arXiv subjects

Swathi Krishna

Publications and source records attributed to Swathi Krishna.

12 recordsLinked to original sources

arXiv2026

Propeller-wing interactions of a lift + cruise eVTOL configuration during transition

The wakes of a lift + cruise aircraft's vertical lift propellers interact with the wing during the transition from hover to forward flight. This study investigates how variations in axial and longitudinal separations between the vertical lift propellers and the wing affect aerodynamic performance during this critical phase. Axial variations were introduced by orienting the motors either upright or inverted at two longitudinal positions relative to the wing. Wind tunnel tests were conducted on the resulting lift + cruise wing configurations over a Reynolds number range of 52,000 to 377,000, using tufts, pressure taps, and load cells to capture aerodynamic behaviour. Results show that the early transition phase (front propeller advance ratios between 0 and 0.25) is particularly sensitive to configuration changes. Rear motor orientation had a pronounced effect: when oriented upright at the shortest longitudinal separation, the active lift propellers increased the lift-drag ratio of the design by approximately 4 units, whereas in the rear motor inverted case the design suffered a 4-point reduction in the lift-drag ratio. Decreasing the longitudinal separation amplified both the beneficial lift contribution of the rear propeller and the generally adverse effects of the front propeller. However, placing the front propeller closer to the wing in the upright orientation enhanced performance at advance ratios between 0.11 and 0.25, due to favourable interactions between the wing and the front propeller's trailing vortices.

physics.flu-dyn↗

arXiv2026

A Low-Fidelity Method for Aerofoil Shape Optimisation for Curvilinear Blade Kinematics

This study develops a low-fidelity framework for aerofoil shape optimisation under curvilinear blade kinematics, using a hovering cyclorotor as a representative case. Aerofoil optimisation can improve cyclorotor efficiency by suppressing leading-edge vortex separation during dynamic stall, but conventional approaches rely on computationally expensive CFD-based optimisation. The proposed method uses a single-streamtube model to estimate the rotor throughflow and optimises the aerofoil camberline using separate leading- and trailing-edge criteria. Assessed across configurations with varying blade counts and chord lengths, the framework consistently identifies aerofoils that improve hover efficiency, quantified by Figure of Merit. For the baseline four-bladed configuration, the low-fidelity optimum achieves 77% of the Figure of Merit improvement obtained using high-fidelity optimisation at a fraction of the computational cost. The analysis also reveals an additional torque-minimising design family at increased chord lengths, highlighting the influence of trailing-edge loading. Aerofoil optimisation is also compared to blade-pitch kinematics optimisation, which improves the efficiency through similar control of the leading-edge vortex separation. While both approaches produce comparable improvements in efficiency, the optimised kinematics substantially reduces thrust. Aerofoil optimisation may therefore be more practical, as maintaining a target thrust with optimised pitch kinematics would require higher rotational speeds, potentially introducing structural and noise issues.

physics.flu-dyn↗

arXiv2026

A Surfactant Prediction Model for Rising Bubbles

Bubbles released from a needle show shape deformations that depend on the surfactant concentration of the surrounding liquid. We develop a model that predicts the surfactant concentration based on experimental early-stage observations of these deformations. Using high-speed imaging, we examine bubbles within the first 144 ms of ascent, corresponding to a vertical rise distance of approximately 40 mm and extract the instantaneous aspect ratio (AR) and analyse its temporal evolution. In clean conditions, bubbles exhibit pronounced shape oscillations resulting from the periodic exchange between surface and kinetic energy. The presence of surfactants leads to an immediate damping of these oscillations, characterised by reduced AR amplitudes and earlier peak deformations. This damping effect intensifies with increasing surfactant concentration until a near-saturation regime is reached, beyond which bubbles remain largely spherical and further increases in concentration produce indistinguishable AR profiles within the early-stage observation window. To develop the prediction model, an aspect-ratio-based analysis methodology is proposed, which yields an empirical relationship capable of estimating surfactant concentrations between 0 ppm and 2.9 ppm. We finally test the reliability of the model on unknown surfactant-laden bubbles. The model successfully detected the presence and relative extent of surfactant contamination as higher concentrations were introduced.

physics.flu-dyn↗

arXiv2026

The Role of Dynamic Stall in Aerofoil Shape Optimisation for Curvilinear Blade Kinematics

This study investigates the influence of aerofoil shape optimisation on blade aerodynamic performance under curvilinear and unsteady kinematics characteristic of vertical-axis turbines and cycloidal propellers. Using a cyclorotor in hover as a representative configuration, aerofoil optimisation was performed using two-dimensional unsteady Reynolds-averaged Navier-Stokes simulations coupled with Kriging. The optimised design was subsequently experimentally assessed through force measurements and flow-field characterisation using particle image velocimetry. Performance was enhanced through the suppression of leading-edge vortex separation during the primary thrust peak. This finding also reveals a governing constraint: the effectiveness of aerofoil optimisation depends on dynamic stall severity. Under light dynamic stall, geometric modification promotes vortex attachment and improves aerodynamic loading. Under deep dynamic stall, flow separation dominates the blade aerodynamics, and aerofoil shape modification cannot suppress leading-edge vortex shedding. The stall severity is regulated by rotor solidity through its influence on the induced throughflow-to-blade-speed ratio and the resulting effective incidence. Aerofoil optimisation is therefore viable primarily in high-solidity configurations that operate within a moderated stall regime. These findings establish a physics-based condition for aerofoil optimisation in curvilinear dynamic stall environments.

