In this article, we look at an application from the maritime industry for a breaking dam flow and the wave impact on an obstacle placed in the flow. Initially, a series of uniform resolution simulations were performed with PreonLab 5.1 to determine the particle size necessary for accurate simulation results.
Subsequently, simulations have also been performed with PreonLab 5.2 to make use of the new Continuous Particle Size (CPS) feature and analyze the benefits this feature provides towards reducing computational effort without compromising on the accuracy of the results.
The simulation results obtained in PreonLab are compared qualitatively and quantitatively with experiment results published in the paper by Kleefsman et.al .
The experiments were performed at the Maritime Research Institute Netherlands (MARIN). All the experimental data generated is also available to download from the ERCOFTAC database .
According to  , the shutter holding the volume of water in place is pulled up almost instantaneously before the water flows across the tank. The influence of varying shutter opening speeds is not investigated with this setup. Hence, the wooden shutter included in the setup in PreonLab (Figure 2) serves rendering purposes alone and has no influence on the simulation. It is assumed that the shutter is opened instantaneously, and that the volume of water breaks down into a flow due to gravity.
Four height sensors (H1, H2, H3 and H4) are placed along the floor of the marine tank and 8 force sensors (P1 – P8) are placed along the surface of the obstacle to measure pressure, matching the measurement locations described in the experimental setup. The sensors have the dimensions
20 mm x 20 mm.
The simulations were run on 1 CPU with 8 cores. The following table gives an overview of the simulation times required for each investigated variant.
The simulation with particle size 40 mm runs in one minute (1 min 7 s) using 8 cores of processing power. While the general behavior of the flow matches the qualitative results from the experiment quite well, the quantitative results for height and pressure are not accurate enough.
This happens, since the particle size (40 mm) is too large compared to the dimensions of the sensors (20 mm x 20 mm). A similar problem occurs when the particle size is reduced to 20 mm, although a marked improvement in the calculated pressure can be observed. Increasing the resolution to 20 mm results in an increase in the number of particles in the simulation and thereby an increase in the total simulation time – as can be seen from the table above.
The simulation variant with a uniform particle size of 10 mm requires about one hundred minutes to reach completion, however, the accuracy of the results improves drastically. These results have been compared with the measured values from the experiment in Figures 5a-5f.
Using CPS, the particle size was then reduced only in the region of interest i.e., a volume around the obstacle, so that the sensors can accurately capture information about the flow characteristics. As a result, the total number of particles and consequently the computational time reduced significantly (compared to the uniform resolution simulation with particle size
It should be noted that a portion of the computational power is required for performing the splitting and merging of particles when using CPS. However, we are able to achieve results with an accuracy, which is similar to that when a uniform resolution with particle size = 10 mm is used – with only about half of the computational time required for simulation.
This shows that CPS can be effectively applied to cases such as the breaking dam flow, by keeping the particle size large for the bulk of the flow and reducing the particle size in the regions which need to be investigated closely.
Figures 3 and 4 show a qualitative comparison between the breaking dam flow simulated with PreonLab and snapshots from the experiment  at 0.4 s and 0.56 s, respectively:
Figure 3: Snapshot of the dam break simulation (right) compared with the experiment (left) at time 0.4 s.
The following video showcases the breaking dam flow with a box-shaped obstacle in the flow simulated with PreonLab for a total of 6 seconds (simulated flow from the uniform resolution simulation has been used for rendering purposes).
The flow simulated with PreonLab is quantitatively compared with the measured data by analyzing the water height at 2 locations in the tank (H2 and H4), and the pressure exerted by the water at 4 locations (P1, P3, P5 and P7) on the obstacle, corresponding to the results published in .
Note: The measured values for pressure at P1, P3, P5 and P7 published in  show an initial value of 1000 Pa acting on the obstacle – even at t = 0 s, where the flow has not reached the obstacle. Since no direct explanation for this pressure value could be found in the paper, this offset of
1000 Pa was deducted from the experiment data used for the comparisons in the following figures.
Figure 5a: Height in (m) after 6 s of simulation at location H2: Comparison between simulation results using a uniform resolution, simulation results using CPS and experimental data.
All the quantitative results show an overall good agreement with the experiment results. A slight delay can be observed with the simulated data around 4 s at height sensor H4 and between 4.5 s and 5 s at height sensor H2 as well as at the pressure sensors. This possibly occurs due to minor differences in the simulated wave propagation between two simulation time steps which are prominently visible due to getting accumulated towards the later part of the simulation. Similar delays can also be observed with the original simulation results by .
The results presented in this article showcase the capabilities of PreonLab for investigating free surface flows.
Using CPS, the computational effort can be kept to a minimum without compromising on the accuracy of the results in the region of interest.
 K. M. T. Kleefsman, G. Fekken, A. E. P Veldman, B. Iwanowski, and B. Buchner, A Volume-of-Fluid based simulation method for wave impact problems. J. Comp. Phys., 206:363–393, 2005
 SPH European Research Interest Community SIG, R.Issa and D. Violeau, Test-Case 2 3D dam breaking, Test 02 (spheric-sph.org)