Date of Award

Summer 8-23-2024

Access Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Aerospace Engineering

Committee Chair

J. Gordon Leishman

Committee Co-Chair

Ebenezer Gnanamanickam

First Committee Member

Richard Prazenica

Second Committee Member

Sandra K.S. Boetcher

Third Committee Member

Robert Minniti

College Dean

James W. Gregory

Abstract

A wind tunnel campaign was conducted to investigate the DARPA Suboff boundary layer. To emphasize the axisymmetric boundary layer, the generic submarine model's sail, appendages, and propeller were omitted. The study focused on the afterbody from 70–95% of the model length to examine the concurrent pressure gradient and wall curvature effects, which are commonplace in engineering. Three particle image velocimetry (PIV) configurations were employed to measure the boundary layer in detail: one spanned the entire afterbody, another had narrow strips sampled at 16 kHz, and the third used two simultaneous orthogonal measurement planes. Considerable effort was made to reduce the laser reflection, allowing seed particles to be measured less than 20 microns above the opaque wall. PIV measurements were acquired at friction Reynolds numbers up to 2,700 or ReL=8 million based on the model length and the free stream, which is substantial for an axisymmetric body. The primary aims were to 1. Elucidate how the statistics and structure of a near-canonical boundary layer were modified along the Suboff afterbody and 2. Determine whether these alterations stemmed from pressure gradient, lateral curvature, or longitudinal curvature depending on their relative spatial patterns. The pressure gradient significantly affected the wall-tangent turbulence intensity, whereas the wall-normal intensity experienced the effects of longitudinal and lateral curvature. A scale decomposition demonstrated that large scales were more influenced by the non-equilibrium flow conditions than the small scales. Further investigation of the large scales relied on two-point statistical methods. Linear stochastic estimation showed that hairpin vortex packets had a significant role, irrespective of the pressure gradient and wall curvature. Consequently, the correlation contours based on the wall-tangent velocity were forward leaning. The near-wall structures were elongated/compressed with minor rotation, whereas structures at the boundary layer edge were rotated with minimal distortion. The resulting length scales were collapsed across all wall-parallel and -normal positions by a pressure gradient parameter, when shifted by roughly one boundary layer thickness to account for the delayed response of the structures. The wall-normal correlation contour's usual column shape was unaffected near the boundary layer edge, whereas the near-wall structure developed, with downstream distance, a peak frequency and a forward-leaning appearance indicative of hairpin packet influence. The peculiar behavior in the cross-correlation of wall-tangent and wall-normal velocities was interpreted as longitudinal streamline curvature effects on the turbulent bulges and valleys, as indicated by a conditional average analysis. This finding motivated a study of the turbulent/non-turbulent interface. A modified interface detection criterion was necessary to account for wall-normal velocity and streamline curvature. The computed interface indicated that the boundary layer was intermittently turbulent in the outer 40–70% depending on the pressure gradient. Across these interfaces were velocity ''jumps" that became more significant with adverse pressure gradients. Additionally, the large-scale motions underlying the interface bulges were gradually enlarged downstream.

Additional Files
Ani1.5.mp4 (643 kB)
Evolution of Ruu structure at 15% boundary layer thickness.
Ani5.mp4 (712 kB)
Evolution of Ruu structure at 50% boundary layer thickness.
Ani8.mp4 (789 kB)
Evolution of Ruu structure at 80% boundary layer thickness.
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