1 Reply Latest reply on Dec 16, 2008 3:11 PM by Bill McEachern

    Amphibian aircraft planing on water

    Brian Robinson
      Hi, I was wondering if there are certain techniques to simulate a boundary layer between water and air. I would like to determine drag and lift forces of the hull of an amphibian aircraft planing on the water, just before lift-off. I placed the model of the aircraft in an assembly file where the top plane represents the surface of the water. I used the assembly cut extrude feature to remove everything above the top plane so that only the portion of the hull in the water remains. I set Analysis Type to External, Fluids to Water, Velocity in Z direction to 88 ft/s, Global Goals to Y and Z Component of Force, and Computational Domain Y max to 0 ft to represent the surface of the water.
      After some investigation I realized the horizontal forces seemed far too high, the vertical forces were negative and that the water flow seemed to be crossing the top plane.
      I then created a simple extrusion to represent the air and specified the bottom face as an ideal boundary wall condition. This corrected the flow lines of the water but the forces still do not make sense.
      What else needs to be set up for a proper determination of drag force?

      Peter Ferreira
        • Amphibian aircraft planing on water
          Bill McEachern
          Hi Peter,

          It sounds to me that what you did was the right way to go. The lift being negative is not unexpected as the velocities at the hull surfaces due to its curvature would induce higher velocities - high velocity = low pressure.

          Obviously, if the free surface problem was modeled there is a hydrodynamic lift component that assists in obtaining a planning condition but I would think the approximation might be too rough to capture that effect. The only componnet of the drag force would be the skin friction and possibly to some exent the pressure drag might be incorporated. However, the wave drag would not be accounted for in any way.

          You should also check to see if turing on the pressure potential siwth has any influence - you do this by turning on gravity and in then in the initial conditions dialog the pressure potential switch should be checked by defualt but you should check it to be sure. I would make a run with this enabled and one without and see if it makes a significant difference. The switch will account for hte pressure differentials due to gravity to be accounted for. I am not sure off he top of my head how significant it would be in you case.

          The other thing you could do is modify the wall you put in to account for the surface of the water and shape it to approximate the free surface deformations if you have any feel for those and see if it provides andything useful. You would be way out on a limb on this one but one nevers knows.