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Nonlinear wave interactions in a transitional hypersonic boundary layer

PHYSICS OF FLUIDS 2022年第11期34卷 1-13页

作者： Zhang, Chuanhong Shi, Zhiwei Nanjing Univ Aeronaut & Astronaut Coll Aerosp Engn 29 Yudao St Nanjing 210016 Jiangsu Peoples R China Peking Univ Coll Engn State Key Lab Turbulence & Complex Syst 5 Yiheyuan Rd Beijing 100871 Peoples R China Nanjing Univ Aeronaut & Astronaut Key Lab Unsteady Aerodynam & Flow Control Minist Ind & Informat Technol 29 Yudao St Nanjing 210016 Jiangsu Peoples R China

The linear and nonlinear evolutions and breakdown of the second modes in hypersonic boundary layers (HBLs) on a flared cone are investigated using Rayleigh-scattering flow visualization and fast-response pressure sensors. Based on two spatially separated pressure signals, cross-bicoherence analysis that permits the distinction of sum- and difference-interactions is utilized to identify the nonlinear interactions. In addition, the visualization temporal and spatial resolution allows fine flow features to be captured to provide additional flow information. Amplitude correlation technique is used to estimate the nonlinear energy transfer between the modes. Our results show that nonlinear interactions between the second mode and the low-frequency wave contribute to the growth of the low-frequency wave, and the difference interactions between the second mode and its first harmonic play a dominant role in modulating the waves in the overall transition process. Amplitude correlation analysis reveals that the spectral energy is nonlinearly transferred from the second mode into its first harmonic and into low-frequency wave, in agreement with the cross-bicoherence analysis. The amplitude modulation of the second mode caused by the difference interaction between the second mode and its first harmonic will reduce the propagation speed of the second mode. However, at the final breakdown stage, this difference interaction vanishes, and the second-mode propagation velocity recovers quickly. Since the frequency of the second mode keeps almost unchanged over the entire transition process, a higher propagation velocity will result in a larger wavelength, indicating an elongation and deformation of the second mode. Eventually, the difference interaction between the second mode and the low-frequency wave accompanying the energy transfer from the second mode to low-frequency waves leads to the final breakdown of the HBLs into a turbulent state. Published under an exclusive license by A

关键词： Hypersonic boundary layers

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A hybrid artificial intelligence control of a turbulent jet: Reynolds number effect and scaling

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JOURNAL OF FLUID MECHANICS 2022年 942卷 A47-A47页

作者： Perumal, A. K. Wu, Z. Fan, D. W. Zhou, Y. Harbin Inst Technol Shenzhen Ctr Turbulence Control Shenzhen 518055 Peoples R China Indian Inst Technol Kanpur Dept Aerosp Engn Jetacoust & Flow Control Lab Kanpur 208016 Uttar Pradesh India

This work aims to investigate experimentally the effect of the Reynolds number Re, based on the nozzle diameter D, on jet mixing manipulation using an unsteady radial minijet. A novel artificial intelligence (AI) control system has been developed to manipulate the jet over Re = 5800-40 000. The system may optimize simultaneously the control law and a time-independent parameter, which dictate the actuation ON/OFF states and amplitude, respectively. The control parameters include the mass flow rate, excitation frequency and diameter ratios (C-m, f(e)/f(0) and d/D) of the minijet to the main jet as well as the duty cycle (alpha) of minijet injection. Jet mixing is quantified using K-e and K-0, where K is the decay rate of the jet centreline mean velocity, and subscripts e and 0 denote the manipulated and unforced jets, respectively. It has been found that the maximum K-e achievable does not vary with Re. Scaling analysis of the huge volume of experimental data obtained from the AI system reveals that the relationship K-e = g(1) (C-m, f(e)/f(0), alpha, d/D, Re, K-0) may be reduced to K-e/K-0 = g(2) (zeta), where g(1) and g(2) are different functions and the scaling factor zeta = (C-m/alpha) (D/d)(l-n )(1/Re)(f(e)/f(e, opt))(m), m and 1 - n are the power indices, and subscript opt denotes the value at which K-e is maximum. The scaling law is discussed in detail, along with the physical meanings of the dimensionless parameters K-e/K-0, zeta, (C-m/alpha) (D/d)(1-n) (1/Re) and (f(e)/f(e)(,opt))(m).

