Limit-cycle oscillations in unsteady flows dominated by intermittent leading-edge vortex shedding

High-frequency limit-cycle oscillations of an airfoil at low Reynolds number are studied numerically. This regime is characterized by large apparent-mass effects and intermittent shedding of leading-edge vortices. Under these conditions, leading-edge vortex shedding has been shown to result in favourable consequences such as high lift and efficiencies in propulsion/power extraction, thus motivating this study. The aerodynamic model used in the aeroelastic framework is a potential-flow-based discrete-vortex method, augmented with intermittent leading-edge vortex shedding based on a leading-edge suction parameter reaching a critical value. This model has been validated extensively in the regime under consideration and is computationally cheap in comparison with Navier-Stokes solvers. The structural model used has degrees of freedom in pitch and plunge, and allows for large amplitudes and cubic stiffening. The aeroelastic framework developed in this paper is employed to undertake parametric studies which evaluate the impact of different types of nonlinearity. Structural configurations with pitch-to-plunge frequency ratios close to unity are considered, where the flutter speeds are lowest (ideal for power generation) and reduced frequencies are highest. The range of reduced frequencies studied is two to three times higher than most airfoil studies, a virtually unexplored regime. Aerodynamic nonlinearity resulting from intermittent leading-edge vortex shedding always causes a supercritical Hopf bifurcation, where limit-cycle oscillations occur at freestream velocities greater than the linear flutter speed. The variations in amplitude and frequency of limit-cycle oscillations as functions of aerodynamic and structural parameters are presented through the parametric studies. The excellent accuracy/cost balance offered by the methodology presented in this paper suggests that it could be successfully employed to investigate optimum setups for power harvesting in the low-Reynolds-number regime

Reduced-order Aeroelastic Model for Limit-cycle Oscillations in Vortex-dominated Unsteady Airfoil Flows

In previous research, Ramesh et al (JFM,2014) developed a low-order discrete vortex method for modeling unsteady airfoil flows with intermittent leading edge vortex (LEV) shedding using a leading edge suction parameter (LESP). LEV shedding is initiated using discrete vortices (DVs) whenever the Leading Edge Suction Parameter (LESP) exceeds a critical value. In subsequent research, the method was successfully employed by Ramesh et al (JFS, 2015) to predict aeroelastic limit-cycle oscillations in airfoil flows dominated by intermittent LEV shedding. When applied to flows that require large number of time steps, the computational cost increases due to the increasing vortex count. In this research, we apply an amalgamation strategy to actively control the DV count, and thereby reduce simulation time. A pair each of LEVs and TEVs are amalgamated at every time step. The ideal pairs for amalgamation are identified based on the requirement that the flowfield in the vicinity of the airfoil is least affected (Spalart, 1988). Instead of placing the amalgamated vortex at the centroid, we place it at an optimal location to ensure that the leading-edge suction and the airfoil bound circulation are conserved. Results of the initial study are promising

Efforts Towards the Total Synthesis of Amphidinolide B

Studies towards the total synthesis of the cytotoxic marine macrolide Amphidinolide B have been disclosed. Catalytic asymmetric AAC methodology has been applied to efficiently generate the C11- and the C18- stereocenters in the requisite fragments 114 and 120 throughƒnbeta-lactones 102 and 81 respectively.An efficient route to install the C14-C15 trisubstituted alkene was realized through a stannylcupration reaction. The Stille and Suzuki cross-coupling methodologies were investigated for the formation of the C13-C15 diene of amphidinolide B. Iodide 90 was coupled with boronic ester 114 via an efficient Suzuki reaction to form a C7-C20 fragment 115. Fragment was further homologated and coupled to sulfone 65 to complete a C1-C20 synthon of amphidinolide B

