Physical mechanism of purely elastic instability in plane Poiseuille flow

Neither Elastic turbulence (ET) or turbulent drag drag reduction (TDR) state for polymeric flow enjoys the good theoretical understanding as compared to the Newtonian turbulence. To understand it, I am using a long straight 2-D channel of large aspect ratio (width/height) of 7 with a height of 0.5 mm and length of 1000 height for the investigation of elastic instabilities. I am tracking the evolution of unstable wave and transition mechanism in 2-D channel flows. I am simultaneously using the pressure drop measurement, Laser doppler velocimetry (LDV) and Particle image velocimetry (PIV) to measure the drag, high temporal resolution velocity from LDV and spatially resolved velocity from PIV to understand and couple the flow structure and the flow drag.

I observed elastic wave and non-linear break down of flow structure to lead to elastic turbulence and observed that elastic wave and turbulent state is coupled. I further studied the relation between wall friction and instability at different Weissenburg number (Wi = polymer relaxation time/characteristic flow time) and observed various flow states.

Effect of human passage on air curtain sealing in the doorways of a building

Heat and mass flow between cold and warm environments due to the pressure difference between both sides. This exchange causes a loss of energy and human comfort in the buildings. To minimize this heat and mass flux, an air curtain is often used as an artificial separation barrier. Although air curtains are mostly used to facilitate passage through the doorway, the effect of human and vehicle traffic on the stability and effectiveness of an air curtain is not well understood. We have conducted laboratory experiments to examine the effect of a person passing through the curtain. We measured the flow rate through and the density across the doorway with and without the air curtain to calculate the effectiveness of an air curtain, and find that the effectiveness is decreased by the passage of a person and that the effect increases with increasing walking speed. We visualized the jet and wake using dye to determine how the air curtain is deflected by the passage of a person.

Time resolved particle image velocimetry has also been done to study the flow structure and entrainment due to the interaction of the air curtain and wake of the cylinder. We also observed that the effect is independent of the direction of travel, a result of the relatively fast walking speed compared with the stack-driven exchange flow under normal circumstances. We studied it’s feasibility in increasing containment effectiveness of isolated hospital rooms. Finally, we compared the lab scale measurement with the real scale air curtain measurements at TNO, Delft, Fluent simulation and theoretical models. Subsequently, we studied the effect of heavier curtain fluid on stability and effectiveness of air curtain.

Origin of thin film circular hydraulic jump

I have also worked on experimental investigation of the unsteady behavior of circular and two dimensional hydraulic jump and developed a theoretical model. Surfactant, Acetic-acid, and Propanol are separately mixed in water to vary the surface tension and viscosity of the liquid so that we could study the effect of fluid properties on hydraulic jump.

Interaction of bubbles with vortical structures

Bubbly turbulent flows occur in a variety of industrial, naval and geophysical problems. In these flows, the bubbles in the flow interact with turbulence and/or vortical structures present in the continuous phase, resulting in bubble motion and deformation, and at the same time modifying the turbulence and/or vortical structures. Despite the fact that this has been a subject of interest for some time, mech- anisms of bubble break-up due to turbulence and turbulence modulation due to bubbles are not well understood.

To help understand this two-way coupled problem, we study in my thesis, the interaction of single and multiple bubbles with vortical structures; the thesis being broadly divided in to three parts. In the first part, we study the interaction of a single bubble with a single vortical structure, namely a vortex ring, formed in the continuous phase (water). This may be thought of as a simplified case of the interaction of bubbles with vortical structures in any turbulent flow. We then proceed to study the interaction of a single bubble with vortical structures present in a fully developed turbulent channel flow, and then finally to the case of a large number of bubbles injected in to a fully developed turbulent channel. In all the cases, the bubble motions and deformations are visualized using high speed visualization, while the flow field information is obtained using time-resolved Particle-Image Velocimetery (PIV) in the first two cases, and from pressure drop measurements within the channel in the latter case.

Super-hydrophobic surfaces for drag reduction

I have also worked on sustainability of air pockets in the crevices on super-hydrophobic surface by controlling the air saturation level of the incoming water flow to get sustained drag reduction. It has been shown there that the shape and depth of air water interface affects the wall drag.

Investigating the flow physics of air curtains using experimental and numerical methods

The flow induced by air curtains comprises of competing effects of inertia contained within a planar jet and the buoyancy force acting across it. The relative magnitude of transverse stack effect largely dictates the jet establishment and its evolution over time. In FE²B, we deploy experimental as well as numerical tools to unravel the flow physics associated with such flows of practical importance in environmental sciences. Through conductivity and density measurements, we obtain quantitative estimates of bulk fluid exchange whereas particle image velocimetry provides insight into the flow field evolution. This data is complemented by the Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) based modelling of the air curtain flows. While the numerical solvers are able to provide the three-dimensional velocity and density fields which are more arduous to obtain through experiments, the laboratory measurements provide time resolved data to critically analyse the transient flow patterns.

