摘要:

A new two‐fluid model is developed to describe the nonlinear interaction of acoustic waves and vortices. Analytical and computational results are presented for a sound pulse interacting with and being modified by a vortex. A novel numerical method based on a particle‐in‐cell discretization of the acoustic field is developed and used to study the nonlinear scattering of sound by a cylindrical vortex. Equations for the sound wave packet propagating in an axially symmetric mean flow are integrated analytically. Nonlinear modification of the vortex flow by the high‐frequency sound is found to be mediated by growing pressure disturbances generated by the radiative forcing on the high gradient regions of the acoustic pulse. The total energy of the vortex mean flow grows monotonically, as the acoustic component loses its energy. The changes in the kinetic and internal energies of the vortex are greater than the changes in its total energy, although these changes are reversible in lowest order of the nonlinear vortex–acoustic interaction. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868769

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Employing an aqueous acid‐base reaction, the minimum mixing rate of turbulent vortex rings was investigated in a water tank. Vortex rings were generated by a simple apparatus with a cylindrical geometry. The released fluid surrounding the vortex core mixed very rapidly when compared with the fluid in the toroidal core. Moreover, the fluid within the core did not mix uniformly in the azimuthal direction. The normalized distance a vortex ring must travel, in order to completely mix with the ambient fluid to a specific volumetric ratio, depends on the aspect ratio of the generating cylinder. Scaling arguments are presented that relate the above distance to the spreading rate and the generating apparatus parameters. Due to the very small net entrainment rate of vortex rings, the detrainment of core material cannot be ignored when the mixing rate of the core is considered. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868752

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A nonuniform magnetic field induces an inhomogeneous ferroparticle distribution in a magnetic fluid (MF). As a consequence the magnetic field influences MF flows. In particular, we consider an oscillatory pipe flow in a stationary nonuniform magnetic field subject to the quasielastic magnetic force. Concentration inhomogeneities can be formed from both single particles and many‐particle drops, which appear as a result of the field‐induced MF phase separation. Kinematic considerations show that only the drops contribute to the magnetic force. Our theory is in good agreement with recent experimental results. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868686

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Coefficients of series expansions of turbulent velocity fluctuations in the viscous wall region are used to generate an arbitrary but quantitative measure of the time‐average strength of the near‐wall quasi‐streamwise vortices, which appear as ‘‘streaks’’ in flow visualization. Existing databases from direct numerical simulations of wall bounded turbulence are used to compute some estimates. The results show that the strength of the streaks is Reynolds‐number‐dependent, even in simple flows, as well as flow‐dependent, contrary to traditional law‐of‐the‐wall arguments. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868687

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Streamwise high vorticity rolls and streaks in the turbulent channel flows have been the subject of many studies due to their important role in turbulence production, as a result of sweeping, ejection, and bursting of these structures. Understanding the physics of these streamwise structures is important in controlling drag producing events. Investigations of the average streak spacing of the low‐speed streaks have resulted in the generally accepted range of &lgr;+=&lgr;¯u&tgr;/&ngr;=100±20, where &lgr;¯ is the mean spanwise spacing between streaks, normalized to the viscous length &ngr;/u&tgr;. It is also reported, fory+≤30, that the streak spacing grows nearly linearly with distance from the wall. The previous studies mostly have focused on distances close to the wall. Here we report on correlation measurements extended into the log layer, which show that the linear growth of the vortex diameter and the streak spacing extends well in the log layer. Arguments are presented to distinguish these two measures. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868688

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The global evolution of variable density turbulent jets is now quite well documented, showing that the entrainment of external fluid into these jets is considerably modified by density variations. But, to our knowledge, no specific study has so far been devoted to the intermittent region of such flows. For constant density flows, the radial evolutions of the velocity variances are known to follow the so‐called Phillips’ relations in this interface region. The main objective of the present work is to investigate whether density variations affect properties of the interface. It is found that Phillips’ relations are also valid in the presence of large‐density variations, albeit their extent is slightly different. In relation to this, the structure of turbulence is almost unchanged, even in the outer region where large‐scale structures are dominating the flow mixing properties, and departure from isotropy for the Reynolds stresses is rather similar for all jets within the range of density ratios considered here. Therefore, except in the very near‐field region, most of the influence of density variations can be taken into account by considering only the different evolutions of the flow characteristics on the jet axis. The implication of our results for various aspects associated with these flows, such as in modeling, is also discussed. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868689

