Research Topics

Modeling Entrainment Rate In Heated Plumes and Jets

In the 2014 report of the Intergovernmental Panel on Climate Change, modeling of atmospheric clouds, which play a crucial role in the Earth’s radiation budget, was seen as one of the biggest challenges. One of the key assumptions in parameterization of atmospheric clouds is that the lateral entrainment rate of surrounding air into the cloud is a constant with height. Moreover, the lateral boundary of a convective cloud is usually assumed to be straight (i.e. that its effective cross-sectional radius does not change with height). However, observations, as well as laboratory experiments, have shown that these assumptions may not hold, and that, in fact, there may be a significant variation of the radius of a cloud, as well as the entrainment rate, with height. The main objective of this work is to formulate consistent models for lateral entrainment rate in plumes and jets that are volumetrically heated above a certain height. The volumetric heating essentially mimics the release of latent heat due to condensation of droplets, which occurs at the base of atmospheric clouds. We are attempting to construct a self-consistent 1-D model for this jet/plume, where the entrainment rate is not prescribed externally. The 1-D cloud model is to be validated against in-house Large Eddy Simulation (LES), as well as observational data. The LES model is also being used to examine the validity of assumptions underlying the 1-D model. The in-house LES solver has been developed at I.I.T. Bombay with Ph.D. student Chandra Shekhar Pant, who is also co-advised by Prof. Amit Agrawal from I.I.T. Bombay.

References

1. Pant, C.S., Bhattacharya, A., (2018) "Evaluation of an energy consistent entrainment model for volumetrically forced jets using large eddy simulations",Physics of Fluids, 30, 105107

2. Pant, C.S., Bhattacharya, A., (2016) "A viscous sponge layer formulation for robust large eddy simulation of thermal plumes", Computers and Fluids, 134, 177-189

Core Annular Flow of Oil and Emulsion

Emulsions are ubiquitous in the processing industry, and typically have very high viscosity (1000-50000 centipose). Pumping such fluids through pipes can require a very high pressure drop (and therefore power input). One solution towards reducing pumping cost is to transport the emulsion as a Core Annular Flows (CAF), in which the high-viscosity fluid is lubricated at the pipe walls by a low-viscosity fluid (e.g. water). The low-viscosity fluid is injected at the entrance of the pipe. The primary research question we are trying to address in the short term is: for a given mass flow rate of the emulsion, how does the optimal pressure drop and power to pump it as a CAF depend on the visco-plastic properties of the emulsion. Towards this goal, my Ph.D. student, Sumit Tripathi, has built an experimental setup with 6 meter long test section for generating CAF, in which emulsion is the core fluid. A mixer has also been fabricated to make upto 50 liter batch of emulsion. Emulsifiers, provided by Orica mining company in Australia, allow us to make high internal phase water-in-oil emulsions, which have especially large viscosity and yield stress. In preliminary results, we have observed that, for low viscosity of the core fluid, the core-annulus interface displays high amplitude waves, whereas, for large viscosity of the core fluid, the interface stays almost straight. For large core viscosity and high flow rates of the annular fluid, we also observe cork-screw waves in the core. These interface patterns in turn affect the pressure drop required to pump the core. The rheology of the emulsion has been studied in detail at Prof. Rico Tabor’s lab at Monash University. The Ph.D. student in this project was co-advised by Prof. Ramesh Singh from I.I.T. Bombay. Fabrication of the setup has been funded by the Orica Mining company, via the I.I.T.B.-Monash Academy.

Video: APS DFD Gallery of Fluid Motion

• 1. ReferencesTripathi, S., Bhattacharya, A., Singh, R., Tabor, R.F. (2017) "Rheological behavior of high internal phase water-in-oil emulsions: Effects of droplet size, phase mass fractions, salt concentration and aging", Chemical Engineering Sciences, DOI:https://doi.org/10.1016/j.ces.2017.09.016

     • Featured on cover page of "Chemical Engineering Sciences"

  2. Tripathi, S., Tabor, R.F., Singh, R., Bhattacharya, A. (2017) "Characterization of interfacial waves and pressure drop in horizontal oil-water core-annular flows", Physics of Fluids, DOI: http://dx.doi.org/10.1063/1.4998428

Formulation of An Efficient Hybrid Method For Simulation Of Particulate Flows

Many instances involving fluid flow around arbitrary shaped particles with small surface features can be found in both nature and technological applications (e.g. dispersed multiphase flows, biological swimmers, particle separation). Conventional CFD methods (e.g. Finite Difference method) may be used for solving such problems; however, for particles having fine surface features, usual CFD methods become computationally intractable due to the requirement for extremely fine grids near the particle surface. We have devised an efficient method to solve such problems, where a Finite Difference solver is coupled to a Boundary Integral (BI) solver. Such an approach allows a coarse finite difference grid even close to the particle surface, since the BI solver is able to resolve the particle surface with high resolution. This method may be used to solve full Navier Stokes equation, and is more efficient than conventional CFD over a range of Reynolds numbers. We are currently working towards making the method even more efficient. This work was performed with the help of an M.Tech student, Tejas Kesarkar.

References

1. Bhattacharya, A., Kesarkar, T., (2016) "Numerical simulation of particulate flows using a hybrid of finite difference and boundary integral methods", Physical Review E., Vol.94, No.4, DOI: 10.1103/PhysRevE.94.043309

Energy Extraction From Vortex Induced Vibration of Bi-stable Bluff Bodies

Energy extraction from fluid flows using Vortex Induced Vibrations (VIV) of flexible structures is being

seen as a viable option for renewable energy generation due to the large efficiency (~30%) demonstrated in

experiments. Moreover, the small size of such devices allow for much higher power extraction density compared

to conventional turbines. In our group, we have been exploring energy extraction via VIV of bi-stable objects.

