This paper considers the effect of flow advection on entropy waves. The interest is in whether entropy waves persist in gas turbine combustors, between the flame, where they are generated, and the combustor exit, where their acceleration generates acoustic waves (known as ‘entropy noise’ or ‘indirect combustion noise’). Entropy wave advection within a simplified fully developed turbulent channel-flow simulation is investigated. Entropy wave dissipation is found to be negligible, with loss of entropy wave strength caused predominantly by mean flow shear dispersion. The impulse response of entropy perturbations downstream of where they are generated is shown to be well modelled by a Gaussian profile in time. This yields a (different) Gaussian form for the inlet–outlet transfer function of entropy perturbations. For representative gas turbine flows, the magnitude of this transfer function is such that significant entropy wave strength will remain at the combustor exit, confirming that entropy-generated acoustic waves are likely to be important.
The objective of this numerical study is to increase the base pressure on a backwardfacing step via linear feedback control, to be ultimately translated to a drag reduction on a blunt-based bluff body. Two backward-facing step cases are simulated: a laminar two-dimensional (2D) flow at a Reynolds number of Re θ = 280, and a turbulent three-dimensional (3D) flow at Re θ = 1500 using large-eddy simulation. The control is effected by a full-span slot jet with zero-net-mass-flux, and two jet locations are examined. Linear system identification is performed to characterize the flow response to actuation, used to synthesize a control law. The control strategy is based on the premise that an attenuation of the instantaneous pressure fluctuations on the base of the step should lead to an increase in the time-averaged base pressure. Open-loop harmonic forcing is examined within a broad frequency range for both the 2D and 3D flows, which are found to respond differently to actuation. The controllers based on disturbance attenuation lead to sensible increases in base pressure (up to 70 % in 2D and 20 % in 3D) with higher efficiency than the best results achieved in open-loop. The results support the conjecture about the link between the base pressure fluctuations and mean, although it is shown that such a black-box model approach is not suitable for optimization without further physical insight.
The pressure drag of blunt bluff bodies is highly relevant in many practical applications, including to the aerodynamic drag of road vehicles. This paper presents theory revealing that a mean drag reduction can be achieved by manipulating wake flow fluctuations. A linear feedback control strategy then exploits this idea, targeting attenuation of the spatially integrated base (back face) pressure fluctuations. Large-eddy simulations of the flow over a D-shaped blunt bluff body are used as a test-bed for this control strategy. The flow response to synthetic jet actuation is characterised using system identification, and controller design is via shaping of the frequency response to achieve fluctuation attenuation. The designed controller successfully attenuates integrated base pressure fluctuations, increasing the time-averaged pressure on the body base by 38%. The effect on the flow field is to push the roll-up of vortices further downstream and increase the extent of the recirculation bubble. This control approach uses only body-mounted sensing/actuation and input-output model identification, meaning that it could be applied experimentally.
Perovskite has been known as a promising novel material for photovoltaics and other fields because of its excellent opto-electric properties and convenient fabrication. However, its stability has been a widely known haunting factor that has severely deteriorated its application in reality. In this work, it has been discovered for the first time that perovskite can become significantly chemically unstable with the existence of a temperature gradient in the system, even at temperature far below its thermal decomposition condition. A study of the detailed mechanism has revealed that the existence of a temperature gradient could induce a mass transport process of extrinsic ionic species into the perovskite layer, which enhances its decomposition process. Moreover, this instability could be effectively suppressed with a reduced temperature gradient by simple structural modification of the device. Further experiments have proved the existence of this phenomenon in different perovskites with various mainstream substrates, indicating the universality of this phenomenon in many previous studies and future research. Hopefully, this work may bring deeper understanding of its formation mechanisms and facilitate the general development of perovskite toward its real application.
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