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Daniel Guggenheim School of Aerospace Engineering

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https://hdl.handle.net/1853/70742

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College of Engineering

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Aerospace Design Group

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Aerospace Systems Design Laboratory (ASDL)

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Air Transportation Laboratory

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Cognitive Engineering Center

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Space Systems Design Laboratory (SSDL)

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Publication Search Results

Now showing 1 - 10 of 21

EXPERIMENTAL STUDY OF LOW REYNOLDS NUMBER MULTIROTOR AERODYNAMIC INTERACTIONS

(Georgia Institute of Technology, 2019-05-23) Shukla, Dhwanil P.

In recent years, Vertical Take-off and Landing (VTOL) rotor Unmanned Aerial Vehicles (UAVs) have gained importance in various application suited to their small size and relatively cheap construction. Among rotor UAVs, multirotor UAVs are easier to control, and hence popular. Unfortunately, the small size rotor UAVs have poor aerodynamic performance due to viscosity dominated losses and the lack of understanding of rotor-rotor and rotor-airframe interactions. The existing tools developed for solving helicopter flows are inapplicable in this regime because of the invalidity of the underlying assumptions based on ignoring viscosity. Hence, there is an urgent need for characterizing the low Reynolds number multirotor flows which will enable UAV design optimization to make UAVs more capable. The present work explored some prominent flow field and aerodynamic interaction phenomena typical to small size multirotor UAVs. This was done through performance measurements and flow diagnostics over five different setups: single rotor, coaxial rotor, quadrotor, modular bi-rotor, and a rotor-box. Various vortex-vortex, vortex-vortex sheet, blade-vortex, vortex-duct, and vortex-box interactions were observed and interpreted. The implications of such interactions on vehicle performance and noise were also studied. This work serves as a platform over which further detailed studies into specific aspects of the whole problem can find inspiration. Most of the current findings can be applied to the UAV design process in their present form too.
Aerodynamics and aeroacoustic sources of a coaxial rotor

(Georgia Institute of Technology, 2018-04-10) Schatzman, Natasha Lydia

Vehicles with coaxial, contra-rotating rotor systems (CACR) are being considered for a range of applications, including those requiring high speed and operations in urban environments. Community and environmental noise impact is likely to be a concern in these applications. Design parameters are identified that effect the fundamental aerodynamics and fluid dynamic features of a CACR in hover, vertical, and edgewise flight. Particular attention is paid to those features affecting thickness, loading, blade vortex interaction (BVI), and high speed impulsive (HSI) noise. Understanding the fluid dynamic features is a precursor to studying the aeroacoustics of a coaxial rotor. Rotor performance was computed initially using Navier-Stokes solver with prescribed blade section aerodynamic properties, the results validated against generic experimental test cases. The fluid dynamics of blade interactions was simplified and broken into a 2-D blade crossing problem, with crossing locations and velocity fields from the rotor results. Two trains of 8 airfoils passing were simulated to understand the effects due to shed vorticity. The airfoils are displaced vertically by a distance equivalent to the typical spacing between the upper and lower rotors of a coaxial system. A 2D potential flow code and 2D OVERFLOW compressible-flow Navier-Stokes solver were used to investigate the complex coaxial rotor system flow field. One challenge of analyzing the CACR is the difficulty in envisioning all the possible interactions and their possible locations as flight conditions and rotor designs change. A calculation tool has been developed to identify time and location of blade overlap. The tool was then integrated with a wake aerodynamics model to identify locations and instances of upper rotor tip vortex interaction with a lower rotor blade. This tool enables rapid identification of different types of BVI based on relative rotor orientation. Specific aerodynamic phenomena that occur for each noise source relevant to CACR are presented, along with computational tools to predict these occurrences.
Vortex aerodynamics of rotors at high advance ratios

(Georgia Institute of Technology, 2018-03-05) Hiremath, Nandeesh Science

Rotor operation at high advance ratio is important for high-speed and compound rotorcraft concepts. The operation of helicopter rotors in reverse flow has taken on new significance in the context of co-axial rotors. A rotor moving edgewise at a high advance ratio, encounters reverse flow on parts of the retreating portion of the rotor disc. Predicting rotor stability and pitch link loads is complicated by the presence of unsteady pitch, yaw and rotation effects. Predictions using comprehensive codes have shown large differences from full-scale experimental data. Prior approaches have modeled these using flow separation with airfoil data modified for yaw, vortex shedding, dynamic pitch oscillations and reverse dynamic stall of an airfoil. However, the highly 3-dimensional flow phenomena do not conform to approaches based on 2-dimensional airfoil aerodynamics. The present work delineates the nature of flow around a rotating blade in reverse flow by integrating the results from fixed wing experiments with rotating wing experiments. The work focuses on a strong 3D vortex similar to those seen on delta wings that would develop over the sharp edge at high yaw, providing an avenue for vortex lift aerodynamic analyses. The fixed wing and rotating wing experiments were performed on a tethered rotor blade with NACA0013 profile. Fixed-wing results from load measurements and flow visualization showed that the sharp-edge vortex (SEV) is a primary feature in reverse flow when the blade is yawed either forward or backward. The aerodynamic loads conform with analytical model using Polhamus Suction Analogy, thus showing significant contributions from vortex-induced lift and pitching moments. In summary, it is apparent that the reverse flow regime should be modeled and analyzed as a case of SEV formation under the very sharply swept blade immediately after 180 degrees azimuth. The SEV evolves as the sweep decreases with increasing azimuth. In the regime before 240 degrees, an attached, strengthening SEV may be expected. At some moderate sweep (azimuth beyond 240 degrees in our case) the vortex bursts and detaches from the surface. Thereafter it convects with the blade, but induces strong pressure effects on the blade surface even as far as 300 degrees azimuth. The blunt edge flow is highly 3-dimensional, and has much less flow separation and unsteadiness than might be predicted from 2-dimensional airfoil aerodynamics.
Radial flow effects on a retreating rotor blade

