Weiyu Lu’s research while affiliated with Nanjing Tech University and other places

Single periodic excitation is widely used in unsteady flow separation control. However, in some two-dimensional (2D) or three-dimensional (3D) flow separation scenarios, dual excitations are necessary or advantageous. Referring to dual-excitation flow separation control, a nonlinear reduced-order model is established in this study to describe the interaction between dual external excitations with the internal flow instability of the separated flow and to provide physical insight into flow control mechanisms. The model is based on the Navier-Stokes equation, flow images of typical 2D or 3D flow separation, and the Stuart–Landau model. Additionally, a comprehensive index is proposed to evaluate flow control performance reflected by the reduced-order model, which accounts for flow losses from time-averaged and fluctuating flow, thus providing a more comprehensive evaluation of the model characteristics. Furthermore, we compare the results of the model to some numerical and experimental results of typical 2D and 3D flow separation control with dual excitations. The agreement of the model results and the numerical and experimental results verify the reduced-order model and its comprehensive index to a certain degree.

This study presents a new concept of self-driven pulsed jet flow control on a compressor stator blade. This passive unsteady flow control method has the advantage that neither external flow nor electrical source is needed. This study’s preliminary proof-of-concept study on a low-speed compressor stator blade is performed using numerical simulation. When the pulsed jet frequency is 100 Hz, the optimum control performance and control efficiency are reached, and the total pressure loss coefficient is reduced by 8.9%. As the valve’s rotational speed increases, the pulsed jet’s momentum coefficient decreases gradually. The analysis of the unsteady characteristics of the self-driven pulsed jet shows that the jet velocity is close to a periodic square wave signal, and the typical reduced jet velocity ranges from approximately 0.15 to 0.8. Moreover, the time-averaged driving torque on the valve depends on the rotational speed and is relevant to the self-starting and self-driven characteristics of this passive flow control method. Under different bearing resistance torque, the self-driven valve behaves differently in three cases that can self-start and be self-driven, cannot self-start but can be self-driven, and cannot self-start nor be self-driven.

The tip leakage vortex (TLV) induced by the tip clearance flow has a significant impact on the performance of centrifugal compressors, causing impeller flow losses and reducing the stall margin. To solve this problem, an unsteady flow control technology called the NCFC method is proposed based on the concept of negative circulation control, realized by a vortex generator placed in a tube connected with the shroud through a hole. The approach is derived from a theoretical study of the compressor TLV by introducing a two-dimensional vortex model. A numerical simulation is then performed to verify the effectiveness of the NCFC method. The result shows that the NCFC method can greatly stabilize the flow field at the blade tip and improve the stall margin and efficiency of the compressor without reducing the total pressure ratio of the compressor, which has the characteristics of both unsteadiness and negative circulation effect. In addition, a HC method with only unsteady excitation effect is also studied for comparison, which only slightly stabilizes the blade tip flow and increases the stall margin of the compressor, suggesting that the NCFC is more effective than the HC. Finally, it is highly recommended to improve the efficiency of any unsteady jet/suction and separation flow interaction.

In this study, a novel fluidic oscillator suitable for use as a key component of a flow control device is proposed and investigated through numerical simulations. The new layout adds resonators to a typical fluidic oscillator with dual feedback channels, and the length of the feedback loop is designed to be adjustable. This fluidic oscillator with movable feedback channels and resonators can generate a jet with an adjustable frequency, and it has smaller total pressure loss than the baseline model. Numerical results show that the movement of the feedback channels regulates the degree of coupling between the feedback channels and resonators to generate different orders of jet frequencies. This self-excited fluidic oscillator with adjustable jet frequency is more adaptive than typical designs when dealing with complex flow separation conditions, and it will be more stable because the frequency adjustment requires neither high-frequency movable mechanisms nor external energy input. Moreover, the frequency switching phenomenon is observed in special cases, which may help improve the efficiency of the compressor blades with a drastically changed dominant frequency under off-design conditions or with multiple dominant frequencies, such as tip leakage flow and shock–boundary layer interaction.

A two-dimensional vortex model is introduced in this paper in order to understand the characteristics of the shedding vortex in a blade-divergent passage and to mitigate or suppress it by appropriate methods. The performance of this model under the influence of three typical external factors is studied, namely, the main flow extrusion effect, viscous effect, and transport effect. Based on the analysis, a negative circulation unsteady flow control technique is proposed to compensate for the viscous effect, which is known as NCFC. Numerical simulation is performed to verify the effectiveness of the NCFC method. The results show that the NCFC method is superior to the conventional unsteady flow control for improving the performance of the blade-divergent passage in most cases. In addition, there is an optimum injection to suppress the shedding vortex with NCFC, which is about 0.2% of the main flow mass, and NCFC shows to be more efficient than conventional flow control in weakening the shedding vortex. Furthermore, NCFC can effectively inhibit separation flow and is shown to be insensitive to the injection flow mass. Finally, the NCFC method is highly recommended to adapt to the fact that the working conditions often change in practice.

