实验流体力学

Contents

2023, 37(4)

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Abstract:

A review on the development of oscillating jets

WU Mengwei, XU Minyi, MI Jianchun

2023, 37(4): 1-17. doi: 10.11729/syltlx20230022

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Abstract:

The flow control method can effectively promote the mixing of the injected fluid with the surrounding fluid, and it has important applications in various fields, such as energy, chemical industry, aerospace and precision manufacturing, etc. Therefore, it has received widespread attention from scholars worldwide. In order to better understand the current situation and future development trend of oscillatory jets, this paper is the first to comprehensively summarize and classify the seven excitation methods of oscillatory jets, such as microjet, mechanical, feedback, blunt V-nozzle, impact V-plate, cavity self-excited flapping, film self-excited flapping, etc. A comparison of the principles of the different excitation methods and the characteristics of the resulting oscillating jets is conducted, pointing out the problems of the oscillating jet technology itself and the limitations of the industrial application of related technologies, and giving forecasts and suggestions for the development of the oscillating jet technology.

2023, 37(4): 1-2.

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Abstract:

Effect of flow around a wall-mounted 2D square cylinder on WSS in a TBL

ZHANG Zhihao, FU Qixing, WANG Qingyang, XU Shengjin

2023, 37(4): 18-28. doi: 10.11729/syltlx20230035

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Abstract:
In this paper, the effect of flow around a wall-mounted 2D square cylinder on WSS (Wall Shear Stress) in a TBL (Turbulent Boundary Layer) is studied using TR–PIV and hot wires. The Reynolds number, defined by the cylinder width and the free stream velocity, is fixed at 1.1 × 10⁴. There exist two types of large-scale flow structures, i.e., vortex I which denotes those moving towards the wall, and vortex II which denotes those moving downstream. The flow structures have significant effects on the flow and WSS: increasing the streamwise velocity gradient that results in a sharply increased WSS; reducing the streamwise velocity gradient that results in a sharply decreased WSS; and changing the WSS direction. The study is helpful to understand the physics of surface erosion, pollutant accumulation, flow energy loss, etc.

Research progress on rotor reverse flow and dynamic stall flow control methods

LI Guoqiang, ZHAO Xinhai, YI Shihe, SONG Kuihui, ZHAO Guangyin

2023, 37(4): 29-47. doi: 10.11729/syltlx20230054

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Abstract:
When a helicopter flies forward at high speed with heavy load, the blade pitch changes greatly and dynamic stall is prone to occur. The lower rotational speed of the inner section of the trailing blade leads to the formation of a reverse flow zone under the superposition of the incoming flow, resulting in a reduction in the aerodynamic efficiency of the blade. The problems of blade fatigue failure and lift reduction hinder the further improvement of helicopter performance. Flow control methods have great potential in improving the aerodynamic characte-ristics of airfoils, and are effective ways to improve the rotor aerodynamic efficiency and ensure helicopter safety and stability. In this paper, the formation mechanism and unsteady flow characteristics of the reverse flow zone and dynamic stall are firstly described, and the research results of two special aerodynamic phenomena are summarized. On this basis, a comparative analysis of flow control methods such as variable airfoil configuration, surface mechanical devices, air-blowing control, plasma actuator, synthetic jet actuator, and trailing edge flap on the mechanism of rotor dynamic stall and reverse flow control is conducted, and the effects of control parameters and flow field parameters on control effectiveness are summarized. Finally, the remain-ing problems and solutions in the application of various flow control methods are prospected.

