First Advisor

Xiaowei Zhu

Term of Graduation

Summer 2025

Date of Publication

9-10-2025

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Mechanical Engineering

Department

Mechanical and Materials Engineering

Language

English

Subjects

drag, stratified turbulence, turbulent boundary layers

Physical Description

1 online resource (vii, 54 pages)

Abstract

Riblets are micro-groove elements which can reduce the skin friction drag coefficient for wall-, channel-, and pipe-flows. It is unclear how robust these drag-reduction capabilities are when external forces are acting on the fluid and affecting turbulent structures. Here, buoyancy forces are introduced by varying the wall Richardson number (Riτ = 0, 18, 60) to create unstable stratification. Direct numerical simulations (DNS) are performed on streamwise-aligned riblets of triangular and bladed geometries in a horizontal half-channel pressure-driven turbulent flow (Reτ = 395). The Fukagata-Iwamoto-Kasagi (FIK) identity is used to decompose how changes in stresses contribute to changes in the skin friction drag coefficient. It is observed that an increase in the Richardson number increases drag, regardless of riblet geometry. The largest contributor to the coefficient of drag and changes in drag is the turbulence contribution from Reynolds stresses. The riblet size providing the greatest drag reduction for a given Richardson number remained the same, with only one clear exception being bladed riblets at Riτ = 60. Smaller riblet sizes experience increases in drag like that of a smooth wall, while larger riblet sizes have an additional increase in drag that grows linearly with its groove size. The drag change for a given riblet geometry and Richardson number can be modeled as a piecewise function of its groove size. The cross-stream plane shows that riblets that experience an additional size-dependent drag increase due to the Richardson number have increased Reynolds stresses near their groove walls. Reynolds stresses were found to decrease or remain the same in groove centers and above riblet tips, regardless of geometry type, groove size, or magnitude of the Richardson number. Regions of dispersive stresses and riblet-generated vorticity increase in the grooves as riblet size and Richardson number increases. These results may help better understand how the drag-related mechanisms of riblets behave across a variety of environments and provide insight into the significance of external forces when designing riblets.

Rights

In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).

Persistent Identifier

https://archives.pdx.edu/ds/psu/44127

Download

Share

COinS