关键词： mixing enhancement control theory jets machine learning

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Aerodynamic characteristics of unsymmetrical aerofoil at various turbulence intensities

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Chinese Journal of Aeronautics 2019年第11期32卷 2395-2407页

作者： S.ARUNVINTHAN S.NADARAJA PILLAI Turbulence & Flow Control Lab School of Mechanical Engineering SASTRA Deemed University

A series of wind tunnel tests were performed to investigate the effect of turbulent inflows on the aerodynamic characteristics of the unsymmetrical airfoil at various turbulence intensities and Reynolds number. To assess the aerodynamic characteristics, surface pressure measurements were made over the unsymmetrical airfoil surface by using a simultaneous pressure scanner MPS4264 of Scanivalve make. Self-generated passive grids made of parallel arrays of round bars were placed at four different locations to generate various turbulence Intensities(TI) in the wind tunnel. The location of the passive grid has been normalized in terms of considering the distance between the entry of the test section and the leading edge of the model. Based on the wind tunnel results, by comparing the baseline without grid low turbulence case TI = 0.51% with other turbulence generated cases like TI = 4.68%, 4.73%, 6.04% and 8.46% at different Reynolds number, it is found that the coefficient of lift increases with the increase in the turbulence intensity. Results also reveal that the flow featuring turbulence can effectively delay the stall characteristics of an airfoil by attaching the flow over the airfoil for an extended region. Additionally, attempts were made to understand the influence of turbulence on the aerodynamic hysteresis.

关键词： Aerodynamic efficiency Aerodynamic hysteresis Passive grid Simultaneous pressure distribution turbulence intensity

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Effect of surface blowing on aerodynamic characteristics of tubercled straight wing

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Chinese Journal of Aeronautics 2019年第5期32卷 1111-1120页

作者： N.GANESH S.ARUNVINTHAN S.NADARAJA PILLAI Turbulence and Flow Control Lab School of Mechanical Engineering SASTRA Deemed University

Recent research proves that wings with leading-edge tubercles have the ability to perform efficiently in post-stall region over the conventional straight wing. Moreover, the conventional straight wing outperforms the tubercled wing at a pre-stall region which is quintessential. Even though tubercled wing offers great performance enhancement, because of the complexity of the flow, the trough region of the tubercled wing is more prone to flow separation. Henceforth, the present paper aims at surface blowing – an active flow control technique over the tubercled wing to enhance the aerodynamic efficiency by positively influencing its lift characteristics without causing any additional drag penalty. flow parameters like blowing velocity ratios and the location of blowing were chosen to find the optimised configuration keeping the amplitude and frequency of the leading-edge tubercles constant as 0.12 c and 0.25 c respectively. Numerical investigations were carried out over the baseline tubercled wing and tubercled wing with surface blowing at various blowing jet velocity ratios 0.5, 1 and 2 over four different chordwise locations ranging from 0.3 c to 0.8 *** results confirm that blowing at various x/c with different blowing velocity ratios performs better than the conventional tubercled wing. Comparatively, blowing velocity ratio 2 at 0.3 c shows peak performance of about 28% enhancement in the lift characteristics relative to the baseline model. Particularly, in the pre-stall region, 25–50% increase in aerodynamic efficiency is evident over the tubercled wing with surface blowing compared with the baseline case. Additionally,attempts were made to delineate the physical significance of the flow separation mechanism due to blowing by visualizing the streamline pattern.

关键词： Active flow control Aerodynamic efficiency Biomimetics Blowing Tubercles

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GRADIENT-FREE OPTIMIZATION IN THERMOACOUSTICS: APPLICATION TO A LOW-ORDER MODEL

GRADIENT-FREE OPTIMIZATION IN THERMOACOUSTICS: APPLICATION T...

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ASME Turbomachinery Technical Conference and Exposition (Turbo Expo) - Sustainable Energy - Accelerating the Transition by Advancing Turbine Technology

作者： Reumschuessel, Johann Moritz von Saldern, Jakob G. R. Li, Yiqing Paschereit, Christian Oliver Orchini, Alessandro Tech Univ Berlin Chair Fluid Dynam Berlin Germany Tech Univ Berlin Lab Flow Instabil & Dynam Berlin Germany Harbin Inst Technol Ctr Turbulence Control Shenzhen Guangdong Peoples R China

ISBN: (纸本)9780791884942

Machine learning and automatized routines for parameter optimization have experienced a surge in development in the past years, mostly caused by the increasing availability of computing capacity. Gradient-free optimization can avoid cumbersome theoretical studies as input parameters are purely adapted based on output data. As no knowledge about the objective function is provided to the algorithms, this approach might reveal unconventional solutions to complex problems that were out of scope of classical solution strategies. In this study, the potential of these optimization methods on thermoacoustic problems is examined. The optimization algorithms are applied to a generic low-order thermoacoustic can-combustor model with several fuel injectors at different locations. We use three optimization algorithms the well established Downhill Simplex Method, the recently proposed Explorative Gradient Method, and an evolutionary algorithm - to find optimal fuel distributions across the fuel lines while maintaining the amount of consumed fuel constant. The objective is to have minimal pulsation amplitudes. We compare the results and efficiency of the gradient-free algorithms. Additionally, we employ model-based linear stability analysis to calculate the growth rates of the dominant thermoacoustic modes. This allows us to highlight general and thermoacoustic-specific features of the optimization methods and results. The findings of this study show the potential of gradient-free optimization methods on combustor design for tackling thermoacoustic problems, and motivate further research in this direction.

