10:30
Wall-bounded flows 4
Chair: Christoph Egbers
10:30
15 mins
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Generalized diagnostic scaling for high-order moments in turbulent boundary layers
Ramis Orlu, Antonio Segalini, Joseph Klewicki, P. Henrik Alfredsson
Abstract: The present work builds upon the diagnostic plot for the streamwise turbulence intensity [Alfredsson & Örlü, 2010] and generalises it for higher-order (even and odd) moments providing a general description of the probability density distribution of streamwise velocity fluctuations. Turbulent boundary layers (up to a friction Reynolds number of 20'000) are employed and demonstrate its feasibility to scale data throughout the overlap and outer region.
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10:45
15 mins
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Skin-Friction Measurements on Mathematically Generated Roughness in a Turbulent Channel Flow
Julio M. Barros, Michael P. Schultz, Karen A. Flack
Abstract: Engineering systems are affected by surface roughness, however, predicting frictional drag has proven to be challenging.
The present work takes a systematic approach by generating and manufacturing surfaces roughness where surface statistics, such as rms, skewness and power-spectral density can be controlled.
The frictional drag on these surfaces is measured in a turbulent channel flow facility.
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11:00
15 mins
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Structure and dynamics of turbulent flows over highly permeable walls
Wim-Paul Breugem, Mehdi Niazi Ardekani, Gerrit E. Elsinga
Abstract: Highly porous materials are found in various industrial applications and environmental flows. In previous studies it was found that a turbulent flow along a highly porous wall experiences a higher skin friction as compared to a solid wall with similar surface roughness when the so-called permeability Reynolds number (Re_K) is larger than O(1). The main objective of the present study was to gain understanding of the characteristic structures and auto-generation mechanisms of turbulence for Re_K >> 1. To this purpose the Volume-Averaged Navier-Stokes (VANS) equations were solved in a Direct Numerical Simulation (DNS) of a turbulent flow through a plane channel with an upper solid wall and a lower porous wall at Re_K = 5.91. The DNS results are in good agreement with available Particle Image Velocimetry (PIV) data for the same flow geometry. A linear stochastic estimation technique was used to capture the structure associated with the characteristic ejection event that contributes most to the Reynolds shear stress near the porous wall. This structure is similar to a horseshoe vortex. Contrary to the conventional hairpin vortex found near solid walls, this horseshoe vortex has a significantly higher inclination angle with the wall and its legs are much shorter. The latter is consistent with the observed absence of low and high-speed streaks near highly permeable walls. Next, the auto-generation mechanisms of the horseshoe vortex were studied in another DNS in which the horseshoe vortex was released in the Reynolds-averaged flow field obtained from the former DNS. Two distinct auto-generation mechanisms were observed: (1) the generation of new structures at the upstream end of the horseshoe vortex, which evolve rapidly into a turbulent spot with an arrowhead shape, and (2) the interaction of the horseshoe vortex with spanwise oriented Kelvin-Helmholtz vortex rollers originating from the inflexion point in the mean velocity profile near the porous wall.
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11:15
15 mins
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Near Wall PIV-Measurements on the Windward Slope of a Hill
Daniel Quosdorf, Ulrich Schuster, Michael Manhart
Abstract: The turbulent flow over periodic hills was measured near to the wall, using planar Particle-Image-Velocimetry (PIV) at high spatial resolution. Our focus is on the near wall turbulence structure on the windward slope of the hill. For large-eddy simulation (LES) we suspect that, if this was not predicted accurately, it affects the prediction of the velocity profiles over the hill crest which in turn will affect the recirculation length downstream of the hill. Regarding the time averaged velocities, we were able to resolve the linear viscous region of the boundary layer. The velocity distribution and also the Reynolds stress does not comply with the law of the wall as it is valid for a turbulent boundary layer at equilibrium.
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11:30
15 mins
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Direct numerical simulation of turbulent Couette-Poiseuille flow with zero skin friction
Gary N Coleman, Philippe R Spalart
Abstract: The near-wall scaling of mean velocity U(y) is addressed for the case of zero skin friction on one wall of a fully turbulent channel flow. The present DNS results can be added to the evidence in support of the conjecture that U is proportional to sqrt(y) in the region just above the wall at which the mean shear dU/dy = 0.
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11:45
15 mins
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PRESSURE DROP AND TURBULENCE STATISTICS IN TRANSPIRED PIPE FLOW
Francisco J. S. Bandeira, Juliana B. R. Loureiro, Atila P. Silva Freire
Abstract: Measurements of turbulent flow in a horizontal pipe subjected to wall transpiration are presented. Results include data on global flow rates and pressure drop, and local mean and fluctuating velocity profiles. Two distinct flow transpiration rates are studied, $v_w^{++}$ = $v_w$/$U_m$ = 0.0005 and 0.001. The effects of flow transpiration on the friction-coefficient are compared with theoretical predictions. The theory furnishes predictions accurate to 3\%.
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12:00
15 mins
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Counter-gradient diffusion of Reynolds stress in turbulent Couette flow with forward-facing step
Yohei Morinishi, Daiki Yoshikawa, Shinji Tamano
Abstract: The turbulent Couette flow with a forward-facing step (sudden contraction) is investigated experimentally and numerically. The particle image velocimetry (PIV) measurement and direct numerical simulation (DNS) prove peculiar turbulence behavior, the counter-gradient diffusion of the Reynolds stress, near the front part of the corner separation on the forward step. The negative turbulence production which follows the counter-gradient diffusion is then evaluated through quadrant analysis. The negative contribution of Q1 and Q3 events inside the shear layer increases with decreasing the Reynolds number. The effect makes the counter-gradient diffusion rather pronounced in lower Reynolds number flows.
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12:15
15 mins
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Turbulence Modeling for Oscillatory Pipe Flow
Alexander Shapiro, David Greenblatt, Gershon Grossman
Abstract: Oscillating pipe flows, with zero mean velocity, are common in engineering systems and particularly in Stirling-based engines and heat pumps, e.g. pulse-tube cryogenic coolers. These flows are particularly challenging to model because their flow state depends on both the Reynolds number and the dimensionless frequency or Womerlsey number. A central challenge in modeling these systems is that the Reynolds number varies greatly within the cycle, often crossing from laminar to turbulent flow regimes. Indeed, flows that have super-critical Reynolds numbers may pass between states a total of four times; two laminar-turbulent transitions and two turbulent-laminar transitions. Contrary to a steady flow in which the transition between the laminar and the turbulent regimes is affected only by the Reynolds number, for oscillating flow the transition is affected by a combination of Reos and a dimensionless frequency-based number such as Womersley (Wo). The objective of this work is to develop and validate a computational method that solves the incompressible oscillating flow equations for different combinations of Reos and Wo.
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