The three-dimensional flow around a spherical bubble moving steadily in a viscous linear shear flow is studied numerically by solving the full Navier-Stokes equations. The bubble surface is assumed to be clean so that the outer flow obeys a zero-shearstress condition and does not induce any rotation of the bubble. The main goal of the present study is to provide a complete description of the lift force experienced by the bubble and of the mechanisms responsible for this force over a wide range of Reynolds number (0.1 Re 500, Re being based on the bubble diameter) and shear rate (0 Sr 1, Sr being the ratio between the velocity difference across the bubble and the relative velocity). For that purpose the structure of the flow field, the influence of the Reynolds number on the streamwise vorticity field and the distribution of the tangential velocities at the surface of the bubble are first studied in detail. It is shown that the latter distribution which plays a central role in the production of the lift force is dramatically dependent on viscous effects. The numerical results concerning the lift coefficient reveal very different behaviours at low and high Reynolds numbers. These two asymptotic regimes shed light on the respective roles played by the vorticity produced at the bubble surface and by that contained in the undisturbed flow. At low Reynolds number it is found that the lift coefficient depends strongly on both the Reynolds number and the shear rate. In contrast, for moderate to high Reynolds numbers these dependences are found to be very weak. The numerical values obtained for the lift coefficient agree very well with available asymptotic results in the low-and high-Reynolds-number limits. The range of validity of these asymptotic solutions is specified by varying the characteristic parameters of the problem and examining the corresponding evolution of the lift coefficient. The numerical results are also used for obtaining empirical correlations useful for practical calculations at finite Reynolds number. The transient behaviour of the lift force is then examined. It is found that, starting from the undisturbed flow, the value of the lift force at short time differs from its steady value, even when the Reynolds number is high, because the vorticity field needs a finite time to reach its steady distribution. This finding is confirmed by an analytical derivation of the initial value of the lift coefficient in an inviscid shear flow. Finally, a specific investigation of the evolution of the lift and drag coefficients with the shear rate at high Reynolds number is carried out. It is found that when the shear rate becomes large, i.e. Sr l O(1), a small but consistent decrease of the lift coefficient occurs while a very significant increase of the drag coefficient, essentially produced by the modifications of the pressure distribution, is observed. Some of the foregoing results are used to show that the well-known equality between the added mass coefficient and the lift coefficient holds only in the limit of w...

An experimental investigation of the flow generated by a homogeneous population of bubbles rising in water is reported for three different bubble diameters (d= 1.6, 2.1 and 2.5 mm) and moderate gas volume fractions (0.005 ≤ α ≤ 0.1). The Reynolds numbers,Re=V0d/ν, based on the rise velocityV0of a single bubble range between 500 and 800. Velocity statistics of both the bubbles and the liquid phase are determined within the homogeneous bubble swarm by means of optical probes and laser Doppler anemometry. Also, the decaying agitation that takes place in the liquid just after the passage of the bubble swarm is investigated from high-speed particle image velocimetry measurements. Concerning the bubbles, the average velocity is found to evolve asV0α−0.1whereas the velocity fluctuations are observed to be almost independent of α. Concerning the liquid fluctuations, the probability density functions adopt a self-similar behaviour when the gas volume fraction is varied, the characteristic velocity scaling asV0α0.4. The spectra of horizontal and vertical liquid velocity fluctuations are obtained with a resolution of 0.6 mm. The integral length scale Λ is found to be proportional toV02/gor equivalently tod/Cd0, wheregis the gravity acceleration andCd0the drag coefficient of a single rising bubble. Normalized by using Λ, the spectra are independent on both the bubble diameter and the volume fraction. At large scales, the spectral energy density evolves as the power −3 of the wavenumber. This range starts approximately from Λ and is followed for scales smaller than Λ/4 by a classic −5/3 power law. Although the Kolmogorov microscale is smaller than the measurement resolution, the dissipation rate is however obtained from the decay of the kinetic energy after the passage of the bubbles. It is found to scale as α0.9V03/Λ. The major characteristics of the agitation are thus expressed as functions of the characteristics of a single rising bubble. Altogether, these results provide a rather complete description of the bubble-induced turbulence.

