We work on a variety of research problems related to energy and the environment. Our primary focus is on understanding the effects of heterogeneity and mixing on porous media flows that are governed by the density and/or concentration differences. The majority of the sub-surface flows like what develops during carbon sequestration or groundwater contamination are characterized by plumes or gravity currents and mixing between the injected and ambient fluids. In our research, we investigate the fundamental nature of these flows using theoretical and experimental techniques. Below are some highlights of the research we do and their applications. Scroll down to the bottom for applications.
Plumes are defined as a primarily vertical buoyancy-driven flow, generated from a local source. The buoyancy difference between the source and ambient fluids may be caused by a difference either in the solute concentration (haline plumes) or temperature (thermal plumes). However, if the source has both concentration and temperature differences, the plume is of thermohaline type. Because of the difference in the concentration or temperature between the plume and ambient fluids, an entrainment of the latter into the former occurs resulting in an increase in the volume flux of the plume. Notice that the plume diameter increases in the downstream direction which is indicative of an entrainment of (clear) ambient fluid into the (dark) plume fluid. In our research work, we try to investigate the behavior of plumes and quantify the extent of entrainment, theoretically and experimentally. We study the early-time and late-time regimes separately and particularly focus on the effects of dispersion rather than diffusion.
Gravity currents are primarily horizontal flow, driven by the density difference between the contaminated and ambient fluids. A gravity current is created by dense fluid in a lighter ambient, over an impermeable or a less permeable boundary. Most of the previous theoretical models assumed that there is a sharp interface between the gravity current and the ambient, such that there is no mixing between the two. However, based on our laboratory experiments we show that the sharp-interface assumption is not valid and, in fact, a significant amount of mixing happens, particularly in heterogeneous porous media. We use light-attenuation based laboratory experiments to quantify these effects.
Mixing of solute in the sub-surface flows, for example in the leakage of contaminants into groundwater, is quantified by a dispersion coefficient that depends on the dispersivity of the medium and the transport velocity. It has been reported from the field data that the dispersivity increases with the scale of flow, especially in a heterogeneous medium. Based on our theoretical model and laboratory experiments, we try to investigate the asymmetry obtained in the anomalous dispersion and quantify the value of dispersivity for various levels of heterogeneity in the medium.
Double-diffusive convection
In case when there are sources of heat and concentration together, double-diffusive convection occurs, which, as the name suggests, consists of both thermal convection as well as solute convection. Both are physically coupled such that the development of one affects the development of the other. We use laboratory experiments and numerical modelling to study this problem, particularly in the context of nuclear waste storage, wherein the case of nuclides leakage may result in such convection.
Groundwater contamination
Leakages from the waste piles are the most common source of groundwater contamination. They are usually heavier than the groundwater and creates dense plumes, which also gets dragged by the background flow. We study the effects of mechanical dispersion on the rate and range of mixing.
Image credit: Talabi & Kayode (JWRP, 2019).
Seawater intrusion into groundwater
Groundwater near the coastal areas is usually high in salt concentration due to the intrusion of seawater. Moreover, they usually happen in the form of gravity currents or longitudinal mechanical dispersion. We estimate the true extent of the contamination in terms of length scale and concentration.
Image credit: USGS (through Google).
With the aim of mitigating global warming, CO2 is captured from the atmosphere and injected underground into permeable aquifers at the depth of around 1000-2000 m. The injected CO2 develops buoyancy-driven plumes and gravity currents in the saturated brine. For the long term feasibility of Carbon Capture and Sequestration, it is important that CO2 mixes with the brine and remains trapped in the aquifers. We study this problem in the context of mixing in buoyancy-driven flows.
Geothermal energy recovery
Geothermal heat is a promising source of green energy. To harvest geothermal energy, cold water is injected through a well into the hot aquifers and extracted through another well. Due to the temperature gradient between the injected and ambient water, buoyancy-driven convection occurs, which plays important role in enhancing the harvesting efficiency. Results from our research are applicable in estimating the harvesting efficiency.
Image credit: CHPM (through IE and Google).