Flowfield and friction characteristics downstream of mirco vortex generator in turbulent boundary layer

ZHANG Yi, PAN Chong, DOU Jianyu, ZHANG Miao

2023, 37(4): 48-58. doi: 10.11729/syltlx20230027

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Abstract:
The present work uses the stereoscopic particle image velocimetry and calibration-free dual hot-film wall shear stress measurement sensor to measure the flowfield and friction at downstream of the one array of forwards wedge Micro Vortex Generator (MVG) in the turbulent boundary layer at moderate Reynolds number. The result of flowfield measurement shows that MVG produces the streamwise velocity defect regions and streamwise vortices pairs in downstream time-averaged flowfield, which causes the second outer-peak in the spanwise pre-multiplied energy spectra. The result of proper orthogonal decomposition shows that the contribution of energy of structures induced by MVG is equivalent to the that of large-scale structures and very large-scale structures in the smooth-wall turbulent boundary layer, which also significantly affects the spatial distribution of the near-wall structures. The friction measurement experiment shows that MVG array with higher height and closer spanwise arrangement has higher friction drag reduction. The drag reduction effect of MVG lasts downstream to 80 times of its own characteristic height.

AI-based real-time noise reduction of flow field pressure signals under plasma electromagnetic interference

CHEN Jie, ZONG Haohua, SONG Huimin, LIANG Hua, LIU Shimin, FANG Ziqi

2023, 37(4): 59-65. doi: 10.11729/syltlx20230030

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Abstract:
For the reliable sensing requirements of closed-loop active flow control, a real-time noise reduction method based on the artificial neural network was proposed for solving the plasma actuation electromagnetic interference on flow field signals. Taking the dynamic pressure sensor installed on the cylinder surface as the experimental subject, the “dense peak” type noise signals of alternating current dielectric barrier discharge (AC–DBD) and the “sparse spike” type noise signals of nanosecond pulsed dielectric barrier discharge (NS–DBD) were collected respectively. Artificial synthetic noise signals were used for supervised learning, and the generalization of the artificial neural network model was tested and verified. The results show that this method can effectively suppress the influence of electromagnetic interference caused by plasma actuation and restore the real pressure signal. It has better denoising performance on the AC–DBD “dense peak” type noise signal. The denoised signal is smoother and better fitted with the real one. This model is also applied to the real flow field pressure measurement, and the accuracy of the denoising network prediction is further verified by comparing the mean value of the denoised signal and the real signal.

Passive control on flow past a circular cylinder with bionic nylon wires

CHEN Wenli, LIN Longhan, DENG Zhi, GAO Donglai

2023, 37(4): 66-75. doi: 10.11729/syltlx20230019

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Abstract:
We investigated the control effectiveness and mechanism of the control of the circular cylinder flow field using bionic nylon wires inspired by bird feathers by wind tunnel tests. In this experiment, at a Reynolds number of 2.67 × 10⁴, the bionic nylon filament was arranged at the front station of the cylinder and the length ratio L/D between nylon wire length and cylinder diameter was used as the characteristic parameter. The surface pressure measurement system was used to obtain the pressure coefficients around the cylinder to analyze the aerodynamic forces acting on the cylinder. The two-dimensional flow field information of the cylinder was obtained by a high-speed Particle Image Velocimetry (PIV) measurement system, and the Proper Orthogonal Decomposition (POD) was used to obtain the instantaneous and time-averaged characteristics of the flow field. The results show that at L/D < 0.6, the control effectiveness of nylon wires is limited because the nylon-induced vortex structures cannot reach the wake field. At L/D > 1.0, the nylon wires can significantly reduce the turbulent kinetic energy and Reynolds stress of the cylindrical wake field and suppress the lift and drag coefficient distributions around the circular cylinder. And at high values, nylon wires can inhibit the interaction between shear layers and thus change the von Kármán vortex shedding pattern of the cylinder.

