scholarly journals Flight Data-Based Wind Disturbance and Air Data Estimation

Atmosphere ◽  
2021 ◽  
Vol 12 (4) ◽  
pp. 470
Author(s):  
Zhenxing Gao ◽  
Haofeng Wang ◽  
Zhiwei Xiang ◽  
Debao Wang
Keyword(s):  
Turbulence Model ◽  
Wind Field ◽  
Estimation Accuracy ◽  
Small Scale ◽  
Prevailing Wind ◽  
Wind Disturbance ◽  
Flight Data ◽  
Von Karman ◽  
Wide Range ◽  
Von Kármán

The instantaneous wind field and air data, including true airspeed, angle of attack, angle of sideslip, cannot be measured and recorded accurately in wind disturbance. A new air data and wind field estimation method is proposed based on flight data in this study. Since the wind field is the horizontal prevailing wind added by turbulence, the slowly time-varying prevailing wind and small-scale turbulence are described by the exponentially correlated stochastic wind model and von Karman turbulence model, respectively. The system update equation of air data is built based on inertial measurements instead of the complex aerodynamic and aero-engine model of aircraft. Benefitted by the post-analysis characteristics of flight data, a forward–backward filtering algorithm was designed to improve the estimation accuracy. Simulation results indicate that the forward–backward filter integrated with the von Karman turbulence model can reduce the estimation error and ensure filtering stability. A further test with actual flight data shows that the forward–backward filter is not only able to track the wide-range change in prevailing wind but also reduce the adverse effects of uncertain disturbance on estimation accuracy.

scholarly journals Elastohydrodynamics of a pre-stretched finite elastic sheet lubricated by a thin viscous film with application to microfluidic soft actuators

2019 ◽  
Vol 862 ◽  
pp. 732-752 ◽  
Author(s):  
Evgeniy Boyko ◽  
Ran Eshel ◽  
Khaled Gommed ◽  
Amir D. Gat ◽  
Moran Bercovici
Keyword(s):  
Closed Form ◽  
Closed Form Solution ◽  
Finite Domain ◽  
Viscous Film ◽  
Elastic Sheet ◽  
Von Karman ◽  
Von Kármán Equations ◽  
Wide Range ◽  
Soft Actuators ◽  
Von Kármán

The interaction of a thin viscous film with an elastic sheet results in coupling of pressure and deformation, which can be utilized as an actuation mechanism for surface deformations in a wide range of applications, including microfluidics, optics and soft robotics. Implementation of such configurations inherently takes place over finite domains and often requires some pre-stretching of the sheet. Under the assumptions of strong pre-stretching and small deformations of the lubricated elastic sheet, we use the linearized Reynolds and Föppl–von Kármán equations to derive closed-form analytical solutions describing the deformation in a finite domain due to external forces, accounting for both bending and tension effects. We provide a closed-form solution for the case of a square-shaped actuation region and present the effect of pre-stretching on the dynamics of the deformation. We further present the dependence of the deformation magnitude and time scale on the spatial wavenumber, as well as the transition between stretching- and bending-dominant regimes. We also demonstrate the effect of spatial discretization of the forcing (representing practical actuation elements) on the achievable resolution of the deformation. Extending the problem to an axisymmetric domain, we investigate the effects arising from nonlinearity of the Reynolds and Föppl–von Kármán equations and present the deformation behaviour as it becomes comparable to the initial film thickness and dependent on the induced tension. These results set the theoretical foundation for implementation of microfluidic soft actuators based on elastohydrodynanmics.

Existence of a sharp transition in the peak propulsive efficiency of a low- pitching foil

2016 ◽  
Vol 800 ◽  
pp. 307-326 ◽  
Author(s):  
Anil Das ◽  
Ratnesh K. Shukla ◽  
Raghuraman N. Govardhan
Keyword(s):  
Power Coefficient ◽  
Energetic Cost ◽  
Mean Power ◽  
Thrust Coefficient ◽  
Propulsive Efficiency ◽  
Von Karman ◽  
Wide Range ◽  
The Mean ◽  
The One ◽  
Von Kármán

We perform a comprehensive characterization of the propulsive performance of a thrust generating pitching foil over a wide range of Reynolds ($10\leqslant Re\leqslant 2000$) and Strouhal ($St$) numbers using a high-resolution viscous vortex particle method. For a given $Re$, we show that the mean thrust coefficient $\overline{C_{T}}$ increases monotonically with $St$, exhibiting a sharp rise as the location of the inception of the wake asymmetry shifts towards the trailing edge. As a result, the propulsive efficiency too rises steeply before attaining a maximum and eventually declining at an asymptotic rate that is consistent with the inertial scalings of $St^{2}$ for $\overline{C_{T}}$ and $St^{3}$ for the mean power coefficient, with the latter scaling holding, quite remarkably, over the entire range of $Re$. We find the existence of a sharp increase in the peak propulsive efficiency ${\it\eta}_{max}$ (at a given $Re$) in the $Re$ range of 50 to approximately 1000, with ${\it\eta}_{max}$ increasing rapidly from about 1.7 % to the saturated asymptotic value of approximately $16\,\%$. The $St$ at which ${\it\eta}_{max}$ is attained decreases progressively with $Re$ towards an asymptotic limit of $0.45$ and always exceeds the one for transition from a reverse von Kármán to a deflected wake. Moreover, the drag-to-thrust transition occurs at a Strouhal number $St_{tr}$ that exceeds the one for von Kármán to reverse von Kármán transition. The $St_{tr}$ and the corresponding power coefficient $\overline{C_{p,}}_{tr}$ are found to be remarkably consistent with the simple scaling relationships $St_{tr}\sim Re^{-0.37}$ and $\overline{C_{p,}}_{tr}\sim Re^{-1.12}$ that are derived from a balance of the thrust generated from the pitching motion and the drag force arising out of viscous resistance to the foil motion. The fact that the peak propulsive efficiency degrades appreciably only below $Re\approx 10^{3}$ establishes a sharp lower threshold for energetically efficient thrust generation from a pitching foil. Our findings should be generalizable to other thrust-producing flapping foil configurations and should aid in establishing the link between wake patterns and energetic cost of thrust production in similar systems.

