The emergence of bubble-induced scaling in thermal spectra in turbulence

We report on the modification of the spectrum of a passive scalar inside a turbulent flow by the injection of large bubbles. Although the spectral modification through bubbles is well known and well analysed for the velocity fluctuations, little is known on how bubbles change the fluctuations of an approximately passive scalar, in our case temperature. Here we uncover the thermal spectral scaling behaviour of a turbulent multiphase thermal mixing layer. The development of a -3 spectral scaling is triggered. By injecting large bubbles (Re-bub = O(10(2))) with gas volume fractions alpha up to 5 %. For these bubbly flows, the -5/3 scaling is still observed at intermediate frequencies for low alpha but becomes less pronounced when alpha further increases and it is followed by a steeper -3 slope for larger frequencies. This -3 scaling range extends with increasing gas volume fraction. The -3 scaling exponent coincides with the typical energy spectral scaling for the velocity fluctuations in high-Reynolds-number bubbly flows. We identify the frequency scale of the transition from the -5/3 scaling to the -3 scaling and show how it depends on the gas volume fraction.