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Efficiently and accurately calculating the plasma transport process is one of the difficulties in aerospace plasma application simulation, especially in the magnetic sail spacecraft applications that have a huge size. This paper develops a three-dimensional trilinear anisotropic immersed-finite-element direct-implicit particle-in-cell (IFE-DIPIC) model to solve the problem of large-scale, long-term evolution plasma with complex interfaces. The model uses the DIPIC method to track the motion of particles in the plasma while simulating the anisotropic electric field containing an interface by using a modified trilinear anisotropic IFE method. Compared to the previous models, the developed model in this paper allows for the use of larger spatial and time steps in the Cartesian meshes without inducing numerical divergence. Using an interface-independent mesh avoids redundant interpolation in the PIC method, further improving efficiency. These advantages significantly improve the efficiency in solving actual complex three-dimensional plasma physics problems. The accuracy, efficiency, stability, and applicability of the proposed model are proved through numerical examples and the application in magnetic sail. The simulation results indicate that the developed model can efficiently simulate the actual working conditions of magnetic sails. The performance is significantly influenced by both the direction and magnitude of the magnetic moment. [ABSTRACT FROM AUTHOR]

A framework of physically consistent homogenization and multiscale modeling (PCHMM) for reduced-order analysis of plate/shell structures is developed in this paper. To address the inapplicability of conventional periodic boundary conditions and Hill's condition involved in homogenization of shear-deformable shell structures, the paper proposes physically consistent boundary conditions and modified Hill's condition for plate/shells. Unlike the PCHMM method for beams, considering the contradiction between high-order displacement fields induced by shear forces and low-order kinematic assumptions, additional constraints are applied to the plate/shell structure sectional strains during the solution of perturbation fields. The correctness and effectiveness of the proposed plate/shell PCHMM framework and method are verified by typical numerical examples. The proposed theory can also be conveniently embedded into commercial finite element software for homogenization and multiscale analysis of structures such as microscale metamaterials like lattice plates and large complex structures like aircraft fuselage sections. [ABSTRACT FROM AUTHOR]

This paper examines the application of Gauss's principle of least constraint to the classical problem of the lift of a two-dimensional airfoil in ideal, incompressible fluid flow, including the role of the empirical Kutta condition that determines the airfoil circulation in classical theory. Gauss's principle demonstrates that the fluid pressure field consists of two components: a constraint pressure field that enforces the continuity and nonpenetration constraints and an impressed pressure field of arbitrary strength that satisfies but does not influence any constraints. The analysis shows that a flow solution that minimizes the difference between the acceleration and the impressed pressure gradient is a valid solution of Euler's equation. Furthermore, Gauss's principle proves that ideal potential flow airfoil theory is incomplete and that a first principles closure condition cannot exist within potential flow. This reaffirms the view that an external, empirical condition is needed to complete classical airfoil theory, i.e., the Kutta condition. The paper also investigates and disproves a new variational theory of lift developed from Hertz's principle of minimum curvature that is claimed to provide a closure condition based on first principles to replace the empirical Kutta condition of classical airfoil theory. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive experimental analysis of the evolutionary modal characteristics of a highly flexible wing that exhibits bending-torsion coupling-driven instability. By implementing operational modal analysis on responses triggered by a combination of an external pulse-like stimulation and turbulence within the flow, this paper presents the airspeed-driven variations of the modal frequencies, damping ratios, and the underlying modal coupling behavior, leading to instability. This analysis is extended to varying the wing root pitch angles through which the effects of geometrical nonlinearity are exercised. Their effects are particularly noted on the hump feature of the airspeed-driven damping ratio locus of the mode responsible for instability. The decreasing critical damping ratio is shown to result in amplified turbulence-driven responses, which pose significant challenges to identification procedures by masking the visibility of other modes. Furthermore, through a novel technique used to analyze the modal coupling, the relative phase and magnitude properties of the coupled bending-torsion composition of the critical mode before and at flutter onset are evidenced experimentally. It is demonstrated that these relative participation measures provide a strong indication of the response content of the limit cycle oscillations that emerge after the flutter speed. [ABSTRACT FROM AUTHOR]

A lightweight, 12 L liquid hydrogen fuel tank has been designed, fabricated, and tested with the aim of optimizing boil-off rates while minimizing the weight of a complete propulsion system intended to be integrated into an unmanned aerial vehicle. After looking at several different insulation schemes, the system was optimized as two concentric, lightweight titanium cylinders with high vacuum and multilayer insulation in between. The thickness of the multilayer insulation and the design of support structures were optimized to reduce the overall weight of the tank. This paper focuses on characterizing the boil-off rate of a small liquid hydrogen tank in relation to ambient temperature and fuel level. Additionally, it addresses the analysis of the instrumented transfer line between the reservoir and the operating fuel cell, presenting pressure and temperature measurements for each component of the transfer line. The efficiency of the heat exchanger under natural and forced convection is also discussed. The key contribution of the experimental campaign lies in demonstrating an average boil-off rate of 19.5 g/h over 45 h. This significantly surpasses the limits of ultra-long-range flight achievable with alternative electric energy sources. Furthermore, the paper highlights the importance of the variable boil-off with time and its impact on aircraft performance. [ABSTRACT FROM AUTHOR]

This paper introduces a novel approach to constrain limit-cycle oscillations in design optimization. The approach builds upon a limit-cycle oscillation constraint that bounds the recovery rate to equilibrium, circumventing the need for bifurcation diagrams. Previous work demonstrated the constraint using approximate recovery rates obtained by evaluating the system state velocity for prescribed states. This work proposes a fully nonlinear matrix pencil method that accurately evaluates the recovery rate based on transient simulations. The proposed method captures the amplitude variation in the recovery rate using a short time window that slides along the time history of a quantity of interest. This sliding-window matrix pencil method is first verified for a typical section model. Sensitivity analyses identify guidelines to obtain accurate recovery rates efficiently. The system is then optimized subject to limit-cycle oscillation, flutter, and side constraints, and the results are compared with the ones based on approximate recovery rates. The sliding-window matrix pencil method allows the optimizer to produce a less conservative design while preventing limit-cycle oscillations at desired operating conditions and amplitudes. The approach introduced in this paper can facilitate the inclusion of limit-cycle oscillation considerations in the design phase of a broad class of nonlinear systems. [ABSTRACT FROM AUTHOR]

