Research

I use self-written MPI-parallelized numerical codes, asymptotic and singular perturbation theories, and scaling arguments to tackle various problems related to turbulence. You can find the repositories containing some of these codes on my GitHub. I also explain my findings using nice visualizations.

Internal Tides and the Internal Wave Continuum - C. Rajarshi, Jim Thomas

One-third of the net power put in the ocean comes from internal tides that form due to the gravitational pull of the Sun, the Moon, and other celestial objects. These internal tides control turbulent mixing throughout the ocean by exchanging energies with other internal waves, and slowly evolving balanced eddies. Such energy interactions at large scales differ considerably from those at small-scales. Due to computational limitations, small-scale interactions are poorly captured in current global ocean models. We investigate energy exchanges of internal tides with balanced flow across length scales. At large length scales, tide-eddy interaction is small, with eddies transferring energy to large scales, hence preserving their coherent structures. At smaller length scales, strong tide-eddy interactions break the large-scale eddies and increase turbulent mixing. Such interactions not only generate waves with harmonics of the tidal frequency, but also energize rapidly evolving non-wave motions. This leads to a continuum in the frequency spectra. By quantifying pathways for tidal energy exchanges across length and timescales, our study offers new insights into small-scale turbulent mixing in the ocean.

Under review.

Caustics of finitely dense inertial particles - C. Rajarshi, Rama Govindarajan

Caustics occur where inertial particles suspended in flows collide. They can be identified in a continuum as regions where the divergence of the particle velocity field diverges to negative infinity. We show that less dense particles need to go through stronger strain to form caustics. However, not all particles going through such high strain undergo caustics, and we propose a mechanism for this.

ArXiV: 2601.14179. Physical Review Fluids, in press.

Active Caustics - R Chajwa, C. Rajarshi, R Govindarajan, S Ramaswamy

In this work, we show that extreme clustering can arise without mechanical inertia for self-propelled particles (SPPs). We establish that a singular perturbation is at the heart of caustics formation by SPPs around a single vortex, and our numerical studies of SPPs in two-dimensional Navier-Stokes turbulence show intense caustics in straining regions of the flow, peaking at intermediate levels of self-propulsion.

ArXiV: 2310.01829. Phys. Rev. Fluids 11, 033104 [Editor's suggestion].

Pathways to thermalization in Spectrally Truncated Euler equation - C. Rajarshi, P Anand, Md Saif Khan, SS Ray.

We explore how the spectrally truncated Euler equation thermalizes and how similar it is to decaying turbulence. We uncover scalings different from decaying turbulence, and show that despite the system having a constant flux across wavenumbers, it does not show a $k^{-5/3}$ energy spectra.

ArXiV: 2512.10788. Philosophical Transactions of the Royal Society A, in press.

Growth rate of 'Lucky drops' in turbulence - C. Rajarshi, Rama Govindarajan

In a sea of tiny droplets, a larger drop, owing to its larger inertia, has more potential to collide with smaller drops, coalesce with them, and grow. We derive an equation of motion for these big, lucky droplets. In 2D turbulence, we show that their growth scales with the smaller drops’ inertia, and point to the advantage enjoyed by luckier drops starting in vortical regions.

In progress.

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