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Icing is ubiquitous in nature and engineering applications, and imposes threats to road and air transportations, wind energy infrastructures, etc. However, current active de‐icing solutions, especially the most popular one, i.e., heating, suffer from high energy consumption whilst passive methods are often ineffective at high‐speed, long‐term, or large‐particle conditions. Herein, a promising strategy adopting magnetic‐responsive microfins (MRS) featuring reversible deformations is developed for de‐icing. A novel micro‐scale ice shoveling effect induced by the localized destruction of the ice adhesion interface owing to the inhomogeneous deformation is demonstrated, and its dependence on the ice particle size and temperature is investigated. An analytical model is proposed to describe the mechanism of this effect, showing a linear relation between the position of the magnet and the induced force agreeing well with experiments, leading to a system straightforward to predict and control. Specifically, the de‐icing capacity of the surface becomes prominent when small‐scale ice particles merge to large ones, providing a promising solution for applications on aircraft, wind turbines, etc., as the first of its kind to remove large particles under high‐speed conditions effectively. [ABSTRACT FROM AUTHOR]

Reduced order modeling (ROM) techniques, such as proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), have been widely used to analyze stationary flows. Neural networks such as autoencoders are effective to reduce the dimension of non-stationary flow, but their larger storage requirements compared to POD and DMD compromise the expectations on ROM. The present work aims at compressing the autoencoder model via two distinctively different approaches, i.e., pruning and singular value decomposition (SVD). The developed algorithm is then applied to reconstruct the flow fields of typical stationary (i.e., a laminar cylinder flow and two turbulent channel flows) and non-stationary (i.e., a laminar co-rotating vortex pair) examples. It is shown that pruning and SVD reduce the size of the autoencoder network to 6% and 3% for the two simple laminar cases (or 18% and 13%, 20%, and 10% for the two complex turbulent channel flow cases), respectively, with approximately the same order of accuracy. Therefore, the proposed autoencoders optimized by the network pruning and SVD lead to effective ROM of both stationary and non-stationary flows although they require more iterations to converge than conventional methods. [ABSTRACT FROM AUTHOR]

Proper orthogonal decomposition (POD) serves as a principal approach for modal analysis and reduced-order modeling of complex flows. The method works robustly with most types of fluid data and is fundamentally trusted. While, in reality, one has to discern the input spatiotemporal data as passively contaminated globally or locally. To understand this problem, we formulate the relation for uncertainty transmission from input data to individual POD modes. We incorporate a statistical model of data contamination, which can be independently established based on experimental measurements or credible experiences. The contamination is not necessarily a Gaussian white noise, but a structural or gusty modification of the data. Through case studies, we observe a general decaying trend of uncertainty toward higher modes. The uncertainty originates from twofold: self-correlation and cross correlation of the contamination terms, where the latter could become less influential, given the narrow correlation width measured in experiments. Mathematically, the self-correlation is determined by the inner product of the data snapshot and the mode itself. Therefore, the similarity between the input data and the resulting POD modes becomes a critical and intuitive indicator when quantifying the uncertainty. A scaling law is shown to be applicable for self-correlation that promotes fast quantification on sparse grids. [ABSTRACT FROM AUTHOR]

We propose a spatial-temporal multi-fidelity Gaussian process regression framework for the fusion of flow field data with various availabilities and fidelities but not sufficiently large to train neural networks commonly encountered in fluid mechanics studies. For example, fluid experiments lead to data with high fidelity but sparse in time and space, while most of the numerical data are generally regarded as less accurate but are spatially temporally continuous. The proposed framework aims at generating a new set of fused data by combining the merits of those in the spatial-temporal space. Numerical simulations [e.g., direct numerical simulation (DNS), large eddy simulation, Reynolds-averaged Navier–Stokes] of flow around a National Advisory Committee for Aeronautics 0012 airfoil are performed to collect the original raw data with various fidelities, and a fraction of the DNS result is used to mimic the high-fidelity but sparse experimental data. It is found that the accuracy of the fused data increases with the density of high-fidelity points until reaching a threshold, above which the fusion accuracy becomes insensitive. This limit can be overcome by introducing extra dimensions, such as the gradients of the low-fidelity data field. By examining the error fields, it is found that the high-fidelity points can tune low-fidelity fields but only within a limited local region. The accuracy can be firmly improved by introducing more high-fidelity points or higher levels of spatial gradients if the data set captures the temporal development. [ABSTRACT FROM AUTHOR]