We are motivated by prior work which shows that bi-stable mass-spring systems can be used to harvest vibrational

energy over a large range of frequencies. Our recent work in this area, involves the study of the vortex-induced

angular oscillation of an elliptical prism, in which the central axis of the prism is fixed (and perpendicular to the flow), and a torsional spring is attached to the prism. This configuration leads to two stable points for the orientation of the prism. We find that autorotation of the prism (which arise due to bi-stability and inertia) can lead to large angular oscillations, which in turn can lead to enhanced power extraction efficiency. Currently, we are studying the same effect for the VIV of a cylinder attached to a bi-stable linear spring, and comparing its power extraction efficiency to a VIV of a cylinder attached to a linear spring. Our goal is to understand the mechanisms due to which a bi-stable spring may enhance the power output efficiency, or increase the range of parameters over which this efficiency is maximized. The Ph.D. student working on this project, Rameez Bahadurshah, is being co-advised by Prof. Rajneesh Bhardwaj from I.I.T.B.

References

1. Bhattacharya, A., Sorathiya, S., (2016) "Power Extraction From Vortex-Induced Angular Oscillations of Elliptical Cylinder", Journal of Fluids and Structures, 63, 140-154

2. Badhurshah, R., Bhardwaj, R., Bhattacharya, A., (2019) "Lock-in regimes for Vortex-Induced Vibrations of a cylinder attached to a bistable spring", Journal of Fluids and Structures, https://doi.org/10.1016/j.jfluidstructs.2019.102697

My PhD and Post-doctoral Work

Modeling turbulence in wall-bounded flows (PhD Work): Turbulent flows consist of a large range of time and length scales. Direct Numerical Simulation of turbulent flows are therefore very expensive. My research on turbulence focuses on developing Large Eddy Simulation (LES) models. In LES, only the large scales of turbulence are evolved, while the effect of the small ("sub-grid") scales on the large scales are modeled. Typically, to construct the sub-grid model, some universal scale-similar behavior needs to be assumed for the small scales. Scale-similar behavior can be justified in isotropic turbulence. In free-shear flows, local isotropy, and therefore, scale-similarity, can be assumed, below a certain length scale. However, for wall-bounded flows, scale similarity breaks down close to the wall, due to the presence of an external length-scale (i.e. distance from the wall). In my research work, I have been developing "Optimal LES" (OLES) models for LES [1], which use stochastic estimation to construct models for the subgrid stresses. OLES models require multi-point turbulence velocity correlations as inputs, and therefore this work also involves efforts towards representing these correlations [2]. I have also been working on the"Near Wall Modeling" problem, which consists of estimating wall shear stresses (e.g. viscous drag) in LES of channel flows using variational methods [3].

References

1. Moser, R.D., Malaya, N.P., Chang, H., Zandonade, P.S., Vedula, P., Bhattacharya, A., Haselbacher, A., (2009), “Theoretically based optimal large-eddy simulation”, Physics of Fluids 21, 105104.

2. Bhattacharya, A., Kassinos, S.C., Moser, R.D., (2008), “Representing anisotropy of two-point second-order turbulence velocity correlations using structure tensors”, Physics of Fluids 20, 101502.

3. Bhattacharya, A., Das, A., Moser, R.D., (2008), “A filtered-wall formulation for large-eddy simulation of wall-bounded turbulence”, Physics of Fluids 20, 115104.

Chemical signaling between self-Propelled objects (Post-doctoral work): In the recent past, experimentalists have been able to synthesize self-propelled nano-rods and environment sensitive micro-capsules. These micro-scale synthetic objects have the potential to mimic biological cells in unprecedented ways, primarily due to their size, and their ability to manipulate chemical signals. Using computational models, it is possible to show novel collective behavior for a collection of environment-sensitive microcapsules that release chemicals, which in turn modulate the adhesion of the surface in their surroundings. Specifically, it is possible for an array microcapsules to move along the surface collectively [1]. Also, if two typesof chemicals are involved, then a stationary array of microcapsules can transmit signals over long distances [2].

References

1. Bhattacharya, A., Balazs, A. C., (2010), “Designing microcapsule arrays that propagate chemical signals”, Physical Review E 82, 021801.

2. Bhattacharya, A., Usta, O.B., Yashin, V.V., Balazs, A., (2009), “Self-sustained motion of a train of haptotactic microcapsules”, Langmuir 25(17), 9644-9647.

Transport of particles via actuated cilia (Post-doctoral Work): Recent advances have made it possible to synthesize arrays of microscopic hair-like filaments that are either self-actuated or, actuated via external magnetic and electric fields. The motion of these actuated artificial cilia is similar to that of biological cilia, and it is possible to functionalize the cilia with "sticky" tips. Thus, artificial cilia can potentially interact with, and manipulate, micron sized particles, either via hydrodynamic forces or via adhesive contact. Using numerical simulations, it can be shown that rigid particles are transported at different rates, depending on the Young's modulus of the cilia and the adhesion level [1]. At a certain range of adhesion and cilia stiffness, the particle is "propelled" by the cilia. For compliant particles, results show that [2], for a given stiffness, floppy particles can travel faster or slower than rigid particles, depending on cilia stiffness.

References

1. Bhattacharya, A., Buxton, G.A., Usta, O. Berk, Balazs, A.C., (2012) “Propulsion and trapping of micro-particles by active cilia arrays”, Langmuir, 28(6), 3217-26.

2. Bhattacharya, A., Balazs, A.C., (2013), "Stiffness-Modulated Motion of Soft Microscopic particles Over Active Adhesive Cilia", Soft Matter DOI:10.1039/C3SM00028A.