Water Rivulets Research Articles

Mucilaginibacter rivuli sp. nov., isolated from a water rivulet.

Strain HMF5004T was isolated from a rivulet located in Yongin, Republic of Korea. Phylogenetic analyses based on 16S rRNA gene sequences showed that strain HMF5004T belonged to the genus Mucilaginibacter. Strain HMF5004T was closely related to Mucilaginibacter paludis (97.7%) and Mucilaginibacter gracilis (97.2%). The values of average nucleotide identity and digital DNA-DNA hybridization between strain HMF5004T and M. paludis were 72.8 and 19.2 %, respectively. Cells of strain HMF5004T were Gram-stain-negative, rod-shaped, non-motile, catalase-positive and oxidase-positive. The DNA G+C content of strain HMF5004T was 42.4 mol%. Strain HMF5004T had menaquinone-7 as a major quinone. The major cellular fatty acids included iso-C15 : 0, summed feature 3 (C16 : 1 ω7c and/or C16 : 1 ω6c) and anteiso-C15 : 0. The polar lipids of strain HMF5004T contained phosphatidylethanolamine, five unidentified aminolipids, one unidentified aminophospholipid and four unidentified polar lipids. On the basis of the evidence presented in this polyphasic taxonomic study, strain HMF5004T is considered to represent a novel species for which the name Mucilaginibacter rivuli sp. nov. is proposed. The type strain is HMF5004T (=KCTC 82633T=NBRC 115091T).

Optimization of Multiple Helical Fillets Surface to Suppress Rain-wind Vibration of Stay Cables: A Wind Tunnel Investigation

Aims: To develop a new cable surface for control rain wind induced cable vibration of stay cable. Background: Stay cables are light and vulnerable structures. Therefore, it can be easily excited by wind or rain wind interaction. Stay cables wrapped with a single helical fillet have been proposed so far. However, these countermeasures could prevent cable vibration well, especially in dry conditions. Objective: Therefore, the objective of this research is to develop a new cable surface to control not only RWIV but also dry galloping of stay cable. Methods: A wind tunnel test will be used to investigate the RWIV characteristics and its new countermeasure. Results: First, a rain wind-induced vibration of circular stay cable was reproduced in a wind tunnel environment. The effect of upper and lower water rivulets was examined to understand their role on RWIV better. A rainfall simulator was employed to generate artificial rainfall for two different wind tunnel experiments. Finally, to control the RWIV of stay cables, novel multiple helical surface fillets were used. The upper and lower rain rivulets can play a significant role in energizing the RWIV. It was also found that the multiple helical surface fillets can suppress the cable vibration well both in wet and dry conditions. Conclusion: Multiple helical fillets cable surface could successfully prevent both the RWIV and dry galloping. To fabricate helical fillet cable to control cable vibration, 04 to 12 fillets with sizes ranging from 3mmx7.5mm to 5mmx7.5mm and a pitch of 2.95D-4.78D (D: cable diameter) are the most influential parameters and suggested herewith for practical application.

Stochastic simulation of rain intrusion through small defects due to water rivulet overpressure. Introducing a driving rain leakage potential

There is a need of upgrading the old building stock with respect to the thermal insulation of the building envelope and specifically the façades. There are several systems on the market, and some are quite new and innovative. To bring down the cost some of the systems many are based on prefabricated moisture tight insulated units. This means that in case there is moisture tight barrier on the interior side, two moisture tight barriers surround the wall structure. The leakage of driving rain into the structure then represents a major threat to the durability of these systems. This paper investigates the pressure build up in water rivulets running down a façade acting together with the wind pressure. A driving rain leakage potential is introduced. Using real weather data years and Monte Carlo Simulations, the mean and standard deviation of the annual leakage through small hole is estimated. The examples show that the leakage can reach a level 0-0.5 liter/year for a hole with a diameter of 1-2 mm, and 0.5-3 liter/year for a diameter of 3-4 mm.