Quantum sensors represent a new generation of sensors with improved precision, accuracy, stability, and robustness to environmental effects compared to their classical predecessors. After decades of laboratory development, several types of quantum sensors are now commercially available or are part-way through the commercialization process. This paper provides a brief description of the operation of a selection of quantum sensors that employ the principles of atom-light interactions and discusses progress toward packaging those sensors into products. This paper covers quantum inertial and gravitational sensors, including gyroscopes, accelerometers, gravimeters, and gravity gradiometers that employ atom interferometry, nuclear magnetic resonance gyroscopes, atomic and spin-defect magnetometers, and Rydberg electric field sensors. [ABSTRACT FROM AUTHOR]

This paper proposes a Reynolds number Re scaling for the number of grid points Ncv required in wall-modeled Large Eddy Simulation (WMLES) of turbulent boundary layers (TBL) to accurately capture the regions of flow separation. Based on the various time scales in a nonequilibrium TBL, a definition of the near-wall "underequilibrium" scales is proposed (in which "equilibrium" refers to a quasi balance between the viscous and the pressure gradient terms). This length scale is shown to vary with Reynolds number as lp~Re-2/3. A-priori analysis demonstrates that the resolution (Δ) required to reasonably predict the wall stress in several nonequilibrium flows is at least ?(10)lp, irrespective of the Reynolds number and Clauser parameter. Further, a-posteriori studies (on the Boeing speed bump, Song- Eaton diffuser, Notre-Dame Ramp, and the backward-facing step) show that scaling Δ such that Δ/lp is independent of Reynolds number results in accurate predictions of separation for the same "nominal" grid across different Reynolds numbers. Finally, we suggest that near separation and reattachment points, Ncv for WMLES scale as Re4/3, which is more restrictive than the previous estimates (~Re¹) by Choi and Moin (Choi, H., and Moin, P., "Grid-Point Requirements for Large Eddy Simulation: Chapman's Estimates Revisited," Physics of Fluids, Vol. 24, No. 1, 2012, Paper 011702) and Yang and Griffin (Yang, X. I. A., and Griffin, K. P., "Grid-Point and Time-Step Requirements for Direct Numerical Simulation and Large-Eddy Simulation," Physics of Fluids, Vol. 33, No. 1, 2021, Paper 015108). [ABSTRACT FROM AUTHOR]

DLR, German Aerospace Center, has developed a low-noise retrofit to reduce noise generation of a conventional midrange transportation aircraft, i.e., measures to the airframe and engine exhaust nozzle. Selected measures have been installed onboard of DLR's flying testbed Advanced Technology Research Aircraft and been subject to a flyover noise campaign. The aircraft in its original configuration and the modified aircraft have been tested and measured in flyover campaigns in 2016 and 2019, respectively. Initial simulation models have been derived from the results and previous component simulations and wind tunnel tests to account for the new reduction measures within DLR's system noise prediction tool Parametric Aircraft Noise Analysis Module (PANAM). This paper presents the developed measures as selected for the flight test campaigns. An overview of the conducted flight tests is provided. The overall noise simulation process, including aircraft and engine design, flight simulation, and noise prediction, is presented. Simulation results obtained from this process are then compared to the measured levels from the flight campaigns. Estimated simulation uncertainties from PANAM can now be directly compared to differences between measured and simulated levels. Finally, a parameter study is conducted in order to assess the reduction potential of the novel retrofit measures along different simulated flight procedures. Spatial noise distribution in sound exposure level contours is assessed. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of a nonlinear static deformation on the dynamic aeroelastic behavior of very flexible wings. The objects of study are the Pre-Pazy and Pazy wings, which are highly flexible wings presented by the Large Deflections Working Group of the Third Aeroelastic Prediction Workshop. A nonlinear static aeroelastic coupling analysis is performed, coupling a nonlinear beam formulation with the Vortex Lattice Method. The natural frequencies and mode shapes of the wing are obtained considering the predeformed structure, and these results serve as input data for the dynamic aeroelastic model, which considers the Unsteady Vortex Lattice Method as the aerodynamic model and serves to yield the aeroelastic responses in modal coordinates. The frequency spectrum of each time response serves as input for a system identification method, which considers the Least Squares Complex Frequency-domain estimator. The velocity-damping-frequency diagrams are obtained from the identified frequencies and damping ratios, and the critical flutter condition is determined. The flutter speeds obtained agree with the references, and the differences are up to 77% for the Pre-Pazy wing and 4% for the Pazy wing. A significant reduction in those speeds is observed with an increase in the angle of attack. [ABSTRACT FROM AUTHOR]

This paper studies transition trajectory optimization for a tiltwing electrical vertical takeoff and landing (eVTOL) aircraft configuration and evaluates the benefits of equipping it with fluid injection active flow control (AFC). Using a three-degree-of-freedom model of the aircraft's longitudinal dynamics, both hover-to-cruise (forward) and cruise-to-hover (backward) transition are investigated for a range of center-of-gravity (CG) positions to determine minimum-energy trajectories that achieve zero altitude variation while avoiding wing aerodynamic stall. It is demonstrated that constant-altitude transition can be achieved in the forward direction (with or without AFC), whereas the use of AFC in backward transition reduces the total altitude change by 24%. A comparative constant-altitude backward transition solution using a different airfoil with a higher stall limit suggests that a redesign of the tail wing of this unique tiltwing aircraft could better leverage the benefits of using AFC in this application. Additionally, it is also shown that, in the case of forward transition, AFC can be used to decrease the total transition time and increase the range of CG positions where constant-altitude transition can occur. [ABSTRACT FROM AUTHOR]

Airfoil design using numerical optimization has addressed the problem of minimizing drag for a given lift constraint for one or more flight conditions. However, designers often want to avoid separation at off-design conditions regardless of the drag. This paper develops a separation constraint formulation for airfoil shape optimization. We perform multipoint optimizations and demonstrate how a separation constraint yields practical airfoil shapes. We compare results from single-point optimizations with and without a leading-edge radius constraint, which is a simple approach used as a surrogate to avoid separation, to an optimization where separation is constrained at a low-speed high-lift off-design point. Constraining the leading-edge edge radius improves the high-lift performance of the airfoil but not as much as constraining separation at the off-design point. We also compare the results of separation-constrained optimizations with optimizations where the drag at an off-design point is included in the objective. The latter formulation also produces optimal designs that are separation-free at the off-design points. However, the separation-constrained formulation produces airfoils with better cruise performance. Finally, we demonstrate the separation constraint efficacy at the off-design point with multiple cruise points. The results help guide parameterization and constraint formulation for airfoil design optimization problems. [ABSTRACT FROM AUTHOR]

