Computational Mechanics Research and Support for Aerodynamics ...

efine the rate function to improve the match to experimental results using the results of the CFD
simulation. The CFD simulation yields the bed shear at the current scour depth at each time step. The
laboratory time at which the simulated scour depth was reached can be computed from Equation (2.3),
for times greater than 1800 s. Also for times greater than 1800 s, the bed recession rate at the depth
from the simulation can be calculated from Equation (2.4). This procedure yields a new value for bed
recession rate corresponding to the bed shear at the maximum depth for each time step. These new
values can be used to improve the parameters for the entrainment rate function. As noted in Section
2.1, there are no experimental data for the first 1800 s, and the erosion rate at the deepest point as a
function of time, Equation (2.4) has a physically unrealistic singular point at time equal to zero. In the
absence of experimental data, some reasonable assumptions are needed to determine an erosion rate
as a function of bed shear during the first 1800 s that will result in the simulation matching the
experimental scour depth within the range of uncertainty at 1800 s. The procedure for achieving this
goal is currently under development.
The streamwise velocity distribution in the vicinity of the flooded bridge deck at the initial unscoured
state is shown in Figure 2.4. It clearly shows accelerated flow under the deck and a much higher velocity
near the bed than in the upstream. Figure 2.5 shows the streamwise velocity distribution after the scour
hole has fully formed. The accelerated flow under the deck is significantly reduced, and the near bed
boundary layer is thicker, which yields a reduced shear stress peak under the bed and near zero erosion
rate.
Figure 2.4: Initial streamwise velocity distribution around bridge deck
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