Date

2015

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Banerjee, Arindam

Other advisers/committee members

Rockwell, Donald; Neti, Sudhakar; Diplas, Panos

Abstract

A marine hydrokinetic turbine (MHkT) operating in its natural environment is subjected to dynamic effects due to variations in its operating conditions. Though the flow velocity and direction are predictable and do not undergo drastic variations as in wind turbines (wind gusts and changes in wind direction), a denser working medium due to placement in water imposes higher structural loading on turbine blades. Furthermore, a variation in water depth alters the turbine depth of immersion during operation. These dynamic conditions change the ratio between the turbine rotation disc (area) and channel area that affects blockage and hence the performance characteristics. Furthermore, variations in blade tip clearance from free surface is hypothesized to affect flow-field and performance characteristics of a marine hydrokinetic turbine, especially those operating in a shallow channel. Significant flow acceleration occurs in and around the turbine rotation plane; the magnitude of which depends on size of the turbine relative to the channel cross-section and is commonly referred to as solid blockage. In addition, the wake behind the turbine creates a restriction to the flow called wake blockage. Understanding the effects of solid and wake blockages in presence of free surface proximity on performance of MHkT is crucial for deployment and efficient operation of individual turbines. In addition, since the free surface is deformable, it is expected to modify both near and far-wake characteristics behind a turbine which must be accounted for in order to develop efficient models of MHkT farms where an array of such devices are deployed in the river bed.In this dissertation, the dynamic interaction of a stall regulated, fixed pitch, horizontal axis MHkT with its environment is addressed through closely coupled experimental, computational and analytical studies. The analytical model was developed based on blade element momentum theory to investigate effect of rotational speed, flow speed, blade pitch, chord length, twist angle on hydrodynamic and structural performance of turbine. The model was further extended to perform one way fluid-structure interaction analysis and a multi-objective optimization (with Genetic Algorithm) study for improving hydrodynamic and structural performance of river-turbine-prototype. To validate the results of analytical model, a three dimensional coupled- computational fluid dynamics-finite element analysis scheme was developed in ANSYS Workbench. In addition, a coupled experimental and computational study was carried out with the objective of unraveling the influence of boundary proximity and blockage effects on the turbine performance. Experimental efforts consisted of performance measurements with a lab-scale prototype in a water channel at various flow velocities, rotational speeds, and depths of immersions to understand effects of free surface proximity, Reynolds number, and Froude number on power and thrust developed by the turbine. The experimental measurements were complimented by a steady state and transient CFD analysis for flow-field characterization behind the turbine. Further, to quantify the influence of free surface proximity on blockage effects, two different models were developed: first for a closed-top channel, and second an open surface water channel (free surface proximity environment). The blockage effects were quantified in terms of percentage change in flow velocity, power coefficient, and thrust coefficient compared to an unblocked, non-free surface environment. In addition, to understand the mechanism responsible for variation of power coefficient with rotational speed and free surface proximity, stereoscopic particle image velocimetry measurements were carried out in the near-wake region of turbine at various rotational speeds and blade tip-free surface clearances. Time averaged measurements were carried out to determine the ensemble-averaged statistics of flow quantities such as mean velocity, strain rates, Reynolds stresses, and turbulence parameters at various turbine operating conditions. In addition to free-run PIV, a phase locked PIV measurements were carried out in the wake region to study transient phenomena like wake development and propagation process, tip and hub vortices formation and propagation.A reduction in tip-depth of immersion was observed to improve the turbine performance until it reached an optimum depth beyond which a reduction in performance was observed due to free surface interaction with wake and bypass region. For low tip clearance ratios, a significant drop (up to 5 to 10% of channel depth) in free surface was observed (from both experimental investigations and transient CFD analysis) behind the turbine with complex three dimensional flow structures that lead to a skewed wake affecting its expansion and restoration process. The percent change in power coefficient (with respect to unblocked, non-free surface environment) was found to be dependent on flow velocity, rotation speed and free surface to blade tip clearance. Flow field visualization, based on SPIV, showed presence of slower wake at higher rotational velocities and increased asymmetry in wake at high free surface proximity. In addition, significant difference in flow structures was observed between upper and lower bypass regions.

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