scholarly journals Characterizing seismic scattering in 3D heterogeneous Earth by a single parameter

2020 ◽  
Author(s):  
Jagdish Chandra Vyas ◽  
Martin Galis ◽  
Paul Martin Mai
Keyword(s):  
Standard Deviation ◽  
Correlation Length ◽  
Hurst Exponent ◽  
Seismic Waves ◽  
The Other ◽  
Small Scale ◽  
Seismic Scattering ◽  
Von Karman ◽  
Velocity Variations ◽  
Von Kármán

<p>We analyze the power spectral density (PSD) of von Karman autocorrelation function (ACF) to derive a theoretical parameter which characterizes the scattering of seismic wavefield due to random heterogeneities in 3D Earth structure. We then verify our analytical findings by performing ground-motion simulations. We characterize scattering using root-mean-square (RMS) fluctuations of normalized seismic wave speed, which represents wavefield scattering due to random heterogeneities in 3D Earth under the diffraction condition. The isotropic von Karman ACF is parameterized by correlation length a, standard deviation σ, and Hurst exponent H. To compute the RMS value, we simplify the von Karman PSD for three regimes: k·a ≫ 1 (λ ≪ a), k·a ≈ 1 (λ ≈ a) and k·a ≪ 1 (λ ≫ a), where λ is wavelength and k wavenumber of the seismic waves. The analysis of the RMS values reveals that 1) scattering is proportional to the standard deviation σ of small-scale velocity variations in all three regimes, 2) scattering is inversely proportional to the correlation length in the k·a ≫ 1 regime, but directly proportional to the correlation length in the other two regimes, 3) a small Hurst exponent H for the k·a ≫ 1 regime leads to scattering controlled solely by the standard deviation of small-scale velocity variations (for the other two regimes, it leads to weaker scattering). The seismic scattering effectively vanishes for H approaching zero. Our theoretical findings are purely physics based and are furthermore verified by 3D high resolution numerical simulations. Hence, we developed solid physics-based understanding of 3D seismic scattering due to random heterogeneities in the Earth which will be helpful for future modeling studies.</p>

scholarly journals Approaches for Reduced-Order Modeling of Electrically Actuated von-Karman Microplates

2016 ◽  
Vol 12 (1) ◽  
Author(s):  
Shahid Saghir ◽  
M.I. Younis
Keyword(s):  
Finite Element ◽  
Finite Element Model ◽  
Electrostatic Force ◽  
Element Model ◽  
Reduced Order Models ◽  
Reduced Order ◽  
Linear Eigenvalue Problem ◽  
Von Karman ◽  
Wide Range ◽  
Von Kármán

This article presents and compares different approaches to develop reduced-order models for the nonlinear von-Karman rectangular microplates actuated by nonlinear electrostatic forces. The reduced-order models aim to investigate the static and dynamic behavior of the plate under small and large actuation forces. A fully clamped microplate is considered. Different types of basis functions are used in conjunction with the Galerkin method to discretize the governing equations. First, we investigate the convergence with the number of modes retained in the model. Then for validation purpose, a comparison of the static results is made with the results calculated by a nonlinear finite element model. The linear eigenvalue problem for the plate under the electrostatic force is solved for a wide range of voltages up to pull-in. Results among the various reduced-order modes are compared and are also validated by comparing to results of the finite-element model. Further, the reduced-order models are employed to capture the forced dynamic response of the microplate under small and large vibration amplitudes. Comparison of the different approaches is made for this case.

A Scale-Adaptive Turbulence Model Based on the k-Equation and Recalibrated Reynolds Stress Constitutive Relation

2016 ◽  
Vol 138 (6) ◽  
Author(s):  
Yang Zhang ◽  
Jun-Qiang Bai ◽  
Jing-Lei Xu
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Turbulence Model ◽  
Boundary Layer Flow ◽  
Dissipation Rate ◽  
Turbulent Dissipation ◽  
Turbulent Dissipation Rate ◽  
Von Karman ◽  
Layer Flow ◽  
Von Kármán

An algebraic relationship between turbulent dissipation rate and von Karman length are used to dismiss the transport equation of turbulent dissipation rate in standard k−ε (SKE) turbulence model. Meanwhile, a recalibrated Bradshaw's assumption is built based on the data from a boundary layer flow of turbulent flat plate simulated by direct numerical simulation (DNS). The JL model is reformed to a one-equation model which only depends on the turbulent energy, so the new model can also be called kinetic-energy dependent only (KDO) turbulence model. As the KDO model is using the von Karman length scale, it can automatically adjust to fit the resolved structures of the local flow. Results will be shown for the boundary layer flow on a turbulent flat plate, and the external flows of an NACA4412 airfoil, an ONERA-M6 wing, a three dimension delta wing, and an NACA0012 airfoil at deep stall.

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