Drag coefficient

About: Drag coefficient is a research topic. Over the lifetime, 14471 publications have been published within this topic receiving 303196 citations. The topic is also known as: drag factor.

Viscoplastic flow around a cylinder in an infinite medium

Abstract: The purpose of the numerical simulations carried out in this study is to determine the flow characteristics of a Herschel–Bulkley viscoplastic fluid around a cylinder in an infinite medium. Inertia is assumed to be negligible. Two types of boundary conditions are considered: the fluid adheres or slips (zero tangential stress) on the cylinder wall. Finite-element modelling involves regularising the Herschel–Bulkley model, as proposed by Papanastasiou [J. Rheol. 31 (1987) 385]. The effect of the yield stress value and shear-thinning index on the kinematic field and drag exerted on the cylinder were explored systematically. The location, dimension and kinematics of the rigid zones were determined. The results are compared with available theoretical data.

Turbulent drag reduction and multistage transitions in viscoelastic minimal flow units

Abstract: The observation that addition of a minute amount of flexible polymers to fluid reduces turbulent friction drag is well known. However, many aspects of this drag reduction phenomenon are not well understood; in particular, the origin of the maximum drag reduction (MDR) asymptote, a universal upper limit on drag reduction by polymers, remains an open question. This study focuses on the drag reduction phenomenon in the plane Poiseuille geometry in a parameter regime close to the laminar–turbulent transition. By minimizing the size of the periodic simulation box to the lower limit for which turbulence persists, the essential self-sustaining turbulent motions are isolated. In these ‘minimal flow unit’ (MFU) solutions, a series of qualitatively different stages consistent with previous experiments is observed, including an MDR stage where the mean flow rate is found to be invariant with respect to changing polymer-related parameters. Before the MDR stage, an additional transition exists between a relatively low degree (LDR) and a high degree (HDR) of drag reduction. This transition occurs at about 13%–15% of drag reduction and is characterized by a sudden increase in the minimal box size, as well as many qualitative changes in flow statistics. The observation of LDR–HDR transition at less than 15% drag reduction shows for the first time that it is a qualitative transition instead of a quantitative effect of the amount of drag reduction. Spatio-temporal flow structures change substantially upon this transition, suggesting that two distinct types of self-sustaining turbulent dynamics are observed. In LDR, as in Newtonian turbulence, the self-sustaining process involves one low-speed streak and its surrounding streamwise vortices; after the LDR–HDR transition, multiple streaks are present in the self-sustaining structure and complex intermittent behaviour of the streaks is observed. This multistage scenario of LDR–HDR–MDR recovers all key transitions commonly observed and studied at much higher Reynolds numbers.