LHA Airwake Wind Tunnel and CFD Comparison with and Without Bow Flap

Abstract

In order to evaluate the accuracy of computational fluid dynamics (CFD) for flow over a ship geometry, a comparison with an existing wind tunnel test of a simplified LHA hull with wind straight down the bow is made. One of the U.S. Navy's main interests in ship airwakes is the effect they have on aircraft operating nearby. In the interest of providing a more uniform flowfield in the vicinity of the ship, the wind tunnel test also investigates the effect of adding a downward -deflected flap to the bow, meant to mimic the rounded deck edge at the bow of later LHD -class ships, in contr ast to the sharp 90 -degree corner found on LHA - and earlier LHD -class ships. The CFD portion of this test also evaluates the effectiveness of the bow flap to show if CFD can match the trends of such geometry modifications. Furthermore, the CFD test investi gates the sensitivity of the solution to the grid resolution at the bow, where a large separation bubble affects a large portion of the flight deck, and will also determine if filtering the CFD data to mimic the performance of the wind tunnel equipment alt ers the results. This study seeks to validate CFD against wind tunnel data for this geometry and reinforce the wind tunnel assessment of the bow flap’s effectiveness. I. Introduction All branches of the U.S. military share an interest in the operation of air craft in the vicinity of ships. The airwake shed as wind flows over a ship can greatly influence the safety and ease of such operation. Features such as antennae, masts, radomes, deck equipment, and superstructures generate a complex flowfield that varies substantially with wind speed and direction. Due to the expense of repeating flight tests over different combinations of wind speed and direction, and with different aircraft types, CFD becomes an important tool in reducing such a test matrix by screening test conditions to those most crucial, and by evaluating ship geometries to predict problems and establish best design practices early in a ship's planning stage. To date, wind tunnels have borne much of the load for ship airwake testing, but they suffer f rom several drawbacks. Obviously, wind tunnels must use very small scale factors for ships, but they cannot achieve the airspeed required to match full -scale Reynolds numbers (Re). The small scale also decreases the period and size of pertinent flow featur es, making them difficult to detect. In addition, wind tunnel tests often require lengthy surveys or prior knowledge of the flowfield to find and adequately resolve off -body flow phenomena. In contrast, CFD can run any arbitrary physical scale, output resu lts at a user -determined frequency, and provide results over the entire solution space as a matter of course. However, wind tunnels certainly still provide an invaluable service in validating CFD and in testing conditions for which CFD is currently not sui table.