Some Comparative Studies on Electromagnetic Sensor Heads in Laminar and Near-Turbulent Flows in a Towing Tank

The paper describes a series of towing tank observations, made on electromagnetic current sensor heads in laminar and near turbulent flows in order to ascertain the relative merits of different head shapes. Three types of head were studied: discus, annulus and sphere. Measurements in laminar flow were of linearity, noise, directional responses, stability and dynamic response; a dye solution was used as an aid in flow visualization. The heads were used to investigate the known wake structure of a towed cylinder and comparisons were made with propeller and thermistor sensors in the higher levels of turbulence generated by a submerged jet. A weighting function is derived from the sensor mean velocity profile with reference to the propeller output. Any systematic change in instrumental sensitivity is shown to be small for all three heads although differences emerge in responses to laminar flow. Although the tests were devised with practical operating conditions in mind, it is emphasised that such measurements are complementary to evaluation at sea.