M1 phase MoVTeNb mixed oxides exhibit unique catalytic properties that lead to high C2H4 yields in oxidative conversion of C2H6 at moderate temperatures. The role of the heptagonal channel micropores of the M1 phase in regulating reactivity and selectivity is assessed here using reactant size-dependent kinetic probes and density functional theory (DFT) treatments for C2H6 and cyclohexane (C6H12) activations inside and outside the micropores. The sizes of C2H6 and the micropores suggest a tight guest–host fit, but C6H12 cannot access intrapore sites. Measured C2H6 to C6H12 activation rate ratios on MoVTeNbO are much higher than those measured on nonmicroporous vanadium oxides (VO x /SiO2) and estimated by DFT on external surfaces, suggesting that most C2H6 activations on MoVTeNbO occur inside the micropores under typical conditions. C2H6 exhibits higher activation energy than C6H12 on VO x /SiO2, consistent with the corresponding C–H bond strengths; the activation energy difference between C2H6 and C6H12 is lower on MoVTeNbO because micropores stabilize C–H activation transition states through van der Waals interactions. Product selectivities for C2H6 and C6H12 suggest that the ability of VO x /SiO2 to activate C–H bonds and resist O-insertion in products is similar to the external surfaces of MoVTeNbO, but the micropores in the latter oxides are more selective for C–H activation. DFT calculations show that the tight confinement in micropores hinders the C–O contact necessary for O-insertion. These insights provide guidance for utilizing shapes and sizes of confining voids to mitigate selectivity limitations dictated by thermodynamics of sequential oxidation reactions and electronic properties of redox catalysts.

In recent years, the data-driven turbulence model has attracted widespread concern in fluid mechanics. The existing approaches modify or supplement the original turbulence model by machine learning based on the experimental/numerical data, in order to augment the capability of the present turbulence models. Different from the previous researches, this paper directly reconstructs a mapping function between the turbulent eddy viscosity and the mean flow variables by neural networks and completely replaces the original partial differential equation model. On the other hand, compared with the machine learning models for the low Reynolds (Re) number flows based on direct numerical simulation data, high Reynolds number flows around airfoils present the apparent scaling effects and strong anisotropy, which induce large challenges in accuracy and generalization capability for the machine learning algorithm. We mainly concentrate on the high Reynolds number turbulent flows around the airfoils and take the results calculated by the computational fluid dynamics solver with the Spallart-Allmaras (SA) model as training data to construct a high-dimensional data-driven network model based on machine learning. The radial basis function neural network and the auxiliary optimization methods are adopted, and the individual models are built separately for the flow fields of the near-wall region, wake region, and far-field region. The training data in this paper is extracted from only three subsonic flow fields of NACA0012 airfoil. The data-driven turbulence model can be applied to various airfoils and flow states, and the predicted eddy viscosity, lift/drag coefficients, and skin friction distributions are all in good agreement with the results of the original SA model. This research demonstrates the promising prospect of machine learning methods in future studies about turbulence modeling.

Piezochromic fluorescent (PCF) materials with distinct multicolor switching have attracted wide attention in many fields such as optoelectronic devices and deformation detection. However, few PCF materials with low-pressure stimuli and good recoverability have been reported. A highly sensitive and easily recoverable PCF molecular system that can switch between green (G) and orange (O) emissions upon an extremely low piezoresponsive (PR) of 0.5 MPa and heating at 120 °C is demonstrated. A mechanistic study combining X-ray diffraction analysis and the theoretical calculations reveal that a slight change in slipping-angle of π-stacks induced by mechanical pressure amplifies the exciton couplings from G to O J-aggregates, leading to not only distinct PCF switching but also high emission efficiencies >0.5 owing to superradiance of J-aggregate excitons. Benefiting from low MPa PR, high emission efficiency, and good recoverability applications including haptic sensors and anti-counterfeiting application are demonstrated. This research introduces the effect of stimuli-responsive excitonic coupling as new design guidance for developing PCF materials with low-pressure stimuli, high emission efficiency, and good recoverability.

