Guy Dumas

Computational Fluid Dynamics Analysis of a Hydrokinetic Turbine Based on Oscillating Hydrofoils

The performance of a new concept of hydrokinetic turbine using oscillating hydrofoils to extract energy from water currents (tidal or gravitational) is investigated using URANS numerical simulations. The numerical predictions are compared with experimental data from a 2 kW prototype, composed of two rectangular oscillating hydrofoils of aspect ratio 7 in a tandem spatial configuration. 3D computational fluid dynamics (CFD) predictions are found to compare favorably with experimental data especially for the case of a single-hydrofoil turbine. The validity of approximating the actual arc-circle trajectory of each hydrofoil by an idealized vertical plunging motion is also addressed by numerical simulations. Furthermore, a sensitivity study of the turbine’s performance in relation to fluctuating operating conditions is performed by feeding the simulations with the actual time-varying experimentally recorded conditions. It is found that cycle-averaged values, as the power-extraction efficiency, are little sensitive to perturbations in the foil kinematics and upstream velocity.

Gurney Flap Scaling for Optimum Lift-to-Drag Ratio

This note aims at providing evidence that there exists a flow-based scaling for the Gurney flap heights that yield an increase in lift-to-drag performance compared with the baseline airfoil at the same angle of attack (beneficial Gurney flaps). The results presented here, support this statement and further suggest that the boundary-layer thickness δ, measured at the trailing edge on the pressure side of the baseline airfoil, is not only an appropriate flow-based normalization for the flap height but is also a proper order of magnitude for the flap height providing the largest increase in the lift-to-drag ratio (optimum Gurney flap)

Three-Dimensional Effects on an Oscillating-Foil Hydrokinetic Turbine

Three-dimensional hydrodynamic losses are assessed in this investigation for a foil oscillating sinusoidally in a combined heave and pitch motion with large amplitudes. Simulations are performed using a unsteady Reynolds-Averaged-Navier-Stokes (URANS) solver on an oscillating foil in a power-extraction mode; thus acting as a hydrokinetic turbine at high Reynolds number. Foils of various aspect ratios (span to chord length ratio) are considered, both with and without endplates for one representative operation point. Hydrodynamic forces and extracted power are compared with results from the equivalent two-dimensional (2D) computations. It is found that the relative drop of performance (cycle-averaged power extracted) due to 3D hydrodynamic losses can be limited to 10% of the 2D prediction when endplates are used on a foil of aspect ratio greater than ten. The practical consideration of an oscillating-foil hydrokinetic turbine operating in an imperfectly-aligned upstream water flow is also addressed with simulations considering an upstream flow at a yaw angle up to 30° with respect to the foil chord line. Effects on performance are found to be proportional to the projected kinetic energy flux.

Development of a parallel plate flow chamber for studying cell behavior under pulsatile flow.

&NA; The design of a new parallel plate perfusion chamber for cell behavior studies involving pulsatile flowrates is presented. It was based on fluid mechanical considerations to ensure a region of regular and uniform shear stress at the wall. A numeric solution of the flow was performed to study the effect of pulsating flow on the entrance length. Dye injection investigations in the chamber showed laminar and uniform flow in the culture region under steady state conditions. ASAIO Journal 1995;41:876‐883.

Parametric study of H-Darrieus vertical-axis turbines using CFD simulations

A parametric study of vertical axis turbines of the H-Darrieus type is conducted using state-of-the-art Computational Fluid Dynamics (CFD) and the k-ω Shear Stress Transport RANS model in its unsteady form. Although most parameters have previously been investigated individually, the effect of solidity, number of blades, tip speed ratio, Reynolds number, fixed blade pitch angle, and blade thickness on the aerodynamic efficiency of the turbine is evaluated using the same performance evaluation set-up in order to determine what would be the best aerodynamic configuration and operation parameter in a given application. The quantitative impact of 3D effects associated with the blade aspect ratio and the use of end-plates is also investigated. For high-Reynolds applications, optimal radius-based solidity is found to be around σ=0.2, while higher solidities show a lower maximum efficiency than what was previously published using simpler streamtube based methods. In 3D, a small blade aspect ratio ( AR=7) leads to...

