Flow over large-scale low-order topographies: impact of heterogeneities
A systematic experimental investigation was carried out to provide phenomenological and quantitative insight on the flow over large-scale topographies characterized by few dominant modes. Particle image velocimetry (PIV) was used in a refractive-index-matching (RIM) channel to study the dynamics of the flow over a series of topographies consisting of (i) 2D and 3D wavy walls and (ii) homogeneous and heterogeneous canopies. The focus of these experiments is to characterize and quantify the impact of specified heterogeneities on the mean flow field and turbulence quantities. In the first set of experiments, the surface heterogeneity is given by a single spanwise sinusoidal mode superimposed on a 2D wavy wall; this allowed to uncover the impact of topography three-dimensionality on the flow field. In the second set of experiments, the flow over an array of rigid elements with uniform height, i.e. homogeneous canopy, was compared to that over a heterogeneous canopy composed of elements with varied heights. The impact of three-dimensionality on the flow field over the wavy walls was investigated in multiple regimes, including developed and developing turbulent flows as well as the transition to turbulence. The results show that, within the developed region, the 3D wall induces distinctive spanwise flows within the topography that lead to drag and turbulence reduction compared with the 2D counterpart. In the developing regime, the turbulent boundary layer is highly sensitive to the large-scale topography. The integral parameters such as displacement and momentum thicknesses are significantly more modulated by the topography in the 2D case due to relatively larger pressure variations in the streamwise direction. The three-dimensionality of the topography also impacts the mechanisms by which the boundary layer transitions to turbulence. This phenomenon over the 2D topography occurs due to an inflection point in the velocity profile resulting from flow separation within the troughs. In contrast, the transition to turbulence is significantly delayed due to the lack of such instability over the 3D wall. The flow over the canopies was studied under various submergence depths common in riverine environments. Here, the submergence depth is defined as the ratio of the flow depth to the canopy height. Turbulence statistics complemented with quadrant analysis and proper orthogonal decomposition reveal richer flow dynamics induced by the element height heterogeneity. Topography-induced spatially-periodic mean flows occur in the heterogeneous canopy. Additionally, non-vanishing vertical velocity is maintained across the entire length of the heterogeneous canopy with increased levels at lower submergence depths. Further modulations were induced in the turbulent kinetic energy, Reynolds shear stress and the canopy mixing layer. The results from both sets of experiments indicate a significant impact of surface heterogeneity on the flow dynamics over large-scale topographies. This includes considerable changes in the mean flow, production and levels of turbulence as well as turbulent transport, which is of high importance in environmental and industrial applications.
Turbulence, Boundary layer, Topography, Canopy flows, Environmental flows
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign