Orientadores: Edvaldo Sabadini, Marcelo G. de Oliveira / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-04T14:22:34Z (GMT). No. of bitstreams: 1 Alkschbirs_MelissaInger_D.pdf: 20703558 bytes, checksum: 093e76dc07818947b87478f9a799ad47 (MD5) Previous issue date: 2004 / Resumo: O efeito da redução de atrito hidrodinâmico em soluções poliméricas nas estruturas produzidas pelo impacto de gotas foi determinado por meio da análise de imagens obtidas utilizando uma câmera CCD e um programa de tratamento de imagens. O impacto de uma gota contra uma superficie líquida causa o fenômeno conhecido como splash, onde duas principais estruturas são formadas: a coroa e o jato Rayleigh. A altura máxima atingida pelo jato Rayleigh foi utilizada como parâmetro para determinar a redução de atrito hidrodinâmico, proporcionada pela presença de pequenas quantidades (ppm) de polímeros de elevada massa molar presentes em solução. A capacidade redutora de atrito hidrodinâmico do poli( óxido de etileno ), PEO, o mais eficiente agente redutor de atrito, foi estudada em função da qualidade do solvente, da temperatura, da concentração, da massa molar e da flexibilidade intrínsica da cadeia polimérica. As modificações decorrentes de alguns destes fatores sobre o raio de giração do polímero e, conseqüentemente sobre o tempo de relaxação e a viscosidade elongacional, são responsáveis pelas modificações morfológicas observadas no jato Rayleigh. Estudos temporais da evolução do splash também foram desenvolvidos, onde se procurou correlacionar a taxa de deformação do líquido com o tempo de relaxação da cadeia polimérica. O presente trabalho mostra, de forma inédita, que é possível utilizar o splash nos estudos sobre a redução de atrito hidrodinâmico / Abstract: The presence of very small amounts (ppm) of high-molecular weight polymers in a solution produces high levels of drag reduction in a turbulent flow. This phenomenon, termed as the Toms Effect, was studied using images of the impact of a small drop against shallows liquid surfaces, both liquids containing a drag reducer agent. After the impact a crown and a cavity are created and the collapse of these structures impels a liquid column, named as Rayleigh jet. This phenomenon is termed splash. The amplitude reached by the Rayleigh jet was used to estimate the energy of the drop stored in the liquid; therefore, the maximum height of the jet allow us to determine the percentage of drag reduction. The results were discussed in terms of different parameters such as polymer concentration, molecular weight in the poly(ethylene oxide), PEO, the most efficient drag reducer agent in aqueous system. The splash in aqueous polymeric solution is dominated by the elongational viscosity and therefore, the polymer relaxation time has an important role in the process. We consider that the main contribution of this work to the drag reduction field is the new approach proposed to investigate this old hydrodynamic phenomenon / Doutorado / Físico-Química / Doutor em Quimica

The remarkable efficacy with which normal surface mass transpiration (blowing and suction) alters a given base flow to achieve a desired predefined objective has motivated several investigations on drag reduction, self-propulsion and suppression of separation and wake unsteadiness in bluff body flows. However, the energetic efficiency, a critical parameter that determines the true efficacy and in particular practical feasibility of this control strategy, has received significantly less attention. In this work, we determine the optimal zero net mass transpiration blowing and suction profiles that minimize net power consumption while reducing drag or enabling self-propulsion in typical bluff body flows. We establish the influence of prescribed blowing and suction profiles on the hydrodynamic loads and net power consumption for a representative bluff body flow involving flow past a stationary two-dimensional circular cylinder. Using analysis based on Oseen’s equations, we find that all the symmetric modes, except the first one, lead to an increase in the net power consumption without affecting hydrodynamic drag. The optimal blowing and suction profile that yields minimum power consumption is such that the normal stress acting on the cylinder surface vanishes identically. Furthermore, we show that a self-propelling state corresponding to zero net drag force is attained when the first mode of blowing and suction profile is such that the flow field be-comes irrigational. Based on these findings we employ direct numerical simulation tools to decipher the Reynolds number dependence of the optimal profiles and the associated power consumption for both drag reduction and self-propulsion. For a typical Reynolds number, the time-averaged drag coefficient first decreases due to vortex shedding suppression, then increases and eventually decreases again after attaining a local maximum as the strength of the first mode is increased. The net power consumption continues to decrease with an increase in the strength of the first mode before reaching a minima after which it rises continuously. For a Reynolds number of 1000 over fifteen fold reduction in drag is achieved for an optimal blowing and suction profile with a maximum radial surface velocity that is nearly 1.97 times the free stream velocity. Next, to establish whether or not higher modes play a role in decreasing net power consumption at finite Reynolds number, we perform theoretical analysis of a configuration similar to the one described above for a spherical body. At zero Reynolds number, as a result of mode independence, we show that surface blow-ing and suction of any form that involves second or higher order axisymmetric or non-axisymmetric modes does not contribute to drag and only leads to an increase in total power consumption. However, at finite Reynolds number, using analysis based on Oseen’s equations, we find that the second and higher modes contribute substantially to the optimal profiles. Finally to understand the effects of a change in shape we consider generalization of the above analysis to axisymmetric prolate and oblate spheroidal bodies. We find that for a general axisymmetric body with non-constant curvature, the optimal drag reducing and self-propelling blowing and suction profiles for minimum power consumption contain second and higher-order modes along with the first mode even when the Reynolds number is zero. The net decrease in power consumption with the use of second and higher order modes exceeds 33% for a disk-like low aspect ratio self-propelling oblate spheroid. Moreover, we perform comparisons between blowing and suction and tangential surface velocity based boundary deformation propulsion mechanisms. Below an aspect ratio of 0.56 we find blowing and suction mechanism to be more efficient for self-propulsion of an oblate spheroid. In contrast, for a self-propelling pro-late spherical micro-swimmer, we show that the tangential surface tread milling consumes less power irrespective of the aspect ratio.

