## Publications

13 results found

Ray PK, Bouvier D, Papageorgiou DT, 2023, Flow of shear-thinning liquids in channels with superhydrophobic surfaces, *Journal of Non-Newtonian Fluid Mechanics*, Vol: 319, Pages: 1-16, ISSN: 0377-0257

We investigate the influence of shear-thinning on fully-developed flow in channels with a spanwise-periodic array of longitudinal superhydrophobic surfaces (SHSs) on each wall. The Carreau constitutive equation is used to model shear thinning, and the influence of shear thinning on the volume flow rate and slip length are examined. For weakly shear-thinning fluids where the maximum possible reduction in the viscosity is small, asymptotic analysis is used to derive integral expressions for the flow rate and slip length which can be evaluated using the Newtonian solution. As observed in previous related studies, there is a particular Carreau number where the slip length is maximum and the SHSs are most effective. Numerical simulations of the full momentum equation are used for strongly shear-thinning liquids, and Carreau model parameters are chosen to match measurements for whole blood and a xanthan gum solution. The slip length again reaches a maximum for a particular Carreau number, and the explanation for these peaks is similar for both weakly- and strongly-shear thinning liquids. At small Carreau numbers, high values of shear are needed to reduce the viscosity and increase the flow rate. Such shear is generated at the SHS groove edges where there is an abrupt change in the boundary conditions. In the absence of SHSs, there is little shear thinning at low Carreau numbers since the shear rate is relatively small. When the Carreau number is large, there is less advantage to using SHSs as the shear generated at the solid wall of a conventional channel is sufficiently large for tangible viscosity reduction and flow enhancement. This difference for low and large Carreau numbers results in a peak slip length at an “intermediate” Carreau number (typically between 1 and 3). Reducing the solid fraction leads to large increases in the shear generated near the solid wall which, in turn, leads to substantial increases in the slip length. Modifying the deformation o

Vigna-Gómez A, Murillo J, Ramirez M,
et al., 2023, Design and analysis of tweet-based election models for the 2021 Mexican legislative election, *EPJ Data Science*, Vol: 12, Pages: 1-17, ISSN: 2193-1127

Modelling and forecasting real-life human behaviour using online social media is an active endeavour of interest in politics, government, academia, and industry. Since its creation in 2006, Twitter has been proposed as a potential laboratory that could be used to gauge and predict social behaviour. During the last decade, the user base of Twitter has been growing and becoming more representative of the general population. Here we analyse this user base in the context of the 2021 Mexican Legislative Election. To do so, we use a dataset of 15 million election-related tweets in the six months preceding election day. We explore different election models that assign political preference to either the ruling parties or the opposition. We find that models using data with geographical attributes determine the results of the election with better precision and accuracy than conventional polling methods. These results demonstrate that analysis of public online data can outperform conventional polling methods, and that political analysis and general forecasting would likely benefit from incorporating such data in the immediate future. Moreover, the same Twitter dataset with geographical attributes is positively correlated with results from official census data on population and internet usage in Mexico. These findings suggest that we have reached a period in time when online activity, appropriately curated, can provide an accurate representation of offline behaviour.

Sharma A, Ray PK, Papageorgiou DT, 2019, Dynamics of gravity-driven viscoelastic films on wavy walls, *Physical Review Fluids*, Vol: 4, Pages: 063305-1-063305-26, ISSN: 2469-990X

