Uttam K. Chakravarty

Aerodynamic control of micro air vehicle wings using electroactive membranes

Dielectric elastomer materials are ideal candidates for developing high-agility micro air vehicles due to their electric field–induced deformation. Consequently, the aero-structural response and control authority of the dielectric elastomer material, VHB 4910, are characterized on an elliptical membrane wing. An experimental membrane wing platform was constructed by stretching VHB 4910 over a rigid elliptical wing-frame. The low Reynolds number (chord Reynolds number < 106) and aerodynamics of the elliptical wing were characterized when different electrostatic fields were applied to the membrane. We observe an overall increase in lift with maximum gains of 20% at an applied voltage of 4.5 kV and demonstrate the ability to delay stall. The time-averaged aerodynamic surface pressure is also investigated by comparing sting balance data and membrane deformation measured using visual image correlation. The experimental results are compared to a nonlinear finite element membrane model to further understand the effects of aerodynamic load and electric fields on membrane displacements. Model predictions of surface pressure provide insight into how the electrostrictive constitutive relations influence the fluid–structure interactions of the membrane. This is validated by comparing lift predictions from the model with time-averaged wind tunnel lift measurements near stall.

A Structural Dynamic Analysis of a Crane Fly Forewing

This paper describes a method for conducting the structural dynamic analysis of a crane fly (family Tipulidae) forewing. Wing geometry is captured via microcomputed tomography (micro-CT) scan. A finite element (FE) model of the forewing is developed from the reconstructed model of the micro-CT scan to determine the natural frequencies and mode shapes of the wing in vacuum. The FE model is validated by comparing the first three natural frequencies of a tubular cantilever beam with similar dimensions and properties of a vein of the wing to the analytical solution obtained from Euler beam theory. The natural frequency of the wing increases with mode in vacuum. The first three mode shapes of the wing are characterized by bending and torsional deformation responses. Furthermore, a simulation of the fluid-structure interaction (FSI) of the forewing under steady and unsteady conditions at low freestream velocities, similar to those encountered by insects in regular flight, is performed by coupling the FE model of the wing with a computational fluid dynamics model. For steady air flow, the FSI simulations reveal that high magnitude deformation regions develop at the center of the wing and that the deformation increases nonlinearly with increasing freestream velocity. For unsteady air flow, a more uniform deformation distribution is observed along the span wise direction of the wing.

A fluid–structure interaction approach to investigate the quenching characteristics of a steel tube with temperature dependent properties

Quenching is commonly used for improving material properties of steel tubes because of their numerous applications. However, quenching generates some residual stress and deformation in the material due to rapid temperature fluctuations. The properties of the steel are strong functions of these variable temperatures and therefore, the estimated stress and deformation by constant property or static quenching analysis are not very realistic. This study describes the first extensive study of the quenching process of a steel tube including temperature dependent properties by three liquid quenchants using the dynamic fluid–structure interaction quench model. The cooling characteristics of the three liquid quenchants are compared to each other along with the transient temperature distributions in the steel tube. The time-varying nodal, axial, and radial residual stress and deformation of the tube are studied. It is found that, the effectiveness of quenching does not depend only on a particular quenchant, but also on the temperature-varying properties of the steel and the uniformity of the cooling which ultimately determine the criteria for selecting a suitable quenchant for a specific purpose.

Thermo-Fluid Characterizations of Ti-6Al-4V Melt Pool in Powder-Bed Electron Beam Additive Manufacturing

The powder-bed electron beam additive manufacturing (EBAM) process is one of the relatively new additive manufacturing (AM) technologies in which the metal powder is melted in a vacuum environment utilizing a high-energy heat source to fabricate metallic parts in a layer by layer manner. Different metallic alloys (especially, high entropy alloys such as Ti-6Al-4V) have been widely studied as a powder-bed material for the EBAM. Despite the unique advantages of designing complex geometry and tooling-free manufacturing, there are still considerable challenges in the EBAM, e.g., obtaining desired metallurgical behavior, part accuracy, reliability, and quality consistency. Therefore, a better understanding of the thermo-fluid and mechanical properties of the EBAM process is indispensable to meet the challenges. In this study, transient computational fluid dynamics (CFD) modeling of Ti-6Al-4V melt pool has been done using ANSYS Fluent 15.0 to characterize the process parameters associated with the EBAM process including the melt pool geometry, beam power, beam speed, beam diameter, and temperature profile along the melt scan. In fact, the dynamics and the solidification of the melt pool have been investigated numerically and results for cooling rate, variation in density, pressure, velocities, and liquid fraction have been obtained to illustrate the versatility of the analysis.Copyright

Vibration Analysis of a Composite Helicopter Rotor Blade at Hovering Condition

The helicopter is an essential and unique means of transport nowadays and needs to hover in space for considerable amount of time. During hovering flight, the rotor blades continuously bend and twist causing an increased vibration level that affects the structural integrity of the rotor blade leading to ultimate blade failure. In order to predict the safe allowable vibration level of the helicopter rotor blade, it is important to properly estimate and monitor the vibration frequencies. Therefore, the mathematical model of a realistic helicopter rotor blade composed of composite material, is developed to estimate the characteristics of free and forced bending-torsion coupled vibration. The cross-sectional properties of the blade are calculated at first and are then included in the governing equations to solve the mathematical model. The natural frequencies and mode shapes of the composite helicopter rotor blade are evaluated for both the nonrotating and rotating cases. The time-varying bending and torsional deflections at the helicopter rotor blade tip are estimated with suitable initial conditions. The validation of the model is carried out by comparing the analytical frequencies with those obtained by the finite element model.