Investigation on performance enhancement of flap based on dual synthetic jets

ZHANG Jianyuan, LUO Zhenbing, PENG Wenqiang, LIANG Ruiqi, DENG Xiong, WANG Wanbo, ZHAO Zhijie, LIU Jiefu

2023, 37(4): 76-86. doi: 10.11729/syltlx20230046

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Abstract:
When the aircraft is in the process of taking off, landing and flexible maneuvering, flow separation occurs above the flap and rudder which is caused by the large deflection angle. It reduces the performance of the flap and rudder, or even makes them ineffective. In order to solve this problem, a performance enhancement technology of flap based on array dual synthetic jets was proposed. Aiming at seamless flap, the influence of different dual synthetic jets driving parameters on the lift force and flap performance was investigated. The investigation results show that the dual synthetic jets can generate periodic vortex structures above the flap surface, which enhanced the momentum exchange between the low-velocity air in the boundary layer and the main flow. These vortex structures can also strengthen the ability of the boundary layer to resist the adverse pressure gradient. The array dual synthetic jets at the flap can effectively increase the lift force and enhance the flap performance. The flap performance enhancement is more effective when the dimensionless driving frequency is 3.89 and the momentum coefficient is 3.01 × 10^–3. The integrated model of the array dual synthetic jets actuator combined with the wing was designed and fabricated, and the flight test was carried out. The rolling angular velocity could achieve 15.69 (°)/s, which verifies the feasibility and effectiveness of performance enhancement of flap based on dual synthetic jets.

Parameter influence and optimization of energy conversion efficiency of synthetic jet actuators

WANG Lei, LI Zhe, FENG Lihao

2023, 37(4): 87-95. doi: 10.11729/syltlx20230039

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Abstract:
Piezoelectric-driven synthetic jet actuators have been widely used in various flow control areas due to the characteristics of no air supply, high jet velocity and response frequency. The exit peak velocity and energy conversion efficiency are important indicators to measure the performance of piezoelectric-driven actuators. When a higher exit velocity is obtained, the energy conversion efficiency of the existing piezoelectric-driven synthetic jet actuators is low. To improve the performance of synthetic jet actuators, the exit velocity and power are measured by the hot-wire anemometer and power meter, respectively. The effects of the exit length, exit neck length, cavity height and piezoceramics thickness on the performance of actuators are analyzed. It is found that for different configuration parameters, the exit peak velocity displays a similar trend with power. Based on the optimized actuator configuration, the exit peak velocity is increased, and the energy conversion efficiency is improved in comparison to the previous results with the maximum increment of 233.3%, thereby reducing the energy consumption of actuators.

Development of forebody asymmetric vortex control based on alternating synthetic jet and the verification on model free flight

LI Linkai, HUANG Zi, GU Yunsong, PENG Zhenjun, ZHANG Zongyuan, LEI Yu

2023, 37(4): 96-104. doi: 10.11729/syltlx20230042

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Abstract:
In order to apply the forebody vortex flow control method to high Angle of Attack (AoA) flight control of the aircraft, an asymmetric vortex control technique based on the Alternating Synthetic Jet (ASJ) flow was proposed and developed. A set of airborne alternating synthetic jet control device and a free flight verification model aircraft were developed. The feasibility of using the forebody vortex control method to realize tail spin out and high Angle of Attack attitude control was verified by semi-free flight in the wind tunnel and model free flight in open airspace. Meanwhile, by means of the flight measurement and control system and the airborne pressure measurement system, the aircraft attitude, vortex position and body surface pressure can be measured synchronously, which can effectively evaluate the efficiency of the vortex control technology. Wind tunnel semi-free flight test results show that the alternating synthetic jet can effectively control the relative position of the forebody vortices at 60° Angle of Attack, which can generate yaw moment and realize heading control at high Angle of Attack. In the flight test, the technology can realize the change of tail spin under the failure of conventional rudder, and the controllable tail spin angular velocity can reach 173 (°)/s. Based on this technology, the verification model aircraft can perform fast yaw control when flying at high Angle of Attack, and the time delay from control input to yaw angular velocity change is less than 0.5 seconds.

