- Title
- Chacterisation of single and binary phase turbulence in an oscillating grid system
- Creator
- Hoque, Mohammad Mainul
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2017
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The characteristics of single- and binary-phase turbulence in an oscillating grid system were investigated experimentally by using time-resolved, non-intrusive particle image velocimetry (PIV) technique. Experiments were conducted in three different fields of view (FoV) namely: 10 mm × 10 mm, 30 mm × 30 mm and 60 mm × 60 mm in the center of the tank for grid oscillation frequency 0 to 5 Hz. The grid Reynolds number (Reg) and Taylor Reynolds number (Reλ) ranges were 1080–10800 and 12–60, respectively. Specifically, the focus was on: (i) characterisation of single-phase homogeneous isotropic flow based on specific energy dissipation rate; (ii) modulation of turbulence due to particle-fluid and bubble-fluid interaction; and (iii) quantification of scaling properties of pressure spectrum for single- and binary-phase flow. In order to understand the homogeneous and isotropic turbulence inside the oscillating grid system turbulent length scales, isotropy ratio (IR = vrms/urms), specific energy dissipation rate, and energy spectra were determined from the measured experimental fluctuating velocity field for different grid Reynolds numbers. Fluctuating velocity was found to increase linearly with increasing Reg which was in agreement with the previous reported results. It was found that the turbulence length scale decreased with increase in grid oscillation frequency. The isotropy ratio ranged from 0.77-0.85 which indicated the presence of isotropic homogeneous turbulence in the system. The energy dissipation rate of single-phase flow was determined using the following methodologies: (i) dimensional analysis; (ii) velocity gradient; (iii) structure function; and (iv) energy spectrum. In general, the specific energy dissipation rate increased with increase in grid oscillation frequency. It was found that the specific energy dissipation rates were different for each of the four methodologies. Whilst the analysis identified uncertainties in all four approaches, it was concluded that the energy spectrum methodology was likely to be most reliable since it was able to satisfy the energy balance of the system—this was not possible for other three methods. Moreover, the energy spectra exhibited a slope close to Kolmogorov’s -5/3 in the inertial subrange. White noise was observed in the dissipation range, which was able to be removed by an exponential filter. The modulation of homogeneous and isotropic turbulence was experimentally investigated in the presence of a single stationary particle. The particle diameter varied in the range of 1 to 8 mm (~ 10 to 77 times larger than the flow Kolmogorov length scale). It was found that the fluid-only fluctuating velocity increased by up to 2-25 percent depending upon the particle diameter. The isotropy ratio of the fluid-only phase also increased with the size of the particle; but was much less influenced by the Reynolds number of the grid, Reg. The energy dissipation rate of the fluid-only phase increased with increase in particle size; and followed a power law trend with grid Reynolds number. Longitudinal and transverse integral length scales were determined using the autocorrelation function for both fluid-only and particle-fluid case. The fluid-only phase integral length scales followed a power law dependency with Reg, and decreased when a particle was present. Both longitudinal and transverse energy spectrums in the inertial subrange exhibited a slope less steep than the -5/3 predicted by Kolmogorov when a particle was present. It is thought that the particle presence resulted in the production of turbulence in the inertial region, leading to an energy enhancement in that part of the spectrum. Finally, turbulence intensity was determined as a function of the particle-diameter-to-integral-length-scale ratio, dp/Lx; and it was found that below dp/Lx = 0.41 the turbulence intensity was attenuated, and above this dp/Lx value the turbulence intensity was enhanced. As per the particle-fluid experiments, modulation of homogeneous and isotropic turbulence was also studied due to bubble-fluid interaction. The bubble equivalent spherical diameter was varied in the range 2.7-3.52 mm, that corresponded to approximately 26-34 times larger than the Kolmogorov length scale. In the presence of a bubble, the single-phase fluctuating velocity along the transverse direction was found to be significantly enhanced when compared to that in the longitudinal direction. The presence of the bubble also influenced the isotropy of the fluid flow field; whereby at low grid Reynolds number the isotropy ratio increased with increase in the bubble equivalent spherical diameter, whilst at high Reg the isotropy ratio showed no significant bubble equivalent spherical diameter dependence. The specific energy dissipation rate was found to be influenced by the shape of the bubble, and followed a (positive exponent) power law dependence with the bubble equivalent spherical diameter. Conversely, the integral length scale of the single-phase decreased with increase in the bubble equivalent spherical diameter. It also followed that the spectral slope was less steep than -5/3 in the inertial subrange—corresponding to an enhancement of energy—for both longitudinal and transverse energy spectra. Any effect of bubble size on the energy spectrum in the dissipative region could not be conclusively demonstrated due to the presence of white noise. Finally, the pressure spectrum for both single- and binary-phase flows was obtained by taking the fast Fourier transformation (FFT) of the instantaneous pressure field which was computed from the measured, instantaneous 2D velocity field. It was found that in the inertial subrange the pressure spectra exhibited a -7/3 slope for single-phase flow, whilst that for the binary-phase flow exhibited a less steep slope. The pressure-based integral length scale as well as the Taylor microscale were estimated from autocorrelation function and pressure spectrum, respectively. For single-phase flow, at low grid Reynolds number, the pressure-integral-length-scale-to-velocity-integral-length-scale ratio was found to be constant at around 0.67; whilst the pressure Taylor microscale was approximately 79 percent of the velocity Taylor microscale. Both of these values were consistent with theoretical predictions and published direct numerical simulation results. Finally, a methodology has been proposed whereby the specific energy dissipation rate can be computed from the pressure spectrum. It was found that the values obtained from this approach were approximately 25 percent higher than those calculated directly from velocity spectrum.
- Subject
- turbulence; energy dissipation rate; turbulence modulation; homogeneous; isotropy; FFT; thesis by publication; PIV; oscillating grid; particle-fluid interaction; turbulence intensity; integral length scale; energy spectrum; pressure spectrum; bubbly-flow phase
- Identifier
- http://hdl.handle.net/1959.13/1335955
- Identifier
- uon:27517
- Rights
- Copyright 2017 Mohammad Mainul Hoque
- Language
- eng
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