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Effects of multi-scale and regular grid geometries on decaying turbulence
No full textThe influence of a multi-scale fractal based geometry on the decay of turbulence is investigated by comparing the turbulence produced by a square fractal element grid to that produced by two regular grids with similar physical properties. Comparison of the grid wakes at constant grid Reynolds number, ReM, identifies that in the far field both regular grids produce comparable or higher turbulence intensities and local Reynolds numbers, Re?, than the square fractal element grid. This result is illustrative of a limitation of multi-scale geometries to produce the oft-quoted high levels of turbulence intensity and Re?. In the far field, the spectra are approximately collapsed at all scales for all three grids at a given Re?. When a non-equilibrium near field spectrum with ?uv??0 is compared to a far field spectrum at the same Re? but with ?uv??0, it is shown that their shapes are markedly different and that the non-equilibrium spectrum has a steeper slope, giving the appearance of being nearer k?5/3, although there is no theoretical expectation of an inertial range at such locations in the flow. However, when a non-equilibrium spectrum with ?uv??0 is compared to a far field spectrum at the same Re?, they are once again collapsed. This is shown to be related to non-zero Reynolds shear stress at scales that penetrate the scaling range for the present experiment, and hence the influence of shear is not limited to the largest scales. These results demonstrate the importance of local properties of the flow on the turbulence spectra at given locations in the inherently inhomogeneous flow found in the non-equilibrium region downstream of grids. In particular, how the presence of local shear stress can fundamentally change the shape of the spectra at scales that can be mistakenly interpreted as an inertial range
Dataset for Characterizing the surface texture of a dense suspension undergoing dynamic jamming
No full textThis dataset supports the publication:
'Characterizing the surface texture of a dense suspension undergoing dynamic jamming' by Olav Rømcke, Ivo R. Peters and R. Jason Hearst, Experiments in Fluids (2021)</span
Dataset for Getting jammed in all directions: Dynamic shear jamming around a cylinder towed through a dense suspension
No full textThis dataset supports the publication:
'Getting jammed in all directions: Dynamic shear jamming around a cylinder towed through a dense suspension' by Olav Rømcke, Ivo R. Peters and R. Jason Hearst, Physical Review Fluids (2021)</span
Dataset for Collision of Dynamic Jamming Fronts in a Dense Suspension
No full textThis dataset supports the publication:
'Collision of Dynamic Jamming Fronts in a Dense Suspension' by Olav Rømcke, Ivo R. Peters and R. Jason Hearst, Physical Review Fluids (2021)</span
Effect of turbulence on the wake of a wall-mounted cube
Get PDFThe influence of turbulence on the flow around a wall-mounted cube immersed in a turbulent boundary layer is investigated experimentally with particle image velocimetry and hot-wire anemometry. Free-stream turbulence is used to generate turbulent boundary layer profiles where the normalised shear at the cube height is fixed, but the turbulence intensity at the cube height is adjustable. The free-stream turbulence is generated with an active grid and the turbulent boundary layer is formed on an artificial floor in a wind tunnel. The boundary layer development Reynolds number () and the ratio of the cube height () to the boundary layer thickness () are held constant at and . It is demonstrated that the stagnation point on the upstream side of the cube and the reattachment length in the wake of the cube are independent of the incoming profile for the conditions investigated here. In contrast, the wake length monotonically decreases for increasing turbulence intensity but fixed normalised shear---both quantities measured at the cube height. The wake shortening is a result of heightened turbulence levels promoting wake recovery from high local velocities and the reduction in strength of a dominant shedding frequency
Characterizing the surface texture of a dense suspension undergoing dynamic jamming
No full textAbstract: Measurements of the surface velocity and surface texture of a freely propagating shear jamming front in a dense suspension are compared. The velocity fields are captured with particle image velocimetry (PIV), while the surface texture is captured in a separated experiment by observing a direct reflection on the suspension surface with high-speed cameras. A method for quantifying the surface features and their orientation is presented based on the fast Fourier transform of localized windows. The region that exhibits strong surface features corresponds to the the solid-like jammed region identified via the PIV measurements. Moreover, the surface features within the jammed region are predominantly oriented in the same direction as the eigenvectors of the strain tensor. Thus, from images of the free surface, our analysis is able to show that the surface texture contains information on the principle strain directions and the propagation of the jamming front. Graphic Abstract: [Figure not available: see fulltext.]</p
Getting jammed in all directions: dynamic shear jamming around a cylinder towed through a dense suspension
No full textExperimental results of towing a cylinder through a dense suspension of cornstarch and sucrose-water are presented. Focus is placed on the jamming fronts that exist in such systems. The literature has concentrated on the propagation of the jammed region under pushing, pulling, or shearing conditions independently. How the different fronts interact and if the fronts are symmetric when generated simultaneously has remained unexplored. Investigating this is our main goal. With the current setup, we are able to view a continuous, quasi-two-dimensional field around the cylinder. As such, a way of generating jamming fronts is presented whereby pushing, pulling, and shearing can be examined simultaneously. In agreement with previous studies, the front propagates roughly twice as fast in the longitudinal direction compared to the transverse direction, which is attributed to a single underlying onset strain, regardless of orientation from the cylinder. Although the jamming front shows nearly perfect transverse symmetry, there is clear longitudinal asymmetry. This is evident in the velocity and strain fields, and is also detectable in the front propagation velocity and onset strain.</p
Dataset for "Uniform momentum zones in a turbulent boundary layer subjected to freestream turbulence"
No full textThis is the dataset accompanying the following paper:
R. J. Hearst, C. M. de Silva, E. Dogan & B. Ganapathisubramani, 2021, Journal of Fluid Mechanics, “Uniform momentum zones in a turbulent boundary layer subjected to freestream turbulence.” DOI: 10.1017/jfm.2021.102</span
Collision of Dynamic Jamming Fronts in a Dense Suspension
No full textDynamic jamming is a phenomenon whereby a dense suspension switches from a fluidlike to a solidlike state when subjected to sufficient stress and deformation. Large enough systems show that this transition is accompanied by a distinct jamming front. We present an experimental study where two jamming fronts are created simultaneously using two cylinders moving in parallel. We focus our observations on the collision of the jammed regions when the two fronts meet. Surprisingly, our measurements, combining surface texture visualization and time-resolved particle image velocimetry, show the formation of an unjammed region contained within the otherwise jammed suspension
Modelling high Reynolds number wall-turbulence interactions in laboratory experiments using large scale free-stream turbulence
No full textA turbulent boundary layer subjected to free-stream turbulence is investigated in order to ascertain the scale interactions that dominate the near-wall region. The results are discussed in relation to a canonical high Reynolds number turbulent boundary layer because previous studies have reported considerable similarities between these two flows. Measurements were acquired simultaneously from four hot-wires mounted to a rake which was traversed through the boundary layer. Particular focus is given to two main features of both canonical high Reynolds numberboundary layers and boundary layers subjected to free-stream turbulence: (i) the footprint of the large scales in the logarithmic region on the near-wall small scales, specifically the modulating interaction between these scales, and (ii) the phase difference in amplitude modulation. The potential for a turbulent boundary layer subjected to free-stream turbulence to “simulate” high Reynolds number wall-turbulence interactions is discussed. The results of this study have encouraging implications for future investigations of the fundamental scale interactions that take place in high Reynolds number flows as it demonstrates that these can be achieved at typical laboratory scales
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