Towards Reconciling the Large-Scale Structure of Turbulent Boundary Layers in the Atmosphere and Laboratory

Abstract

A collaborative experimental effort employing the minimally perturbed atmospheric surface-layer flow over the salt playa of western Utah has enabled us to map coherence in turbulent boundary layers at very high Reynolds numbers, Re τ ∼ O(10 6 ). It is found that the large-scale coherence noted in the logarithmic region of laboratory-scale boundary layers are also present in the very high Reynolds number atmospheric surface layer (ASL). In the ASL these features tend to scale on outer variables (approaching the kilometre scale in the streamwise direction for the present study). The mean statistics and two-point correlation map show that the surface layer under neutrally buoyant conditions behaves similarly to the canonical boundary layer. Linear stochastic estimation of the three-dimensional correlation map indicates that the low momentum fluid in the streamwise direction is accompanied by counter-rotating roll modes across the span of the flow. Instantaneous flow fields confirm the inferences made from the linear stochastic estimations. It is further shown that vortical structures aligned in the streamwise direction are present in the surface layer, and bear attributes that resemble the hairpin vortex features found in laboratory flows. Ramp-like high shear zones that contribute significantly to the Reynolds shear-stress are also present in the ASL in a form nearly identical to that found in laboratory flows. Overall, the present findings serve to draw useful connections between the vast number of observations made in the laboratory and in the atmosphere.

A New Miniature Wind Turbine for Wind Tunnel Experiments. Part II: Wake Structure and Flow Dynamics

“…In order to elucidate the possible origin of the high-speed region observed in Figure 17a,c, one can notice that the flow in both cases is generally directed from the high-speed region to the low-speed one at around turbine hub-height level, and it gradually changes direction with increasing height. This is similar to the flow motion between well-known high-and low-speed streaks, known as very-large-scale motions (VLSMs), in turbulent boundary layers [45][46][47]. In addition, the size of high-speed regions shown in Figure 17a,c is in the same order of magnitude as the values reported in the literature for the size of VLSMs in boundary-layer flows (e.g., [45]).…”

Coherent structures in flow over hydraulic engineering surfaces

“…Support for this also comes from atmospheric surface layer and laboratory measurements as described in Hambleton et al (2006) and Hutchins et al (2012), as shown in Fig. 6, where simultaneous x − y and x − z plane three-component velocity measurements reveal signatures entirely consistent with the superstructure events consisting of an organized array of packet structures.…”

“…Here, the black isocontours show swirl strength, indicating the corresponding location of vortical structures with the low-speed (blue) and high-speed (red) regions. After Marusic and Adrian (2013) Figure 6 Top panel: instantaneous velocity fluctuations in the streamwise-wall-normal (x-y) plane and instantaneous streamwise velocity fluctuations in the streamwise/spanwise (x-y) planes for data from laboratory PIV (Hambleton et al 2006) and for the atmospheric surface layer using an arrays of sonic anemometers (Hutchins et al 2012). High-positive w regions are indicated by red, while blue denotes highly negative w regions.…”

Reynolds number trend of hierarchies and scale interactions in turbulent boundary layers

“…This figure has reference curves corresponding to a constant physical inclination angle θ, which were constructed via τ U ∞ /δ = (z w − z)/ tan(θ )/δ. Typically, angles of θ ≈ 16 • have been reported for the u component of velocity [5]. Figure 3b shows that the shift is only weakly dependent on scale, and so the large scales are said to be non-dispersive.…”