The increasing complexity of aircraft systems and the shift toward electrical aircraft are driving the introduction of entirely new concepts in aircraft systems and airframe design; it is therefore likely that the evaluation of aircraft systems and their integration with the airframe in the conceptual design phase will play an ever more crucial role in determining the feasibility of a newly proposed aircraft. This paper proposes a bio-inspired design algorithm for allocating space within the airframe to interconnections that supply aircraft systems. The proposed method is an adaptation of an existing pathfinding algorithm, with a novel mechanism to avoid overlap between systems that require segregation. The input required for the proposed method is likely to be available at the conceptual design phase. A detailed description of the proposed method is given, followed by a demonstration using a design problem based on an existing aircraft. Results of the demonstration show that the proposed method is capable of handling redundant systems and their segregation by reducing the overlap between different systems. [ABSTRACT FROM AUTHOR]

In the past decade, CubeSats have undergone a revolution, moving from university research projects to enabling industry opportunities and government missions. In 2014, the Jet Propulsion Laboratory, California Institute of Technology (JPL/Caltech) initiated a research and technology development effort to advance CubeSat communication capabilities. One of the critical thrusts was the Ka-band parabolic deployable antenna (KaPDA). This antenna started with the ambitious goal of fitting a 42 dB, 0.5 m, 35 GHz antenna in a 1.5U canister. At that time, there had been minimal development in high-gain CubeSat antennas, critical for high-data-rate communications and remote sensing science. A Ka-band high-gain antenna would provide a 10,000 times increase in data communication rates over an X-band patch antenna and a 100 times increase over state-of-the-art S-band parabolic antennas. This paper discusses designing, building, integrating, and operating the flight antenna from a mechanical perspective, its final performance, and lessons learned. KaPDA enabled the RainCube mission, the first Earth Science CubeSat to have an active instrument. RainCube was launched in May 2018, making KaPDA the second deployable parabolic antenna to fly on a CubeSat and the first to operate in Ka-band, enabling follow-on opportunities for high-rate antenna communications and remote sensing science. [ABSTRACT FROM AUTHOR]

This paper deals with the analysis of wave propagation characteristics in various prestressed structures with geometric nonlinearities using the Carrera Unified Formulation (CUF). CUF provides a versatile platform to model a wide range of structures and nonlinearities that can take care of all wave propagation aspects. In this work, different geometric nonlinearities for which representative governing equations have been derived and numerical solutions have been obtained through a unified approach are considered. The study investigates in detail the effect of prestress and geometric nonlinearity on wave propagation behavior. The results indicate that prestress has a very influential effect on modal frequency and dispersion characteristics for wave propagation. Specifically, three CUF-modeled beams are considered herein, having a sandwich, metallic portal, and metallic box cross section, respectively. Initially, the principal cross-sectional modal shapes of the unstressed, linear, and full nonlinear (i.e., full three-dimensional Green-Lagrange strain matrix) beam with a prestress are investigated, among which torsional and flexural modes can be recognized. Afterward, the equilibrium curves of such structures for various geometrical nonlinear approximations are traced, highlighting that most types of nonlinearity induce a hardening behavior in the system, which increases with the preload, directly leading to a variation in modal frequencies. The dispersion relations of the full nonlinear structure examined as a function of the applied preload are further compared, enriching the investigation by exploiting wave finite element method capabilities. This knowledge paves the way toward the design and optimization of prestressed systems with enhanced acoustic performance, and that fosters the development of sound absorption, noise insulation, and structural isolation. [ABSTRACT FROM AUTHOR]

This paper focuses on extending coupled structural-thermal reduced order modeling approaches for the prediction of the nonlinear geometric response of heated structures to efficiently account for temperature-dependent structural and thermal properties. Structurally, a linear dependence of the elasticity tensor and thermal expansion coefficient is assumed consistently with typical material behavior. The resulting structural reduced order model (ROM) equations exhibit polynomials of the temperature generalized coordinates, the coefficients of which are readily identified. A different strategy is proposed for conductance and capacitance properties which typically depend nonlinearly on temperature. It relies on splitting the temperature generalized coordinates into a small set of dominant ones having a nonlinear effect on the ROM conductances and capacitances and a much larger set affecting them linearly. The inclusion of uncertainty in the structural ROM is also addressed extending earlier work limited to temperature independent properties. These strategies are exemplified on a representative hypersonic vehicle panel undergoing a full mission profile and with aero-thermal-structural coupling. The ROM predictions are observed to be very close to the full finite element ones for the mean model. Moreover, an uncertainty analysis demonstrates the strong sensitivity of the response to the structural model in the strongly nonlinear regions of the trajectory. [ABSTRACT FROM AUTHOR]

Accurately characterizing the dynamic load environment is vital for the structural optimization and fatigue life assessment of liquid rocket engines, but the difficulty in precisely measuring or identifying such distributed loads is substantial. This paper proposed a method to determine the equivalent concentrated loads of liquid rocket engines at main vibration sources using the direct inverse method in the frequency domain. Response verification is performed to confirm the effectiveness of the equivalent loads by assessing the consistency between responses due to them and actual distributed loads. A pumped liquid rocket engine is investigated for specific research. Responses from the hot-fire test are used to identify equivalent loads at three main vibration sources, which are the gas generator, turbine shell, and combustion chamber. The results indicate that the errors in responses induced by the equivalent loads and the actual distributed loads are generally within ±4 dB. To further validate the equivalent loads, an experiment is conducted, scaling and applying the equivalent loads to the rocket engine. The resulting responses, after amplification, align well with those obtained from the hot-fire test, confirming the validity of the proposed method. However, response verification errors escalate significantly when nonprimary sources are included in the equivalent locations. [ABSTRACT FROM AUTHOR]

This paper presents a novel algorithm for aeroelastic flutter early detection. Two new features for flutter onset detection are presented. Flutter early warning is accomplished using only measured signals, with essentially no prior knowledge needed about the aircraft or the flutter mechanism involved. The algorithm consists of three stages: 1) extraction of regularity features, 2) calibration by addition of white noise to nominal measurements, and 3) thresholding. Four types of datasets were used: a) synthetic data, b) simulated data generated using aeroelastic response simulations to stochastic gusts, c) measured data from a wind tunnel experiment, and d) flight test data including actual flutter onsets. The algorithm was shown to be able to flag an impending flutter event before critical onset occurs. [ABSTRACT FROM AUTHOR]

