arXiv subjects

Charul Gupta

Publications and source records attributed to Charul Gupta.

5 recordsLinked to original sources

arXiv2026

Vortex ring formation from the interaction of a cavitation bubble with a confined air bubble: experiments and a timing criterion

We study vortex ring formation arising from the interaction between a cavitation bubble and a confined air bubble in a cylindrical blind hole, using high-speed shadowgraphy imaging. As the cavitation bubble grows above the hole, it drives a downward flow that compresses the air bubble at the base. The air bubble subsequently expands, expelling the overlying liquid column upward as a coherent slug; impact of this slug on the far boundary of the collapsing cavitation bubble produces a vortex ring. Parametric experiments across the dimensionless stand-off distance $\mathcal{H} = h/R_{\max}$ and the air bubble fill fraction $\mathcal{B} = (d_\text{hole} - d_\text{top})/d_\text{hole}$ identify three regimes: (i) liquid column impact during collapse, producing a vortex ring ($\mathcal{H} \lesssim 0.5$, $\mathcal{B} \lesssim 0.5$); (ii) late impact near the end of collapse (large $\mathcal{H}$); and (iii) direct air bubble impact after bypassing the liquid column (large $\mathcal{B}$), with neither (ii) nor (iii) producing a ring. Two one-dimensional models, based on the Rayleigh-Plesset equation and isentropic air bubble expansion, predict the liquid column impact location and its speed $U_\text{lc}$, respectively. A dimensionless timing parameter $\Pi = (h + R_{\max}) / (U_\text{lc} \cdot t_\text{cav}/2)$, comparing the liquid column travel time to the cavitation collapse half-period, distinguishes the three regimes: ring formation occurs for $1 \lesssim \Pi \lesssim 1.5$. The ring propagates from the hole at an initial speed of $5$ m/s, decelerating quadratically, and breaks apart via azimuthal instabilities at $Re \approx 4500$.

physics.flu-dyn↗

arXiv2026

Inertial effects on flow dynamics near a moving contact line

This study investigates the role of inertia in moving contact lines using experiments, theoretical analysis, and numerical simulations. Experiments are conducted using a plate immersion configuration over a wide range of Reynolds numbers from $O(10^{-3})$ to $O(10)$. Flow configurations and quantitative measurements are obtained using high-speed imaging and particle image velocimetry. The streamfunction contours reconstructed from the experimental velocity fields are compared with the viscous modulated wedge solution (viscous-MWS) and inertial-MWS theory. Experimental observations show that the streamfunction contours agree well with viscous predictions at low Reynolds numbers; however, systematic deviations emerge as the Reynolds number increases. The inertial-MWS theory, an inertial extension of the Huh and Scriven framework, accounts for these deviations, but only within a narrow range of Reynolds numbers $10^{-1} < Re < 1$. At higher Reynolds numbers, inertial theory fails to accurately capture the deviations in the streamfunction contours observed in the experiments. Moreover, simulations conducted using the volume of fluid method support our findings, exhibiting deviations in streamfunction contours consistent with experimental observations. We demonstrate that inertia does not fundamentally alter the underlying flow configuration but instead induces a systematic deviation in the streamfunction contours. At finite $Re$, the interfacial speed transitions from a nearly constant value in the viscous regime to a monotonic decay along the interface. These findings expose the need for more sophisticated models of moving contact lines.

physics.flu-dyn↗

arXiv2025

An Investigation of Flow and Interface Dynamics Near a Moving Contact Line at Obtuse Contact Angles

The flow near a moving contact line is primarily governed by three key parameters: viscosity ratio, dynamic contact angle, and inertia. While the behavior of dynamic contact angles has been extensively studied in earlier experimental and theoretical works, quantitative characterization of flow configurations remains limited. The present study reports detailed measurements of flow fields, interface shapes, and interfacial speeds in the low to moderate Reynolds number ($Re$) regimes using particle image velocimetry (PIV) and high-resolution image analysis. The investigation is restricted to dynamic contact angles greater than $90^{\circ}$. In the low-$Re$ regime, excellent agreement is observed between measured streamfunction contours and the modified viscous theory of Huh \& Scriven \cite{huh1971hydrodynamic} that accounts for a curved interface. Theoretical models such as the DRG formulation, using a single fitting parameter, accurately predict interface shapes even at finite $Re$. The interfacial speed away from the contact line compares favorably with theoretical predictions, whereas a pronounced deceleration is observed close to the contact line. Complementary Volume-of-Fluid (VoF) based numerical simulations were performed using identical geometric and material parameters to validate and extend the experimental observations. The simulations successfully reproduce the interface topology, flow structure, and the deceleration of the interfacial velocity near the contact line, providing strong support to the experimental findings. We argue that this rapid reduction in speed, observed both in experiments and simulations, is critical to the resolution of the long-standing moving contact line singularity.

physics.flu-dyn↗

arXiv2024

An experimental investigation of flow fields near a liquid-liquid moving contact line

A moving contact line occurs at the intersection of an interface formed between two immiscible liquids and a solid. According to viscous theory, the flow is entirely governed by just two parameters, the viscosity ratio, $\lambda$, and the dynamic contact angle, $\theta_d$. While a majority of experimental studies on moving contact lines involve determining the relationship between the dynamic contact angle and capillary number, a few studies have focused on measuring the flow field in the vicinity of the contact line involving liquid-gas interfaces. However, none of the studies have considered liquid-liquid moving contact lines and the present study fills this vital gap. Using particle image velocimetry, we simultaneously measure the velocity field in both the liquid phases using refractive index matching techniques. The flow field obtained from experiments in both phases is directly compared against theoretical models. Measurement of interface speed reveals that material points rapidly slow down as the contact line is approached. Further, the experiments also reveal the presence of slip along the moving wall in the vicinity of the contact line suggesting a clear mechanism for how the singularity is arrested at the contact line.

physics.flu-dyn↗

arXiv2023

An experimental study of flow near an advancing contact line: a rigorous test of theoretical models

The flow near a moving contact line depends on the dynamic contact angle, viscosity ratio, and capillary number. We report experiments involving immersing a plate into a liquid bath, concurrently measuring the interface shape, interfacial velocity, and fluid flow using digital image processing and particle image velocimetry. All experiments were performed at low plate speeds to maintain small Reynolds and capillary numbers for comparison with viscous theories. The dynamic contact angle, measured in the viscous phase, was kept below $90^{\circ}$, an unexplored region of parameter space. An important aim of the present study is to provide valuable experimental data using which new contact line models can be developed and validated. Interface shapes reveal that the strong viscous bending predicted by theoretical models is absent in the experimental data. The flow field is directly compared against the prediction from the viscous theory of \cite{huh1971hydrodynamic} but with a slight modification involving the curved interface. Remarkable agreement is found between experiments and theory across a wide parameter range. The prediction for interfacial speed from \cite{huh1971hydrodynamic} is also in excellent agreement with experiments except in the vicinity of the contact line. Material points along the interface were found to rapidly slow down near the contact line, thus alleviating the singularity at the moving contact line. To the best of our knowledge, such a detailed test of theoretical models has not been performed before and we hope the present study will spur new modeling efforts in the field.

physics.flu-dyn↗