摘要:

An experimental study of turbulent primary breakup at the free surface of plane liquid wall jets along smooth walls in still air at normal temperature and pressure is described. The study seeks a better understanding of spray formation processes in marine environments, such as in bow sheets. The measurements involved initially nonturbulent annular liquid wall jets, to approximate plane liquid wall jets, with the growth of a turbulent boundary layer along the wall initiated by a trip wire. Pulsed shadowgraphy and holography were used to observe liquid surface properties as well as drop sizes and velocities after turbulent primary breakup. Test conditions included several liquids (water, glycerol mixtures and ethyl alcohol), liquid/gas density ratios of 680–980, wall jet Reynolds numbers of 17 000–840 000 and Weber numbers of 6 100–57 000, at conditions where direct effects of liquid viscosity were small. Measurements included the following: location of the onset of surface roughness, drop size and velocity distributions after breakup, flow properties at the onset of breakup, and mean drop sizes and velocities after breakup. In general, the measurements were correlated successfully based on phenomenological theories. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869639

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Direct numerical simulations were performed in order to investigate the evolution of turbulence in a stably stratified fluid forced by nonvertical shear. Past research has been focused on vertical shear flow, and the present work is the first systematic study with vertical and horizontal components of shear. The primary objective of this work was to study the effects of a variation of the angle &thgr; between the direction of stratification and the gradient of the mean streamwise velocity from&thgr;=0,corresponding to the well-studied case of purely vertical shear, to&thgr;=&pgr;/2,corresponding to purely horizontal shear. It was observed that the turbulent kinetic energyKevolves approximately exponentially after an initial phase. The exponential growth rate &ggr; of the turbulent kinetic energyKwas found to increase nonlinearly, with a strong increase for small deviations from the vertical, when the inclination angle &thgr; was increased. The increased growth rate is due to a strongly increased turbulence production caused by the horizontal component of the shear. The sensitivity of the flow to the shear inclination angle &thgr; was observed for both low and high values of the gradient Richardson number Ri, which is based on the magnitude of the shear rate. The effect of a variation of the inclination angle &thgr; on the turbulence evolution was compared with the effect of a variation of the gradient Richardson number Ri in the case of purely vertical shear. An effective Richardson numberRieffwas introduced in order to parametrize the dependence of the turbulence evolution on the inclination angle &thgr; with a simple model based on mean quantities only. It was observed that the flux Richardson numberRifdepends on the gradient Richardson number Ri but not on the inclination angle &thgr;. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869640

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We present results of numerical simulations of turbulence control in saltwater channel flows using electromagnetic (EM) forces. The control actuators are millimeter-sized micro-tiles flush mounted in the lower channel wall. This arrangement closely models one of the experimental designs proposed and developed by Bandyopadhyay at NUWC. We have studied two main secondary flow patterns which we denote by UV and WV (i.e., predominantly streamwise/normal and spanwise/normal) induced by both static and pulsed EM forcing. We have observed low net drag reduction, with a maximum of approximately 1&percent;. This may be within the uncertainty of our computations. However, we have also found regions of localized reduction/increase in wall shear stress as high as±11&percent;versus the uncontrolled flow. Also, in every simulation with control we have observed a consistent (albeit small) reduction in skin friction which increases confidence in the results. The method of pulsing the EM force did not result in any observable resonance effects, at the low Reynolds numbers of this study. The mean turbulence intensities appear to be only weakly correlated with the reduction in viscous drag. The change in net drag does not appear to scale linearly on the magnitude of the EM forcing in the cases we have considered. Flow visualizations in the both the UV and WV cases indicate that the mean secondary flow above the actuators consists of a pair of near-wall oppositely oriented streamwise vortices which induce a flow where the normal velocity is wall-ward and is accompanied by strong spanwise wall jets. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869641

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this paper we examine how the flow in the wall region can be predicted from realistic (partial) measurements. The underlying motivation of this study is the improvement of control schemes for near-wall flows. We propose a method based on the proper orthogonal decomposition which provides estimated amplitudes for the coherent structures (i.e., the large scales) of the flow from wall measurements. The method is tested for the direct numerical simulation of a minimal flow unit. The large scales obtained by reconstruction from wall data are compared to those of the velocity field in the wall layer. The dominant structures—the streamwise streaks—are well recovered, the cross-stream motions less so since they are associated with higher-order structures unaccounted for in our truncation. We defined “rescaled” eigenfunctions to try to improve the representation of the cross-stream components of the flow. Aliasing effects due to realistic (large) sensor spacings were examined. We find that the spanwise spacing is the limiting factor for the estimation, so that a realistic grid will affect the reconstruction by at least 20&percent; compared to full wall information. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869642

