Stage Of Laminar-turbulent Transition Research Articles

Evolution of kinematic structure of the flow behind a transverse rib for transitional flow regimes

Results are presented on visualization of a separated flow behind two kinds of transverse ribs in a channel for a range of Reynolds numbers covering the different stages of laminar-turbulent flow transition. The data was obtained on dynamics of kinematic structure of flow and on evolution of large-scale transverse vortex structures which were generated in the mixing layer during late stages of laminar-turbulent transition. The qualitative estimates were obtained for the vortex generation frequency and velocity of their convective transfer. The features of flow structure were identified for flow behind tested shapes of ribs.

Vortical structure dynamics of separated channel flow behind a semicylindrical rib at late stages of laminar-turbulent transition

Vortical structure dynamics of separated channel flow behind a semicylindrical rib at late stages of laminar-turbulent transition

Non-equilibrium, nonlinear critical layers in laminar-turbulent transition

We describe some recent developments of high-Reynolds-number asymptotic theory for the nonlinear stage of laminar-turbulent transition in nearly parallel flows. The classic weakly nonlinear theory of Landau and Stuart is briefly revisited with the dual purposes of highlighting its fundamental ideas, which continue to underlie much of current theoretical thinking, as well as its difficulty in dealing with unbounded flows. We show that resolving such a difficulty requires an asymptotic approach based on the high-Reynolds-number assumption, which leads to a nonlinear critical-layer theory. Major recent results are reviewed with emphasis on the non-equilibrium effect. Future directions of investigation are indicated.

Numerical Modeling of the Development of a Streaky Structure in a Boundary Layer at a High Freestream Turbulence Level

The disturbances generated by external turbulence in the boundary layer on a flat plate set suddenly in motion are determined. A turbulent flow calculated by direct numerical simulation is taken as the initial conditions. The solution obtained simulates the initial stage of laminar-turbulent transition in the flat-plate boundary layer at a high turbulence level in the oncoming flow. The solution makes it possible to estimate the effects of different factors, such as nonstationarity, nonlinearity, and the parameters of the freestream velocity fluctuation spectrum, on disturbance enhancement in the boundary layer.

Spatial Evolution of Nonstationary Perturbations in Hamel Flow

The propagation of small perturbations in longitudinally inhomogeneous flows is investigated. The evolution of the perturbations is studied with reference to the radial flow of a viscous incompressible fluid between plane nonparallel walls, the simplest inhomogeneous flow. Using a generalized method of variation of constants, the corresponding boundary-value problem is reduced to an infinite-dimensional evolutionary system of ordinary differential equations for the complex amplitudes of the eigensolutions of a locally homogeneous problem. Physically, the method can be interpreted as a representation of the perturbation evolution process via two concomitant processes: the independent amplification (attenuation) of normal modes of the locally homogeneous problem and the rescattering of these modes into each other on local inhomogeneities of the base flow. The calculations show that reduced versions of the method are promising for describing the linear stage of laminar-turbulent transition in a boundary layer.

Experimental and computational study of laminar cavity flows at hypersonic speeds

This paper presents a combined experimental/computational study of a surface cavity in a low Reynolds number Mach 9 flow. The geometry is based on a body of revolution, which produces highly two-dimensional time-averaged flow for all experimental test cases. A range of cavity length-to-depth ratios, up to a maximum of 8, is investigated. These correspond to ‘closed’ cavity flows, with the free shear layer bridging the entire cavity. For most cases the free shear layer is laminar. However, there is evidence of three-dimensional unsteadiness which is believed to be the consequence of Taylor–Görtler-type vortex formation. The effect of this is first detected on the cavity floor but progressively spreads as the cavity length is increased. For the longest cavities the flow is also influenced by the early stages of laminar–turbulent transition in the free shear layer.

Improved methods for measuring laminar-turbulent intermittency in boundary layers

The last stage of laminar-turbulent transition in boundary layers is commonly described using the spatial distribution of intermittency. Existing methods for computing this laminar-turbulent intermittency from experimental data involve the use of thresholds that are set by the experimentalist using subjective comparisons to the raw signal. These threshold settings cannot be reproduced by other experimentalists using different equipment, so a precise determination of intermittency is not currently possible. This note reports on a new method of determining boundary-layer intermittency that appears to be objective and reproducible. The wall shear was measured in a flat-plate boundary layer using hot-film sensors. Probability density functions (PDF's) of the calibrated wall shear are remarkably consistent. A correlation to the turbulent portion of these PDF's is given. The consistency observed in these PDF's suggests an objective and reproducible setting for the laminar-turbulent cutoff threshold. Intermittencies determined using this method can be compared quantitatively, with any differences being caused only by experimental error or differences in the flows. The universality of the method can be determined through comparisons to measurements in other flows.

Multiple instabilities of three-dimensional boundary layers along a concave wall

The initial stage of laminar turbulent transition in boundary layers on a swept wing is governed by two types of instability, namely, the Tollmien-Schlichting instability based on the effect of viscosity and the cross-flow instability associated with the twisted velocity distribution of three-dimensional boundary layers. If the flow is along a locally concave surface, there is another possibility, that of Taylor–Görtler instability induced by centrifugal force. The strength of these instabilities mainly depends upon local Reynolds number, magnitude and direction of the cross-flow, and local curvature of the wall. Thus a model system of linear equations is proposed to describe the initial development of disturbances through the three instabilities, and its eigensolutions are used to evaluate stability limits of the Falkner Skan-Cooke flow to those disturbances. The three types of disturbances can be distinguished from each other by definite differences in the wavenumber and frequency regions. Three subregions in the parameter plane of cross-flow magnitude and wall curvature are determined in correspondence to the three instabilities, each of which gives the lowest value of critical Reynolds numbers in its own Subregion.

Velocity disturbance structure in the nonlinear stage of laminar-turbulent transition

The Herbert theory [1, 2] of laminar turbulent transition is generalized by taking into account the development of the secondary disturbances with allowance for their higher harmonics. The results of calculations of the development of three-dimensional pulsations, based on this theory, are in a agreement with the Herbert results in the initial stage of development and with the results of direct numerical modeling, obtained in this paper, and with experiment [3] in the later nonlinear stages of transition.

Behavior of secondary flows in a rotating pipe

The later stages of laminar-turbulent transition are studied by direct numerical integration of the Navier-Stokes equations. A sequence of flow regimes which replace each other as the Reynolds number increases is obtained. In the coordinate system moving with the traveling wave this sequence includes steady, periodic, quasiperiodic and chaotic motions. The chaotic motion is preceded by synchronization of the frequencies of the quasiperiodic regime.