arXiv subjects

Hridey Narula

Publications and source records attributed to Hridey Narula.

3 recordsLinked to original sources

arXiv2026

Bridging Filtering and Point-Splitting Approaches for Variable-Density Flows

Energy transfer in turbulent flows is typically described either through correlation functions, via the Kármán-Howarth-Monin relation, or through a scale-by-scale budget of the filtered energy (Frisch 1995). For constant-density homogeneous and isotropic turbulence, the equivalence between these two descriptions is well understood. In compressible turbulence, however, several definitions of filtered energy exist, and for most definitions the associated formulation in terms of correlation functions is unclear. We develop a general empirical framework, supported by theoretical arguments and numerical simulations, to determine the multipoint correlation functions corresponding to any filtered energy. We then show that the Favre filtered energy -- defined as the ratio of the squared filtered momentum to the filtered density -- corresponds to an infinite series of multipoint correlation functions. This is achieved by expanding the Favre velocity as a power series in local density fluctuations. The expansion reveals the contributions of the subgrid-scale fluctuations of velocity and density to the Favre velocity. We verify the proposed expansion for the buoyancy and pressure contributions for three-dimensional buoyancy-driven bubbly flows with a large density contrast ($10^2$) between the liquid and the bubble phase.

physics.flu-dyn↗

arXiv2026

Gappy Reconstruction of Bubbly Flows by Guided Diffusion Models

Experiments in multiphase flows are often limited in their ability to simultaneously obtain velocity measurements in different phases. At the same time, flow reconstruction from phase-limited measurements is a challenging problem due to the substantially different velocity statistics across the phases. We address this problem for buoyancy-driven bubbly flows in the pseudo-turbulence regime by using a guided diffusion model. We train the model using two-dimensional slices of the velocity field extracted from fully resolved three-dimensional direct numerical simulations. The model generates physically realistic velocity fields both unconditionally and when conditioned on the surrounding liquid flow. The reconstructed bubble-phase velocity field accurately reproduces key statistical features of the flow. We further show that a simple patching procedure for adjacent two-dimensional slices enables a reasonable reconstruction of the three-dimensional flow inside a bubble. These results establish the potential of diffusion models to serve as generative priors for three-dimensional turbulent multiphase flows, opening a route toward the reconstruction of unobserved or experimentally inaccessible velocity fields from sparse, partial, or phase-limited measurements.

physics.flu-dyn↗

arXiv2026

Scale-by-scale energy transfers in bubbly flows

Buoyancy-driven bubbly flows naturally have spatially-dependent density fields, which allow for multiple definitions of the scale-dependent (or filtered) energy. A priori, it is not obvious which of these provide the most physically apt scale-by-scale budget. In the present study, we compare two such definitions, based on (a) filtered momentum and filtered velocity (Pandey et al. 2020), and (b) Favre filtered energy (Aluie 2013; Pandey et al. 2023). We also derive a Kármán-Howarth-Monin (KHM) relation using the momentum-velocity correlation function and contrast it with the scale-by-scale energy budget obtained in (a). We find that for the volume fraction and Atwood number explored, irrespective of the definition, energy transfers due to the advective nonlinearity and surface tension are identical. However, discrepancies arise for the buoyancy and pressure contributions. We show that the Favre filtered definition is the more appropriate choice, within which buoyancy injects energy, pressure transfers energy to large scales, and both advective nonlinearity and surface tension transfer energy downscales where it is dissipated by viscosity.

physics.flu-dyn↗