Experimental study on surface arc plasma actuation-based hypersonic boundary layer transition flow control

Effective control of hypersonic transition is essential.In order to avoid affecting the structural proflle of the aircraft,as well as reducing power consumption and electromagnetic interference,a low-frequency surface arc plasma disturbance experiment to promote hypersonic transition was carried out in theΦ0.25 m double-throat Ludwieg tube wind tunnel at Huazhong University of Science and Technology.Contacting printed circuit board sensors and non-contact focused laser differential interferometry testing technology were used in combination.Experimental results showed that the low-frequency surface arc plasma actuation had obvious stimulation effects on the second-mode unstable wave and could promote boundary layer transition by changing the spectral characteristics of the second-mode unstable wave.At the same time,the plasma actuation could promote energy exchange between the second-mode unstable wave and other unstable waves.Finally,the corresponding control mechanism is discussed.

Supersonic boundary layer transition induced by self-sustaining dual jets

To promote high-speed boundary layer transition,this paper proposes an active self-sustaining dual jets(SDJ)actuator utilizing the energy of supersonic mainflow.Employing the nanoparticle-based planar laser scattering(NPLS),supersonic flat-plate boundary layer transition induced by SDJ is experimentally investigated in an Ma-2.95 low-turbulence wind tunnel.Streamwise and spanwise NPLS images are obtained to analyze fine flow structures of the whole transition process.The results reveal the transition control mechanisms that on the one hand,the jet-induced shear layer produces unstable Kelvin–Helmholtz instabilities in the wake flow,on the other hand,the jets also generates an adverse pressure gradient in the boundary layer and induce unstable streak structures,which gradually break down into turbulence downstream.The paper provides a new method for transition control of high-speed boundary layer,and have prospect both in theory and engineering application.

The effect of G?rtler instability on hypersonic boundary layer transition

The evolution of Gortler vortices and its interaction with other instabilities are investigated in this paper. Both the Mack mode and the Gortler mode exist in hypersonic boundary-layer flows over concave surfaces, and their interactions are crucially important in boundary layer transition. We carry out a direct numerical simulation to explore the interaction between the GOrtler and the oblique Mack mode. The results indicate that the interaction between the forced Gortler mode and the oblique Mack mode promotes the onset of the transition. The forced oblique Mack mode is susceptible to nonlinear interaction. Because of the development of the GOrtler mode, the forced Mack mode and other harmonic modes are excited.

Selection and Impact of an Aerofoil Leading Edge on Boundary Layer Transition

The choice of leading-edge aspect ratio (AR) plays a crucial role when planning boundary layer wind tunnel tests on a flat plate. Poor selection of the leading-edge profile hampers effectiveness of the experiment and increases testing costs associated with interchanging of leading edges to attain accurate results. Thus, the appropriate selection of the leading edge is a very crucial part of the wind tunnel experiment process. It is argued that the curvature of the leading edge and thus the AR is of paramount importance to achieve accurate results from the wind tunnel testing. In this project, seven different elliptical leading edges were tested, and their performance was compared with an ideal leading edge with zero thickness. Experiments and computation have been done for leading edges ranging from AR6 to AR20. Results were evaluated for boundary layer transition onset location, and it was found that AR20 has the least influence on the flow structure when compared to the ideal leading edge. A study of the flow structure at the stagnation point indicates an increase in adverse pressure gradient with an increase in the AR but also shows a decrease in the size of the stagnation region. The presence of a higher AR leading edge reduces the turbulent spot production rate, which is one of the primary causes of boundary layer transition. This paper presents a correlation that enables aerodynamicists to quantify the impact of the leading-edge AR on transition. A typical case is also presented to compare the relative performance of a wedge and the higher AR leading edge, which provides a choice between an elliptical or a wedge-shaped leading edge.

An Investigation of Innovative Experimental and Numerical Techniques to Detect Boundary Layer Transition

Different methods to detect boundary layer transition are investigated within the scope of this paper. Laminar and turbulent boundary layers exhibit a significantly different behavior, not only regarding skin friction but also for heat-transfer which affects the blade cooling design. The present work presents a novel and non-intrusive measurement technique to detect the transition, based on acoustic concepts. The reliability of the technique was investigated by means of boundary layer measurements over a fiat plate in subsonic flow conditions. After a preliminary assessment with a conventional Preston tube, a row of microphones were installed along the plate to correlate transition pressure fluctuations. To provide a comprehensive representation of the experiment, dedicated measurements with a fast response aerodynamic pressure probe were performed to determine the turbulence intensity and the dissipation rate upstream of the flat plate. The experimental results were systematically compared with calculations performed with three different computational fluid dynamics solvers （ANSYS-Fluent, ANSYS-CFX, OpenFOAM） and using both the k-k1-ω and the γ-Reθ transition models. Results show a fair agreement between CFD （computational fluid dynamics） predictions and the acoustic technique, suggesting that this latter might represent an interesting alternative option for transition measurements.