Bayesian optimization (BO) is one of the most powerful strategies to solve computationally expensive-to-evaluate blackbox optimization problems. However, BO methods are conventionally used for optimization problems of small dimension because of the curse of dimensionality. In this paper, a high-dimensionnal optimization method incorporating linear embedding subspaces of small dimension is proposed to efficiently perform the optimization. An adaptive learning strategy for these linear embeddings is carried out in conjunction with the optimization. The resulting BO method, named efficient global optimization coupled with random and supervised embedding (EGORSE), combines in an adaptive way both random and supervised linear embeddings. EGORSE has been compared to state-of-the-art algorithms and tested on academic examples with a number of design variables ranging from 10 to 600. The obtained results show the high potential of EGORSE to solve high-dimensional blackbox optimization problems, in terms of both CPU time and the limited number of calls to the expensive blackbox simulation. [ABSTRACT FROM AUTHOR]

The efficient design of air intake is crucial for high-speed airbreathing propulsion systems. This paper presents a novel design method that integrates the recently introduced internal conical flow M (ICFM) basic flowfield (Musa et al., "New Parent Flowfield for Streamline-Traced Intakes," AIAA Journal, Vol. 61, No. 7, 2023, pp. 2906-2921) with the pressure-corrected osculating axisymmetric flows and streamline tracing techniques. The new design method takes into account the effects of lateral pressure gradients by incorporating pressure corrections into the original design method using crossflow velocity information from the ICFM flowfield. Additionally, new entrance and throat profiles are introduced for designing two hypersonic internal waverider intakes with shape transition. One intake is designed using the new method, and the other using the original method. Viscous simulations have been performed at design (i.e., Mach 6.0) and off-design (i.e., Mach 4.0 and 3.5) conditions. The results indicate that the pressure-corrected intake exhibits superior performance in terms of total pressure recovery and reduced flow distortion. This reveals that the new design method can achieve a smooth shape and efficient aerodynamic transitions between the intake entrance and throat. The new entrance and throat profiles realized better performance than the typical ones. The startability analysis reveals that the Van Wie curve controls the considered intakes. Thus, combining the pressure-corrected osculating axisymmetric flows with the ICFM flowfield enhances the performance of hypersonic intakes, making this approach a promising avenue for future hypersonic vehicles. [ABSTRACT FROM AUTHOR]

The chemical reaction and cold wall effects in hypersonic reacting turbulent boundary layers (HRTBLs) cause the failure of the existing wall function method. Therefore, this paper proposes an improved wall function method to simulate skin friction cf and wall heat flux qw in HRTBLs accurately and efficiently. Firstly, the temperature-velocity relation is extended to a generalized enthalpy-velocity relation to comprehensively involve the chemical reaction and cold wall effects. Then, a new unified velocity law of the wall for the inner layer of HRTBLs is theoretically derived. Finally, an improved wall function method is designed with the new laws of the wall. Numerical experiments of typical HRTBLs show that, compared with the existing wall function method, the improved wall function method can significantly reduce the errors of cf and qw from 8.0% and 10.2% to 0.4% and 0.5%, respectively, when the near-wall grids are relaxed to the viscous sublayer, while from 11.0% and 15.3% to 1.2% and 5.8%, respectively, when the near-wall grids are further relaxed to the logarithmic layer, bringing 2.7 and 15 times enhancement of the simulation efficiency. Moreover, by introducing the reference point, the improved wall function method removes the boundary layer edge quantities while ensuring accurate results. [ABSTRACT FROM AUTHOR]

Propeller-wing-flap systems are subject to complex aerodynamic interactions between each part of the system. Although the propeller-wing interaction in cruise conditions is well defined, the high-lift condition is relatively unexplored. Effective analysis of the complex aerodynamic relationship between propeller, wing, and flap is being impeded by a lack of understanding of the underlying mechanisms. In this paper, we therefore investigate the effects of a 2D jet impinging on a multisection airfoil. We quantify which factors that define a jet-wing-flap configuration dominate lift, drag, and moment responses. We further investigate interactions between these factors and discuss how they affect the flow. We find that the jet velocity ratio is by far the dominant factor in lift, drag, and moment responses, but it does not have strong interactions with other factors. The sensitivities of the multi-element airfoil do not change significantly when impinged upon by a jet, except when critical Mach numbers are exceeded. This strongly affects the aerodynamic response and dominant sensitivities. We furthermore conclude that the immersion of the flap is a key aspect when it comes to augmenting the lift by increasing the dynamic pressure in the flowfield. The conclusions from this paper can provide key insights for propeller-wing-flap flows. [ABSTRACT FROM AUTHOR]

Aircraft propulsion electrification is currently being considered by industry and academia as one of the most promising strategies to reduce air transport emissions and increase overall efficiency levels. In the past decade, several papers were published on this subject, with the majority indicating encouraging fuel burn benefits versus conventional, fossil-fuel-based propulsion systems when future technologies, novel aircraft configurations, and synergistic propulsive-airframe integration are employed. However, a much smaller effort has been applied to the economic aspects of hybrid and fully electric propulsion, which are crucial for a successful product introduction. The present paper describes the modeling of a baseline general-aviation-type aircraft and its propulsion system retrofit with electrified architectures, exploring different electrification strategies for a fixed airframe design. Analyses are performed at the aircraft level, comparing recurring and cash operating costs for several cost and durability scenarios. While considerable CO2 reductions may be achieved in some electrification strategies, aircraft performance is significantly penalized, and important improvements in economic figures of merit are needed in order to make electrified propulsion cost-competitive. Electrified architectures tend to increase costs: turboelectric increases recurring equipment costs, while hybrid-electric increases recurring and direct maintenance costs, especially at higher degrees of energy hybridization. [ABSTRACT FROM AUTHOR]