M1-phase MoVTeNb mixed oxides contain V-oxo species isolated by dispersion in the Mo-oxo framework and one-dimensional heptagonal micropores that tightly enclose C2H6 molecules. These oxides catalyze C2H6 oxidation with C2H4 selectivity much higher than V2O5 oxides containing continuous V-oxo domains without micropores. Here, effects of the structures of VO x domains and of the micropores on the selectivity are discerned using (i) measured rate constant ratios and activation barrier differences relevant to selectivity on the two oxides and (ii) density functional theory (DFT) analysis of steps mediating C–H activation in C2H6 and C2H5 radicals and unselective C–O bond formations in C2H5 radicals and C2H4 molecules on (001) surfaces of both oxides and in pores of MoVTeNb oxides. The DFT-derived values of kinetic parameters representing C2H4 selectivities and activation energy differences between C2H4 formation and C–O bond formation steps on V2O5(001) are similar to measured values. In contrast, for MoVTeNb oxides, the DFT-derived selectivity inside the pores is much higher than measurements, while that on the (001) surfaces is much lower, suggesting that measured selectivity represents contributions from C–H activations inside the pores and unselective steps inside pores as well as on (001) surfaces. The selectivity on (001) surfaces is similar in V2O5 and MoVTeNb oxides, indicating that the isolation of V-oxo domains within this surface leads to only small changes in selectivity, while the pores lead to much higher selectivity. The descriptors of the selectivity trends on such transition metal oxide surfaces are derived by examining C2H4 epoxidation and C2H6 C–H activation transition-state energies and molecule-surface van der Waals (vdW) interactions and steric forces that influence these energies on a variety of O atoms with different electronic and structural properties on (001) surfaces and inside the pores. High C2H4 selectivity requires that the O atoms in oxides exhibit lower tendency to form C–O bonds in C2H4 than to activate C–H bonds in C2H6, which depends strongly on the H atom addition energies of oxides and O atom coordination. V2–O–V tri-coordinated and V–O–V or V–O–Mo bridging O atoms require significantly greater energy penalty than VO terminal O atoms for the metal–oxygen framework distortions required for forming C–O bonds; these distortion energies reflect steric hindrance to forming C–O bonds, which leads to higher epoxidation transition-state energy and indicates higher C2H4 selectivity in tri-coordinated and bridging O atoms. The high selectivity inside the heptagonal pores originates from the inaccessibility to terminal O atoms in addition to much stronger vdW interactions and more significant steric distortion energies in tight pores. These analyses suggest that H atom addition energies, vdW interaction energies, and catalyst distortion energies are relevant descriptors of selectivity for both intrapore and external O atoms.

Transonic buffet is a phenomenon of aerodynamic instability with shock wave motions which occurs at certain combinations of Mach number and mean angle of attack, and which limits the aircraft flight envelope. The objective of this study is to develop a modelling method for unstable flow with oscillating shock waves and moving boundaries, and to perform model-based feedback control of the two-dimensional buffet flow by means of trailing-edge flap oscillations. System identification based on the ARX algorithm is first used to derive a linear model of the input–output dynamics between the flap rotation (the control input) and the lift and pitching moment coefficients (system outputs). The model features a pair of unstable complex-conjugate poles at the characteristic buffet frequency. An appropriate reduced-order model (ROM) with a lower dimension is further obtained by a balanced truncation method that keeps the pair of unstable poles in the unstable subspace but truncates the dynamics in the stable subspace. Based on this balanced ROM, two kinds of feedback control are designed by pole assignment and linear quadratic methods respectively. These independent designs, however, result in similar suboptimal static output feedback control laws. When introduced in numerical simulations, they are both able to completely suppress the buffet instability. Furthermore, the resulting controllers are even able to stabilize buffet flows with nonlinear disturbances and in off-design flow conditions, thus implying their robustness. The analysis of the feedback control laws indicates that parameters (frequency and phase) corresponding to the ‘anti-resonance’ of the linear input–output model are vital for optimal control. The best performance is obtained when the control operates close to the ‘anti-resonance’, which is supported by the optimal frequency and the phase of the open-loop control as well as by the optimal phase of the closed-loop control.

