Browsing by Author "Alexander, W. Nathan"
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- Design and Performance Analysis of Quadfoil: A Novel Aerofoil-Shaped DroneDeshmukh, Sarthak Dinesh (Virginia Tech, 2024-09-25)This project presents a comprehensive exploration of the Quadfoil drone, an innovative quadrotor design developed by the Uncrewed Systems Laboratory (USL) at Virginia Tech to address limitations of traditional quadcopters. Unlike conventional quadrotors, which rely solely on rotor-generated lift, the Quadfoil features a central lifting body, enhancing aerodynamic efficiency by reducing drag and generating additional lift during forward flight. This design results in an approximate 27% increase in efficiency at optimal forward flight speeds, enabling extended range and endurance while retaining essential VTOL (Vertical Take-Off and Landing) capabilities. The study begins with a literature review of UAV design evolution, detailing advancements from basic quadrotor models to contemporary hybrid UAVs and aerodynamic structures. Following this, the design and development process of the Quadfoil is discussed, including critical decisions on airfoil selection, materials, and structural innovations to optimize for low Reynolds number flight conditions. Key elements of the design include the use of carbon fiber composites for weight reduction and enhanced structural integrity, as well as the selection of the ARA-D 20% airfoil for its favorable lift-to-drag ratio. The Quadfoil’s innovative design also allows for significant internal space, facilitating the integration of advanced electronics and avionic systems. The research includes extensive experimental flight testing to assess the Quadfoil’s performance in terms of energy efficiency, power consumption, and flight dynamics. These tests demonstrated the Quadfoil’s ability to maintain lower power consumption in sustained forward flight compared to a conventional quadcopter, with a notable improvement in energy efficiency. Data gathered from the proof-of-concept prototype and scaled-up iterations is thoroughly analyzed, identifying areas for further optimization. The findings conclude with a discussion on the Quadfoil’s broader implications for UAV technology. The research highlights potential applications in fields such as surveillance, cargo transport, and critical medical payload delivery, showcasing the Quadfoil’s advantages in both range and endurance. The project aims to contribute to the advancement of UAV technology by providing a solid foundation for continued innovation and development.
- Excitation of Acoustic Surface Waves by TurbulenceDamani, Shishir (Virginia Tech, 2021-07-28)Acoustic metamaterials have been shown to support acoustic surface waves when excited by a broadband signal in a quiescent environment and these waves could be manipulated by varying the geometry of the structure making up the metamaterial. The study presented here demonstrates the generation of trapped acoustic surface waves when excited by a turbulent flow source. The metamaterial and flow were interfaced using a Kevlar covered single cavity whose Kevlar side faced the flow to ensure no significant disturbance to the flow and the other side was open to a quiescent (stationary) environment housing the metamaterial. Acoustic measurements were performed very close to the surface of the metamaterial in the Anechoic Wall Jet Facility at Virginia Tech using two probe-tip microphones and correlation analysis yielded the structure of the surface waves. Two different metamaterials; slotted array and meander array were tested and characterized by their dispersion relations, temporal correlations, and spatial-temporal structure. The measurements proved the existence of surface waves with propagating speeds of a tenth of the speed of sound, when excited by a turbulent boundary layer flow. These waves were much weaker than the overlying flow exciting them but showcased excellent attenuation properties away from the source of excitation. Measurements along the length of the unit-cell geometry of the metamaterial demonstrated high coherence over a range of frequencies limited by the dimension of the cell. This was a surprising behavior provided the cavity was excited by a fully developed turbulent flow over a flat plate and indicated to an area averaging phenomenon. A wall normal two-dimensional particle image velocimetry (2D-PIV) measurement was performed over the Kevlar covered cavity and a smooth surface to study the effects of the cavity on the flow. The field of view was the same for both cases which made direct flow comparison possible. Flow characteristics such as the boundary layer profiles, Reynolds stress profiles and fluctuating velocity spectrum were studied over the cavity and at downstream locations to quantify the differences in the flows. The boundary layer profiles collapsed in the inner region of the boundary layer but there were small differences in the outer region. The Reynolds stress profiles were also very similar with differences within the uncertainties of processing the images and it reflected similar average behavior of the flow over a smooth wall and a Kevlar covered cavity. The fluctuating velocity spectrum studied over the cavity location showed some differences at low frequencies for all wall normal locations while at higher frequencies the differences were within ±3 dB. These measurements showcased the underlying physics behind the interaction of acoustic metamaterials and turbulent boundary layer flows creating possibilities of using these devices for flow control although further analysis/optimization is needed to fully understand the capabilities of these systems. The demonstration of no significant effect on flow by the Kevlar covered cavity stimulated development of sensors which can average over a region of the wall pressure spectrum.
