|Year : 2011 | Volume
| Issue : 1 | Page : 47-51
Wake Vortex Propagation around a Wing Wall Abutment
Upain Kumar Bhatia1, Baldev Setia2
1 Department of Civil Engineering, M.M. Engineering College Mullana, Ambala, India
2 Department of Civil Engineering, National Institute of Technology, Kurukshetra, Haryana, India
|Date of Web Publication||4-Jan-2011|
Upain Kumar Bhatia
Department of Civil Engineering, M.M. Engineering College Mullana, Ambala
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Scour around bridge elements found in erodible sediment beds have always been a topic of interest for investigators on the subject. An insight into the mechanism of flow modification responsible for initiating and continuing the process of scour is the only way to understand and curb this undesirable hydraulic phenomenon. In this article, results of an experimental work on initiation and propagation of a vortex in the wake zone of a wing wall type abutment model are presented. Hydrogen bubble flow visualization technique is used for observing the vortex, and photographs for the initiation, strengthening, and diffusion of wake vortex are presented and discussed in a sequential manner.
Keywords: Flow visualization, physics of flow modification, hydrogen bubble technique
|How to cite this article:|
Bhatia UK, Setia B. Wake Vortex Propagation around a Wing Wall Abutment. J Eng Technol 2011;1:47-51
| 1. Introduction|| |
The fact that our bridges are not 100% safe against failure is the most potential source of motivation toward research on scour prediction and prevention. Many researchers have devoted their time and attention to the prediction of scour and its extent around any hydraulic structure that obstructs the stream flow. The main cause of scour around any such structure is a vortex that forms around the structure due to three-dimensional separation of flow , . A bridge is one such important structure that is threatened by scouring. Hydraulic failure of a bridge may occur due to failure of its piers or abutments, both essential parts of a bridge. Although abutment failure has been commonly observed, it is evident from open literature that research on bridge piers has received greater attention relative to the bridge abutments. During the last 5 decades many researchers have studied the scour around abutments and conducted experiments to understand the mechanism of scour , .
Earlier studies in this direction were carried out by Ahmad (1953), Liu et al. (1961), Laursen (1963), Wong (1982), Dongol (1984), Tey (1984), Kwan (1984, 1988), Kandasamy (1989, 1998), Lim (1997) Lim et al. (1998), Barkdoll (2005) etc. (refer Singh Kulbir 2007)  . Melville, In India, Ganga Pul (Ganga bridge) at Mokameh (River Ganga) and a bridge on River Tapi failed due to scouring around bridge piers and abutments. An abutment of a bridge across McLeod River, Alberta [Figure 1], was outflanked during design flood event. Most researchers have concentrated on the prediction of scour around the abutments , . Experimental studies in understanding the mechanism of the scouring have been relatively lesser. The present article is the result of an experimental investigation aimed at understanding and highlighting the vortex system formed in the wake zone of an abutment and bottom vortex responsible for initiating and abetting scour around abutment, ultimately culminating into its failure. Efforts have been made to understand the physics of flow modification around the abutments as a result of flow-bed-structure interaction. Studies have been carried out on a rigid bed using the hydrogen bubble flow technique. The authors have tried to visualize and present the formation of wake vortices in a sequential manner.
| 2. Scheme of Experimentation|| |
Experiments were conducted in Fluid Mechanics Laboratory of M.M. Engineering College, Mullana, in a recirculating flume (3 m long, 0.25 m wide, 0.35 m deep). The flume [Figure 2] had some portion of glass on the sides to facilitate visualization flow modification inside the channel. The discharge in the flume was maintained with the help of a centrifugal pump. The discharge was controlled by a valve on the delivery pipe. The flow depth was controlled with the help of an under-sluice type tail gate. For measurement of discharge, the flume was linked with a collecting tank at the end. The waves generated at the inlet were suitably dampened by placing some floating planks of wood. The flow conditions were maintained steady and uniform throughout the experimental run. Experiments were carried out at a Reynolds number, Re = 4394. [Figure 3] gives the schematic of the experimental setup.
The model used for the study, was a wooden model of a wing wall type abutment of the dimensions as given in [Figure 4].
| 3. Hydrogen Bubble Technique|| |
This technique is employed for flow visualization if the flowing medium is water. The water is dissociated into its constituents, hydrogen and oxygen, by the process of electrolysis. The setup for generating sufficient hydrogen for the study was developed by the authors in the laboratory of Fluid Mechanics of M.M.E.C., Mullana. A view of Hydrogen Bubble Generator is presented in [Figure 5].
Since ordinary tap water was used as the flowing medium, it was observed that it got easily dissociated into hydrogen and oxygen, hydrogen being twice in volume as compared with oxygen. To accelerate this process of electrolysis, table salt (Sodium Chloride) was added to the flume water. A copper sheet was used as anode, while a thin steel wire (0.14 mm diameter) served as cathode. The idea behind keeping the cathode in the form of a thin wire is that the hydrogen bubbles generated should be small enough in size so as not to be affected by buoyancy and to mainly follow the flow. Also, the cathode being in the form of a thin wire, hydrogen is generated in the form of a sheet of fine bubbles. Since the hydrogen is insoluble in water, this sheet could be illuminated by an external light source and hence could be easily visualized, photographed, and video-graphed.
