Application and Verification of Sub-Grid Scale Boundary Conditions for the Prediction of Antenna Wake Flowfields

Abstract

It has been shown through wind tunnel tests that “sma ll” geometric features such as antennas and masts can influence the mean and turbulent wake signature behind large naval vessels. Modeling these geometric features using standard computational fluid dynamics techniques would require wrapping either structu red or unstructured computational grids around these very complicated structures. This is a very costly approach for many reasons. As an alternative, sub -grid scale boundary conditions have been developed with the intention of capturing at least the first order effects of the presence of the masts and antennas. A test case using a generic naval vessel mast was developed. A wind tunnel model of the antenna geometry was created and hot -wire anemometry data were collected. The CFD predictions were shown to co mpare well with the wind tunnel data for the parameters of concern. I. Introduction he dynamic interface between a surface ship and an aircraft during approach, landing and departure operations is comprised of the cause and effect relationship between the a irwake and turbulence produced by the ship’s above water structure (topside design), ship motion, sea state and the aircraft’s flying qualities and performance. As ship topside designs evolved aircraft designers continued to improve the handling qualities and performance needed to operate an aircraft in the highly turbulent ship approach and landing environment. An aircraft’s approach and departure characteristics in the ship environment are degraded by the effect of the ship’s airwake on the aircraft’s ae rodynamics, engine performance, and consequent aircraft control response. The airwake velocity field near the ship and over the deck is extremely complex. Vortices generated by the ship’s superstructure are influenced by the shape and size of the superstru cture, the direction and magnitude of the Wind -Over -Deck (WOD), ship motion, hull shape, and sea state. This complex flowfield directly affects the pilot’s workload on approach and departure for both fixed wing and rotary wing aircraft. Wind tunnel tests 1 have shown that “small” geometric details on ships can significantly affect the character of the airwake. In this context, “small” geometric features are judged relative to the size of the main island structure and include masts, antenna arrays, armament, etc. These features can be geometrically complex and present an extremely difficult computational gridding problem for both grid construction and required grid density. The objective of this work is to develop and verify computational fluid dynamics meth ods to approximate the presence of “small” geometric features. The success of the technique is based on the ability to predict time -average velocity (momentum) deficit, turbulence intensity and frequency content of the far field wake. For this application , far field is assumed to be at least 200 feet downstream of the structure in full -scale. It is assumed that the chosen metrics will capture the flowfield elements that may affect flight operations. The far field wake is targeted since aircraft in normal f light operations would generally not encounter a mast wake until hundreds of feet downstream of the mast.