Michael L. Jonson

A Design of a Hydrofoil Family for Current-Driven Marine-Hydrokinetic Turbines

The natural kinetic motion of oceans, rivers, and other bodies of water is a promising resource for sustainable power production. Rotor-based marine and hydrokinetic (MHK) turbines generate electricity from river, tidal, and other water currents, operating analogously to wind turbines in air. An MHK rotor designer can draw upon a vast body of general purpose and wind power specific airfoils, but application specific hydrofoils can more optimally meet the needs of MHK power. We present the MHKF1 family of hydrofoils, designed upon experience drawn from wind turbine airfoils and incorporating hydro-specific considerations. The MHKF1 hydrofoils were developed to balance the following design objectives: (1) basic hydrodynamic performance with lift to drag ratio (l/d) as a key metric, (2) limited sensitivity to soiling because of biofouling concerns and the high cost of maintenance in the marine environment, (3) sufficient thickness for structural efficiency, (4) good stall characteristics, (5) hydrodynamic and geometric compatibility such that the different hydrofoils of the family can be applied on the same rotor blade, (6) low susceptibility to cavitation, and (7) low susceptibility to singing. While the first five criteria are common to wind turbine airfoil design, the last two are specific to operation in water. Cavitation, the formation of bubbles within a fluid, can have numerous detrimental effects including erosion of impinged surfaces, degraded performance, vibration, and noise. The minimum surface pressure of the MHKF1 hydrofoils were managed to reduce the likelihood of cavitation. Singing, a hydroacoustic/hydroelastic phenomenon of the trailing edge of hydrofoils, results in noise and vibration. To suppress singing, trailing edge thicknesses were increased and hydrofoil variants were designed with “anti-singing” profiles. The MHKF1 hydrofoils were developed with a combination of inverse and direct design methods using XFOIL and various routines for parameterizing hydrofoil geometries and surface velocity distributions. Performance was further evaluated with OVERFLOW, a Reynolds averaged Navier Stokes computational fluid dynamics code.Copyright

Influence of blade Solidity on Marine Hydrokinetic Turbines.

Marine hydrokinetic (MHK) devices are currently being considered for the generation of electrical power in marine tidal regions. Turbulence generated in the boundary layers of these channels interacts with a turbine to excite the blades into low-to mid-frequency vibration. Additionally, the self-generated turbulent boundary layer on the turbine blade excites its trailing edge into vibration. Both of these hydrodynamic sources generate radiated noise. Being installed in a marine ecosystem, the noise generated by these MHK devices may affect the fish and marine mammal well-being. Since this MHK technology is relatively new, much of the design practice follows that from conventional horizontal axis wind turbines. In contrast to other underwater turbomachines like conventional merchant ships that have solid blades, wind turbine blades are made of hollow fiberglass composites. This paper systematically investigates the contrast of this design detail on the blade vibration and radiated noise for a particular MHK turbine design.Copyright

AN ACOUSTIC SUPERPOSITION METHOD FOR COMPUTING STRUCTURAL RADIATION IN SPATIALLY DIGITIZED DOMAINS

This paper presents a new method for computing acoustic fields of structural radiators based on a coustic s uperposition methods using m eshless, spatially d igitized d omains (ASMDD). Here the system matrices are assembled knowing only coordinate points in 3D space that describe the geometry of the radiating structure. In contrast to conventional methods, ASMDD does not require numerical, high orders of integration over elemental surfaces to populate system matrices. The system’s Green functions are computed simply between source and receiver locations at their respective points. A new derivation provides an analytical solution for coincident source and receiver points where the Green function is singular. The digital domain of ASMDD is a uniform distribution of points equidistant in the x, y, and z directions. The centroid of each activated voxel (used only as a means for visualizing the 3D surface) represents a point on the structural surface being modeled. Work in this paper exploits the inherent uniformity of neighboring points to formulate a locally determined outward-pointing, surface normal needed for acoustic radiation problems. The ability of the calculated surface normals to model the curvature of the continuous radiating surface depends on the density of the meshless grid, i.e., higher curvature requires higher grid densities. The attractiveness of the digital domain approach is its simplicity for morphing of structural shapes in optimization. Shape iterations in the digitized space reduce to a simple process of activating or deactivating selected points in a contiguous manner depending on the desired shape during an optimization. As an example, the ASMDD formulation is used to compute the modal radiation from a square plate in an infinite and cubic baffle. The ASMDD surface points are shown to blend seamlessly with the surface vibration results of the plate generated via meshless structural dynamics (M eshless L ocal P etrov G alerkin method - MLPG). This is achieved by solving the modal radiated acoustic power from the plate where the surface velocity is specified by the modal results determined by the MLPG method. The sound power calculations are in good agreement with those generated via conventional BEM codes.© 2007 ASME

