Bubbly flow physics for applications in aerated hydroturbines and underwater transport

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My doctoral research aims at speed enhancement and controllability of next-generation high–speed underwater vehicles for naval defense applications and environment-friendly power generation through next-generation aerating hydroturbines. These research objectives are addressed through the investigations into fundamental fluid dynamics of bubbles at different size scales when air is entrained inside water. A large bubble is used to envelop an underwater vehicle causing tremendous reduction in flow resistance while the small bubbles are employed for aeration applications in a hydroturbine. Our experiments have provided critical insights into the design and development of operational strategies and models for these novel technologies. Specifically, a novel aspect of my research pertains to the visualization and quantification of the internal flows inside supercavity bubbles and drops. In order to get physical insights into such internal flows, we are currently developing techniques to carry out particle tracking across such gas-liquid interfaces.

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Underwater ventilation refers to the entrainment of an air stream inside water, conventionally employed in a number of applications ranging from aeration in common household aquariums to wastewater treatment, ship hull drag reduction etc. My doctoral research focusses on two very novel applications of underwater ventilated flows: supercavitation and bubbly flows. Both these applications employ bubbles at different size scales: supercavitation regime deals with bubbles at large size range, whereas small size bubbles are of concern in the bubbly flow regime. The drag-reduction capability of supercavitation can be best illustrated through the fascinating underwater motion of penguins as shown in Figure 1. The typical speed of a penguin underwater ranges from 1 – 3 meters/sec. However, when chased by a predator, a penguin can triple its speed in short bursts to escape or launch itself out of water at speeds as high as 9 meters/sec. It performs this neat trick by fluffing its feathers to use air to generate a buffer between their bodies and the water around them. The air entrained in the feathers (during diving) is broken into 

small microbubbles and covers the majority of the penguin’s body, thus allowing the penguin to cruise effortlessly inside an air cavity of bubbles. This is so because the friction drag experienced by a body inside water is significantly higher than the drag experienced in air (compare the ease in moving your hand in and outside water!). Evidently, this is a major bottleneck in the attainment of high speeds by underwater vehicles as compared to their aerial counterparts (For instance, the speed of a typical commercial airplane is 200 – 300 meters/sec while peak speed of a submarine stands at a mere 13 meters/sec). It is clear then that similar to penguins, the swift movement of vehicles underwater necessitates a large air cavity (also called a ‘supercavity’) which can envelop the entire vehicle. This is typically done by blowing some non-condensable gas like air or rocket exhaust at the front part of the vehicle body as shown in Figure 2.This technology, called ‘ventilated supercavitation’ is of immense promise for naval applications and underwater travel (For more insight, please refer to our research featured in the WIRED magazine).

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However, there are several challenges involved in taking this technology from a concept to reality. The first challenge relates to the prediction of ventilation demand required to establish a supercavity under different flow conditions. This is crucial since it determines the amount of gas to be carried on-board. The second challenge pertains to the determination of ventilation demand to sustain a supercavity under different sea-states, particularly unsteady state conditions caused due to wave train on the sea surface. The vehicle might encounter unsteadiness at various phases of operation and under variable environments. Such unsteady phenomena can increase the ventilation demand and cause wetting of the body, resulting in increased drag, cavity breakup or even damage to the vehicle (Imagine the vehicle fins colliding with water at a speed of 300 meters/second!). These need to be avoided through a series of control strategies under various flow environments(e.g. vehicle rising or dipping under the ocean, vehicle riding under the effect of surface waves etc.). Such optimal control strategies are governed by distinct factors such as energy efficiency, maneuverability or noise reduction (a supercavity formation suppresses underwater noise caused by the vehicle). Finally, the most important and intricate challenge is vehicle control and maneuverability. Here, total ventilation demand is not the only parameter of interest, but it is of great significance to understand how the ventilation gas distributes itself inside the cavity and how the supercavity closes at the rear portion. This knowledge can enable us to steer the vehicle safely or to further reduce the ventilation demand for maintaining a supercavity, provided the ventilation gas is entrained at some ‘strategic’ sites of the vehicle. To locate such ‘selective’ ventilation sites requires an in-depth understanding of the flow and pressure distribution within a cavity under various unsteady conditions. The proposed research envisages a study of the flow inside a cavity in sync with the unsteady cavity pressure measurements to fully characterize such processes and suggest control strategies for the operation of underwater vehicles.

