Vinod K. Lakshminarayan

Computational Investigation of Micro-Scale Coaxial Rotor Aerodynamics in Hover

In this work, a compressible Reynolds-averaged Navier-Stokes solver is used to investigate the aerodynamics of a microscale coaxial-rotor configuration in hover, to evaluate the predictive capability of the computational approach and to characterize the unsteadiness in the aerodynamic flowfield of the microscale coaxial systems. The overall performance is well-predicted for a range of rpm and rotor spacing. As the rotor spacing increases, the top-rotor thrust increases and the bottom-rotor thrust decreases, while the total thrust remains fairly constant. The thrusts approach a constant value at very large rotor spacing. Top rotor contributes about 55 % of the total thrust at smaller rotor spacing, which increases to about 58% at the largest rotor separation. The interaction between the rotor systems is seen to generate significant impulses in the instantaneous thrust and power. Unsteadiness is mainly caused due to blade loading and wake effect. Additional high-frequency unsteadiness was also seen due to shedding near the trailing edge. The phasing of the top vortex impingement upon the bottom rotor plays a significant role in the amount of unsteadiness for the bottom rotor. Interaction of the top-rotor tip vortex and inboard sheet with the bottom rotor results in a highly three-dimensional shedding on the upper surface of the blade in the outboard region and a two-dimensional shedding on the lower surface at the inboard portion of the blade. The wake of the top rotor contracts faster compared with that of the bottom rotor because of the vortex-vortex interaction. Further, the top-rotor wake convects vertically down at a faster rate due to increased inflow.

An ALE formulation of embedded boundary methods for tracking boundary layers in turbulent fluid-structure interaction problems

Embedded Boundary Methods (EBMs) for Computational Fluid Dynamics (CFD) are usually constructed in the Eulerian setting. They are particularly attractive for complex Fluid-Structure Interaction (FSI) problems characterized by large structural motions and deformations. They are also critical for flow problems with topological changes and FSI problems with cracking. For all of these problems, the alternative Arbitrary Lagrangian-Eulerian (ALE) methods are often unfeasible because of the issue of mesh crossovers. However for viscous flows, Eulerian EBMs for CFD do not track the boundary layers around dynamic rigid or flexible bodies. Consequently, the application of these methods to viscous FSI problems requires either a high mesh resolution in a large part of the computational fluid domain, or adaptive mesh refinement. Unfortunately, the first option is computationally inefficient, and the second one is labor intensive. For these reasons, an alternative approach is proposed in this paper for maintaining all moving boundary layers resolved during the simulation of a turbulent FSI problem using an EBM for CFD. In this approach, which is simple and computationally reasonable, the underlying non-body-fitted mesh is rigidly translated and/or rotated in order to track the rigid component of the motion of the dynamic obstacle. Then, the flow computations away from the embedded surface are performed using the ALE framework, and the wall boundary conditions are treated by the chosen Eulerian EBM for CFD. Hence, the solution of the boundary layer tracking problem proposed in this paper can be described as an ALE implementation of a given EBM for CFD. Its basic features are illustrated with the Large Eddy Simulation using a non-body-fitted mesh of a turbulent flow past an airfoil in heaving motion. Its strong potential for the solution of challenging FSI problems at reasonable computational costs is also demonstrated with the simulation of turbulent flows past a family of highly flexible flapping wings.

Assessment of transition model and CFD methodology for wind turbine flows

A detailed evaluation of the predictive capability of a Reynolds Averaged Navier Stokes (RANS) solver with a transition model is performed for wind turbine applications. The performance of the computational methodology is investigated in situations involving attached flow as well as incipient and massive flow separation and compared with experiment. Two-dimensional simulations on wind turbine airfoil sections are seen to qualitatively and quantitatively predict the onset of transition to turbulence and provide significantly improved lift and drag predictions when compared to simulations that assume fully turbulent flow. In three-dimensional wind turbine simulations, detailed validation studies of the integrated loads and sectional pressure coefficient also show definite improvements at wind speeds at which separation is incipient or confined to a small portion of the blade surface. At low wind speeds, for which the flow is mostly attached to the blade surface, and at high wind speeds, for which it is massively separated, the transition model produces similar results to a fully turbulent calculation. Overall, the performance of the transition model highlights the necessity of such models while also pointing out the need for further development.