Spatiotemporal CO₂ monitoring as a step for climate change and global health pandemic research

Humans release carbon dioxide (CO₂) as part of their respiration with an average flow rate of 0.5 m³/hr. Tracking the dynamics of these exhaled CO₂ in buildings can answer many questions related to building ventilation and airborne transmission of diseases. If you ask ChatGPT, Climate change and Global health pandemics are two of the top three challenges for which the world has to be prepared. The concept of building ventilation is closely interrelated with these two big-picture challenges. Therefore, we are monitoring this human exhaled CO₂ in buildings using low-cost sensors in both space and time.

Extracting the physics from spatiotemporal data has its implications in understanding the building design constraints, policy making and infection transmission reduction. Many low-cost CO₂ measurement monitors are designed and developed by our own in our lab and we are calibrating it using tracer gas decay experiments. We installed these developed devices in classrooms for understanding the spatial distribution, and stratification of CO₂. Moreover we are relating temperature and humidity data with CO₂ dynamics.

Acoustic radiation of bubbles and vortices

The core looks at the principles governing and behaviour and flow structure of sound waves as they travel through water, providing an understanding of the sea or ocean acoustic environment. Sound propagation adheres to the same fundamental principle in the air but has distinct differences in water's viscous and elastic nature. The molecular density provides faster wave travelling characteristics for water than air. The sound waves encounter various interfaces leading to reflection and refraction, making it crucial in sonar applications. These sound waves dissipate energy while travelling in water due to absorption and scattering. The higher frequencies are absorbed more readily, leading to effective communication and detection systems change. The sound travelling profile varies with depth, temperature and salinity. This sound speed affects sound propagation and can create sound channels where waves travel long distances with minimal attenuation. Marine life, geological activity, human activities like drilling, sonar operation and ocean currents generate underwater acoustic noise.

Bio-fluid mechanics and cancer dynamics

For a long time, researchers all over the world have been trying hard to understand and fight against cancer, which is a very complicated disease. One big problem they're facing is figuring out how cancer cells can stay alive, move around in the body, and get into blood vessels despite the strong forces of blood flow. Recently, researchers have started using a mix of ideas from biology, physics, and engineering called "bio-fluid mechanics" to help them understand this problem better.

Cloud microphysics

The human relationship with rain is intricate because rain serves both as an inconvenience and a necessity. Therefore, curiosity about the process of rain formation shouldn’t be surprising. The urge to understand rain is so strong that scientists have persisted in their efforts, despite each new study revealing increasing complexity. Initial studies were conducted before a full understanding of the turbulent fluid environment, a defining characteristic of rain clouds, was achieved. Fortunately, the proliferation of advanced experiments has enabled researchers to delve more deeply into the added complexity that air turbulence brings to the rain formation puzzle. Even today, despite the rapid growth in computer processing speed, the wide range of scales involved in the precipitation process requires computing power that may not be available for decades. Equally challenging are experimental observations. Making precise measurements of the small-scale precipitation processes from a platform that must endure the harsh conditions of clouds is dangerous and expensive. However, precisely these types of challenges make this problem so engaging, and this enduring interest has persisted for decades. At present, the scientific community has a solid qualitative understanding of how precipitation forms. However, some parts of the process still lack the necessary understanding to make accurate quantitative predictions. Therefore, at the Fluid Mechanics Laboratory in the Department of Applied Mechanics at IIT Delhi, we are investigating the behavior of droplets in turbulent environments. These investigations are directly relevant to understanding the collision-coalescence process that takes place during the creation of warm rain.

Energy extraction via vortex induced oscillation and vibrations

A novel approach rooted in harvesting energy from wind and current through vortex-induced motions has emerged recently. Our research endeavours align with this paradigm and entail the utilization of flat plates and cylindrical bluff bodies within a controlled wind tunnel environment. The objective is to capitalize on fluid-induced motions to harness energy effectively. In the specific context of flat plate autorotation, we systematically explore the effects of non-dimensional parameters to delineate distinct regimes encompassing oscillations, autorotation, and stationary states. Central to this investigation is the comprehensive characterization of energy extraction mechanisms facilitated by vortex-induced angular motion. The research methodology encompasses meticulous flow visualization and diagnostic techniques to elucidate intricate fluid-structure interactions, vortex-shedding phenomena, and the intricate mechanisms underlying energy transfer.