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Moving over to the bubble formation and physics at small size scale, my doctoral research also investigates bubbly flow which is relevant for applications in next-generation aerated hydroturbines. Aeration in hydroturbines is of great concern and interest because the water discharged by hydropower facilities is of increasing concern due to its effect on downstream water quality. This decreased water quality arises from two different mechanisms: increased dissolved gases such as nitrogen over high spillways and greatly diminished oxygen content in the water discharged from hydroturbines to the downstream environment. The impoundments necessary for creating the hydraulic head to operate conventional hydroturbines can degrade water quality. The residence time of water within these reservoirs is long and processes such as respiration by aquatic plant and animal life, biodegradation of organic materials in the sediments, oxygen-consuming chemical reactions, etc. can decrease the DO levels, especially at greater depths (i.e. the hypolimnion) within the reservoir. Thermal stratification due to solar heating enhances conditions for low DO in the hypolimnion. Such a system, being hydrodynamically stable, inhibits mixing between layers and isolates the bottom water from atmospheric oxygen. Only surface waters are replenished with oxygen through gas transfer processes resulting from wave action. As shown in Figure 3, Hydropower projects often have hydroturbine intakes located in the hypolimnion where DO levels may drop to anoxic conditions. Hypolimnetic anoxia in turn leads to trace metals, nutrients, and hydrogen sulfide being released from sediments and a drop in the pH of the water endangering fish and other aquatic life in downstream rivers. US Environmental Protection Agency has set minimum standards for dissolved oxygen content in the downstream water.  

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The auto venting turbine (AVT) has been proposed as a cost-effective and promising technology that can be employed to mitigate the problems associated with low DO concentration in the hydroelectric releases. The AVT is a self-aspirating hydroturbine designed to aerate the turbine discharge through ports located at low pressure regions (See Figure 4), which are connected to the atmosphere. Air released to the water from these ports breaks up into small bubbles due to the water's high velocity and turbulence. While oxygen transfer is augmented by the high interfacial area of these bubbles, there is a concomitant decrease in the turbine efficiency due to the increased void fraction in the flow. Relatively limited research has been undertaken on optimizing the performance of auto venting systems. Broadly, there are three important factors influencing the performance of an AVT : the quantity of entrained air, the bubble sizes resulting from competing breakup and coalescence processes, and the rate of oxygen transfer from the bubbles. In my thesis, I have investigated these different factors in detail, beginning with the development of required measurement and post-processing techniques.

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Research done

         Supercavitation regime: I began my doctoral research by exploring the physics of ventilation processes in a supercavity. My initial research on different models of supercavitating vehicles showed that the ventilation required to sustain a supercavity is significantly less compared to the amount of ventilation required to generate it. This discovery was truly revolutionary in terms of its impact on the reduced ventilation gas requirement to be carried on-board in a vehicle. However, the reason for it was not well understood until I investigated this phenomena with a critical emphasis on cavity closure. The supercavity closure, although studied since last fifty years, was not well understood since most of the researchers present empirical observations without investigating the fundamental physics. As a result, a lot of discrepancies exist in the literature regarding the factors that determine supercavity closure. In a novel research (published in Journal of Fluid Mechanics), I put forward three major contributions to the field of supercavitation [1]: (i) I presented a hypothesis and experimental proof of the phenomena that led to less ventilation to sustain a supercavity than to generate it. (ii) I proposed a unified theory of predicting supercavity closure that reconciled all the observations on closure since last five decades in a single whole, and (iii) I discovered new closure mechanisms (as shown in Figure 5), which further corroborated the proposed theory. This was followed by another important contribution to the field that provides a unique insight and caution for the operation of underwater vehicles moving close to the sea surface. To study this, I designed an experimental setup for replicating sea-states and studying the effect of sea waves on supercavity motion and stability. My experiments revealed that such a flow unsteadiness, unexpectedly causes change in supercavity closure and the ventilation demand [2].

