Session HU: Multiphase Flows V

Perturbed Partial Cavity Drag Reduction at High Reynolds Numbers

Ventilated partial cavities were investigated at Reynolds numbers to 80 million. These cavities could be suitable for friction drag reduction on ocean going vessels and thereby lead to environmental and economical benefits. The test model was a 3.05 m wide by 12.9 m long flat plate, with a 0.18 m backward-facing step and a cavity-terminating beach, which had an adjustable slope, tilt and height. The step and beach trapped a ventilated partial cavity over the longitudinal mid-section of the model. Large-scale flow perturbations, mimicking the effect of ambient ocean waves were investigated. For the conditions tested a cavity could be maintained under perturbed flow conditions when the gas flux supplied was greater than the minimum required to maintain a cavity under steady conditions, with larger perturbations requiring more excess gas flux to maintain the cavity. High-speed video was used to observe the unsteady three dimensional cavity closure, the overall cavity shape, and the cavity oscillations. Cavities with friction drag reduction exceeding 95{\%} were attained at optimal conditions. A simplified energy cost-benefit analysis of partial cavity drag reduction was also performed. The results suggest that PCDR could potentially lead to energy savings.   

Stability Analysis of Superhydrophobic Friction Reduction Polymeric Microchannels

Superhydrophobic surfaces are surfaces where fluid contact angle is larger than 150$^{\circ}$. Superhydrophobic states which allow water droplets to fall off at low sliding angles are termed as Cassie state. It is widely known that drag/friction reduction is closely related to liquid under Cassie state, and studies have been widely performed to achieve such effects. Our research goal is to develop superhydrophobic microfluidic channels with trenches on the side walls and observe the stability of the air pockets formed within these trenches. We have prepared PDMS(poly-dimethylsiloxane) substrates with different trench aspect ratio of 1:1, 1:2, 1:500 and 1:3000. As the aspect ratio of the trench decreases, the pressure in the air pockets tends to resist wetting. However, once penetration of the water into the air pocket occurred, the shallow trenches were wetted in a rapid fashion while the deep trenches were wetted at a slower rate. A compression model of the air pockets as a function of pressure difference and volume change of the air pockets was also developed. In the theoretical model, the air in the pockets is assumed to be an ideal gas. This model was compared and validated against the experimental results.   

Heating Effects In Very Rough Polymeric Microchannels

Slip in internal flows is known to reduce friction and thus reduce the required pumping power. One method to achieve slip is by `roughening' the surface to induce Cassie state. The Cassie state is a phenomenon in which a liquid rests on top of a rough surface with a gas layer formed underneath. Our research goal is to develop a highly rough microfluidic channel and study the heating effects on the air pockets trapped between the roughness elements. We have prepared a PDMS (poly-dimethylsiloxane) microfluidic channel with trenches on the side walls. The channel dimension is 100um x 110um (width x height), and the dimensions of each trench are 30um x 60um x 110um (width x length x height. As the heat flux into the microfluidic channel increases the air trapped on the trenches expanded increasing the volume of the void. In order to prevent the expanding air from invading the liquid flow layer the pressure drop was increased. Therefore by heating the channel the wetting of air pockets can be prevented even under higher pressure drops, thus maintaining the two phase flow and significantly reducing the friction coefficient.   

Micro-PIV Measurements near a Moving Contact Line

The displacement of one fluid by an immiscible second fluid (i.e. dynamic wetting), governs many natural and technological processes. Despite extensive studies, understanding and modeling the displacement process remains one of the outstanding problems in fluid mechanics. In this work, we explore the physics of the moving contact line (the idealized line of intersection between two fluids and a solid) by measuring velocities near the moving contact line with micron resolution particle image velocimetry. The measured flow is generated by dynamic wetting in a glass microchannel. The microchannel is mounted on an automated microscope stage with precise velocity control allowing for the static placement of the contact line within the field of view. Full-field velocity measurements near the contact line were made in water/glycerol and fructose/glucose/water solutions. Preliminary results appear to show remarkable similarity to controversial theoretical predictions.   

Film Deposition in the Presence of a Moving Contact Line

Film deposition experiments are performed in circular glass capillaries of 500 $\mu $m diameter. Two surface wettabilities are considered, contact angle $\theta $ = 30$^{\circ}$ for water on glass and $\theta $ = 105$^{\circ}$ when a hydrophobic coating is applied. It was observed that the liquid film deposited as the meniscus translates with a velocity U presents a ridge which also moves in the direction of the flow. The ridge is bound by a contact line moving at a velocity U$_{CL}$ as well as a front of velocity U$_{F}$, and it translates over the deposited stagnant film. The behavior of the ridge presents striking dissimilarities when the wettability is changed. Both U$_{CL}$ and U$_{F}$ are approximately twice as large for the non-wetting case at the same capillary number Ca. Classical film deposition theory does not account for the existence of a contact line and it assumes perfect wetting. In contrast, the contact line dynamics fundamentally alter the deposition physics by causing the film to be non-stagnant. As a consequence the non-wetting film is significantly thicker that the Bretherton prediction. Taylor bubbles also form due to the growth of the ridge and are differentiated by wettability, being much shorter and presenting a thicker film in the non-wetting case. The dynamics of the contact line is studied experimentally and a criterion is proposed to explain the occurrence of a shock in the non-wetting film.   

