Abstract

This investigation studies in detail the changes occurring in near wake dynamics associated with base drag reduction on bluff bodies achieved via boundary layer modification. A wedge model and a flat plate model are used to study bluff bodies over a range of Reynolds number of 0.8 × 10 ⁶–0.3 × 10⁶. The boundary layer on both these models is modified using fore-body surface roughness, by applying sand grains with a mean diameter of 2.2 mm to their fore-body surfaces. Particle Image Velocimetry (PIV) measurements, as well as simultaneous PIV and time-resolved base pressure measurements, are performed on these models in order to determine the near wake changes associated with base drag reduction. For the wedge model, the PIV measurements capture a downstream region of x/h = 0-1.5, whereas, for the flat plate model, the PIV measurements are performed at three different distances downstream, x/h = 0-7, 7-14 and 14-21. The PIV results are analyzed using Proper Orthogonal Decomposition (POD) in order to identify the changes in coherent structures associated with base drag reduction. It is observed from the POD results that, when there is a reduction in base drag, the vortex shedding in the near wake becomes more organized, its strength decreases, and the vortex convects faster. This result differs from those obtained in earlier studies, where base drag reduction is achieved by interrupting vortex shedding.

In order to reconstruct near wake dynamics simultaneous PIV and time-resolved pressure measurements are used. Using Linear Stochastic Estimation (LSE)-POD technique, the cycle-to-cycle variation in vortex shedding is accurately captured. It is observed from the reconstruction that for the smooth test cases, the cycle-to-cycle variations in the vortex shedding are significantly higher, as compared to the rough test cases.