arXiv subjects

Sriram P. Kalathoor

Publications and source records attributed to Sriram P. Kalathoor.

9 recordsLinked to original sources

arXiv2026

Route survival and spectral modification of finite-depth salt-finger plume forests under imposed mean shear

Salt-finger plume forests in a finite layer can differ in strength and in the route by which interfacial activity becomes vertically connected. We use direct three-dimensional simulations to test whether such a route is a short-lived realization-specific transient or a persistent route family under an added mean-shear perturbation. The baseline route atlas holds density ratio, diffusivity ratio, Prandtl number, interface thickness, roughness amplitude, domain, and resolution fixed while varying the imposed interfacial roughness spectrum. Low-mode roughness forms a broad connecting endpoint, high-annulus roughness forms a localized route-memory endpoint, and mixed roughness forms a delayed scale-transfer route. A second mixed realization preserves continuous active-width, spectral, and transport measures after $t=45$, with mean absolute differences of $3.1\%$ in $w$-active width, $1.6\%$ in salinity-active width, $2.8\%$ in broad spectral fraction, and $3.6\%$ in salt flux, while shifting the binary scalar-contact label. We then impose an initial tanh mean shear on the mixed route. The full-resolution shear case reaches $t=60$ and preserves finite-depth reach: first velocity contact occurs at $t=57.75$, first salinity contact occurs at $t=59.5$, and both times match the unsheared mixed reference. The spectral branch is redistributed. At $t=60$, the broad fraction is $1.116$ times the mixed value, the intermediate fraction is $0.530$ times the mixed value, and the short-wave fraction is $1.278$ times the mixed value. In this finite-depth configuration, route survival means preserved reach and contact timing with a changed spectral pathway.

physics.flu-dyn↗

arXiv2026

Observability of Finite-Depth Double-Diffusive Exchange from Sparse Temperature-Salinity Measurements

Double-diffusive interfaces can support exchange histories that are spatially organized but only sparsely observed. A resolved three-dimensional calculation contains the route by which scalar gradients broaden, remain compact, connect with remote parts of a finite-depth layer, or organize horizontally, whereas field products usually provide profiles, repeated casts, autonomous-float records, or hydrographic sections. We ask which route-relevant features of finite-depth double-diffusive exchange remain observable after that measurement reduction. Four controlled finite-depth exchange histories are treated as known truth fields and sampled with vertical-profile, profile-bundle, vertical-coarsening, Argo-style, and section-like observing formats. The resulting observables are compared with selected Ice-Tethered Profiler, Argo, and CCHDO/GO-SHIP products to place the synthetic measurements in realistic observing contexts. Isolated profiles are weak route identifiers: at the final comparison time, strict case accuracy remains below 0.48, route-family accuracy is about 0.53--0.56 in leave-one-out profile classification, and 22 of 36 profiles are closer to a different-route profile than to a same-route profile under the current feature set. Profile bundles are substantially more informative. In the bundle-enumeration framework, the one-profile late-time route-family baseline is 0.694, while two-, three-, and four-profile bundles reach route-family accuracies of 0.917, 0.961, and 0.994. Coarse vertical sampling can preserve broad route separation while distorting local interface-width estimates, and section-like sampling adds horizontal-scale information only when station spacing resolves the relevant mode. The useful observing unit for hidden finite-depth double-diffusive exchange is therefore an ensemble or section, not an isolated cast.

physics.geo-ph↗

arXiv2026

Interfacial Spectral Memory as a State Variable for Finite-Depth Salt-Finger Exchange

Thermohaline interfaces in the ocean are often treated through local double-diffusive favorability, yet finite interfaces can also inherit roughness from prior waves, stirring, intrusions, and earlier mixing events. Such inherited geometry can matter because salt fingering does not develop from a flat abstract surface in many geophysical settings. We use controlled three-dimensional direct simulations to test whether the spectral state of a finite rough interface changes the pathway by which salt-finger activity develops between adjacent layers. The density ratio, diffusivity ratio, Prandtl number, interface thickness, roughness amplitude, domain, resolution, and analysis window are held fixed; only the imposed roughness spectrum and, for one pair, the realization are changed. Broad low-mode memory produces the largest cumulative salt exchange and the earliest finite-depth contact. High-annulus memory remains localized and intermediate-scale dominated. Mixed memory produces delayed scale transfer and scalar-rich structure that is robust in integrated exchange and broad-memory measures across a second realization, while local plume timing and probe amplitudes remain realization-sensitive. The simulations therefore support treating interfacial spectral memory as an additional state variable for finite-depth double-diffusive exchange, complementary to local thermodynamic descriptors.

