McCorquodale, Mark W.
(2018)
Interaction between oscillating-grid turbulence and a solid impermeable boundary.
PhD thesis, University of Nottingham.
Abstract
The interaction of a boundary with turbulence is a defining feature of many turbulent flows, resulting in a turbulent boundary layer which plays a prominent role in the production and dissipation of turbulence. Commonly, this interaction is dominated by the effects of mean shear. However, more subtle aspects of the interaction, such as effects associated with turbulent motions impinging onto the boundary, are still thought to play a key role in giving rise to the boundary layer structure. Unfortunately, these aspects of the interaction are currently poorly understood. A better understanding of these aspects of the interaction may be derived by isolating them from the effects of mean shear through the study of zero-mean-shear turbulence interacting with a boundary.
This study reports experimental work investigating the interaction between oscillating-grid turbulence (OGT) and a solid impermeable boundary. OGT is a commonly used experimental tool that produces a turbulent flow which is approximately homogeneous and isotropic in planes parallel to the oscillating grid but which is inhomogeneous in planes perpendicular to the oscillating grid. Throughout this study, instantaneous velocity measurements of the flow are obtained by applying two-dimensional particle imaging velocimetry to the vertical plane through the centre of the oscillating grid.
A detailed preliminary study to characterise the flow generated by the OGT apparatus is initially performed. Visualisation of the flow close to the oscillating grid indicates that large-scale circulations are induced in OGT by the merging of grid-induced jets close to the tank walls. The installation of an open-ended cuboidal `inner box' below the grid is shown to inhibit the merging of these jets, thereby resulting in a more regular jet structure close to the oscillating grid and a corresponding reduction in mean flow within the inner box. It is also found that, contrary to assumptions in the literature, this amendment to the standard OGT apparatus is most effective when the top of the inner box is located close to the oscillating grid. The reduction in mean flow intensity that results from the use of a correctly installed inner box brings about a turbulent flow in which the mean flow velocity components are small compared to velocity fluctuations, thereby enabling a meaningful comparison to be made with zero-mean-shear turbulence.
Consequently, the interaction between OGT and a solid impermeable boundary is studied to derive insight into the mechanisms governing the interaction of zero-mean-shear turbulence with boundaries. Results indicate that a critical aspect of the interaction is the blocking of a boundary-normal flux of turbulent kinetic energy across the boundary-affected region, which acts to increase the magnitude of the boundary-tangential turbulent velocity components, relative to the far-field trend, but not the boundary-normal turbulent velocity component. This feature arises as a result of the anisotropic nature of the flow produced by OGT, whereby the turbulent fluctuations decay with distance normal to and away from the oscillating grid, and would not be present in a turbulent flow that was otherwise homogeneous above the boundary-affected region of the flow. This observation provides new insight into the validity of well-established models of the interaction of zero-mean-shear turbulence and a solid impermeable boundary and provides a physical mechanism that explains the disparity in previously reported measurements relating to this problem.
The results reported are also in support of intercomponent energy transfer mechanisms previously proposed to govern the interaction of zero-mean-shear turbulence with boundaries, including viscous and `return-to-isotropy' mechanisms. That is, within a thin region adjacent to the boundary, approximately equal in thickness to the viscous sublayer, the data indicate that turbulent motions incident towards the boundary are more energetic than motions away, which are characteristics of an intercomponent energy transfer primarily driven by the viscous dissipation of turbulent kinetic energy. In addition, at the edge of the boundary-affected region, where the magnitude of the boundary-tangential turbulent velocity components exceeds the magnitude of the boundary-normal turbulent velocity component, results indicate that an intercomponent energy transfer occurs from the boundary-tangential turbulent velocity components to the boundary-normal turbulent velocity component in a so-called `return-to-isotropy' energy transfer. However, the data also indicate the presence of an additional intercomponent energy transfer, from the boundary-normal turbulent velocity component to the boundary-tangential turbulent velocity components over a thin region outside the viscous sublayer. Comparison to previously published results of related studies indicates that this mechanism is also prevalent in previous work, but is not captured within existing models of intercomponent energy transfer at the boundary. Results further indicate that the intercomponent energy transfer mechanisms are not independent of the blocking of the boundary-normal turbulent kinetic energy flux. That is, the blocking of the boundary-normal turbulent kinetic flux promotes anisotropy within the boundary-affected region of the flow and thereby induces a stronger `return-to-isotropy' energy transfer mechanism. Hence, the effect of a solid impermeable boundary on turbulent velocity components in zero-mean-shear turbulence depends critically on the nature of the original turbulent field (i.e. homogeneous or inhomogeneous turbulence).
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