Experimental results of towing a cylinder through a dense suspension of cornstarch and sucrosewater 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-2D field around the cylinder. As such, a new 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.
The role of the Lagrangian mean flow, or drift, in modulating the geometry, kinematics and dynamics of rotational and irrotational deep-water surface gravity waves is examined. A general theory for permanent progressive waves on an arbitrary vertically sheared steady Lagrangian mean flow is derived in the Lagrangian reference frame and mapped to the Eulerian frame. A Lagrangian viewpoint offers tremendous flexibility due to the particle labelling freedom and allows us to reveal how key physical wave behaviour arises from a kinematic constraint on the vorticity of the fluid, inter alia the nonlinear correction to the phase speed of irrotational finite amplitude waves, the free surface geometry and velocity in the Eulerian frame, and the connection between the Lagrangian drift and the Benjamin–Feir instability. To complement and illustrate our theory, a small laboratory experiment demonstrates how a specially tailored sheared mean flow can almost completely attenuate the Benjamin–Feir instability, in qualitative agreement with the theory. The application of these results to problems in remote sensing and ocean wave modelling is discussed. We provide an answer to a long-standing question: remote sensing techniques based on observing current-induced shifts in the wave dispersion will measure the Lagrangian, not the Eulerian, mean current.
When surface waves interact with ambient turbulence, the two affect each other mutually. Turbulent eddies get redirected, intensified and periodically stretched and compressed, while the waves suffer directional scattering. We study these mutual interactions experimentally in the water channel laboratory at the Norwegian University of Science and Technology (NTNU) Trondheim. Long groups of waves were propagated upstream on currents with identical mean flow but different turbulence properties, created by an active grid at the current inlet. The subsurface flow in the spanwise–vertical plane was measured with stereo particle-image velocimetry. Comparing the subsurface velocity fields before and after the passage of a wave group, a strong enhancement of streamwise vorticity is observed which increases rapidly towards the surface for $k_0z\gtrsim -0.3$ ( $z$ , vertical distance from still surface; $k_0$ , carrier wavenumber) in qualitative agreement with theory. Next, we measure the broadening of the directional wave spectrum at increasing propagation distance. The rate of directional diffusion is greatest for the turbulent case with the highest energy at the longest length scales whereas the highest total turbulent kinetic energy overall did not produce the most scattering. The variance of directional spectra is found to increase linearly as a function of propagation time.
The influence of upstream turbulence on the flow produced by a plane jet is investigated experimentally with hot-wire anemometry and smoke flow visualisation. An innovative active grid, where each wing can be independently controlled, is used to change the upstream turbulence conditions. Three cases are investigated: a canonical reference case, a case with the same integral scale as the reference case but an order of magnitude increase in turbulence intensity, and a case with both an order of magnitude increase in turbulence intensity and an order of magnitude increase in integral scale compared to the reference case. It is demonstrated that the wake width increases with turbulence intensity, but decreases with integral scale for constant turbulence intensity. In addition, the positional variability of the wake width is highest with high turbulence intensity and a short integral scale. Along the jet centreline, the potential core region is shorter with elevated upstream turbulence intensity; this is reflected in both the mean velocity and the variance. The decay of the centreline mean velocity is also retarded by incoming turbulence. In all, increased incoming turbulence results in increased jet spreading, and a shorter integral scale further increases the spreading. Graphic abstract
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