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M. M. Bandi

Publications and source records attributed to M. M. Bandi.

At least 19 recordsLinked to original sources

Rician Distribution as a Physically Interpretable Model for Wind-Speed Statistics

The statistics of atmospheric wind variations are commonly modeled using Gaussian or Weibull forms, which often trade physical interpretability against statistical accuracy, especially in the distribution tails. Here we derive a Rician distribution for wind speed from a simple physical model based on two orthogonal Gaussian velocity components with a non-zero mean in the preferred direction. Using wind-speed records from four geographically distinct wind farms, we show that the Rician model consistently outperforms the Gaussian model and remains competitive with the Weibull model. The same behavior persists when the data are partitioned into monthly windows, where the Rician parameters also provide a transparent description of seasonal and geographic variability, compared to Weibull parameters. In addition, the model naturally connects Gaussian-like and Weibull-like regimes through the Rician parameter ratio $\mu/\sigma$, making the Rician distribution a compact and physically interpretable two-parameter model for wind-speed statistics.

physics.data-an

A Two-Regime Statistical Framework for Wind-Power Distributions: From Wind-Speed Fluctuations to Turbine Control

Wind-power variability is a major challenge for the reliable integration of utility-scale wind energy into modern power systems. Although wind-speed statistics are often described by simple parametric distributions, translating these statistics into turbine-level power fluctuations is nontrivial because the relationship between wind speed and power is highly nonlinear and changes across different turbine operating regimes. Here, we develop a two-regime statistical framework for wind-power distributions. In the aerodynamic operating regime, between the cut-in and rated speeds, the turbine power follows an approximate cubic dependence on wind speed. Starting from a physically motivated Rician model for the wind-speed magnitude, we derive an analytical expression for the corresponding wind-power distribution using a nonlinear change of variables. In the control-dominated near-rated regime, where active blade-pitch and generator control regulate the turbine output, the aerodynamic transformation is no longer applicable. Instead, we characterize the power deficit relative to the rated power and show empirically that its continuous tail is well described by a bounded stretched-exponential distribution for both individual turbines and wind-farm ensembles. These results provide a physically interpretable statistical description of wind-power fluctuations across the full operational range of utility-scale wind turbines.

physics.data-an

Collective and nonlinear structure of wind power correlations

We describe the correlation structure of wind power fluctuations in a farm of 80 turbines, sampled over 5 years. We report the presence of universal, collective, and nonlinear correlations, responsible for the excess persistency and intermittency of farm-aggregated power output. A first cross-correlation analysis of turbine production reveals a dynamical scaling transition (\`a la Family-Vicszek) from local decoherence to large-scale turbulence-driven scaling, and responsible for the geographical smoothing effect, previously reported beyond farm scale [M. M. Bandi, Phys. Rev. Lett. 118, 028301 (2017)]. A second bivariate analysis shows the long-range correlation of non-Gaussian features, responsible for their amplification in total farm output. These findings provide a new perspective on wind power variability, highlighting the importance of nonlinear correlations in power production dynamics. By better characterising these fluctuations, our results can inform strategies for grid management, storage optimization, and wind farm design, ultimately improving the integration of wind energy into modern power systems.

physics.flu-dyn

Electrocharged facepiece respirator fabrics using common materials

Face masks in general, and N95 filtering facepiece respirators (FRs) that protect against SARS-Cov-2 virion in particular, have become scarce during the ongoing COVID-19 global pandemic. This work presents practical design principles for the fabrication of electrocharged filtration layers employed in N95 FRs using commonly available materials and easily replicable methods. The input polymer is polypropylene or polystyrene, and can include discarded plastic containers of these materials, and the fabrication setup is based on the cotton candy (CC) principle. The primary parameters underlying the CC principle are translated to simple design rules that allow anyone to construct their own fabrication system from common parts, or employ a commercial cotton candy machine with minimal modifications. Finally, basic characterization results for structural and filtration properties of electrocharged fabrics made using the CC principle are detailed.

