Kenneth T. Christensen

Statistical evidence of hairpin vortex packets in wall turbulence

The structure of velocity in the outer region of turbulent channel flow ( y + [gsim ] 100) is examined statistically to determine the average flow field associated with spanwise vortical motions. Particle image velocimetry measurements of the streamwise and wall-normal velocity components are correlated with a vortex marker (swirling strength) in the streamwise–wall-normal plane, and linear stochastic estimation is used to estimate the conditional average of the two-dimensional velocity field associated with a swirling motion. The mean structure consists of a series of swirling motions located along a line inclined at 12°–13° from the wall. The pattern is consistent with the observations of outer-layer wall turbulence in which groups of hairpin vortices occur aligned in the streamwise direction. While the observational evidence for the aforementioned model was based upon both experimental and computational visualization of instantaneous structures, the present results show that, on average, the instantaneous structures occur with sufficient frequency, strength, and order to leave an imprint on the statistics of the flow as well. Results at Re τ = 547 and 1734 are presented.

Population trends of spanwise vortices in wall turbulence

The present effort documents the population trends of prograde and retrograde spanwise vortex cores in wall turbulence outside the buffer layer. Large ensembles of instantaneous velocity fields are acquired by particle-image velocimetry in the streamwise–wall-normal plane of both turbulent channel flow at

Spatial structure of a turbulent boundary layer with irregular surface roughness

\hbox{\it Re}_\tau\equiv u_*\delta/\nu=570

Outer-layer similarity in the presence of a practical rough-wall topography

, 1185 and 1760 and a zero-pressure-gradient turbulent boundary layer at

The velocity and acceleration signatures of small-scale vortices in turbulent channel flow

\hbox{\it Re}_\tau=1400

Two-color laser-induced fluorescent thermometry for microfluidic systems

, 2350 and 3450. Substantial numbers of prograde spanwise vortices are found to populate the inner boundary of the log layer of both flows and most of these vortices have structural signatures consistent with the heads of hairpin vortices. In contrast, retrograde vortices are most prominent at the outer edge of the log layer, often nesting near clusters of prograde vortices. Appropriate Reynolds-number scalings for outer- and inner-scaled population densities of prograde and retrograde vortices are determined. However, the Re

High‐Throughput Printing via Microvascular Multinozzle Arrays

_\tau=570

Low-order representations of irregular surface roughness and their impact on a turbulent boundary layer

channel-flow case deviates from these scalings, indicating that it suffers from low- Re effects. When the population densities are recast in terms of fractions of resolved prograde and retrograde spanwise vortices, similarity is observed for

Observation of yeast cell movement and aggregation in a small-scale MHz-ultrasonic standing wave field.

100\,{ in channel flow and in both flows for

Spatial signatures of retrograde spanwise vortices in wall turbulence

100\,{ over the Re