The recent interest in the development of supersonic transport raises concerns about an increase in community noise around airports. As noise certification standards for supersonic transport other than Concorde have not yet been developed by the International Civil Aviation Organization, there is a need for a physics-based scaling rule for supersonic transport takeoff noise performance. Assuming supersonic transport takeoff noise levels are dominated by the engine mixed jet velocity and the aircraft-to-microphone propagation distance, this paper presents a reduced-order model for supersonic transport takeoff noise levels as a function of four scaling groups: cruise Mach number, takeoff aerodynamic efficiency, takeoff speed, and number of installed engines. This paper finds that, as cruise Mach number increases, supersonic transport takeoff noise levels increase while their thrust cutback noise reduction potential decreases. Assuming constant aerodynamic efficiency, takeoff speed, and number of installed engines, the takeoff noise levels and noise reduction potential of a Mach 2.2 aircraft are found to be ∼15.3  dB higher and ∼19.2  dB less compared to a Mach 1.4 aircraft, respectively. This scaling rule can potentially yield a simple guideline for estimating an approximate noise limit for supersonic transport, depending on their cruise Mach number. [ABSTRACT FROM AUTHOR]

Aircraft are subject to small disturbances during taxiing, many of which are accidental and difficult to explain. Although nonlinear factors are considered in traditional analysis, Coulomb friction is generally ignored, and the interaction and common effects of nonlinear factors are mostly not discussed. In this paper, the dynamic model of shimmy is established, and the influence of Coulomb friction on shimmy is studied by using bifurcation theory and the structural mechanics analysis method. The results show that the system with Coulomb friction has subcritical Hopf bifurcation and that the system with square damping has supercritical Hopf bifurcation. Although the two do not change the stability of the system, their cooperation can improve the stability of the system. The Coulomb friction torque of the system has a complex piecewise functional relationship with the stability distance, rake angle, and acceleration. Some combinations will lead to very low Coulomb friction and deteriorate the anti-interference ability of the landing gear. This paper provides theoretical basis and support for the rational design of structural parameter collocation, enhancing the antidisturbance ability of the system during constant-speed or variable-speed taxis and explaining the shimmy phenomenon in the process of variable-speed taxis of some aircraft. [ABSTRACT FROM AUTHOR]

This paper examines the performance of a 3D printed acoustic metamaterial as an acoustic treatment for aircraft engine nacelles in the Advanced Noise Control Fan. As the level of air travel continues to increase, so too does the demand for better noise-reduction technologies for aircraft. Engines are one of the two main sources of noise generated by aircraft, with fan noise, in particular, being of concern due to its broadband and tonal contributions. Small and lightweight methods of addressing both broadband and tonal noise are necessary due to the limitations presented by the current engine design. Presented in this paper is a novel acoustic metamaterial that has undergone design optimization for broadband noise reduction. The final design was produced using 3D printing and tested using the Advanced Noise Control Fan at the University of Notre Dame. It was found that the material is capable of reducing the first harmonic of the blade passing frequency by up to 18.5 dB, with an overall noise reduction of 3.7 dB. [ABSTRACT FROM AUTHOR]

The structural safety factor is an essential parameter in aircraft design, representing the ratio of the design load to the operating load. Traditional design methods rely on subjective determination of safety factor values based on experience, lacking objectivity in quantifying uncertainty. However, with advancements in aircraft design technology and increasing competition in the commercial space market, new-generation hypersonic aircraft with complex load environments require a more optimal approach. Applying a uniform safety factor to each component within subregions of the aircraft leads to overly conservative results and impacts flight performance. To address this limitation, a design scheme that incorporates subregional, differentiated safety factors is necessary. This approach allows for better material utilization and ensures compliance with safety requirements. This paper utilizes reliability-based design optimization theory to consider uncertainty in structural systems. It establishes a mapping relationship between structural reliability and differentiated safety factors, providing safety under uncertainty while guaranteeing weight reduction. Additionally, this paper develops a subregional, differentiated safety factors distribution program to determine the safety factors of different subregions of the structure. Consequently, a refined subregional differentiated safety factors scheme that balances safety and economy is derived. [ABSTRACT FROM AUTHOR]

This paper presents a nonlinear-model-inversion control law to suppress stall flutter of an airfoil with active trailing-edge morphing. First, a nonlinear aeroelastic model is proposed utilizing two nonlinear autoregressive neural networks with exogenous inputs, which are used to predict aerodynamic moments on an airfoil due to large-amplitude oscillation and camber morphing, respectively. Afterward, a nonlinear-model-inversion control system is designed upon the mentioned aeroelastic system to suppress stall flutter via the camber morphing. A fluid-structure-control (FSC) coupling strategy is developed with structure and control systems embedded in the high-fidelity computational-fluid-dynamics environment to validate the control effect. The FSC high-fidelity simulations show that the nonlinear-model-inversion controller can suppress pitching oscillation completely, whereas a linear proportional-derivative controller without time delay only performs a limited suppression rate by 25.6%. The flowfield evolution result infers that the active camber morphing can generate a converse training-edge vortex, which counteracts the leading-edge vortex during stall flutter. From the perspective of an energy hysteresis, active camber morphing works well by converting injected aerodynamic energy from positive to negative. [ABSTRACT FROM AUTHOR]

Quantum computing has become a pivotal innovation in computational science, offering novel avenues for tackling the increasingly complex and high-dimensional optimization challenges inherent in engineering design. This paradigm shift is particularly pertinent in the domain of structural optimization, where the intricate interplay of design variables and constraints necessitates advanced computational strategies. In this vein, the gate-based variational quantum algorithm utilizes quantum superposition and entanglement to improve search efficiency in large solution spaces. This paper delves into the gate-based variational quantum algorithm for the discrete variable truss structure size optimization problem. By reformulating this optimization challenge into a quadratic, unconstrained binary optimization framework, we bridge the gap between the discrete nature of engineering optimization tasks and the quantum computational paradigm. A detailed algorithm is outlined, encompassing the translation of the truss optimization problem into the quantum problem, the initialization and iterative evolution of a quantum circuit tailored to this problem, and the integration of classical optimization techniques for parameter tuning. The proposed approach demonstrates the feasibility and potential of quantum computing to transform engineering design and optimization, with numerical experiments validating the effectiveness of the method and paving the way for future explorations in quantum-assisted engineering optimizations. [ABSTRACT FROM AUTHOR]