cond-mat.soft

Collective dynamics in two-dimensional aggregations with competing interactions

A generic aggregate forming system in two dimensions (2D) is studied using canonical ensemble constant temperature molecular dynamics simulation. The aggregates form due to the competition between short range attraction and long range repulsion of pair-wise interactions. Choosing the appropriate set of interaction parameters, we focus on characterising the collective dynamics in two specific morphologies, {\it viz.} compact and string-like aggregates. We focus on the temporal evolution of the mobility of an individual particle and the dynamic change in its nearest neighbourhood, measured in terms of the Debye-Waller factor ($\bar{u}_i^2$) and the non-affine parameter ($χ$), respectively (defined in the text), and their interrelation over several lengths of observation time $τ_w$. The distribution for both measures are found to follow the relation: $P(x;τ_w) \sim τ_w^{-γ}x$ for the measured quantity $x$. The exponent $γ$ is equal to $2$ and $1$ respectively, for the compact and string-like morphologies following the ideal fractal dimension of these aggregates. A functional dependence between these two observables is determined from a detailed statistical analysis of their joint and conditional distributions. The results obtained can readily be used and verified by experiments on aggregate forming systems such as globular proteins, nanoparticle self-assembly etc. Further, the insights gained from this study might be useful to understand the evolution of collective dynamics in diverse glass-forming systems.

cond-mat.soft

Force Distributions in Frictional Granular Media

We report a joint experimental and theoretical investigation of the probability distribution functions (pdf's) of the normal and tangential (frictional) forces in amorphous frictional media. We consider both the joint pdf of normal and tangential forces together, and the marginal pdf's of normal forces separately and tangential forces separately. A maximum entropy formalism is utilized for all these cases after identifying the appropriate constraints. Excellent agreements with both experimental and simulational data are reported. The proposed joint pdf (which appears new to the literature) predicts giant slip events at low pressures, again in agreement with observations.

cond-mat.soft

Force Chain Evolution in a Two-Dimensional Granular Packing Compacted by Vertical Tappings

We experimentally study the statistics of force-chain evolution in a vertically-tapped two-dimensional granular packing by using photoelastic disks. In this experiment, the tapped granular packing is gradually compacted. During the compaction, the isotropy of grain configurations is quantified by measuring the deviator anisotropy derived from fabric tensor, and then the evolution of force-chain structure is quantified by measuring the interparticle forces and force-chain orientational order parameter. As packing fraction increases, the interparticle force increases and finally saturates to an asymptotic value. Moreover, the grain configurations and force-chain structures become isotropically random as the tapping-induced compaction proceeds. In contrast, the total length of force chains remains unchanged. From the correlations of those parameters, we find two relations: (i) a positive correlation between the isotropy of grain configurations and the disordering of force-chain orientations, and (ii) a negative correlation between the increasing of interparticle forces and the disordering of force-chain orientations. These relations are universally held regardless of the mode of particle motions with/without convection.

cond-mat.soft

Training, Memory and Universal Scaling in Amorphous Frictional Granular Matter

We report a joint experimental and theoretical investigation of cyclic training of amorphous frictional granular assemblies, with special attention to memory formation and retention. Measures of dissipation and compactification are introduced, culminating with a proposed scaling law for the reducing dissipation and increasing memory. This scaling law is expected to be universal, insensitive to the details of the elastic and frictional interactions between the granules.

cond-mat.soft

Hydrodynamic signatures of stationary Marangoni-driven surfactant transport

We experimentally study steady Marangoni-driven surfactant transport on the interface of a deep water layer. Using hydrodynamic measurements, and without using any knowledge of the surfactant physico-chemical properties, we show that sodium dodecyl sulphate and Tergitol 15-S-9 introduced in low concentrations result in a flow driven by adsorbed surfactant. At higher surfactant concentration, the flow is dominated by the dissolved surfactant. Using Camphoric acid, whose properties are {\it a priori} unknown, we demonstrate this method's efficacy by showing its spreading is adsorption dominated.

physics.flu-dyn

Influence of surface tension in the surfactant-driven fracture of closely-packed particulate monolayers