Model reduction in discrete-vortex methods for unsteady airfoil flows

Discrete-vortex methods are a class of low-order methods widely used to study unsteady aerodynamic phenomena. However, these methods demand high computational costs when subject to large number of vortices in the flowfield. This calls for model reduction in discrete-vortex methods. A model-reduction technique is applied to a recently developed discrete-vortex method in which the criticality of the leading-edge suction parameter (LESP) controls the initiation and termination of leading-edge vortices (LEVs). This method, called the LESP-modulated discrete-vortex method (LDVM), has been successfully used in recent work to study unsteady airfoil flows with LEV shedding. In this research, model reduction in the LDVM is achieved by amalgamating suitable pairs of discrete vortices identified through a condition that requires that the velocity at the airfoil leading edge is not affected by amalgamation. The amalgamated vortex is placed at an optimal location to ensure that the bound circulation and the leading-edge suction are conserved. The reduced-order model is able to predict the flow features and the force and moment coefficients in good agreement with the full model while having significantly lower runtimes. Use of physical quantities like leading- edge suction and bound circulation enables the easy implementation of this model-reduction strategy in other computational methods based on discrete-vortex elements

Modeling intermittent leading-edge vortex shedding in unsteady airfoil flows with reduced-count discrete vortices

A discrete-vortex method for unsteady airfoil flows with intermittent leading-edge vortex (LEV) shedding was proposed by Ramesh et al. (JFM, 2014). Two novelties were introduced: (i) LEV shedding is initiated using discrete vortices whenever the Leading Edge Suction Parameter (LESP), which is a measure of leading-edge suction, exceeds a critical value, and (ii) the strength of the discrete vortices is determined such that the LESP maintained at the critical value during the shedding process. Although results from this low-order method agree with CFD and experiments, the increasing vortex count with time increases the computational cost. The large number of shed vortices from the TE can be reduced through traditional techniques such as amalgamation and deletion, as they typically convect away from the airfoil and interact only weakly with the airfoil vorticity. The LEV, on the other hand, interacts strongly with the airfoil, and has a large influence on the forces. An approach to reduce the vortex count is desired. Inspired by Wang and Eldredge (TCFD, 2013), we propose a model that has just a single vortex to model an active LEV. The varying strength of this free vortex is determined using our LESP criterion. Results from the method for unsteady airfoil motions are promising

CFD Analysis of Circulation Control Airfoils Using Fluent

In an effort to validate computational fluid dynamics procedures for calculating flows around circulation control airfoils, the commercial flow solver FLUENT was utilized to study the flow around a general aviation circulation control airfoil. The results were compared to experimental and computational fluid dynamics results conducted at the NASA Langley Research Center. The current effort was conducted in three stages: 1. A comparison of the results for free-air conditions to those from experiments. 2. A study of wind-tunnel wall effects. and 3. A study of the stagnation-point behavior

Computation vs. Experiment for High-Frequency Low-Reynolds Number Airfoil Plunge

We seek to extend the literature on sinusoidal pure-plunge of 2D airfoils at high reduced frequency and low Reynolds number, by including effects of camber and nonzero mean incidence angle. We compare experimental results in a water tunnel using dye injection and 2D particle image velocimetry, with a set of computations in 2D – Immersed Boundary Method and unsteady Reynolds-Averaged Navier Stokes. The Re range is from 10,000 to 60,000, based on free stream velocity and airfoil chord, chosen to cover cases where transition in attached boundary layers would be of some importance, and where transition would only occur in the wake. Generally at high reduced frequency there is no Reynolds number effect. Mean angle of attack has significance, notionally, depending on whether it is below or above static stall. Computations were found to agree well with experimentally-derived velocity contours, vorticity contours and momentum in the wake. As found previously for the NACA0012, varying Strouhal number is found to control the topology of the wake, while varying reduced amplitude and reduced frequency together, but keeping Strouhal number constant, causes wake vortical structures to scale with the reduced amplitude of plunge. Flowfield periodicity – as evinced from comparison of instantaneous and time-averaged particle image velocimetry – is generally attained after two periods of oscillation from motion onset

Measurement of Elastic Microfence Deflection for Aerodynamic Flow Sensing

Bio-inspired artificial hair sensors have the potential to detect aerodynamic flow features such as stagnation point, flow separation, and flow reattachment that could be beneficial for ight control and performance enhancement of aircraft. In this work, elastic microfence structures were tested on a at-plate setup. The microfences were fabricated from a two-part silicone molded against a template patterned by laser ablation. The response of the microfences to different freestream velocities and to flow reversal at the sensor were recorded via an optical microscope