Microbubble Drag Reduction in Rough Walled Turbulent Boundary Layers

Abstract

Experiments were conducted in the 12-inch diameter tunnel at ARL/PSU using the tunnel wall boundary layer facility to determine the influence of surface roughness on microbubble drag reduction. To accomplish this, carbon dioxide was injected through a slot at rates of 0.001 m3/s to 0.011 m3/s, and the resulting skin friction drag measured on a 317.5 mm long by 152.4 mm span balance. In addition to the hydrodynamically smooth balance plate, additional plates were covered with roughly 75, 150 and 300 micron grit. Over the speed range tested of 7.6, 10.7 and 13.7 m/s, the roughness ranged from smooth to fully rough. Not only was microbubble drag reduction achieved over the rough surfaces, but the percentage drag reduction at a given gas flow rate was larger for larger roughness. A new scaling parameter that collapses all of the data is also introduced.

Assessments of Bubble Dynamics Model and Influential Parameters in Microbubble Drag Reduction

“…Good agreement was achieved between the simulations with the single phase model and experimental data of Deutsch et al (2003). This good agreement was observed for both free stream velocity as well as surface roughness effect studies.…”

“…The results of our model are very close to those of Kunz et al model, however, both models over predict experimentally observed drag reduction by as much as 50% in some cases. To facilitate comparison of simulations with experimental study of Deutsch et al (2003) on the influence of surface roughness on microbubble drag reduction a slightly modified computational domain was used. This domain is shown schematically in Figure 6.…”

“…This domain is shown schematically in Figure 6. The dimensions of this domain were selected to closely approximate experimental conditions reported by Deutsch et al (2003). A 127×64 grid was used for this domain.…”

“…Next, the value of ω at the wall is specified as Simulation results were validated by comparing the results for smooth wall cases with experimental data of Deutsch et al (2003). This comparison is shown in Figure 7, which shows integrated drag coefficient over "Wall 4" flat plate section normalized by its single-phase value as a function of the volumetric flow rate of gas injection.…”

“…A comparison of simulation results with the experimental data of Deutsch et al (2003) is shown in Figure 16. The computational domain shown in Figure 6 was used for these simulations as this domain closely mimics experimental conditions of Deutsch et al (2003).…”