A dual-step cylinder is a canonical geometry commonly encountered in many practical flows. It consists of a large diameter cylinder (D) attached coaxially to the mid-span of a small diameter cylinder (d). This work provides a comprehensive description of the flow development, classifies common wake regimes and considers the associated structural loading on a dual-step cylinder. The influence of the aspect ratio of the large diameter cylinder ($L/D$) and diameter ratio ($D/d$) is studied experimentally for a Reynolds number of ReD = 2100, 1.33≤ D/d ≤ 4 and 0.2≤ L/D ≤ 5. The flow evolution and structural loading are quantified via a combination of flow visualization, Laser Doppler velocimetry, particle image velocimetry measurements and multi-component force balance measurements. Through a detailed analysis of the results, six distinct flow regimes are identified based on observed changes in the flow development downstream of the large diameter cylinder. The findings are distilled into a map of flow regimes that provides a framework for analysis of the dual-step cylinder wakes and incorporates limiting cases of this geometry, namely, uniform circular cylinders, cantilevered cylinders, cylinders with two free ends, coin-like cylinders and single-step cylinders. The identified flow regimes are also related to changes in structural loading.
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Morton et al. (2020) studied this question.
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