摘要:

Measurements of the charge and size of heptane droplets generated by electrostatic sprays showed that the droplet charge‐to‐volume ratio is a monotonically decreasing function of size. In the useful range of electrospray operation, characterized by droplets smaller than the size of the orifice from which the liquid is issued, it was found that the larger were the droplets the closer they were to the Rayleigh limit. In particular, when droplets had charging levels between 70% and 80% of such limit, they were observed to rupture because the repulsive force due to surface charge evidently overcame surface tension. The rupture phenomenon, here termed Coulomb fission, was also captured in microphotographs that typically showed a droplet with one or two, diametrically opposed, conical protrusions terminating in a fine jet ejecting a stream of much smaller, apparently equisized offsprings. The process appeared swift and, yet, well ordered, quite different from the common view of a violent, convulsive explosion. Corroborating evidence on the disruption pattern was also gathered by quantitative measurements of the evolution of the droplet size distribution in evaporating sprays using phase Doppler anemometry (PDA). Implications of these findings are finally discussed in the context of a particular application of electrostatic sprays, electrospray ionization, a technique that is revolutionizing the mass‐spectrometric analysis of large biomolecules.

ISSN:1070-6631    

DOI:10.1063/1.868037

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Using a zeroth‐order entropy production model, a relationship between the second viscosity, the relaxation time for a nonequilibrium process, and turbulence is determined from the macroscopic entropy rate equation. For turbulence generated reaction rate distortion, enhanced second viscosities can be predicted for turbulent fluid systems with long relaxation times and nonzero velocity divergence.

ISSN:1070-6631    

DOI:10.1063/1.868038

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An approximate expression for the history force on a spherical bubble is proposed for finite Reynolds number, Re. At small time, the history‐force kernel is a constant, which decreases with increasing Re, and the kernel decays ast−2for large time. For an impulsively started flow over a bubble, accurate finite difference results show that the history force on the bubble decays ast−2at large time. Satisfactory agreement is observed between the presently proposed history force and the numerical solution.

ISSN:1070-6631    

DOI:10.1063/1.868039

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The fluid transfer mechanisms between a channel and a cavity are investigated by laser‐induced fluorescence for a Reynolds number based on cavity height of 600. In particular, this work examines the effect of upstream geometry on the flushing of the cavity. Oscillation of the shear‐layer and Ekman‐layer pumping are the dominant mechanism for fluid transfer in most cases. The flapping shear layer impinging on the trailing edge of the cavity creates a net inflow of channel fluid into the central portion of the cavity. A resulting outflow of cavity fluid is observed at the cavity end walls. Geometrical modifications investigated include shortening the cavity length to isolate the Ekman layer from the channel wall boundary layer, placing a cylinder directly upstream of the cavity leading edge, and serrating the upstream edge of the cavity. A cylinder placed upstream increases the flushing rate a maximum of 400%. A serrated leading edge increases the flushing rate 200%. The shorter cavity illustrates the effect of the channel side walls on the cavity flow without affecting the mean flushing rate. Finally, three‐dimensional (3‐D) effects are shown to have a dominant influence on the flushing rate.

ISSN:1070-6631    

DOI:10.1063/1.868040

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The linear stability analysis of a simple model of a swirling jet illuminates the competition and interaction of centrifugal and Kelvin–Helmholtz instabilities. By employing potential theory, analytical expressions are derived for the growth rate and propagation velocity of both axisymmetric and helical waves. The results show that centrifugally stable flows become destabilized by sufficiently short Kelvin–Helmholtz waves. The asymptotic limits demonstrate that for long axisymmetric waves the centrifugal instability dominates, while long helical waves approach the situation of a Kelvin–Helmholtz instability in the azimuthal direction, modulated by a stable or unstable centrifugal stratification. Both short axisymmetric and short helical waves converge to the limit of a plane Kelvin–Helmholtz instability feeding on the azimuthal vorticity.

ISSN:1070-6631    

DOI:10.1063/1.868041

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The response of the two‐dimensional velocity field to a single eigenmode driving a fluid flow is analyzed. It is shown that independent of its amplitude such a driving force cannot lead to Kraichnan’s inertial range spectrum. At least a pair of eigenmodes, one acting as a power source, and the other as a power sink, is necessary to obtain a separation of length scales and an accompanying statistically steady inertial spectral range.

ISSN:1070-6631    

DOI:10.1063/1.868042

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Direct numerical simulation data obtained for a zero pressure gradient turbulent boundary layer at a moderate Reynolds number indicate a significant increase in the anisotropy of the near‐wall region when suction is applied uniformly at the wall. This anisotropy is reflected in the approach of the Reynolds stress tensor towards a one‐component state.

ISSN:1070-6631    

DOI:10.1063/1.868043

出版商:AIP    

年代:1994

数据来源: AIP