Hydrogen Bubble Technique

The Federal Highway Administration's Turner-Fairbank Highway Research Center Hydraulics Laboratory has developed a Particle Imagery Velocimetry (PIV) technique for flow visualization and analysis.

Flow visualization is a long-established tool for supporting fluid mechanics research.  Recent advances in physical modeling instrumentation have facilitated flow visualization to support research. 

Flow visualization is currently performed using the hydrogen bubble technique whereby a platinum wire cathode is mounted vertically upstream of a test structure (e.g., pier, abutment) in the flume. A graphite rod is used as an anode. Electrolysis of the water in the flume produces hydrogen bubbles small enough (Ø < 0.025 mm) that their buoyancy may be ignored within the visualized area. By employing pulsed time line markers with a specially built hydrogen bubble generator, it is possible to visualize the phase speed of the diving current formed at the face of the structure. This diving current is usually responsible for bridge scour.

Flow visualization provides many advantages for physical modeling: 

• It gives better insight into flow patterns caused by obstructions. These insights can form the basis for numerical models and can be used to validate current numerical models. 
• It can give quick feedback for flow alteration ideas. 
• It can reduce experiment time because fixed bed experiments can be performed to evaluate flow patterns. Scour magnitude can then be inferred from the flow patterns. Currently, most hydraulics laboratories perform movable bed experiments and measure scour holes after long run times (up to 48 hours or longer per experiment). 
• The hydrogen bubble technique can eventually be used in conjunction with PIV to measure velocity structures.

To see an animated example of velocity vectoring, view this "AVI" file. (2,746 KB)

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