Dynamic stall process on a finite span model and its control via synthetic jet actuators

Abstract

An experimental study of the process by which dynamic stall occurs on a finite span S809 airfoil was conducted at the Center for Flow Physics and Control at Rensselaer Polytechnic Institute. Understanding the flow field around a dynamically pitching airfoil helped in controlling the dynamic stall process through active flow control via synthetic jet actuators. The three component, two dimensional flow fields were measured with a stereoscopic particle image velocimetry system. This study demonstrated that, through the introduction of periodic momentum near the leading edge of this model, the evolution of the dynamic stall vortex, which forms and convects downstream under dynamic conditions, could be delayed or suppressed in favor of the preservation of a trailing edge vortex that arises due to trailing edge separation and recirculation in the time averaged sense. This process seems to be the result of changing how the flow field transitions from trailing edge separation to a fully separated flow. In a phase-averaged sense, absent of flow control, this process is defined by the creation of a phase averaged leading edge recirculation region, which interacts with the trailing edge separation. Through the introduction of momentum near the leading edge, this process can be altered, such that the phase averaged trailing edge separation region is the dominant structure present in the flow. Additionally, a cursory investigation into the instantaneous flow fields was conducted, and a comparison between the phase averaged flow field and instantaneous fields demonstrated that while similar effects can be observed, there is a significant difference in the flow field observed in the instantaneous fields versus the phase averaged sense. This would imply that a different method of analyzing dynamic stall from PIV measurements may be necessary.

Wind tunnel quantification of dynamic stall on an S817 airfoil and its control using synthetic jet actuators

“…Although an analysis of the phase averaged flow field has been the focus of the current study to this point, and provides some insight into the average behavior of the unsteady flow fields, an investigation into the instantaneous flow fields can further describe the unsteady aerodynamics experienced by a wind turbine blade in dynamic stall. In fact, phase averaging can wash out many of the smaller scale vortical features associated with dynamic stall that contribute to the load root‐mean‐square (RMS) . To analyze the instantaneous images, the Γ 1 criterion, which describes a normalized angular momentum of an area in the flow, is used to identify the vortices .…”

“…In fact, phase averaging can wash out many of the smaller scale vortical features associated with dynamic stall that contribute to the load root-mean-square (RMS). 11,31 To analyze the instantaneous images, the Γ 1 criterion, which describes a normalized angular momentum of an area in the flow, is used to identify the vortices. 32 This method was previously used by Taylor and Amitay 11 on the basis that the local flow field in the separated region has low velocity relative to the laboratory reference frame, such that a Galilean variant method successfully identifies them; this was shown on a variety of instantaneous images to predict instantaneous vortex centers well.…”

Quantification of the S817 airfoil aerodynamic properties and their control using synthetic jet actuators

“…Boundary layer control generally represents devices that energize the boundary layer by adding vorticity or momentum. This can be done through devices such as deployable vortex generators, dynamic pins, suction/blowing, plasma actuators, or synthetic jets …”

“…In fact, for the cases shown, lift enhancement was increased by 400% when the synthetic jets were pulse modulated at , nearly 3 times as effective as high‐frequency ( ) actuation, with 25% of the jet momentum coefficient. Additionally, pulse modulation was used by Taylor et al to show a 50% reduction in lift hysteresis from dynamic stall compared with the continuously ( ) actuated synthetic jet, while consuming significantly less power.…”

“…This can be done through devices such as deployable vortex generators, [7][8][9] dynamic pins, 10 suction/blowing, [11][12][13][14] plasma actuators, 15,16 or synthetic jets. [17][18][19] Although significant laboratory research has been conducted on these devices to show their potential for altering flowfields, full-scale field tests demonstrating and analyzing the cost and benefits of active flow control devices on wind turbines are much less frequent. Rice et al 20 demonstrated the ability to install synthetic jets and dynamic vortex generators on a residential scale wind turbine, monitor blade deflection, and actuate flow control in a closed loop fashion, showing moderate reduction in blade vibrations during dynamic stall and reduced separation as observed through tuft flow visualizations.…”