The linear stability and nonlinear dynamics of viscoelastic liquid films flowing down inclined surfaces with sinusoidal topography are investigated. The Oldroyd-B constitutive model is used and numerical solutions of a long-wave nonlinear evolution equation for the film thickness, introduced by Dávalos-Orozco [L. A. Dávalos-Orozco, Stability of thin viscoelastic films falling down wavy walls, Interfacial Phenom. Heat Transfer 1, 301 (2013)], provide insight into the influence of elasticity and wall topography on the nonlinear film dynamics, while Floquet analysis of the linearized evolution equation is used to study the onset of linear instability. Focusing initially on inertialess films (with zero Reynolds number), linear stability results are organized into three regimes based on the wall wavelength. For sufficiently short and sufficiently long wall wavelengths, the onset of instability is not tangibly affected by the topography. There is however an intermediate range of wavelengths where, as the wall wavelength is increased, the critical Deborah number for the onset of instability first decreases (topography is destabilizing) and then increases sufficiently for topography to be stabilizing (relative to the flat wall). Solutions to a perturbation amplitude equation indicate that the character of the instability changes substantially within this intermediate range; topography induces streamwise variations in the base-state velocity at the free surface which couple with perturbations and substantially influence the instability growth rate. Very similar trends are observed for Newtonian films and variations in the critical Reynolds number. Simulations of the full nonlinear evolution equation produce a broad range of nonlinear states including traveling waves, time-periodic waves, and chaos. Perturbations to the film generally saturate at higher amplitudes for cases with larger linear growth rates, e.g., with increasing Deborah number or for a destabiliz

Ray PK, Hauge J, Papageorgiou D, 2017, Nonlinear interfacial instability in two-fluid viscoelastic Couette flow, *Journal of Non-Newtonian Fluid Mechanics*, Vol: 251, Pages: 17-27, ISSN: 0377-0257

Weakly-nonlinear interfacial instabilities in two-fluid planar Couette flow are investigated for the case where one layer is thin. Taking this thin-layer thickness as a small parameter, asymptotic analysis is used to derive a nonlinear evolution equation for the interface height valid for wavelengths that scale with the channel height. Consequently, the influence of the thick layer is felt through a non-local coupling term which is obtained by solving a system of linear equations which are a simplified viscoelastic analogue to the Orr–Sommerfeld equation. The evolution equation allows for the clear identification of the influence of normal stresses at the interface on both the initial instability and the subsequent nonlinear dynamics. Results from numerical simulations illustrate: (1) an array of non-stationary states including traveling waves and chaos, (2) competition between elastic instability and instability due to viscosity stratification, and (3) the accuracy of a simplified ’localized’ evolution equation (derived using a long-wave approximation to the coupling term) when either the elasticity of the thick-layer fluid is sufficiently weak or the elasticities of the two fluids are sufficiently well-matched.

Ray PK, Zaki TA, 2015, Absolute/convective instability of planar viscoelastic jets, *PHYSICS OF FLUIDS*, Vol: 27, ISSN: 1070-6631

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Ray PK, Zaki TA, 2014, Absolute instability in viscoelastic mixing layers, *PHYSICS OF FLUIDS*, Vol: 26, ISSN: 1070-6631

Mahak M, Tucker PG, Ray PK, 2012, Cost-Effective Hybrid RANS-NLES Method for Jet Turbulence and Noise Prediction, ASME 2012 Gas Turbine India Conference, Publisher: American Society of Mechanical Engineers

<jats:p>Jets at higher Reynolds numbers have a high concentration of energy in the small scales in the nozzle vicinity. This is challenging for LES, potentially placing severe demands on grid density. To circumvent this, we propose a novel procedure based on well known Reynolds number (Re) independence of jets. We reduce the jet Re whilst rescaling the boundary layer properties to maintain incoming boundary layer thickness consistent with high Re jet. The simulations are carried out using hybrid largeeddy simulation type of approach which is incorporated by using near wall turbulence model with modified properties. No Subgrid Scale (SGS) model is used in these simulations. Hence, they effectively become Numerical Large Eddy Simulation (NLES) with Reynolds-averaged Navier-Stokes (RANS) covering the full boundary layer region. The noise post processing is carried out using Ffowcs-Williams-Hawking (FWH) approach. The simulations are made for Mach numbers (M) of 0.75 and 0.875. The results for Overall Sound Pressure Level (OASPL) are observed to be within 2–3% accuracy range and directivity of sound is also captured accurately for both the cases. The low Re simulations hence, can be more beneficial in saving time and cost of the simulation while providing reasonably accurate results.</jats:p>

Tucker P, Eastwood S, Klostermeier C,
et al., 2012, Hybrid LES Approach for Practical Turbomachinery Flows—Part II: Further Applications, *Journal of Turbomachinery*, Vol: 134, ISSN: 0889-504X