A centrifugal compressor of a micro turbine generator system is investigated by the theoretical model and numerical analysis to explore the characteristics of the tip leakage vortex as the centrifugal compressor approaches stall. The numerical simulation results show the cross-sectional shape of the tip leakage vortex is elliptical, and its long and short axes are gradually stretched as the compressor approaches stall. Moreover, the vortex trajectory is inclined to the pressure side of the adjacent blade. In addition, the Kirchhoff elliptical vortex model is introduced to analyze the flow passage constriction effect, the passage vortex squeezing effect, and the leakage flow translation effect. Results show that there is no upper limit for the flow passage constriction effect on the tip leakage vortex. Furthermore, relative to the original vortex, the minimum constriction effect depends on the axis ratio of the elliptical tip leakage vortex. The passage vortex has an expansion effect on the tip leakage vortex rather than a squeezing effect, which is limited and also depends on the axis ratio of the ellipse. However, the effect magnitude of the leakage flow depends on the scales both of the long and short axes, which also have no upper limit.

A two-equation nonlinear reduced-order model is established to describe the interaction between external periodic excitation from a pulsed jet and unsteady flow separation. This model, which is based on a 2D Navier–Stokes equation, the Stuart vortex row model, and the Stuart–Landau model, can approximately describe the typical phenomena occurring in pulsed jet flow separation control. These phenomena include the frequency-dependent, threshold, and lock-on effects, which are related to the frequency, intensity, and phase of excitation. Two indices, namely, the maximum Lyapunov exponent and the entrainment degree, are introduced to identify the mechanism of flow control via external periodic excitation. These indices are used to reflect the order degree and momentum transfer of the flow field. A comparison of the results of the model and those of a numerical simulation in a curved diffuser shows that the nonlinear reduced-order model is effective for qualitatively featuring the behavior of pulsed jet flow separation control. Moreover, the mechanism of pulsed jet flow separation control is explained via the model. Three aspects (i.e., flow instability, flow field ordering, and momentum transfer) are assumed to function together, resulting in an efficient flow control performance.

The giant and sustainable salinity gradient energy broadly occurs when mixing solution sources with different concentrations can be potentially harvested through the intensively studied membrane-based indirect power generation technologies. This kind of technology commonly has two functional submodules, namely the osmosis submodule to induce driven forces and the electric submodule to produce electricity. However, almost all relevant reviews only concentrate on the traditional pressure retarded osmosis (PRO) technology without involving newly emerged ones such as the forward osmosis-electrokinetic (FO-EK) technology, leading to outdated and incomplete knowledge in this field. Therefore, this study is going to provide a comprehensive and up-to-date review of the membrane-based indirect power generation technologies through adequately outlining most related research. The authors not only provide a systematic overview of the theoretical background and the development of the state-of-the-art membrane-based indirect power generation technologies but also highlight their essential characterizations. Meanwhile, the challenges and the optimizing strategies in aspects of efficient semipermeable membranes, available fluid materials, and operation conditions, as well as future promising applications in different scenarios are also elaborated in detail.

This paper presents a new concept of self-excited unsteady flow control technique on a compressor blade using a passive fluidic oscillator. This concept, as an unsteady flow control method, has the advantages that no additional air source or power supply needs to be introduced and no complex actuators or movable parts are included. Numerical simulations show that the passive fluidic oscillator will reduce the total pressure loss coefficient of the compressor cascade by 4.2% to 2.8% when the inlet Mach number of the compressor blade ranges from 0.28 to 0.39. The time domain and frequency spectrum analysis prove the unsteadiness of the jet produced by the passive fluidic oscillator. Within the Mach number scope, the reduced jet velocity equals approximately 0.68 to 0.7 and the reduced jet frequency equals nearly 1.1 to 1.2. This finding agrees with the order-of-magnitude study result that the jet parameters are compatible with the compressor (the reduced jet velocity and frequency are in the order of magnitude of 1) at the design point, and it conforms to the self-adaptivity of the passive fluidic oscillator in a certain degree (the reduced jet velocity and frequency are nearly invariant) at off-design points (providing small changes in angles of attack), which most passive flow control methods do not share.

The use of pulsed suction is an effective unsteady flow control method. However, compared with steady suction, which simply eliminates low-energy boundary layer, pulsed suction flow control presents unique phenomena and complex control mechanisms that are not explained fully. Thus, in this paper, pulsed suction flow control in a compressor cascade is investigated by numerical simulation. Numerical results show that the control performance depends on the suction frequency, momentum, and location, as any unsteady active flow control method. To further reveal the mechanism and explain the phenomena of pulsed suction, a nonlinear reduced-order model for pulsed suction flow control is established on the basis of the Navier–Stokes equation, Stuart vortex row model, Oseen vortex model, and Stuart–Landau equation. The entrainment degree and maximal Lyapunov exponent are used as indices to evaluate the flow losses and control performance in this model. By solving the equations of this model under different parameters, the model presents similar phenomena as found in the numerical simulations with various suction frequencies, momentum, and locations. Thus, it is innovative that the model with calibrated parameters has the ability to roughly predict control performance of unsteady pulsed suction in an efficient way. Also, based on this reduced-order model, one can explain that, unlike steady suction, chaos control, momentum transfer and lock-in mechanisms are especially relevant with the control physics of pulsed suction.