摘要:

An experimental study has been performed on a shear-driven three-dimensional turbulent boundary layer in equilibrium over a rotating disk. The objective of the present study is to investigate the modification of the near-wall coherent structure in the three-dimensional turbulent boundary layer developed in this flow. Two-point velocity measurements are carried out with two X-wire probes which are aligned in the local wall-parallel mean velocity direction, which show that strong asymmetries exist in the velocity cross-correlation coefficient in the spanwise direction, and also in the conditionally averaged velocity signals in the vicinity of a strong ejection or sweep, as was shown by Littell and Eaton [J. Fluid Mech.266, 175 (1994)]. However, a quadrant analysis reveals that the Reynolds-shear-stress-producing eddies are nearly symmetric in the spanwise direction and those asymmetries are attributed to the changes in the negative-Reynolds-shear-stress-producing motions which have less relation to the streamwise coherent structures. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869750

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

This paper examines the trajectory and entrainment characteristics of a round jet in crossflow. A series of large eddy simulations was performed at Reynolds numbers of 1050 and 2100 and at jet to crossflow velocity ratios of 2.0 and 3.3. Trajectories, which are defined based on the mean streamlines on the centerplane, all collapse to a single curve far from the jet exit, and this curve can be represented with a power law fit. Within this power law region, entrainment of crossflow fluid is shown to be the primary mechanism by which the jet trajectory is determined. Upstream of the power law region, near the jet exit, jet trajectory varies from changes in pressure drag and from differences in the turbulence intensities in the incoming pipe flow. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869751

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The evolution of the invariants (RandQ) of the velocity gradient tensor in homogeneous isotropic turbulence is investigated using data from direct numerical simulation (DNS). The concepts of conditional average time rate of change of the invariants and conditional mean trajectories (CMT) in invariant phase space are introduced to study the dynamics of this flow. The resulting dynamical system in the(R,Q)phase space is a clockwise spiral with a stable focus at the origin, illustrating that in the mean, the cyclic sequence of topological evolution following a fluid particle is unstable-node/saddle/saddle (UN/S/S)→stable-node/saddle/saddle (SN/S/S)→stable-focus/stretching (SF/S)→unstable-focus/contracting (UF/C). The mean rates of change ofRandQ, i.e.,R˙,Q˙,are found to be negligible near the right branch of the null discriminant(D=0)curve, indicating that this curve is an attractor in the(R,Q)space. The effects of both the diffusion term and the anisotropic part of the pressure Hessian term on the dynamics of the invariants have also been analyzed using the conditional averages. Both contributions are found to be important in the dynamics of the velocity gradient invariants. Based on these results the extent of the validity of the model equations governing the evolution ofRandQproposed by Cantwell [Phys. Fluids A4, 782 (1992)] and Dopazo &etal; [“Velocity gradients in turbulent flows. Stochastic models,” Ninth Symposium on “Turbulent Shear Flows,” Kyoto, Japan, 1993, pp. 26-2-1–26-2-5] are discussed. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869752

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

From phenomenological considerations, Marble and Broadwell1have introduced the flame surface density concept to describe nonpremixed turbulent combustion. In this approach, the averaged reaction rate is modeled as the product of the local reaction rate per unit of flame area,m˙F,by the flame area per unit volume,&Sgr;¯.This approach is attractive because it decouples the chemical problem(m˙F)from the description of the turbulence combustion interaction(&Sgr;¯).In this paper, a theoretical analysis providing two alternatives exact, but unclosed, balance equations for the flame surface density&Sgr;¯is first developed. Then, direct numerical simulations of a spatially developing turbulent reacting mixing layer are used to analyze the structure of the reaction zone and to propose and validate closures for the&Sgr;¯-balance equations. The flame surface density concept is found to provide a relevant description for nonpremixed turbulent flames. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869753

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

This paper describes a laboratory study on the evolution of a point turbulent plume placed at the free surface of a homogeneous fluid layer in the presence of background rotation. It is shown that the plume initially evolves as if there is no rotation. However, the rotational effects become important after the plume descends a vertical distancehc1≈3.3(B/&OHgr;3)1/4for a normalized time&OHgr;tc1≈2.4,whence the vertical descent rate of the plume is reduced while maintaining approximately the same lateral growth rate. Here &OHgr; is the rate of background rotation andBis the specific buoyancy flux of the plume. The rotational effects inhibit the lateral growth of the plume at a time&OHgr;tc2≈5.5,when the maximum plume width isbc≈1.4(B/&OHgr;3)1/4.Thereafter, the vertical descent continues and the plume evolves into a cylindrical shape while developing a cyclonic circulation in and around it, except near the plume front. Upon reaching the bottom surface after traveling a fluid depth ofH,the plume deflects, propagates horizontally, and becomes unstable breaking up into anticyclonic eddies. Studies carried out for the case ofH<hc1show that this instability is initiated at a horizontal length scale proportional to the Rossby deformation radius of the deflected flow, and hence it is of baroclinic type. These eddies appear to align vertically with the cyclonic eddies formed by the barotropic instability of the surface rim current, thus producing heton-like structures. The influence of the diameterd0of the plume on the flow evolution is also studied, and it is shown that plumes with aspect ratioh/d0<12(wherehis the vertical extent) can be approximated as point plumes. Scaling arguments are advanced to explain the results. Some geophysical applications of the study are also discussed. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869754

