Low-frequency, large-amplitude fluctuations of the laminar boundary layer.

AIAA Journal ◽  
1966 ◽  
Vol 4 (6) ◽  
pp. 994-1001 ◽  
Author(s):  
W. S. KING
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Large Amplitude ◽  
Low Frequency ◽  
Amplitude Fluctuations

The late stages of transition induced by a low-amplitude wavepacket in a laminar boundary layer

1997 ◽  
Vol 340 ◽  
pp. 395-411 ◽  
Author(s):  
KENNETH S. BREUER ◽  
JACOB COHEN ◽  
JOSEPH H. HARITONIDIS
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Wavelet Transforms ◽  
Random Phase ◽  
Low Frequency ◽  
Small Scale ◽  
Frequency Spectra ◽  
Weakly Nonlinear ◽  
Lower Amplitude ◽  
Low Amplitude

The evolution of a wavepacket in a laminar boundary layer is studied experimentally, paying particular attention to the stage just prior to the formation of a turbulent spot. The initial stages of development are found to be in very good agreement with previous results and indicate a stage in which the disturbance grows according to linear theory followed by a weakly nonlinear stage in which the subharmonic grows, apparently through a parametric resonance mechanism. In a third stage, strong non-linear interactions are observed in which the disturbance develops a streaky structure and the corresponding wavenumber–frequency spectra exhibit an organized cascade mechanism in which spectral peaks appear with increasing spanwise wavenumber and with frequencies which alternate between zero and the subharmonic frequency. Higher harmonics are also observed, although with lower amplitude than the low-frequency peaks. The final (breakdown) stage is characterized by the appearance of high-frequency oscillations with random phase, located at low-speed ‘spike’ regions of the primary disturbance. Wavelet transforms are used to analyse the structure of both coherent and random small-scale structure of the disturbance. In particular, the breakdown oscillations are also observed to have a wavepacket character riding on the large-amplitude primary disturbance.

Simulations of bypass transition

2001 ◽  
Vol 428 ◽  
pp. 185-212 ◽  
Author(s):  
R. G. JACOBS ◽  
P. A. DURBIN
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Free Stream ◽  
Low Frequency ◽  
Numerical Data ◽  
Bypass Transition ◽  
Mean Velocity ◽  
Free Stream Turbulence ◽  
Turbulent Inflow ◽  
Irregular Motion

Bypass transition in an initially laminar boundary layer beneath free-stream turbulence is simulated numerically. New perspectives on this phenomenon are obtained from the numerical flow fields. Transition precursors consist of long backward jets contained in the fluctuating u-velocity field; they flow backwards relative to the local mean velocity. The jets extend into the upper portion of the boundary layer, where they interact with free-stream eddies. In some locations a free-stream perturbation to the jet shear layer develops into a patch of irregular motion – a sort of turbulent spot. The spot spreads longitudinally and laterally, and ultimately merges into the downstream turbulent boundary layer. Merging spots maintain the upstream edge of the turbulent region. The jets, themselves, are produced by low-frequency components of the free-stream turbulence that penetrate into the laminar boundary layer. Backward jets are a component of laminar region streaks.A method to construct turbulent inflow from Orr–Sommerfeld continuous modes is described. The free-stream turbulent intensity was chosen to correspond with the experiment by Roach & Brierly (1990). Ensemble-averaged numerical data are shown to be in good agreement with laboratory measurements.

Calculation of the primary trajectories of spores ejected into still air and laminar flow

1988 ◽  
Vol 233 (1273) ◽  
pp. 477-486 ◽  
Keyword(s):  
Boundary Layer ◽  
Gravitational Field ◽  
Laminar Flow ◽  
Laminar Boundary Layer ◽  
Low Frequency ◽  
Viscous Friction ◽  
Spherical Particles ◽  
Horizontal Wind ◽  
Boundary Layer Flows ◽  
Family Of Curves

The plane ballistic trajectories followed by small spherical particles representing a variety of spores ejected into still air are determined for motion in a constant gravitational field and resisted according to the Stokes law of viscous friction. These trajectories are presented in terms of a universal one-parameter formula together with a family of curves defining the ground ranges likely to be achieved in a majority of practical cases. Consideration is also given to the movement of spores ejected into linear laminar boundary-layer flows that are both steady and periodic in time in the downstream velocity. The solutions show that projection to the largest possible vertical and downstream altitude, and also the presence of temporal periodic horizontal wind motion of low frequency and favourable phasing, are both factors of significance in augmenting the ground range achieved.

Response of a compressible laminar boundary layer to free-stream vortical disturbances

2007 ◽  
Vol 587 ◽  
pp. 97-138 ◽  
Author(s):  
PIERRE RICCO ◽  
XUESONG WU
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Free Stream ◽  
Three Dimensional ◽  
Low Frequency ◽  
Boundary Region ◽  
Free Stream Turbulence ◽  
Local Boundary ◽  
The Individual ◽  
Vortical Disturbance

As a first step towards understanding the role of free-stream turbulence in laminar–turbulent transition, we calculate the fluctuations induced by free-stream vortical disturbances in a compressible laminar boundary layer. As with the incompressible case investigated by Leibet al. (J. Fluid Mech. vol. 380, 1999, p. 169), attention is focused on components with long streamwise wavelength. The boundary-layer response is governed by the linearized unsteady boundary-region equations in the typical streamwise region where the local boundary-layer thickness δ* iscomparable with the spanwise length scale Λ of the disturbances. The compressible boundary-region equations are solved numerically for a single Fourier component to obtain the boundary-layer signature. The root-mean-square of the velocity and mass-flux fluctuations induced by a continuous spectrum of free-stream disturbances are computed by an appropriate superposition of the individual Fourier components.Low-frequency vortical disturbances penetrate into the boundary layer to form slowly modulating streamwise-elongated velocity streaks. In the compressible regime, vortical disturbances are found to induce substantial temperature fluctuations so that ‘thermalstreaks’ also form. They may have a significant effect on the secondary instability. The calculations indicate that for a vortical disturbance with a relatively large Λ, the induced boundary-layer fluctuation ultimately evolves into an amplifying wave. This is due to a receptivity mechanism, in which a vortical disturbance first excites a decaying quasi-three-dimensional Lam–Rott eigensolution. The latter then undergoes wavelength shortening to generate a spanwise pressure gradient, which eventually converts the Lam–Rott mode into an exponentially growing mode. The latter is recognized to bea highly oblique Tollmien–Schlichting wave. A parametric study suggests that this receptivity mechanism could be significant when the free-stream Mach number is larger than 0.8.

Automatic control of laminar boundary-layer transition

AIAA Journal ◽  
1997 ◽  
Vol 35 ◽  
pp. 85-90
Author(s):  
P. A. Nelson ◽  
M. C. M. Wright ◽  
J.-L. Rioual
Keyword(s):  
Boundary Layer ◽  
Automatic Control ◽  
Laminar Boundary Layer ◽  
Boundary Layer Transition ◽  
Layer Transition
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