On the coupling mechanism of rain–wind two-phase flow induced cable vibration: A wake-dynamics perspective

Large amplitude rain–wind induced vibration (RWIV) of stay cables of long-span cable-stayed bridges has been a major concern in the past three decades, yet its excitation mechanism has not been clarified thoroughly. Recent numerical simulations and experimental work showed that when the RWIV of a cable is developing, the dominant frequency of its unsteady lift forces is notably decreased, i.e., the conventional Strouhal law of cylinder flow is obeyed. In some experimental studies, however, the small-scale vortical structures, which are believed to be closely associated with the low-frequency fluctuating components, are visualized qualitatively in the cable wake during the RWIV. In the present work, the first experimental observation of the dynamic wake behind a cable during the RWIV is presented. The RWIV is reappeared based on a stay cable model supported by springs in wind tunnel tests. The dynamic characteristics of the water rivulet on the upper surface of the cable are recorded by a high-speed camera, and the detailed wake flow of the cable is captured by employing the high-speed particle image velocimetry technique. We focus on the low-frequency synchronization between the cable vibration, upper-rivulet movement, and the wake dynamics to investigate the excitation mechanism of the RWIV. A three-phase (gas–liquid–solid) coupling scenario is finally proposed to explain this interesting phenomenon as low-frequency resonance between fluids and the cable structure.

Role of dynamic water rivulets in the excitation of rain–wind-induced cable vibration: A critical review

It has been more than 30 years since Hikami Y and Shiraishi N (1988) Rain–wind-induced vibrations of cable-stayed bridges. Journal of Wind Engineering and Industrial Aerodynamics 29: 409–418 first reported the rain–wind-induced vibration (RWIV) of stay cables in the construction stage of Meikonishi Bridge, Japan. After that, considerable research efforts have been devoted to understanding the RWIV of stay cables, and the role of the upper rivulet has been gradually realized and studied. This study presents a selective review on recent progress of RWIV and its controversial excitation mechanism. The available knowledge and up-to-date understanding of this complex fluid-structure interaction are presented in some detail. The formation, dynamics of water rivulet, and its role in affecting the near-wall boundary layer properties and in the excitation scenario of RWIV are of particular interest in this study. Finally, some limitations of previous studies are concluded, with some perspective suggestions for further study of excitation mechanism of RWIV.

Solidification of a rivulet: shape and temperature fields

Abstract

An experimental study to characterize the effects of initial ice roughness on the wind-driven water runback over an airfoil surface

In the present study, an experimental study was conducted to characterize the effects of initial ice roughness on the transient behaviors of surface water/ice run-back over a NACA 23012 airfoil model under a glaze icing condition. The experimental study was conducted in the Icing Research Tunnel of Iowa State University (i.e., ISU-IRT). A digital image projection (DIP) technique was applied to provide non-intrusive, temporally-and-spatially-resolved measurements of the thickness distributions of the dynamic water/ice flows over the airfoil surface. Two typical surface morphologies were observed for the surface water runback over the airfoil models: water film flow and water rivulets flow. While the surface water film flow modulated with one primary wave and multiple secondary waves is observed at lower wind speed conditions (i.e., U∞ = 10 m/s), the water rivulets flow is observed at relatively higher wind speed conditions (i.e., U∞ = 15 m/s). The initial ice roughness is found to retard and shorten the primary wave formation in the water film flow. It is also found that the initial ice roughness could trap and decelerate the water flow and decrease the inertia force in the film front, which essentially delays the formation of the rivulets. The water rivulet flow trapped by the initial roughness was found to have a meandering behavior, due to which, the initially formed narrow rivulets merged into wider rivulets as they move downstream. By recognizing the film/rivulets boundary during the dynamic surface water/ice runback process, the quantitative details were extracted, i.e., the formation, transition, and development of the rivulet flows. The initial ice roughness was found to have a significant effect on the rivulet characteristics (e.g., rivulet width, spacing, and height).

Thickness Measurement of Water Film/Rivulets Based on Grayscale Index

This study proposed a nonintrusive and cost-efficient technique to measure the thickness of a thin water film/rivulet based on the grayscale index. This technique uses millions of probes and only needs a digital camera, fill lights, and pigment. For water colored with diluted pigment, the grayscale index of the water captured by a digital camera depends on the water thickness. This relationship can be utilized to measure the water thickness through digital image processing. In the present study, the relationship between the grayscale index and water thickness was theoretically and experimentally investigated. Theoretical derivation revealed that when the product of water thickness and the color density approaches to 0, the grayscale index is inversely proportional to the thickness. The experimental results show that under the color density of 0.05%, the grayscale index is inversely proportional to the thickness of water film when the thickness is less than 6 mm. This linear relationship was utilized to measure the distribution and profile of a water rivulet flowing on the lower surface of a cable model.