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

This article considers the topology of the vortex regimes generated in harbor-like basins by the external potential long shore current at large Reynolds numbers. The proposed theory discusses the issues of what solution compatible with the Prandtl-Batchelor theorem for inviscid fluids, and under what conditions, may be realized as an asymptotic state of the open hydrodynamical system. The analysis developed is based on the variational principle. We formulated a validity criterion according to which stationary regimes of dissipative systems may be considered as extremals of a variational inviscid problem. In particular, such a situation is possible when the dissipative functionals represent some functions of motion invariants of the same problem. It is shown that the steady state corresponds to the circulational regime in which the system has minimal energy with the fixed enstrophy. This state is fixed by the Reynolds number. The approach that is formulated is applied to the model of a rectangular harbor-like basin in order to obtain the relation between the Reynolds number, the geometry factor and the topological number characterizing the number of vortex cells. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869755

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Thermal instabilities in the form of two-dimensional convection rolls in a rotating annulus with the flat rigid ends and a moderate gap are investigated by both asymptotic and numerical methods. It is shown that the thin Ekman boundary layers at the ends of the annulus, to which the convection rolls attach, play an active controlling role. An asymptotic theory for an asymptotically large Taylor numberTis developed to obtain complete analytical solutions of the convection rolls, indicating at leading orderRc=R0+C(T1/4/&lgr;),&ohgr;=0,where&lgr;is the aspect ratio,&ohgr;is the frequency,Rcis the critical Rayleigh number with the presence of the Ekman boundary layers andR0is the critical Rayleigh number without the influence of the boundary layers. WhileR0can be determined exactly by using Bessel functions as the eigenfunction, constantCis obtained by matching the interior convection rolls to explicit solutions of the Ekman boundary layers. In the corresponding numerical analysis, convection solutions in a rotating annulus are calculated up toT=108with&lgr;=1.The analytical and numerical convection solutions are then compared to show a remarkable quantitative agreement whenT>106.©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869756

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We consider two-dimensional waves in a rectangular container which is periodically excited along its length in the horizontal direction. In general a standing wave of odd mode number whose frequency is close to the forcing frequency is excited. However, we show in this paper that the neighboring even mode, though not directly excited, may be excited through an energy transfer from the odd mode. As a result, the wave response becomes superposition of two standing waves which are not in general in phase with each other. Consequently the mixed-mode wave motion is not standing waves but traveling waves. We employ a perturbation method to derive amplitude equations governing the dynamics of these two modes. Studies of the steady-state solutions and their stability lead to bifurcation diagrams showing the sequences of the events leading to the instability and the parameters for which the standing waves become unstable. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869757

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We consider the motion of a spherical annulus of fluid driven by the slightly differential rotation of the confining spherical shells. The novelty of our analysis is the imposition of a strong axisymmetric potential magnetic field on a conducting annulus bounded by insulators. The strength of the field is measured by its ability to dominate the dynamics of the fluid shell, being much larger than the viscous and Coriolis forces. The fluid flow is structured such as to minimize the effect of the strong field and the associated induction currents. The thrust of the analysis is the identification of regions of the fluid shell in which the azimuthal flow and generated field are relatively uniform. These regions are bounded by shear layers in which the azimuthal flow and field vary rapidly. Thickness of the shears is about a square root of a Hartmann layer thickness. These Hartmann boundary layers control main streams in the magnetohydrodynamic flow. An imposed axial magnetic field produces two rigid-body rotating regions that have different rotation rates outside and inside the axial cylinder tangent to the inner sphere. The azimuthal magnetic field is proportional to the cylindrical radius and is expelled from the outside of the tangent cylinder. For an imposed dipole magnet, similar regions with rigid-body rotation are divided by a shear that has a shape of a lobe touching the outer equator. Very weak meridional fluid flux is inversely proportional to a cube of the imposed magnetic field for these degenerated cases. In a quadrupole magnetic field, two lobe-like regions are symmetric with respect to the shell’s equator, touch the outer sphere, and rotate with the inner sphere. Outside these regions, differential rotation is valuable and a weak meridional fluid flux is inversely proportional to a square of the imposed magnetic field as in the general case. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869758

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Opposition control is a simple method used to attenuate near-wall turbulence and reduce drag in wall-bounded turbulent flows [H. Choi, P. Moin, and J. Kim, J. Fluid Mech.262, 75 (1994)]. This method employs blowing and suction at the wall in opposition to the wall-normal fluid velocity a small distance from the wall. Results from direct numerical simulations of turbulent channel flow indicate that, when the control at the wall is based on detection of the wall-normal velocity in a plane sufficiently close to the wall, the opposition control strategy establishes a “virtual wall,” i.e., a plane that has approximately no through flow, halfway between the detection plane and the wall. As a consequence, drag is reduced about 25&percent;. When the detection plane is at a greater distance from the wall, a virtual wall is not established, and the blowing and suction increase the drag significantly. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869759

出版商:AIP    

年代:1998

数据来源: AIP