Thesis title: Dispersed bubble and particle-laden turbulent fows in the two-way coupling regime
The momentum exchange of bubble and particle laden incompressible turbulent flows is investigated
by means of Direct Numerical Simulations (DNS), employing the Eulerian-Lagrangian approach.
The Exact Regularised Point Particle method (ERRP) is used to achieve the inter-phase momentum
coupling between the two phases. The first part of the research deals with bubble-laden turbulent
homogeneous shear flow. The aim of this study consists in addressing the modulation of shear turbulence
and the bubble clustering geometry in presence of different inter-phase momentum coupling
conditions. Suspensions with different combinations of void fraction and Kolmogorov-based Stokes
number, in the dilute regime, are studied. Bubbles suppress the turbulent kinetic energy and turbulent dissipation as well.
Turbulent modulation occurs via the direct change of the Reynolds shear
stress. In fact, the bubble energy source is proved to be negligible in the scale-by-scale turbulent
energy budget. The bubble clustering, in agreement with the literature, occurs in the form of thin
elongated structures. The clusters are aligned with principal strain direction of the mean fow, as
usual in shear flows. The bubble clustering and turbulent modifcation are strictly related: both
increase with the Stokes number and are independent of the void fraction, in the range of parameters
considered in our simulations. The data show that the turbulent modification is disadvantaged when
the bubble distribution is homogeneous (i.e. small Stokes number). Finally, the small scale bubble
clustering is slightly reduced by two-way coupling effects even though the clustering anisotropy
still persists at small scales as it occurs for inertial particles.
In the next stage of the research,
the objective is to study multiphase wall-bounded turbulent flows. Under the same flow rate, the
dispersed phase can either reduce, as in bubbly-flows, or increase, as in particles-laden flows, the
viscous wall drag. However, it is well acknowledged that bubbles must be large, and deformable,
in order to reduce the viscous resistance in wall turbulence. On the other hand it is known that
small inertial particles lead to a wall drag increase. Since we are interested on important turbulence
modifications, the second part of the research is devoted to particle-laden wall turbulence flows.
In this new investigation, the turbulence modulation is addressed in an particle-laden annular pipe
flow, via Direct Numerical Simulation (DNS). The alteration of the heat exchange induced by the
different turbulent mixing is studied as well. The turbulence modulation induced by small particles
is addressed for the first time in the annular geometry, in the context of Direct Numerical Simulations.
A wall correction is included in ERPP in order to take into account wall effects in the particle
disturbance. The research also focuses on the particle preferential concentration close to the wall,
the so-called turbophoresis. The relation between the particle concentration and the friction wall
drag and heat exchange modification is explored. The first and second moment statistics, the two-point
correlation functions and the energy spectra are studied. The two-way coupled momentum
exchange leads up to 30% wall drag increase. The phenomenon is controlled by the particle mass loading and the wall radius ratio
Ri /Ro , where Ri is the internal wall radius and Ro the external one.
The mechanism leading to the increase of resistance is attributed to the modified Reynolds
The heaviest suspensions show a drastic modification of the coherent structures by
the external wall, although the flow is altered in the whole annular pipe. The TKE significantly
increases close the external wall, while it is suppressed close the internal wall. The increase of the
heat-exchange, induced by the different turbulent mixing, is small, below 5 %.
In the annular pipe the dispersed phase preferentially migrates toward the external wall.
In fact, the internal peak of the particle concentration is up to 100 times lower than the external one.
Moreover, the findings suggest that the particle concentration is largely overestimated in the central
and internal regions, in the one-way coupling regime ( i.e. no turbulence modification ). In fact, the
particle feedback promotes the turbophoresis of the external wall, while the particle accumulation
close the internal side is attenuated.