The Experts below are selected from a list of 321 Experts worldwide ranked by ideXlab platform
Markus J Kloker - One of the best experts on this subject based on the ideXlab platform.
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receptivity of a swept wing boundary layer to micron sized discrete Roughness elements
Journal of Fluid Mechanics, 2014Co-Authors: Holger B E Kurz, Markus J KlokerAbstract:The receptivity of a laminar swept-wing boundary layer to a spanwise array of circular Roughness elements is investigated by means of direct numerical simulations (DNS). The initial amplitude of a steady crossflow mode generated by the shallow Roughness elements does not vary strictly linearly with the Roughness Height, as often assumed. Rather, a fundamental, superlinear dependence of the receptivity amplitude on the Roughness Height is found. In order to account for shape effects, the Roughness geometry is Fourier decomposed to its spanwise spectral content, and elements with a reduced spectrum are investigated. If only modes are present that synthesise a regular structure of alternating bumps and dimples of equal shape and size, the receptivity amplitude is strictly linear for each mode and nominal Roughness Heights up to at least 15 % of the local displacement thickness.
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Effects of a Discrete Medium-Sized Roughness in a Laminar Swept-Wing Boundary Layer
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2014Co-Authors: Holger B E Kurz, Markus J KlokerAbstract:Direct numerical simulations of the effects of a cylindrical Roughness element in the laminar 3-d boundary layer on the upper surface of a swept wing are performed. The Roughness element generates streamwise vortices, where one is persistently growing in streamwise direction due to crossflow instability. By varying the Roughness Height, the onset of secondary instability of this crossflow vortex and ultimate transition to turbulence varies also in streamwise direction. When reaching the “effective”, i.e. the flow-tripping Roughness Height, the linear crossflow-instability regime is bypassed and breakdown to turbulence occurs in close vicinity to the element due to a global instability.
Holger B E Kurz - One of the best experts on this subject based on the ideXlab platform.
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receptivity of a swept wing boundary layer to micron sized discrete Roughness elements
Journal of Fluid Mechanics, 2014Co-Authors: Holger B E Kurz, Markus J KlokerAbstract:The receptivity of a laminar swept-wing boundary layer to a spanwise array of circular Roughness elements is investigated by means of direct numerical simulations (DNS). The initial amplitude of a steady crossflow mode generated by the shallow Roughness elements does not vary strictly linearly with the Roughness Height, as often assumed. Rather, a fundamental, superlinear dependence of the receptivity amplitude on the Roughness Height is found. In order to account for shape effects, the Roughness geometry is Fourier decomposed to its spanwise spectral content, and elements with a reduced spectrum are investigated. If only modes are present that synthesise a regular structure of alternating bumps and dimples of equal shape and size, the receptivity amplitude is strictly linear for each mode and nominal Roughness Heights up to at least 15 % of the local displacement thickness.
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Effects of a Discrete Medium-Sized Roughness in a Laminar Swept-Wing Boundary Layer
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2014Co-Authors: Holger B E Kurz, Markus J KlokerAbstract:Direct numerical simulations of the effects of a cylindrical Roughness element in the laminar 3-d boundary layer on the upper surface of a swept wing are performed. The Roughness element generates streamwise vortices, where one is persistently growing in streamwise direction due to crossflow instability. By varying the Roughness Height, the onset of secondary instability of this crossflow vortex and ultimate transition to turbulence varies also in streamwise direction. When reaching the “effective”, i.e. the flow-tripping Roughness Height, the linear crossflow-instability regime is bypassed and breakdown to turbulence occurs in close vicinity to the element due to a global instability.
Nian-sheng Cheng - One of the best experts on this subject based on the ideXlab platform.
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Representative Grain Size and Equivalent Roughness Height of a Sediment Bed
Journal of Hydraulic Engineering, 2016Co-Authors: Nian-sheng ChengAbstract:AbstractThe representative grain size of a sediment bed can be defined as the mean-volume diameter, which varies with the geometric standard deviation of sediment size. The obtained results explain why the representative sediment size can be approximated to be D84 for typical gravel beds. The analysis is finally extended to quantify the dependence of the equivalent sand Roughness on the geometric standard deviation of grain size distribution.
