Mechanism of Drag Reduction by Dimple Structures on a Sphere

Abstract

The purpose of the present study is to clarify the mechanism of drag reduction for a sphere with arc type dimples. The sphere has 328 dimples of different depths uniformly distributed on its surface. The present study measured the pressure and velocity distributions inside and between the dimples, and visualized the flow on the sphere surface by an oil film method. The results indicated that separation bubbles were generated inside the dimples and transformed a laminar boundary layer into a turbulent boundary layer. Compared to a smooth sphere, the critical Reynolds number decreased and the separation point shifted further downstream. Therefore, the drag coefficient of a dimpled sphere was smaller than that of a smooth sphere. The magnitude of the decrease in the critical Reynolds number was found to increase with dimple depth. However, the separation point shifted to the upstream side and the drag coefficient became larger in the super-critical Reynolds number region.

Programmable Aerodynamic Drag on Active Dimpled Cylinders

“…[5] Aoki et al [17] later discovered that deeper dimples cause for the separation bubble to occur further upstream (closer to the front of the body), explaining the lower critical Reynolds number and the higher minimum Cd observed for the specimens with deeper dimples. [17] Terwagne et al [18] systematically examined the effect of the dimple depth on aerodynamic drag by making use of a wrinkling instability of a thin-stiff shell bound to a thick-soft substrate to generate a dimpled pattern with tunable depth on spherical specimens. Dimples formed and their amplitude could be varied by depressurizing a spherical cavity inside the samples to induce increasing levels of compression.…”

“…All rights reserved 5 and a higher minimum drag coefficient. [17,18] Therefore, at high Re a shallower dimpled specimen may have a lower Cd than a deeper dimpled one. However, at a lower Re, the opposite may be true.…”

“…[15,16] Several studies have investigated the drag on bluff bodies in the neighborhood of this drag crisis. [1,3,5,6,[15][16][17][18] In a seminal study, Achenbach [15,16] performed wind tunnel experiments on both smooth and uniformly roughened spheres and observed that as the Re was increased, past a drag coefficient minimum, a steep increase and subsequent plateau of Cd were observed in the supercritical and transcritical regimes, respectively. Subsequently, Bearman and Harvey [1] A u t h o r M a n u s c r i p t…”

Investigation on Drag Reduction Characteristics and Shell‐Side Hydrodynamics in Hollow Fiber Membrane Modules

“…Knowledge and experiences in novel membrane modules fabrication engineering remain limited in the open literature . Recently, some scholars identified that, hydrodynamics flow control in HFM shell‐side can change the flow drag over the boundary surface of the circular cylindrical fibers wall and can improve energy utilization efficiency in separation process . Yang et al.…”

Evaluation of Flight Trajectory and Unsteady Fluid Forces on Kicked Non-Spinning Soccer Ball by Digital Image Analysis

“…For example, the effects of dimple shape and the spin on the flying distance of golf ball were studied [1,2] and the influence of seams and rotation on the aerodynamic properties were investigated for baseball [3,4]. Furthermore, the aerodynamic characteristics were evaluated for soccer balls [5][6][7][8][9][10][11][12].…”