Large Amplitude, Low Frequency Solutions for a Certain Class of Laminar Boundary-Layer Problems

AIAA Journal ◽  
1974 ◽  
Vol 12 (3) ◽  
pp. 265-266
Author(s):  
JAMES C. WILLIAMS
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Large Amplitude ◽  
Low Frequency ◽  
Boundary Layer Problems

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.

Sign in / Sign up

Export Citation Format

Share Document