Prediction of Boundary Layer Transition Based on Modeling of Laminar Fluctuations Using RANS Approach

This article presents a linear eddy-viscosity turbulence model for predicting bypass and natural transition in boundary layers by using Reynolds-averaged Navier-Stokes (RANS) equations. The model includes three transport equations, separately, to compute laminar kinetic energy, turbulent kinetic energy, and dissipation rate in a flow field. It needs neither correlations of intermittency factors nor knowledge of the transition onset. Two transition tests are carried out: flat plate boundary layer under zero ...

Progress in flight tests of hypersonic boundary layer transition

Boundary layer transition(BLT)can cause a sharp rise in heat flux and skin friction,which can seriously affect the flight performance and safety of hypersonic flight vehicles.Therefore,the mechanism,prediction and control of transition have become important issues that must be dealt with for the development of advanced flight vehicles,and it is also a research hotspot of particular interest to major aerospace countries.Compared to other transition research approaches,model flight tests can better present the transition problems under real flight conditions,thus have been carried out extensively over the past 30 years.The United States,Germany,France,Australia,and other countries have carried out transition research based on flight tests,such as the Pegasus wing-glove crossflow transition and the Hypersonic Boundary Layer Transition(HyBOLT)transition control flight test of the United States,the joint research project of the Hypersonic International Flight Research and Experimentation-1(HIFiRE-1)circular cone and the HIFiRE-5 elliptic cone transition flight tests between the United States and Australia,the flight test of compression surface transition of the scramjet forebody(LEA)in France and so on.Although these flight tests suffered various setbacks,they still obtained valuable transition data.Recently,the United States is carrying out the concave-surface transition flight tests of Hypersonic Boundary Layer Transition(BOLT)and BOLT-II.Since its first model flight test mission for verification purpose launched successfully in 2015,several hypersonic BLT flight tests have been conducted by China Aerodynamics Research and Development Center(CARDC).The flight tests have measured valid transition data under flight conditions,obtained the transition front and its dynamical variation on blunt cones at various angles of attack and a lifting body Hypersonic Transition Research Vehicle(HyTRV).The crossflow traveling waves in high-altitude flight were measured for the first time,and our understanding of hypersonic BLT has been greatly improved.

A symmetry-based length model for characterizing the hypersonic boundary layer transition on a slender cone at moderate incidence

The hypersonic boundary layer(HBL)transition on a slender cone at moderate incidence is studied via a symmetry-based length model:the SED-SL model.The SED-SL specifies an analytic stress length function(which defines the eddy viscosity)describing a physically sound two-dimensional multi-regime structure of transitional boundary layer.Previous studies showed accurate predictions,especially on the drag coefficient,by the SED-SL for airfoil flows at different subsonic Mach numbers,Reynolds numbers and angles of attack.Here,the SED-SL is extended to compute the hypersonic heat transfer on a 7∘half-angle straight cone at Mach numbers 6 and 7 and angles of attack from 0∘to 6∘.It is shown that a proper setting of the multi-regime structure with three parameters(i.e.a transition center,an after-transition near-wall eddy length,and a transition width quantifying transition overshoot)yields an accurate description of the surface heat fluxes measured in wind tunnels.Uniformly good agreements between simulations and measurements are obtained from windward to leeward side of the cone,implying the validity of the multi-regime description of the transition independent of instability mechanisms.It is concluded that a unified description for the HBL transition of cone is found,and might offer a basis for developing a new transition model that is simultaneously of computational simplicity,sound physics and greater accuracy.

Numerical Investigation on Two-dimensional Boundary Layer Flow with Transition

As a basic problem in many engineering applications, transition from laminar to turbulence still remains a difficult problem in computational fluid dynamics （CFD）. A numerical study of one transitional flow in two-dimensional is conducted by Reynolds averaged numerical simulation （RANS） in this paper. Turbulence model plays a significant role in the complex flows＇ simulation, and four advanced turbulence models are evaluated. Numerical solution of frictional resistance coefficient is compared with the measured one in the transitional zone, which indicates that Wilcox （2006） k-ω model with correction is the best candidate. Comparisons of numerical and analytical solutions for dimensionless velocity show that averaged streamwise dimensionless velocity profiles correct the shape rapidly in transitional region. Furthermore, turbulence quantities such as turbulence kinetic energy, eddy viscosity, and Reynolds stress are also studied, which are helpful to learn the transition＇s behavior.

DNS study on bursting and intermittency in late boundary layer transition

Experimental and numerical investigations have suggested the existence of a strong correlation between the passage of coherent structures and events of bursting and intermittency. However, a detailed cause-and-effect study on the subject is rarely found in the literature due to the complexity and the nonlinear multiscale nature of turbulent flows. The primary goal of this research is to explore the motion and evolution of coherent structures during late transition, whose structure is much more ordered than that of fully developed turbulence, and their relationship with events of bursting and intermittency based on a verified high-order direct numerical simulation(DNS). The computation was carried out on a flat plate at Reynolds number 1000(based on the inflow displacement thickness) with an inflow Mach number 0.5. It is concluded that bursting and intermittency detected by stationary sensors in a transitional boundary layer actually result from the passage and development of vortical structures, and it would be more rational to design transitional turbulence models based on modelling the moving vortical structures rather than the statistical features and experimental experiences.

