摘要:

An experimental study of an incompressible circular jet in a crossflow and theoretical analysis based on inviscid flow models are described. The jet exits from a rigidly mounted pipe projecting distant from the floor of a tunnel carrying a steady stream of water; density of the jet and the stream are the same. The results of scalar and velocity measurements and visualizations showed that the jet bifurcated into two separated, counterrotating arms for values of &egr;=U∞/UJET, the ratio of the mean crossflow velocityU∞to the mean jet discharge velocityUJET, less than or equal to 0.25. The angle of separation between the two arms of the bifurcated jet was found to vary inversely with &egr;. For higher values of &egr; the jet does not bifurcate but is dominated by a different mode of instability. The structure of the flow field, which is different for bifurcated and nonbifurcated jets, comprised a variety of vortical structures which survived for very long distancesxbeyondx/2a≳400, whereais the radius of the jet exit andxis distance downstream from the jet axis. The location of the point of bifurcation is predicted from consideration of potential flow models and the characteristics of bifurcating elliptical jets. The location of the point of bifurcation is more distant from the jet exit for smaller values of &egr;, and experimental results were in good agreement with the theoretical predictions. The initial jet trajectory is shown to be associated with the presence in the wake of vorticity shed from the pipe. The near‐field geometry and centerline trajectory of the jet are also found to be in accord with predictions in that it is observed thatz∼x1/2andz∼&egr;−1. Dilutions of bifurcated jets are found to be greater than for nonbifurcated jets. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868859

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Results of an experimental investigation of the inhomogeneous, three‐dimensional flow around a surface mounted cube in a channel are presented. LDA measurements of single‐point velocity correlations are used to determine the production, convection and transport of the turbulence kinetic energy,k, in the obstacle wake. The turbulence dissipation rate is obtained as a closing term to the balance of thek‐transport equation. The results provide some insight to the evolution of the turbulence dissipation rate from the near field recirculation zone to the asymptotic wake. Also presented is a comparison between measured and modeled transport terms. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868860

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The consistency of second‐order closure models with results from hydrodynamic stability theory is analyzed for the simplified case of homogeneous turbulence. In a recent study, Speziale, Gatski, and Mac Giolla Mhuiris [Phys. Fluids A2, 1678 (1990)] showed that second‐order closures are capable of yielding results that are consistent with linear stability theory for the case of homogeneous shear flow in a rotating frame. It is demonstrated in this paper that this success is due to the fact that the stability boundaries for rotating homogeneous shear flow are not dependent on the details of the spatial structure of the disturbances. For those instances where they are—such as in the case of elliptical flows where the instability mechanism is more subtle—the results are not so favorable. The origins and extent of this modeling problem are examined in detail along with a possible resolution based on Rapid Distortion Theory (RDT) and its implications for turbulence modeling. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868861

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This paper describes experimental results measured in a low‐shear turbulent boundary layer. The low‐shear condition is exerted after the boundary layer reaches Re&thgr;&bartil;2000 and has the effect of removing the inner layer; thus, these are the first results to show the behavior of an outer‐layer‐only turbulent boundary layer. The removal of the inner layer causes the gradual decay of the turbulent stresses over eleven boundary‐layer thicknesses (roughly 20 large‐eddy length scales) of streamwise distance, with the decay beginning at the wall and propagating into the outer flow with increasing downstream distance. However, the structure of the outer layer is little affected by the perturbation, as demonstrated by stress (anisotropy) ratios, quadrant analysis, and spectral measurements. Although the lack of near‐wall production implies this flow must eventually decay into isotropic turbulence, this decay occurs relatively slowly because the dissipation is also greatly reduced with the decrease in near‐wall shear. In addition, the outer‐layer production is significant in maintaining the turbulence level. These results show that, once formed, the outer‐layer characteristics are not explicitly dependent on the presence of the inner layer. These results are compared with similar studies of isotropic turbulence near a shear‐free wall. Very close to the wall the two flows both show that the normal‐stress components respond differently to the presence of the wall. However, away from the wall the isotropic results underpredict the distance to which the tangential stresses are damped by the impermeability condition at the wall. Finally, the results show general similarities to those in a boundary layer just downstream of reattachment, after a similar low‐shear condition over the separation bubble. This raises the possibility that many of the important features of the reattaching flow can be captured by the present, simpler experiment. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868862

