Session R04: Gust Encounters and Interactions

Blade-Vortex Interaction on a Twisting Flat Plate

Vortex dynamics during parallel blade-vortex interactions (BVIs) of a tandem-wing configuration were studied in a towing tank using particle image velocimetry (PIV) and force sensor data. Vortices were generated by applying a rapid pitching motion to a leading rigid flat plate using a servo motor. The generated vortices collide with a trailing flat plate undergoing dynamic twisting. The interaction between the vortex and leading edge, the interaction between the vortex and the boundary layer, and the subsequent evolution of the vortex over the surface of the trailing plate were investigated for varying twisting kinematics. Vortex trajectory, direction of rotation, and strength were obtained from PIV data. The results were used to study the mechanisms of vortex decay and the effects of key parameters on vortex dynamics. The subsequent effects on the hydrodynamic forces experienced by the trailing plate are studied by analyzing the force sensor data in conjunction with PIV results.   

Gust Mitigation Through Spanwise Twisting

Wind gust encounters impose a highly unsteady and unpredictable environment for air vehicles. Small aircrafts are particularly vulnerable to gust-induced disturbances due to their limited size and weight, resulting in pronounced fluctuations during flight. This study proposes a new concept that involves implementing a twisting mechanism along the wingspan, enabling active control of individual wing sections' angles of attack. By adapting to varying gust conditions, this twist-based spanwise pitch control system demonstrates promising potential in enhancing small aircraft's stability and safety during flight. We present our experimental results of time resolved lift and flow field measurements to demonstrate the effects of spanwise twisting for gust mitigation.   

An investigation of the dynamic response of a laminar separation bubble on a NACA 0015 wing forced with a periodically unsteady freestream

The aerodynamic performance of conventional wing sections at transitional Reynolds numbers has been shown to be dominated by the formation of laminar separation bubbles (LSBs) on both the suction and pressure sides of the wing. More specifically at low angles of attack LSBs typically form near the wing trailing edge; increasing the effective wing thickness and altering the effective camber, which can cause significant deviations in the lift slope classical theory. The size and position of the LSBs is strongly Reynolds number dependent, which makes them highly susceptible to unsteady variations in the freestream flow. To better understand this dynamical coupling a finite span NACA 0015 wing section at a fixed angle of attack of α = 2° is exposed to an unsteady freestream, driven in a periodic fashion in the range of 75,000 < Re < 200,000 for reduced frequencies from 0.04 < k < 0.12. The LSB dynamics are experimentally captured through the synchronized measurement of time-resolved surface pressure and planar particle image velocimetry measurements. The periodic roll-up and shedding spanwise oriented coherent vortical structures within the LSB are observed whose convective spacing dynamically varies with the unsteady freestream conditions.   

Unsteady lift estimation using distributed pressure sensing in the presence of uncertainty

The estimation of fluid dynamic loads using distributed pressure sensors is of practical interest to engineers designing systems that are meant to operate in unsteady environments. In this work, a wing model instrumented with 48 pressure sensors is used to experimentally evaluate the effectiveness of different sensing configurations. Simultaneous pressure and force data are collected for wings undergoing unsteady kinematics as well as encountering transverse gusts. Methods of varying fidelity are assessed in their ability to predict lift from sparse measurements. Emphasis is placed on discovering configurations that minimize the uncertainty of the estimated loads, across a variety of unsteady flows.   

Unsteady flows can help turbines surge ahead in power production

To augment the performance of energy-harvesting turbines in real flow conditions, the effects of temporal variations in the streamwise inflow must be considered. These unsteady dynamics can lead to enhancements or losses in the time-averaged power extraction of a turbine, depending on the characteristics of the flow perturbation and turbine aerodynamics, and alter the flow conditions downstream of the turbine. To quantify and predict these phenomena, an analytical framework for the time-varying and time-averaged power extraction of a turbine in a periodically varying streamwise inflow is derived and validated in wind-tunnel experiments. This framework can also be used to predict the time-varying flow properties upstream of the turbine. Extensions of this modeling paradigm to the wake region downstream of the turbine are then explored and compared with flow-field measurements of a periodically surging turbine in an optical towing tank using two-dimensional particle-image velocimetry. The phase-averaged wake measurements help parameterize the effects of unsteady inflow dynamics on the time-averaged wake profile and on the contributions of vortical structures to the evolution of the wake. These results inform control schemes for enhancing the power extraction of individual turbines in unsteady flow conditions, as well as minimizing unsteady wake losses on downstream turbines in an array.   

On the response of the thin airfoil under multiscale gusty flows

Greenberg and Isaacs' theories are typically used for sinusoidal incoming flows, and their practicality in predicting lift overshot for thin airfoils with small angles of attack has been well established. In this study, we conducted experiments using a moving net to generate a multiple scale sinusoidal free stream. The main objective was to assess Isaacs' theories under random multiple scale waves with varying combined reduced frequencies, while maintaining constant small angles of attack on a NACA 0012 airfoil. The results showed the validity of Isaacs' theory when applied to multiple scale sinusoidal free stream conditions. First, the underlying reason was theoretically analyzed; second, to further validate the classic theory, we employed a time-resolved particle image velocimetry method to analyze the evolution of the flow field. Additionally, the momentum conservation approach was utilized on the control surface to quantify the lift coefficient contributions. Comparisons of the velocity magnitude evolution served as the foundation for understanding the airfoil-gust interaction response.