Forest of hairpins in a low-Reynolds-number zero-pressure-gradient flat-plate boundary layer

2009 ◽  
Vol 21 (9) ◽  
pp. 091106 ◽  
Author(s):  
Xiaohua Wu ◽  
Parviz Moin
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Flat Plate ◽  
Low Reynolds Number ◽  
Plate Boundary ◽  
Low Reynolds ◽  
Flat Plate Boundary Layer

Direct numerical simulation of turbulence in a nominally zero-pressure-gradient flat-plate boundary layer

2009 ◽  
Vol 630 ◽  
pp. 5-41 ◽  
Author(s):  
XIAOHUA WU ◽  
PARVIZ MOIN
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Stress ◽  
Pressure Gradient ◽  
Flat Plate ◽  
Plate Boundary ◽  
Transitional Region ◽  
Bypass Transition ◽  
Number Range ◽  
Flat Plate Boundary Layer

A nominally-zero-pressure-gradient incompressible boundary layer over a smooth flat plate was simulated for a continuous momentum thickness Reynolds number range of 80 ≤ Reθ ≤ 940. Transition which is completed at approximately Reθ = 750 was triggered by intermittent localized disturbances arising from patches of isotropic turbulence introduced periodically from the free stream at Reθ = 80. Streamwise pressure gradient is quantified with several measures and is demonstrated to be weak. Blasius boundary layer is maintained in the early transitional region of 80 < Reθ < 180 within which the maximum deviation of skin friction from the theoretical solution is less than 1%. Mean and second-order turbulence statistics are compared with classic experimental data, and they constitute a rare DNS dataset for the spatially developing zero-pressure-gradient turbulent flat-plate boundary layer. Our calculations indicate that in the present spatially developing low-Reynolds-number turbulent flat-plate boundary layer, total shear stress mildly overshoots the wall shear stress in the near-wall region of 2–20 wall units with vanishing normal gradient at the wall. Overshoots as high as 10% across a wider percentage of the boundary layer thickness exist in the late transitional region. The former is a residual effect of the latter. The instantaneous flow fields are vividly populated by hairpin vortices. This is the first time that direct evidence (in the form of a solution of the Navier–Stokes equations, obeying the statistical measurements, as opposed to synthetic superposition of the structures) shows such dominance of these structures. Hairpin packets arising from upstream fragmented Λ structures are found to be instrumental in the breakdown of the present boundary layer bypass transition.

scholarly journals Persistence of the laminar regime in a flat plate boundary layer at very high Reynolds number

2006 ◽  
Vol 10 (2) ◽  
pp. 63-96 ◽  
Author(s):  
Jovan Jovanovic ◽  
Bettina Frohnapfel ◽  
Emir Skaljic ◽  
Milenko Jovanovic
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Flat Plate ◽  
High Reynolds Number ◽  
Plate Boundary ◽  
Laminar Regime ◽  
High Reynolds ◽  
Very High ◽  
Flat Plate Boundary Layer

Roughness receptivity and shielding in a flat plate boundary layer

2015 ◽  
Vol 777 ◽  
pp. 430-460 ◽  
Author(s):  
Matthew S. Kuester ◽  
Edward B. White
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Reynolds Number ◽  
Flat Plate ◽  
Plate Boundary ◽  
Boundary Layer Transition ◽  
Shielding Effect ◽  
Transient Growth ◽  
The Impact ◽  
Flat Plate Boundary Layer

Surface roughness can affect boundary layer transition by acting as a receptivity mechanism for transient growth. While experiments have investigated transient growth of steady disturbances generated by discrete roughness elements, very few have studied distributed surface roughness. Some work predicts a ‘shielding’ effect, where smaller distributed roughness displaces the boundary layer away from the wall and lessens the impact of larger roughness peaks. This work describes an experiment specifically designed to study this effect. Three roughness configurations (a deterministic distributed roughness patch, a slanted rectangular prism, and the combination of the two) were manufactured using rapid prototyping and installed flush with the wall of a flat plate boundary layer. Naphthalene flow visualization and hotwire anemometry were used to characterize the boundary layer in the wakes of the different roughness configurations. Distributed roughness with roughness Reynolds numbers ($\mathit{Re}_{kk}$) between 113 and 182 initiated small-amplitude disturbances that underwent transient growth. The discrete roughness element created a pair of high- and low-speed steady streaks in the boundary layer at a sub-critical Reynolds number ($\mathit{Re}_{kk}=151$). At a higher Reynolds number ($\mathit{Re}_{kk}=220$), the discrete element created a turbulent wedge 15 boundary layer thicknesses downstream. When the distributed roughness was added around the discrete roughness, the discrete element’s wake amplitude was decreased. For the higher Reynolds number, this provided a small but measurable transition delay. The distributed roughness redirects energy from longer spanwise wavelength modes to shorter spanwise wavelength modes. The presence of the distributed roughness also decreased the growth rate of secondary instabilities in the roughness wake. This work demonstrates that shielding can delay roughness-induced transition and lays the ground work for future studies of roughness-induced transition.

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