Session L4: General Fluid Dynamics I: Drag Reduction

Drag reduction due to spatial thermal modulations

It is demonstrated that a significant drag reduction for pressure driven flows can be realized by applying spatially distributed heating. The heating creates separation bubbles that separate the stream from the bounding walls and, at the same time, alters distribution of the Reynolds stress providing a propulsive force. The strength of this effect is of practical interest for heating with the wave numbers 0(1) and for flows with small Reynolds numbers and, thus, it is of potential interest for applications in micro-channels. The strength of the effects can be increased by using heating with a non-zero mean. The drag reducing effect increases proportionally to the second power of the heating intensity. This increase saturates if the heating becomes too intense.   

Modeling drag reduction by slippery surfaces comprised of microridges with two fluids

Theoretical analysis, numerical simulations, and experimental study are developed to predict drag reduction possessed by slippery surfaces, which entrap a second immiscible fluid within their micropores. The aim is to improve our understanding of the slip-flow and drag-reduction effects in terms of surface morphology and properties of both fluids. Stokes flow is simulated over surfaces with microstructure of both streamwise and spanwise ridges configurations. The entrapped fluid circulation between the microridges is also simulated for different geometries of the cavity. For validation, the results of the theoretical model are compared to those of the numerical simulations, and also compared to the available results of previous studies reported in the literature. Scaling laws are obtained for the reduction in shear stress at the surface in terms of the generic surface characteristics (surface roughness and both fluids' properties). These predictions are compared to the experimental data of rheological tests performed on fabricated samples. The models allow using different kinds of fluids with a wide range of viscosities. This work could be utilized to design slippery surfaces such as superhydrophobic and omniphobic coatings to maximize drag reduction.   

Groove Optimization for Drag Reduction

It has been shown that long-wavelength, longitudinal grooves reduce pressure losses in laminar, pressure driven flows. This work is focused on the search for the groove shapes that maximize the reduction of such losses. It is shown that the optimal shapes can be characterized using reduced geometry models involving just a few Fourier modes. Two classes of grooves have been considered, i.e. equal-depth grooves, which have the same height and depth, and unequal-depth grooves. It has been shown that the optimal grooves in the former cases are characterized by a certain universal trapezoid. There exists an optimum depth in the latter case and this depth, combined with the corresponding groove shape, defines the optimal geometry; this shape is well-approximated by a delta function. The maximum possible drag reduction has been determined for the optimal shapes. The analysis has been extended to kinematically-driven flows. It has been shown that in this case the longitudinal grooves always increase flow resistance regardless of their shape.   

Drag reduction using slippery liquid infused surfaces

A new method for passive drag reduction is introduced. A surface treatment inspired by the Nepenthes pitcher plant, previously developed by Wong et al. (2011), is utilized and its design parameters are studied for increased drag reduction and durability. Nano- and micro-structured surfaces infused with a lubricant allow for mobility within the lubricant itself when the surface is exposed to flow. The mobility causes slip at the fluid-fluid interface, which drastically reduces the viscous friction. These new surfaces are fundamentally different from the more conventional superhydrophobic surfaces previously used in drag reduction studies, which rely on a gas-liquid interface. The main advantage of the liquid infused surfaces over the conventional surfaces is that the lubricant adheres more strongly to the surface, decreasing the risk of failure when exposed to turbulence and other high-shear flows. We have shown that these surfaces can reduce viscous drag up to 20{\%} in both Taylor-Couette flow and in a parallel plate rheometer.   

Drag reduction using a multi-cavity at the afterbody

We present a numerical study on the drag reduction of a two-dimensional bluff body with a blunt trailing edge, which has a chord length $L$, body height $H$ and spanwise width $W$, being $H/W\ll 1$, aligned with a turbulent incompressible free-stream of velocity $U_{\infty}$, density $\rho$ and viscosity $\mu$. In particular, an extensive parametric study is performed numerically using the IDDES turbulent model, at a Reynolds number, $Re=\rho U_{\infty}H/\mu=20000$, to analyze the effect on the drag coefficient $C_D$ of both a single-cavity as a multi-cavity of variable depth $h$ at the base of the body. It is observed within the range, $0\le h/H \le 0.2$, that $C_D$ decreases monotonically reaching an asymptotic value in both cases. In turn, shorter cavity depths are necessary to reach the same drag reduction with a multi-cavity than with a single-cavity. On the other hand, the temporal evolution of the drag coefficient shows a lower standard deviation with a multi-cavity than with a single-cavity, which is manifested in the flow as a wake with a lower level of disorder.   