关键词： Optimization Thermoacoustics Fuels Combustion chambers Algorithms Optimization algorithms Surges Evolutionary algorithms Gradient methods Design Fuel injectors Machine learning Stability

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Aerodynamics of ducted re-entry vehicles

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Chinese Journal of Aeronautics 2020年第7期33卷 1837-1849页

作者： C.ANBU SERENE RAJ M.NARASIMHAVARADHAN N.VAISHNAVI S.ARUNVINTHAN A.AL ARJANI S.NADARAJA PILLAI Turbulence&Flow Control Lab School of Mechanical EngineeringSASTRA Deemed UniversityTamil Nadu 613401India College of Engineering Prince Sattam Bin Abdelaziz University(PSAU)Alkharj 11942Saudi Arabia

Ballistic parameter plays a major role in determining the re-entry *** ballistic coefficient offers an optimal re-entry,wherein the vehicle decelerates higher up in the atmosphere thereby decreasing the imposed aerothermal *** current computational study proposes an add-on,to the existing Orion-based re-entry vehicle:a duct circumventing the capsule from the shoulder to the base,to improve the aerocapture ability of the re-entry *** design cases are categorised based on a non-dimensional parameter termed the Annular Area Ratio(AAR).Dragand ballistic coefficient of the Ducted Re-entry Vehicles(DRVs)at various Mach numbers are evaluated and compared with those of the baseline *** results show that the proposed design increases the drag for all the AARs considered in the subsonic *** the supersonic regime,ducted models of higher AAR are more promising with the increase in Mach *** also exhibit lower ballistic coefficients than their baseline counterparts.

关键词： Aerocapture Annular area ratio(AAR) Ballistic coefficient CFD Re-entry

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Aerodynamic characteristics of variously modified leading -edge protuberanced (LEP) wind turbine blades under various turbulent intensities

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JOURNAL OF WIND ENGINEERING AND INDUSTRIAL AERODYNAMICS 2020年 202卷 104188-104188页

作者： Arunvinthan, S. Pillai, S. Nadaraja Cao, Shuyang SASTRA Deemed Univ Sch Mech Engn Turbulence & Flow Control Lab Thanjavur 613401 Tamil Nadu India Tongji Univ State Key Lab Disaster Reduct Civil Engn Shanghai 200092 Peoples R China

A series of wind tunnel tests were performed to investigate the effect of turbulent inflows on the aerodynamic characteristics of variously modified leading-edge protuberanced (LEP) wing configurations at various turbulence intensities. A self-developed passive grid made of parallel arrays of round bars was placed at different locations of the wind tunnel to generate desired turbulence intensity. The aerodynamic forces acting over the baseline straight leading-edge model and the modified small, medium and high LEP wing configurations were obtained from the surface pressure measurements made over the wing at different turbulence intensities using MPS4264 Scanivalve simultaneous pressure scanner corresponding to a sampling frequency of 700Hz. All the test models were tested for a wide range of angles of attack ranging between −45° ≤ α ≤ 45° at turbulence intensities varying between 0.51% ≤ TI ≤ 4.92%. Results reveal that the time-averaged mean coefficient of lift (C_L) increases with the increase in the turbulence intensity associated with smooth stall characteristics rendering the modified LEP test models advantageous. Furthermore, based on the surface pressure coefficients the underlying dynamics behind the stall delay tendency were discussed. Additionally, attempts were made to statistically quantify the aerodynamic forces using standard deviation at both the pre-stall and the post-stall angles.

关键词： turbulence intensity Passive grid Simultaneous pressure distribution Standard deviation Leading-edge protuberance

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Skin-friction measurements using oil-film interferometry in hypersonic transitional boundary layer flows

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AEROSPACE SCIENCE AND TECHNOLOGY 2024年 149卷

作者： Zhang, Chuanhong Ni, Huibin Shi, Zhiwei Wu, Puyuan Nanjing Univ Aeronaut & Astronaut Coll Aerosp Engn 29 Yudao St Nanjing 210016 Jiangsu Peoples R China Nanjing Univ Aeronaut & Astronaut Key Lab Unsteady Aerodynam & Flow Control Minist Ind & Informat Technol 29 Yudao St Nanjing 210016 Jiangsu Peoples R China Peking Univ Coll Engn State Key Lab Turbulence & Complex Syst 5 Yiheyuan Rd Beijing 100871 Peoples R China

