Search

The decay of a two-dimensional solitary wave into three-dimensional waves in film flow along a vertical plate has been studied using a high-speed laser-induced fluorescence technique that allows high spatial and temporal resolution measurements of the thickness of wave liquid films. It has been shown that the three-dimensional wave packet resulting from the decay of a two-dimensional solitary wave has a strong perturbing effect on the falling film and leads to the formation of rivulets on its surface.

The results of an experimental study of the waves on the surface of a vertically falling liquid film in the range of the Reynolds numbers of film flow 80 < Re < 420 have been presented. The experiments have been carried out with the use of the field optical diagnostic methods with high spatial and time resolution. Generation of capillary ripples with a small wavelength has been detected at the crests of large three-dimensional waves. It is in these areas of three-dimensional waves where so-called “turbulence spots,” the occurrence of which is associated with a transition to a turbulent flow mode, are observed, as noted in the literature. Long-lived rounded depressions with characteristic transverse dimensions of 1–3 mm, which are formed in the region of the interaction of capillary precursors and then propagate as independent structural elements of the flow, have also been recorded.

Fluid velocity measurement in liquid films is a challenging task due to the small film thickness and large transverse gradients of velocity. While this problem was solved for the case of two-dimensional waves by several research groups, measurements in three-dimensional waves impose additional difficulties that we managed to overcome in the course of this study. We used a single light field camera for simultaneous particle tracking velocimetry and laser-induced fluorescence film thickness measurements. In this way, obtained velocity fields were related to the wavy interface shape. The applicability of the proposed methodology was demonstrated in the test experiments, which were carried out for two-dimensional regular waves. Measurements in three-dimensional waves were performed for the fluids with different viscosities and surface tensions but with close values of the Kapitsa number. In all investigated cases, the flow structure in different parts of waves is similar. Comparison of the experimentally obtained velocity fields with calculations by weighted residual integral boundary layer model shows qualitative agreement.

In this paper we present the results of experimental investigation of the effect of ionic surfactants (sodium dodecyl sulfate and cetrimonium bromide) at various concentrations on the vertical film flow in the range of Reynolds numbers 10 < Re < 50. The laser-induced fluorescence technique was used for instant film thickness measurements. The results are compared with data obtained for non-ionic Triton X-100. For all surfactants the rivulets formation and the dumping of wave motion are observed. The difference in the action of ionic and non-ionic surfactants on wave motion is revealed. Such deviations indicate that for ionic surfactants the surface tension relaxation process (at least in the case of film flows) is not fully described by diffusion processes, but has a more complex nature.

Experimental study of wave evolution on the surface of isothermal liquid films falling down vertical plate conducted by means of field measuring system has revealed three typical scenarios of the evolution in the range of low and moderate liquid flow rates. At low liquid flow rates transverse modulation of 2D waves develops without formation of 3D waves. Two dimensional waves break up into 3D waves only when Reynolds number Re of the film flow exceeds some threshold value which depends on physical properties of working fluid. The fully developed steady-state 3D wave regimes are observed when Re exceeds another threshold value. The obtained results allow to determine the boundaries within which the adequate comparison between modeling and experiments on 3D wave flow of liquid films is possible.

We have simultaneously measured the velocity vector and film thickness fields in three-dimensional (3D) waves formed upon the decay of regular 2D waves on a liquid film flowing down a vertical plate. Velocity fields at various distances from the plate surface in various regions of a 3D wave are constructed. It is shown that the liquid flow in the wave has a complex structure with regions of backflow and spanwise streams, which mainly occur under the main crest of the wave.

Averaged fields of thickness of an isothermal vertically flowing liquid film, obtained by the laserinduced fluorescence method, have been analyzed. The chosen minimum averaging time interval, at which the wave motion is completely averaged, allows one to observe rivulet dynamics in a transverse direction. It is found that short-lived rivulets, which are chains of no less than five to eight waves with similar transverse coordinates, are dominant structures on the film surface at Reynolds number Re > 50.