The aim of this paper is to investigate the linear and weakly nonlinear dynamics in flow over a flat-plate with leading edge. Linear optimal and suboptimal inflow perturbations are obtained using a Lagrangian multiplier technique. In particular, the suboptimal inflow conditions and the corresponding downstream responses are investigated in detail for the first time. Unlike the suboptimal dynamics reported in other canonical cases such as the backward-facing step flow, the growth rate of the suboptimal perturbation is in the same order as the optimal one, and both of them depend on the lift-up mechanism even though they are orthogonal. The suboptimal mode has an additional layer of vorticity that penetrates into the boundary layer farther downstream, generating a second patch of high- and low-speed streaks. The farther suboptimal ones spread to the free-stream without entering the boundary layer. The weakly nonlinear dynamics are examined by decomposing the flow field into multiple orders of perturbations using the Volterra series. Small structures in the higher order perturbations mainly concentrate in the region farther away from wall, suggesting a mechanism of outward perturbation developments, which is opposite with the well reported inward development of perturbations, i.e., from free-stream to boundary layer. The significance of these modes is then demonstrated through a prediction of flow field from the inflow condition by exploiting the orthogonality of the modes. [ABSTRACT FROM AUTHOR]

A four-species plasma-fluid model is used to investigate the previously unexplained reversal of flow direction that occurs when the diameter of millimetric annular dielectric barrier discharge (DBD) plasma actuators reduces. We found that the transformation from a strong outward jet to a weak inward one is induced by disproportionate reduction of the negative radial electrohydrodynamic forcing component during the positive sub-cycle. This change of forcing is due to the increased curvature of the electric field lines, which leads to a higher rate of dielectric charging and thus an increasingly intermittent discharge structure. The flow reversal also depends on the thickness of the electrode and the ambient oxygen content via similar discharging mechanisms. This study indicates that a thinner electrode or higher oxygen fraction is favorable when using DBD plasma actuators in milli/microscale flow control problems since it maintains the strong outward jet at smaller diameter and with higher efficiency. [ABSTRACT FROM AUTHOR]

The development of free-stream disturbances in flow over a vertically vibrating flat plate with a slender leading edge is investigated. The evolution of the optimal inflow perturbation that results in the maximum amplification is computed to investigate the effect of the plate vibration on the development of free-stream disturbance, secondary instability of streaks and subsequently the bypass transition to turbulence. It is observed that the plate vibration leads to periodic change of the angle of attack, shifting the free-stream disturbance to the upper or lower side of the plate. Therefore, the development of steady inflow perturbations, which receive the largest amplification, is interrupted by the vibration, and the perturbation amplification via the lift-up mechanism is weakened. The vibration brings a second peak of perturbation growth at the vibration frequency, leading to high-frequency free-stream perturbations penetrating into the base boundary layer, which is not observed in flow over a stationary plate owing to the sheltering mechanism. This resonance of the flow perturbation and the vibrating plate is explained by the staggering effect of the leading edge. Further, the direct numerical simulations with the optimal inflow perturbation imposed on the inflow boundary show that the vertical vibration of the plate leads to streamwise periodic vorticity near the edge of the boundary layer. This inhomogeneity of the streamwise vorticity brings about streamwisely localized distortion of the low-speed streaks and, thus, an intermittent secondary instability. Therefore, before the streaks break down to turbulence, they undergo several rounds of secondary instabilities, resulting in an elongated bypass transition process. [ABSTRACT FROM AUTHOR]

We propose an image-based flow decomposition developed from the two-dimensional (2-D) tensor empirical wavelet transform (EWT) (Gilles, IEEE Trans. Signal Process., vol. 61, 2013, pp. 3999–4010). The idea is to decompose the instantaneous flow data, or their visualisation, adaptively according to the averaged Fourier supports for the identification of spatially localised structures. The resulting EWT modes stand for the decomposed flows, and each accounts for part of the spectrum, illustrating fluid physics with different scales superimposed in the original flow. With the proposed method, decomposition of an instantaneous three-dimensional (3-D) flow becomes feasible without resorting to its time series. Examples first focus on the interaction between a jet plume and 2-D wake, where only experimental visualisations are available. The proposed method is capable of separating the jet/wake flows and their instabilities. Then the decomposition is applied to an early-stage boundary layer transition, where direct numerical simulations provided a full dataset. The tested inputs are the 3-D flow data and their visualisation using streamwise velocity and $\lambda _{2}$ vortex identification criterion. With both types of inputs, EWT modes robustly extract the streamwise-elongated streaks, multiple secondary instabilities and helical vortex filaments. Results from 2-D stability analysis justify the EWT modes that represent the streak instabilities. In contrast to proper orthogonal decomposition or dynamic modal decomposition that extract spatial modes according to energy or frequency, EWT provides a new strategy for decomposing an instantaneous flow from its spatial scales. [ABSTRACT FROM AUTHOR]