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New formulation for the effective relative Roughness Height of open channel flows with submerged vegetation
Advances in Water Resources, 2015Co-Authors: Hao-ran Shi, Wenxin Huai, Zhengwei Xiong, Nian-sheng ChengAbstract:Abstract The friction factor f is a crucial parameter for scaling friction resistance and energy loss in open channel flows [11] , [16] . We propose an auxiliary bed for the distinctive flow structure arising from mutual effects of flow and vegetation. Using the auxiliary bed, a two-layer dynamic model with meaningful physical significance is established to modify equations for channelized flows with vegetation. This new approach divides the vegetated flow into two parts: a basal layer and a suspension layer. A parameter named ‘effective relative Roughness Height’, analogous to the relative Roughness Height of unvegetated open channels, is proposed to predict the friction factor of the suspension layer. By analyzing hundreds of datasets, a series of formulae are proposed based on the application of the effective relative Roughness Height. Comparisons show that the new formulae yield more precise estimates of the flow rate, Manning coefficient and Chezy coefficient than previously published formulae on the subject.
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Representative Roughness Height of submerged vegetation
Water Resources Research, 2011Co-Authors: Nian-sheng ChengAbstract:[1] Roughness length scale is important in the evaluation of resistance caused by submerged vegetation in open channel flows. By transforming the concept of hydraulic radius, a representative Roughness Height is proposed in this study for quantifying effect of submerged vegetation on flow resistance in the surface layer. The proposed Roughness Height is characterized by its proportionality to both stem diameter and vegetation concentration and performs better than other length scales in collapsing resistance data collected under a wide range of vegetation conditions. An approach is then developed for estimate of the average flow velocity and thus resistance coefficients for both cases of rigid and flexible vegetation. Comparisons are also made between the present study and other four formulas available in the literature. This study also shows that all the formulas, if simplified for some simple conditions, can be unified in a general form.
Jonathan Connelly - One of the best experts on this subject based on the ideXlab platform.
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examination of a critical Roughness Height for outer layer similarity
Physics of Fluids, 2007Co-Authors: Karen A Flack, Michael P Schultz, Jonathan ConnellyAbstract:The existence of a critical Roughness Height for outer layer similarity between smooth and rough wall turbulent boundary layers is investigated. Results are presented for boundary layer measurements on flat plates covered with sandgrain and woven mesh with the ratio of the boundary layer thickness to Roughness Height (δ∕k) varying from 16 to 110 at Reθ=7.3×103–13×103. In all cases tested, the layer directly modified by the Roughness (the Roughness sublayer) is confined to a region <5k or <3ks from the wall (where ks is the equivalent sandgrain Roughness Height). In the larger Roughness cases, this region of turbulence modification extends into the outer flow. However, beyond 5k or 3ks from the wall, similarity in the turbulence quantities is observed between the smooth and rough wall boundary layers. These results indicate that a critical Roughness Height, where the Roughness begins to affect most or all of the boundary layer, does not exist. Instead, the outer flow is only gradually modified with increas...
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Examination of a critical Roughness Height for outer layer similarity
Physics of Fluids, 2007Co-Authors: Karen A Flack, Michael P Schultz, Jonathan ConnellyAbstract:The existence of a critical Roughness Height for outer layer similarity between smooth and rough wall turbulent boundary layers is investigated. Results are presented for boundary layer measurements on flat plates covered with sandgrain and woven mesh with the ratio of the boundary layer thickness to Roughness Height (δ∕k) varying from 16 to 110 at Reθ=7.3×103–13×103. In all cases tested, the layer directly modified by the Roughness (the Roughness sublayer) is confined to a region
J.l. Bhagoria - One of the best experts on this subject based on the ideXlab platform.
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A Numerical Investigation of Turbulent Flows through an Artificially Roughened Solar Air Heater
Numerical Heat Transfer Part A: Applications, 2014Co-Authors: Anil Singh Yadav, J.l. BhagoriaAbstract:A numerical investigation of turbulent flows through a solar air heater roughened with semicircular sectioned transverse rib Roughness has been executed. The physical problem is represented mathematically by a set of governing equations, and the transport equations are solved using the finite element method. The numerical results show that the flow-field, average Nusselt number, and average friction factor are strongly dependent on the relative Roughness Height. The thermohydraulic performance parameter is found to be the maximum for the relative Roughness Height of 0.042. Comparisons with previously published work are performed and were found to be in excellent agreement.
