10:30
15 mins
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Downstream Evolution of Perturbations in a Zero Pressure Gradient Turbulent Boundary Layer
Eduardo Rodriguez Lopez, Paul J. K. Bruce, Oliver R. J. Buxton
Abstract: This abstract examines the evolution of perturbations generated by various trips in a turbulent boundary layer. Measurements taken using hot wire anemometry show that the evolution towards the natural state is strongly dependent on the formation mechanisms of the boundary layer, this being different for different wall normal distribution of the trips' blockage. It is observed that standard boundary layer properties are recovered, after an adaptation region, with $175\%$ higher momentum thickness than the natural case. Two-point measurements with time resolved velocity in the inner region are studied to explore the different formation mechanisms.
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10:45
15 mins
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LES OF MODERATE REYNOLDS NUMBER TURBULENT PIPE FLOWS
Cheng Chin
Abstract: Wall-resolved large-eddy simulation of fully developed turbulent pipe flows are performed using a spectral vanishing viscosity
approach. Turbulence statistics are compared with direct numerical simulation and hot-wire experimental data at similar friction
Reynolds numbers Re = 1002. Turbulence statistics of streamwise velocity show good agreement up to the fourth order. The results
highlight the feasibility of using wall-resolved large-eddy simulation to accurately investigate turbulent pipe flow at Reynolds numbers
not currently feasible for direct numerical simulation. Further simulations have been performed at Re ≈ 2000, preliminary results
compared well to DNS data and will be presented in the conference and full paper.
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11:00
15 mins
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ON RESUSPENSION OF SPHERICAL PARTICLES FROM ROUGH AND SMOOTH SURFACES BY A WALL-NORMAL VORTEX
Ron Shnapp, Alex Liberzon
Abstract: A three-dimensional particle tracking velocimetry (3D-PTV) method was applied to study in details the single-particle
resuspension (lift-off) events of large spherical particles. In order to increase statistics, we have utilized a wall-normal, tornado-like
vortex that lifted the particles from surfaces of different roughness. Analyzing the three-dimensional particle trajectories, velocities
and accelerations before, during and after the resuspension events, the resuspension efficiency was obtained directly. The results show
that the lift-off efficiency from the rough surface is significantly higher than from the smooth one. We suggest a simplified mechanistic
model based on the mobility parameter that can predict this peculiar effect. This study can lead to improved modeling of resuspension
rates from smooth and rough beds.
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11:15
15 mins
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NEGATIVE STREAMWISE SKIN-FRICTION IN NEAR-WALL TURBULENCE
Christoph Bruecker
Abstract: The recent discovery of rare backflow events in turbulent boundary layer flows based on the analysis of DNS data has raised again the need of experimental visualizations of wall-shear stress (WSS) fields in such unsteady flows. Of importance is the localization of critical points which are thought to strongly correlate with large-scale events in the log-layer. Up to now there is no experimental prove of these rare events and their topological patterns in the skin-friction field. Their existence in a turbulent boundary-layer flow is shown herein by means of imaging with 2D arrays of flexible micropillars attached at the wall.
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11:30
15 mins
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Direct numerical simulation of open-channel flow in the fully rough regime
Marco Mazzuoli, Markus Uhlmann
Abstract: The Direct Numerical Simulation (DNS) of an incompressible open channel flow over a layer of rigid spherical roughness elements fixed on the wall in well-packed square arrangement has been performed which shows values of the Reynolds number Rb ~ 6900 (ks+ ~ 100) and produces a mean velocity defect ∆U+ ~ 7 in the range of the fully-rough regime. First and second moment statistics of the velocity field have been analyzed. The stress distribution as well as the hydrodynamic force and torque acting on the surface of individual roughness elements have been computed. Results are presently shown and compared with those obtained in the context of a previous DNS performed in the transitionally rough regime.
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11:45
15 mins
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Secondary Flows in Boundary Layers over Streamwise-aligned Wall-roughness
Christina Vanderwel, Bharathram Ganapathisubramani
Abstract: The cause of recently observed spanwise-alternating low- and high-momentum pathways [1,2,3] to appear across the entire turbulent boundary layer formed over rough surfaces is experimentally investigated by measuring the flows over roughness elements with systematically varied spanwise spacing. We found that the secondary flows associated with these low- and high-momentum pathways only appeared when the spanwise spacing of roughness elements was roughly proportional to the boundary layer thickness, and did not appear for cases with spacing much less than the boundary layer height. This suggests that the ratio of the spanwise spacing to the boundary layer thickness, as opposed to the roughness height (or width), is the most important parameter to predict the occurrence of these secondary motions in turbulent boundary layers over rough walls.
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12:00
15 mins
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EFFECTS OF EXTERNAL DISTURBANCES ON TURBULENT BOUNDARY LAYERS
Eda Dogan, Ronald Hanson, Bharathram Ganapathisubramani
Abstract: The state of a turbulent boundary layer developing under the influence of different types of freestream turbulence (FST) is examined. Different FST conditions with different length scales and turbulence intensity levels are generated using an active grid. Experiments are performed using two different techniques allowing simultaneous measurements of the freestream and the boundary layer: single component hot-wire anemometry and multi-camera planar Particle Image Velocimetry (PIV). Penetration level of the freestream into the boundary layer and the effect on the near wall region are shown to depend on the turbulence characteristics of the freestream. These interactions will further be investigated using spectral analysis from hot-wire measurements and also coherent structures associated with these interactions will be studied using PIV data.
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12:15
15 mins
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DIRECT NUMERICAL SIMULATION OF AN EQUILIBRIUM ADVERSE PRESSURE GRADIENT TURBULENT BOUNDARY LAYER AT THE VERGE OF SEPARATION
Vassili Kitsios, Callum Atkinson, Juan Sillero, Guillem Borrell, Ayse Gungor, Javier Jimenez, Julio Soria
Abstract: The statistics and structure of a self-similar equilibrium adverse pressure gradient (APG) turbulent boundary layer (TBL) are investigated using a direct numerical simulation (DNS) of the flow at the verge of separation. Flow simulations are performed using the DNS TBL code of Simens et al. [8] and Borrell et al. [1] with the desired equilibrium APG achieved via the use of a tailored far-field boundary condition. The APG TBL develops over a momentum thickness based Reynolds number of Reθ ≈ 2000 to 6000, achieving a region of constant friction coefficient (Cf ), pressure velocity (up) and shape factor (H). One- and two-point statistics are presented under both inner wall (uτ ) and pressure velocity (up ) based scaling.
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