Michael P. Kinzel

Numerical Investigation of Miniature Trailing-Edge Effectors on Static and Oscillating Airfoils

The aerodynamics of deployable devices based on Gurney flaps, miniature trailing-edge effectors, or MiTEs, is explored for rotorcraft applications using computational fluid dynamics (CFD). A Gurney flap is a small aerodynamic device, with a height of only a few percent of the airfoil chord, mounted normal to the airfoil surface near the trailing edge. A MiTE extends the Gurney flap concept by making it deployable. To support future design efforts, and because fully dynamic wind-tunnel experiments would be difficult and costly, the unsteady behavior of MiTEs is studied for a static airfoil and during airfoil-pitch oscillations using the Navier-Stokes numerical solver OVERFLOW2. Static wind-tunnel tests from low-speed wind-tunnel experiments that explore the effects of Gurney flaps at a chord Reynolds number of 1.0×10 are used for the calibration and verification of the CFD. The results indicate similar effects along with qualitatively correct behavior, and the trends in the predicted dynamic performance are reasonable in that they agree with unsteady circulatory theory. In addition, wind-tunnel experiments of an oscillating rotorcraft airfoil using Gurney flaps through dynamic stall are also used for validation. These experiments verify the CFD models, allowing the extension of the studies to examine the potential of MiTE for rotorcraft. Much insight is gained on the unsteady aerodynamic behavior through transonic flow conditions, at high angles of attack, and with variations in the chord-wise placement of MiTEs. Additionally, CFD is used to show that MiTEs have the ability to be used as an active stall control device. These studies show that there does not appear to be any significant shortcoming of MiTEs potential for rotorcraft.

Active Gurney Flaps: Their Application in a Rotor Blade Centrifugal Field

Miniature trailing-edge effectors are segmented gurney flaps that can deploy to achieve multipurpose functions, such as performance enhancement, noise/vibration control, and/or load control on rotor blades. The unsteady aerodynamics of miniature trailing-edge effectors and a deployable plain flap (with an equivalent lift gain) are quantified experimentally at a reduced frequency of 0.21 and a Reynolds number of 1×106. These experiments are also simulated using computational fluid dynamics. The combination of the wind tunnel experiments and computational fluid dynamics are used to quantify the aerodynamic effects of miniature trailing-edge effector deployment to compare their unsteady aerodynamics to plain flaps, and to evaluate the fluid dynamics of miniature trailing-edge effectors against experimental data. The current experiments display unsteady aerodynamics that corroborate previous computational fluid dynamics findings that indicate that miniature trailing-edge effectors shed on-surface vortices dur...

A Finite-Volume Approach to Modeling Ice Accretion

In this work we present a novel, generalized, multiscale physics, unstructured finite-volume, CFD approach for simulating ice accretion on aircraft. A multi-physics solver that evaluates the (1) air flow, (2) droplet trajectories, (3) surface-liquid flow, (4) solidification, and (5) computes the deformed ice shape, is presented. Initial results show promise in the developed methods and solvers, that are expected to later be extended for future rotorcraft ice-accretion analysis. Initial validation cases are presented for the various components of the solver, and compare reasonable well with LEWICE and experiments for simple geometries. This initial capability displays a capability that could be extended, in future efforts, with more detailed models and provide ice shapes of similar quality as the current methodologies, while providing a capability that extends to more complex configurations such as rotorcraft.

Space-Time Loadings on Wind Turbine Blades Driven by Atmospheric Boundary Layer Turbulence

the interactions between the spatio-temporal loadings on wind turbine blade blades and the turbulence structure of the neutral and moderately convective atmospheric surface layer by combining the Blade Element Method incorporated in the FAST/AeroDyn codes from NREL with a dynamic stall model with large-eddy simulation (LES) of the atmospheric boundary layer (ABL). The inow conditions were obtained from high-resolution LES interpolated to the turbine blade. The central aim of our analysis is to search for and quantify direct causal relationships between specic space-time variabilities in the turbulent inow velocity eld and the spatio-temporal variability of forces on the turbine blades, and the integrations along the blade span that produce time variations in bending moment at the hub and shaft torque. A related interest is the impact of an accurate versus inaccurate predictions of shear rate by the LES. We nd that atmospheric turbulence is a major contributor to blade loadings and that the distribution of force uctuations is sensitive to the specic structure of ABL turbulence. A well designed, accurate LES model has signicant advantages for quantifying the role of atmospheric turbulence on wind turbine performance.

Comparing Unsteady Loadings on Wind Turbines using TurbSim and LES Flow Fields

Unsteady loading on wind turbine blades due to atmospheric turbulence may be a cause for higher than anticipated wind turbine downtime. The National Renewable Energy Laboratory (NREL) has produced a turbulence simulator, TurbSim, for use in wind turbine development and analysis. We compare the turbulence created by TurbSim with atmospheric turbulence created with low-dissipation Large-Eddy Simulation of the canonical moderately convective atmospheric boundary layer. This turbulence is used to create inow conditions to NREL’s FAST code to study the dierences in wind turbine loadings. Through examination of dierent wind turbine parameters, we observe dierences between the kinematic and dynamic turbulence ow elds. In particular, we nd that the predicted mean values for rotor power and lift are very similar between the two turbulence elds. However, the variance inherent in the LES turbulence is not found within the TurbSim kinematic turbulence. Matching the TurbSim mean Reynolds stresses to those of the LES ow eld does not cause the correlations between wind velocity and turbine loadings to match. We conclude that TurbSim is a reasonable tool for some wind turbine analysis applications, but it does not fully capture the variance associated with the canonical moderately convective atmospheric boundary layer.

