Mapping flow distortion on oceanographic platforms using computational fluid dynamics

Ocean-Atmosphere Fluxes Eddy correlation (EC) is the most direct method to measure fluxes of trace gases over the Earth's surface. In its simplest form, an EC setup consists of a gas sensor and a sonic anemometer. EC is commonly used on land, but its adaptation at sea has proven difficult because of the marine environment, the motion of the research platform (ship or buoy), and flow distortion. Flow distortion occurs when streamlines circumvent the research platform, which may lead to significant errors in the calculation of the gas transfer velocity. This paper uses computational fluid dynamics (CFD) to simulate the errors in wind speed measurements caused by flow distortion on the R/V Celtic Explorer. Numerical measurements were obtained from the finite volume CFD code OpenFOAM, which was used to simulate the velocity fields. This was done over a range of orientations in the test domain from 60° to +60°, in increments of 10°. The simulation was also set up for a range of velocities, ranging from 5 m s 1 to 25 m s 1 in increments of 0.5 m s 1 The numerical analysis showed close agreement to experimental measurements to within a 12% mean difference prediction of flow distortion effects. Other aspects resulting from flow distortion that were investigated using the CFD tools included development of a correction method for flow distortion effects for in situ wind speed measurements; analysis of ideal positioning of anemometers; vertical tilt orientation of the vessel to inflow; meteorological mast design; and mast instrumentation setups.