Longevity and drag reduction of omniphobic surfaces

Omniphobic surfaces, which consist of an omniphobic lubricant impregnated into a micro/nanoscale textured substrate, have been shown to repel a wide range of liquids [Wong et. al (Nature 2011)]. Here, experiments are performed on these surfaces to investigate the drag reduction as well as the time-dependent omniphobicity in the presence of flow. Drag measurements are performed in number of different flows including parallel plate and Taylor-Couette rheometers, pipe flow, and bluff body flows. The longevity of the surfaces are measured using three techniques: (i) an in situ noninvasive optical method to characterize the the loss of lubricant with time; (ii) thin-film interferometry measurements of the lubricant thickness versus time; and (iii) goniometer measurements of the time-dependent threshold sliding angle as well as contact-angle hysteresis. The impact of the substrate morphology on the drag reduction and longevity is observed both with and without flow in the surrounding water environment. This work could help to investigate ways of enhancing the drag-reducing properties of omniphobic surfaces by controlling their morphologies.   

Drag Reduction On Multiscale Superhydrophobic Surfaces

Fluid drag reduction is of great interest in a variety of fields, including hull engineering, microfluidics, and drug delivery. We fabricated samples with multi-scale superhydrophobic surfaces, which consist of hexagonally self-ordered microscopic spikes grown via anodization on macroscopic grooves cut in aluminum. The hydrodynamic drag properties were studied with a cone-and-plate rheometer, showing significant drag reduction near 15{\%} in turbulent flow and near 30{\%} in laminar flow. In addition to these experiments, numerical simulations were performed in order to estimate the slip length at high speeds. Furthermore, we will report on the progress of experiments with a new type of surface combining superhydrophobic surfaces like those discussed above with Slippery Liquid Infused Porous Surfaces (SLIPS), which utilize an oil layer to create a hydrophobic self-repairing surface. These ``Super-SLIPS'' may combine the best properties of both superhydrophobic surfaces and SLIPS, by combining a drag reducing air-layer and an oil layer which may improve durability and biofouling resistance.   

Numerical investigation of drag in regular arrays of circular cylinders

The temporal and spatial distribution of the drag exerted by regular arrays of circular cylinders is closely investigated by a series of numerical simulations. Applicability of the macroscopic drag-force model for the flow is also examined.   

Effective Medium Theory for Drag Reducing Micro-patterned Surfaces in Turbulent Flows

Inspired by the lotus effect, many studies in the last decade have focused on micro- and nano-patterned surfaces. They revealed that patterns at the micro-scale combined with high contact angles can significantly reduce skin drag. However, the mechanisms and parameters that control drag reduction, e.g. Reynolds number and pattern geometry, are still unclear. We propose an effective medium representation of the micro-features that treats the latter as a porous medium, and provides a framework to model flows over patterned surfaces in both Cassie and Wenzel states. Our key result is a closed-form expression for the skin friction coefficient in terms of frictional Reynolds (or Karman) number in turbulent regime. We apply the proposed model to turbulent flow over superhydrophobic ridged surfaces. The model predictions agree with laboratory experiments for Reynolds numbers ranging from 3000 to 10000.   

Fly in Atmosphere by Drag Force -- Easy Thrust Generation Aircraft Engine Based Physics

This paper aims to present to the science community another way to fly in atmosphere, a way which is much more cheaper, efficient, safe and easy. Over the years scientists have been trying to find a way to built the vertically taking off vehicles but there have been no satisfactory success(what have been found was very expensive), Even aircrafts we know now need very sophisticated and expensive engines and not efficient enough. This way of flying may help our governments to spend less money on technologies and will help people to travel at very low prices so that, it may be a solution to the crisis which the world faces nowadays. In other words, it is my proposal to the next generation technologies we was looking for for years because everything can fly from the car to the trucks, the spaceships and even the hotels maybe constructed and fly as we construct the ships which sail in the oceans. My way of flying will have many applications in all the aspect of travel as it is going to be explained.