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The similarity form of the scalar-variance spectrum at high Schmidt numbers is investigated for nonstationary turbulence. Theoretical arguments show that Batchelor scaling may apply only at high Reynolds numbers. At low Reynolds numbers, Batchlor scaling is not possible unless the turbulence is stationary or the enstrophy decays asymptotically ast−2.When this latter condition is satisfied, it is shown from an analysis using both the Batchelor and Kraichnan models for the scalar-variance transfer spectrum that thek−1power law in the viscous-convective subrange is modified. Results of direct numerical simulations of high Schmidt number passive scalar transport in stationary and decaying two-dimensional turbulence are compared to the theoretical analysis. For stationary turbulence, Batchelor scaling is shown to collapse the spectra at different Schmidt numbers and ak−1viscous-convective subrange is observed. The Kraichnan model is shown to accurately predict the simulation spectrum. For nonstationary turbulence decaying at constant Reynolds number for which the enstrophy decays ast−2,scalar fields for different Schmidt numbers are simulated in situations with and without a uniform mean scalar gradient. The Kraichnan model is again shown to predict the spectra in these cases with different anomalous exponents in the viscous-convective subrange. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869643

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Numerical experiments have been performed to study the geometric collision rate of heavy particles with finite inertia. The turbulent flow was generated by direct numerical integration of the full Navier-Stokes equations. The collision kernel peaked at a particle response time between the Kolmogorov and the large-eddy turnover times, implying that both the large-scale and small-scale fluid motions contribute, although in very different manners, to the collision rate. Both numerical results for frozen turbulent fields and a stochastic theory show that the collision kernel approaches the kinetic theory of Abrahamson [Chem. Eng. Sci.30, 1371 (1975)] only at very large&tgr;p/Te,where&tgr;pis the particle response time andTeis the flow integral time scale. Our results agree with those of Sundaram and Collins [J. Fluid Mech.335, 75 (1997)] for an evolving flow. A rapid increase of the collision kernel with the particle response time was observed for small&tgr;p/&tgr;k,where&tgr;kis the flow Kolmogorov time scale. A small inertia of&tgr;p/&tgr;k=0.5can lead to an order of magnitude increase in the collision kernel relative to the zero-inertia particles. A scaling law for the collision kernel at small&tgr;p/&tgr;kwas proposed and confirmed numerically by varying the particle size, inertial response time, and flow Reynolds number. A leading-order theory for small&tgr;p/&tgr;kwas developed, showing that the enhanced collision is mainly a result of the nonuniform particle concentration that results from the interaction of heavy particles with local flow microstructures. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869644

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Vorticity is deposited baroclinically by shock waves on density inhomogeneities. In two dimensions, for a planar “slow–fast” interface, we present analytical results for &sgr;, the circulation deposited per unit unshocked interface length, within the regular refraction regime. The parameters that describe the interaction are the Mach number(M), the density ratio of the two gases (&eegr;, &eegr;<1), the local angle between the shock and the interface (&agr;), and the ratio of specific heats of the two gases(&ggr;0,&ggr;b). For weak shocks &sgr; scales as&sgr;∝(1−&eegr;−1/2)&xgr;(M)sin &agr;and for strong shocks&sgr;→K(&eegr;,&agr;,&ggr;)/(1−&xgr;(M)).For scaling purposes, the gases are assumed to have the same &ggr;.K(&eegr;,&agr;,&ggr;)is a function of the density ratio, the interface angle, and the ratio of specific heats &ggr; [Eq. (4.6)] and&xgr;(M)is the normalized pressure gradient across the shock. The planar interface approach is used to find formulas to calculate the total circulation deposited on sinusoidal interfaces. To validate the formulas, numerical simulations of the compressible Euler equations were made using a second-order Godunov code. Simulations were done for1.05⩽M⩽3.0and&eegr;=0.14,0.33 and 0.65. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869649

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The final stage of sedimentation of a spherical particle moving along the axis of a conical vessel containing a viscous incompressible fluid is studied both theoretically by lubrication analysis and experimentally by laser interferometry. The particle settling velocity varies liked5/2,wheredis the gap. There is an excellent agreement between this result from lubrication theory and experiment, the upper bound being for a gap of about1/30radius and the lower practical bound being at the size of the particle roughness. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869790

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The spreading of surfactant coated thin liquid films is often accompanied by an instability producing significant film corrugation, fingering and branching. Marangoni stresses, responsible for the rapid and spontaneous spreading, are suspected as the main cause of unstable flow. Traditional eigenvalue analysis of a self-similar solution describing Marangoni driven spreading has predicted only stable modes. We present results of a transient growth study which reveals enormous amplification of initially infinitesimal disturbances in the film thickness. This analysis provides, for the first time, evidence of an instability resembling experimental patterns. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869645

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:1070-6631    

DOI:10.1063/1.869646

出版商:AIP    

年代:1998

数据来源: AIP