Direct numerical simulation of turbulent flows with cloud-like off-source heating

Abstract

Direct numerical solutions of the incompressible Navier–Stokes equations have been obtained under the Boussinesq approximation for the temporal evolution of a turbulent jet-like flow subjected to off-source volumetric heating, of the kind that occurs in a cloud due to latent heat release on condensation of water vapour. The results show good qualitative agreement with available experimental data on spatially growing jets. Thus, heating accelerates the flow and arrests jet growth; and turbulence velocities increase with heating but not as rapidly as mean velocities, so normalized intensities drop. It is shown that the baroclinic torque resulting from the heating enhances the vorticity dramatically in all three directions, with a preferential amplification at the higher wavenumbers that results in a rich fine structure at later times in the evolution of the jet. Streamwise vortex pairs, rendered stronger by mean flow acceleration, appear to be responsible for large expulsive motions at certain transverse cross-sections in the ambient fluid near the heated flow; together with the disruption of the toroidal component of the coherent vorticity achieved by heating, this results in an entraining velocity field that is qualitatively different from that around unheated turbulent jets. This mechanism may provide a plausible explanation for the experimentally observed drop in entrainment with off-source heating.

Laboratory simulations show diabatic heating drives cumulus-cloud evolution and entrainment

“…Qualitatively similar behavior is observed in the present experiments on steady diabatic flows, in which rough estimates of the turbulent mixing time scale indicate that it decreases approximately by a factor of two (from 1.5 s to 0.7 s) with height over the extent of the HIZ (for details on the estimates of τ mix see SI Text, section 6 and Table S2). This is consistent with lower dilution due to the distortion [owing to axial acceleration (47,51)] and disruption of coherent structures (10,48), accompanied by faster mixing due to intensification of small-scale vorticity caused by the baroclinic torque (48), both resulting from off-source diabatic heating. These physical effects make conditions favorable for more homogeneous mixing as we move up beyond cloud base.…”

Droplet growth in warm turbulent clouds

“…Elavarasan et al, 1995;Bhat and Narasimha, 1996; and numerical simulations (e.g. Basu and Narasimha, 1999;. Here, additional energy is injected into a welldeveloped jet far from the source and over a finite streamwise extent.…”

Direct numerical simulation of a turbulent axisymmetric jet with buoyancy induced acceleration

“…Specifically, [5,6] had reported a decrease in their normalized fluctuations, whereas the experimental findings in [3] have indicated that the normalized rms for streamwise velocity increases. The present computations, albeit at a higher heating rate, suggest a complex behavior of the rms fluctuations for the streamwise velocity within the HIZ that possibly contributed to the contradictory results reported earlier.…”

“…Such a three-way coupling of the momentum, scalar and energy equations has not been attempted by previous DNS investigations of heated jets. For example, Basu and Narasimha [5] did not solve for the scalar field and instead used a time-averaged heat injection profile in their computations.…”

“…For example, Basu and Narasimha [5] performed DNS computations at Re = 1600 to investigate the effect of heating on jets, and the effect of different heating profiles on the jet. Their formulation consisted of a temporal analogue of the spatial problem.…”