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Okinawa in the subtropical islands enclosed in the ocean has a problem that corrosion of structures progresses quickly because of high temperature, high humidity and adhesion of sea-water mists flying from sea. Author is interested in corrosion of bridge made of weatherability steel. Therefore, it needs to investigate the flow structure around bridge beams and behavior of sea-water mist (droplet). In this paper, flow visualization and PIV are attempted to understand the flow structures around bridge beams and, furthermore, numerical approach of motion of droplets is attempted to understand the collision of sea-water mists on the bridge wall.

Okinawa is located in the southern area in Japan and is subtropical islands enclosed in the ocean. There, therefore, is a problem that corrosion of the structures progresses quickly because of high temperature, high humidity and adhesion of sea-water mist flying from sea. For instance, since the Benoki Bridge located in Kunigami village, Okinawa, was built by weatherability steel in 1981, it collapsed by remarkable corrosions in July, 2009 [

Author is interested in the relationship between flow structure around bridge beams and corrosion of sea-water mist. Therefore, two approaches are attempted. One is flow visualization and PIV to understand the flow structures inside bridge beams and another is numerical approach to understand the collision of sea-water mists on beams walls.

bridge beams. ^{4}.

The inside flow images are captured by high-speed camera (Phantom V710, Frame rate: 3000 fps, image size: 1280 × 800 pixels). The flow is illuminated by CW Nd: YAG (Output power: 8 W, wave length: 532 nm) as laser sheet light with about 1 mm width. For seeding, smoke generated by heating the glycol solution is derived from three locations. One is at upstream of bridge beam, the others are on wall in behind of two cavities.

Whole area including the two cavities is captured to investigate the relationship between their upstream and downstream cavities. Calculated areas are cropped to two areas of A (upstream cavity) and B (downstream cavity) areas as shown in

Above mentioned flow visualization was executed in the wind tunnel, but it is difficult to use the sea-water mist as tracer particle. Because wind tunnel was corroded by salt, author, therefore, attempts numerical approach to understand the collision of sea-water mists. Also, velocity data obtained by PIV are used in a whole flow field.

These behaviors of sea-water mists are given by equation of translate motion of particle (or droplet) [

d d t ( ρ l V l u l ) + d d t ( β ρ G V l u l ) + ρ l V l g + V l ∇ p − V l μ ( ∇ 2 u G ) + 1 2 ρ G ( π r l 2 ) C D | u l − u G | ( u l − u G ) + F L = 0 . (1)

Thin of plate | t | 8 mm |
---|---|---|

Width of whole bridge model | W 1 | 400 mm |

Width of each beam | W 2 | 125 mm |

Width of pendent part | W 3 | 63 mm |

Width of below fringe | W 4 | 16 mm |

Height of beam | H | 100 mm |

Length of whole bridge model (Depth direction in | D | 500 mm |

Height from the fringe to flat plate | L | 50, 100, 200 mm |

where, 1st term to 7th terms mean inertia force, added inertia force, gravity force, pressure gradient force, viscos force, drag force and lift force. u is velocity vector, r and d are a radius and a diameter of particle, p is the pressure, g is gravity acceleration vector, ρ is density, μ is viscosity, V is volume of a particle, and β is virtual mass coefficient. Drag force coefficient C D used equation known as Newtonian resistance and equation proposed by Shiller and Naumann [

In real case, the diameter of the sea-water mist has the range of a few micro-meters to several ten micro-meters. In this simulation, a diameter of droplet d sets to 10 μm and the standard deviation of droplet σ_{d} sets to 5 μm. Initial positions of droplets are located randomly in whole cavities. Time marching method set to 1st order Euler method. Droplet position in the next step is calculated by velocity u l given by Equation (1) and the time interval dt = 5 ms. Interaction between droplets don’t consider. When droplets impinge to wall, it is judged that droplets bonded to the wall. At the time of impinging, bonded wall position (left, center and right wall) and impinging wall height are recorded to investigate a degree for each height of wall.

From

respectively. These results are obtained as time average of 3000 frame velocity vectors (1 second). Color bar means u/U which is defined the rate of the magnitude of each velocity vector to the inlet velocity U. That is, blue means 0m/s and red mean 5 m/s.

From

Flow of

The separated flow goes over the downstream cavity and the right wall. Therefore, flow in the downstream cavity rotates to counter-clockwise, countercurrent flow under the center wall are obtained from the downstream cavity to the upstream cavity, and flow in the upstream cavity rotates to clockwise. The magnitudes of velocities in two cavities are slower than ones in

L/H | Area A | Area B |
---|---|---|

0.5 | ○ | |

1 | ○ | |

2 | × |

○ means counter clockwise flow; × means clockwise flow.

structure as shown in

Numerical approach of sea-water mist particles is executed. Figures 6(a)-(d) and Figures 7(a)-(d) show the probability density function (PDF) distribution of the collision of sea-water mist particles at the all side wall at the L/H = 0.5 and 2.

Flow both of their upstream and downstream cavities rotates to counter clockwise, mist particles which flow into these cavities from main flow impinge to right wall in these cavities. As shown in

In

particles move to downward slowly and to almost parallel along the left wall. PDF distribution is almost equivalent in whole height.

The author was interested in the relationship between corrosion of sea-water mist and flow structure. Two approaches were attempted. One was flow visualization and PIV to understand the flow structures around bridge beams, and another was numerical approach of collision of sea-water mists.

From the visualization and PIV, authors revealed that flow at L/H = 0.5 rotates to counter-clockwise in two cavities and also flow at L/H = 2 rotates to counter-clockwise in the downstream cavity and to clockwise in the upstream cavity and countercurrent flow from the downstream cavity to the upstream cavity.

From the numerical approach on collision of water-sea mist particle, probability density function of collision was higher at the impinging point in two cavities and it was easy for corrosion to progress in the position.

Ishikawa, M. (2017) A Study on Flow Structure around a Bridge Beam and Behavior of Sea Water Mist. Open Journal of Fluid Dynamics, 7, 340-347. https://doi.org/10.4236/ojfd.2017.73022