No abstract

The problem of a drop of arbitrary density and viscosity moving close to a vertical wall under the effect of buoyancy is analysed theoretically. The case where the suspending fluid is at rest far from the drop and that of a linear shear flow are both considered. Effects of inertia and deformation are assumed to be small but of comparable magnitude, so that both of them contribute to the lateral migration of the drop. Expressions for the drag, deformation and migration valid down to separation distances from the wall of a few drop radii are established and discussed. Inertial and deformation-induced corrections to the drag force and slip velocity of a buoyant drop moving in a linear shear flow near a horizontal wall are also derived.

The three-dimensional flow past two identical spherical bubbles moving side by side in a viscous fluid is studied numerically by solving the full Navier–Stokes equations. The bubble surface is assumed to be clean so that the outer flow obeys a zero-shear-stress condition. The present study describes the interaction between the two bubbles over a wide range of Reynolds number ($0.02\,{\le}\,Re\,{\le}\,500$, $Re$ being based on the bubble diameter and rise velocity), and separation $S$ ($2.25\,{\le}\,S\,{\le}\,20$, S being the distance between the bubble centres normalized by the bubble radius). The flow structure, the vorticity field, the sign of the interaction force and the magnitude of the drag and lift forces are analysed; in particular the latter are compared with analytical expressions available in the potential flow limit and in the limit of low-but-finite Reynolds number. This study sheds light on the role of the vorticity generated at the bubble surface in the interaction process. When vorticity remains confined in a boundary layer whose thickness is small compared to the distance between the two bubbles, the interaction is dominated by the irrotational mechanism that results in an attractive transverse force. In contrast, when viscous effects are sufficiently large, the vorticity field about each bubble interacts with that existing about the other bubble, resulting in a repulsive transverse force. Computational results combined with available high-Reynolds-number theory provide empirical expressions for the drag and lift forces in the moderate-to-large Reynolds number regime. They show that the transverse force changes sign for a critical Reynolds number whose value depends on the separation. Using these computational results it is shown that, depending on their initial separation, freely moving bubbles may either reach a stable equilibrium separation or move apart from each other up to infinity.

The self-propulsion of a spherical squirmer -a model swimming organism that achieves locomotion via steady tangential movement of its surface -is quantified across the transition from viscously to inertially dominated flow. Specifically, the flow around a squirmer is computed for Reynolds numbers (Re) between 0.01 and 1000 by numerical solution of the Navier-Stokes equations. A squirmer with a fixed swimming stroke and fixed swimming direction is considered. We find that fluid inertia leads to profound differences in the locomotion of pusher (propelled from the rear) versus puller (propelled from the front) squirmers. Specifically, pushers have a swimming speed that increases monotonically with Re, and efficient convection of vorticity past their surface leads to steady axisymmetric flow that remains stable up to at least Re = 1000. In contrast, pullers have a swimming speed that is non-monotonic with Re. Moreover, they trap vorticity within their wake, which leads to flow instabilities that cause a decrease in the time-averaged swimming speed at large Re. The power expenditure and swimming efficiency are also computed. We show that pushers are more efficient at large Re, mainly because the flow around them can remain stable to much greater Re than is the case for pullers. Interestingly, if unstable axisymmetric flows at large Re are considered, pullers are more efficient due to the development of a Hill's vortex-like wake structure.

No abstract

The expression of the hydrodynamic force experienced by a spherical bubble having a variable radius and moving in a viscous incompressible liquid is derived analytically in two different asymptotic situations. The solution is obtained by rewriting the initial problem in a frame of reference where the bubble has a fixed radius and where the relevant dimensionless parameters are conserved. It is shown that when the assumption of unsteady creeping motion is valid, the bubble radius variation combined with a constant rise velocity produces a nonzero history force. This contribution can have a significant effect on the bubble motion, especially for a collapsing bubble.