Ultrasonic Lift Measurement Technique for Flow-Induced Structural Vibrations

Deficiencies exist in our current ability to measure lift forces in wind-tunnel experiments on vibrating structures in a fluid flow. The ultrasonic lift measurement (ULM) technique has been previously developed to measure time-averaged fluid circulation and lift on stationary structures. The ULM technique is based on measuring transit times of acoustic pulses along paths enclosing the structure. A quasi-steady method based on the Kutta–Joukowski theorem has been used in the past to convert fluid circulation to lift values in ULM studies. In this paper, the largely unstudied extension of the ULM technique to measure unsteady lift forces in flows involving structural vibration is considered. Analytic methods are developed that can be used to properly convert the instantaneous circulation measurements (attainable from ULM experiments) to lift values. These unsteady methods are validated by using numerical simulations of flows over flat plates undergoing oscillating motion. It is shown that the addition of unsteady terms provides a method with improved accuracy over the previous quasi-steady assumption. The methods are also applied to ULM data from an oscillating airfoil experiment in a preliminary study.