Bubbly turbulent flow in a channel is investigated using interface-resolved direct numerical simulation. An efficient coupled level-set volume-of-fluid solver based on a fast Fourier transform algorithm is implemented to enable a high resolution and fast computation at the same time. Up to 384 bubbles are seeded in the turbulent channel flow corresponding to 5.4% gas volume fraction. Bubbles are clustered in the channel center due to the downward flow direction. The bubbles induce additional pseudo-turbulence in the channel center and are also able to attenuate the energy in the boundary layer by reducing the shear production. Turbulent kinetic energy budget indicates a significant buoyancy production in the channel center. A local equilibrium between buoyancy production and dissipation is observed here besides the shear production peak in the boundary layer. Comparing the local production and dissipation indicates a coexistence of boundary layer turbulence near the wall and bubble-induced pseudo-turbulence in the channel center. The liquid phase and gas phase are coupled through the complex liquid-gas interface. Local flow topology analysis is depicted in the liquid phase around the bubbles as well as in the gas phase. The flow topology of the liquid phase and the gas phase differs from each other significantly. Local dissipation is more dominant in the liquid phase near the bubble interface, whereas local enstrophy is preferred in the gas phase. In the liquid phase, a high dissipation event is preferred close to the interface, whereas a high enstrophy event is dominant away from the interface.

@en

Flow and heat transfer of merging and bouncing droplets are studied for different Weber and Reynolds numbers and eccentricities of droplets by means of direct numerical simulation. Droplets are allowed to deform under the hydrodynamic forces of the surrounding flow. A coupled level-set and volume of fluid (CLSVOF) method is used to capture the highly deformable topology of the droplets. The method is coupled with a fast pressure solver, developed by Dodd and Ferrante, 2014, in order to circumvent the expensive iterative solvers commonly used. The temperature distribution inside the droplet and its consequent effect on the Nusselt number is studied. The results show that the Reynolds number and the initial configuration of droplets is of more important than the effect of the surface tension which governs the extent of deformations.

@en

This work investigates fully developed turbulent flows of carbon-dioxide close to its vapour-liquid critical point in a channel with a hot and a cold wall. Two direct numerical simulations are performed at low Mach numbers, with the trans-critical transition near the channel centre and the cold wall, respectively. An additional simulation with constant transport properties is used to selectively investigate the effect of the non-linear equation of state on turbulence. Compared to the case where the pseudo-critical transition occurs in the channel center, the case with the pseudo-critical transition close to the cold wall reveals that compressibility effects can exist in the near-wall region even at low Mach numbers. An analysis of the velocity streaks near the hot and the cold walls also indicates a greater degree of streak coherence near the cold wall. A comparison between the constant and variable viscosity cases at the same Reynolds number, Mach number and having the same isothermal wall boundary conditions reveals that variable viscosity increases turbulence near the cold wall and also causes higher velocity gradients near the hot wall. We also show that the extended van Driest transformation results in a better agreement of the velocity profile with the log-law of the wall compared to the standard van Driest transformation. The semi-locally scaled turbulent velocity fluctuations and the turbulent kinetic energy budgets on the hot and the cold sides of the channel collapse on top of each other, thereby establishing the validity of Morkovin’s hypothesis.

@en

We use direct numerical simulations to study the effect of thermal boundary conditions on developing turbulent pipe flows with fluids at supercritical pressure. The Reynolds number based on pipe diameter and friction velocity at the inlet is Reτ0=360 and Prandtl number at the inlet is Pr0=3.19. The thermodynamic conditions are chosen such that the temperature range within the flow domain incorporates the pseudo-critical point where large variations in thermophysical properties occur. Two different thermal wall boundary conditions are studied: one that permits temperature fluctuations and one that does not allow temperature fluctuations at the wall (equivalent to cases where the thermal effusivity ratio approaches infinity and zero, respectively). Unlike for turbulent flows with constant thermophysical properties and Prandtl numbers above unity – where the effusivity ratio has a negligible influence on heat transfer – supercritical fluids shows a strong dependency on the effusivity ratio. We observe a reduction of 7 % in Nusselt number when the temperature fluctuations at the wall are suppressed. On the other hand, if temperature fluctuations are permitted, large property variations are induced that consequently cause an increase of wall-normal velocity fluctuations very close to the wall and thus an increased overall heat flux and skin friction.@en

Fluids at supercritical pressure undergo a continuous phase from a liquid to a gas state if the fluid is heated above the critical pressure. During this phase transition the thermophysical properties of the fluid vary significantly within a narrow temperature range across the pseudo-critical temperature T pc  Tpc (pseudo-critical temperature is defined as the temperature at which the specific heat at constant pressure (c p  cp ) attains its peak value). Figure 1 shows the variation of thermophysical properties of CO 2  2 at a thermodynamic supercritical pressure P 0  P0 = 80 bar (P critical =73.773bar Pcritical=73.773bar ) as a function of temperature (Int J Thermophys 24, 1–39 (2003)) [1]. These characteristics make supercritical fluids appealing in many industrial applications, such as: desorption, drying and cleaning in extraction processes; pharmaceutical industry; in power cycles as working fluids (Renew Sustain Energy Rev 14, 3059–3067 (2010)) [2] , (Nucl Technol 154, 283–301 (2006)) [3] and biodiesel production ( Fuel 80, 225–231 (2001)) [4].@en