An Analysis of Skimboard Hydrodynamics N. D. Barnett1 and E. Gutierrez-Miravete*2 1 General Dynamics-Electric Boat, 2Rensselaer at Hartford *Corresponding author: 275 Windsor Street, Hartford, CT 06120, [email protected] Introduction A skimboard is a device approximately 1-2 m in length and 1m diameter that is used to plane along the shoreline for a short distance (10 m max). The boards are usually employed in less than 2 inches (50mm) of water. The flow is similar to the flow underneath a surfboard, or powerboat, except the flow is bounded by very shallow water. This project simulates the flow under a skimboard using CFD in COMSOL Multiphysics, and validates the analysis by comparison to results obtained in previous works. Future work resulting from this project includes the possibility of expanding the techniques learned to other types of skimming hulls, such as surfboards, jet skis, and small power boats. Figure 1 shows a schematic representation of a skimboard sliding over a thin water layer from right to left. The skimming effect is obtained from the relative motion of the tilted board over the water surface. The upward directed jet at the bow of the board is the result of the mass flow imbalance underneath. The goal of this project was to investigate the hydrodynamic performance of a simple skimboard as a function of the various parameters illustrated in the figure. Figure 1: Schematic representation of a skimboard Use of COMSOL Multiphysics The Steady State Fluid Dynamics module in COMSOL Multiphysics was used to create a two-dimensional model of the skimboard-water layer system based on the schematic representation of Fig. 1. Regarding boundary conditions, no-slip, non-moving walls boundary conditions are designated as u = 0. Free surfaces are assumed to be symmetry boundaries, and are defined by having no shear, as in . For moving walls, the nodes on the boundary are given the velocity of that boundary (u= u w). For sliding walls, the nodes are given the tangential velocity entered and it is assumed to have no-slip. Localized mesh refinement was used to resolve the flow in selected areas of the system and mesh independent results were obtained by using ~ 1500 elements. Expected Results (Optional) The COMSOL model was used to test many different scenarios in order to determine the set of parameter values and boundary conditions that produced the best agreement with the results of prior studies and with physical intuition. Figure 2 shows a typical FE mesh used in our calculations and Figure 3 shows the computed pressure distribution underneath the skimboard. It is interesting to note that the resulting pressure hill yields enough force to withstand the weight of a 65 kg person traveling on top of the board. Figure 2. Finite Element Mesh Figure 3. Computed Pressure (Pa) underneath the skimboard Conclusion Useful information about the flow underneath a planning skimboard can be obtained from simple CFD models. Use of an appropriate set of boundary conditions resulted in a computed lift similar to that noted in previous theoretical studies and consistent with physical observation. For future consideration many other cases can be studied. For example, the flow along the bottom of a jet ski (hull and the jet) could be visualized quickly and compared to the models of previous successful and failed. Reference 1. Tuck, E.O. Dixon, A. Surf skimmer hydrodynamics. Journal of Fluid Mechanics. 205, 581-592, 1 February 1989.