Recent Advancements in the Helios Rotorcraft Simulation Code

An overview of new capabilities recently included in the HPCMP CREATE TM -AV Helios high-delity rotorcraft simulation code is presented. These include a new implicit obody Cartesian solver to support both steady solutions and time-accurate RANS/DES in the wake, a new body hierarchy formulation to support coupled wing/rotor aeroelastics and maneuvering ight, a new strand-based near-body solver intended to support enhanced automation and accuracy, the inclusion of two new unstructured near-body solvers { FUN3D from NASA and kCFD from CREATE-AV Kestrel. New unsteady particle tracing and moving contour plane capability have been added to runtime-based in situ visualization. Example application of these capabilities are presented.

Computational Analysis of Shrouded Wind Turbine Configurations

Computational analysis of di↵user-augmented turbines is performed using high resolution computations of the Reynolds Averaged Navier–Stokes equations supplemented with a transition model. Shroud geometries, generated by the extrusion of airfoil profiles into annular wings, are assessed based on their ability to capture mass-flow through the interior of the shroud. To this end, axisymmetric calculations of high-lift airfoil sections are performed. The amplification of mass flow through a shroud is found to increase nearly linearly with radial lift force, and nonlinear e↵ects are examined in terms of the location of the leading edge stagnation point. Of the shapes considered, the Selig S1223 high-lift lowRe airfoil is found to best promote mass flow rate. Following this, full three-dimensional simulations of shrouded wind turbines are performed for selected shroud geometries. The results are compared to bare turbine solutions. Augmentation ratios (defined as the ratio of the power generated by a shrouded turbine to the Betz limit) of up to 1.9 are achieved by the shrouded turbines. Peak augmentation occurs at the highest wind speed for which the flow over the bare turbine blade stays attached. Flow fields are examined in detail and the following aspects are investigated: regions with flow separation, the development of averaged velocity profiles, and the interaction between the helical turbine wake and shroud boundary layer. Finally, power augmentation is demonstrated to continue increasing at high wind velocities, at which the turbine blade would otherwise stall, if a constant tip speed ratio is maintained.

Fundamental Understanding of the Physics of a Small-Scale Vertical Axis Wind Turbine with Dynamic Blade Pitching: An Experimental and Computational Approach

This paper describes the systematic experimental and computational (CFD) studies performed to investigate the performance of a small-scale vertical axis wind turbine (VAWT) utilizing dynamic blade pitching. A VAWT prototype with a simplified blade pitch mechanism was designed, built and tested in the wind tunnel to understand the role of pitch kinematics in turbine aerodynamic efficiency. The ability of the present pitch mechanism to change the blade pitch phasing instantaneously in order to adapt to changes in wind direction is the key to maximizing power extraction in urban environments. A CFD model was developed and the model predictions correlated extremely well with test data. The validated CFD model was used to develop a fundamental understanding of the physics of power extraction of such a turbine. Both experimental and CFD studies showed that the turbine efficiency is a strong function of blade pitching amplitude, with the highest efficiency occurring around ±20◦ to ±25◦ amplitude. The optimum tip speed ratio (TSR) depends on the blade pitch kinematics, and it decreased with increasing pitch amplitude for the symmetric blade pitching case. CFD analysis showed that the blade extracts all the power in the frontal half of the circular trajectory, however, it loses power into the flow in the rear half. One key reason for this being the large virtual camber and incidence induced by the flow curvature effects, which slightly enhances the power extraction in the frontal half, but increases the power loss in the rear half. Fixed-pitch turbine investigated in the present study also showed lower efficiency compared to the variable pitch turbines owing to the massive blade stall in the rear half, caused by the large angle of attack and high reverse camber. Maximum achievable CP of the turbine increases with higher Reynolds numbers, however, the fundamental flow physics remains relatively same irrespective of the operating Reynolds number. This study clearly indicates the potential for major improvements in VAWT performance with novel blade kinematics, lower chord/radius ratio, and using cambered blades.

