No description available.

No description available.

An investigation of turbulent structure modification of a boundary layer for a fully developed channel flow by microbubble injection close to the upper wall was carried out using Particle Image Velocimetry (PIV). Two-dimensional velocity components in an x-y plane at Reynolds number of 5128 based on the half height of the channel and bulk velocity were measured. Microbubbles, with an average diameter of 30 ??m were produced by electrolysis and injected in the buffer layer. Different values of the void fraction were attained and used to evaluate the effects of the presence of microbubbles and their concentration within the boundary layer. A reduction in drag was observed due to the injection of microbubbles. Drag reduction augments as the value of the void fraction increases. Furthermore, increases in both the non-dimensional values of streamwise and normal turbulent intensities, normalized by the friction velocity were observed with the void fraction growth. A gradual decrease in the Reynolds shear stresses was achieved as the void fraction increases. This effect is due to a ??decorrelation?? or ??decoupling?? between the streamwise and normal fluctuating velocities. Modifications in the length and time scales due to the presence of microbubbles were detected by calculating two-point correlation coefficients in one and two dimensions and the autocorrelation coefficient at various locations within the measurement zone. Streamline length and time scales were increased. On the contrary, the normal length and time scales were decreased. The vorticity and strain rate values decreased with the injection of microbubbles. Turbulent energy production was also decreased within the boundary layer. Quadrant analysis was used to find out the contribution of the u?? and v?? fluctuating velocity components to the Reynolds stress. The presence of microbubbles reduces the contribution to the Reynolds stresses by Q4 events (sweeps), which are responsible for the production of skin friction. Vortical structure detection in the measurement area was pursued. The structure with and without the microbubble injection is compared. In this study the presence of microbubbles within the boundary layer has produced several modifications in the flow structure as well as reduction in the drag.

No description available.

No description available.

Drag reduction in wall-bounded flows can be achieved by the passive flow control technique using riblets and surface grooves aligned in the mean direction of an overlying turbulent flow. They were inspired by the skin of fast sharks covered with small longitudinal ribs on their skin surfaces. Although it was found that the drag reduction depends on the riblets’ geometrical characteristics, their physical mechanisms have not yet been fully understood in the scientific terms. Regarding riblets sizing, it has been critically explained in the literature how riblets with vanishing size interact with the turbulent flow and produce a change in the drag proportional to their size. Their shapes are focused upon because these are most significant from a technological perspective, and also less well understood. Different riblet shapes have been designed, some with complicated geometries, but except for the simple ones, such as U and V grooves, there has not been enough study regarding shape features. Therefore, special effort is undertaken to the design of an innovative type of ribleted surface, e.g. the Serrate-Semi-Circular shape, and its effect on the skin friction and drag reduction. In this work, the possible physical mechanisms of riblets for turbulent drag reduction have been explored. The modelling and experiments concerning the relationship between the riblets features and the turbulent boundary layer structure have also been reviewed. Moreover, numerical simulations on riblets with different shapes and sizes are presented and studied in detail. An accurate treatment based on k-ε turbulence model was adopted to investigate the flow alteration and the consequent drag reduction on ribleted surfaces. The interaction of the overlying turbulent flow with riblets and its impact on their drag reduction properties are further investigated. In addition, the experimental facilities, instrumentation (e.g. hotwires) and measurement techniques (e.g. time-averaged turbulence structure) have been employed to experimentally investigate the boundary layer velocity profiles and skin friction for smooth and micro-structured surfaces (the proposed riblet shape, respectively and the presented new design of riblets with serration inside provides 7% drag reduction. The results do not show significant reduction in momentum transfer near the surface by riblets, in particular, around the outer region of the turbulent boundary layer. Conclusions with respect to the holistic investigation on the drag reduction with Serrate-Semi-Circular riblets have been drawn based on the research objectives as achieved. Recommendations for future work have been put forward particularly for further future research in the research area.

No description available.

Superhydrophobic surfaces are able to entrap gas pockets in-between surface roughness elements when submerged in water. These entrapped gas pockets give these surfaces the potential to reduce drag due to the overlying flow being able to locally slip over the gas pockets, resulting in a mean slip at the surface. This thesis investigates the different effects that slip and the texturing of the surface have on turbulence over superhydrophobic surfaces. It is shown that, after filtering out the texture-induced flow, the background, overlying turbulence experiences the surface as a homogeneous slip boundary condition. For texture sizes, expressed in wall units, up to $L^+ \lesssim 20$ the only effect of the surface texture on the overlying flow is through this surface slip. The direct effect of slip does not modify the dynamics of the overlying turbulence, which remains canonical and smooth-wall-like. In these cases the flow is governed by the difference between two virtual origins, the virtual origin of the mean flow and the virtual origin experienced by the overlying turbulence. Streamwise slip deepens the virtual origin of the mean flow, while spanwise slip acts to deepen the virtual origin perceived by the overlying turbulence. The drag reduction is then proportional to the difference between the two virtual origins, reminiscent of drag reduction using riblets. The validity of slip-length models to represent textured superhydrophobic surfaces can resultantly be extended up to $L^+ \lesssim 20$. However, for $L^+ \gtrsim 25$ a non-linear interaction with the texture-coherent flow alters the dynamics of the background turbulence, with a reduction in coherence of large streamwise lengthscales. This non-linear interaction causes an increase in Reynolds stress up to $y^+ \lesssim 25$, and decreases the obtained drag reduction compared to that predicted from homogeneous slip-length models.