It is well known that the occurrence of vortex-induced vibration (VIV) at a subcritical Reynolds number, which is lower than 47, is induced by fluid-structure interaction. However, for the free flow, this phenomenon disappears at a Reynolds number about 20. The current study provides an explanation to the disappearance of the VIV by capturing the evolution of the purely fluid modes at Reynolds numbers between 12 and 55. To ensure accurate mode extraction, the dynamic mode decomposition technique is utilized. Results show that the stable von Kármán vortex shedding mode exists in a subcritical flow regime but becomes less distinct as the Reynolds number decreases. When the Reynolds number is lower than 18, this mode nearly vanishes, characterizing the lower boundary of fluid-structure instability.

Mechanistic details and product distributions for C3H8–O2 reactions catalyzed by gas-phase NO x species are compared to reactions on solid V2O5 catalysts. C3H8 conversions are greatly enhanced by addition of small concentrations of NO to C3H8–O2 mixtures without solid catalysts because homogeneous catalytic redox cycles involving oxidation of NO to NO2, reduction by H-addition to form HONO and release of OH radicals facilitate abstraction of H-atoms from strong C–H bonds in C3H8. NO x -mediated conversions exhibit C3H6 selectivity values among the highest reported at similar oxidative conditions because OH radicals are strong abstractors that activate C–H bonds via early transition states that do not exhibit significant bond elongation, which dampens bond-strength sensitivity and the preference to activate weak allylic C–H bonds in C3H6 products over strong bonds in C3H8. C3H8 conversion rates increase with residence time and NO, O2, and H2O pressures and exhibit supra-linear dependence on C3H8 pressure with trends analogous to homogeneous systems involving H-abstraction by OH radicals but significantly different from V2O5 catalysts that exhibit linear C3H8 pressure dependence and weak sensitivity to the other parameters. Temperature dependencies of rate-constant-ratios representing activation energy differences between primary and secondary C3H8 and allylic C3H6 C–H bonds in NO x -catalyzed routes are much weaker than V2O5, consistent with dampened bond-strength sensitivity that is also observed in density functional theory estimates of these differences for OH radicals. NO x mediation enables efficient alkane activation providing high productivity and yields at moderate temperatures, which is important for chemical transformations requiring rate-limiting C–H activation.

Frequency lock-in can occur on a spring suspended airfoil in transonic buffeting flow, in which the coupling frequency does not follow the buffet frequency but locks onto the natural frequency of the elastic airfoil. Most researchers have attributed this abnormal phenomenon to resonance. However, this interpretation failed to reveal the root cause. In this paper, the physical mechanism of frequency lock-in is studied by a linear dynamic model, combined with the coupled computational fluid dynamics/computational structural dynamics (CFD/CSD) simulation. We build a reduced-order model of the flow using the identification method and unsteady Reynolds-averaged Navier–Stokes computations in a post-buffet state. A linear aeroelastic model is then obtained by coupling this model with a degree-of-freedom equation for the pitching motion. Results from the complex eigenvalue analysis indicate that the coupling between the structural mode and the fluid mode leads to the instability of the structural mode. The instability range coincides with the lock-in region obtained by the coupled CFD/CSD simulation. Therefore, the physical mechanism underlying frequency lock-in is caused by the linear coupled-mode flutter – the coupling between one structural mode and one fluid mode. This is different from the classical single-degree-of-freedom flutter (e.g. transonic buzz), which occurs in stable flows; the present flutter is in the unstable buffet flow. The response of the airfoil system undergoes a conversion from forced vibration to self-sustained flutter. The coupling frequency certainly should lock onto the natural frequency of the elastic airfoil.