Direct numerical simulations of Taylor–Couette flow: Effect of dimple roughness on torque response

Torque response in Taylor–Couette (TC) flow is governed by the interaction among cylinder geometry, boundary layer dynamics, and flow regime. The influence of dimple roughness on TC flow, particularly regarding torque modification and the associated flow structures, remains insufficiently explored. In this study, direct numerical simulations are performed for laminar and turbulent TC flows at inner-cylinder Reynolds numbers of 400 and 3960, respectively, with a radius ratio of η=0.714. Validation tests and cylinder-height sensitivity analyses confirm the accuracy and robustness of the numerical approach. The results show that torque modification arises from the balance between friction-induced torque reduction and pressure-induced torque enhancement. Dimples tend to torque reduction in laminar regime, with a maximum reduction of 8.3% in Nuω, whereas they produce torque enhancement in the turbulent regime, reaching up to 11.2%. Taylor vortices significantly modulate both wall shear stress and pressure along the axial direction, giving rise to height-dependent variations in torque response. These findings elucidate the underlying mechanisms of torque modification induced by dimple roughness in TC flow and offer physical insights into the design of energy-efficient rotating machinery, such as electric motors.