physics.flu-dyn↗

arXiv2026

Interfacial Roughness Spectra and Finite-Depth Salt-Finger Mixing at a Two-Layer Thermohaline Interface

Salt fingering drives diapycnal scalar exchange across thermohaline interfaces that are statically stable but double-diffusively unstable. Oceanic interfaces are finite-depth structures and may carry roughness inherited from waves, shear, intrusions, or prior mixing. We test how the horizontal spectrum of that roughness controls the route from a two-layer interface to a finite-depth salt-finger plume forest. Direct simulations of the modeled Boussinesq equations are performed at $\mathrm{Pr}=7$, $τ=0.01$, and $\mathrm{R}_ρ=1.2$, with matched domain, grid, amplitude, boundary treatment, and analysis measures. The imposed spectra are high-annulus, low-mode, and mixed; a second mixed realization tests robustness. The imposed spectrum selects distinct routes to vertical exchange. High-annulus roughness remains compact and branch-locked through $t=60$, without a tracked broad-branch transition. Low-mode roughness begins on the broad branch, produces the strongest salinity transport at $t=45$, and reaches the finite-depth boundary region first. Mixed roughness follows a velocity-led pathway: vertical velocity selects the broad branch before salinity, while salinity develops the richest planform spectral population. At $t=45$, the mixed salinity effective mode count is $86.66$, compared with $3.26$ for high-annulus forcing and $5.46$ for low-mode forcing. Angular and signed-branch measures show branch-dependent diagonal organization, and probe/volume measures show that local plume-passage asymmetry does not imply large global upper/lower imbalance. The replicate preserves the mixed route with shifted transition times. Thus a finite-depth thermohaline interface can retain spectral memory, controlling whether salt-finger mixing remains localized, penetrates rapidly, or forms a scalar-rich plume forest through delayed modal handoff.

physics.flu-dyn↗

arXiv2026

Planar Lagrangian transport and scalar-gradient organization in a turbulent reacting shear layer

We analyze planar Lagrangian transport and scalar-gradient organization in a supersonic, reacting hydrogen-air temporal mixing layer using time-resolved mid-plane data from a three-dimensional direct numerical simulation. The analysis combines forward/backward finite-time Lyapunov exponent (FTLE) fields, operational FTLE-ridge skeletons, Cauchy-Green deformation measures, shear-LCS metrics, and planar hyperbolic geodesic-LCS extraction to examine how finite-time stretching structures the reacting shear layer. The time-resolved FTLE ridges identify repelling and attracting finite-time transport skeletons in the constrained two-dimensional slice, from which ridge geometry, intersection occupancy, persistence, and scalar-conditioned transport are quantified. Hyperbolic geodesic LCS are extracted from Cauchy-Green tensors reconstructed from planar flow maps as strainlines seeded at high-$λ_{\max}$ normal maxima, providing a variational counterpart to the operational FTLE-ridge skeleton. We then relate the transport skeleton to temperature, mixture fraction, and a reaction intermediate. The results show localized forward/backward ridge overlap, strong scalar-gradient enrichment, finite-time geodesic LCS that occupy the same high-strain transport skeleton, residual direction-dependent separation from a time- and cross-stream-stratified null model, and scalar-response lags that remain compact relative to decorrelation and FTLE-integration scales. Together, these results provide a transport-oriented characterization of coherent structures and their role in mid-plane mixing within a compressible reacting shear flow.

physics.flu-dyn↗

arXiv2026

Receptivity and Biorthogonal Decomposition in a Reacting Temporal Mixing Layer

We examine receptivity and biorthogonal decomposition in a reacting temporal mixing layer using direct and adjoint eigenmodes of a finite-thickness compressible linearized operator built from the mean reacting base state. The analysis focuses on the Kelvin--Helmholtz branch and asks how the reacting base state modifies the selected temporal instability, where localized forcing most efficiently excites it, and how strongly the associated modal family is represented in time-resolved planar simulation data. Receptivity maps are constructed for mass, momentum, thermal, and mixture-fraction forcing channels using an energy-weighted adjoint projection, with biorthogonality enforced by the corresponding direct--adjoint inner product. A complementary biorthogonal decomposition provides modal amplitudes and cumulative few-mode reconstructions at the fundamental streamwise wavenumber. The finite-thickness branch is interpreted against a compressible vortex-sheet reference built from the outer-stream states. The reacting layer supports an unstable finite-thickness Kelvin--Helmholtz family over low-to-moderate wavenumbers even though the discontinuous reference is essentially neutral. Mass forcing leads the raw localized receptivity maps, mixture-fraction forcing follows through composition-pressure coupling, and chemistry-weighted thermal forcing identifies the strongest thermochemical support of the same family. The results show how distributed reacting thermodynamics reorganize compressible shear-layer instability and how that reorganized branch remains embedded in the nonlinear flow.