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My investigations into supercavitation phenomena till now have been truly insightful and suggest the need for carrying out physics based measurements in the internal regions of supercavity. Towards this end, I have already started designing experiments to study the variation of flow structures and pressure distribution within the supercavity, with some preliminary success. The completion of this ongoing study will not only enrich the field of supercavitation but also mark the end of my thesis research.

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Bubbly Flow regime: As far as second goal of my thesis is concerned, I began my research by developing tools to study and characterize the dense bubbly flows. Bubbly flows are ubiquitous in various engineering and industrial applications. The extraction of the information of size, shape, number of bubbles and bubble velocity field in a dense bubbly flow is challenging owing to the limitations in experimental techniques. Further, the conventional image analysis techniques are computationally intensive and can sometimes take more than several minutes per image, making real-time monitoring of such flows impossible. My thesis research thus began with the development of suitable experimental and analysis techniques to characterize such high void-fraction flows. I employed Shadow imaging velocimetry technique in my experiments and developed an integrative image analysis technique to monitor such dense bubbly flows in real time by bringing down the analysis time of bubble images to less than a second per image. This measurement technique has been published in Chemical Engineering Science [3]. Thereafter, I used this technique for quantitative analysis of aeration statistics and capabilities for turbine blade hydrofoil designs, providing crucial datasets for the design of auto-vented turbines which can be used by turbine manufacturing companies to validate their simulations [4,5]. However, the turbine industry is also interested in getting a theoretical benchmark for bubble size and its corresponding efficiency of oxygen replenishment in the wake. Thus, I took my research a step ahead in addressing the needs of the industry and developed a mathematical oxygen transfer model to determine bubble-water oxygen transfer in the wake of auto-venting hydroturbine [6]. Following this, I studied the bubble breakup and coalescence mechanisms in great depth to develop a theoretical framework to predict bubble size in the wake [7]. In conjunction with the experimental datasets, these two models will amply address the needs of the industry in designing auto-venting turbines.

 Please take a look at my publications to get the relevant references.

Ongoing/Future Research

My ongoing research concerns with the supercavitation regime of bubbly flow. I am currently studying the close inter-relation between air entrainment, internal flow within a supercavity and supercavity closures under both steady and unsteady sea states. Our recent study has examined interesting behaviors in the formation and collapse behaviors of a ventilated supercavity under steady and unsteady flow conditions and points out the crucial factors that need to be taken into account for the estimation of gas storage requirements [8]. In our previous studies, we have underscored the importance of supercavity internal flow as a determining factor in phenomenon like ventilation hysteresis, or closure transitions etc. The current focus of my research is to substantiate this hypothesis by conducting systematic experiments to visualize the internal flow structures of a supercavity for different closures. The change in supercavity internal flow during closure transitions is of particular interest. Figure 6 below shows a schematic for our ongoing experiments.

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Essentially, the above setup shows a schematic for a 2-view particle tracking technique. Another important factor to be kept in mind is the need for correcting the optical distortion caused due to the presence of an air-water interface. Currently, we are also working on developing a ray optics based technique to correct the distorted particle images. 

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One of the major problems encountered, however, is the reduced light intensity inside the supercavity as can be seen from Figure 7 above. The insufficient light intensity makes it difficult to carry out the particle tracking experiments inside a supercavity. Although attempts are underway to circumvent this limitation using several approaches, we are also exploring the possibility of characterizing internal flows using our experimental and distortion-correction technique in the phenomenon of liquid drop impacting a surface. Future work involves application of these techniques to a supercavity under different regimes. It is hoped that these novel flow visualization and quantification techniques inside a supercavity will yield significant insights into the supercavity flow physics. 

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My current research also hinges around characterizing the internal motions of liquid drops upon impact with a solid surface. In this study, we employ laser induced fluorescence and particle tracking to visualize and quantify the flow structures developed inside an impacting drop. Further, we have developed a distortion correction algorithm for the particle images used for tracking. The goal of our experiments is to relate the internal kinetic energy developed within a drop to the kinetic energy deficit observed between the impact and recoil. Our experiments have shown the dependence of such internal flow structures on a non-dimensional “impact factor” (a function of Reynolds and Weber numbers), with the number of vortices developed inside the drop increasing with increase in impact factor, eventually causing separation into droplets and splash.