A novel approach based on the local entropy generation rate, also known as the second law analysis (SLA), is proposed to compute and visualize the flow resistance in mass transfer through a pipe/channel with a sudden contraction component (SCC) at low Reynolds number (Re) featuring velocity slip. The linear Navier velocity slip boundary condition is implemented using the explicit scheme. At small Reynolds number, i.e., Re ≤ 10.0, the flow resistance coefficient of the SCC, KSCC, is found to be a function of the dimensionless velocity slip length L s l i p * and Re−1, and gradually increase to a constant value at contraction ratio Rarea ≥ 8, reaching a formula K S C C = (0.4454 L s l i p * 3 − 1.894 L s l i p * 2 + 2.917 L s l i p * + 8.909) / R e. Over this range of Re, the equivalent length of the flow resistance is almost independent of Re, while out of this range, the equivalent length increases monotonically with Re. Moreover, the dimensionless drag force work around the SCC is negative and reaches a minimum at a critical L s l i p * . The SLA reveals that the regions affected by the SCC mainly concentrate around the end section of the upstream pipe/channel rather than the initial partition of the downstream section reported in large Re turbulent flow, and this non-dimensional affected upstream length increases with L s l i p * . The fluid physics are further examined using SLA to evaluate the energy loss over the entire domain, decomposed as the viscous dissipation inside the domain and the drag work on the wall boundary. [ABSTRACT FROM AUTHOR]

Flow structures in the wake of a wind turbine are analysed by performing spatio‐temporal Koopman decomposition (STKD). STKD is a new method to decompose the flow field as a combination of travelling waves and to extract the frequency and wavenumbers (characterizing a specific spatial direction) of the dominant waves. The results indicate that the complex unsteady behaviour of the turbine wake flow is mainly driven by a single wave travelling downstream, combined with smaller amplitude traveling waves representing the flow rotation. This result agrees with the general description of the wake of a wind turbine regarded as the combination of self‐sustained oscillations and the development of upstream external noise, which are represented by the dominant traveling wave and the small‐magnitude ones, respectively. [ABSTRACT FROM AUTHOR]

Sensitivity of drag to tangential surface motion is calculated in flow past a circular cylinder in both two- and three-dimensional conditions at Reynolds number Re≤1000. The magnitude of the sensitivity maximises in the region slightly upstream of the separation points where the contour lines of spanwise vorticity are normal to the cylinder surface. A control to reduce drag can be obtained by (negatively) scaling the sensitivity. The high correlation of sensitivities of controlled and uncontrolled flow indicates that the scaled sensitivity is a good approximation of the nonlinear optimal control. It is validated through direct numerical simulations that the linear range of the steady control is much higher than the unsteady control, which synchronises the vortex shedding and induces lock-in effects. The steady control injects angular momentum into the separating boundary layer, stabilises the flow and increases the base pressure significantly. At Re=100, when the maximum tangential motion reaches 50% of the free-stream velocity, the vortex shedding, boundary-layer separation and recirculation bubbles are eliminated and 32% of the drag is reduced. When the maximum tangential motion reaches 2.5 times of the free-stream velocity, thrust is generated and the power savings ratio, defined as the ratio of the reduced drag power to the control input power, reaches 19.6. The mechanism of drag reduction is attributed to the change of the radial gradient of spanwise vorticity (∂rζ^) and the subsequent accelerated pressure recovery from the uncontrolled separation points to the rear stagnation point. [ABSTRACT FROM AUTHOR]

The interaction of a vortex and a pipe flow, modelled as the Lamb-Oseen vortex and the Poiseuille flow, respectively, is investigated by means of stability analyses and direct numerical simulations (DNS). From the distribution of the most unstable mode, it is observed that the instability is induced by the combination of the radial gradients of the base azimuthal and axial velocity components, e.g. an axial (or azimuthal) vorticity perturbation acts on the axial (or azimuthal) base velocity via a lift-up effect to generate axial (or azimuthal) velocity streaks, which are further stretched by the base azimuthal (or axial) velocity to create azimuthal (or axial) vorticity. This lift-up-stretch mechanism is confirmed in DNS of the model base flow initially perturbed by the most unstable mode. After nonlinear saturation, the perturbations decay since the flow no longer supports instability after sufficient radial mixing induced by the lift-up of the azimuthal and axial velocity components. These observations suggest that the vorticity outside the vortex core can be suppressed by instabilities if a streamwise boundary layer flow exists outside the core. [ABSTRACT FROM AUTHOR]

