Research Interests

Classical turbulence in simple and active binary fluids

Binary fluid is a two-component fluid ranging from a mixture of two incompressible fluids, e.g., oil and water, to complex fluids such as active fluids. Simple binary fluids contain microstructures such as droplets and sharp interfaces, while active fluids contain living microorganisms or nonliving self-propelled rods or entities. The dynamics of a binary fluid can be described by a composition field (describing the chemical composition of two fluids) and by a mean velocity. Sharp variation in composition field across interfaces generates diffusion currents, which in turn drive the velocity field by exerting feedback stress.  Below the critical temperature, binary fluid flow stabilizes by phase separation via coarsening or domain growth. For active binary fluid, this phase separation arises from purely repulsive interactions among the active particles but through density-dependent self-propelled velocities termed motility-induced-phase-separation (MIPS). In the presence of a high Reynolds number, classical turbulence, the phase separation was arrested. We are interested in studying the effect of turbulence on coarsening, spectral energy transfers, and scaling laws in terms of exact laws of the cascading invariants and direct numerical simulations, etc.

Relaxation of turbulent binary fluids

Turbulent relaxation is a process where the system self-organizes when the turbulent forcing is quenched. The problem of turbulent relaxation started almost 70 years ago when Chandrasekhar and Woltjer found some regular patterns in cosmic observations of the Crab Nebula. Such regular patterns are the force-free regions of the magnetic field and were considered to arise from turbulent relaxation. There were many attempts to describe such self-organization in neutral fields and plasmas using selective decay theories. However, they were unable to predict more general pressure-balanced relaxed states observed later in simulations and experiments. Very recently, a unified principle, namely the principle of vanishing nonlinear transfer (PVNLT), has been proposed, which successfully explains such relaxed states unambiguously. It is very interesting to study such relaxation in the binary fluids, which are certainly different from neutral fluids and plasmas due to the presence of interfaces.  Simulating CHNS equations, we showed that the bulk and the interfacial relaxation are different but can be explained via a universal pathway based on our recently proposed principle of vanishing nonlinear transfers. Here, we observe direct evidence of such pressure-balance relaxation since the binary fluid interfaces are found to follow a Helmholtz-like pressure-balance type of relaxation.

Active turbulence 

Classical-high Reynolds number turbulence is characterized by strong non-linear interactions with a well-separated inertial range and celebrated Kolmogorov’s (K41) scaling laws. Active matter (living and non-living) is comprised of self-propelled entities, have the ability to convert chemical/ambient energy into directed motion. In general, at high enough concentration, these active systems (e.g., dense bacterial suspensions, microtubule networks) organize themselves into complex patterns characterized by collective behavior, hydrodynamic instabilities, enhanced fluid mixing, etc. These low Reynolds number flow patterns qualitatively resemble the high Reynolds number turbulent patterns, hence termed active turbulence. The complete physical understanding and governing scaling laws behind active turbulence are quite unclear. I am interested in studying various types of active matter, corresponding non-equilibrium hydrodynamic models, and hydrodynamic instabilities based on their symmetry, swimming mechanism, and the nature of momentum conservation and exploring universal characteristics in terms of spectral scaling laws, etc.

Publications 

• Universal energy cascade in homogeneous binary fluid turbulence: A direct comparison of different exact relations, Nandita Pan and Supratik Banerjee, Physical Review Fluids,  10, 064615  (2025)

• Stationary and non-stationary energy cascades in homogeneous ferrofluid turbulence, Sukhdev Mouraya, Nandita Pan, Supratik Banerjee, Physical Review Fluids,  9, 094604 (2024)

• Universal relaxation of turbulent binary fluids, Nandita Pan, Supratik Banerjee, and Arijit Halder,  Commun Phys 7, 4 (2024)

• Universal turbulent relaxation of fluids and plasmas by the principle of vanishing nonlinear transfers, Supratik Banerjee, Arijit Halder and Nandita Pan, Physical Review E (Letts.), 107 (4), L043201 (2023)

• Exact relations for energy transfer in simple and active binary fluid turbulence, Nandita Pan and Supratik Banerjee, Physical Review E, 106 (2), 025104 (2022)

• Effect of plasma and beam parameters on focal dimensions in micrometer charged particle optics: enhanced nonlinear demagnification below the Debye length, Sanjeev Kumar Maurya, Sushanta Barman, Nandita Pan, and Sudeep Bhattacharjee, Physics of Plasmas 26, 063103 (2019) 

• Utilization of wind energy in vehicles, Nivedita Pan and Nandita Pan, Student Journal of Physics 5 (3), 305 (2015) (Symposium Abstract)