Wavelet analysis study of microbubble drag reduction in a boundary channel flow
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Particle Image Velocimetry (PIV) and pressure measurement techniques were performed to investigate the drag reduction due to microbubble injection in the boundary layer of a fully developed turbulent channel flow. Two-dimensional full-field velocity components in streamwise-near-wall normal plane of a turbulent channel flow at Reynolds number of 5128 based on the half height of the channel were measured. The influence of the presence of microbubbles in the boundary layer was assessed and compared with single phase channel flow characteristics. A drag reduction of 38.4% was achieved with void fraction of 4.9%. The measurements were analyzed by studying the turbulence characteristics utilizing wavelet techniques. The wavelet cross-correlation and auto-correlation maps with and without microbubbles were studied and compared. The two-dimensional and threedimensional wavelet maps were used to interpret the results. The following observations were deduced from this study: 1. The microbubble injection within the boundary layer increases the turbulent energy of the streamwise velocity components of the large scale (large eddy size, low frequency) range and decreases the energy of the small scale (small eddy size, high frequency) range. 2. The wavelet cross-correlation maps of the normal velocities indicate that the microbubble presence decrease the turbulent energy of normal velocity components for both the large scale (large eddy size, low frequency) and the small scale (small eddy size, high frequency) ranges. 3. The wavelet auto-correlation maps of streamwise velocity shows that the intensities at low frequency range were increased with microbubble presence and the intensities at high frequency range were decreased. 4. The turbulent intensities for the normal fluctuating velocities at both low frequency and high frequency range were decreased with microbubble injection. This study presents the modifications in the characteristics of the boundary layer of channel flow which are attributed to the presence of microbubbles. Drag reduction studies with microbubble injections utilizing wavelet techniques are promising and are needed to understand the drag reduction phenomena.
Zhen, Ling (2004). Wavelet analysis study of microbubble drag reduction in a boundary channel flow. Master's thesis, Texas A&M University. Texas A&M University. Available electronically from