The Effect of Aspect Ratio on the Drag of Bare Cylinders

Abstract

The drag coefficient for long-slender structures that is typically provided in design guidance has been determined from test specimens of sufficient length that they are unaffected by the aspect ratio (L/D), whereby they are considered to be of “infinite” length. However, it is apparent from the literature that aspect ratio does have a significant non-linear effect at short L/D ratios. DNV provides guidance on the aspect ratio effect on the drag coefficient of a cylinder, for which no experimental source data has been cited. The DNV design guidance has wide usage in the offshore industry and merits critical evaluation. This paper critically reviews the literature and presents the results of a series of tow tank experiments performed by the authors. A series of tow tank tests of a surface-piercing cylinder has been undertaken using a range of aspect ratios as well as testing the effect of various end conditions, where the effects of VIV and ventilation has been deemed insignificant. Correlation of the various data sets of the literature and the experimental test programme provides the basis for developing an alternate design guidance curve for the effect of aspect ratio on the drag coefficient of cylinders.

Numerical study of flows for slub yarn transmission in an air jet

“…In addition, the change rate for the coefficient of drag force presented a descending tendency gradually with an increase in the number of yarn faults. 25,27…”

“…The high value of the air pressure will increase the value of the air velocity, and therefore affect the value of the friction force created from the contact of the yarn and the air. 6,[25][26][27][28][29][30] In the meantime, the increase of the air velocity leads to a change in the yarn surface hairiness reducing the yarn skin friction coefficient.…”

“…In addition, the change rate for the coefficient of drag force presented a descending tendency gradually with an increase in the number of yarn faults. 25,27 In the case of slub yarn, the diameter is not constant and varies along the yarn. The slub also may have the same diameter or different diameters along the yarn length with periodical equal distance or it may be variable, depending on the slub distribution desired pattern for the final fabric, either woven or knitted fabric.…”

“…The value of k 0 was found to be a function of (L/d) as its value approaches the value of one for the value of (L/d) ¼ 100. 24,27 Cd 1 is the mean drag coefficient for a cylinder of infinite length (unaffected by aspect ratio). 27 The air drag force will depend on the slub diameter as the value of (b ¼ k/L) increases.…”

“…24,27 Cd 1 is the mean drag coefficient for a cylinder of infinite length (unaffected by aspect ratio). 27 The air drag force will depend on the slub diameter as the value of (b ¼ k/L) increases. The value of the Reynolds number is expected to be higher.…”