The identification of uniform momentum zones in wall-turbulence, introduced by Adrian, Meinhart & Tomkins (J. Fluid Mech., vol. 422, 2000, pp. 1-54) has been applied to turbulent channel flow, revealing a large 'core' region having high and uniform velocity magnitude. Examination of the core reveals that it is a region of relatively weak turbulence levels. For channel flow in the range Re τ = 1000-4000, it was found that the 'core' is identifiable by regions bounded by the continuous isocontour lines of the streamwise velocity at 0.95U CL (95 % of the centreline velocity). A detailed investigation into the properties of the core has revealed it has a large-scale oscillation which is predominantly anti-symmetric with respect to the channel centreline as it moves through the channel, and there is a distinct jump in turbulence statistics as the core boundary is crossed. It is concluded that the edge of the core demarcates a shear layer of relatively intense vorticity such that the interior of the core contains weakly varying, very low-level turbulence (relative to the flow closer to the wall). Although channel flows are generally referred to as 'fully turbulent', these findings suggest there exists a relatively large and 'quiescent' core region with a boundary qualitatively similar to the turbulent/non-turbulent interface of boundary layers, jets and wakes.

Here, we detail and analyse a multi-resolution particle image velocity measurement that resolves the wide range of scales prevalent in a zero pressure gradient turbulent boundary layer at high Reynolds numbers (up to Reτ ≈ 20 000). A unique configuration is utilised, where an array of eight high resolution cameras at two magnification levels are used simultaneously to obtain a large field of view, while still resolving the smaller scales prevalent in the flow. Additionally, a highly magnified field of view targeted at the near wall region is employed to capture the viscous sublayer and logarithmic region, with a spatial resolution of a few viscous length scales. Flow statistics from these measurements show good agreement with prior, well resolved hot-wire anemometry measurements. Analysis shows that the instantaneous wall shear stress can be reliably computed, which is historically known to be challenging in boundary layers. A statistical assessment of the wall shear stress shows good agreement with existing correlations, prior experimental and direct numerical simulation data, extending this view to much higher Reynolds numbers. Furthermore, conditional analysis using multiple magnification levels is detailed, to study near-wall events associated with high skin friction fluctuations and their associated overlaying structures in the log region. Results definitively show that the passage of very large-scale positive (or negative) velocity fluctuations are associated with increased (or reduced) small-scale variance in wall shear stress fluctuations.

No abstract

No abstract

In this study, we describe a multi-camera large field-of-view (FOV) planar-PIV experiment to capture the wide range of scales that coexist in high Reynolds number turbulent boundary layers. The proposed measurements are designed to capture spatial flow features over a greater range than current common practices, and at significantly lower cost. With this goal in mind, specialist PIV cameras are substituted with modern consumer full-frame digital cameras, which are typically available at a fraction of the cost, with higher resolution sensors. These cameras are configured to capture single-frame double-exposed images (DE-PIV), but at a much higher spatial resolution than what is available from specialist PIV cameras that capture double-frame single-exposure images (SE-PIV). This work discusses a set of simulations and experiments to quantitatively assess the quality of the PIV velocity fields from these two approaches for large field-of-view measurements. Our findings confirm that despite the known loss-of-accuracy associated with DE-PIV, the use of high-resolution cost-effective consumer cameras provides an economically feasible PIV solution with the necessary performance and accuracy for high spatial range measurements in wall-bounded turbulent flows.