Surface Piercing Drag in CFD

Drag force on surface piercing flat plates oriented normal to a flow has been modeled using Ansys CFX v12.1 and compared to experimental results. Surface piercing refers to a body which is subjected to a two phase external flow, with the surface being the boundary between the two phases, in this case water and air. It is known that at high velocities flow behind a water piercing flat plate will separate due to a low pressure region created in the wake of the plate. This is known as the mechanism of ventilation. Once ventilation occurs, profile drag is decreased on the plates, due to the back side of the plate being exposed to air and thus atmospheric pressure. However, for surface piercing flow energy is dissipated in the form of waves and spray which increases the drag. The resulting change in drag can be expressed as a surface piercing drag coefficient of a plate, which changes as a function of speed or Froude Number and has been empirically shown to fluctuate from approximately 1.1 to 1.7 for a square plate. (Hoerner) The aim of the report is to verify and present the CFD results for surface piercing drag at a range of speeds from 0–10 m/s (0–36 kph). The results for a fully simulated domain are compared to experimental results at speeds of 0–3 m/s. One rectangular plate of aspect ratio 1 was tested in a towing tank and speed was limited. For speeds of 1–3 m/s the comparison was within 10%.