Experimental study on the thickness dependence of bionics coverts for the wing stall control

HUANG Yijun, GONG Xu’an, MA Xingyu, JIANG Nan

2023, 37(4): 105-115. doi: 10.11729/syltlx20230028

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Abstract:
Bio-inspired by the covert feathers on bird wings, the artificial coverts of different thicknesses were designed by using flexible materials, and they were installed at different locations on the upper side of a NACA0018 wing model at a high angle of attack. In the wind tunnel experiments, the hot-wire anemometer was used to measure the velocity distributions in the wake flow, and therefore the time-averaged and turbulent fluctuation velocities were obtained. The flow separation control effectiveness of the different thicknesses were analyzed by the time-averaged velocity profiles, the root-mean-square velocitydistributions of the turbulent fluctuations as well as their Power Spectral Density (PSD). The results show that, the thin coverts near the leading-edge effectively reduce the distance between the leading-edge shear layer and the upper surface, whereas installed near the trailing-edge, the flow field around the airfoil has mere change. On the other hand, for the thick coverts, the flow separation control effectiveness is better than those near the leading-edge. Based on the multi-scale wavelet analysis, the artificial coverts improve the transformation of low-frequency large-scale coherent structures to high-frequency small-scale ones by adaptively fluttering and flapping motions, which is highly effective for flow separation control.

Sweeping jet control mechanism and its application in flapless flight control

QIU Zihao, LI Ziyan, ZHOU Kaiwen, WANG Shiqi, LIU Yingzheng, WEN Xin

2023, 37(4): 116-125. doi: 10.11729/syltlx20230045

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Abstract:
The flapless aerodynamic control technology of aircraft has the advantages of low additional weight, excellent control performance and suitability for stealth design by using the active flow control method of jets and eliminating the traditional mechanical flap surface. This paper presents the application of sweeping jets in flapless aerodynamic layouts to address the problem of high air consumption caused by the low control efficiency of existing steady jet control methods. This paper outlines the current research status of two flapless control technologies, i.e., circulation control and fluidic thrust vectoring, and then discusses the mechanism and advantages of the sweeping jet in terms of control area, enhanced flow mixing, and frequency regulation. The mechanism and performance of the sweeping jet in flapless flight control are presented in detail in terms of circulation control and thrust vectoring.

Active drag reduction for a D-shaped cylinder flow using Coanda pulsation jets

ZHANG Shixiong, BAI Honglei

2023, 37(4): 126-136. doi: 10.11729/syltlx20230053

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Abstract:
A D-shaped cylinder can be considered as one of the typical models for bluff bodies; flow separation from the trailing edges and near-wake flow structures are internally linked with aerodynamic forces acting on the D-shaped cylinder. Based on Coanda pulsation jets and Genetic Algorithms (GAs), this work is focused on the active control of the D-shaped cylinder flow for drag reduction. Wind tunnel experiments are conducted at a Reynolds number Re = 1.8 × 10⁴, which is based on the incoming freestream velocity and height of the D-shaped cylinder. Being placed on the upper and lower sides of the cylinder base, the Coanda pulsation jets are composed of 1/4-parts of a circular cylinder (radius is 0.2H) and horizontal slot jets. Control parameters include the driving pressure of the jet, pulsation frequency, duty cycle, and the phase shift of the lower and upper jets. The time-averaged base pressure of the D-shaped cylinder, which is connected with the drag force, is chosen to be the objective function of GAs. Results from this work indicate that GAs are robust to identify the optimum control parameters (i.e., driving pressure of the jet is 1.94 times atmospheric pressure, non-dimensional pulsation frequency is 0.27, duty cycle is 37% and phase shift is 136°), resulting in a recovery of the base pressure up to 61% (corresponding to a drag reduction up to 23%), associated with a high efficiency of 45%; meanwhile, it is observed that large-scale near-wake structures of the D-shaped cylinder are impaired, with altered shedding frequency and phase difference, by the Coanda pulsation jets.

2023 Vol. 37, No. 4