This paper investigates an output-based approach for flutter bifurcation analysis of single-bay panels in supersonic flow. The approach leverages bifurcation forecasting, a class of methods to predict bifurcation diagrams using prebifurcation output data. This work is the first study into this approach applied to panel limit-cycle oscillations, building on previous efforts focused on geometrically nonlinear wings and propeller-nacelle systems. The study uses output data from transient simulations of single-bay panels at as few as two preflutter dynamic pressures, which are selected using an eigenvalue-based criterion that ensures consistent prediction accuracy across panel configurations. The approach captures the bifurcation type and amplitude variation of limit-cycle oscillations around the flutter point for a variety of materials, boundary conditions, thermal loads, and cross-stream curvatures. This approach can facilitate nonintrusive panel limit-cycle oscillation analyses for parametric studies and design. [ABSTRACT FROM AUTHOR]

The pursuit of lightweight structures has propelled spacecraft with large flexible appendages to the forefront of modern aerospace engineering. Flexoelectric actuators, with their inherent self-sensing and self-actuating capabilities, perfectly align with the demand for lightweight structures. This paper presents a dynamic model and a flexoelectric vibration control method tailored for spacecraft equipped with single-sided flexible appendages. The spacecraft system is a central rigid hub with flexible cantilever beam appendages that allow for single-axis rotation. The flexible beam has a flexoelectric element attached to it. Leveraging Hamilton's principle, the equations of motion are derived, accounting for flexoelectric effects, the centrifugal stiffening effect, and the coupling between rigid body attitude and flexible beam vibration. A closed-loop proportional-derivative controller is crafted for attitude control of the system. Additionally, a flexoelectric patch is employed as an actuator for active vibration control of the flexible beam. The precision and efficacy of the proposed model and control scheme are meticulously validated through numerical simulation. The results indicate that the flexoelectric patch can effectively manage the vibration of the flexible beam, consequently impacting the attitude control performance of the system. Furthermore, the flexoelectric effect of the system under static conditions is scrutinized, and the influence of the flexoelectric actuator on the rigid-flexible coupling characteristics of the system is thoroughly analyzed. These analyses not only confirm the feasibility of flexoelectricity in rigid-flexible coupled systems, but also open avenues for applications and optimizations in intelligent control methods for flexible appendages. [ABSTRACT FROM AUTHOR]

Many scientific space missions need highly precise attitude and orbit control or ultrafine drag compensation, which relies on the variable-thrust propulsion technology operating at the micronewton level. Cold gas thruster (CGT) is a very promising solution, mainly because of its high reliability. One of the keys to the success of micronewton variable-thrust CGT is to understand the flow in its nozzle, whose configuration is much more complex than traditional CGT nozzles. This paper applies kinetic-based multiscale models to investigate the gas flow in the complex nozzle of a micronewton variable-thrust CGT. The simulations reveal that the flow simultaneously experiences all kinds of regimes from continuum to free molecular. The continuum breakdown is likely to occur near the throat region due to large gradients of flow variables and in the expander due to low gas density. Frictional choking is observed, and the nozzle length can be optimized to improve the thruster performance. Nozzle performance measures such as thrust, discharge coefficient, and thrust efficiency are found to change only with the throat Knudsen number 𝐾⁢𝑛𝑡. The performance curves can be divided into two sections at 𝐾⁢𝑛𝑡≃0.1, and thereby an empirical piecewise formula for thrust prediction is proposed [ABSTRACT FROM AUTHOR]

This paper reports on the implementation and assessment of a wall-modeled large-eddy simulation (WMLES) methodology in an unstructured-grid, node-centered flow solver, FUN3D, that is developed and supported at the NASA Langley Research Center. Finite-volume (FV) and finite-element (FE) discretization schemes considered in the study provide formal second-order spatial accuracy. Large-eddy simulations (LES) resolve large-scale turbulent-flow features, and small-scale effects are modeled using the Vreman subgrid-scale model. At solid-wall boundaries, a shear-stress model is employed to provide a proper boundary-flux closure. The nonlinear equations are integrated in time using either an optimized backward difference formula or an implicit multistage Runge-Kutta temporal scheme. The implicit equations at each time step are solved by strong nonlinear iteration schemes. WMLES demonstrations are shown for two high-lift configurations, namely, the McDonnell Douglas 30P30N multi-element airfoil and a NASA High-Lift Common Research Model. Results show that the WMLES approaches implemented in the FV and FE discretization methods produce consistent solutions and are capable of capturing key aerodynamic characteristics and flow structures for high-lift configurations at a wide range of angles of attack, including maximum-lift conditions. In the 30P30N example, correct trends in the variations of integrated aerodynamic forces and moments, surface pressure distributions, and boundary-layer profiles are captured as the Reynolds number is increased. [ABSTRACT FROM AUTHOR]

The blended-wing-body (BWB) is hoped to be the main configuration of next-generation transport aircraft. However, its large flat belly may cause a violent skipping motion during the ditching on water. In this paper, the skipping motions of a BWB aircraft ditching on calm and wavy water are numerically investigated. The finite volume method with the volume-of-fluid approach is employed to solve the unsteady Reynolds-averaged Navier-Stokes equations. A coupled method between global moving mesh and overset mesh is proposed to avoid a large background domain and improve the prediction accuracy of the free surface, and its accuracy is well validated by predicting the motions of a skipping stone, the hydrodynamic load and pressure on a flat plate in high-speed ditching, and the fifth-order Stokes wave in a dynamic wave tank. At the beginning of every surfing stage, the flat belly of BWB aircraft impacts heavily on the water surface at a smaller pitch angle and larger speed, resulting in the local vertical overload peak, which is much larger than the peak in the porpoising motion. The wavy water surface intensifies the heave and pitch motions and increases the risk of the cockpit diving into the water. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on the anti-icing performance of full-scale rotating spinners with different cone angles. The experiments were conducted in a 3m×2m ice wind tunnel (IWT), and the spinners were equipped with hot-air anti-icing systems. Three different cone angles (80°, 70°, and 60°) were examined. The surface temperature of the spinners was measured using an infrared (IR) camera, and ice accretion and water film flow were recorded using high-speed cameras. Numerical simulations were also performed to analyze the water collection coefficients and anti-icing heat energy. The results showed that a larger cone-angle spinner has a larger water collection coefficient but a smaller heat-transfer coefficient and total surface area, resulting in better anti-icing performance for the same conditions. Additionally, the results showed that the residual ice on the spinner exhibited small, white, discretely distributed icicles. Furthermore, the surface temperature in the icing region of the 60° spinner was found to be almost equal to the total temperature of the incoming flow, indicating rime-ice accretion even under glaze-ice conditions. This observation can be attributed to the low water collection coefficient in this region. [ABSTRACT FROM AUTHOR]

