10 research outputs found
A self-sustaining process theory for uniform momentum zones and internal shear layers in high Reynolds number shear flows
Many exact coherent states (ECS) arising in wall-bounded shear flows have an
asymptotic structure at extreme Reynolds number Re in which the effective
Reynolds number governing the streak and roll dynamics is O(1). Consequently,
these viscous ECS are not suitable candidates for quasi-coherent structures
away from the wall that necessarily are inviscid in the mean. Specifically,
viscous ECS cannot account for the singular nature of the inertial domain,
where the flow self-organizes into uniform momentum zones (UMZs) separated by
internal shear layers and the instantaneous streamwise velocity develops a
staircase-like profile. In this investigation, a large-Re asymptotic analysis
is performed to explore the potential for a three-dimensional, short
streamwise- and spanwise-wavelength instability of the embedded shear layers to
sustain a spatially-distributed array of much larger-scale, effectively
inviscid streamwise roll motions. In contrast to other self-sustaining process
theories, the rolls are sufficiently strong to differentially homogenize the
background shear flow, thereby providing a mechanistic explanation for the
formation and maintenance of UMZs and interlaced shear layers that respects the
leading-order balance structure of the mean dynamics
Near-surface particle image velocimetry measurements in a transitionally rough-wall atmospheric boundary layer
Efficacy of single-component MTV to measure turbulent wall-flow velocity derivative profiles at high resolution
An investigation of channel flow with a smooth air-water interface
Published online: 9 June 2015.
This article belongs to a Topical Collection of articles entitled Extreme Flow Workshop 2014. Guest Editors: I. Marusic and B. J. McKeon.Experiments and numerical simulation are used to investigate fully developed laminar and turbulent channel flow with an air鈥搘ater interface as the lower boundary condition. Laser Doppler velocimetry measurements of streamwise and wall-normal velocity components are made over a range of Reynolds number based upon channel height and bulk velocity from 1100 to 4300, which encompasses the laminar, transitional and low Reynolds numbers turbulent regimes. The results show that the airflow statistics near the stationary wall are not significantly altered by the air鈥搘ater moving interface and reflect those found in channel flows. The mean statistics on the water interface side largely exhibit results similar to simulated Poiseuille鈥揅ouette flow (PCF) with a solid moving wall. For second-order statistics, however, the simulation and experimental results show some discrepancies near the moving water surface, suggesting that a full two-phase simulation is required. A momentum and energy transport tubes analysis is investigated for laminar and turbulent PCFs. This analysis builds upon the classical notion of a streamtube and indicates that part of the energy from the pressure gradient is transported towards the stationary wall and is dissipated as heat inside the energy tubes, while the remainder is transmitted to the moving wall. For the experiments, the airflow energy is transmitted towards the water to overcome the drag force and drive the water forward; therefore, the amount of energy transferred to the water is higher than the energy transferred to a solid moving wall.Reza Madad, John Elsnab, Cheng Chin, Joseph Klewicki, Ivan Marusi