Significant effort has been placed on the development of surfaces which reduce the amount of drag experienced by a fluid as it passes over the surface. Alterations to the fluid itself, as well as the chemical and physical composition of the surface have been investigated with varying success. Investigations into turbulent drag reduction have been mostly limited to those involving bubbles and riblets. Superhydrophobic surfaces, which combine hydrophobic surface chemistry with a regular array of microfeatures, have been shown to provide significant drag reduction in the laminar regime, with the possibility of extending these results into turbulent flows. Direct numerical simulations are used to investigate the drag reducing performance of superhydrophobic surfaces in turbulent channel flow. Slip velocities, wall shear stresses, and Reynolds stresses are considered for a variety of superhydrophobic surface microfeature geometry configurations at friction Reynolds numbers of Re = 180, Re = 395, and Re = 590. This work provides evidence that superhydrophobic surfaces are capable of reducing drag in turbulent flow situations by manipulating the laminar sublayer and turbulent energy cascade. For the largest micro-feature spacing of 90 microns an average slip velocity over 80% of the bulk velocity is obtained, and the wall shear stress reduction is found to be greater than 50%. The simulation results suggest that the mean velocity profile near the superhydrophobic wall continues to scale with the wall shear stress, but is offset by a slip velocity that increases with increasing micro-feature spacing.

The purpose of this thesis work was to design a new shape for the BYU Supermileage vehicle in order to improve its fuel efficiency. Computational Fluid Dynamics (CFD) was used to obtain the coefficient of drag (CD) and drag area of the current baseline vehicle at a Reynolds number of 1.6x10^6 and 8.7x10^5. Then a new shape was developed using mesh morphing software. The new shape was imported into the CFD program and the drag figures and airflow plots from the modified design were compared with the baseline vehicle. Scale models of the vehicles were also printed using a 3D printer in order to perform wind tunnel testing. The models were installed in the wind tunnel and the coefficient of drag and drag area were compared at a Reynolds number around 8.7x10^5.It was found from the CFD results that the new vehicle shape (labelled Model C) caused a 10.8% reduction in CD and a 17.4% reduction in drag area under fully laminar flow. Smaller drag reductions were observed when the flow was fully turbulent. From the wind tunnel comparisons, it was found that Model C reduced CD by 5.3% and drag area by 11.4%, while the fully laminar CFD results at Re = 8.7x10^5 showed that Model C reduced CD by 9.8% and drag area by 15.9%. Smaller drag reductions were again observed for fully turbulent flow. Thus in order to improve the aerodynamic performance, the current vehicle shape should be changed to match that of Model C, and laminar flow should be encouraged over as much of the wetted area as possible.

One approach recently proposed for reducing the frictional resistance to liquid flow in microchannels is the patterning of micro-ribs and cavities on the channel walls. When treated with a hydrophobic coating, the liquid flowing in the microchannel wets only the top surfaces of the ribs, and does not penetrate into the cavities, provided the pressure is not too high. The net result is a reduction in the surface contact area between channel walls and the flowing liquid. For micro-ribs and cavities that are aligned normal to the channel axis (principal flow direction), these micropatterns form a repeating, periodic structure. This thesis presents numerical results of a study exploring the momentum and thermal transport in a parallel plate microchannel with such microengineered walls. The liquid-vapor interface (meniscus) in the cavity regions is approximated as flat in the numerical analysis. Two conditions are explored with regard to the cavity region: 1) The liquid flow at the liquid-vapor interface is treated as shear-free (vanishing viscosity in the vapor region), and 2) the liquid flow in the microchannel core and the vapor flow within the cavity are coupled through the velocity and shear stress matching at the interface. Predictions reveal that significant reductions in the frictional pressure drop (as large as 80%) can be achieved relative to the classical smooth channel Stokes flow. In general, reductions in the friction factor-Reynolds number product (fRe) are greater as the cavity-to-rib length ratio is increased (increasing shear-free fraction), as the relative module length (length of a rib-cavity module over the channel hydraulic diameter) is increased, as the Reynolds number decreases, and as the vapor cavity depth increases. The thermal transport results predict lower average Nusselt (Nu) numbers as the cavity-to-rib length ratio is increased (increasing shear-free fraction), as the relative module length (is increased, and as the Reynolds number decreases with little dependence on cavity depth. The ratio of Nu to fRe was evaluated to characterize the relative change in heat transfer with respect to the reduction in driving pressure. Results show that the benefits of reduction in driving pressure outweigh the cost of reduction in heat transfer at higher Reynolds numbers and narrower relative channel widths.

