Coherent Structure Interaction During Unsteady Separation

Unsteady flow separation in rotationally augmented flow fields plays a significant role in the aerodynamic performance of industrial rotors, including wind turbines. Current computational models underestimate the aerodynamic loads due to the inaccurate prediction of the emergence and severity of unsteady flow separation, especially in response to a sudden change in the effective angle of attack. Through the use of time-resolved particle image velocimetry (PIV), coherent structure formation during the unsteady separation over an experimental wind turbine blade is examined. Time-dependent empirical mode decomposition results during a dynamic pitching cycle give insight into the spatio-temporal scales that influence the transition from attached to separated flow. Empirical mode decomposition (EMD) modes are represented as two-dimensional fields and are analyzed together with Lagrangian coherent structures, the spatial distribution of vortices, the location of the separation point, and velocity contours focusing on the role of vortex shedding and shear-layer perturbation in unsteady flow separation, stall, and reattachment. The combination of these analytical techniques provides experimental evidence that the location of the separation point and the stability of the shear layer are directly influenced by the presence of vortices. Within this paper, each of the scales represented by the EMD are directly connected to the size of the vortices present, from the smallest representing a vortex radius to the largest reaching to two full vortex diameters. The velocity scales and spatial scales provided by the EMD modes are also found to supply valuable inputs into the identification of Lagrangian coherent structures within each of the PIV snapshots. This indicates that the scales captured by the EMD can be used to extract important turbulent scales present at the point of flow separation where the vortices are created, providing relenvant insight into the separation dynamics of the airfoil.

Dynamic stall of an experimental wind turbine blade

Karen Ann J Mulleners

To understand the complex flow phenomena over wind turbine blades during stall development, a scaled three-dimensional non-rotating blade model is designed to be dynamically similar to a rotating full-scale NREL 5 MW wind turbine blade. A time-resolved particle image velocimetry (PIV) investigation of flow behavior during the stall cycle examines the processes of stall development and flow reattachment. Proper orthogonal decomposition (POD) and vortex detection techniques are applied to the PIV fields to quantify relevant flow characteristics such as vortex size, separation angle, and separation point throughout a dynamic pitching cycle. The behavior of the POD coefficients provides time scales for the transitional stages which are quantified and compared, revealing that transition from attached flow to full stall is delayed to higher angles of attack and occurs at a higher rate than the transition from full stall to attached flow. The instantaneous flow fields are then reconstructed using the first four POD modes to demonstrate their prominent roles throughout the stall cycle and their ability to capture the general separation behavior over the blade surface. (C) 2016 AIP Publishing LLC.

Amer Inst Physics2016

The onset of dynamic stall revisited

Karen Ann J Mulleners

Dynamic stall on a helicopter rotor blade comprises a series of complex aerodynamic phenomena in response to the unsteady change of the blade's 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; a proper orthogonal decomposition of the velocity field revealed the energetically dominant coherent flow patterns and their temporal evolution. Based on the complementary information obtained by these methods the dynamics and interaction of vortical structures were analysed within a single dynamic stall life cycle leading to a classification of the unsteady flow development into five successive stages: the attached flow stage; the stall development stage; stall onset; the stalled stage; and flow reattachment. The onset of dynamic stall 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. The instantaneous effective unsteadiness was defined as a single representative parameter to describe the influence of the motion parameters. Dynamic stall onset was found to be promoted by increasing unsteadiness. The mechanism that results in the detachment of the dynamic stall vortex from the airfoil was identified as vortex-induced separation caused by strong viscous interactions. Finally, a revised criterion to discern between light and deep dynamic stall was formulated. © Springer-Verlag 2011.

2012

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

The onset of dynamic stall revisited

Karen Ann J Mulleners

2012