Channels, Fall 2020

Channels • 2 020 • Volume 5 • Number 1 Page 4 Fig. 3 Low Drag Cart Throughout the physical testing, the groove parameters were varied to isolate the effects of certain characteristics, but unless otherwise stated, the grooves were 2.8 mm in width, 0.39 mm in depth and spaced evenly with 11.25° between the deepest part of each groove. The cylinders were fabricated using resin 3D printing to obtain detailed groove profiles difficult to manufacture conventionally or with fused deposition modeling (FDM) 3D printing. These circular cylinders were tested and compared to the research published in [1]. The purpose of this procedure was to determine if the low drag cart was capable of recording the slight changes in drag caused by surface groove drag reduction. However, because the published values [1] used 2D CFD, but the wind tunnel tests included the 3D effect of flow over the ends of the cylinders, the goal for this part of the testing process was to verify that a similar trend could be found. Such a trend was observed, and the data was recorded in Table I. TABLE I C OMPARING D RAG C OEFFICIENTS FROM S ONG AND W IND T UNNEL E XPERIMENTATION Model Type 2D Song [1] C d 3D Experimental C d Smooth 1.21 0.768 Model A 0.83 0.672 Model B 0.785 0.553 Following the verification procedure for the measurement device and wind tunnel, the experimentation transitioned to testing of elliptical cylinders to determine which parameters would optimize drag reduction. In the elliptical models, the varied parameters were the groove depth and number of grooves (N). The aspect ratio for the elliptical model was 0.75 with the minor (vertical) axis length maintained at 38.1 mm and the major (horizontal) axis at 50.8 mm. Instead of using 11.25° as the measurement for groove spacing, the arclength between the groove centers on the circular model (3.7 mm) was used unless otherwise specified. In the circular cylinders, Model B included an angle to denote how far the grooves went along the surface of the cylinder. For an ellipse, however, the definition of the grooved region for Model B was the total number of grooves. Each model has a groove centered along the stagnation point. Therefore, the number of grooves for Model B is always odd. Fig. 4 shows a representative cross section of the base models used for the elliptical experimentation.