Unsteady Lift of Thick Airfoils in Turbulent Flow

Measurements of the unsteady lift forces acting on airfoils in turbulent flow were made to determine the effect of thickness on the gust response and validate a previously developed analytical model. A family of NACA 65-series uncambered airfoils with a range of thickness-to-chord ratios were tested in a water tunnel with grid-generated turbulence. Piezoelectric force gages were used to measure the spectral density of the unsteady lift, and the system was calibrated using an impulse force hammer. An accelerometer-based multiple coherence noise removal technique was employed to eliminate background noise contamination from the facility. The experimental results are shown to agree well with an analytical model of the unsteady lift based on turbulence ingestion theory and an incompressible gust response which accounts for airfoil thickness.Copyright

Preferred Vehicle Scaling

Properly designed and geometrically similar small-scale models can provide insight into component and system performance of large vehicles for a variety of engineering disciplines for a fraction of the cost compared to full scale fabrication and testing. Using the density, specific heat, magnetic permeability of the fluid, the rotation rate, and the size, the author provides non-dimensional performance properties broadly encompassing the hydrodynamic, acoustic, electro-magnetic, thermal, and structural fields. The preferred numbers based on a geometric series as devised by Renard are introduced. The author then provides a method for defining a mean and standard deviation of a size differences from a standard geometric series. In particular, statistics for customary and metric material specifications of sheet metal, square and round stock, electrical wire, fasteners, beams, and pipes are quantified. The study concludes that preferred numbers are best for standard material sizes and scales and suggests potential procedures for improving systems engineering.Copyright

Parameterization of a Multi-Directional Tidal Turbine Performance Using Computational Fluid Dynamics

High resolution RANS CFD analysis is performed to support the design and development of the Ocean Renewable Power Company (ORPC) TidGen™ multi-directional tidal turbine. Two-dimensional and three-dimensional unsteady, moving-mesh CFD is utilized to parameterize the device performance and to provide guidance for device efficiency improvements.The unsteady CFD analysis was performed using a well validated, naval hydrodynamic CFD solver and implementing dynamic overset meshes to perform the relative motion between geometric components. This dynamic capability along with the turbulence model for the expected massively separated flows was validated against published data of a high angle of attack pitching airfoil.Two-dimensional analyses were performed to assess both blade shape and operating conditions. The blade shape performance was parameterized on both blade camber and trailing edge thickness. The blades shapes were found to produce nearly the same power generation at the peak efficiency tip speed ratio (TSR), however off-design conditions were found to exhibit a strong dependency on blade shape. Turbine blades with the camber pointing outward radially were found to perform best over the widest range of TSR’s. In addition, a thickened blade trailing edge was found to be superior at the highest TSR’s with little performance degradation at low TSR’s.Three-dimensional moving mesh analyses were performed on the rotating portion of the full TidGen™ geometry and on a turbine blade stack-up. Partitioning the 3D blades axially showed that no sections reached the idealized 2D performance. The 3D efficiency dropped by approximately 12 percentage points at the peak efficiency TSR. A blade stack-up analysis was performed on the complex 3D/barreled/twisted turbine blade. The analysis first assessed the infinite length blade performance, next end effects were introduced by extruding the 2D foil to the nominal 5.6m length, next barreling was added to the straight foils, and finally twist was added to the foils to reproduce the TidGen™ geometry. The study showed that making the blades a finite length had a large negative impact on the performance, whereas barreling and twisting the foils had only minor impacts. Based on the 3D simulations the largest factor impacting performance in the 3D turbine was a reduction in mass flow through the turbine due to the streamlines being forces outward in the horizontal plane due to the turbine flow resistance. Strategies to mitigate these 3D losses were investigated, including adding flow deflectors on the turbine support structure and stacking multiple turbines in-line.Copyright

