dsd-int 2014 - symposium next generation hydro software (nghs) - how to set up a typical coastal -...
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Grid design in estuaries and lagoons
using Delft3D Flexible Mesh
Bas van Maren, Arnold van Rooijen, Arthur van Dam,
Giselle Lemos (Technital), Herman Kernkamp
Introduction
Delft3D-FLOW * D-Flow FM **
Morphodynamics 2015?
Sand-mud interaction 2016?
Vegetation
3D flow
Resolution
Numerical aspects: conveyance & definition of fluxes
Case studies: Wadden Sea & Venice Lagoon
* Delft3D-FLOW = hydrodynamic simulation engine of Delft3D 4
** D-Flow FM = hydrodynamic simulation engine of Delft3D Flexible Mesh
Model resolution
This presentation:
- Short introduction on computational methods in D-Flow FM related
to model resolution (conveyance and 2nd order fluxes)
- Comparison of D-Flow FM – Delft3D-FLOW, for two lagoons:
- Wadden Sea
- Venice Lagoon
5
Delft3D-FLOW: tile depths - uniform friction and depth per cell
D-Flow FM: bed levels at cell corners. 2D analytical conveyance -
compute friction integral along entire cell’s edge, based on
bathymetry at cell’s corner points.
Model resolution: conveyance
KfKI, Bremerhaven, 2 November 2011 6
Delft3D-FLOW,
3 cells
The computed discharge
does not converge when
increasing # cells, when
using tile depths.
Correct discharge ≈ 497 m3/s
KfKI, Bremerhaven, 2 November 2011 7
The computed discharge
does not converge when
increasing # cells, when
using tile depths.
Correct discharge ≈ 497 m3/s
Delft3D-FLOW,
48 cells
KfKI, Bremerhaven, 2 November 2011 8
The computed discharge
now does converge
when using 2D
conveyance.
Correct discharge ≈ 497 m3/s
D-Flow FM,
48 cells
KfKI, Bremerhaven, 2 November 2011 9
The computed discharge
now does converge
when using 2D
conveyance.
Correct discharge ≈ 497 m3/s
D-Flow FM,
3 cells
Less curvilinear cells needed in
D-Flow FM compared to
Delft3D-FLOW because of
friction formulation
Model resolution: triangular or curvilinear
Less curvilinear cells needed in
case of simple topographies
Channels in an D-Flow FM model are preferentially
designed with a curvilinear grid
But also: larger cells larger
timestep possible
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Triangular grids lead to
cross-flow numerical diffusion
Model resolution: triangular or curvilinear
Channels in an D-Flow FM model are preferentially
designed with a curvilinear grid
Model resolution: conclusions
- Less curvilinear cells needed in D-Flow FM compared to
Delft3D-FLOW because of the bed schematization (conveyance)
- Curvilinear cells are more efficient than triangular cells for simple
geometry
- Less grid cells needed
- Larger grid cells larger timestep possible
- Triangular grids lead to cross-flow numerical diffusion
Use curvilinear grids when
possible and triangular grids
when needed
Case study: the Wadden Sea
Curvilinear grid
(Borsje et al. 2008) Unstructured grid
Grid Time step
Delft3D-FLOW curvilinear 1 min
D-Flow FM: CL curvilinear 1 min
D-Flow FM unstructured ≈ 20 sec (CFL-
condition based)
Case study: the Wadden Sea
- Delft3D-FLOW model most accurate
- Related to numerical settings optimization needed in the
D-Flow FM model (and practical experience)
RMSE (cm) Den
Oever
Harlingen Kornwerder
zand
Delft3D-FLOW 9.9 6.8 8.2
D-Flow FM: CL 10.0 8.7 9.1
D-Flow FM 11.1 8.8 12.4
Case study: the Wadden Sea
- D-Flow FM is 2.5 times faster than Delft3D-FLOW for the
curvilinear grid
- The new D-Flow FM model is much slower, because of much
higher resolution
Model run Wall clock time # time steps x
1000
# grid cells
Delft3D-FLOW 143 m 176 20829
D-Flow FM: CL 63 m 187 20829
D-Flow FM 464 m 602 45134
Case study: the Venice Lagoon
- Venice lagoon model setup in
Delft3D-FLOW and D-Flow FM
(various configurations, see
presentation Giselle Lemos)
- Continuous improvements in the past
years
Case study: the Venice Lagoon
10 november 2014
VENICE LAGOON: SOUTHERN PART 3D-FLOW VENICE MODEL: SOUTHERN PART D-FLOW VENICE MODEL: SOUTHERN PART
Triangular cells used as ‘glue’.
Curvilinear cells when possible,
triangular when needed
Case study: the Venice Lagoon – curvilinear grid
10 november 2014
D-Flow FM and Delft3D-FLOW
give similar results on the same
curvilinear grid, but D-Flow FM
is 2 times faster
Case study: the Venice Lagoon – new grid
Fluxes Water levels
New grid: D-Flow FM slightly
better, but computationally more
demanding
Conclusions
D-Flow FM is more accurate in complex topographies less grid
cells required
D-Flow FM is faster combined with less grid cells the model should
be much faster
Case studies: D-Flow FM is >2 times faster on same curvilinear grid
and comparably accurate
Pitfall: increase the horizontal resolution (too much…) resulting in
(much) slower models
Setting up an D-Flow FM grid takes time – think carefully before actual
grid design
Need to improve hands-on experience for accurate numerical settings
Use curvilinear grids when possible and triangular grids when needed
(‘glue’)