Velocity moment analysis on oscillatory flow above self-formed vortex ripples

Wu, Heng

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https://hdl.handle.net/2142/50465 Copy

Description

Title

Velocity moment analysis on oscillatory flow above self-formed vortex ripples

Author(s)

Wu, Heng

Issue Date

2014-09-16

Director of Research (if dissertation) or Advisor (if thesis)

Garcia, Marcelo H.

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Oscillatory flow
• Velocity moments

Abstract

This thesis investigated the flow characteristics of oscillatory flow above self-formed vortex ripples. Using the data collected by Admiraal et al. (2006), phase-averaged and time-averaged mean velocities, root-mean-square speeds, turbulent kinetic energy, Reynolds shear stress, and higher order velocity moments including skewness factors, flatness factors, and turbulent diffusion factors were studied for nine different Reynolds numbers. The study on phase-averaged skewness factors indicated that the area of skewed streamwise velocity oscillated around the ripple. The wall-normal velocity is skewed to the negative near the ripple and positive away from it. The contours of streamwise and transverse turbulent diffusion factors are similar due to the comparable magnitude of root-mean-square velocities in the two directions. All the non-zero regions of phase-averaged velocity moments oscillated with the flow, suggesting that turbulence was transported by the oscillatory flow. There are no significant effects of Reynolds number among the nine experiments. For the time-averaged variables, the asymmetry phenomenon (Musalem-Jara, 2006) is observed for streamwise mean velocity, Reynolds shear stress, streamwise skewness factor, streamwise flatness factor, streamwise and transverse turbulent diffusion factors. The time-averaged turbulent kinetic energy was dominated by streamwise root-mean-square velocity. In addition, from the skewness and flatness factors, it can be found that the time-averaged streamwise velocity is close to a Gaussian distribution in the area above the ripple crest and deviates from it on either side of the ripple crest. The degree of deviation from Gaussian distribution of the time-averaged wall-normal velocity increases with the wall-normal velocity. A band where the transverse velocity is closest to Gaussian distribution is near the crest of the ripple.

Graduation Semester

2014-08

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http://hdl.handle.net/2142/50465

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Copyright 2014 Heng Wu

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