A phase-field model is used to capture the surfactant-driven formation of fracture patterns in particulate monolayers. The model is intended for the regime of closely-packed systems in which the mechanical response of the monolayer can be approximated as a linearly elastic solid. The model approximates the loss in tensile strength of the monolayer as the surfactant concentration increases through the evolution of a damage field. Initial-boundary value problems are constructed and spatially discretized with finite element approximations to the displacement and surfactant damage fields. A comparison between model-based simulations and existing experimental observations indicates a qualitative match in both the fracture patterns and temporal scaling of the fracture process. The importance of surface tension differences is quantified by means of a dimensionless parameter, revealing thresholds that separate different regimes of fracture. These findings are supported by newly performed experiments that validate the model and demonstrate the strong sensitivity of the fracture pattern to differences in surface tension.

cond-mat.soft

Dynamics of a Camphoric Acid boat at the air-water interface

We report experiments on an agarose gel tablet loaded with camphoric acid (c-boat) set into self-motion by interfacial tension gradients at the air-water interface. We observe three distinct modes of c-boat motion: harmonic mode where the c-boat speed oscillates sinusoidally in time, a steady mode where the c-boat maintains constant speed, and a relaxation oscillation mode where the c-boat maintains near-zero speed between sudden jumps in speed and position at regular time intervals. Whereas all three modes have been separately reported before in different systems, we show they belong to a common description. Through control of the air-water surface tension with Sodium Dodecyl Sulfate (SDS), we experimentally deduce the three self-propulsive modes result from surface tension difference between Camphoric Acid (CA) and the ambient surroundings.

physics.flu-dyn

Sensitivity of Granular Force Chain Orientation to Disorder-induced Metastable Relaxation

A two-dimensional system of photoelastic disks subject to vertical tapping against gravity was experimentally monitored from ordered to disordered configurations by varying bidispersity. The packing fraction $ϕ$, coordination number $Z$, and an appropriately defined force chain orientational order parameter $S$, all exhibit similar sharp transition with small increase in disorder. A measurable change in $S$, but not $ϕ~\&~Z$, was detected under tapping. We find disorder-induced metastability does not show configurational relaxation, but can be detected via force chain reorientations.

cond-mat.soft

Monami as an oscillatory hydrodynamic instability in a submerged sea grass bed

The onset of monami ~-- the synchronous waving of sea grass beds driven by a steady flow -- is modeled as a linear instability of the flow. Unlike previous works, our model considers the drag exerted by the grass in establishing the steady flow profile, and in damping out perturbations to it. We find two distinct modes of instability, which we label Mode 1 and Mode 2. Mode 1 is closely related to Kelvin-Helmholtz instability modified by vegetation drag, whereas Mode 2 is unrelated to Kelvin-Helmholtz and arises from an interaction between the flow in the vegetated and unvegetated layers. The vegetation damping, according to our model, leads to a finite threshold flow for both these modes. Experimental observations for the onset and frequency of waving compare well with model predictions for the instability onset criteria and the imaginary part of the complex growth rate respectively, but experiments lie in a parameter regime where the two modes can not be distinguished. % The inclusion of vegetation drag differentiates our mechanism from the previous linear stability analyses of monami.

physics.flu-dyn

A Minimal description of morphological hierarchy in two-dimensional aggregates

A dimensionless parameter $Λ$ is proposed to describe a hierarchy of morphologies in two-dimensional (2D) aggregates formed due to varying competition between short-range attraction and long-range repul- sion. Structural transitions from finite non-compact to compact to percolated structures are observed in the configurations simulated by molecular dynamics at a constant temperature and density. Configurational randomness across the transition, measured by the two-body excess entropy $S_2$, exhibits data collapse with the average potential energy $\bar{\mathcal{E}}$ of the systems. Independent master curves are presented among $S_2$, the reduced second virial coefficient $B_2^*$ and $Λ$, justifying this minimal description. This work lays out a coherent basis for the study of 2D aggregate morphologies relevant to diverse nano- and bio-processes.

cond-mat.soft

Transition in heterogeneous dynamics across the morphological hierarchy in two-dimensional aggregates

