An experimental investigation of rotational shear flows with application to turbulent drag-reducing surfaces and liquids.

摘要

Drag due to turbulent shear stress plays a prominent role in limiting the utility and efficiency of various fluid-based applications. Any reduction in this drag is of great interest from a commercial standpoint and has been a major topic of research for several decades. The focus of the research here is the measurement of shear stress in turbulent cylindrical shear flows with an emphasis on studying the performance of drag-reducing surfaces (like riblets) and drag-reducing liquids (like dilute polymer solutions and worm-like micellar solutions) in this type of flow.; A customized experimental instrument, called the "rotating cylinder drag balance," is presented and tested for consistency and reliability of shear stress data as measured from a smooth control surface. The instrument consists of concentric cylinders with a fluid annulus between them. The outer cylinder is driven by a motor, causing the test fluid to apply a measurable shear stress to the inner cylinder. Relevant flow parameters, such as curvature and annulus gap size, are controlled by varying the size of the cylinders. The apparatus is calibrated for laminar flow and turbulent flow data is compared to historical results. Experimental uncertainties are also considered, including the effects of frictional heating of the test fluid those related to the design of the instrument.; The instrument is demonstrated as a predictor of drag reduction by testing V-groove riblet film applied to the cylinder surfaces and compared to results in literature for the case of planar flow. Dilute solutions of poly(ethylene-oxide) and the surfactant cetylpiridinium chloride are also studied from an exploratory perspective to evaluate their potential as drag reducers. This is done by establishing the criteria for which drag reduction occurs and subsequently ends for each type of liquid as well as by determining the effects of flow curvature on their performance. Results for the polymer solutions are compared to those in literature for planar flow experiments, and the study of the surfactant solutions presents new information regarding the formation and degradation of the worm-like micelles which provide the drag reducing effect.