physics.flu-dyn↗

arXiv2024

Relative hyperbolicity of ascending HNN extension of groups

We prove that for a finitely generated group G with a free factor system and an injective endomorphism that preserves the free factor system, the ascending HNN extension of G is hyperbolic relative to a collection of maximal parabolic subgroups. As a corollary, we see that if an injective endomorphism of a finite rank free group F is exponentially growing, the ascending HNN extension of F is relatively hyperbolic.

math.GR↗

arXiv2022

To tread or not to tread: comparison between water treading and conventional flapping wing kinematics

Hovering insects are limited by their physiology and need to rotate their wings at the end of each back and forth motion to keep the wing's leading edge ahead of its trailing edge. The wing rotation at the end of each half-stroke pushes the leading edge vortex away from the wing which leads to a loss in the lift. Unlike biological fliers, human-engineered flapping wing micro air vehicles have different design limitations. They can be designed to avoid the end of stroke wing rotation and use so-called water-treading flapping kinematics. Flapping wings using conventional flapping kinematics have a designated leading and trailing edge. In the water-treading mode, the role of the leading and trailing edges are continuously alternated throughout the stroke. Here, we compare velocity field and force measurements for a rectangular flapping wing conducting normal hovering and water-treading kinematics to study the difference in fluid dynamic performance between the two types of flapping kinematics. We show that for similar power consumption, the water-treading mode produces more lift than the conventional hovering mode and is 50% more efficient for symmetric pitching kinematics. In the water-treading mode, the leading edge vortex from the previous stroke is not pushed away but is captured and keeps the newly formed leading edge vortex closer to the wing, leading to a more rapid increase of the lift coefficient which is sustained for longer. This makes the water-treading mode a promising alternative for human-engineered flapping wing vehicles.

physics.flu-dyn↗

arXiv2022

Relatively hyperbolic metric bundles and Cannon-Thurston map

Given a metric (graph) bundle $X$ over $B$ where all the fibres are strongly relatively hyperbolic and nonelementary we show that, under certain conditions, $X$ is strongly hyperbolic relative to a collection of maximal cone-subbundles of horosphere-like spaces. Further, given a coarsely Lipschitz qi embedding $i: A\to B$, we show that the pullback $Y$ is strongly relatively hyperbolic and the map $Y\to X$ admits a Cannon-Thurston (CT) map. As an application, we prove a group-theoretic analogue of this result for a relatively hyperbolic extension of groups.

math.GR↗

arXiv2021

Twisted Conjugacy in Big Mapping Class Groups

Let $G$ be a group and $\varphi$ be an automorphism of $G$. Two elements $x, y$ of $G$ are said to be $\varphi$-twisted conjugate if $y=gx\varphi(g)^{-1}$ for some $g\in G$. A group $G$ has the $R_{\infty}$-property if the number of $\varphi$-twisted conjugacy classes is infinite for every automorphism $\varphi$ of $G$. In this paper we prove that the big mapping class group $MCG(S)$ possesses the $R_{\infty}$-property under some suitable conditions on the infinite-type surface $S$. As an application we also prove that the big mapping class group possesses the $R_\infty$-property if and only if it satisfies the $S_{\infty}$-property.

math.GR↗

arXiv2020

Pullbacks of metric bundles and Cannon-Thurston maps

In this version of the paper the exposition is improved and gaps in some of the arguments filled following referee comments. We also include an appendix explaining the equivalence of flaring conditions for a metric bundle and the canonical metric graph bundle associated to it.

math.GT↗

arXiv2019

Tunable magnetization in nanoscale LuFeO3: Role of morphology, ortho-hexa phase ratio and local structure

We have observed enhancement and shift in the spin reorientation transition temperature as a consequence of coexistence of orthorhombic and hexagonal phases and higher aspect ratio in nanoscale LuFeO3. Nanoparticles and nanofibers of LuFeO3 are considered for this work. Nanoparticles have 75 % orthorhombic phase and 25 % hexagonal phase, while nanofibers have 23% orthorhombic phase and 77%-hexagonal phase. Larger aspect ratio in case of nanofibers is seen to help strain-stabilize the hexagonal phase in the material. Magnetic measurements show significant difference in the magnetic behavior and spin reorientation temperature; 183K for the nanoparticle case and 150K for the case of nanofibers. Moreover, the ferromagnetic moment is two order of magnitude higher for nanofibers than that of nanoparticles, In hexagonal phase, frustration of triangular lattice, works against the long range ordering while magnetic anisotropy works in favor of the long range ordering, which contributes towards the enhanced and anomalous magnetic behavior in case of fibers. X -ray absorption near edge spectroscopy (XANES) at the Fe K-edge has been used to probe the symmetry driven dynamics of Fe 3d- 4p orbitals. It established that due to noncentrocymmetry of the Fe atom, the nanofibers have decreased 3d-4p orbital mixing and reduced crystal field splitting energy, which are also contributing factor for the enhanced magnetic behaviour.

cond-mat.mtrl-sci↗

arXiv2018

A Limit Set Intersection Theorem for Graph of Relatively Hyperbolic Groups

Let $G$ be a relatively hyperbolic group that admits a decomposition into a finite graph of relatively hyperbolic groups structure with quasi-isometrically (qi) embedded condition. We prove that the set of conjugates of all the vertex and edge groups satisfy the limit set intersection property for conical limit points. This result is motivated by the work of Sardar for graph of hyperbolic groups.

math.GR↗

arXiv2016

Palindromic widths of graph of groups

We prove that the palindromic width of HNN extension of a group by proper associated subgroups is infinite. We also prove that the palindromic width of the amalgamated free product of two groups via a proper subgroup is infinite (except when the amalgamated subgroup has index two in each of the factors). Combining these results it follows that the palindromic width of the fundamental group of a graph of groups is mostly infinite.

math.GR↗