arXiv subjects

Arun Balakrishna

Publications and source records attributed to Arun Balakrishna.

4 recordsLinked to original sources

arXiv2026

Gravity Wave Interactions in the Stratocumulus-Topped Boundary Layer

This work studies the breakup propensity of the stratocumulus-topped boundary layer (STBL) interacting with gravity waves using large-eddy simulation with a uniform vertical grid of $5$ m and horizontal spacing of $30$ m. A radiative-convective equilibrium (RCE) state is constructed to enforce stationarity in the STBL, and the gravity waves are introduced via a vertical momentum forcing mimicking a packet of plane waves. A nondimensionalization involving the inversion height and mean horizontal base wind as length and velocity scales is proposed to provide a framework to analyze the forcing parameter space. The magnitude of the scaled forcing amplitude ($\mathcal{A}$) is critical in understanding various STBL breakup conditions. Classification of breakup was based on the reduction of the liquid water path for each forced STBL case. We found that breakup did not occur for $\mathcal{A}<1$ and observed modest reductions in cloud for $1<\mathcal{A}<2$, but the deck recovered to the stationary state slowly after the single-period forcing ceased. Fixing $\mathcal{A}\sim 2$ showed that forcings with longer duration and wider locality promote breakup. However, when the forcing is a linear combination of waves of two different periods, the percentage of cleared cloud dramatically increases, though recovery of RCE is still observed in some cases. $\mathcal{A}\geq2.5$ marks a critical threshold by which the STBL breaks up entirely and remains patchy. We further explore the connection between these bulk breakup results and the turbulent state by examining energy budgets and the anisotropy induced by the forcing.

physics.flu-dyn↗

arXiv2025

Instability and breaking of internal waves in a horizontal shear layer

The behaviour of internal waves propagating in a background shear flow is studied in the case where the direction of shear is orthogonal to gravity. Ray-tracing theory is used to predict properties of the wave state at locations where instability occurs. Local wave energy growth is found to result from two distinct mechanisms: an increase in wave steepness due to refraction by the shear, or an increase in streamwise velocity perturbations due to wave advection of the background flow. Based on the initial conditions, a dimensionless perturbation energy ratio $F$ is constructed to predict the relative importance of these two mechanisms in facilitating wave-breaking. When $F$ is small and waves become locally steep, perturbation kinetic and potential energy remain approximately equipartitioned and subsequent instabilities are expected to develop due to a combination of shear and convection. On the other hand, as $F$ increases, kinetic energy dominates and wave advection of momentum may instead cause breaking to become increasingly driven by enhanced vertical shear. To test these predictions, fully nonlinear direct numerical simulations are conducted, spanning a range of wave-breaking dynamics. Good qualitative agreement with the theory is found despite substantial departures from the underlying assumptions. Wave breaking leads to significant turbulent dissipation, which in some cases greatly exceeds the initial wave energy. Momentum and energy transfers between the wave, background flow and turbulence are found to be sensitive to the dynamics of breaking, as are the mixing properties.

physics.flu-dyn↗

arXiv2025

Large eddy simulation of ocean mesoscale eddies

Mesoscale eddies produce lateral (2D) fluxes that need to be parameterized in eddy-permitting (1/4-degree) global ocean models due to insufficient horizontal resolution. Here, we systematically apply methods from the 3D LES community to parameterize lateral vorticity fluxes produced by mesoscale eddies leveraging an explicit filtering approach together with a dynamic procedure. The developed subfilter closure is implemented into the GFDL MOM6 ocean model and is evaluated in an idealized double-gyre configuration, both a-priori and a-posteriori. For sufficiently resolved grids, the LES simulations converge to the filtered high-resolution data. However, limitations in the proposed closure are observed when the filter scale approaches the energy-containing scales: the a-priori performance drops and a-posteriori experiments fail to converge to the filtered high-resolution data. Nevertheless, the proposed closure is accurate in predicting the mean flow in a-posteriori simulations at all resolutions considered (1/2-1/8 degrees). Finally, we propose parameterizing the thickness fluxes using a Bardina model which further improves simulations at the coarsest resolutions (1/2-1/3 degrees).

physics.ao-ph↗

arXiv2019

Effect of mounting strut and cavitator shape on the ventilation demand for ventilated supercavitation

The present work reports behaviors regarding the formation and collapse of a ventilated supercavity while varying the cavitator shapes, including triangle, disk, and cone and varying mounting struts. Three cavitators with the same frontal area are fabricated with 3D printing and mounted on a forward facing model (FFM). The ventilation requirements to generate (C_Qf) and sustain (C_Qc) a supercavity are tested over a wide range of Froude number (Fr) for each cavitator and compared with backward facing model(BFM). Compared to the triangle and disk cavitators, the cone-shaped cavitator requires the least amount of air to generate a supercavity in nearly all of the tested flow regime except very high Fr. The C_Qc of disk FFM is lower than that of its BFM counterpart at small Fr and exceeds the BFM C_Qc with further increase of Fr. The cone cavitator has the smallest C_Qc among all the cavitators across the range of Fr in our experiments. Simultaneous internal pressure and cavity dimension measurements are conducted to elucidate the cavity sustenance behaviors. The cone-generated cavity yields a significantly smaller maximum diameter and a shorter half-length. Cavity geometric information and cavity pressure measurements with high-speed imaging of re-entrant jet are employed to estimate the re-entrant jet momentum under different Fr for disk and cone cavitators. The estimated re-entrant jet momentum shows reasonable match with the ventilation air momentum under C_Qc in lower Fr for both cavitator cases, with the disk cavitator case yielding significantly stronger re-entrant jet, providing support to the re-entrant jet mechanism governing on the cavity collapse. Our study sheds some light on the cavitator design and ventilation strategy for a supercavitating vehicle in practice.

physics.flu-dyn↗