Title page for ETD etd-08252011-081702


Type of Document Master's Thesis
Author Rusche, Max Thomas
URN etd-08252011-081702
Title Structure of Turbulent Boundary Layers and Surface Pressure Fluctuations on a Patch of Large Roughness Elements
Degree Master of Science
Department Aerospace and Ocean Engineering
Advisory Committee
Advisor Name Title
Simpson, Roger L. Committee Chair
Devenport, William J. Committee Member
Schetz, Joseph A. Committee Member
Keywords
  • turbulence
  • roughness
  • surface pressure
  • velocity spectra
  • boundary layer
Date of Defense 2011-07-28
Availability unrestricted
Abstract
Measurements were made in a zero pressure gradient turbulent boundary layer over two

roughness patches containing hemispherical and cubical elements. The elements were 3

mm in height and spaced 16.5 mm apart in an array containing 7 streamwise rows and 6

spanwise columns for a total of 42 elements per patch. The boundary layer thickness was

approximately 60 mm, so the ratio of element height to that thickness was a large amount

at k= = 1=20. A three velocity component laser Doppler velocimeter measured instantaneous

velocities. Mean flow and turbulence statistics were calculated as well velocity

energy spectra. Surface pressure fluctuations were measured using a two-microphone

subtraction method.

The results show that hemispherical elements produce larger turbulence quantities in

their wakes compared to the cubes. This is due to the hemispheres having a frontal area

nearly 60% larger than that of the cubes. The turbulence levels behind the hemispheres

is a maximum behind the first streamwise row of elements, and decreases afterwards.

The cubical elements maintain a nearly constant amount of turbulence in their wake,

signifying little interaction between cubical elements. Surface pressure fluctuations vary

little in the streamwise direction of the patches. The hemispherical elements produce

a larger sound pressure level behind them than the cube elements do. Velocity spectra

results show large normal stress energy for regions at and below the element height. The

energy for locations high in the boundary layer increases as the flow moves downstream.

Coherency plots show that there is a large correlation between the turbulent structure and

production of shear stress at the roughness height. Any measurements taken at or below

the roughness height are highly correlated under 10 kHz, while locations higher in the

boundary layer are correlated under 2 kHz.

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