Session: 02-04-01: Experimental Studies in Structural Dynamics

Paper Number: 133511

133511 - Experimental Investigation of Shock-Induced Aeroelastic Instabilities 

Interest in the nonlinear, dynamic aeroelastic behavior of hot, flight-weight aerospace structures has increased dramatically in the past decade. While various multi-fidelity simulation strategies have been developed to study the aeroelastic responses, very few experiments exhibiting aeroelastic instabilities exist in the literature. Even fewer experiments include the effects of shock/boundary layer interactions (SBLIs). The data/knowledge obtained from these experiments are needed to understand the mechanisms governing aeroelastic instabilities and guide the development/validation of computational tool sets. Towards this need, the Air Force Research Laboratory Structural Sciences Center previously initiated an experimental campaign in the Research Cell 19 (RC-19) wind tunnel targeting aeroelastic instabilities. The primary objectives of this ongoing campaign were to: (1) observe/measure the effect of complex flow physics on thin-walled structures; (2) develop/implement a systematic testing methodology to identify dynamic instabilities and explore the post-instability responses; (3) investigate severe structural events (e.g. large-amplitude dynamics and failure); and (4) refine full-field, non-contact measurement techniques to characterize the aerodynamic loads and associated structural responses. Early entries in RC-19 primarily relied on three-dimensional digital image correlation (DIC) and high-speed, z-type Schlieren imaging to measure the full-field panel responses and flow characteristics, respectively. Discrete, surface mounted instruments were also used to monitor to panel dynamics and operating conditions (i.e. cavity pressure and temperature differential between the panel and frame). The effects of heated flow, turbulence, shock/boundary layer interaction (SBLI), and flow separation on the aeroelastic behavior of the panel were considered. In the absence of SBLI, the thermally buckled panel experienced bi-stable, cross-well oscillations with cavity pressure modulation. Both periodic and chaotic motions were observed. Shock-induced periodic limit cycle oscillations were also measured for the first time using DIC for both attached and separated SBLIs.

Recenet entries in RC-19 have focused on refining and introducing new full-field measurement techniques including particle image velocimetry (PIV), fast-reacting pressure sensitive paint (PSP), high-speed focusing Schlieren, and thermal imaging (FLIR). Focusing Schlieren and PSP were used to characterize the time-averaged and fluctuating loads as well as the three-dimensional shock structures in the absence of structural compliance. Full-field measurements of the temperature differential between the panel and frame were obtained from FLIR data. Note that comparisons between simulations and experiments previously identified the temperature differential as a key parameter when simulating aeroelastic instabilities of buckled structures. Increasingly complex loading conditions were also explored in greater detail. For a majority of the operating conditions with a separated SBLI, measured time histories of the thin panel dynamics revealed small-amplitude forcing about a stationary deformation. It was hypothesized that these motions were due to pressure fluctuations from the turbulent boundary layer and/or separated SBLI. However, careful modulation of the cavity pressure revealed the presence of bi-stable, cross-well oscillations for a small range of temperature differentials which were captured for the first time using DIC and FLIR. Intermittent snap-through motions were initially observed for higher temperature differentials and eventually evolved into continuous motions as the temperature differential decreased. The DIC data illustrated that the panel oscillations were centered about the mid-chord, coincident with the shock impingement location and the associated separation bubble. The observed aeroelastic behavior was indicative of strong coupling between the separated SBLI and panel motions. Post-instability oscillations in the presence of a swept, attached SBLI were also captured for the first time. As expected, the swept shock produced a strong, three-dimensional loading environment leading to the excitation of spanwise structural modes and the modification of the aeroelastic instability boundaries. Simple and complex periodic motions were observed with modulation of the cavity pressure and temperature differential.

Presenting Author: Kirk Brouwer AFRL

Presenting Author Biography: Dr. Brouwer received his PhD in Aerospace Engineering from The Ohio State University in 2018. His dissertation research focused on reduced-order modeling of aerodynamic loads for aerothermoelastic simulations in high-speed flows. Since 2019 he has worked with the AFRL Structural Sciences Center (SSC) primarily conducting aerothermoelastic experiments in various wind tunnels and running companion simulations. His current research areas include experimental and computational aerothermoelasticity, nonlinear structural dynamics, shock/boundary layer interaction, and reduced-order modeling.