physics.flu-dyn↗

arXiv2026

Near-Field Combustion-Noise Source Dynamics in a Reacting Supersonic Temporal Mixing Layer

Compressibility and chemical reactions in reacting flows provide source mechanisms for pressure fluctuations whose signatures depend on the flow, source distribution, and acoustic environment. Bounded flows can sustain strong feedback and narrowband tones, whereas boundary-free flows more often exhibit broadband source activity distributed across frequency. Near-field combustion-noise source dynamics are examined in a supersonic reacting hydrogen-air temporal mixing layer using high-fidelity time-resolved direct numerical simulation data. Pressure, heat-release, and dilatation fields are used to identify how localized reacting structures, compressive disturbances, broadband spectral content, and burst-driven temporal organization interact. The results show weak pressure--heat-release coherence concentrated within selected low-frequency bands, with no collapse onto a single dominant mode. Combustion intermittency modulates the near-field pressure response within these bands and is accompanied by burst-driven amplitude modulation and transient trajectory sensitivity, while the overall dynamics remain broadband and bounded. A planar source-radiation-potential projection further shows moderate low-frequency angular bias associated with the heat-release source distribution. Information-theoretic measures indicate that pressure fluctuations share more statistical structure with heat release than with dilatation. The analysis characterizes source-side organization and near-field pressure response in the sampled DNS of a reacting shear layer, providing a basis for subsequent observer-based or acoustic-analogy radiation calculations.

physics.flu-dyn↗

arXiv2026

Event-level compression--chemistry coupling in a supersonic reacting temporal mixing layer

Compression and heat release interact intermittently in high-speed reacting shear layers, and whole-field averages can obscure their coupling. We examine this coupling in a supersonic reacting hydrogen--air temporal mixing layer using time-resolved mid-plane slices from a three-dimensional direct numerical simulation. A fixed dilatation threshold identifies connected compression events, and exothermic heat release and mixture-fraction-gradient activity are then conditioned on the evolving event population. The record separates into startup ($t^*<5$), transition ($5\le t^*<20$), and developed ($t^*\ge 20$) regimes, with the developed regime carrying the persistent compression--chemistry interaction. In this regime, compression appears as a population of intermittent events with no single structure dominating the field. Stronger exothermic response is associated with larger maximum-event area, larger event count, greater compression--heat-release overlap, and smaller distance from compression to the most exothermic regions. Scalar-gradient amplification peaks near zero lag relative to compression-area excursions, whereas the strongest exothermic response precedes peak compression coverage by $Δt^*\approx -0.85$. These results show that compression organizes chemistry most clearly through event population, overlap, proximity, and lag, providing an event-level description of compression--chemistry coupling in an open supersonic reacting shear layer.

physics.flu-dyn↗

arXiv2026

Filtering effects on entropy transport and entropy-production structure in a supersonic reacting shear layer

Spatial filtering is examined in time-resolved mid-plane DNS fields of a supersonic reacting shear layer using a sequence of box filters. The analysis tracks a nondimensional entropy-like scalar $s^\ast$, its in-plane material derivative $D s^\ast / D t^\ast$, a residual $Π_s^\ast$, and viscous and conductive entropy-production diagnostics, $σ_μ^\ast$ and $σ_k^\ast$. Filtering changes $s^\ast$ only weakly, attenuates $D s^\ast / D t^\ast$ and the strongest tails of $σ_μ^\ast$ and $σ_k^\ast$, but broadens the residual distribution and increases the residual RMS with filter width. The residual remains concentrated in the layer core that carries the largest mechanical and thermal activity. Conditional statistics show that $|Π_s^\ast|$ rises with both entropy-production intensity and entropy-gradient strength. Spectral and structural diagnostics show that increasing filter width removes high-wavenumber content and simplifies the geometry of the high-$|Π_s^\ast|$ sets. Coarser filtering therefore increasingly distorts entropy transport preferentially through the most dynamically and thermally active structures, rather than uniformly across the plane.

physics.flu-dyn↗