Effect of a surfactant Triton X-100 on three-dimensional wave regimes of the falling liquid film flow was experimentally studied. The shadow method and laser induced fluorescence measurements revealed that for surfactant concentrations at which complete suppression of wave motion is not observed, the flow characteristics at the three-dimensional wave regime differ from the characteristics of a pure water flow. Most likely, these differences are associated with differences in the structure of flows under the three-dimensional wave, which were revealed by simultaneous measurements of the thickness and volumetric measurements of the velocity in the wave at the initial section of the flow. For films with the surfactant concentration of 2000 mg/l, the capillary ripples in front of the three-dimensional wave are weakly expressed, there are no backflows, and transverse flows in the area of the capillary precursors are much weaker than in the case of pure water.

This Letter presents the results of an experimental study of the wave regimes of the liquid film flow over the vertical plate within a range of moderate Reynolds numbers using liquids with different physical properties. The analysis of statistical characteristics of wave fields has made it possible to elaborate the classification of three-dimensional wave regimes in a range of liquid flow rates under study. Steady-state threedimensional wave regimes are shown to occur at Reynolds numbers that are significantly higher than the Reynolds number of a transition from a two-dimensional to three-dimensional wave flow.

In this paper we present results of experimental investigation of the influence of soluble surfactant (Triton X-100) on wave characteristics of vertically falling liquid film in the range of film flow Reynolds number 25 < Re < 95. Shadowgraph technique is used to analyze wave patterns on the whole length of the test section (140 cm). Film thickness fields obtained with the help of high-speed Laser Induced Fluorescence (LIF) technique in two areas in the bottom part of test section allow analyzing statistical characteristics of the film flow. Expectedly, adding surfactant suppresses wave motion. For some surfactant concentrations the absence of waves is observed in the whole test section. For other concentrations the wave repetition rate decreases with distance from the liquid inlet. Analysis of spectral characteristics reveals that for water with some concentrations of surfactant, as for pure liquids, the steady-state three-dimensional wave regimes are observed. The characteristics in such flow regimes differ from the characteristics of pure water flow.

We present the results of studying the influence of surfactants on three-dimensional waves, in particular, on the flow structures in the wave. Volumetric fluid velocity measurements in waves, as well as the determination of their shape, were carried out using a light field camera. An aqueous solution of the nonionic surfactant Triton-X was used as a working fluid. The presence of surfactant leads to a significant change in the structure of the transverse flow regions under the main hump of the wave. Besides, capillary precursor and backflow in it are suppressed.

We present the results of an experimental investigation of the transition from a two-dimensional to three-dimensional (3D) flow in a liquid film flowing down a vertical plate of large dimensions. It is established that this transition is accompanied by the formation of rivulets. At large distances from the initial region, the wave regime can be almost completely suppressed in inter-rivulet regions.

Two-phase flow in a rectangular short horizontal channel 200 μm high was studied experimentally. The use of the fluorescent method made it possible to reveal flow of liquid in the channel and to determine its characteristics quantitatively. The existence of the regime of separate (stratified) flow is established. Based on analysis of previous investigations and newly obtained data, it is shown that a change in the height of the horizontal channel has a substantial effect on the boundaries between the regimes. The region of the churn regime increases with decreasing thickness of the channel.

Results of an experimental investigation of the features of evolution of a solitary three-dimensional (3D) wave on the surface of a vertically falling liquid film heated by a constant heat flux are presented. It is established that the wave weakly influences the film surface temperature. The interaction of a 3D wave with thermocapillary rivulet structures on the falling film surface results in that the wave is pulled toward the center of a rivulet.

We have investigated the effect of soluble surfactant Triton X-100 on the initial development of waves on vertically falling liquid films. Field method of laser induced fluorescence was used to obtain instantaneous wave patterns forming at different concentrations of the surfactant. These concentrations were chosen on the basis of simple consideration that most pronounced influence of the surfactant on the wave characteristics should be observed when relaxation time of surface tension (dependent on surfactant concentration) is comparable with characteristic time of wavy motion. At low and moderate surfactant concentrations damping of waves is observed, which coincides with the results of other authors. At large surfactant concentration the waves start to grow again, but without initial stage of two dimensional wave development typical of pure liquids. The absence of this initial stage is due to the formation of rivulets on the free surface of films in the presence of the surfactant. The influence of the surfactant concentration on characteristics of rivulets and waves is described.

The transition from a regular two-dimensional to three-dimensional (3D) flow in vertically falling liquid films has been experimentally studied. This transition is accompanied by a significant redistribution of liquid in the lateral direction. Characteristic shapes of 3D structures developing in the course of the transition are described.

