摘要:

Patterns of laminar flow development in the entrance region of an elliptical slit have been determined experimentally by hot‐wire anemometry. The inlet cross section is circular, tapering downstream into elliptical sections of increasing eccentricity. The cross‐sectional area is maintained constant throughout, to facilitate an assessment of wall effects on the developing flow patterns. Flow development along the major axis is found to lag the change in geometric contour, so that axisymmetric inlet profiles do not acquire the elliptical symmetry predicted by conformal mapping. Nevertheless, flow distributions are invariant in the mean: axial partial flows through downstream elliptical rings are nearly the same as on corresponding circular rings at the inlet. This near‐invariance holds approximately even for uniformly‐curved pipe sections in which intense swirl is present, and so may have more general application to conduit flows than just for the examples reported here.

ISSN:0031-9171    

DOI:10.1063/1.865575

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

This Letter is concerned with the effect of nonlinear critical layers on spatially growing Tollmien–Schlichting waves in a Blasius boundary layer. It is shown that they effectively eliminate the upper branch of the neutral stability curve predicted by strictly linear theory.

ISSN:0031-9171    

DOI:10.1063/1.865576

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A new, self‐similar, turbulent jet flow is postulated in which the global vorticity &OHgr; is a constant following any vortex. In contrast, conventional self‐similar flows all exhibit &OHgr; inversely proportional to the vortex age. In order for the new, postulated flow to exist, the nozzle exit speed must increase exponentially with time. It is shown that this circumstance may strongly inhibit the entrainment of ambient fluid into the jet.

ISSN:0031-9171    

DOI:10.1063/1.865577

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

From the direct interaction approximation (DIA) equations for strong turbulence it is shown that low‐frequency fluctuations constituting two‐dimensional strong turbulence in a magnetized plasma scatter off each other to develop a finite spread in wave vectors along the magnetic field and are thereby Landau‐damped. A transport equation in (k⊥,k∥) space is developed that determines the wave spectrum under these conditions.

ISSN:0031-9171    

DOI:10.1063/1.865525

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Analytical solutions of the Navier–Stokes equations for flows in circular pipes whose cross‐sectional area varies slowly in the axial direction are given. Specific shapes of the pipe wall are determined to give axial velocity profiles that are independent of the Reynolds number.

ISSN:0031-9171    

DOI:10.1063/1.865526

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The scalar field (heat or species concentration) of a small‐source laminar starting plume was measured at a single dimensionless buoyancy injection rate for two values of the ratio of momentum diffusivity to scalar diffusivity, Pr=7 and 300. Digital image processing was used to extract concentration contours from the interferograms, which considerably reduced the data reduction time and permitted an improved resolution. The effect of Pr was clearly visible in the scalar contours, especially in terms of the entrained fluid that is found to be undiluted far inside the cap of the higher Pr. The scalar contours were not self‐similar when a single length scale was used, but the data collapsed well using a two‐length scale normalization. The convective velocity of the cap was also measured for the two cases, suggesting a possible logarithmic effect of Pr. An analysis confirms the speculation. It was also found that about 37% of the total injectionrateinto the starting plume is convected into the cap. This work is the first experimental investigation demonstrating the effect of change in Pr on the motion of small buoyancy‐source laminar gravitational convection phenomena.

ISSN:0031-9171    

DOI:10.1063/1.865527

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Using a Green’s function approach with a dipole formulation, it is possible to find the shape of a two‐dimensional steady Taylor bubble satisfying Bernoulli’s equation. This is done for the case of bubbles rising in a vertical channel. Our inviscid treatment leads to an infinite set of shapes. Using experimental evidence, however, it is possible to obtain one shape and a comparison with experiments is presented. The potential bubble so chosen has the distinction that it is the only one in the family of potential curves with a circular nose. An integral mass and momentum balance analysis in the liquid region between the free surface and the walls is also conducted, which takes fluid viscosity into account. By requiring a match between the two techniques at the nose, a viscous bubble is obtained that agrees well with both the potential bubble and experiments.

ISSN:0031-9171    

DOI:10.1063/1.865528

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The representation of an inviscid three‐dimensional incompressible flow by vortex singularities is considered and shown to lead to dynamical inconsistencies.

ISSN:0031-9171    

DOI:10.1063/1.865529

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A family of exact solutions of the Euler equations is presented: they are generalizations of the Kirchhoff vortex toNconfocal ellipses. Special attention is given to the caseN=2, for which the stability is analyzed with a method similar to the one used by Love [Proc. London Math. Soc.1, XXV 18 (1893)] for the Kirchhoff vortex. The results are compared with those for the corresponding circular problem.

ISSN:0031-9171    

DOI:10.1063/1.865530

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The stability analysis of boundary layers over curved surfaces is presented. A simple inviscid stability criterion is developed and used to identify shear layers which are potentially unstable with respect to the Go¨rtler‐type disturbances. A flow is stable if the velocity magnitude increases with distance away from the wall in the case of convex surfaces and decreases in the case of concave surfaces. The flows with nonmonotonic velocity distributions are therefore always potentially unstable regardless of the type of surface being considered. When viscous stability theory is applied to selected boundary layers the results confirm conclusions based on the inviscid theory. Detailed calculations are carried out for the Blasius boundary layer (monotonic velocity distribution) and a wall jet (nonmonotonic velocity distribution). Results suggest that flows with monotonic velocity distributions become unstable earlier than flows with nonmonotonic velocity distributions.

ISSN:0031-9171    

DOI:10.1063/1.865531

出版商:AIP    

年代:1986

数据来源: AIP