Dynamic Stall Characteristics and Modelling of Time-Varying Pitching Kinematics

TL;DR

This study tackles dynamic stall under nonlinear pitching kinematics by combining controlled experiments with a time-resolved force model. It demonstrates that stall onset delay follows a universal decay with a characteristic pitch rate defined at static stall, and that acceleration mainly affects the stall angle rather than the delay. The generalized Goman-Khrabrov model, originally using a single lag tied to pitch rate, is extended with a modified effective-angle definition to separately represent reaction and vortex-formation delays, yielding significantly improved predictions for nonlinear motions. The findings advance dynamic-stall prediction for complex kinematics and offer a robust benchmark for validating first-order unsteady aerodynamic models in engineering systems with time-varying pitching.

Abstract

We present an experimental investigation examining how the complexity of pitching kinematics influences dynamic stall characteristics, including the stall delay and aerodynamic force response. The study examines whether the pitch rate defined at the static stall angle adequately characterises time-varying pitching kinematics for stall onset prediction. We then evaluate the performance of the generalised Goman-Khrabrov model in predicting force responses of nonlinear pitching motions and propose necessary modifications to extend the model's applicability to complex kinematics.

Figures (6)

• Figure 1: (a) Schematic of the test setup in the water channel, (b) the prescribed quadratic pitching motions. The colourbar indicates the non-dimensional pitch acceleration $\frac{\ddot{\alpha}c^2}{2U_\infty^2}$.
• Figure 2: Dynamic stall onset detection based on lift coefficient. The shaded region indicates the pitching duration. The instant of exceeding the critical stall angle (${\alpha}_{\hbox{ss}}$) and dynamic stall onset (${\alpha}_{\hbox{ds}}$) at reaching the maximum lift coefficient (${C}_{\hbox{L,max}}$) are indicated with vertical lines.
• Figure 3: Example results for linear and nonlinear pitching motions, (a) pitching kinematics, (b) pitch rate variation, (c) lift coefficient versus angle of attack, (d) lift coefficient versus convective time shifted by the instant of exceeding static stall angle.
• Figure 4: Stall characteristics for quadratic pitching motions. (a) Stall onset delay, (b) dynamic stall angle and (c) the maximum lift coefficient versus non-dimensional pitch rate at static stall angle. The colourbar indicates the non-dimensional acceleration. Results from linear pitch motions are shown for reference using $\blacktriangle$.
• Figure 5: Goman-Khrabrov model results for nonlinear motions following the two modelling approaches compared to experimental results for three representative cases presented in \ref{['fig:forces']}, (a) a decelerating motion, (b) a linear motion, (c) an accelerating pitch motion. Insets are zoomed in views of the lift peaks and post-stall behaviour. The label Original model refers to the approach proposed by Ayancik2022Ayancik2022, and Modified model corresponds to the Goman--Khrabrov model with the modification proposed in the present work.