Due to the increase of application in a number of emerging technologies, a growing amount of research has focused on the reduction of drag in microfluidic transport. A novel approach reported in the recent literature is to fabricate micro-ribs and cavities in the channel wall that are then treated with a hydrophobic coating. Such surfaces have been termed super- or ultrahydrophobic and the contact area between the flowing liquid and the solid wall is greatly reduced. Further, due to the scale of the micropatterned structures, the liquid is unable to wet the cavity and a liquid meniscus is formed between ribs. This creates a liquid-vapor interface at the cavity regions and renders surfaces with alternating regions of no-slip and of reduced shear on the microscale. This thesis reports the numerical study of hydrodynamically fully-developed laminar and turbulent flows through a parallel plate channel with walls exhibiting micro-ribs and cavities oriented parallel to the flow direction, where fully developed turbulent flow is considered in a time-averaged sense. Three laminar flow models are implemented to investigate the liquid-vapor interface and to account for the effects of the vapor motion in the cavity regions. For each of the laminar flow models, the liquid-vapor interface was idealized as a flat interface. As a benchmark for the proceeding laminar flow models, the first model considers the case of a vanishing shear stress at the interface between the liquid and vapor domains. Effects of the vapor motion in the cavity are then accounted for in a one-dimensional cavity model where the vapor velocity is considered to be dependent on the wall normal coordinate only, followed by a two-dimensional cavity model that accounts for the vapor velocity's dependence on the transverse coordinate as well. The vapor cavity is modeled analytically and is coupled to the liquid domain by equating the fluid velocities and shear stresses at the liquid-vapor interface. In the turbulent flow model the liquid-vapor interface is idealized as a flat interface with a zero shear stress boundary condition. In general the numerical predictions show a reduction in the total frictional resistance as the cavity width is increased relative to the channel width, the channel height-to-width aspect ratio is decreased, and the vapor cavity depth is increased. The frictional resistance is also reduced with increased Reynolds number in the turbulent flow case. In the range of parameters examined for each fluid flow regime, reductions in drag as high as 91% and 90% are reported for the laminar flow and turbulent flow models, respectively. Under similar conditions however, the turbulent flow results indicate a greater reduction in flow resistance than for the laminar flow scenario. Based on an analysis of the obtained data, analytical expressions are proposed for both laminar and turbulent flow which facilitates the prediction of the frictional resistance.

In the context of building a framework for active flow control of turbulent boundary layers in wings, a set of large-eddy simulation (LES) are implemented in OpenFOAM. The flow around a NACA4412 wing profile is simulated at 5° angle of attack and Re_c = 400˙000. Validation of the uncontrolled flow results is performed with respect to the dataset generated by Vinuesa et al. (2018) at the same aerodynamic configuration. Afterwards, two different flow control strategies are analyzed over the suction side (SS) of the wing to yield skin friction drag reduction and an overall improvement of the aerodynamic efficiency. The region subject to the actuation spans 0.25 x_ss/c to 0.:86 x_ss/c, where c is the chord length of the wing. In the current setup, uniform blowing (BLW) and suction (SCT) control schemes show close agreement with the trends presented by Atzori (2021). Indeed, BLW decreases the viscous drag, but increases its pressure contribution and penalizes the lift, thus lowering the global efficiency of the wing, while SCT has an opposite effect. Thus, these methods behave similarly to pressure gradients (PGs) conditions, as BLW enhances the APG, whereas SCT damps it. The streamwise travelling waves strategy is then assessed for three set-ups characterized by different phase speeds. A consistent skin friction drag reduction and efficiency improvement are observed for two cases, while milder benefits are recorded even when drag increase was expected. Trends which have already been reported in the literature by Quadrio et al. (2009) and Skote (2014) are identified, i.e. the effects of this actuation to be mainly enclosed in the viscous sub-layer and the gross amount of drag reduction to be dependent on the wave relative speed; however, it is believed that the PGs conditions over the SS of the wing significantly alters the outcomes of the chosen parameters. Eventually, Reynolds averaged Navier-Stokes (RANS) simulations are performed to assess their accuracy with respect to the generated LES set-up, in the effort to enable a multi-fidelity approach for future works.

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