Results of systematic investigations of the characteristics of stationary solitary three-dimensional (3D) waves in vertically falling films of liquids with various physical properties are presented. Experimentally determined characteristics of the stationary 3D waves are compared to theoretically predicted values. A domain of film flow regimes is established in which the theory well agrees with experiment.

Results of experimental investigation of regular wave conditions for straight rivulets falling down a vertical plane are presented. Field measurements of the local thickness of rivulets were carried out with the help of laser-induced fluorescence. Data on the wave structure of rivulets is shown in a wide range of frequencies of regular waves for different Reynolds numbers of the flow and contact wetting angles. It was observed for all studied wave conditions that transverse width of rivulets and wetting angles are insensitive to the phase of propagating waves. Moreover, it was found that the wave structure of rivulets differs significantly for the cases of low and high values of the contact angle.

Experimental study of two-phase flow in the short rectangular horizontal channel with height 440 μm has been performed. Characteristics of liquid motion inside the channel have been registered and measured by the Laser Induced Fluorescence technique. New information has allowed determining more precisely the characteristics of churn regime and boundaries between different regimes of two-phase flow. It was shown that formation of some two-phase flow regimes and transitions between them are determined by instability of the flow in the lateral parts of the channel.

A two-phase (liquid-gas) flow in a short horizontal slit channel of a rectangular cross section with a height of 440 μm has been experimentally studied using a fluorescent method, which allowed the flow pattern to be monitored and its quantitative characteristics to be measured. It is established that two-phase flow regimes in this channel substantially differ from the classical regimes of flow in extended channels of large cross section. It is demonstrated that the formation of various two-phase flow regimes and the transitions between them in short narrow rectangular slits are determined by instability of the liquid-gas flow at the side-walls of the channel.

This article presents the results of experimental and theoretical study of 3D instability of waves in liquid films and rivulets, falling down a vertical plate. The method of laser induced fluorescence was used to obtain instant shapes of three dimensional waves in a liquid films and rivulets and to investigate the regularities of formation of 3D wave patterns. In particular, during 2D-3D wave transition in the film flow, we observed a strong transverse redistribution of liquid leading to the formation of rivulets on the surface of isothermal liquid film, which is a phenomenon not described previously. The results of simulation of 2 D and 3D regular waves in the single rivulet are compared with the authors’ previous experimental data.

Results of simultaneous liquid velocity and film thickness fields measurements in the three dimensional waves are presented. The three dimensional waves formed as a result of decay of the forced two dimensional waves were studied. Liquid velocity fields were reconstructed at different distance from the film surface with reference to the specific area of the wave. It was shown that flow in the three dimensional wave has complicated structure with existence of areas with backflow and transverse flows of opposite directions. Main areas of transverse flows existence are lying under main humps of three dimensional waves.

Three-dimensional waves appear at the final stage of the evolving wave regime of a fluid-film flow [1]. At this stage, for values of the Reynolds number of the film flow that are sufficiently low (Re < 100), the wave regime manifests itself as a soliton gas. The film surface is covered by numerous horseshoe-shaped waves, which propagate over a thin residual layer, randomly interacting with each other and explicitly demonstrat

This article presents the results of experimental investigations of the process of transition from two-dimensional (2D) to three-dimensional(3D)waves in liquid films falling down a vertical plate. The method of laser induced fluorescence was used to obtain instant shapes of three dimensional waves and to investigate the regularities of formation of 3Dwave patterns arising due to transverse instability of 2D waves. The obtained results were compared to the results from the published literature on the modeling of 3Dwave regimes of filmflow. Although many details of 3Dwave patterns correspond well, there are a few significant distinctions between our experiments and modeling. In particular, during 2D-3D wave transition, we observed a strong transverse redistribution of liquid leading to the formation of rivulets on the surface of isothermal liquid film, which is a phenomenon not described previously. Possible discrepancies between modeling and experiments, including applicability of boundary layer models and downstream periodic boundary conditions, are discussed. The authors hope that the results presented in the article are of interest not only for modeling of filmflows but also for practical applications because at large distances from the film inlet due to 2D-3D wave transition the local flow rates can differ several times at the transverse distances of about 1 cm, which is an effect that cannot be neglected.

In this Letter we present novel results of the experimental study of three-dimensional waves evolving from a localized disturbance on a vertically falling film at low Reynolds numbers 1.25