Critical evolution of leading edge suction during dynamic stall

Dynamic stall dominates the aerodynamic performance, the robustness, and the wake dynamics of vertical axis wind turbines. To better assess the dynamic stall onset and its associated unsteady effects, this paper analyzes experimentally the evolution of the leading edge suction vector on a sinusoidally pitching airfoil based on time-resolved surface pressure measurements and particle image velocimetry. During the dynamic stall stage, we linked the shear layer evolution with the evolution of the leading edge suction. The dynamic stall development prior to dynamic stall onset consists of two stages: a primary instability stage and a vortex formation stage. The transition between the stages is marked by a maximum in the leading edge suction. During the primary instability stage, the leading edge suction increases linearly while the shear layer height with respect to the airfoil's surface also increases linearly. During the vortex formations stage, the leading edge suction decreases linearly while the shear layer rolls up and creates a dynamic stall vortex. The maximum leading edge suction increases nearly linearly with the normalized effective unsteadiness. The leading edge suction at dynamic stall onset appears to be independent of unsteadiness of the pitching motion.

Unsteady fluid dynamics around a hovering flat plate wing

Swathi Badala Krishna

Insect flight is characterised by complex time-dependent flows in response to the unsteady wing movements. Biological fliers exploit the unsteady flow fields to modulate aerodynamic forces, thereby displaying unmatched flight performance, especially in hover. Naturally, this has inspired the creation of engineering models to replicate the flight behaviour. An in-depth understanding of the flow fields generated during hover and their dependence on the kinematics is paramount to achieve this goal. The two main kinematic components of a hovering wing are the stroke, which refers to the back-and-forth motion, and wing rotation, which refers to the change in angle of attack. The phase relation between stroke and rotation is quantified in terms of phase-shift and is broadly classified into symmetric, advanced, and delayed rotation. The phase-shift and duration of rotation, together referred to as rotational timing, are investigated in this bio-inspired study. The objective is to characterise the effect of rotational timing on the aerodynamic forces and the flow fields generated by a hovering wing. The unsteady flow around a hovering flat plate wing that mimics hoverfly kinematics has been investigated experimentally using particle image velocimetry and direct force measurements. The measurements are conducted at a Reynolds number of Re=220 and a reduced frequency of k=0.32 in order to dynamically match a hoverfly. The Lagrangian finite-time Lyapunov exponent method is used to analyse the unsteady flow fields by identifying dynamically relevant flow features such as the primary leading edge vortex (LEV), secondary vortices, and topological saddles, and their evolution within a flapping cycle. Firstly, the flow and force behaviour was characterised for a typical flapping cycle. The flow evolution in a symmetric, fast rotation is divided into four stages that are characterised by the LEV emergence, growth, lift-off, and breakdown and decay. Tracking saddle points is shown to be helpful in defining the LEV lift-off which occurs at the maximum stroke velocity. The flow fields are correlated with the aerodynamic forces revealing that the maximum lift and drag are observed just before LEV lift-off, which corresponds to the maximum stroke velocity. Secondly, the effect of phase-shift on the formation and evolution of lift-enhancing flow structures are discussed. Two advanced and delayed rotations are compared. The flow development stages and forces are similar for all rotations but the timing of stages varies. The evolution of forces and flow strongly depend on the stroke velocity. Thirdly, the dependence of the flow and force evolution on the stroke velocity was substantiated by doubling the rotational duration in the symmetric rotation. It was found that the timing of the flow stages altered, whereas the flow and forces mostly evolved similarly to that of a fast rotation. The fast rotation, however, produces higher maximum lift and drag compared to the slow rotation. Lastly, the effect of phase-shift on the aerodynamic characteristics of a slow rotation is further explored. The slow rotation cases exhibit distinct flow patterns for varying phase-shifts unlike the fast rotations, in terms of the formation, evolution and breakdown of the flow structures as well as the timing. The forces also show distinct trends for varying phase-shifts and strongly depend on the angle of attack along with the stroke velocity in the slow

EPFL2017

A time-resolved dynamic stall investigation based on coherent structure analysis

Karen Ann J Mulleners

Dynamic stall on an airfoil comprises a series of complex aerodynamic phenomena in response to an unsteady change of the angle of attack. It is accompanied by a lift overshoot and delayed massive flow separation with respect to static stall. The classical hallmark of the dynamic stall phenomenon is the dynamic stall vortex. The flow over an oscillating OA209 airfoil under dynamic stall conditions was investigated by means of unsteady surface pressure measurements and time-resolved particle image velocimetry. The characteristic features of the unsteady flow field were identified and analysed utilising different coherent structure identification methods. An Eulerian and a Lagrangian procedure were adopted to locate the axes of vortices and the edges of Lagrangian coherent structures, respectively. Additionally, the velocity field was subjected to a proper orthogonal decomposition yielding the energetically dominant coherent flow patterns and their temporal evolution. The complementary information obtained by these methods provided deeper insight into the spatiotemporal evolution of vortical structures within a single dynamic stall life cycle. In particular, the onset of dynamic stall, generally defined as the detachment of the primary stall vortex, was specified here based on a characteristic mode of the proper orthogonal decomposition of the velocity field. Variations in the flow field topology that accompany the stall onset were verified by the Lagrangian coherent structure analysis. Furthermore, a subtle but significant change was observed in the orientation of the trajectories of the vortices that originate at the very leading edge shortly before and after stall onset. The mechanism that results in the detachment of the dynamic stall vortex from the airfoil was identified as a vortex induced separation caused by strong viscous interactions.

2010

Stall Delay and Leading-Edge Suction for a Pitching Airfoil with Trailing-Edge Flap

Julien Dominique Claude Deparday, Guosheng He, Karen Ann J Mulleners

The flow around a pitching NACA0015 airfoil with an oscillating trailing-edge flap is experimentally investigated to characterize the influence of the flap kinematics on the development of dynamic stall. We specifically focus on the timing of the stall development and critical values of the leading-edge suction parameter as potential stall triggers. In general, static or dynamic trailing-edge flap motions strongly affect the entire lift response but preserve the characteristic evolution of the leading-edge suction parameter. The influence of the flap on the critical values of the leading-edge suction parameter and their timing is more subtle. Variations in the magnitude of the leading-edge suction are related to the instantaneous geometric effective angle of attack. A thin-airfoil theory-based model that linearly superimposes the effect of the flap on the response for a fixed flap in its neutral position correctly predicts the maximum values of the leading-edge suction parameter. The timing at which these maxima are reached corresponds to the stall delay and is dominated by the prestall temporal evolution of the effective angle of attack. Our results suggest that dynamic stall development is governed by characteristic stall delays rather than critical values of the maximum leading-edge suction parameter.

2020

Unsteady fluid dynamics around a hovering flat plate wing

Swathi Badala Krishna

EPFL2017