The spectra of the Batchelor vortex are obtained by discretizing its linearized evolution operator using a modified Chebyshev polynomial approximation at a Reynolds number of 1000 and zero azimuthal wavenumber. Three types of eigenmodes are identified from the spectra: discrete modes, potential modes and free-stream modes. The discrete modes have been extensively documented but the last two modes have received little attention. A convergence study of the spectra and pseudospectra supports the classification that discrete modes correspond to discrete spectra while the other two modes correspond to continuous spectra. Free-stream modes have finite amplitude in the far field whilst potential modes decay to zero in the far field. The free-stream modes are therefore a limiting form of the potential modes when the radial decay rate of velocity components reduces to zero. The radial form of the free-stream modes with axial and radial wavenumbers is investigated and the penetration of the free-stream mode into the vortex core highlights the possibility of interaction between the potential region and the vortex core. A wavepacket pseudomode study confirms the existence of continuous spectra and predicts the locations and radial wavenumbers of the eigenmodes. The pseudomodes corresponding to the potential modes are observed to be in the form of one or two wavepackets while the free-stream modes are not observed to be in the form of wavepackets. [ABSTRACT FROM PUBLISHER]

Both the asymptotical stability criterion and the bounded stability criterion of the coning motion for wrap around fin (WAF) rockets are proposed through the analysis of coning motion equations, which can be easily used to determine the existence of the coning motion during the rocket design. The correctness of the criterions is verified by mathematical simulation examples of a WAF rocket with different setting angles. It is also found that the setting angle of WAF has great effects on the rolling moment and side moment of the rocket. [ABSTRACT FROM AUTHOR]

A light detection and ranging (LIDAR) wind profiler was used to estimate the wind speed in the southern coast of Santa Catarina State, Brazil. This profiler was installed on a coastal platform 250 m from the beach, and recorded wind speed and direction from January 2017 to December 2018. The power generation from three wind turbines was simulated, to obtain estimations of the average power, energy generation and capacity factor, as well as to assess the performance of a hypothetical wind farm. The scale and shape parameters of the Weibull distribution were evaluated and compared with those of other localities in the state. The prevailing winds tend to blow predominantly from the northeast and southwest directions. Wind magnitudes are higher for the NE and SW ocean sectors where the average wind power density can reach 610–820 W m−2. The Vestas 3.0 turbine spent the largest percentage of time in operation (>76%). The higher incidence of strong northeasterly winds in 2017 and more frequent passage of cold fronts in 2018 were attributed to the cycle of the South Atlantic subtropical high. The results demonstrate a significant coastal wind power potential, and suggest that there is a significant increase of resources offshore. [ABSTRACT FROM AUTHOR]

Three-dimensional vortical structures and their interaction over a low-aspect-ratio thin wing have been studied via particle image velocimetry at the chord Reynolds number of 10 5 . The maximum lift of this thin wing is found at an angle of attack of 42 ∘ . The flow separates at the leading-edge and reattaches to the wing surface, forming a strong leading-edge vortex which plays an important role on the total lift. The results show that the induced velocity of the tip vortex increases with the angle of attack, which helps reattach the separated flow and maintains the leading-edge vortex. Turbulent mixing indicated by the high Reynolds stress can be observed near the leading-edge due to an intense interaction between the leading-edge vortex and the tip vortex; however, the reattachment point of the leading-edge vortex moves upstream closer to the wing tip. [ABSTRACT FROM AUTHOR]

Highlights from the article: The cover image is based on the Research Article I New method to capture traveling waves in flow passing a wind turbine i , by Soledad Le Clainche et al., https://doi.org/10.1002/we.2331. Cover image © Soledad Le Clainche, Xuerui Mao, José M. Vega.

Coning motion has been detected in flight experiments of many spinning projectiles for decades and the stability analysis of this motion has been extensively studied recently. In this paper, a novel optimal control algorithm based on stability analyses is developed to suppress the coning motion. A Lagrangian functional is built to compute the optimal control that minimizes the coning motion for a given control cost. Then a linearized approach, which produces accurate enough control within the range of parameters considered, is proposed to reduce the computational cost. It is analytically presented that the optimal solution of this linearized optimization is unique at any given values of control cost. The analytical analysis also indicates that there exists a direct solution of the control that approaches the linearized optimal control when the control cost is small enough and that this direct solution can be obtained much more efficiently than the linearized optimal control. The controllability problem associated with this optimal control algorithm is also discussed and the results shed lights on the choice of numerical parameters in the optimization. These optimal control algorithms are implemented to calculate the optimal control that minimizes the coning motion induced by initial disturbances to a wrap-around-fin missile. [ABSTRACT FROM AUTHOR]