Two-dimensional (2D) particulate aggregates formed due to competing interactions exhibit a range of non-equilibrium steady state morphologies from finite-size compact crystalline structures to non-compact string-like conformations. We report a transition in heterogeneous microscopic dynamics across this morphological hierarchy as a function of decreasing long-range repulsion relative to short-range attraction at a constant {\it low} density and temperature. Following a very slow cooling protocol to form steady state aggregates, we show that geometric frustration inherent to competing interactions assures non-ergodicity of the system, which in turn results in long-time sub diffusive relaxation of the same. Analysing individual particle trajectories generated by molecular dynamics, we identify {\it caging} dynamics of particles in compact clusters in contrast to the {\it bonding} scenario for non-compact ones. Finally, by monitoring temperature dependence, we present a generic relation between diffusivity and structural randomness of the aggregates, irrespective of their thermodynamic equilibrium.

cond-mat.soft

Grid-scale Fluctuations and Forecast Error in Wind Power

The fluctuations in wind power entering an electrical grid (Irish grid) were analyzed and found to exhibit correlated fluctuations with a self-similar structure, a signature of large-scale correlations in atmospheric turbulence. The statistical structure of temporal correlations for fluctuations in generated and forecast time series was used to quantify two types of forecast error: a timescale error ($e_τ$) that quantifies the deviations between the high frequency components of the forecast and the generated time series, and a scaling error ($e_ζ$) that quantifies the degree to which the models fail to predict temporal correlations in the fluctuations of the generated power. With no $a$ $priori$ knowledge of the forecast models, we suggest a simple memory kernel that reduces both the timescale error ($e_τ$) and the scaling error ($e_ζ$).

physics.data-an

Fragility and hysteretic creep in frictional granular jamming

The granular jamming transition is experimentally investigated in a two-dimensional system of frictional, bi-dispersed disks subject to quasi-static, uniaxial compression at zero granular temperature. Currently accepted results show the jamming transition occurs at a critical packing fraction $ϕ_c$. In contrast, we observe the first compression cycle exhibits {\it fragility} - metastable configuration with simultaneous jammed and un-jammed clusters - over a small interval in packing fraction ($ϕ_1 < ϕ< ϕ_2$). The fragile state separates the two conditions that define $ϕ_c$ with an exponential rise in pressure starting at $ϕ_1$ and an exponential fall in disk displacements ending at $ϕ_2$. The results are explained through a percolation mechanism of stressed contacts where cluster growth exhibits strong spatial correlation with disk displacements. Measurements with several disk materials of varying elastic moduli $E$ and friction coefficients $μ$, show friction directly controls the start of the fragile state, but indirectly controls the exponential slope. Additionally, we experimentally confirm recent predictions relating the dependence of $ϕ_c$ on $μ$. Under repetitive loading (compression), the system exhibits hysteresis in pressure, and the onset $ϕ_c$ increases slowly with repetition number. This friction induced hysteretic creep is interpreted as the granular pack's evolution from a metastable to an eventual structurally stable configuration. It is shown to depend upon the quasi-static step size $Δϕ$ which provides the only perturbative mechanism in the experimental protocol, and the friction coefficient $μ$ which acts to stabilize the pack.

cond-mat.soft

Shock driven jamming and periodic fracture of particulate rafts

A tenuous monolayer of hydrophobic particles at the air-water interface often forms a scum or raft. When such a monolayer is disturbed by the localized introduction of a surfactant droplet, a radially divergent surfactant shock front emanates from the surfactant origin and packs the particles into a jammed, compact, annular band with a packing fraction that saturates at a peak packing fraction $ϕ^*$. As the resulting two-dimensional, disordered elastic band grows with time and is driven radially outwards by the surfactant, it fractures to form periodic triangular cracks with robust geometrical features. We find the number of cracks $N$ and the compaction band radius $R^*$ at fracture onset vary monotonically with the initial packing fraction ($ϕ_{init}$). However, its width $W^*$ is constant for all $ϕ_{init}$. A simple geometric theory that treats the compaction band as an elastic annulus, and accounts for mass conservation allows us to deduce that $N \simeq 2πR^*/W^* \simeq 4πϕ_{RCP}/ϕ_{init}$, a result we verify both experimentally and numerically. We show the essential ingredients for this phenomenon are an initially low enough particulate packing fraction that allows surfactant driven advection to cause passive jamming and eventual fracture of the hydrophobic particulate interface.

cond-mat.soft