Bleed-air control of ram-air parafoils is a lightweight and cost-effective mechanism that enables significant landing accuracy improvements compared to traditional trailing edge brake control due to the addition of direct glide slope control. Single-surface parafoil canopies have been shown to be lightweight and possess similar flight behavior as their ram-air counterparts. The design of bleed-air spoilers is more challenging for single-surface canopies, which lack the internal ram-air that provides a high pressure differential at any chordwise location. This paper explores bleed-air spoilers on single-surface parafoils with a focus on turn rate and glide slope control capabilities. Through a flight test campaign, it was shown that bleed-air spoilers on a single-surface parafoil provide both turn rate control from asymmetric vent opening and glide slope change from symmetric vent opening. Vents located far forward at the 10% chord location yield substantial control response with a maximum turn rate response of 38 deg/s and a maximum glide slope change of 58%. Vents located farther aft at 30, 50, and 70% chords demonstrated diminished control authority. Varying the spanwise locations of the spoilers revealed that the outermost vents had negligible lateral and longitudinal control authority. [ABSTRACT FROM AUTHOR]

This paper presents a structural optimization of a 3D-printed thermoplastic-core propeller blade for high-altitude unmanned aerial vehicles using a genetic algorithm. It aims to minimize the weight of the blade by creating longitudinal holes and spars and to minimize the blade tip deflection during operation at an altitude of 16 km. The objective is to check the validity of a 3D-printed material modeling approach for large-scale and more complex 3D-printed parts, such as hollow twisted blades. The approach is based on attentively selecting the printing parameters that reduce the anisotropy of the 3D-printed parts. The linear isotropic behavior of 3D-printed Polylactic Acid and Acrylonitrile Butadiene Styrene M30 tensile samples, which have been tested at ambient and low temperatures (-60°C), is utilized to numerically predict the experimental deformation and natural frequencies of 3D-printed substitute and twisted full blades with good accuracy. The validated linear isotropic model of each material is then used to evaluate the objectives and the constraints of the two-objective optimization process using one-way fluid-structure interaction. Four optimized blades have been selected from the Pareto fronts as candidates, printed, and tested in bending and vibration. The numerical and experimental results of the candidate blades in terms of deformation and natural frequencies are in good agreement, proving the validity of the present macroscale approach for large-scale hollow twisted blades. [ABSTRACT FROM AUTHOR]

This paper presents a method for predicting the buckling load of stiffened panels using multifidelity modeling based on quadratic neural networks (QNNs) with adaptive activation functions. The effectiveness of the proposed approach is demonstrated through a series of simulations on a range of stiffened panel configurations, and the results are compared to those obtained from traditional multifidelity and high-fidelity models in terms of accuracy and computational efficiency. Numerical experiments demonstrate that the model can accurately and efficiently predict the buckling load of stiffened panels while significantly reducing the computational cost of evaluating the surrogate model. In particular, the proposed adaptive quadratic neural networks (AQNNs) model achieves convergence approximately three times faster and four times less trainable parameters compared to traditional artificial neural networks while maintaining the same validation loss. This approach can significantly improve the design and optimization of aerospace structures by easily and quickly exploring various design configurations and finding stable and efficient configurations. This study highlights the potential of a new multifidelity modeling framework for predicting the buckling load and collapse load of aerospace structures by enhancing convergence speed and prediction accuracy while reducing the computational complexity of neural networks. [ABSTRACT FROM AUTHOR]

Monitoring the three-dimensional formation of ice layers on airfoils during icing wind tunnel experiments is extremely challenging. For the first time, this paper demonstrates the use of a single plenoptic camera to perform transient, nonintrusive in situ measurements of ice crystal accretion. Experiments have been conducted in the Altitude Icing Wind Tunnel at the National Research Council of Canada under different icing conditions to assess the potential of the new technique. Using the camera in a close-up configuration, the results show the evolution of the three-dimensional shape of the accreted ice in high spatial and temporal resolution and in absolute metric units. The computed surface meshes allow for a detailed analysis in terms of ice shape, surface area, and ice volume. Posttest shapes are compared to measurements taken using a commercial laser scanner. Although not rated for ice surfaces, this device is used as a reference to compare the detailed surface structure after registering the data sets. The results of the two methods are in good agreement and show a mean relative deviation of the plenoptic camera of about 0.15 mm. [ABSTRACT FROM AUTHOR]

Spatiospectral lobes are significant contributors to noise radiated from full-scale tactical aircraft. Prior studies have explored lobe frequency-domain characteristics, but a joint time-frequency domain analysis has the potential to further describe these phenomena and connect them to source-related events in the time waveform. This paper uses acoustical data collected from a 120-microphone array near a T-7A-installed F404 engine to characterize the spatiospectral lobes in combinations of the time, frequency, and spatial domains. An event-based beamforming method is used in conjunction with a wavelet transform to determine propagation angles and event source locations corresponding to each of the lobes. Temporospectral events in the wavelet transform are then analyzed using Markov chains. Finally, spatiospectral maps created from the measured data are decomposed into individual lobes using events in the wavelet transform as a guide. The spatiospectrotemporal combination of these three analyses shows that the lobes originate from multiple, overlapping regions along the jet lipline and that each lobe has its own peak radiation angle. Additionally, events corresponding to the spatiospectral lobes occur intermittently and at different times from each other, leading to bursts of acoustic energy with rapidly changing directivities. [ABSTRACT FROM AUTHOR]

The paper illustrates the effect of temperature on composite material properties and its influence on the response of composite plates. Two composites considered include laminae with uniaxially oriented and with in-plane randomly oriented fibers. Analytical solutions are presented for a uniformly heated large-aspect-ratio plate deforming into a cylindrical surface. The solutions are demonstrated for thermal buckling and natural frequencies for the cases of controlled loading and controlled displacements. Micromechanical residual and lifetime thermally induced stresses at the interface of fibers and matrix are assessed to predict a possible onset of local damage after the curing and during lifetime. The material considered in numerical examples consists of silicon carbide fibers and a titanium alloy matrix. The moduli of elasticity and shear as well as the coefficients of thermal expansion of this composite are significantly affected by temperature. The thermally induced microscopic radial stress at the fiber-matrix interface is high, so it should be monitored in silicon fiber titanium matrix composites to avoid the onset of local damage. In conclusion, accounting for the effect of temperature on material properties is highly desirable since it produces more accurate solution than those utilizing the properties at the room temperature. [ABSTRACT FROM AUTHOR]

