Inspired by fluorescein visualisations of an evolving boundary layer developing under a towed 5m long plate, we present a video sequel\(-\)now with temporally resolved velocity information. Again, a stationary imaging system is utilised but this time with a high-speed PIV configuration. This unique frame of reference allows us to capture the evolution of large-scale coherent structures which appear almost stationary, providing a different visual perspective as compared to a more traditional frame of reference where fluid passes over a stationary solid body. The acquisition of temporally resolved velocity information enables us to visualise a range of fundamental processes found in a developing boundary layer, for instance vortical motions, momentum transport, structural arrangement, interfacial bulging and entrainment. Particle and scalar concentration visualisations derived from the acquired velocity information are used in conjunction with instantaneous velocity information to provide a unique perspective on the dynamical processes of an evolving turbulent boundary layer.
Description of Experiment
Experiments are performed in a tow tank facility with dimensions of 60 x 2 x 2m (length x width x height). The test surface consists of a plate measuring 5m in length and 1.2m in width with an elliptical leading edge. A zero pressure gradient boundary layer which develops under the bottom surface of this plate is tripped using a 1mm diameter trip wire. The developing turbulent boundary layer is then captured using a time-resolved PIV system which consists of a Photonics DM20-527 dual head Nd:YLF laser that delivers 100 mJ/pulse at 1000Hz, and two PCO Dimax CMOS cameras with a resolution of 2000 x 2000 pixels each. The high speed laser is fanned into a streamwise / wall-normal sheet illuminating hollow glass spheres. The two cameras are placed side-by-side such that an elongated field of view in the streamwise direction of 170 x 80mm (streamwise x wall-normal) is attained. All acquisitions are acquired at a sampling rate of 1000Hz.
More than 120 tows are performed with 5000 images captured for each sequence covering the entire streamwise domain of the plate from the trip to the trailing edge. For each acquisition the plate is towed using an automated carriage at 1m/s, giving a turbulent boundary layer developing to a friction Reynolds number of \(Re_\tau \approx 2500\). Here,\(Re_\tau = \delta U_\tau/\nu\), where \(\delta\) is the boundary layer thickness, \(U_\tau\) is the wall-shear velocity and \(\nu\) the kinematic viscosity.
By viewing the temporally resolved velocity information, one can visualise the structural arrangement and vortical motions within the layer as it develops. Further, by applying Reynolds decomposition to the velocity information, the velocity fluctuations can be extracted. Inspection of these velocity fluctuations highlights the well-known ejection and sweep events associated with momentum transport across the extent of the boundary layer. Collisions between these events seem to be associated with the formation of inclined shear layers. Scalar dye concentration visualisations are estimated from the velocity field using the 2D advection-diffusion equation. This visualisation illustrates scalar mixing throughout the boundary layer. Characteristic features such as turbulent bulges at the edge of the layer and strong vortical eruptions at the wall are well highlighted by these visualisations. Pseudo particle visualisations, again calculated from the velocity field, accentuate the role of these processes in the entrainment of irrotational freestream fluid into the boundary layer.