Experimental investigation and modal analysis of the wake behind a flat-back Ahmed body

Abstract

To enhance the aerodynamic performance of a vehicle, a fundamental understanding of the flow is required. This can be achieved through flow characterization of simplified models such as the Ahmed body. However, uncertainties regarding the topology of flow motions in the wake persist due to the limitations of previous measurement and analysis methods. In this research, the three-dimensional wake flow behind a flat-back Ahmed body is investigated at ReH = 10,000, based on the freestream velocity and the height of the body, H. The flow was measured using planar and time-resolved tomographic particle image velocimetry measurements, and subsequently analyzed using two modal decomposition techniques. The current proper orthogonal decomposition (POD) analysis indicates that the most energetic flow motion is attributed to the well-known bi-stability, accounting for 9.1% of the total turbulent kinetic energy of the wake flow. It is followed by other flow motions with energy below 1.5%, such as vortex shedding. In the current study, the lack of temporal coherence in spatial patterns obtained from POD was addressed by employing its space-time formulation, known as spectral proper orthogonal decomposition (SPOD). The coherent flow motions at each discrete StH within the range of 0.007 to 1.228 are characterized using SPOD analysis, where StH is the Strouhal number based on H. The analysis has discerned the known flow motions, including bi-stability, vortex shedding, and shear layer instability, each occupying a specific StH range. The bi-stability mode demonstrates the switching between two asymmetric states of the wake. Each asymmetric state is characterized by the presence of a skewed toroid vortex attached to a pair of vertically oriented counter-rotating streamwise vortices. The bistable motion also results in significant modifications to the separation bubble, which causes the separation bubble to enlarge and elongate in the streamwise direction. Furthermore, the results reveal that the vortex shedding mechanism and shear layer instability are primarily associated with the shedding of a pair of counter-rotating quasi-streamwise vortices and toroidal vortices, respectively. Additionally, a new type of motion, namely the swirling motion, is identified at StH between 0.014 and 0.123. Unlike the bi-stability mode, the swirling motion leads to the wake alternating between a symmetric and an asymmetric state. Its topology is characterized by two sets of counter-rotating streamwise structures that have varying orientations depending on the wake state. The swirling motion comprises both rotational motion in the crossflow plane and streamwise motion of the wake barycentre. This flow motion bears similarities to the bubble pumping and switching attempts mentioned in earlier studies. Overall, this study contributes to a more comprehensive understanding of the wake of a flat-back Ahmed body by detailing the spatiotemporal information of the high-energy flow motions.