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The turbulent wake of a flat plate aligned with a uniform water flow and extending through the free surface was investigated experimentally. Laser‐Doppler velocimetry (LDV) measurements show good agreement with published data for a two‐dimensional wake, except in a shallow layer near the free surface. In this surface layer, the wake width is observed to double while the wake centerline velocity remains essentially unchanged from its value at depth, resulting in a wake momentum deficit that is twice that at depth. Instantaneous, full‐field measurements of the velocity were made using digital particle image velocimetry (DPIV) to elucidate the role of vortical structures in the development of the surface layer. DPIV measurements reveal that in the deep wake, vortex structures predominantly of opposite sign exist on opposite sides of the wake centerline and contribute to a velocity and vorticity field that is two‐dimensional in the mean. However, near the surface, vortex structures tend to become either surface‐normal or surface‐parallel and contribute to a velocity and vorticity field that is highly three‐dimensional in the mean. The resulting surface layer is characterized by surface–parallel structures interacting with their images above the surface to retard and widen the surface flow to a depth comparable to the size of the vortex structures. Histograms taken from many independent DPIV realizations of the flow characterize the distribution of vorticity in the wake and verify a mean flow consistent with the LDV measurements. The existence of these structures in the surface layer is further confirmed by flow visualization using laser‐induced fluorescence. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868866

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The Rayleigh–Taylor instability of an interface separating fluids of distinct density is driven by an acceleration across the interface. Low order statistical moments of fluctuating fluid quantities characterize the hydrodynamics of the mixing zone. A new model is proposed for the momentum coupling between the two phases. This model is validated against computational data for compressible flows, including flows near the incompressible limit. Our main result is a zero parameter first order closure for ensemble averaged two phase flow equations. We do not, however, fully solve the closure problem, as the equations we derive are missing an (internal) boundary condition along any surface for which either phase goes to zero volume fraction. In this sense, the closure problem is reduced from a volume to a surface condition, rather than being solved completely. We compare two formulations of the statistical moments, one based on two phase flow and the other on turbulence models. These formulations describe different aspects of the mixing process. For the problem considered, the two phase flow moments appear to be preferable, in that they subsume the turbulence moments but not conversely. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868863

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The potential for transient growth in compressible boundary layers is studied. Transient amplification is mathematically associated with a non‐orthogonal eigenvector basis, and can amplify disturbances although the spectrum of the linearized evolution operator is entirely confined to the stable half‐plane. Compressible boundary layer flow shows a large amount of transient growth over a wide range of parameter values. The disturbance size is here measured by a positive definite energy like quantity that has been derived such that pressure‐related transfer terms in its evolution equation mutually cancel. The maximum of the transient growth is found for structures which are independent of the streamwise direction and is found to scale withR2. This suggests that the transient growth originates from the same lift‐up mechanism found to give large growth in incompressible shear flows. The maximum growth is also found to increase with Mach number. In compressible flow, disturbances that experience optimal transient growth can be excited naturally by a non‐linear interaction of oblique unstable first mode waves. Thus, a triggering of transient growth may account for the difference in timescales between the fast oblique breakdown process and traditional secondary instability. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868864

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Direct numerical simulation data for two‐point velocity and vorticity correlations at small separations near the centerline of a fully developed turbulent channel flow are more closely approximated by axisymmetry than isotropy. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868865

出版商:AIP    

年代:1996

数据来源: AIP

ISSN:1070-6631    

DOI:10.1063/1.869133

出版商:AIP    

年代:1996

数据来源: AIP