title: Bluff-body wake encounter and tandem wing-tail wake dynamics in forced harmonic pitch
Creators: name: Tan, Kin Jon Benjamin
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: vortex shedding, wake dynamics, wake encounter, bluff-body, tandem flows, forced harmonic motion, overset grid, OpenFOAM.

description: Wake phenomena and its encounter with downstream bodies are of engineering importance as it can affect aerodynamic loads. This can induce unfavourable-or even dangerous-conditions to aircraft through a loss of lift, or stability and control. Such scenarios range from the local interactions between an aircraft wing and its empennage, to wakes emanated from buildings acting on a helicopter fuselage downstream. Therefore, wake physics must be evaluated to predict any consequential aerodynamic effects. Furthermore, accurately modelling turbulent wake regimes are challenging, as obtaining physically meaningful data requires high-fidelity techniques. In this thesis, canonical cases under static and dynamic conditions focusing on wake encounters are investigated computationally to address these concerns. This is achieved using a Detached-Eddy Simulation (DES) approach that will be validated with a static case before expanding to include overset methods to induce dynamic grid motion on a tandem configuration. Validation is performed by investigating the vortex shedding behaviour and wake characteristics of a separated flow over a square beam bluff-body. Spectral analyses of surface-forces reveal von Karman street dynamics with frequency correlations in the freestream-parallel and cross-stream directions. Metrics are verified against benchmark experimental data, where a considerable aerodynamic impact is implied by both first- and second-moment wake statistics up to a measured downstream distance of six characteristic lengths. The extent of numerical treatment is further demonstrated through validation of its shear-stresses, an auto-correlation function of point-probed velocities, while coherence is observed as peak frequencies correspond to surface vortex shedding frequency. This case is then subsequently used as a wake generator for the investigation of its aerodynamic impact downstream. A NACA0012 wing-section is placed three characteristic lengths downstream for insights on the aerodynamic effects of the bluff-body wake encounter. Time-averaged surface-forces on the wing-section are evaluated against a wake-free condition of the airfoil for reference. The wake encounter demonstrates a decrease in overall pressure distribution from wake-induced separation, with a strong correlation in lift response with the bluff-body vortex shedding dynamics. Instantaneous contours reveal flow behaviour resembling those expected of heave dynamics caused by the alternating vortex street. As oscillatory lift characteristics are induced, the work proposes approximating this response with the Sear's and Theodorsen's functions represented as a relative harmonic motion to the wake based on reduced frequency. Finally, expanding this framework to include overset grids accomplishes dynamic motion for forced harmonic pitching on a tandem wing-tail configuration. This subjects the horizontal stabilizer to a wake induced by pitch oscillations of its main wing located upstream. As this is a single rigid system with a rotational centre on the wing chord, it is observed that the moment arm translates to a coupled pitch-heave motion at the tail. In addition, a separated wake with characteristic leading and trailing edge vortices (LEV/TEVs) is emanated from the wing at the higher angles-of-attack in the harmonic pitch cycles. This leads to a direct correlation in wing-tail dynamics where the tail lift response can be distinguished into two components; the combination of its pitch-heave directly contributed by the tail moment arm, and a gust component by periodically encountering the separated wing wake. The combination of these mechanisms synthesises the tails response from both forced harmonic motion and wing wake interaction, and is shown to be significant to the entire (wing-tail) system. This contributes to novel insights on wake interactions, as the computational framework advances the understanding of tandem aerodynamic relationships under dynamic conditions.