere suddenly begins to vibrate periodically and vigorously in a zigzag trajectory within a vertical plane. The critical mass is nearly constant over two ranges of Reynolds number (m * crit ≈ 0.4 for Re = 260-1550 and m * crit ≈ 0.6 for Re > 1550). We do not observe helical or spiral trajectories, or indeed chaotic types of trajectory, unless the experiments are conducted in disturbed background fluid. The wakes for spheres moving rectilinearly are comparable with wakes of non-vibrating spheres. We find that these wakes comprise single-sided and double-sided periodic sequences of vortex rings, which we define as the 'R' and '2R' modes. However, in the zigzag regime, we discover a new '4R' mode, in which four vortex rings are created per cycle of oscillation. We find a number of changes to occur at a Reynolds number of 1550, and we suggest the possibility of a resonance between the shear layer instability and the vortex shedding (loop) instability. From this study, ensuring minimal background disturbances, we have been able to present a new regime map of dynamics and vortex wake modes as a function of the mass ratio and Reynolds number {m * , Re}, as well as a reasonable collapse of the drag measurements, as a function of Re, onto principally two curves, one for the vibrating regime and one for the rectilinear trajectories.

show abstract

Section: Introduction and Preliminary Resultsmentioning

confidence: 98%

The effect of Reynolds number on the dynamics and wakes of freely rising and falling spheres

2010

View full text Add to dashboard Cite

show abstract

“…MacCready & Jex give simple qualitative explanations of the complex coupling or interaction between transverse sphere motion, rotational sphere motions and the unsteady wake flow behind the sphere. Scoggins (1964Scoggins ( ,1967 has also reported that the unsteady transverse motions of rising balloons of near spherical shape are much greater when the Reynolds number is supercritical, Rcrit ci 4 x 105. Scoggins (1964) reported tests of a spherical balloon with large roughness elements (conical paper cups) glued to the surface and showed that the random irregularities of the motion of the rising balloon are much reduced in the supercritical range of Reynolds number.…”

Section: Introductionmentioning

confidence: 99%

Aerodynamic lift and moment fluctuations of a sphere

Willmarth

Enlow

1969

J. Fluid Mech.

View full text Add to dashboard Cite

Measurements are reported of the fluctuating lift acting on a sphere and the moment acting about the centre of a sphere at supercritical Reynolds numbers (R > 4 × 105). The lift and moment fluctuations are random functions of time which scale with the free-stream dynamic pressure and sphere dimensions. The power spectra of the lift and moment also scale with the above parameters and with the Strouhal number, nd/U. The spectra contain a maximum spectral density at very low frequencies (nd/U < 0·0003) and do not reveal appreciable effects of vortex shedding at discrete frequencies.Hot wire anemometers were placed near the surface but outside the boundary layer along a great circle in the meridian plane in which the lift was measured. The fluctuating velocity component near the surface on the upstream hemisphere in this meridian plane is highly correlated with the fluctuating lift in the same meridian plane. The correlation betweeen the lift and tangential velocity near the surface suggests that the fluctuating lift is produced by the component of fluctuating bound vorticity about the sphere that is normal to the meridian plane in which the lift force is measured. The fluctuating moment measured about an axis passing through the centre of the sphere and perpendicular to the above meridian plane is almost perfectly correlated with the fluctuating lift (the measured correlation coefficients were 0·99 and 1·00). The fluctuating moment coefficient is very small ($\sqrt{C^2_m}\simeq 5\times 10^{-4}$) compared to the fluctuating lift coefficient ($\sqrt{C^2_L}\simeq 6\times 10^{-2}$). The exceptional correlation between the random lift and moment suggests that the unsteady moment about the sphere centre (which can be produced only by shear stress fluctuations) is caused by the fluctuations of bound vorticity (residing in the boundary layer and wake) that are responsible for the unsteady lift.

show abstract

“…Allen (1900) studied rising paraffin particles in aniline at Re,< 30 and found no difference in the movement of light and heavy spheres. Scoggins (1964) observed the free rise of spherical meteorological balloons. They showed a more or less spiral trajectory under very stable atmospheric conditions (Figure 1).…”

mentioning

confidence: 96%