On boundary damping to reduce the rain\u2013wind oscillations of an inclined cable with small bending stiffness

In this paper, a model will be derived to describe the rain–wind-induced oscillations of an inclined cable. Water rivulets running along the cable and aerodynamics forces acting on the cable are taken into account to describe these oscillations. A boundary damper is assumed to be present near the lower endpoint of the cable. For a linearly formulated initial-boundary value problem for a tensioned beam equation describing the in-plane transversal oscillations of the cable, the effectiveness of this damper is determined by using a two-timescales perturbation method. It is shown how mode interactions play an important role in the dynamic behaviour of the cable system. Some resonant and non-resonant cases have been studied in detail.

Multi-modal vortex- and rain–wind- induced vibrations of an inclined flexible cable

Abstract We reproduce multi-modal vortex-induced vibrations (VIVs) and multi-modal rain–wind-induced vibrations (RWIVs) of an inclined and yawed cable in wind tunnel tests. The flexible cable model has low mass and low damping. First, the cable model is kept dry and exposed to uniform airflow; it experiences first-, second- and third-mode VIVs with the increase of wind speed. The structural responses of VIVs are analysed and the frequency lock-in phenomenon is observed for different modes of VIVs of the flexible cable. In addition to VIVs of the dry cable, RWIVs are excited by guiding water rivulets on the cable surface from a water tube. The first-, second- and third-mode RWIVs of the flexible cable are observed and identified at a much higher range of incoming wind speeds than that of the VIVs. To further explore the origin of the multi-modal behaviours of RWIVs, the upper rivulet is guided and restricted to form locally along the cable. Experimental results reveal that the higher- and multiple-mode RWIVs can be excited by a local rivulet, even under a lower wind speed. Finally, RWIVs are compared to VIVs to uncover their underlying similarities and relationships.

Multi-mode responses, rivulet dynamics, flow structures and mechanism of rain-wind induced vibrations of a flexible cable

We successfully excite the multi-mode rain-wind induced vibrations (RWIVs) of a flexible stay cable in a wind tunnel by guiding water rivulets along the cable surface. The first, second and third mode RWIVs are excited respectively by increasing the incoming wind speed. The movement of upper rivulet during the first, second and third mode RWIVs are recorded with a high-speed camera. By employing a computer vision based recognition technique, the upper-rivulet dynamics of different modes of RWIVs are recognized and analyzed. The wake flow structures around the dry cable and the cable suffering from RWIVs are measured by using a particle image velocimetry (PIV) system. Experiment results show that the oscillation frequency of the upper rivulet is synchronized with the cable vibration frequency for the first, second and third mode RWIVs. Based on experiment results, bubble burst is proposed as a possible mechanism for the excitation of RWIVs.

Varieties of the gas driven water rivulet flow regimes in the minichannels

This paper reports on an experimental study of the gas (nitrogen) driven water rivulet flow regimes in the minichannel. The width of the liquid nozzle is 10 mm, the height and width of minichannel are 1.61 ±0.02 mm and 30 mm correspondently. The new flow regime has been found by authors. Its occurrence is connected with influence of relatively large static wetting angle of water on the flow and comparably small height of the channel. It was named “rivulet bridge”. The paper presents a map of the various rivulet flow regimes, characterized by different geometry and different kinds of surface deformations at temperature 40°C.

