Bubble-induced turbulence depends on gas volume fraction when Re ~ O(1)

When a swarm of bubbles rises in an otherwise stagnant fluid at a moderate Reynolds number, the wake behind the bubbles interacts with one another giving rise to the emergence of k^-3 scaling of the energy spectra of velocity fluctuations. Interestingly this behavior does not depend on the gas volume fraction. Mazzitelli and Lohse (PRE, 2009) argued that for point-bubbles the energy spectra do not show a k^-3 scaling because of the absence of wake. We report the first experimental evidence that bubbles with Re ~ O(1) do not show a the expected and confirm that the energy spectra does depend on the number of bubbles. The energy dissipation rate is determined from the decaying turbulence behind the bubble swarm using particle image velocimetry (PIV). As the gas volume fraction increases, the energy spectrum eventually attains the k^-3 scaling even for suhc small values of Re.