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Velocity autocorrelations and the mean‐square displacements of fluid particles are obtained for decaying, isotropic homogeneous turbulence by numerical simulation of the flow field, using 1283and 2563grids, and tracking several tens of thousands of fluid particles, using a third‐order interpolation scheme. A self‐preserving Lagrangian velocity autocorrelation coefficient is found in terms of a dimensionless time variables, defined byds=dt/Ts(t), under the observation of a power‐law energy decay and the assumption thatTs(t) is proportional to the Lagrangian integral timescaleTL. This timescale is in turn assumed to be proportional to the length scale of the energy‐containing eddiesLe∼K3/2/&egr; divided by the turbulent velocityu′, whereK=3/2u′2is turbulent energy and &egr; is the energy dissipation rate. ©1995 American Institute of Physics. 

ISSN:1070-6631    

DOI:10.1063/1.868780

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The effect of suction, applied through a short porous wall strip, on a low Reynolds number self‐preserving turbulent boundary layer has been quantified by measuring the local wall shear stress and the main Reynolds stresses downstream of the strip. When the suction rate is sufficiently high, pseudo‐relaminarization occurs almost immediately downstream of the strip. Farther downstream, transition occurs followed by a slow return to a fully turbulent self‐preserving state. During relaminarization, the measured skin friction coefficientcffalls below the level corresponding to the no suction value, reaching a minimum where transition begins. An empiricalcfdistribution is proposed that groups together results obtained at different streamwise stations and different suction rates. Of all the measured Reynolds stresses, the longitudinal turbulence intensity recovers relatively quickly from the change in boundary conditions while the wall‐normal turbulence intensity and the Reynolds shear stress are significantly affected by the suction. The Reynolds shear stress, which is negligible during relaminarization, has the slowest recovery. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868690

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The reflection of a normal shock wave from the end wall of a two‐dimensional channel has been numerically simulated to investigate the unsteady, viscous interaction aspects of shock bifurcation. The numerical simulation implements a data‐parallel version of the Flux‐Corrected Transport algorithm that has been coupled to the viscous transport terms of the Navier–Stokes equations. All numerical simulations were performed on the Connection Machine, the CM‐5. The results indicate that the shear layer in the bifurcation zone is unstable, and the large and small scale vortices lead to complex flow patterns. In addition, the high‐speed, essentially inviscid flow, which is adjacent to the shear layer, is deflected over this region. As a result, weak shock and expansions waves are generated and a reattachment shock is formed at the trailing edge of the interaction region. The impact of heat transfer, Reynolds number, and incident shock strength on the viscous interaction is also investigated. Heat transfer to the walls weakens the interaction between the boundary layer and the reflected shock. However, the decreased Reynolds number and increased shock strength enhances the interaction. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868691

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The instability of supersonic mixing layers, with velocity profiles possessing a wake component, is investigated using linear, inviscid, spatial theory. The mean‐velocity profile is represented by a hyperbolic‐tangent profile plus a wake component. Such profiles are encountered in the initial region of experimental supersonic shear‐layer flows, as well as in envisaged hypersonic propulsion systems in which ingested boundary layers generate substantial wake components. Shear‐layer and wake instability modes previously found in incompressible mixing layers are also found in compressible mixing layers. The existence of a wake component in the velocity profile renders the mixing layer more unstable at all free‐stream Mach numbers. For convective Mach numbers exceeding unity, the shear‐layer mode splits into two supersonic modes, and the mixing layer becoming more unstable with increasing wake deficit. The wake mode becomes less unstable and eventually stable with increasing compressibility, i.e., increasing convective Mach numbers. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868692

出版商:AIP    

年代:1995

数据来源: AIP