Unsteady Force Measurement for a Beam Using Small Piezoelectric End Sensors

A new method to measure the total unsteady lift force across a propeller blade is presented in this paper. Unsteady forces across propeller blades are generated from the interaction of the blade boundary with a rotating pressure field associated with the propeller. The oscillating nature of the unsteady forces, particularly at higher harmonics, suggests that the unsteady lift fluctuations nearly cancel out over the blade span, and that it is possible to find the total unsteady force across the propeller from parameters at the root and tip. These parameters were determined from an approximation provided by the Method of the Stationary Phase. A newly designed apparatus for the measurement of total unsteady force across a propeller blade based on this theory is described in detail. For future experimental validation of the newly designed sensors, a propeller blade is modeled as a uniform beam, and a known unsteady force is generated across the beam surface.Copyright

Fluctuating Shear Stress Calibration Method Using a Channel Flow

Fluctuating wall shear stress causes vibration and radiated noise from a structure. In the past wall shear stress has been measured indirectly using hot wires and hot films. Recently direct shear sensors have been developed. In this paper a calibration device consisting of a 305 mm × 60 mm × 5 mm channel filled with glycerin is used to calibrate a direct shear stress sensor with amplitudes up to 10 Pa of shear stress over a frequency range from 10 Hz to 1 kHz. The analytically known flow field caused by an oscillating plate 5 mm from the sensor is verified using laser Doppler velocimetry (LDV). The flow field is derived using a frequency-wavenumber approach thereby allowing for a known spatial and temporal field to be generated by specifying a derived plate vibration.© 2010 ASME

Numerical Characterization of the Background Noise Generated by the ARL/Penn State Garfield Thomas Water Tunnel Impeller

The Pennsylvania State University Applied Research Laboratory has a 1.22 meter (48 inches) diameter closed loop water tunnel. The flow is driven by an axial pump with three blade rows and powered by a 1.5 MW motor. The four blades of the impeller have the highest relative velocities of any lifting surface in the facility, and generate input acoustic power which acts as a background noise floor in tunnel measurements of turbomachinery and vehicle body performance. In this paper, we investigate the sound power radiated by the impeller and its propagation through the water tunnel. Trailing edge hydrodynamic forcing functions are computed for the impeller based on local relative velocities and turbulence properties. For lower frequencies, these forces are then applied to an experimentally validated structural finite element model (FEM) of the impeller. The input power to the water tunnel is determined using an acoustic boundary element model (BEM) of the impeller. A statistical energy (SEA) model of the water tunnel allows for an estimate of the rms pressure within the water tunnel test section to provide guidance on lowering background noise.Copyright

The Influence of Flow Instability on the Lock-In of Distributed Elastic Resonators

Lock-in occurs between many different types of flow instabilities and structural-acoustic resonators. Factors that describe the coupling between the fluid and structure have been defined for low flow Mach numbers. This paper discusses how different flow instabilities influence lock-in experimentally and analytically. A key concept to the lock-in process is the relative source generation versus dissipation. The type of fluid instability source dominates the generation component of the process, so a comparison between a cavity shear layer instability with a relatively stronger source, for example wake vortex shedding from a bluff body, will be described as a coupling factor. In the fluid-elastic cavity lock-in case, the shear layer instability produced by flow over a cavity couples to the elastic structure containing the cavity. In this study, this type of lock-in was not achieved experimentally. A stronger source, vortex shedding from a bluff body however, is shown experimentally to locks into the same resonator. This study shows that fluid-elastic cavity lock-in is unlikely to occur given the critical level of damping that exists for a submerged structure and the relatively weak source strength that a cavity produces. Also in this paper, a unified theory is presented based on describing functions, a nonlinear control theory used to predict limit cycles of oscillation, where a self-sustaining oscillation or lock-in is possible. The describing function models capture the primary characteristics of the instability mechanisms, are consistent with Strouhal frequency concepts, capture damping, and are consistent with mass-damping concepts from wake oscillator theory. This study shows a strong consistency between the analytical models and experimental results.Copyright