Continuing the blog series on Flow Science’s 35th
anniversary contest, I will cover the case study from the winner
of the contest, Daniel Valero Huerta from FH
Aachen University of Applied Sciences in Germany. His research focuses on
understanding the dispersion of contaminants/discharges in rivers and
estuaries. In this research, FLOW-3D has been extensively used as
the computational model for studying the turbulent dispersion of the
Environmental discharges and outfall structures have been
traditionally designed by means of complex, cost-intensive and time-consuming
experimental studies. Models based on an integral approach are commonly
employed despite their limitations, but contaminant re-entrainment or
strong adverse discharges fall outside the hypothesis of such models. Thus,
using a full 3D model for contaminant dispersion may improve knowledge on the real
contaminant dispersion in rivers and estuaries. Similarly, bounded jets can be
modeled and different diffusor locations can be tested in order to improve the
overall environmental water quality and biotic conditions.
physics and the case study
In this study, a jet discharge was modeled both
experimentally and numerically. Then, an estimation of the turbulent dispersion
in the shear region was obtained. For the turbulence modeling, the Renormalized
Group (RNG) model was employed together with the TruVOF method for tracking the
free surface. A monotonicity-preserving, second-order scheme was employed for
contaminant advection ensuring proper modeling of turbulent transport. FLOW-3D
is a very good choice for the numerical modeling of such engineering problems
because it offers a comprehensive turbulence modeling suite and accurately
estimates the free surface. Another advantage of FLOW-3D is the ability
to use a one-fluid approach because modeling air (a two-fluid problem) is not important
for river contaminant transport problems for practical applications. One-fluid
modeling is a more natural, and efficient approach for hydraulic problems. Figure
1 shows a snapshot of the simulation results.
Figure 1. Top view (top) and side view (bottom) showing the
discharge ejected from an outfall. Complex flow patterns are seen along with
circulation zones between groins (pink blocks).
FLOW-3D offers a comprehensive set of turbulence models.
They can be broadly divided into two categories – Reynold’s Averaged Navier
Stokes (RANS) models and Large Eddy Simulation (LES) models. As the name
suggests, RANS models average out the fluctuating quantities in the governing
equations. LES models, on the other hand, solve for the turbulent motions at
scales resolved by the mesh. RANS models are good for understanding the average
behavior of a flow over a period of time, while LES models are used to describe
individual experiments or significant transient behavior. For this study, the RNG
model, which falls into the category of RANS type models, was used. RNG is an
improved k-ԑ model, with coefficients determined through rigorous statistical
analysis. Other options that could have been used as RANS models are the classical
k-ԑ model or the Wilcox k-ω model. RNG was chosen because it is typically good
for transitional flows.
Free surface tracking
FLOW-3Duses an enhanced variant of Volume-of-Fluid (VOF) technique called
TruVOF®. TruVOF provides a natural way to capture free surfaces and their
evolution with great efficiency. More details on the free surface fluid flow
can be foundhere.
FLOW-3D offers three options for momentum advection based on
the order of accuracy desired. The first order is the simplest and fastest
method. The second order is preferred for minimizing numerical dissipation. The
third option is called second order monotonicity preserving. This method is
second order accurate in space and first order accurate in time. It was used
for this study to properly model the turbulent transport of the contaminant. Preservation
of monotonicity ensures that the quantity gradients are limited to avoid
The animation below shows that the upstream channel flow
deforms the jet, pushing it to the side groin fields where re-circulation takes
Simulation showing the deformation of the jet and circulation zones in two different views.
This bounded jet shows unsteady behavior even for the
statistically steady final solution. For ease of visualization, two
iso-concentration surfaces are shown: C=0.01 (red) and C=0.001 (grey), which act as a representative envelope
of the contaminant reach. The grid space was set to 5 cm for visual comparison
with the experimental results (see animations below).
Animations showing a visual comparison of experimental results (top) and numerical results (bottom)
We see that FLOW-3D captured all the relevant physics
important in modeling turbulent dispersion of environmental discharges. The
results match the experiment to a good degree of accuracy both visually and
In my next blog, I will be talking about another entry from
our 35th anniversary simulation contest, which focuses on modeling Pelton turbines.