Skin -friction measurements in hypersonic transitional boundary layer (HTBL) flows on a flared cone at Mach 6 at zero angle of attack are obtained using oil -film interferometry (OFI). In addition, the amplitude evolution of instability waves is investigated using fast-response pressure sensors and Rayleigh-scattering flow visualization (RSFV). The connection between the skin -friction coefficient and near-wall dynamics during the whole transition process is illustrative and the underlying mechanism of the generation of the wall skin friction is revealed. There exist two peaks of skin -friction rise along the streamwise direction of the model. The first one is at the transitional region (denoted as FS) which is caused by the second -mode instability. The second one is at the region where the transition is completed (denoted as FT). The drastic growth of skin friction near the end of transition is mainly attributed to the rapid boundary-layer breakdown into small scales. It is found that the strong shear in the nearwall region in the quiet zone where dominant hairpin-shaped vortices evolved from the second modes can also cause a local skin -friction rise (denoted as FQ). Further investigations show that in the region between FS and FQ, there is a slight increase in the skin friction, which is associated with the growing low-frequency waves. The skin friction coefficient value decreases slowly after FT but does not reach the expected turbulent theoretical value by the end of the test domain, indicating that the development towards the completely turbulent boundary layer appears to be a gradual process.

关键词： Skin friction

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An investigation of the wakes of stranded cables using particle image velocimetry

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PHYSICS OF FLUIDS 2021年第3期33卷 035132-035132页

作者： Abdelhady, Mohamed Wood, David H. Univ Calgary Lab Turbulence Res Aerodynam & Flow Control LTRAC Mech & Mfg Engn Calgary AB T2L 1Y6 Canada

Stranded cables are used in a wide range of applications, but the flow over them has not been studied extensively. Instead, many applications assume circular cylinder behavior when modeling the wake dynamics. This paper provides a detailed study of the wakes of two stranded cables using two-dimensional particle image velocimetry in the streamwise-normal and streamwise-spanwise planes. The first cable had six outer strands and the second had three. A circular cylinder was also investigated experimentally as to provide a benchmark for comparison. The experiments were done in a water channel at a Reynolds number of similar to 3100, based on the circumscribing diameter. Proper orthogonal decomposition and phase-averaging were used to investigate the coherent and incoherent fields. The results showed that 3x1 with the largest variations in the sectional width experiences a local stream of higher streamwise velocity along the span and that both cables have similar to 20% higher mean spanwise velocity, relative to the cylinder. The stranded cable wakes are dominated by alternatively shed Karman vortices, at a frequency similar to the circular cylinder. However, the Reynolds stresses, the shape factor, and the details of vortex shedding showed substantial alterations associated with the cable strands, including 58% variation, relative to the cylinder, in total, coherent, and incoherent Reynolds stresses for cables along the span. In addition, the cable strands resulted in the elongation and distortion of the mean spanwise vorticity, without changing their magnitude or thickness. Finally, the strands generally increase the magnitudes of turbulent transport and coherent diffusion, and production, especially at the shear layers.

关键词： Cables

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Studying the flow dynamics and heat transfer of stranded conductor cables using large eddy simulations

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PHYSICS OF FLUIDS 2021年第1期33卷 015120-015120页

作者： Abdelhady, Mohamed Wood, David H. Univ Calgary Lab Turbulence Res Aerodynam & Flow Control LTRAC Mech & Mfg Engn Calgary AB T2L 1Y6 Canada

Stranded cables are widely used in applications where their heat transfer and fluid dynamics are important, but they have not been extensively studied. This paper investigates, using large eddy simulations with the dynamic Smagorinsky sub-grid scale model, a helically wound stranded conductor cable in comparison to a circular cylinder at a Reynolds number of 1000 and Prandtl number of 0.7. The cylinder and the cable were normal to the flow. The triply decomposed heat transport equations were derived, and proper orthogonal decomposition was applied to the fluctuating vorticity and temperature fields to determine the total, coherent, and incoherent terms in the heat transport equations. The results showed that the stranded cable, relative to circular cylinder, has (i) three-dimensional mean flow and heat transfer, especially within and around recirculation region, (ii) 9% higher drag and 8% higher base pressure magnitude, (iii) near-stagnant flow in the gaps between the strands, which results in a significant variation in the local Nusselt number, (iv) similar to 15% lower span-wise averaged local Nusselt number in the attached boundary layer, suggesting that surface modifications should be addressed to enhance heat transfer, (v) similar to 36 degrees variation in the separation angle along the span, (vi) 12% higher turbulent kinetic energy and 39% higher spanwise normal Reynolds stresses, (vii) insignificant difference in shedding frequency, suggesting similar flow induced vibrations to the cylinder, (viii) asymmetry in the flow and heat fields around the x axis, (ix) significantly different coherent temperature fields and dynamics, and (x) in general, high heat energy transport close to the cable rear side.

关键词： Large eddy simulation

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