2024/05/14 by Rodrigo Vilumbrales-Garcia, Vilumbrales-Garcia, Rodrigo, Putu Brahmanda Sudarsana +3
Neuroscience · #Tactile and Sensory Interactions
paper · pdf · doi:10.48550/arxiv.2405.08896
Dimples on a sphere's surface can lead to significant drag reduction. However, the optimal dimple depth to minimize the drag varies with the Reynolds number (Re). In this study, a smart surface-morphing technique is devised that can adjust dimple depth based on the flow conditions to minimize drag across a wide range of Re values. By depressurizing the core of a rigid skeleton enclosed with a thin latex membrane, the dimple depth can be precisely controlled in response to flow velocity changes. A comprehensive series of systematic experiments are performed for Reynolds number range of 6×104 ≤ Re ≤ 1.3×105, and dimple depth ratios of 0 ≤ k/d ≤ 2×10-2 using the morphable sphere. It is observed that the dimple depth ratio k/d significantly affects both the onset of the drag crisis and the minimum achievable drag. As k/d increases, the critical Reynolds number for the drag crisis decreases. However, the minimum achievable drag coefficient decreases as k/d increases. By carefully adjusting the k/d to Re using the morphable approach, our experiments show that CD reductions up to 50 % can be achieved when compared to a smooth counterpart for all the Re considered. For a constant Re, drag reduces as k/d increases. However, there is a critical threshold beyond which drag amplification starts to occur. Particle image velocimetry (PIV) reveals a delay in flow separation on the sphere's surface with increasing k/d, causing the separation angle to shift downstream. However, when k/d exceeds the critical threshold, flow separation moves upstream, causing an increase in drag. By using the experimental data, a control model is also developed relating optimum k/d with Re to minimize drag. This model also serves as the basis for adaptive drag control of the sphere for a wide range of Reynolds number.