Novel Time-Resolved Pressure Measurements on an Airfoil at a Low Reynolds Number

Abstract

2This work focuses on time-resolved surface pressure measurements on an airfoil operating in low Reynolds number flows. The experiments were performed on a NACA 0018 airfoil at a chord Reynolds number of 100,000. The results are presented for two angles of attack, eight and twelve degrees, representative of two flow regimes common to airfoil operation at low Reynolds numbers, separation bubble formation and separation without subsequent reattachment. An array of twenty-five microphones was embedded into the airfoil model to facilitate multi-point, time-resolved surface pressure measurements. A comparative analysis of mean pressure, velocity, and time-resolved pressure measurements is presented to determine what flow characteristics can be estimated using embedded pressure sensors. The results show that surface pressure fluctuations increase markedly past the separation location, reflecting the increase in the magnitude of velocity fluctuations in the separated shear layer undergoing laminar-to-turbulent transition. In the separation bubble, surface pressure fluctuations peak just upstream of reattachment, suggesting that time- resolved surface pressure measurements can be used to identify the presence and estimate the length of the separation region. The analysis of simultaneous velocity and pressure measurements shows that the characteristics of flow disturbances growing in the separated shear layer can be estimated from microphone measurements. In particular, the dominant frequency, convective speed, and growth rate of disturbances can be estimated without performing the extensive tests typically required when conventional velocity measurement techniques are used. However, when separation occurs without reattachment, the relatively large distance between the transitioning separated shear layer and the airfoil surface results in lower magnitude surface pressure fluctuations, which substantially increases the uncertainty of estimating flow characteristics based on microphone measurements for this flow regime.