Effect of Flow Curvature on Forward Flight Performance of a Micro-Air-Vehicle-Scale Cycloidal-Rotor

This paper describes the systematic experimental and computational studies performed to obtain a fundamental understanding of the physics behind the lift and thrust production of a cycloidal rotor (cyclorotor) in forward flight for a unique blade pitching kinematics. The flow curvature effect (virtual camber and incidence due to the curvilinear flow) was identified to be a key factor affecting the lift, thrust, and power of the cyclorotor in forward flight. The experimental study involved systematic testing of a micro air vehicle-scale cyclorotor in an open-jet wind tunnel using a custom built three-component balance. The key parameters varied include rotor chord/radius ratio and blade pitching axis location because these two parameters have a strong impact on flow curvature effects. Because of the virtual camber/incidence effects and the differences in the aerodynamic velocities around the azimuth, the blades produce a small downward lift when they operate in the upper half of the circular trajectory and...

Detailed Computational Investigation of a Hovering Microscale Rotor in Ground Effect

A compressible, structured, overset Reynolds-averaged Navier–Stokes-based solver is used to simulate a microhovering rotor in ground effect to demonstrate the capability of the code to provide accurate tip-vortex-flowfield predictions, and to provide a good understanding of the ground–wake interactions in conditions prevalent in helicopter brownout. The performance validation at different rotor heights above the ground shows good correlation with the experimental thrust and power measurements. A detailed comparison of the predicted tip-vortex flowfield shows good agreement with the vorticity contours and radial-velocity profiles obtained from the particle-image-velocimetry measurements during experiments. The examination of the in ground effect tip-vortex flowfield suggests high degree of instabilities close to the ground. The induced velocities arising from the proximity of the rotor tip vortices cause flow separation at the ground. An analysis of the eddy-viscosity contours near the ground indicates hig...

Towards Efficient Parallel-in-Time Simulation of Periodic Flows

In response to the stagnation of computer microprocessor speeds over the past decade, the design emphasis of novel supercomputing architectures has focused primarily on increasing the overall number of available cores and reducing communication bottlenecks. Typically, flow solvers have been able to achieve parallel efficiency using domain decomposition, but this approach has the natural limitation that saturation will manifest itself on a finite number of cores at which point parallel speedup stalls and eventually deteriorates. In order to improve parallel scalability we seek to leverage the existing knowledge base on spatial decomposition, while attempting to exploit additional parallelism in the temporal dimension. Specifically, we explore the case of time periodic flows using Parallel-in-Time (PinT) variants of the Time-Spectral (TS) method. A framework employing a Pythonbased infrastructure is described including a standalone library that can be coupled to existing flow solvers in order to facilitate PinT calculations. A model problem of a periodic density pulse is used to examine the different discretization options. Implications for application to wind turbines and rotors are addressed.

Aerodynamics of a Small-Scale Vertical-Axis Wind Turbine with Dynamic Blade Pitching

This paper describes the systematic experimental and computational studies performed to investigate the performance of a small-scale vertical-axis wind turbine using dynamic blade pitching. A vertical-axis wind turbine prototype with a simplified blade pitch mechanism was designed, built, and tested in the wind tunnel to understand the role of pitch kinematics in turbine aerodynamic efficiency. A computational fluid dynamics model was developed, and the model predictions correlated well with test data. Both experimental and computational fluid dynamics studies showed that the turbine efficiency is a strong function of blade pitching amplitude, with the highest efficiency occurring around ±20 to ±25  deg amplitude. The optimum tip-speed ratio depends on the blade pitch kinematics, and it decreases with increasing pitch amplitude for the symmetric blade pitching case. A computational fluid dynamics analysis showed that the blade extracted all the power in the frontal half of the circular trajectory; however...