a b s t r a c tIn this paper we investigate the dam-break problem for viscoplastic (Herschel-Bulkley) fluids down a sloping flume: a fixed volume of fluid initially contained in a reservoir is released onto a slope and flows driven by gravitational forces until these forces are unable to overcome the fluid's yield stress. Like in many earlier investigations, we use lubrication theory and matched asymptotic expansions to derive the evolution equation of the flow depth, but with a different scaling for the flow variables, which makes it possible to study the flow behavior on steep slopes. The evolution equation takes on the form of a nonlinear diffusion-convection equation. To leading order, this equation simplifies into a convection equation and reflects the balance between gravitational forces and viscous forces. After presenting analytical and numerical results, we compare theory with experimental data obtained with a long flume. We explore a fairly wide range of flume inclinations from 6 • to 24 • , while the initial Bingham number lies in the 0.07-0.26 range. Good agreement is found at the highest slopes, where both the front position and flowdepth profiles are properly described by theory. In contrast, at the lowest slopes, theoretical predictions substantially deviate from experimental data. Discrepancies may arise from the formation of unsheared zones or lateral levees that cause slight flow acceleration.

a b s t r a c tWe report experimental results related to the dam-break problem for viscoplastic fluids. Using image processing techniques, we were able to accurately reconstruct the free-surface evolution of fixed volumes of fluid suddenly released a plane. We used Carbopol Ultrez 10 as a viscoplastic material; its rheological behavior was closely approximated by a Herschel-Bulkley model for a fairly wide range of shear-rates. Varying the Carbopol concentration allowed us to change the yield stress and bulk viscosity. The yield stress ranged from 78 to 109 Pa, producing Bingham numbers in the 0.07-0.35 range. We investigated the behavior of a 43-kg mass released on a plane, whose inclination ranged from 0 • to 18 • . For each run, we observed that the behavior was nearly the same: at short times, the mass accelerated vigorously on gate opening and very quickly reached a nearly constant velocity. At time t = 1 s, independently of plane inclination and yield stress, the mass reached a near-equilibrium regime, where the front position varied as a power function of time over several decades. We did not observe any run-out phase, during which the mass would have gradually come to a halt. The similarity in the flow behavior made it possible to derive an empirical scaling for the front position in the form x f = t 0.275(sin˛) 1/3 (sin˛) 5/4 , where˛and t denote plane inclination and time, respectively, and which holds for sloping beds (˛> 0).

Experiments were undertaken to investigate dam-break flows where a finite volume of highly viscous fluid (glucose with viscosity μ ≈ 350 Pa s) maintained behind a lock gate was released into a horizontal or inclined flume. The resulting sequence of flow-depth profiles was tracked using a three-dimensional visualization system. In the low-Reynolds-number and high-capillary-number limits, analytical solutions can be obtained from the Navier-Stokes equations using lubrication theory and matched asymptotic expansions. At shallow slopes, similarity solutions can also be worked out. While the variation in the front position scaled with time as predicted by theory for both horizontal and sloping flumes, there was a systematic delay in the front position observed. Moreover, taking a closer look at the experimental flowdepth profiles shows that they were similar, but they noticeably deviated from the theoretical similarity form for horizontal planes. For sloping beds, the flow-depth profile is correctly predicted provided that different scalings are used at shallow and large slopes.

The dam-break problem (i.e., the sudden release of a given volume of fluid down a slope) has attracted a great deal of attention from mechanicians and physicists over the past few years, with particular interest devoted to the free-surface profile and the spreading rate. Experimentally, impediments to accurate measurements of the free-surface evolution are numerous because of the significant variations in its curvature and velocity. To accurately measure the surge's free-surface variations with time, we have developed a new imaging system, consisting of a digital camera coupled with a synchronized micromirror projector. The object's surface is imaged into a camera and patterns are projected onto the surface under an angle of incidence that differs from the imaging direction. From the deformed pattern recorded by the camera, the phase can be extracted and, by using unwrapping algorithms, the height can be computed and the free surface reconstructed. We were able to measure the free surface of the flow to within 1 mm over a surface of 1.8 · 1.1 m 2 . Although the techniques used in our system are not new when taken individually, the system in its entirety is innovative and more efficient than most methods used to-date in practical applications.

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Concentrating Solar Power (CSP) plants tend to be located in open areas. This leads to the power plant usually being subjected to high wind speeds without much shelter or protection. In parabolic trough plants the solar field, the collectors and receiver tubes, are affected by the wind on both the structural, as well as the performance level. The collectors must resist the aerodynamic forces caused by the wind, and the airflow around the receiver tube has a cooling effect on it. The effects of the wind on Parabolic Trough Collectors (PTC) were investigated in a parametric study over a large range of pitch and yaw angles. Three different trough geometries were analysed varying the focal length of the parabola, i.e. the depth of the trough. The data were obtained using the Computational Fluid Dynamics (CFD) package ANSYS CFX 15.0 and validated against experimental data.An increase of the depth of the parabolic trough increases the maximum aerodynamic forces on the trough. However, a deeper trough has a sheltering effect on the receiver tube, thus reducing the thermal losses due to forced convection. This effect becomes more important the higher the temperature difference between the receiver and ambient air, and it can also reduce the requirements for highly insulated evacuated receiver tubes, which are a significant cost factor in PTC plants.The highest force coefficients on the PTC are observed at high positive pitch angles and a yaw angle of 0 • . While the aerodynamic loads on the trough reduce significantly with an increase in the yaw angle of the approaching wind, the heat flux around the receiver tube only shows a slight decrease in most cases. At some negative pitch angles an increasing yaw angle leads to higher thermal losses, as a vortex, forming at the leading edge of the trough, causes high air velocities around the receiver.