Accurate measurements of the shape of a mercury drop separated from a smooth flat solid surface by a thin aqueous film reported recently by Connor and Horn (Faraday Discuss. 2003, 123, 193-206) have been analyzed to calculate the excess pressure in the film. The analysis is based on calculating the local curvature of the mercury/aqueous interface, and relating it via the Young-Laplace equation to the pressure drop across the interface, which is the difference between the aqueous film pressure and the known internal pressure of the mercury drop. For drop shapes measured under quiescent conditions, the only contribution to film pressure is the disjoining pressure arising from double-layer forces acting between the mercury and mica surfaces. Under dynamic conditions, hydrodynamic pressure is also present, and this is calculated by subtracting the disjoining pressure from the total film pressure. The data, which were measured to investigate the thin film drainage during approach of a fluid drop to a solid wall, show a classical dimpling of the mercury drop when it approaches the mica surface. Four data sets are available, corresponding to different magnitudes and signs of disjoining pressure, obtained by controlling the surface potential of the mercury. The analysis shows that total film pressure does not vary greatly during the evolution of the dimple formed during the thin film drainage process, nor between the different data sets. The hydrodynamic pressure appears to adjust to the different disjoining pressures in such a way that the total film pressure is maintained approximately constant within the dimpled region.
A simple, low-cost, drag reduction device has been developed for applications in high-speed flows. This low-cost technology is expected to decrease fuel consumption (e.g., for high-speed vehicles such as rockets traveling from a highly overexpanded flow at the sea level to a highly underexpanded flow in outer space). Somewhere in between the over and the underexpansion, the rocket experiences perfectly expanded flows. In this study, only overexpanded and perfectly expanded flows have been considered. A single cylinder with a diameter of 2 mm is rotated clockwise inside the recirculation zone (e.g., in high-speed vehicles) to act as a controller. The base pressure in the dead zone and the wall pressure along the square duct length have been measured with and without control. Experiments were carried out for nozzle pressure ratios (NPRs) of 2, 3, 6 and 7.8. When the cylinder was rotated clockwise as an active controller, the base pressure was found to increase by as much as 56% in the perfectly expanded case and up to 17% in overexpanded flows. This drastic increase in the base pressure is correlated to an equivalent drag reduction. In addition, adding an active control had no negative impact on the main flow field. This is important as any disturbance in the main flow field at high speeds may lead to increased oscillations and vibrations, which if not checked may cause material failures. Rotating the cylinder in the clockwise direction near the wall was found to be very effective for higher
The primary objective of the research was to investigate the ideal resin coating on the mild steel surface among various resin coatings which are in use. These resin coatings are used as an anti-corrosive material for mild steel surfaces with enhanced mechanical properties. The resins (epoxy, polyurethane, polyester, and phenolic) on mild steel surface were applied by the pneumatic spray coating method. In addition, immersion test and salt spray test methods were followed using sodium chloride (NaCl) solution. Furthermore, the rate of corrosion and mechanical properties of mild steel coated with different resins was evaluated by conducting various experiments (immersion test, salt spray test, tensile strength test, and scratch hardness test) and was compared with a bare mild steel surface. The results of the current research showed that the mild steel surface coated with epoxy resin was found to be the most effective corrosion resistance material with better mechanical properties compared to other tested mild steel resin-coated surfaces.
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