Testing and Analysis of an Oscillating Hydrofoils Turbine Concept

A new concept of hydrokinetic turbine using oscillating hydrofoils to extract energy from water currents (tidal or gravitational) is presented, tested and analyzed in the present investigation. Due to its rectangular extraction plane, this technology is particularly well suited for river beds and shallow waters near the coasts. The present turbine is a 2 kW prototype, composed of two rectangular oscillating hydrofoils of aspect ratio 7 in a tandem spatial configuration. The pitching motion of each hydrofoil is coupled to their cyclic heaving motion through four-link mechanisms which effectively yield a one-degree-of-freedom system driving a speed-controlled electric generator. The turbine has been mounted on a custom-made pontoon boat and dragged on a lake at different velocities. Instantaneous extracted power has been measured and cycle-averaged for several water flow velocities and hydrofoil oscillation frequencies. Results are demonstrated to be self-consistent and validate our extensive 2D flow simulation database. The present data show optimal performances of the oscillating hydrofoils concept at a reduced frequency of about 0.12, at which condition the measured power extraction efficiency reaches 40% once the overall losses in the mechanical system are taken into account. Further measurements of power extraction with a single oscillating hydrofoil have also been performed by taking out the downstream hydrofoil of the tandem pair. Those measurements favorably compare, quantitatively, with available 3D CFD predictions. The 40% hydrodynamic efficiency of this first prototype exceeds expectation and reaches levels comparable to the best performances achievable with modern rotor-blades turbines. It thus demonstrates the promising potential of the oscillating hydrofoils technology to efficiently extract power from an incoming water flow.Copyright

A Fluid-Structure Interaction Solver for Nano-Air-Vehicle Flapping Wings

This paper presents the implementation of a uid-structure interaction solver intended to be used for the analysis of exible apping airfoils. The governing equations associated to elastic solids with large deformations but small strains, and to incompressible uids are presented. The solver is implemented in the OpenFOAM software. The uid-structure coupling is handled by an iterative partitioned algorithm where each eld (the solid and the uid) are treated separately. The spatial discretization is achieved with the segregated nite-volume method for both elds. The uid module implements the Navier-Stokes equations using a SIMPLE algorithm whereas the solid module implements the St. Venant Kirchho constitutive law in a Lagrangian formulation where nonlinear and componentcoupled terms are treated iteratively in a xed point manner. Typical test simulations are carried out and results are found to be in good agreement with literature. Finally, preliminary results of exible apping wings are presented, showing that the solver seems well suited for this kind of application.

Simulations of Flow Ingestion and Related Structures in a Turbine Disk Cavity

Preliminary results of unsteady numerical simulations of disk cavity flow in interaction with the main gaspath flow in an axial turbine are presented in this article. A large periodic sector including vanes, blades and disk cavity of approximately 74° has been used in order to allow for the formation of large scale flow structures within the cavity. Three purge flow rates have been tested, namely no purge, low purge and high purge flow rates. Energetic large scale flow structures are detected through flow visualizations for the two lowest purge flow rates. They are found to rotate at an angular velocity slightly less than the rotor speed. The presence of the large scale structures involves important pressure perturbations inside the cavity that may lead to deep mass flow ingress, whereas the unsteady vane-blade interaction seems to cause only shallow ingress. Increasing purge flow rate appears to have a stabilizing effect on the pressure fluctuations inside the cavity and to reduce the intensity of the large scale flow structures.Copyright

Secondary flow and roll cells interaction in high-aspect-ratio rotating turbulent duct flows

End-wall effects for high aspect ratio (AR) turbulent duct flows under moderate spanwise rotation are investigated using Reynolds-Averaged Navier–Stokes (RANS) calculations with a Reynolds stress turbulence closure model. It is shown that despite an important uniformisation of the mean streamwise flow compared to the non-rotating case, the channel flow solution (AR = ∞) is not recovered in practical high AR ducts used in experiments. The unavoidable end-wall generated secondary flow causes transverse advection which is capable of altering the mean velocity profile, even for AR as high as 22. In addition, for Re = 40,000 and Ro = 0.22, persistent longitudinal roll cells are found in the RANS solutions. The results suggest that their interaction with the secondary flow may challenge the prospect of formally reaching a steady, streamwise invariant regime in actual rotating duct experiments.

Impact of Blockage on the Hydrodynamic Performance of Oscillating-Foils Hydrokinetic Turbines

This paper describes a study of the impact of confinement on the hydrodynamic performance of oscillating-foils hydrokinetic turbines (OFHT). This work aims to contribute to the development of standards applying to marine energy converters. These blockage effects have indeed to be taken into account when comparing measurements obtained in flumes, towing tanks, and natural sites. This paper provides appropriate correction formula to do so for OFHT based on computational fluid dynamics (CFD) simulations performed at a Reynolds Number Re = 3 × 106 for reduced frequencies between f* = 0.08 and f* = 0.22 considering area-based blockage ratios ranging from e = 0.2% to 60%. The need to discriminate between the vertical and horizontal confinement and the impact of the foil position in the channel are also investigated and are shown to be of second-order as compared to the overall blockage level. As expected, it is confirmed that the power extracted by the OFHT increases with the blockage level. It is further observed that for blockage ratio of less than e = 40%, the power extracted scales linearly with e. The approach proposed to correlate the performance of the OFHT in different blockage conditions uses the correction proposed by Barnsley and Wellicome and assumes a linear relation between the power extracted and the blockage. This technique is shown to be accurate for most of the practical operating conditions for blockage ratios up to 50%.