Effect of bubble on the pressure spectra of oscillating grid turbulent flow at low Taylor-Reynolds number

For many engineering applications, measurements of velocity and pressure distributions in the system are of fundamental importance to provide insights into the flow characteristics. In the present study, the experiments were carried out in an oscillating grid system in absence and presence of a controlled bubble train (D_b = 2.70 and 3.52 mm) rise using the non-intrusive two-dimensional (2D) particle image velocimetry (PIV) at the low Taylor-Reynolds number (Re_λ) ranging from 12 to 60. Using the measured PIV velocity data, the instantaneous pressure fluctuations were estimated by integrating the full viscous form of the Navier-Stokes (N-S) equation and compared with three dimensional (3D) computational fluid dynamics (CFD) simulations. A spectral slope of -7/3 was found in the inertial subrange of the single-phase pressure spectrum. In contrast, the two-phase pressure spectrum exhibited a slope less steep than -7/3 in the inertial subrange because of the extra production of turbulence in the presence of bubble. For single-phase flow, the ratio of pressure integral length scale to the velocity integral length scale (L_p/L) was found to be a constant of about 0.67, and the pressure Taylor microscale (λ_p) was approximately 0.79 ± 0.03 of the velocity Taylor microscale (λ) for a low Reynolds number. The scaling ratio based on the single-phase experimental results were compared with existing theory and DNS results and found to accord well; however, these ratios deviate from theoretical values for two-phase flow. Also, the energy dissipation rate was evaluated based on the pressure spectrum and found the over-predicted (~ 31%) values relative to those calculated from the velocity spectrum.