Drag reduction is a well-observed phenomenon, it was first observed by the British chemist Toms in 1946, yet its mechanism is still unknown to this day. Polymer Drag reduction has found application in reducing pumping costs for oil pipelines (its use in the Trans Alaska Pipeline has resulted in an increase from 1.44 million bbl./day to 2.1356 million bbl./day), increasing the flow rate in firefighting equipment , and in supporting irrigation and drainage systems. Surfactant drag reducers are used industrially in district heating and cooling systems. Though the fields of Surfactant Drag Reduction and Polymer Drag Reduction are each independently well-developed the effect of their interaction on drag reduction is a less explored phenomenon. Through a well chosen pairing of surfactant and polymer, drag reduction can be maximized while minimizing surfactant and polymer concentrations cutting down on cost and environmental impact. The focus of this work was to determine if there was any positive interaction between the polymers Polyethylene Oxide (PEO) and Anionic PolyAcrylAmide (PAM) and the surfactant Amphosol CG (Cocamidopropyl Betaine) as well as any interaction between the polymers themselves. Both polymers are popular drag reducers while Amphosol is a practically nontoxic (LD50=5g/kg) zwitterionic surfactant and is readily biodegradable. In order to determine if any interaction was present and at what concentration was this most notable 4 techniques were used: Surface tension, Conductivity, Relative Viscosity and Shear Viscosity measurement. From this analysis the polymer Saturation point (PSP), Critical aggregation concentration (CAC) and Critical micelle concentration (CMC) were found as well as the concentrations that optimized the viscosity for the pilot plant runs. The bench scale results were used to pick the optimum concentrations for the polymer surfactant solutions. Pressure readings and flowrate measurements were used to plot the Fanning Friction Factor against the Generalized Reynolds Number for the surfactant polymer mixtures and compared to their pure polymer and surfactant counterparts. The Blasius line was found to hold for water measurements taken and is the base to determine percentage drag reduction. The effect of the presence of amphosol on degradation and overall drag reduction were noted. Other factors considered were pipe diameter and the effect of ionic impurities in the solvent.

Drag reduction in turbulent pipe flow using polymeric and surfactant additives is well known. Although extensive research work has been carried out on the drag reduction behavior of polymers and surfactants in isolation, little progress has been made on the synergistic effects of combined polymers and surfactants. In this work the interactions between drag-reducing polymers and surfactants were studied. The drag-reducing polymers studied were nonionic polyethylene oxide (referred to as PEO) and anionic copolymer of acrylamide and sodium acrylate (referred to as CPAM). The drag-reducing surfactants studied were nonionic ethoxylated alcohol - Alfonic 1412-7 (referred to as EA), cationic surfactant - Octadecyltrimethylammonium chloride in pure powder form (referred to as OTAC-p) and commercial grade cationic surfactant - Octadecyltrimethylammonium chloride in isopropanol solvent - Arquad 18-50 (referred to as OTAC-s). The interactions between polymers and surfactant were reflected in the measurements of the physical properties such as electrical conductivity, surface tension, viscosity and turbidity. The critical micelle concentration (cmc) of the mixed polymer / surfactant system was found to be different from that of the surfactant alone. The viscosity of a polymer solution was significantly affected by the addition of surfactant. Weak interactions were observed for the mixed systems of nonionic polymer - nonionic surfactant and anionic polymer - nonionic surfactant. Due to the wrapping of polymer chains around the developing micelles, a minimum in the viscosity is observed in these two cases. In the case of nonionic polymer / cationic surfactant system, the change in the viscosity was found to depend on the polymer concentration (C) and the critical entanglement concentration (C*). When the polymer concentration (C) was less than C* (C < C*), the plot of the viscosity versus surfactant concentration exhibited a minimum. When C > C*, a maximum in the viscosity versus surfactant concentration plot was observed. The interactions between nonionic polymer and cationic surfactant were observed to increase with the increase in temperature. A large drop in the viscosity occurred in the case of anionic-polymer / cationic-surfactant system when surfactant was added to the polymer solution. The observed changes in the viscosity are explained in terms of the changes in the extension of polymeric chains resulting from polymer-surfactant interactions. The anionic CPAM chains collapsed upon the addition of cationic OTAC-p, due to charge neutralization. The presence of counterion sodium salicylate (NaSal) stabilized the cationic surfactant monomers in the solution, resulting in micelle formation at a surfactant concentration well below the concentration where complete charge neutralization of anionic polymer occurred. Preliminary results are reported on the pipeline drag reduction behavior of mixed polymer-surfactant system. The results obtained using combinations of CPAM / OTAC-p in pipeline flow are found to be in harmony with the interaction study. Due to the shrinkage of CPAM chains upon the addition of OTAC-p, the drag reducing ability of CPAM is compromised.