Analysis of flapping motion of reattaching shear layer behind a two-dimensional backward-facing step

2017 ◽  
Vol 29 (11) ◽  
pp. 115104 ◽  
Author(s):  
Xingyu Ma ◽  
Andreas Schröder
Keyword(s):  
Shear Layer ◽  
Two Dimensional ◽  
Backward Facing Step ◽  
Flapping Motion

The Effect of Inlet Turbulence Intensity on the Reattachment Process Over a Backward-Facing Step

1989 ◽  
Vol 111 (1) ◽  
pp. 87-92 ◽  
Author(s):  
K. Isomoto ◽  
S. Honami
Keyword(s):  
Shear Layer ◽  
Turbulence Intensity ◽  
Negative Correlation ◽  
Free Stream ◽  
Grid Turbulence ◽  
Two Dimensional ◽  
Reattachment Length ◽  
Backward Facing Step ◽  
Strong Negative Correlation ◽  
Separated Shear Layer

Behavior of a separated shear layer over a backward-facing step and its reattachment is presented when a two-dimensional cavity or rod is installed upstream of the step in order to change local turbulence intensity in addition to grid turbulence in the free-stream. The reattachment length has a strong negative correlation with maximum turbulence intensity near the wall at the separation point. Turbulence in the entrainment region immediately downstream of the step plays an important role in determining the reattachment length.

Unsteadiness and convective instabilities in two-dimensional flow over a backward-facing step

1996 ◽  
Vol 321 ◽  
pp. 157-187 ◽  
Author(s):  
Lambros Kaiktsis ◽  
George Em Karniadakis ◽  
Steven A. Orszag
Keyword(s):  
Stability Analysis ◽  
Shear Layer ◽  
Random Noise ◽  
Two Dimensional ◽  
Mean Flow ◽  
Backward Facing Step ◽  
External Excitation ◽  
Dimensional Flow ◽  
Two Dimensional Flow ◽  
The Stability

A systematic study of the stability of the two-dimensional flow over a backward-facing step with a nominal expansion ratio of 2 is presented up to Reynolds numberRe= 2500 using direct numerical simulation as well as local and global stability analysis. Three different spectral element computer codes are used for the simulations. The stability analysis is performed both locally (at a number of streamwise locations) and globally (on the entire field) by computing the leading eigenvalues of a base flow state. The distinction is made between convectively and absolutely unstable mean flow. In two dimensions, it is shown that all the asymptotic flow states up toRe= 2500 are time-independent in the absence of any external excitation, whereas the flow is convectively unstable, in a large portion of the flow domain, for Reynolds numbers in the range 700 [les ]Re[les ] 2500. Consequently, upstream generated small disturbances propagate downstream at exponentially amplified amplitude with a space-dependent speed. For small excitation disturbances, the amplitude of the resulting waveform is proportional to the disturbance amplitude. However, selective sustained external excitation (even at small amplitudes) can alter the behaviour of the system and lead to time-dependent flow. Two different types of excitation are imposed at the inflow: (i) monochromatic waves with frequency chosen to be either close to or very far from the shear layer frequency; and (ii) random noise. It is found that for small-amplitude monochromatic excitation the flow acquires a time-periodic behaviour if perturbed close to the shear layer frequency, whereas the flow remains unaffected for high values of the excitation frequency. On the other hand, for the random noise as input, an unsteady behaviour is obtained with a fundamental frequency close to the shear layer frequency.

COMPUTATIONAL STUDY OF THE NON-ISOTHERMAL TWO-DIMENSIONAL PULSED FLOW OVER A CHANNEL WITH A BACKWARD-FACING STEP

2019 ◽  
Author(s):  
Gabriel Machado dos Santos ◽  
Ítalo Augusto Magalhães de Ávila ◽  
Hélio Ribeiro Neto ◽  
Aristeu Silveira Neto
Keyword(s):  
Computational Study ◽  
Two Dimensional ◽  
Backward Facing Step ◽  
Pulsed Flow

The flow over a backward-facing step under controlled perturbation: laminar separation

1992 ◽  
Vol 238 ◽  
pp. 73-96 ◽  
Author(s):  
M. A. Z. Hasan
Keyword(s):  
Shear Layer ◽  
Low Frequency ◽  
Layer Growth ◽  
Stream Velocity ◽  
Backward Facing Step ◽  
Laminar Separation ◽  
Vortex Merging ◽  
Layer Growth Rate ◽  
Merging Process ◽  
Layer Flow

The flow over a backward-facing step with laminar separation was investigated experimentally under controlled perturbation for a Reynolds number of 11000, based on a step height h and a free-stream velocity UO. The reattaching shear layer was found to have two distinct modes of instability: the ‘shear layer mode’ of instability at Stθ ≈ 0.012 (Stθ ≡ fθ/UO, θ being the momentum thickness at separation and f the natural roll-up frequency of the shear layer); and the ‘step mode’ of instability at Sth ≈ 0.185 (Sth ≡ fh/U0). The shear layer instability frequency reduced to the step mode one via one or more stages of a vortex merging process. The perturbation increased the shear layer growth rate and the turbulence intensity and decreased the reattachment length compared to the unperturbed flow. Cross-stream measurements of the amplitudes of the perturbed frequency and its harmonics suggested the splitting of the shear layer. Flow visualization confirmed the shear layer splitting and showed the existence of a low-frequency flapping of the shear layer.

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