### Working Group on Benchmarking Hydrodynamic Numerical Models

Posted:

**Thu Sep 11, 2008 5:07 pm**Purpose of this working group

A series of three workshops addressed how to advance numerical models for long-water-wave problems (in 1990 at the Wrigley Marine Science Center of the University of Southern California on Catalina Island; in 1995 at Friday Harbor, San Juan Island, WA; and in 2004, again at the Wrigley Marine Science Center). The primary objective of the workshops was to discuss how numerical models could be validated through the use of benchmark problems. The benchmark problem exercises used for the workshops proved extremely useful in identifying absolute and comparative modeling capabilities. Significant improvements have been made, due to advances in both numerical algorithms and computational capabilities, especially for those models based on depth-averaged hydrodynamics theories by solving shallow-water-wave equations, KdV equation, KP equation, Boussinesq equations, and “extended” Boussinesq equations. At the same time, model improvements have given rise to new and important problems. In particular, although there is no question that actual long-wave inundation motions are turbulent – nor that runup flow patterns, impacts, scouring effects, and sediment transport are all affected by turbulence – the quantification of its effects remains an open question. This working group specifically focuses on that problem.

Benchmark problems will be identified so that predictive models can be evaluated, compared qualitatively and quantitatively, and discussed. The basic strategy will be to provide participants with initial and geometric data for the problems, so that predictions at specific locations can be prepared in a specified format. Corresponding laboratory measurements will be presented in the same format during a virtual workshop, allowing the comparison of predictions with measurements in a blind-test format. Targeted numerical models are those based on Reynolds Averaged Navier-Stokes (RANS) equations, k-epsilon type turbulence equations, large-eddy simulations, Marker-and-Cell (MAC) method, and Smooth-Particle-Hydrodynamics (SPH) method.

An example of a possible benchmark is the problem of bore-structure interaction by predicting water-surface variations, the turbulent flow field, and forces exerted by the bore on an object. A corresponding set of laboratory data have already been collected in the Oregon State wave basin. In those experiments, a bore was generated by dam-break, lifting a gate that initially separated the quiescent shallow water from a column of impounded water behind the gate. Temporal and spatial variations of the water-surface elevations were measured quantitatively with a Laser-Induced Fluorescence (LIF) technique (attached Figure 1); flow velocity fields were measured with a combination of Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) (attached Figure 2); and forces on the model column were measured with a miniature load-cell transducer.

What we hope to see posted on this discussion group:

The goal of this working group forum is to develop benchmark problems that can be used to validate analytical and computational models, apply those problems to existing models, and compare the predicted results to those measured in the laboratory. This process will broaden researchers’ understanding of the effects of turbulence in long-wave inundation motions. Topics of discussion include, for example:

* Desirable characteristics for benchmarks

* Suggested benchmarks

* Format and data content of initial conditions

* How model results should be formatted to facilitate comparison

* Preparation of laboratory data for comparison

* Methods for analyzing, evaluating, and comparing of results

* Related issues and research questions

A series of three workshops addressed how to advance numerical models for long-water-wave problems (in 1990 at the Wrigley Marine Science Center of the University of Southern California on Catalina Island; in 1995 at Friday Harbor, San Juan Island, WA; and in 2004, again at the Wrigley Marine Science Center). The primary objective of the workshops was to discuss how numerical models could be validated through the use of benchmark problems. The benchmark problem exercises used for the workshops proved extremely useful in identifying absolute and comparative modeling capabilities. Significant improvements have been made, due to advances in both numerical algorithms and computational capabilities, especially for those models based on depth-averaged hydrodynamics theories by solving shallow-water-wave equations, KdV equation, KP equation, Boussinesq equations, and “extended” Boussinesq equations. At the same time, model improvements have given rise to new and important problems. In particular, although there is no question that actual long-wave inundation motions are turbulent – nor that runup flow patterns, impacts, scouring effects, and sediment transport are all affected by turbulence – the quantification of its effects remains an open question. This working group specifically focuses on that problem.

Benchmark problems will be identified so that predictive models can be evaluated, compared qualitatively and quantitatively, and discussed. The basic strategy will be to provide participants with initial and geometric data for the problems, so that predictions at specific locations can be prepared in a specified format. Corresponding laboratory measurements will be presented in the same format during a virtual workshop, allowing the comparison of predictions with measurements in a blind-test format. Targeted numerical models are those based on Reynolds Averaged Navier-Stokes (RANS) equations, k-epsilon type turbulence equations, large-eddy simulations, Marker-and-Cell (MAC) method, and Smooth-Particle-Hydrodynamics (SPH) method.

An example of a possible benchmark is the problem of bore-structure interaction by predicting water-surface variations, the turbulent flow field, and forces exerted by the bore on an object. A corresponding set of laboratory data have already been collected in the Oregon State wave basin. In those experiments, a bore was generated by dam-break, lifting a gate that initially separated the quiescent shallow water from a column of impounded water behind the gate. Temporal and spatial variations of the water-surface elevations were measured quantitatively with a Laser-Induced Fluorescence (LIF) technique (attached Figure 1); flow velocity fields were measured with a combination of Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) (attached Figure 2); and forces on the model column were measured with a miniature load-cell transducer.

What we hope to see posted on this discussion group:

The goal of this working group forum is to develop benchmark problems that can be used to validate analytical and computational models, apply those problems to existing models, and compare the predicted results to those measured in the laboratory. This process will broaden researchers’ understanding of the effects of turbulence in long-wave inundation motions. Topics of discussion include, for example:

* Desirable characteristics for benchmarks

* Suggested benchmarks

* Format and data content of initial conditions

* How model results should be formatted to facilitate comparison

* Preparation of laboratory data for comparison

* Methods for analyzing, evaluating, and comparing of results

* Related issues and research questions