Secondary mean motions arising in a buoyancy induced flow

Audunson, T.; Gebhart, B.

doi:10.1016/0017-9310(76)90126-5

Cited by 18 publications

(2 citation statements)

References 25 publications

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“…This ' selective amplification ' was first demonstrated by the calculations of Dring & Gebhart (1968). Experiment (Jaluria & Gebhart 1975) and theory (Audunson & Gebhart 1976) have shown that this selectivity persists downstream into the region where nonlinear effects and secondary mean flows develop. This behaviour differs from that of external forcedconvection boundary layers, which amplify a wide band of disturbance component frequencies.…”

Section: Introductionmentioning

confidence: 87%

Instability and disturbance amplification in a mixed-convection boundary layer

Brewster

Gebhart

1991

J. Fluid Mech.

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The stability of a laminar mixed-convection boundary layer adjacent to a vertical isothermal surface is examined, using linear stability theory and the parallel-flow approximations. The analysis is valid when the imposed forced-convection effects are small compared to natural-convection effects. The stability equations are solved numerically for aiding and opposing forced-convection effects, for Pr = 0.733 (air) and 6.7 (water). For aiding mixed convection in air, a new feature was found. A small, separated region of instability arises upstream of the ‘conventional’, or ‘primary’, neutral curve. In this region, selective amplification of a narrow band of disturbance frequencies occurred, but disturbance growth was small. Further downstream, disturbance growth rates in flows with an aiding free stream are slower than in natural convection. The opposite is true for an opposing free stream in air. Selective disturbance amplification occurred downstream for all conditions, as in natural convection. In water, an aiding flow was destablizing compared to natural convection, and an opposing flow was stabilizing. Evidence of a separated upstream region of instability was also found for aiding mixed convection in water. However, converged solutions could not be obtained in this circumstance.

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Section: Introductionmentioning

confidence: 87%

Instability and disturbance amplification in a mixed-convection boundary layer

Brewster

Gebhart

1991

J. Fluid Mech.

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“…Further downstream, nonlinear and three-dimensional effects become important, as evidenced by the earlier experimental studies by Eckert, Hartnett & Irvine (1960) and Colak-Antic (1964) in air and by Szewczyk (1962) in water. Audunson & Gebhart (1976) included spanwise effects in their analysis of disturbance growth in air and found that nonlinear interactions of two-and three-dimensional disturbances generate a mean secondary double longitudinal vortex system. This flow modification then promotes favourable conditions for even more rapid disturbance growth, in forming a region of high shear.…”

Section: Introductionmentioning

confidence: 99%

An experimental determination of transition limits in a vertical natural convection flow adjacent to a surface

Mahajan

Gebhart

1979

J. Fluid Mech.

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This paper reports the results of an experimental investigation to determine transition mechanisms and limits in gases at high pressure levels. We sought also to refine further the parameters for transition, in particular the role of kinematic viscosity. In flow adjacent to a vertical uniform-flux surface in nitrogen, pressures to 16 atm were used. Both mean and disturbance quantities for the temperature and velocity fields were measured for various values of the heat flux, downstream location and ambient pressure level. Hot-wire and fine thermocouple probes were used. We found that the velocity and thermal fields remain closely coupled. Velocity, or fluid-dynamic, transition is immediately followed by thermal transition. Each was detected as a decrease in the rate of increase of both the maximum velocity and the overall temperature difference, respectively, from the laminar downstream trends. Also, the ends of transition for the velocity and the thermal fields, respectively, signalled by no further appreciable change in the intermittency distributions, were simultaneous. These results re-affirm the finding that the events of transition are not correlated by the Grashof number alone. An additional dependence on both downstream location and pressure level arises. A fixed value of the parameter $Q_{BT} = qB^{\frac{2}{15}}= 290$ characterizes the beginning of transition, where q is the fifth root of the local non-dimensional wall heat flux and B is the unit Grashof number. The end of transition, on the other hand, is best correlated by $Q_{ET} = QB^{\frac{1}{30}} = 11.4$, where Q is the fifth root of the local non-dimensional total heat convected in the boundary region. A re-examination of other transition studies, in both gases and liquids, supports these correlations, although many such data were not determined with fast response to local sensors. There remains a small level of uncertainty in establishing exact limits for transition, since the apparently proper standards for determining them are very difficult to apply precisely in experiments. However, such limits are very important in separating regimes of different transport mechanisms.

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Basic Statements and Equations of Free Convection

Free-Convective Heat Transfer

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Secondary mean motions arising in a buoyancy induced flow

Cited by 18 publications

References 25 publications

Instability and disturbance amplification in a mixed-convection boundary layer

Instability and disturbance amplification in a mixed-convection boundary layer

An experimental determination of transition limits in a vertical natural convection flow adjacent to a surface

Basic Statements and Equations of Free Convection

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