Optical characterization of fibers suspensions in turbulent pipe flow

Suspensions of elongated rigid fibers in turbulent flows are commonly encountered in applications of engineering interest, and may exhibit complicated rheological properties depending on the spatial distribution and orientation of the fibers. Despite the practical importance of fiber suspensions there is insufficient experimental data to validate numerical simulations and provide benchmarks. This paper presents an image analysis algorithm used to calculate orientation and distribution of fibers suspended in turbulent pipe flow. The algorithm is validated using artificial images. These images represent three-dimensional randomly orientated ellipsoids illuminated by a laser sheet and projected onto a two-dimensional plane. The error magnitude on the orientation distribution and number density is found by means of Monte Carlo simulations. Experiments are carried out considering small control volume near the pipewall. Results indicate that fibers exhibit preferred spatial orientation close to the pipewall and more randomized orientation close to the centerline, in qualitative agreement with the available numerical simulations.   

On the periodic motion of a disk falling freely in a tube

Freely falling or rising particles in an unconfined low-viscosity fluid otherwise at rest are known to exhibit oscillatory motions, such as helicoidal or zigzag paths. In this work, we characterized experimentally the oscillatory motions of disks falling in a tube at Reynolds numbers 60 $< Re <$ 250, covering both rectilinear and periodic motions. The fall of the bodies (of density close to that of the fluid) was followed by two travelling cameras to determine the body's translation and rotation characteristics. We focused on the effect of the confinement factor (ratio of the diameter of the body, d, to that of the tube, D). The study was carried on for different body's aspect ratio (ratio of its diameter d to thickness h, taken as 3, 6 and 10), since this parameter is known to strongly influence the characteristics of the oscillatory motions observed in the unconfined situation.   

Stretching of flexible molecules inside fluid threads

The evolution of viscoelastic fluid threads undergoing capillary breakup is complex and depends on the delicate balance between capillary, viscous, and elastic stresses which result in behavior that is markedly different from Newtonian fluids such as the ``beads-on-a-string'' phenomenon. Here, we aim to understand the thinning of a fluid thread and the drop breakup process of polymeric fluids in a simple microfluidic device by direct visualization of fluorescent DNA molecules. Molecules are observed to transition from a coiled state to an almost fully stretched state when experiencing extensional flow within the filament. The stretching of flexible molecules under applied viscous stress is characterized by a simple worm-chain model.   

Oil filaments produced by an impeller in a water stirred tank

Oil dispersions in aqueous media produced in stirred tanks are part of many industrial processes. The oil drops size and dispersion stability are determined by the impeller geometry, stirring velocity and the physicochemical properties of the mixture. A critical parameter is the total interfacial area which is increased as the drop size is decreased. The mechanism that disperses the oil and generates the drops has not been completely explained. In the present work, castor oil (1\% v/v, viscosity 500mPa) and water are stirred with a Scaba impeller in a flat bottom cylindrical tank. The process was recorded with high-speed video and the Reynolds number was fixed to 24,000. Before the stirring, the oil is added at the air water interface. At the beginning of the stirring, the oil is suctioned at the impeller shaft and incorporated into the flow ejected by the impeller. In this region, the flow is turbulent and exhibits velocity gradients that elongate the oil phase. Viscous thin filaments are generated and expelled from the impeller. Thereafter, the filaments are elongated and break to form drops. This process is repeated in all the oil phase and drops are incorporated into the dispersion. Two main zones can be identified in the tank: the impeller discharge characterized by high turbulence and the rest of the flow where low velocity gradients appear. In this region surface forces dominate the inertial ones, and drops became spheroidal.   

Improved flying hot-film anemometry in liquid-gas flows

A modified hot-film anemometry technique was used to measure liquid velocity fluctuations resulting from bubble agitation in a liquid-gas flow. The first modification aims to remedy the main drawback in hot-film anemometry measurements in liquid-gas flow: bubble-probe interaction. To improve bubble detection, optical fibers were installed in close proximity to the anemometer sensing element; in this way, the collisions of bubbles with the probe can be detected and removed from the signal. The second modification resolves the poor performance of the probe at small mean liquid velocity. The sensing element is moved at a known rate; subsequently, this translation velocity is removed from the signal leaving only the fluctuating velocity of the liquid. Furthermore, an analysis of the effect of the signal processing parameters, such as detection and signal length threshold, is conducted. The flow conditions at which this technique was tested covered void fractions up to 6\% in nearly monodispersed bubbly flows. The results obtained show good agreement with reported data by other authors in both, variance and spectral density of the liquid velocity. This technique can be used to measure psuedoturbulence in on bubbly flows.