Characteristics of sources and sinks of momentum in a turbulent boundary layer

In turbulent boundary layers, the wall-normal gradient of the Reynolds shear stress identifies momentum sources and sinks (T=∂[−uv]/∂y). These motions can be physically interpreted in two ways {i.)} as contributors to the turbulence term balancing the mean momentum equation, and {ii.)} as regions of strong local interaction between velocity and vorticity fluctuations. In this paper, the space-time evolution of momentum sources and sinks are investigated in a turbulent boundary layer at the Reynolds number (Reτ) = 2700, with time-resolved planar particle image velocimetry in a plane along the streamwise and wall-normal directions. Wavenumber--frequency power spectra of T fluctuations reveal that the wave velocities of momentum sources and sinks tend to match the local streamwise velocity in proximity to the wall. However, as the distance from the wall increases, the wave velocities of the T events are slightly lower than the local streamwise velocities of the flow, which is also confirmed from the tracking in time of the intense momentum sources and sinks. This evidences that momentum sources and sinks are preferentially located in low-momentum regions of the flow. The spectral content of the T fluctuations is maximum at the wall, but it decreases monotonically as the distance from the wall grows. The relative spectral contributions of the different wavelengths remains unaltered at varying wall-normal locations. From autocorrelation coefficient maps, the characteristic streamwise and wall-normal extents of the T motions are respectively 60 and 40 wall-units, independent of the wall distance. Both statistics and instantaneous visualizations show that momentum sources and sinks have a preferential tendency to be organized in positive-negative pairs in the wall-normal direction

Spatial-spectral characteristics of momentum transport in a turbulent boundary layer

wall, longer time and larger length scales exhibit an increasing spectral content. Wave velocities of transport events are estimated from wavenumber–frequency power spectra at different wall-normal locations. Wave velocities associated with Spectral content and spatial organization of momentum transport events are investigated in a turbulent boundary layer at the Reynolds number (Re ) = 2700, with time-resolved planar particle image velocimetry. The spectral content of the Reynolds-shear-stress fluctuations reveals that the largest range of time and length scales can be observed in proximity to the wall, while this range becomes progressively more narrow when the wall distance increases. Farther from the ejection events (Q2) are lower than the local average streamwise velocity, while sweep events (Q4) are characterized by wave velocities larger than the local average velocity. These velocity deficits are almost insensitive to the wall distance, which is also confirmed from time tracking the intense transport events. The vertical advection velocities of the intense ejection and sweep events are on average a small fraction of the friction velocity U , different from previous observations in a channel flow. In the range of wall-normal locations 60 < y+ < 600, sweeps are considerably larger than ejections, which could be because the ejections are preferentially located in between the legs of hairpin packets. Finally, it is observed that negative quadrant events of the same type tend to appear in groups over a large spatial streamwise extent

On near-field coherent structures in circular and fractal orifice jets

To investigate the influence of the orifice geometry on near-field coherent structures in a jet, Fourier-POD is applied. Velocity and vorticity snapshots obtained from tomographic particle image velocimetry at the downstream distance of two equivalent orifice diameters are analysed. Jets issuing from a circular orifice and from a fractal orifice are examined, where the fractal geometry is obtained from a repeating fractal pattern applied to a base square shape. While in the round jet energy is mostly contained at wavenumber m=0, associated to the characteristic Kelvin-Helmholtz vortex rings, in the fractal jet modal structures at the fundamental azimuthal wavenumber m=4 capture the largest amount of energy. The second part of the study focuses on the relationship between streamwise vorticity and streamwise velocity, to characterise the role of the orifice geometry on the lift-up mechanism recently found to be active in turbulent jets. The averaging of the streamwise vorticity conditioned on intense positive fluctuations of streamwise velocity reveals a pair of vorticity structures of opposite sign flanking the conditioning point, inducing a radial flow towards the jet periphery. This pair of structures is observed in both jets, even if the azimuthal extent of this pattern is 30% larger in the jet issuing from the circular orifice. This evidences that the orifice geometry directly influences the interaction between velocity and vorticity

Wide-field Time-resolved Particle Image Velocimetry data of a turbulent boundary layer

This dataset consists of time-resolved PIV data of a turbulent boundary layer over a wide field of view with very good spatial and temporal resolution. The data is only available for the bottom half of the boundary layer. In each Run, each vector field is stored in a .mat file (Matlab). The time delay between each vector field is 0.001 second (1kHz acquisition frequency). The name of the files in each Run contains a number, from 1 to 5004 (except for Run 5, which contains files numbered from 1 to 4883). The file number represents the acquisition order (1 is the first). Each file contains 4 matrices, X, Y, U, V. X and Y are the coordinates of the vectors stored in the matrices of the velocity vectors, U and V. U is the matrix containing the streamwise components of the velocity field, while V is the matrix containing the wall-normal components of the velocity field. The measurement units are in the international system, therefore the coordinates are in meters, while the components of the velocity vectors are in meters per seconds. Data in the matrices are ordered such that increasing rows in the matrices represent increasing wall-normal positions, and increasing columns in the matrices represent increasing streamwise distances. The characteristics of the turbulent boundary layer flow are the following. Free-stream velocity, U_{\inf} = 0.67 m s&minus;1 ; boundary layer thickness, &delta; = 0.1 m; friction velocity, U&tau; = 0.027 m/s; and the Reynolds number, Re_\tau = 2700. If you use this dataset, please cite the following papers: 1) de Kat, R &amp; Ganapathisubramani, B (2015) Frequency-wavenumber mapping in turbulent shear flows. J Fluid Mech 783:166&ndash;190. doi:10.1017/jfm.2015.558. 2) Fiscaletti, D, de Kat R, Ganapathisubramani, B (2018), Spatial-spectral characteristics of momentum transport in a turbulent boundary layer. doi:10.1017/jfm.2017.841 This dataset can be requested via http://library.soton.ac.uk/datarequest</span