Wind-Tunnel Investigation of the Aerodynamic Performance of Surface-Modification Cables

Abstract The wind-induced vibration of stay cables of cable-stayed bridges, which includes rain-wind-induced vibration (RWIV) and dry galloping (DG), has been studied for a considerable amount of time. In general, mechanical dampers or surface modification are applied to suppress the vibration. In particular, several types of surface-modification cable, including indentation, longitudinally parallel protuberance, helical fillet, and U-shaped grooving, have been developed. Recently, a new type of aerodynamically stable cable with spiral protuberances was developed. It was confirmed that the cable has a low drag force coefficient, like an indented cable, and that it prevented the formation of water rivulets on the cable surface. In this study, the stability for RWIV of this cable was investigated with various flow angles and protuberance dimensions in a wind-tunnel test. It was found that the spiral protuberance cable is aerodynamically stable against both RWIV and DG for all test wind angles. The effects of the protuberance dimensions were also clarified.

Active Modal Control of Rain-Wind Induced Vibration of Stay Cables

Rain-wind induced vibrations have been observed on numerous cable-stayed bridges around the world, which has resulted in an urgent need to gain further understanding of the phenomenon and to develop efficient means of mitigating the large-amplitude vibrations. It is believed that this aeroelastic instability problem is due to the formation and oscillation of a water rivulet on the upper cable surface, which changes the cable aerodynamics and absorbs energy into the system. In this paper, an analytical study of the phenomenon is conducted by formulating a finite element model for the cable and the upper rivulet with non-linear coupling effect. Linearization of the equations of motion of the cable-rivulet system is next carried out, based on which active vibration controller can be designed. Pole allocation control method has been employed for mitigating RWIV in the present study, which is able to add sufficient damping to several excited modes. The active controller is designed such that the closed-loop poles are allocated on the left half of the complex plane of eigenvalues. The controller gains obtained from the equivalent linear system are then applied to the original non-linear cable-rivulet system. Simulation results show that the proposed active modal controller performs much better in suppressing RWIV than an optimally tuned linear viscous damper.

Study of the gas driven water rivulet flow regimes in the minichannels

The paper deals with the experimental study of the gas (nitrogen) driven rivulet flow regimes of water outflowing from the nozzle with a width of 10 mm into minichannel with the height of 1.6±0.02 mm and a width of 30 mm. The authors revealed a new flow regime – namely “rivulet bridge” whose occurrence is attributed to the influence of relatively large static wetting angle of water on the flow regimes. The paper presents a map of the various rivulet flow regimes characterized by different surface geometry and types of surface deformations.

Excitation mechanism of rain–wind induced cable vibration in a wind tunnel

The rain-wind induced vibration (RWIV) of cables in bridge and wind engineering has been reported worldwide over the past decades. However, quantitative analyses of the RWIV mechanism using the real water rivulets are rare. The RWIV of a cable model is tested in an open-jet wind tunnel. The movement and the geometry of the upper rivulet and the vibration of the cable are obtained by videogrammetry. The coupling of the upper rivulet and cable vibration is shown to be the main excitation mechanism of RWIV. In particular, the oscillating upper rivulet induces the boundary layer to attach to the cable and generates aerodynamic forces, which produce a positive work and excite the cable to vibrate. In turn, the cable vibration harmonizes the upper rivulet along the cable. To verify the proposed mechanism, a numerical model is established using the aerodynamic coefficients measured in dry cable tests. The numerical results are in agreement with the experiments. The effect of the damping ratio on the RWIV amplitude is also experimentally and numerically investigated.

Multi-Zone Ice Accretion and Roughness Models for Aircraft Icing Numerical Simulation

AbstractA mathematical multi-zone ice accretion model used in the numerical simulation of icing on airfoil surface based on three water states, namely, continuous film, rivulets and beads is studied in this paper. An improved multi-zone roughness model is proposed. According to the flow state of liquid water and film flow, rivulets flow governing equations are established to calculate film mass distribution, film velocity, rivulet wetness factor and rivulet mass distribution. Force equilibrium equations of droplet are used to establish the critical conditions of water film broken into rivulets and rivulets broken into beads. The temperature conduction inside the water layer and ice layer is considered. Using the proposed model ice accretion on a NACA0012 airfoil profile with a 4° angle of attack under different icing conditions is simulated. Different ice shapes like glaze ice, mixed ice and rime ice are obtained, and the results agree well with icing wind tunnel experiment data. It can be seen that, water films are formed on the surface, and heights of the films vary with icing time and locations. This results in spatially-temporally varying surface roughness and heat transfer process, ultimately affects the ice prediction. Model simulations indicate that the process of water film formation and evolution cannot be ignored, especially under glaze ice condition.