Analysis of the Residual Stress and Deformation in a Steel Tube due to Quenching Process Using Different Media

Steel tubes are widely used in industries as machine components and are most common in heavily loaded mechanisms subjected to high dynamic torsional and compressive stress. Hence, they should have higher strength than that of the conventional mechanisms to resist failure. Quenching, an industrial heat treatment process, can improve the microstructure, hardness, toughness, and corrosion and wear resistance of materials. Steel tubes, if quenched, would have desired properties to serve the purposes. However, besides improving material properties, quenching generates some residual stress and deformation in the material due to rapid temperature drop and phase transformation. Therefore, to estimate the temperature distribution, residual stress, and deformation computationally; a three-dimensional fluid-structure interaction model is developed for the steel tube with different quenchants. The quenching characteristics by water, brine, and propylene glycol are estimated and compared with each other. The time-varying nodal temperature distributions in the tube are observed and the critical regions are identified having maximum residual stress and deformation. The time-varying residual stress and deformation at a particular point and along the axial and radial directions of the tube are studied. The convergence of the model is checked and validation of the model is done.Copyright

Characterization of the Electromechanical Response of a Dielectric Elastomer Membrane

Dielectric elastomers hold much promise as smart materials that could rapidly adapt to changes in environmental conditions due to their mechanical response to an electrical input. They belong to the group of electroactive polymers which have unique mechanical properties such as flexibility, light-weight, and electrical field-induced deformation. These characteristics make dielectric elastomers suitable candidates as actuators, sensors, or energy converter media. The objective of this study is to characterize the structural dynamic response of a dielectric elastomer membrane exposed to stagnant air environment and steady airflow at different angles of attack. A simulation of the fluid-structure interaction of the membrane is performed by coupling an electromechanical finite element model of the membrane with a computational fluid dynamics model representing the external flow. From the fluid-structure interaction simulation, the vibration frequencies and mode shapes, the time-varying out-of-plane deformation, and the coefficients of lift and drag are determined. Furthermore, a convergence study and mesh refinement are performed to guarantee mesh independence of the calculations from the fluid-structure interaction simulation. Results indicate that the stiffness of the electroactive membrane decreases nonlinearly with an increase of the applied voltage. The electrostatic force from the applied voltage adds compressive stress to the membrane, effectively softening the membrane, increasing the out-of-plane deformation, and reducing the resonance frequency.© 2014 ASME

Dynamic Response of Biomimetic Hair Receptors in Both Steady and Unsteady Flow Environment

The high performance of nature’s creations and biological assemblies has inspired the development of engineered counter parts that may outperform or provide new capabilities to conventional systems. In particular, the wings of bats contain distributed arrays of micro-scaled flow sensitive hair receptors over their surface, which inspires artificial hair sensors (AHS) development in aerodynamic feedback control designs using the micro-electro-mechanical systems (MEMS). One approach investigates the possibility of installing AHS on the leading edges of the wings of small-scaled unmanned aerial vehicles (UAVs) to improve the aerodynamic control. Our major motivation for the present study is that current mathematical models have limited relevance to aerodynamic situations because they are analyzed in steady or purely oscillatory flows. Our overall objective is to understand AHS fluid-structure interaction (FSI) in flow regimes relevant to small-scaled UAVs, for which we speculate a steady baseline flow perturbed by an oscillatory component is an appropriate flow reference condition. Towards understanding the AHS in this situation, we investigate the dynamic response of a hair receptor in a creeping flow environment with a steady and oscillatory component. We present time varying deflection and bending moment of the artificial hair sensors at different freestream velocities. For this, a three-dimensional FSI model is developed for the flexible hair-structure in the airflow, which is coupled with a finite element model using the computational fluid dynamics (CFD). The Navier-Stokes equations including continuity equation are solved numerically for the CFD model. To describe the dynamic response of the hair receptors, the natural frequencies and mode shapes of the hair receptors, computed from the FSI model, are compared with the excitation frequencies of the surrounding airflow. This model also describes both the boundary layer effects and effects of inertial forces due to FSI of the hair receptors. For supporting the FSI model, the dynamic response of the hair receptor is also validated considering the Euler-Bernoulli beam theory including the steady and unsteady airflow.Copyright