Considerations in coupling LES of the atmosphere to CFD around wind turbines

rst part of this paper, we analyze the role of algorithm in the inaccurate predictions of the mean shear in the surface layer of high Reynolds number ows. Brasseur and Wei 1 have proposed a solution to this problem in the < ReLES parameter space. We perform the same simulation using two algorithms with dierent numerical dissipation characteristics, viz., a spectral algorithm and a nite volume algorithm. We repeat this procedure for two incompressible high Reynolds number ows: channel ow and a neutral atmospheric boundary layer. The increased dissipation in the nite volume algorithm compared to a spectral algorithm acts as an eective lter cuto at a lower wavenumber than the grid can represent. We nd that the nite volume algorithm experiences a lower resolved stress, increased variance in the streamwise velocity and lower variance in the vertical velocity in the surface layer. These combine together to aect the turbulence structure in the surface layer that then aects the whole boundary layer.

Computational Investigations of Air Entrainment, Hysteresis, and Loading for Large-Scale, Buoyant Cavities

A complete physical model of ventilated supercavitation is not well established. Efforts documented display the ability, with a finite volume, locally homogeneous approach, to simulate supercavitating flows and obtain good agreement with experiments. Several modeling requirements appear critical, especially in physical hysteretic conditions or configurations. The hysteresis presented is due to obstruction of the flow with a solid object. The modeling approach taken correctly captures a full hysteresis loop and the corresponding dimensionless ventilation rate to cavity pressure (CQdelta) relationship. This correspondence supports the suggestion that the main mechanism of cavity gas entrainment is via shear layers attached to the cavity walls. With such validated solutions, additional insight into the flow within the cavity is gained.

Jet-Supercavity Interaction: Insights from Experiments

An experimental study was performed to evaluate some of the claims of Paryshev (2006) regarding changes to ventilated cavity behavior caused by the interaction of a jet with the cavity closure region. The experiments, conducted in the 1.22m dia. Garfield Thomas Water Tunnel, were performed for EDD to tunnel diameter of 0.022, Fr = 14.5 and 26.2. The model consisted of a converging-section nozzle mounted to the base of a 27.9mm 37° cone cavitator placed on the tunnel centerline at the end of a 138.4mm long streamlined strut. A ventilated cavity was formed over the model, then an air jet, issuing from a converging nozzle, was initiated. Changes to cavity behavior were quantified in terms of cavitation number, thrust-to- drag ratio, and stagnation pressure ratio at the jet nozzle. The results show that, while the overall trends predicted by Paryshev were observed, the data did not fully collapse, suggesting that many of the effects neglected by Paryshevs model have measureable effect.

Unsteady Aerodynamics of Miniature Trailing-Edge Effectors Based on Indicial Methods

The aerodynamics of miniature trailing-edge effectors (MiTEs), or active Gurney flaps, is explored in many aspects with regards to their applicability to rotorcraft. MiTEs hold strong potential to be used as active devices to improve performance, reduce vibrations, and reduce noise of rotors. Analysis of available computational fluid-dynamic (CFD) predictions for MiTEs positioned upstream from the trailing edge indicates the formation and convection of an unsteady vortex in the lower surface of the airfoil, immediately following a MiTE deployment. This disturbance can introduce non-harmonic components in the aerodynamic response, substantially affecting the loads and complicating the analysis. In order to account for these effects during routine helicopter performance and design studies, available CFD results are used to develop a reduced-order, less computationally-intensive model based on indicial concepts. The model extends a work previously done for trailing-edge MiTEs by incorporating a vortex model to predict the unsteady lift of upstream MiTEs. A physicsbased approach is adopted to minimize the number of constants and improve overall generality. The results from the unsteady-lift model are compared with CFD and experiments for different airfoils, MiTE deployment schedules, MiTE chordwise positions, and Mach numbers. Very good agreement is shown for a wide range of conditions that are typical of rotorcraft.

A Level-Set Approach for Compressible, Multiphase Fluid Flows with Mass Transfer

In this work, a new level-set-based approach is presented and applied to compressible, multiphase flows. Using a species-mass-conservation-based level-set transport equation, several advantages over signed-distancefunction-based methods are demonstrated. Specific improvements include a ghost-fluid-free method for highly compressible problems, extensions to an arbitrary number of species, and finite-rate chemistry mass transfer modeling. While maintaining higher-order numerics, the approach is applicable to three-dimensional, time-accurate/steady, turbulent simulations. Herein, the method is applied to a flow solver that fully couples the mass, momentum, energy, and level-set transport equation; although the methods are equally applicable to segregated flow solvers. The methods are tested for ventilated supercavities, natural cavitation (incompressible, compressible, and thermal), and shock-induced boiling. Lastly, new reinitialization methods are developed specific to cavitating flows that decrease the interface diffusion where needed, while retaining an ability to admit subgrid-scale mixtures. Such an approach enables a more physical solution method for certain classes of multiphase flows. This formulation of the level-set approach is a general, valid, method that could easily be incorporated into any species-mass conservation solver.