Date of Award

5-16-2024

Publication Type

Thesis

Degree Name

M.A.Sc.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Helical strakes;Shallow flow;Wall-mounted

Supervisor

Ram Balachandar

Abstract

The wake dynamics of bed-mounted circular cylinders with and without helical strakes in a shallow flow are investigated experimentally using particle image velocimetry system. The helical strakes consist of a three-start configuration (the number of strakes is equal to three), with a pitch equal to 10d and a height of 0.2d, where d is the diameter of the plain cylinder. In addition, the influence of strake orientation on the flow field is explored by positioning the straked cylinder to align one of the strakes at angles of 0° and 30° with the streamwise component of the velocity at the cylinder-bed junction. In the presence of the strakes, the cylinder wakes displayed elevated turbulence levels, reduced mean velocity and strong mean shear which has profound effects on the vortex formation length and wake characteristics behind the cylinder. The time-averaged statistics that include mean velocities, Reynolds stresses and third-order moments are employed to describe and characterize the flow field. The results reveal that the presence of the strakes significantly alters the wake flow. The strakes induce asymmetric behaviour across the depth of the flow resulting in asynchronous vortex shedding that further disrupts vortex coherence. When aligned at 0° with the flow, the strakes increase the Reynolds stresses throughout the flow depth and enhance their diffusion, which can facilitate better mixing and momentum exchange necessary for heat transfer applications. The impact of bed-induced suppression is reduced significantly when the strake is aligned at 0° with the flow, with a notable increase in energy contributions to turbulent kinetic energy near the bed as revealed by proper orthogonal decomposition. The spatial modes exhibit a wide range of scales across the flow depth for cylinders equipped with strakes.

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