(Georgia Institute of Technology, 2014-01-10) Shankare Gowda, Vrishank Raghav

This work studies the effects of radial flow on the aerodynamic phenomena occurring on a retreating blade with a focus on dynamic stall and reverse flow as applied to both a helicopter rotor in forward flight and a wind turbine operating at a yaw angle. While great progress has been made in understanding the phenomenon of two-dimensional dynamic stall, the effect of rotation on the dynamic stall event is not well understood. Experiments were conducted on a rigid two bladed teetering rotor at high advance ratios in a low speed wind tunnel. Particle image velocimetry (PIV) measurements were used to quantify the flow field at several azimuthal angles on the rotating blade during the dynamic stall event. The effect of centrifugal forces induced ``pure'' radial velocity on the dynamic stall event at 270 degrees azimuth was studied in detail. Further investigation of the radial flow field suggested that the mean radial velocity attenuated on moving outboard due to an apparent shear layer instability and it was demonstrated to be of first order importance in the flow field. These radial flow results prompted an exploration of the flow over a rotating disk to establish similarities of the radial flow over rotating blade in separated flow to that over a rotating disk in separated flow. While a greater part of this work focused on aspects of dynamic stall on the retreating blade, the final parts focus on the exotic flow regime of reverse flow (characterized by flow from the trailing edge to the leading edge of the blade). Aerodynamic loads measurement and surface flow visualization via tufts are used to first quantify the behavior of a static yawed blade in reverse flow. PIV measurements are then used on a static yawed blade and a rotating blade in reverse flow conditions to ascertain the effects of rotation on reverse flow.
Tailored Force Fields for Flexible Fabrication

(Georgia Institute of Technology, 2006-04-11) Wanis, Sameh Sadarous

The concept of tailored force fields is seen as an enabler for the construction of large scale space structures. Manufacturing would take place in space using in-situ resources thereby eliminating the size and weight restriction commonly placed on space vehicles and structures. This thesis serves as the first investigation of opening the way to a generalized fabrication technology by means of force fields. Such a technology would be non-contact, flexible, and automated. The idea is based on the principle that waves carry momentum and energy with no mass transport. Scattering and gradient forces are generated from various types of wave motion. Starting from experiments on shaping walls using acoustic force fields, this thesis extends the technology to electromagnetic fields. The interaction physics of electromagnetic waves with dielectric material is studied. Electromagnetic forces on neutral dielectric material are shown to be analogous to acoustic forces on sound-scattering material. By analogy to the acoustic experiments, force fields obtained by optical tweezers are extended to longer wavelength electromagnetic waves while remaining in the Rayleigh scattering regime. Curing of the surface formed takes place by use of a higher frequency beam that scans the surface and melts a subsurface layer enabling a sintering effect to take place between the particles. The resulting capability is explored at its extremes in the context of building massive structures in Space. A unification of these areas is sought through a generalization of the various theories provided in the literature applicable for each field.
Unsteady Aerodynamics of Rotorcraft at Low Advance Ratios in Ground Effect

(Georgia Institute of Technology, 2006-04-05) Ganesh, Balakrishnan

The aerodynamic characteristics of rotorcraft flying at low speed close to the ground are investigated. This will help better understand and quantify the flowfield structures and unsteadiness associated with various in ground effect flight conditions. This study aims to separate out the various phenomena according to their causal factors. Experimental investigations first involved flow visualization, which helped in identifying the various flight regimes and in getting an approximate estimate of the unsteadiness. The problem was divided into its unsteady and quasi-steady aspects. Hotwire measurements were performed and the unsteadiness in the flow structure was quantified. It was found that there were long time scale and fluctuations in the upwind side of the rotor disk, with significant changes in the in and flow. On the quasi-steady side, the fuselage loads for two fuselage cross-sectional shapes were investigated both in and out of ground effect. The fuselage cross-section shape had a significant effect on the loads felt by the fuselage in ground effect. The power required for the experimental configuration was measured to provide a basis for comparison. Finally, the flowfield around the ground vortex was quantified, and the structure of the ground vortex was investigated using Particle Image Velocimetry. It was found that tip vorticity was ingested by the ground vortex and that the strength of the ground vortex was considerably more than the tip vortex.
Download reduction on a wing-rotor configuation

(Georgia Institute of Technology, 2001-12) Matos, Catherine Anne Moseley
Formation and evolution of tip vortices of an isolated rotor in forward flight

(Georgia Institute of Technology, 2001-12) Wong, Oliver D.
On the flowfield and forces generated by a rectangular wing undergoing moderate reduced frequency flapping at low reynolds number

(Georgia Institute of Technology, 2001-05) Ames, Richard Gene
Computational analysis of stall and separation control in centrifugal compressors

(Georgia Institute of Technology, 2000-05) Stein, Alexander