Showing papers by "Mohamed Gad-el-Hak published in 2011"

Modeling drag reduction and meniscus stability of superhydrophobic surfaces comprised of random roughness

Abstract: Previous studies dedicated to modeling drag reduction and stability of the air-water interface on superhydrophobic surfaces were conducted for microfabricated coatings produced by placing hydrophobic microposts/microridges arranged on a flat surface in aligned or staggered configurations. In this paper, we model the performance of superhydrophobic surfaces comprised of randomly distributed roughness (e.g., particles or microposts) that resembles natural superhydrophobic surfaces, or those produced via random deposition of hydrophobic particles. Such fabrication method is far less expensive than microfabrication, making the technology more practical for large submerged bodies such as submarines and ships. The present numerical simulations are aimed at improving our understanding of the drag reduction effect and the stability of the air-water interface in terms of the microstructure parameters. For comparison and validation, we have also simulated the flow over superhydrophobic surfaces made up of aligned o...

Predicting shape and stability of air-water interface on superhydrophobic surfaces with randomly distributed, dissimilar posts

Abstract: A mathematical framework developed to calculate the shape of the air–water interface and predict the stability of a microfabricated superhydrophobic surface with randomly distributed posts of dissimilar diameters and heights is presented. Using the Young–Laplace equation, a second-order partial differential equation is derived and solved numerically to obtain the shape of the interface, and to predict the critical hydrostatic pressure at which the superhydrophobicity vanishes in a submersed surface. Two examples are given for demonstration of the method’s capabilities and accuracy.

In situ, noninvasive characterization of superhydrophobic coatings.

Abstract: Light scattering was used to measure the time-dependent loss of air entrapped within a submerged microporous hydrophobic surface subjected to different environmental conditions. The loss of trapped air resulted in a measurable decrease in surface reflectivity and the kinetics of the process was determined in real time and compared to surface properties, such as porosity and morphology. The light-scattering results were compared with measurements of skin-friction drag, static contact angle, and contact-angle hysteresis. The in situ, noninvasive optical technique was shown to correlate well with the more conventional methods for quantifying surface hydrophobicity, such as flow slip and contact angle.

Salinity effects on the degree of hydrophobicity and longevity for superhydrophobic fibrous coatings

Abstract: Previous studies on submerged superhydrophobic surfaces focused on performance variables such as drag reduction and longevity. However, to use such surfaces for practical applications, environmental factors such as water salinity must be investigated and understood. In this work, experiments were carried out to investigate the impact of salt (sodium chloride, NaCl) concentrations in aqueous solutions on the hydrophobicity and longevity of polystyrene (PS) fibrous coatings. Rheological studies using salt water as a test fluid were performed to determine the effect of salt concentration on drag reduction. Contact-angle measurements were used to validate the results from the rheometer. In situ noninvasive optical reflection was used to measure the longevity of the coating—time-dependent loss of entrapped air within the coating—as a function of salinity. The superhydrophobic coating used herein consisted of PS fibers that were deposited using DC-biased AC-electrospinning. Electrospinning is scalable and far less expensive than conventional methods (e.g., microfabrication), bringing the technology closer to large-scale submerged bodies such as submarines and ships. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Turbulent boundary layers: is the wall falling or merely wobbling?

Abstract: A quick passage through five centuries of turbulence research highlights the major milestones. The more recent cornerstones include Kolmogorov’s equilibrium theory of turbulence spectrum, the universal logarithmic law of wall-bounded flows, and the proliferation of direct numerical simulations. Evidence of recent fault lines in all three major achievements is presented, but also novel remedies as well as a few contemporary accomplishments are pointed out.

Effects of Outer Scales on the Peaks of Near-Wall Reynolds Stresses

Abstract: The classical view of wall-bounded turbulence suggests that the near-wall region should be scaled with characteristic scales that are closely related to that region. For the last decade, however, alternative concepts considering the in uence of outer scales were proposed. Herein, we show that the peak values of the Reynolds stresses in di erent geometries (e.g., zero-pressure-gradient boundary layers, and pipe and channel ows) collapse in single Reynolds-number-independent curve when scaled with an alternative mixed scaling based on u 3=2 u 1=2 e .

Fabrication and Characterization of Low-Cost Superhydrophobic Coatings

Abstract: Most previous studies dedicated to modeling and testing drag reduction and stability of the air{water interface on submerged superhydrophobic surfaces were conducted using microfabricated coatings produced by placing hydrophobic microposts/microridges arranged in aligned or staggered con gurations on a small-scale at surface. In this paper, we numerically model and experimentally characterize the performance of superhydrophobic surfaces made either using AC-electrospun bers or random deposition of hydrophobic particles. Such fabrication methods are far less expensive than microfabrication, bringing the technology closer to large-scale submerged bodies such as submarines and ships.

Does the Field of Aerospace Sciences Have a Role to Play in Large-Scale Disasters?

Abstract: eld equations, but exact solutions of these often nonlinear dierential equations are impossible to obtain particularly for turbulent ows, and heuristic models together with intensive use of supercomputers are necessary to proceed to a reasonably accurate forecast. In other cases, as for example earthquakes, the precise laws are not even known and prediction becomes more or less a black art. Aerospace engineers are well trained in classical mechanics, and this science may form the foundation for predicting and even controlling large-scale disasters. Viewed as dynamical systems, natural as well as manmade disasters are ripe for opportunities for aerospace scientists and engineers to help, along side civil & environmental engineers, sociologists, psychologists, rst responders, tacticians, logisticians, medical personnel, and the usual plethora of government and private agencies involved in prediction and relief eorts.

Reynolds number dependency of near-wall statistics of zero-pressure-gradient turbulent boundary layer

Abstract: We report high-resolution LDA and HWA measurements of the streamwise velocity component of a flat-plate turbulent boundary layer (ZPG TBL) over a range of momentum thickness Reynolds number from 1,170 to 3,720. The primary objective of this work is to investigate the near-wall behavior and the scaling of high-order statistics. In particular, we are interested in certain Karman number dependencies. The obtained data are in excellent agreement with most recent DNS-results, which allows direct comparison of detailed results such as peak value and position of streamwise stress, wall-values of skewness and flatness factors, and turbulence dissipation rate. The experimental data clearly reveal the failure of classical scaling. An alternative mixed scaling based on u?3/2ue1/2 removes these discrepancies.