| 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 utau/(vg)1/3 independent of surface condition, background water surface tension and injector design. |
