M. Vlachogiannis

Experiments on laminar film flow along a periodic wall

Experimental results are reported on the structure of gravity-driven film flow along an inclined periodic wall with rectangular corrugations. A fluorescence imaging method is used to capture the evolution of film height in space and time with accuracy of a few microns. The steady flow is found to exhibit a statically deformed free surface, as predicted by previous asymptotic and numerical studies. Though usually unstable, its characteristics determine much of the subsequent non-stationary dynamics. Travelling disturbances are observed to evolve into solitary multi-peaked humps, and pronounced differences from the respective phenomena along a flat wall are noted. Finally, a remarkable stabilization of the flow at high Reynolds numbers is documented, which proceeds through the development of a three-dimensional flow structure and leads to a temporary decrease in film thickness and recession of solitary waves.

Solitary waves on inclined films: Flow structure and binary interactions

The downstream evolution of disturbances, introduced at the inlet of a liquid film flowing along an inclined plane wall, is studied numerically by solving the full, time-dependent Navier–Stokes equation. Computational results are validated against the predictions of spatial linear stability analysis and against detailed data of the entire evolution process. The structure of the flow field below the waves is analyzed, and the results are used to test assumptions frequently invoked in the theoretical study of film flow by long-wave equations. An interesting prediction is that solitary waves exhibit strongly nonparabolic velocity profiles in front of the main hump, including a slim region of backflow. The computational scheme is subsequently used to study solitary wave interactions. It is predicted that coalescence (the inelastic collision of two humps) is not inevitable but occurs only when the waves differ appreciably in height. Waves of similar size repel monotonically, whereas for intermediate difference...

Experimental study of inclined film flow along periodic corrugations: The effect of wall steepness

Liquid films on a flat wall at inclination are first destabilized at a criticalRe = 5=6cot . The instability mode is a long-wave disturbance that travels downstream fast enough to result in a convective instability. We presently investigate how the primary instability is modified when the flat wall is substituted by one with periodic corrugations. Film flow along a corrugated wall is studied experimentally in a 800 mm long by 250 mm wide channel, whose inclination can be adjusted from 0-50 . Two walls are considered, both with wavelength 12 mm and height 2 mm: in one case the corrugations are purely sinusoidal and in the other they consist of symmetric step-ups and step-downs (orthogonal shape). Multiple conductance probes are installed at different streamwise locations and record time-series of the local free surface elevation[1]. Consistent with previous studies[2], both corrugated walls are found to result in a strong delay of the primary instability of the liquid film, compared to the classical results for flat substrate. Moreover, a significant effect of inclination angle is observed, with the film becoming progressively more stable at higher inclinations. The stabilization also varies with the shape of corrugations: film flow along the orthogonally corrugated wall is more stable at intermediate inclinations. The above results are clarified and interpreted by the observation of a new instability mode, which manifests as a high-frequency oscillation ( 10 Hz) and corresponds to a traveling wave of small wavelength. The new mode appears to be the primary cause of instability at intermediate and high channel inclinations. This observation supports a computational prediction[3], that film flow along sinusoidal corrugations may be unstable not only to long-wave disturbances, but also to disturbances with the wavelength of the wall. It is conjectured that, through an interaction of inlet disturbances with the deformed steady flow, energy is transferred from long-wave modes to the newly observed short wave mode. Given that the latter is expected to remain stable at higher Re, this tentative mechanism may explain the observed enhanced stability of the film.

Observations of solitary wave dynamics of film flows

Experimental results are reported on non-stationary evolution and interactions of waves forming on water and water-glycerol solution flowing along an inclined plane. A nonlinear wave generation process leads to a large number of solitary humps with a wide variety of sizes. A fluorescence imaging method is applied to capture the evolution of film height in space and time with accuracy of a few microns. Coalescence-the inelastic interaction of solitary waves resulting in a single hump-is found to proceed at a timescale correlated to the difference in height between the interacting waves. The correlation indicates that waves of similar height do not merge. Transient phenomena accompanying coalescence are reported. The front-running ripples recede during coalescence, only to reappear when the new hump recovers its teardrop shape. The tail of the resulting solitary wave develops an elevated substrate relative to the front, which decays exponentially in time; both observations about the tail confirm theoretical predictions. In experiments with water, the elevated back substrate is unstable, yielding to a tail oscillation with wavelength similar to that of the front-running ripples

Effectiveness of a drag reducing polymer: Relation to molecular weight distribution and structuring

Solutions of partially hydrolyzed polyacrylamide were degraded by intermittent circulation through a large pump in a turbulent flow loop. Measurements of pressure drop, fluid turbulence, molecular weight distribution, and viscosity were made. Rheo-optical studies were also carried out to explore the propensity of solutions to form structures under simple shear flow. Degradation was not accompanied by significant changes in the molecular weight distributions. This observation suggests that, for the system studied, clusters or aggregates of polymers have a more important effect on the turbulence than individual molecules. Therefore, degradation occurs by the destruction of these clusters. This result is consistent with the observation that larger drag reductions are realized by the injection of concentrated polymer solutions into a water flow.

Desalination by mechanical compression of humid air

A novel desalination concept is described, combining the principles of humidification-dehumidification and mechanical vapor compression. Thermodynamic analysis indicates the effect of operating conditions on water production capacity and compression energy. A laboratory prototype was built to prove the above concept. Preliminary experiments indicate successful operation and point to improvements in a future design.

Effect of channel width on the primary instability of inclined film flow

A procedure is developed to detect the onset of interfacial instability in inclined film flows (with estimated accuracy better than 5%) and is used to show that the finite width of experimental channels stabilizes the undisturbed liquid film. Deviation from the classical prediction scales inversely with the product of channel width and sine of inclination angle, and for small inclinations and/or narrow channels is of the order of 100%. The effect is tentatively attributed to the influence of sidewalls on the traveling disturbances, which results in curved crestlines and transverse variation of wave characteristics.

Nominally two-dimensional waves in inclined film flow in channels of finite width

Traveling waves in inclined film flow in channels of finite width are never truly two-dimensional (2D) because of a long-range effect of sidewalls. The present study documents the characteristics of the first waves that are observed beyond the primary instability (termed nominally 2D) by taking measurements in a 3000 mm long inclined facility with adjustable width up to 450 mm using a fluorescence imaging technique. It is observed that nominally 2D waves are very persistent structures with their crests attaining a parabolic shape, which is symmetric with respect to the channel centerplane irrespective of the 3D content of the inlet forcing. The apex curvature of the parabola varies inversely with channel width and Reynolds number. The wave height is maximum at the centerplane and decreases to zero at the sidewalls, irrespective of the wetting properties of the system. The linear phase velocity of nominally 2D waves is always lower than predicted by the theory for small amplitude, 2D waves, and significant...

Relation of Polymer Drag Reduction to Structure and Molecular Weight (Keynote)

The influence of molecular weight distribution on polymer drag reduction was studied for solutions of partially hydrolyzed polyacrylamide by analyzing the solution over a period of time during which the polymer was degrading. No relation was observed between the effectiveness of this polymer solution and molecular weight.© 2003 ASME