Skin-Friction Drag Reduction within Turbulent Flows. Skin-Friction Drag Reduction within Turbulent Flows.

Skin-friction Drag Reduction;Polymer Degradation;Air Injection;Turbulent Boundary Layer;Polymer Diffusion;Mechanical Engineering;Engineering;Mechanical Engineering

Elbing, Brian R.Solomon, Michael J. ;

University of Michigan

关键词: Skin-friction Drag Reduction;    Polymer Degradation;    Air Injection;    Turbulent Boundary Layer;    Polymer Diffusion;    Mechanical Engineering;    Engineering;    Mechanical Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/62411/belbing_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In nearly all transportation systems moving in a fluid, skin-friction drag is a major component to the total resistance to motion. Thus reduction of turbulent boundary layer (TBL) skin-friction in external flows is an ongoing research priority. This document describes experimental investigations into two active methods for reducing skin-friction drag (injection of air or polymer solution into a TBL). Direct measurement of TBL skin-friction was the primary diagnostic, but other flow measurements were acquired depending on the specific study (e.g. void fraction, concentration and velocity profiles, fluid rheology, etc.). PDR experiments were conducted at typical laboratory (2.72 cm diameter pipe and 0.9 m long flat plate) and at larger (12.9 m long flat plate) scale to bridge the gap in scale between experimental data (~ 1 m) and real world applications (~100 m). The major insights from these investigations are: (1) initial zone diffusion of polymer scales with the distance from the injector based Reynolds number, non-dimensional volumetric injection flux and injection concentration, (2) intermediate zone diffusion scales with flow and injection conditions (K) and the inner variable scaled roughness height (k+), (3) the percent drag reduction (%DR) equals 80[1 – exp(-0.08 S+)] where S+ is the effective slip when the drag is reduced with polymers or surfactants in channel, pipe or boundary layer flows (4) polymer degradation by chain scission within a TBL is important since at typical flow conditions an order of magnitude reduction in molecular weight is possible and (5) the chain scission can be scaled based on flow conditions.Air injection experiments were a continuation from work reported in Elbing et al. (2008) on a 12.9 m long flat test model. Two drag reduction regimes, bubble drag reduction (BDR) and air layer drag reduction (ALDR) were studied. BDR results showed that the %DR is linearly proportional to the near-wall void fraction. ALDR showed %DR between 80 and 100 and the critical volumetric flux of air required to achieve ALDR scaled with uτ/(νg)1/3 independent of surface condition, background water surface tension and injector design.