High spatial range velocity measurements in a high Reynolds number turbulent boundary layer

Abstract

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.

The statistical behaviour of attached eddies

“…If we then substitute the Reynolds number for h min and h max using (33), this results in the following for z ≪ h max :…”

Concurrent Scale Interactions in the Far-Field of a Turbulent Mixing Layer

“…Figure 9 illustrates the joint pd f s between the large-scale fluctuations u A similar feature is also observed in the amplitude modulation effects of wall bounded flows. 8,9,46 For a LES, the filtered LES equations for an incompressible flow can be written as…”

“…Some recent results, primarily in wall-bounded turbulent shear flows, point to the significance of these interactions. [4][5][6][7][8][9] However, very little information is available on these interactions in other forms of shear flows. In this study, we aim to examine the nature of this interaction between large-and small-scale velocity fluctuations in a turbulent free shear flow.…”

Concurrent scale interactions in the far-field of a turbulent mixing layer

“…A similar feature is also observed in the amplitude modulation effects of wall bounded flows. 8,9,46 For a LES, the filtered LES equations for an incompressible flow can be written as…”