arXiv subjects

Xuanhong An

Publications and source records attributed to Xuanhong An.

4 recordsLinked to original sources

arXiv2022

The Mechanism of Lift and Pitching Moment Reversal

The experiment of separated flow response to a single-burst actuation over a 2-D NACA-0009 airfoil at $12^o$ angle of attack was conducted. The mechanism of the lift and pitching moment reversal following the single-burst actuation was studied. A spatially localized region of high pressure caused by a vortices-induced downwash is responsible for the lift and pitching moment reversal. Proper orthogonal decomposition (POD) of the flowfield shows that mode 2 shares a similar structure that produces the downwash and is responsible for the lift and pitching moment reversal. On the other hand, POD mode 1, which represents the direction and strength of the reverse flow on the suction side of the airfoil is responsible for the lift enhancement.

physics.flu-dyn↗

arXiv2020

Lift Coefficient Estimation for a Rapidly Pitching Airfoil

We develop a method for estimating the instantaneous lift coefficient on a rapidly pitching airfoil that uses a small number of pressure sensors and a measurement of the angle of attack. The approach assimilates four surface pressure measurements with a modified nonlinear state space model (Goman-Khrabrov model) through a Kalman filter. The error of lift coefficient estimates based only on a weighted-sum of the measured pressures are found to be noisy and biased, which leads to inaccurate estimates. The estimate is improved by including the predictive model in an conventional Kalman filter. The Goman-Khrabrov model is shown to be a linear parameter-varying system and can therefore be used in the Kalman filter without the need for linearization. Additional improvement is realized by modifying the algorithm to provide more accurate estimate of the lift coefficient. The improved Kalman filtering approach results in a bias-free lift coefficient estimate that is more precise than either the pressure-based estimate or the Goman-Khrabrov model on their own. The new method will enable performance enhancements in aerodynamic systems whose performance relies on lift.

physics.flu-dyn↗

arXiv2019

A Hybrid Model for Lift Response to Dynamic Actuation On A Stalled Airfoil

The current research focuses on modeling the lift response due to dynamic (time-varying) 'burst-type' actuation on a stalled airfoil. Dynamic `burst-type' actuation exhibits two different characteristic dynamic behaviors within the system, namely the high-frequency and low-frequency components. These characteristics introduce modeling challenges. In this paper, we propose a hybrid model composed of two individual sub-models, one for each of the two frequencies. The lift response due to high-frequency single burst actuation is captured using a convolution model. The low-frequency component due to nonlinear burst-burst interactions are captured using a Wiener model, consisting of linear time-invariant dynamics and a static output nonlinearity. The hybrid model is validated using data from wind tunnel experiments.

physics.flu-dyn↗

arXiv2019

Response of the Separated Flow over an Airfoil to Actuator Bursts

The separated flow response to single and multiple burst mode actuation over a 2-D airfoil at $12^o$ angle of attack was studied experimentally. For the single-burst actuation case, surface pressure signals were correlated with the flowfield observations of the roll-up and convection of a large-scale vortex structure that follows the actuator burst input. A spatially localized region of high pressure occurs below and slightly upstream of a "kink" that forms in the shear layer, which is responsible for the lift reversal that occurs within $2.0t^+$ after the burst signal was triggered. Proper orthogonal decomposition of the single-burst flow field shows that the time-varying coefficients of the first two modes correlate with the negative of the lift coefficient and pitching moment coefficient. The dynamic mode decomposition (DMD) of the single-burst flow field data identified the modes related to the kinetic energy growth of the disturbance. The mode with the largest growth rate had a Strouhal number close to that associated with the separation bubble dynamics. However, when multiple bursts are used to control the separation, the interactions between the bursts were observed which depend on the time intervals between the bursts. The convolution integral and DMD were performed on the multi-burst flow field datasets. The results indicate that the nonlinear burst-burst interaction only affects the reverse flow strength within the separation bubble, which is related to the main trend of the lift. On the other hand, the linear burst-burst interaction contributes to the high-frequency lift variation associated with the bursts.

physics.flu-dyn↗