Three Dimensional Volumetric Velocity Measurements in the Inner Part of a Turbulent Boundary Layer Over a Rough Wall Using Digital HPIV

2010 ◽  
Author(s):  
Siddharth Talapatra ◽  
Joseph Katz
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Three Dimensional ◽  
Sample Volume ◽  
Rough Wall ◽  
Roughness Sublayer ◽  
Uniform Grid ◽  
Roughness Elements ◽  
Particle Pairs ◽  
Particle Images

Microscopic digital Holographic PIV is used to measure the 3D velocity distributions in the roughness sublayer of a turbulent boundary layer over a rough wall. The sample volume extends from the surface, including the space between the tightly packed, 0.45 mm high, pyramidal roughness elements, up to about 5 roughness heights away from the wall. To facilitate observations though a rough surface, experiments are performed in a facility containing fluid that has the same optical refractive index as the acrylic rough walls. Magnified in line holograms are recorded on a 4864×3248 pixel camera at a resolution of 0.67μm/pixel. The flow field is seeded with 2μm silver coated glass particles, which are injected upstream of the same volume. A multiple-step particle tracking procedure is used for matching the particle pairs. In recently obtained data, we have typically matched ∼5000 particle images per hologram pair. The resulting unstructured 3D vectors are projected onto a uniform grid with spacing of 60 μm in all three directions in a 3.2×1.8×1.8 mm sample volume. The paper provides sample data showing that the flow in the roughness sublayer is dominated by slightly inclined, quasi-streamwise vortices whose coherence is particularly evident close to the top of the roughness elements.

On the evolution of a turbulent boundary layer induced by a three-dimensional roughness element

1992 ◽  
Vol 237 ◽  
pp. 101-187 ◽  
Author(s):  
P. S. Klebanoff ◽  
W. G. Cleveland ◽  
K. D. Tidstrom
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Laminar Boundary Layer ◽  
Roughness Element ◽  
Three Dimensional ◽  
Outer Region ◽  
Stream Velocity ◽  
Mean Velocity ◽  
Roughness Elements

An experimental investigation is described which has as its objectives the extension of the technical data base pertaining to roughness-induced transition and the advancement of the understanding of the physical processes by which three-dimensional roughness elements induce transition from laminar to turbulent flow in boundary layers. The investigation was carried out primarily with single hemispherical roughness elements surface mounted in a well-characterized zero-pressure-gradient laminar boundary layer on a flat plate. The critical roughness Reynolds number at which turbulence is regarded as originating at the roughness was determined for the roughness elements herein considered and evaluated in the context of data existing in the literature. The effect of a steady and oscillatory free-stream velocity on eddy shedding was also investigated. The Strouhal behaviour of the ‘hairpin’ eddies shed by the roughness and role they play in the evolution of a fully developed turbulent boundary layer, as well as whether their generation is governed by an inflexional instability, are examined. Distributions of mean velocity and intensity of the u-fluctuation demonstrating the evolution toward such distributions for a fully developed turbulent boundary layer were measured on the centreline at Reynolds numbers below and above the critical Reynolds number of transition. A two-region model is postulated for the evolutionary change toward a fully developed turbulent boundary layer: an inner region where the turbulence is generated by the complex interaction of the hairpin eddies with the pre-existing stationary vortices that lie near the surface and are inherent to a flow about a three-dimensional obstacle in a laminar boundary layer; and an outer region where the hairpin eddies deform and generate turbulent vortex rings. The structure of the resulting fully developed turbulent boundary layer is discussed in the light of the proposed model for the evolutionary process.

Direct numerical simulation of the turbulent boundary layer over a rod-roughened wall

2007 ◽  
Vol 584 ◽  
pp. 125-146 ◽  
Author(s):  
SEUNG-HYUN LEE ◽  
HYUNG JIN SUNG
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Reynolds Stress ◽  
Outer Layer ◽  
Turbulent Transport ◽  
Rough Wall ◽  
Momentum Thickness ◽  
Velocity Fluctuations ◽  
Stress Tensors ◽  
Roughness Elements

The effects of surface roughness on a spatially developing turbulent boundary layer (TBL) are investigated by performing direct numerical simulations of TBLs over rough and smooth walls. The Reynolds number based on the momentum thickness was varied in the range Reθ = 300 ∼ 1400. The roughness elements were periodically arranged two-dimensional spanwise rods, and the roughness height was k = 1.5θin, where θin is the momentum thickness at the inlet, which corresponds to k/δ = 0.045 ∼ 0.125, δ being the boundary layer thickness. To avoid generating a rough-wall inflow, which is prohibitively difficult, a step change from smooth to rough was placed 80θin downstream from the inlet. The spatially developing characteristics of the rough-wall TBL were examined. Along the streamwise direction, the friction velocity approached a constant value, and self-preserving forms of the turbulent Reynolds stress tensors were obtained. Introduction of the roughness elements affected the turbulent stress not only in the roughness sublayer but also in the outer layer. Despite the roughness-induced increase of the turbulent Reynolds stress tensors in the outer layer, the roughness had only a relatively small effect on the anisotropic Reynolds stress tensor in the outer layer. Inspection of the triple products of the velocity fluctuations revealed that introducing the roughness elements onto the smooth wall had a marked effect on vertical turbulent transport across the whole TBL. By contrast, good surface similarity in the outer layer was obtained for the third-order moments of the velocity fluctuations.

The turbulent boundary-layer rough-wall pressure spectrum at acoustic and subconvective wavenumbers

1988 ◽  
Vol 415 (1848) ◽  
pp. 141-161 ◽  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Fluctuations ◽  
Wall Pressure ◽  
Plane Wall ◽  
Rough Wall ◽  
Reynolds Stresses ◽  
Low Frequencies ◽  
Wall Boundary ◽  
Roughness Elements

The rough-wall turbulent boundary-layer wall pressure spectrum differs from that on a smooth wall because the strengths of the turbulence Reynolds stresses responsible for the pressure fluctuations are increased by surface roughness, and because the near fields of those enhanced pressure sources are redistributed in wavenumber by diffraction by the roughness elements. In this paper, analyses are performed that indicate that, except at very low frequencies, the diffractive component dominates the spectrum in the low-wavenumber and acoustic domains. The diffraction calculations are performed exactly for a plane wall roughened by a distribution of rigid hemispherical bosses, and a comparison is made with corresponding predictions obtained when only the first non-trivial term is retained in an expansion of the rough-wall boundary condition about that for a plane wall. These calculations take no account of interstitial flows and possible wake formation by the roughness elements, and an empirical model of the spectrum is proposed to incorporate these effects and recommended for use in applications. The values of adjustable coefficients that enter the proposed formula are estimated by reference to published experimental data on the radiation of sound by a rough-wall boundary layer. Such data are incomplete, however, and further experiments need to be performed to validate the model.

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