Other Techniques employed for flow visualization include Mud-Flow visualization, Wet-Paint technique, Dye injection technique, Reflective powder technique etc ,, .This technique of flow visualization has an advantage over other techniques that the hydrogen is not a pollutant or contaminant. Also, being insoluble in water it leaves the water at some stage of time, when the drag on the bubbles becomes less and the buoyant force starts prevailing. The oxygen generated at the anode is soluble in water and helps in oxidizing many impurities present in the water.
| 4. Results and Discussion|| |
A number of preliminary experiments were conducted and flow modification was keenly observed around the wing wall abutment. Since the turbulence in the flow causes quick diffusion of hydrogen bubbles, and makes it difficult to visualize the flow, the experiments were conducted at a low Reynolds number.
After establishing the optimum setup of flow conditions, lighting conditions, and flow imaging, the test run was conducted and images were captured sequentially with the help of a digital camera. The camera used for this imaging was SONY DSC-W300, with 13.6 effective megapixels resolution, 1/1.7" Super Had/CCD Image Sensor, BIONZ Image Processor, and Carl Zeiss Vario-Tessar Lens. The results so obtained are presented in a sequential manner in [Figure 6]a-j.
|Figure 6: (a– j) Visualization of wake vortex propagation around the wing wall abutment model with the help of hydrogen bubble technique|
Click here to view
It is a well-established fact that whenever flow encounters an obstruction, the flow gets modified three-dimensionally due to an adverse pressure gradient. This modification of flow appears in the form of vortices around the obstruction in the wake zone as well as near the front nose of the obstruction.
The wake vortex generated in the wake zone is discussed here as a 4-stage process.
4.1 Initiation of an Infant Vortex
- Initiation of an infant vortex.
- Strengthening of vortex.
- Growth of vortex
- Diffusion of vortex
The visual observations during the experimental investigation of the present study revealed that the wake vortices are initiated near the point of separation of flow from the structure U2 [Figure 4]. The water mass immediately downstream of the separation point is stagnated by the abutment model, whereas the mainstream passes to the downstream with modified, or somewhat increased velocity. The stagnated water mass, on one side is bound by the solid surface of the abutment model between points U2 and D2, and on the other side it is being sheared by the mainstream. This situation results in the overall rotational effect associated with a drift to the downstream, which initiates an infant wake vortex. [Figure 6]a clearly supports the above aspect. The concentrated hydrogen (white portion) in the area highlighted within a circle very clearly indicates the stagnated water mass being sheared by the mainstream.
4.2 Strengthening of the Wake Vortex
The water mass associated with this vortex continues to shear as it keeps pressing against the mainstream. This causes further strengthening of the vortex. The vortex is visualized generally in an elliptical shape because of the lateral constriction due to mainstream and abutment surface [[Figure 6]b].
4.3 Growth of the Wake Vortex
Water in the mainstream near the abutment model mainly flows inclined to the longitudinal direction. This opens up the gap between the solid surface of abutment and the mainstream. At this stage, the energy of the vortex is transferred to the surrounding water through shear action and the surrounding water also joins the vortex, increasing its size. This aspect of growing size is evident in [Figure 6]c in which a downward bend is clearly visible in the hydrogen streak, just downstream of the vortex under consideration, because of the transfer of energy due to shear in the direction of vortex (anticlockwise in this case).
The stage of gaining strength from the mainstream is gradually transformed to the stage of increasing size of vortex in the opened up area. This also pushes the mainstream away from the abutment model.
Furthermore, when the downstream drifting brings the vortex beyond solid boundary of abutment, the strengthening process is ceased as the vortex is not confined in the constricted area and is no more pressed against the mainstream. [[Figure 6]d]. This releases the compression on the vortex and also the push on the mainstream. As a result the mainstream comes to its normal course that falls nearer to the solid boundary of the abutment model. This increases the pressure gradient near the point of separation U2 and initiates another wake vortex.
4.4 Diffusion of the Wake Vortex
The compressed elliptical vortex under consideration starts taking a circular shape and grows bigger in size by associating more and more surrounding water by shear action [[Figure 6]e]. This reduces the energy per unit mass of water in the vortex and hence the speed of the vortex is reduced. Eventually, the size of the wake vortex grows very big and the downstream drift starts prevailing and the vortex diffuses into the mainstream. This is viewed progressively in [Figure 6]f-j. In all these figures, the initiation and strengthening stages of the next wake vortex are also visible, as discussed in reference to [Figure 6]a-d.
| 5. Conclusions|| |
The phenomena of wake vortex generation and propagation are observed and discussed and supported with flow imaging with the help of the hydrogen bubble flow visualization technique. The hydrogen bubble flow visualization technique is found to be very useful in the understanding of the process of vortex generation and propagation around an obstruction. This technique can also be applied effectively to visualize the flow around spurs, dykes, weirs, orifices, and so on. This technique, being environmental friendly, can be easily adopted for laboratory investigations.
| References|| |
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| Authors|| |
Upain Kumar Bhatia is presently Serving in MMEC, Mullana, as an Associate Professor in the Civil Engineering Department. His area of research is Flow Visualization and Fluvial Hydraulics. He is a life member of the Indian Society of Technical Education (ISTE), the Indian Society for Hydraulics (ISH), and the Indian Water Resources Society (IWRS). He has 16 publications in international and Indian journals and conferences.
Baldev Setia is presently serving as Professor and Head, Civil Engineering Department of National Institute of Technology, Kurukshetra. He has about 25 years of experience in teaching, research, and consultancy. He has published about 60 research papers in international and Indian journals and conferences, including 3 patents protected under the Intellectual Property Rights (India). He has visited USA, UK, Australia, Iran, and Japan to attend international conferences.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]