<jats:p>A hybrid large eddy simulation (LES) related technique is used to explore some key turbomachinery relevant flows. Near wall Reynolds-averaged Navier-Stokes (RANS) modeling is used to cover over especially small scales, the LES resolution of which is generally intractable with current computational power. Away from walls, large eddy type simulation is used but with no LES model (numerical LES (NLES)). Linking of the two model zones through a Hamilton–Jacobi equation is explored. The hybrid strategy is used to predict turbine and compressor end wall flows, flow around a fan blade section, jet flows, and a cutback trailing edge. Also, application of NLES to the flow in an idealized high pressure compressor drum cavity is considered. Generally, encouraging results are found. However, challenges remain, especially for flows where transition modeling is important.</jats:p>

Tucker P, Eastwood S, Klostermeier C, et al., 2010, Hybrid LES Approach for Practical Turbomachinery Flows: Part 2—Further Applications, ASME Turbo Expo 2010: Power for Land, Sea, and Air, Publisher: ASMEDC

<jats:p>A hybrid Large Eddy Simulation (LES) related technique is used to explore some key turbomachinery relevant flows. Near wall RANS modeling is used to cover over especially small scales, the LES resolution of which is generally intractable with current computational power. Away from walls, large eddy type simulation is used but with no LES model (NLES). Linking of the two model zones through a Hamilton-Jacobi equation is explored. The hybrid strategy is used to predict turbine and compressor endwall flows, flow around a fan blade section, jet flows and a cutback trailing edge. Also, application of NLES to the flow in an idealized high pressure compressor drum cavity is considered. Generally, encouraging results are found. However, challenges remain, especially for flows where transition modeling is important.</jats:p>

Ray P, Dawes W, 2009, Detached-Eddy Simulation of Transonic Flow Past a Fan-Blade Section, 15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference), Publisher: American Institute of Aeronautics and Astronautics

Ray PK, Cheung LC, Lele SK, 2009, On the growth and propagation of linear instability waves in compressible turbulent jets, *Physics of Fluids*, Vol: 21

RAY PK, LELE SK, 2007, Sound generated by instability wave/shock-cell interaction in supersonic jets, *Journal of Fluid Mechanics*, Vol: 587, Pages: 173-215, ISSN: 0022-1120

<jats:p>Broadband shock-associated noise is an important component of the overall noise generated by modern airplanes. In this study, sound generated by the weakly nonlinear interaction between linear instability waves and the shock-cell structure in supersonic jets is investigated numerically in order to gain insight into the broadband shock-noise problem. The model formulation decomposes the overall flow into a mean flow, linear instability waves, the shock-cell structure and shock-noise. The mean flow is obtained by solving RANSequations with a <jats:italic>k</jats:italic>-ε model. Locally parallel stability equations are solved for the shock structure, and linear parabolized stability equations are solved for the instability waves. Then, source terms representing the instability wave/shock-cell interaction are assembled and the inhomogeneous linearized Euler equations are solved for the shock-noise.Three cases are considered, a cold under-expanded <jats:italic>M<jats:sub>j</jats:sub></jats:italic> = 1.22 jet, a hot under-expanded <jats:italic>M<jats:sub>j</jats:sub></jats:italic> = 1.22 jet, and a cold over-expanded <jats:italic>M<jats:sub>j</jats:sub></jats:italic> = 1.36 jet.</jats:p><jats:p>Shock-noise computations are used to identify and understand significant trends in peak sound amplitudes and radiation angles. The peak sound radiation angles are explained well with the Mach wave model of Tam & Tanna <jats:italic>J. Sound Vib</jats:italic>. Vol. 81, 1982, p. 337). The observed reduction of peak sound amplitudes with frequency correlates well with the corresponding reduction of instability wave growth with frequency. However, in order to account for variation of sound amplitude for different azimuthal modes, the radial structure of the instability waves must be considered in additionto streamwise growth. The effect of heating on

Ray PK, Cheung LC, Lele SK, 2004, On sound generated by instability wave--shock cell interaction in supersonic jets, 10th AIAA/CEAS Aeroacoustics Conference

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