- Investigating the Aeroacoustic Properties of Kevlar FabricsSzőke, Máté; Devenport, William J.; Nurani Hari, Nandita; Alexander, W. Nathan; Glegg, Stewart A. L.; Li, Ang; Vallabh, Rahul; Seyam, Abdel-Fattah M. (2021-02-08)The aeroacoustic properties of porous fabrics are investigated experimentally in an effort to find a porous fabric as an ideal interface between wind tunnel flow and quiescent conditions. Currently, the commercially available Kevlar type 120 fabric is widely used for similar applications, such as side-walls in hybrid anechoic wind tunnels or as a cover of phased microphone arrays. A total number of 8 fabrics were investigated, namely, four glass fiber fabrics, two plain weave Kevlar fabrics, and two modified plain Kevlar fabrics with their pores clogged. Two, custom-made rigs were used to quantify the transmission loss and self-noise of all eight fabrics. It was found that the pores serve as a low-resistance gateway for sound waves to pass through, hence enabling a low transmission loss. The transmission loss was found to increase with decreasing open area ratio while other fabric properties had a minor impact on transmission loss. The self-noise of the fabrics has also been evaluated and it was found that the thread density (thread per inch) is a primary factor of determining the frequency range of self-noise with the open area ratio potentially playing a secondary role in the self-noise levels. For both metrics, the mass per unit area seemed to play a minor role in the aeroacoustic performances of the fabrics. Finally, surface pressure measurements revealed that the commercially available plain Kevlar (type 120) has no quantifiable effect on the hydrodynamic pressure field passing over the fabric, sug- gesting that Kevlar behaves as a no-slip wall from the flow's perspective when no pressure difference is present on the two sides of the fabric.
- Wall-pressure fluctuations in an axisymmetric boundary layer under strong adverse pressure gradientBalantrapu, N. Agastya; Alexander, W. Nathan; Devenport, William J. (Cambridge University Press, 2023-04)Measurements of fluctuating wall pressure in a high-Reynolds-number flow over a body of revolution are described. With a strong axial pressure gradient and moderate lateral curvature, this non-equilibrium flow is relevant to marine applications as well as short-haul urban transportation. The wall-pressure spectrum and its scaling are discussed, along with its relation to the space-time structure. As the flow decelerates downstream, the root-mean-square level of the pressure drops together with the wall shear stress (t(w)) and is consistently approximately 7t(w). While the associated dimensional spectra see a broadband reduction of over 15 dB per Hz, they appear to attain a single functional form, collapsing to within 2 dB when normalized with the wall-wake scaling where t(w) is the pressure scale and U-e/d is the frequency scale. Here, d is the boundary layer thickness and U-e is the local free-stream velocity. The general success of the wall-wake scaling, including in the viscous f(-5) region, suggests that the large-scale motions in the outer layer play a predominant role in the near-wall turbulence and wall pressure. On investigating further, we find that the instantaneous wall-pressure fluctuations are characterized by a quasi-periodic feature that appears to convect downstream at speeds consistent with the outer peak in the turbulence stresses. The conditional structure of this feature, estimated through peak detection in the time series, resembles that of a roller, supporting the embedded shear layer hypothesis (Schatzman & Thomas, J. Fluid Mech., vol. 815, 2017, pp. 592-642; Balantrapu et al., J. Fluid Mech., vol. 929, 2021, A9). Therefore, the outer-region shear-layer-type motions may be important when devising strategies for flow control, drag and noise reduction for decelerating boundary layers.