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VELOCITY [m/s]
3D NUMERICAL MODELLING OF SPILLWAYS David Menéndez Arán | Damwatch Engineering | [email protected] | 021 751 874
Introduction
Physical and computational hydraulic modelling of spillway flows include several aspects that are challenging to solve. Flow is typically highly turbulent, multi-phase (water
and air) with mixed flow regimes and shockwaves. Approach and tailwater conditions and changes in spillway geometry also influence flow behaviour.
Traditionally, physical hydraulic models have been the industry standard to study flow behaviour on spillways. However, these models can be expensive, time-consuming
and there are many difficulties associated with scale effects. Most physical models built are also only kept for a limited time, limiting future investigations.
Three-dimensional (3D) Computational Fluid Dynamics (CFD) models are currently widely used to complement physical hydraulic modelling, or even replace a
physical model in some cases. Although CFD models have several advantages, they also have limitations which need be understood.
This poster provides discussion on some of the issues regarding CFD modelling of spillway flows, through illustration of two case studies.
Case Study 1
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Dis
tan
ce f
rom
inve
rt a
lon
g z
axis
[m]
Distance along invert [m]
SIDEWALL
1D
3D CENTRELINE
3D LEFT WALL3D RIGHT WALL
FREE
BO
AR
D R
IGH
TW
ALL
z
x
CLLEFT WALL
RIGHT WALL
Chute flow dynamics
Final Remarks
Although there have been significant improvements in 3D CFD models and
the computational capacity to run them in recent decades, many 3D CFD
solutions to spillway flows do not resolve all the physical processes present.
The current industry trend for large spillway projects is to develop a physical
hydraulic model and 3D CFD model in series or parallel. This approach
allows the strength of both physical and numerical models to be utilized.
Relatively quick modifications to the design geometry can be made in a cost-
effective way in the numerical model with a large-scale physical model used
to validate the numerical results. 3D CFD models can also be used to
quantify and correct scale effects in the physical model due to skin friction
and drag effects.
• Figure 1 shows 3D CFD model results for a spillway with a contracting chute
and slightly asymmetric approach flow conditions.
• The water surface elevation cross sections show high shockwaves that
criss-cross the chute and reduce available freeboard.
• Figure 2 shows longitudinal water surface profiles for the 3D CFD
model, and compares them to the results from a one-dimensional (1D)
model.
• Figure 2 indicates that 1D models provide relatively good estimates of mean
flow parameters, but do not capture shockwave and geometric effects that
control the height and maximum hydraulic loading on the sidewalls.
Figure 1
Figure 2
VELOCITY [m/s]
Application
Although 3D CFD models have many applications in the modelling of
spillways, there are some limitations that need to be understood. These
include:
• Modelling air entrainment in steep chutes, hydraulic jumps or downstream
of aerators is important for spillway design. Modelling of these processes
is complex and typically requires a high degree of calibration with
prototype or large-scale physical hydraulic model results. Air entrainment
and jet breakup are critical in the modelling of plunge pools (see Figure 6).
• Study of fluctuating pressures in stilling basins and plunge pools requires
the use Large Eddy Simulation (LES) type models with very fine meshes,
which limits their use in practical engineering applications.
• 3D models generally require fixed geometries. Scour analysis of riverbed
sediments is complex and limited to frictional granular material.
3D
3D
SURFACE ELEVATION
SURFACE VELOCITY
2D
2D
SURFACE ELEVATION
DEPTH-AVERAGED VELOCITY
3D
DEPTH-AVERAGED
VELOCITY [m/s]
SPECIFIC
DISCHARGE [m2/s]
2D
2D
3D
SPECIFIC
DISCHARGE [m2/s]
SURFACE VELOCITY [m/s]
A
A
A
B
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AB
Case Study 2
2D shallow water models vs. 3D CFD models
• Figure 3 compares results for an asymmetric approach channel estimated
with a two-dimensional Shallow Water (2D SW) model and a 3D CFD
model. The general flow features are similar in both models.
• 2D SW models assume hydrostatic pressures and ignore the effect of
vertical curvature of ogee weir crests, so discharge capacities through
control sections are underestimated.
• Figure 4 compares results for a gated spillway chute with piers, an
expansion and a horizontal curve simulated with 2D SW and 3D CFD
models. Figure 5 shows a 3D view of the chute.
• Figure 4 indicates that 2D SW models are capable of approximately
capturing shockwave formation in spillway chutes, but underpredict
velocities and shockwave heights.
Figure 4
Figure 5
• Typically, 2D SW models are useful for rapid analysis and optimisation in
preliminary design, but 3D CFD models provide accurate predictions of
discharge capacity and flow behaviour for complex spillway geometries.
Figure 6
Figure 3