This paper reports a novel cone-ray model of background-oriented schlieren (BOS) imaging that accounts for depth-of-field effects. Reconstructions of the density field performed with this model are far more robust to the blur associated with a finite aperture than conventional reconstructions, which presume a thin-ray pinhole camera. Our model is characterized and validated using forward evaluations of simulated buoyancy-driven flow and both simulated and experimental BOS measurements of hypersonic flow over a sphere. Moreover, the model is embedded in a neural reconstruction algorithm, which is demonstrated with a total variation penalty and the compressible Euler equations. Our cone-ray technique dramatically improves the accuracy of BOS reconstructions: the shock interface is well-resolved in all our tests, irrespective of the camera's aperture setting, which spans f-numbers from 22 down to 4. [ABSTRACT FROM AUTHOR]

The nonparametric probabilistic method (NPM) is a physics-based machine learning approach for model-form (MF) uncertainty quantification (UQ), model updating, and digital twinning. It extracts from reference data information not captured by a real-time computational model and infuses it into a "hyperparameterized," stochastic version of this model by solving an inverse statistical problem formulated using a loss function and a few hyperparameters. The loss function is designed such that the mean value and statistical fluctuations of some quantities of interest predicted using the real-time stochastic model match target values obtained from the reference data. NPM's performance hinges upon the efficient minimization of the loss function, which is unfortunately stochastic and nonconvex and thus prone to getting the optimization procedure trapped in suboptimal local minima. The latter scenario is exacerbated when the reference data is scarce. The paper addresses these issues by adopting the squared quadratic Wasserstein distance as the measure of dissimilarity between two different sets of data and by reformulating NPM's inverse statistical problem as a multimodal data-assimilation problem. The potential of the resulting enhanced NPM for MF-UQ, model updating, and digital twinning is demonstrated using numerical simulations relevant to applications in structural dynamics, including structural health monitoring. [ABSTRACT FROM AUTHOR]

This paper investigates the first failure load of simply supported and clamped laminate and sandwich plates loaded by a distributed normal traction on the top surface. A third-order shear and normal deformable plate theory, the isogeometric basis functions, and five failure criteria, namely, the maximum stress, the Tsai-Wu, the Tsai-Hill, the Hoffman, and the Hashin for laminated plates, and only the Tsai-Hill criterion for sandwich plates are used in this study. Of these, Hashin's criteria distinguish between the fiber and the matrix failure. The in-plane stresses are found from the plate displacements and Hooke's law, and the transverse stresses are recovered by integrating the equilibrium equations through the thickness. Effects of the plate aspect ratio, the fiber angle, the face sheet materials, and the core materials on the first failure load are identified. The computed results are found to agree well with those reported in the literature. Whereas the five failure criteria for laminated plates predict nearly the same value of the first failure load, they do not provide the same location of the failure initiation. [ABSTRACT FROM AUTHOR]

This paper investigates experimentally the structure of turbulent blunt trailing edge body wakes with varying boundary-layer thicknesses controlled through the freestream velocity and the chord length. The intrinsic effect of transition to turbulence on the vortex-shedding frequency, strength, and its three-dimensional structure is explored. At large-scales, the vortex shedding is characterized by significant phase variations along the span, which vary stochastically and are punctuated by vortex dislocations when the phase differences grow large. These features of the vortex shedding are quantitatively examined in a streamwise-spanwise plane of the wake with particle image velocimetry. When the point of transition shifts upstream due to an increasing Reynolds number, greater phase variations in the vortex shedding along the span and more frequent dislocation events are produced. These changes are linked to the spanwise correlation of the streamwise velocity in the wake. In the turbulent boundary-layer regime, it is found that the phase drift does not change significantly. The decline in the spanwise correlation with increasing boundary-layer thickness in this regime is instead linked to the relative strength of the vortex shedding compared to the random turbulent fluctuations in the wake. [ABSTRACT FROM AUTHOR]

This paper focuses on the construction of accurate and predictive data-driven reduced models of large-scale numerical simulations with complex dynamics and sparse training datasets. In these settings, standard, single-domain approaches may be too inaccurate or may overfit and hence generalize poorly. Moreover, processing large-scale datasets typically requires significant memory and computing resources, which can render single-domain approaches computationally prohibitive. To address these challenges, we introduce a domain-decomposition formulation into the construction of a data-driven reduced model. In doing so, the basis functions used in the reduced model approximation become localized in space, which can increase the accuracy of the domain-decomposed approximation of the complex dynamics. The decomposition furthermore reduces the memory and computing requirements to process the underlying large-scale training dataset. We demonstrate the effectiveness and scalability of our approach in a large-scale three-dimensional unsteady rotating-detonation rocket engine simulation scenario with more than 75 million degrees of freedom and a sparse training dataset. Our results show that compared to the single-domain approach, the domain-decomposed version reduces both the training and prediction errors for pressure by up to 13% and up to 5% for other key quantities, such as temperature, and fuel, and oxidizer mass fractions. Lastly, our approach decreases the memory requirements for processing by almost a factor of four, which in turn reduces the computing requirements as well. [ABSTRACT FROM AUTHOR]

The Air Force Research Laboratory/Air Force Office of Scientific Research (AFRL/AFOSR) introduced the Boundary Layer Turbulence (BOLT II) flight experiment to further the understanding and modeling of a hypersonic turbulent boundary layer on a geometry that features concave surfaces with swept leading edges. In support of the flight experiment, this paper identifies and documents the transition instabilities and quantifies the breakdown to turbulence on a 25%-scale BOLT II model in hypersonic flow. Experiments were conducted in the conventional Actively Controlled Expansion wind tunnel facility and the Mach 6 Quiet Tunnel located at the Texas A&M University National Aerothermochemistry and Hypersonics Laboratory. Global surface heating was viewed using infrared thermography with on-surface measurements made using high-frequency Kulite and PCB Piezotronics surface pressure transducers. Modal growth was observed among the sensors both upstream and downstream of the model. Off-surface measurements were made in the mixed-mode region utilizing constant-temperature hot-film anemometry in the Mach 6 Quiet Tunnel. Comparisons were made to the Direct Numerical Simulation (DNS) results from the University of Minnesota. Amplified content was observed between f=53 and 75 kHz in the root-mean-square voltage fluctuations of the hot-film measurements. [ABSTRACT FROM AUTHOR]