Measurement of Transitional Surface Roughness Effects on Flat-Plate Boundary Layer Transition

2019 ◽  
Vol 141 (7) ◽  
Author(s):  
Heechan Jeong ◽  
Seung Woo Lee ◽  
Seung Jin Song
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulence Intensity ◽  
Plate Boundary ◽  
Boundary Layer Transition ◽  
Mean Velocity ◽  
Layer Transition ◽  
Turbulence Intermittency ◽  
Transition Length ◽  
Flat Plate Boundary Layer

An experimental study has been conducted to investigate the effects of transitionally rough surface on the flat-plate boundary layer transition. Transitional boundary layers with three different flat plates (ks+ = 0.07 ∼ 0.19, 2.71 ∼ 7.05, and 13.65 ∼ 41.09) have been measured with a single-sensor hot-wire probe. All of the measurements have been conducted under zero pressure gradient (ZPG) at the fixed Reynolds number (ReL) and freestream turbulence intensity (Tu) of 3.05 × 106 and 0.2%. Transitionally, rough surface does not affect the sigmoidal distribution of turbulence intermittency model; but induces earlier transition onset and shortens the transition length. For all surfaces, streamwise turbulence intensity profiles with similar values of turbulence intermittency are similar for the transition length less than 60%. Therefore, mean velocity profiles with the similar values of turbulence intermittency are similar regardless of surface conditions. However, downstream of 60% of the transition length, mean velocity defect increases as the surface roughness increases. Enhanced diffusion of turbulent kinetic energy from the near wall (y/δ < 0.1) to the outer part (y/δ ≈ 0.4) of the boundary layer due to the surface roughness is responsible for the increased momentum deficit.

Evolution of Lagrangian coherent structures in a cylinder-wake disturbed flat plate boundary layer

2016 ◽  
Vol 792 ◽  
pp. 274-306 ◽  
Author(s):  
Guo-Sheng He ◽  
Chong Pan ◽  
Li-Hao Feng ◽  
Qi Gao ◽  
Jin-Jun Wang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Coherent Structures ◽  
Plate Boundary ◽  
Boundary Layer Transition ◽  
Lagrangian Coherent Structures ◽  
Hydrogen Bubble ◽  
Layer Transition ◽  
Low Speed Streaks ◽  
Flat Plate Boundary Layer

Evolution of Lagrangian coherent structures (LCS) in a flat plate boundary layer transition induced by the wake of a circular cylinder is investigated. Both hydrogen bubble visualization and particle image velocimetry (PIV) techniques are used. It is found that downstream of the cylinder, the disturbance in the boundary layer experiences a fast growth followed by a slow decay in the transition. Lagrangian coherent structures are revealed by qualitative hydrogen bubble visualizations and quantitative finite-time Lyapunov exponents (FTLE) fields derived from the PIV data. The evolution of the LCS is considered from the very beginning of the transition up to when the boundary layer becomes fully developed turbulent flow. The mean convection velocity and average inclination angle of the LCS are first extracted from the FTLE fields. The streamwise length of the low-speed streaks seems to increase, while their spanwise distance decreases in the boundary layer transition. Proper orthogonal decomposition (POD) of the PIV data shows that low-speed streaks associated with the hairpin vortices and hairpin packets are the dominant coherent structures close to the wall in the transitional and turbulent boundary layer. The POD modes also reveal a variety of scales in the turbulent boundary layer. Moreover, it is found that large-scale coherent structures can modulate the amplitude of the small-scale ones.

Boundary Layer Transition Induced by Periodic Wakes

1998 ◽  
Vol 122 (3) ◽  
pp. 442-449 ◽  
Author(s):  
Xiaohua Wu ◽  
Paul A. Durbin
Keyword(s):  
Numerical Simulation ◽  
Boundary Layer ◽  
Direct Numerical Simulation ◽  
Skin Friction ◽  
Plate Boundary ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Layer Transition ◽  
Benchmark Data ◽  
Flat Plate Boundary Layer

Turbulent wakes swept across a flat plate boundary layer simulate the phenomenon of wake-induced bypass transition. Benchmark data from a direct numerical simulation of this process are presented and compared to Reynolds-averaged predictions. The data are phase-averaged skin friction and mean velocities. The predictions and data are found to agree in many important respects. One discrepancy is a failure to reproduce the skin friction overshoot following transition. [S0889-504X(00)00503-1]

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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