An experimental investigation on the surface water transport process over an airfoil by using a digital image projection technique

In the present study, an experimental investigation was conducted to characterize the transient behavior of the surface water film and rivulet flows driven by boundary layer airflows over a NACA0012 airfoil in order to elucidate underlying physics of the important micro-physical processes pertinent to aircraft icing phenomena. A digital image projection (DIP) technique was developed to quantitatively measure the film thickness distribution of the surface water film/rivulet flows over the airfoil at different test conditions. The time-resolved DIP measurements reveal that micro-sized water droplets carried by the oncoming airflow impinged onto the airfoil surface, mainly in the region near the airfoil leading edge. After impingement, the water droplets formed thin water film that runs back over the airfoil surface, driven by the boundary layer airflow. As the water film advanced downstream, the contact line was found to bugle locally and developed into isolated water rivulets further downstream. The front lobes of the rivulets quickly advanced along the airfoil and then shed from the airfoil trailing edge, resulting in isolated water transport channels over the airfoil surface. The water channels were responsible for transporting the water mass impinging at the airfoil leading edge. Additionally, the transition location of the surface water transport process from film flows to rivulet flows was found to occur further upstream with increasing velocity of the oncoming airflow. The thickness of the water film/rivulet flows was found to increase monotonically with the increasing distance away from the airfoil leading edge. The runback velocity of the water rivulets was found to increase rapidly with the increasing airflow velocity, while the rivulet width and the gap between the neighboring rivulets decreased as the airflow velocity increased.

High-Speed Imaging to Quantify Transient Ice Accretion Process over an Airfoil

Ice accretion on aircraft wings poses a performance and safety threat as aircraft encounter supercooled droplets suspended in the cloud layer. The details of the ice accretion depend on the atmospheric conditions and the flight parameters. The icing process on the wing consists of a complex interaction of water deposition, surface water transport, and freezing. The aerodynamics affect the water deposition, the heat and mass transport, and ice accumulation; meanwhile, the accumulating ice affects the aerodynamics. Until now, most experimental measurements of aircraft icing have focused on the final ice shapes formed after exposure for a set duration to icing conditions. This approach fails to capture the transient processes that form the final ice formations. Here, we present experiments conducted in the Iowa State Icing Research Tunnel on a NACA 0012 airfoil to study the transient ice accretion process under varying icing conditions. High-speed video of the icing process was acquired under controlled environmental conditions to quantitatively measure the transient water film runback, rivulet formation, and accumulated ice growth. Image processing techniques were developed to extract physical information from the acquired image sequences of the icing events. The experiments demonstrate how varying the environmental conditions modifies the ice accretion process. It was found that the leading-edge ice growth rate is proportional to water deposition rate and the rivulet spacing and rivulet area coverage decrease with increasing wind speed. The water rivulet runback speed over the airfoil surface was found to increase rapidly with increasing wind speed.

Numerical investigation of the coupled interaction between an unsteady aerodynamic flow field and a water film coating on a circular cylinder

Rain–wind induced vibration is an aeroelastic phenomenon that occurs on the inclined cables of cable-stayed bridges and arises due to the interaction between the unsteady wind loading and the formation of water rivulets on the cable surface. A new numerical method has been developed at the University of Strathclyde to simulate the influence of the external flow field on the rivulet dynamics and vice versa. The approach is to couple a Discrete Vortex Method solver to determine the external flow field and unsteady aerodynamic loading, and a pseudo-spectral solver based on lubrication theory to model the evolution and growth of the water rivulets on the cable surface under external loading. Results of this coupled model are presented, to provide detailed information on the development of water rivulets and their interaction with the aerodynamic field. In particular, the effect of the initial water film thickness and the angle of attack in plane on the resulting rivulets are investigated. The results are consistent with previous full scale and experimental observations with rivulets forming on the upper surface of the cable only in configurations where rain–wind induced vibration has been observed. Additionally, the thickness of the lower rivulet is found to be self-limiting in all configurations. The results demonstrate that the model can be used to enhance the understanding of the underlying physical mechanisms of rain–wind-induced vibration.