Highlights of 2010

Little is known about morphological instability of a solidification front during the crystal growth of a thin film of flowing supercooled liquid with a free surface: for example, the ring-like ripples on the surface of icicles. The length scale of the ripples is nearly 1 cm. Two theoretical models for the ripple formation mechanism have been proposed. However, these models lead to quite different results because of differences in the boundary conditions at the solid–liquid interface and liquid–air surface. The validity of the assumption used in the two models is numerically investigated and some of the theoretical predictions are compared with experiments.

Molecular dynamics simulations were performed to study the spreading characteristics of nano-sized droplets on solid surfaces. The spreading behavior was analyzed in terms of the temporal evolution of the dynamic contact angle and spreading diameter for wettable, partially wettable and non-wettable surfaces. The computational model was validated through qualitative comparison with the measurements of Bayer and Megaridis, and through comparison with existing correlations. The comparison based on the ratio of relevant time scales indicated that for the conditions investigated, the spreading dynamics is governed by inertial and surface forces, with negligible influence of viscous forces. In addition, the simulation results indicated that the dynamic contact angle and spreading diameter, as well as the advancing and receding time periods, exhibit strong dependence on droplet size. These results were further analyzed to obtain correlations for the effect of droplet size on these spreading parameters. The correlations indicated that the normalized spreading diameter and contact angle scale with drop diameter as D_m /D₀ ∝D₀^0.5 and θ_R ∝D₀^0.5, while the advancing and receding time periods scale as t∝D₀^2/3. Global kinetic energy and surface energy considerations were used to provide a physical basis for these correlations. The correlations were also found to be generally consistent with the experimentally observed spreading behavior of macroscopic droplets.

A fixed 25° deadrise angle wedge is allowed to fall from a range of heights into static water. A high-speed (up to 5000 frames s⁻¹) camera is used to visualize the impact and subsequent formation of jet flows and droplets. Unsteady pressure measurements at six locations across the wedge surface are measured at 10 kHz. Two accelerometers (10g, 100g) are mounted above the apex of the wedge and measure the vertical acceleration. A purpose-built position gauge and analysis of the synchronized video allows the wedge motion to be captured. The synchronization of these data with the digital images of the impact makes it particularly suitable for the validation of computational fluid dynamics simulations as well as theoretical studies. A detailed experimental uncertainty analysis is presented. The repeatability of the test process is demonstrated and the measured pressures are comparable to previous studies. A ∼2.5 ms time delay is identified between the point of impact observed from the video and the onset of actual wedge deceleration. The clear definition of the free surface provides insight into jet formation, its evolution and eventual breakdown, further assisting with the development of numerical predictions.

The modulation characteristics of the turbulent wall shear stress measured in a plane diffuser subjected to imposed velocity oscillations are presented. The measurements reported in this paper pertain to nearly 150 different flows: imposed oscillations with three amplitudes and six frequencies, two different diverging channels and four streamwise positions. Only the most significant data are analyzed and discussed. The imposed unsteadiness affects the time-mean flow under the effect of the adverse pressure gradient (APG), in contrast to the canonical unsteady turbulent wall layers. The laminar viscous solution that adequately describes the amplitude and phase of the wall shear stress in channel flows in the high-imposed frequency regime is no longer valid in unsteady turbulent boundary layers subjected to APG. The latter modifies also the behavior of the phase shifts of the modulations in the turbulent quantities. The time lag of the wall shear stress turbulent intensity decreases as the pressure gradient increases. Some of these structural modifications are explained by the effect of the eddy viscosity that plays a key role in the vorticity diffusion process as the APG increases.