POD analysis of the instability mode of a low-speed streak in a laminar boundary layer

The instability of one single low-speed streak in a zero-pressure-gradient laminar boundary layer is investigated experimentally via both hydrogen bubble visualization and planar particle image velocimetry(PIV) measurement. A single low-speed streak is generated and destabilized by the wake of an interference wire positioned normal to the wall and in the upstream. The downstream development of the streak includes secondary instability and self-reproduction process, which leads to the generation of two additional streaks appearing on either side of the primary one. A proper orthogonal decomposition(POD) analysis of PIV measured velocity field is used to identify the components of the streak instability in the POD mode space: for a sinuous/varicose type of POD mode, its basis functions present anti-symmetric/symmetric distributions about the streak centerline in the streamwise component, and the symmetry condition reverses in the spanwise component. It is further shown that sinuous mode dominates the turbulent kinematic energy(TKE) through the whole streak evolution process, the TKE content first increases along the streamwise direction to a saturation value and then decays slowly. In contrast, varicose mode exhibits a sustained growth of the TKE content,suggesting an increasing competition of varicose instability against sinuous instability.

Dynamics of low-speed streak evolution and interaction in laminar boundary layer

The present paper presents an experimental effort on the regeneration process of two low-speed laminar streaks in a zero-pressure-gradient laminar boundary layer. Two vertical thin wires separated by a spanwise distance of 30 mm are used to introduce disturbances of two rolls of transitional Karmain vortex street to the downstream boundary layer. Both hydrogen bubble visualization and particle image velocimetry （PIV） measurement show that two lowspeed streaks are induced through leading-edge receptivity process. As these streaks develop in the downstream, two additional low-speed streaks begin to appear outboard of the flank of the original two, together with complex dynamics of streak splitting and merging. A flow pattern of four streaks aligned along the spanwise direction occurs finally in the far downstream. It is found that besides the mechanisms of streak breakdown, the streak interaction is also an important factor characterizing the instability of low speed streaks and their regeneration process.

不同编织方式下陶瓷基复合材料表面流动特性数值计算研究

Ceramic matrix composites(CMCs) are one of the most promising materials in the field of gas turbines,with superior weight and thermal properties. Its surface morphology is different from the traditional casting airfoil components, which mainly comes from different weaving methods and different braided tow thickness. However, few people have studied the influence of surface morphology of ceramic matrix composites(CMCs) on the development of boundary layer and the resulting flow loss. In this paper, Tex Gen is used to generate different surface morphology structures of ceramic matrix composites(CMCs), and the surface flow characteristics of corresponding CMCs plates are numerically studied. It is found that the slope of the displacement thickness of the woven surface first increases and then decreases in the whole transition interval. Thicker braided tow thickness and denser braiding method will induce earlier flow transition phenomenon and produce greater flow loss;The flow loss on the surface of CMCs plate is mainly composed of the vortex loss in the pit and the boundary layer loss outside the pit, and the boundary layer loss is dominant. The weaving methods has a greater influence on the flow state and flow loss of the boundary layer.

水下航行体湍流数值模拟方法研究进展综述

In this paper,we present an overview of numerical simulation methods for the flow around typical underwater vehicles at high Reynolds numbers,which highlights the dominant flow structures in different regions of interest.This overview covers the forebody,midbody,stern,wake region,and appendages and summarizes flow phenomena,including laminar-to-turbulent transition,turbulent boundary layers,flow under the influence of curvatures,wake interactions,and all associated complex vortex structures.Furthermore,the current issues and challenges of capturing these flow structures are addressed.This overview provides a deep insight into the use of numerical simulation methods,including the Reynolds-averaged Navier–Stokes(RANS)method,large eddy simulation(LES)method,and the hybrid RANS/LES method,and evaluates their applicability in capturing detailed flow features.

On the Setup of a Test Bench for Predicting Laminar-to-Turbulent Transition on a Flat Plate

At turbomachinery relevant flow conditions the boundary layers are often transitional with laminar-to-turbulent transition occurring. The characteristics of the main flow can depend highly on the state of the boundary layer. Therefore it can be vitally important for the designer to understand the process of laminar-to-turbulent transition and to determine the position and length of the transitional region. In this paper the flow over a flat plate is experimentally studied in order to investigate and better understand transitional flow. Preston tube measurements as well as a thermographic camera system were performed for two different inlet velocities in order to determine the position of the transitional zone. The results of the experiment are compared to numerical flow solutions using a common transition model to determine its capability. The simulation has been performed with the two commercial codes CFX and Fluent by Ansys and an in-house code called LINARS. As a result of this study, a better understanding of the experimental and numerical methods for determining transition shall be given.

Investigation of influence of local cooling/heating on nonlinear instability of high-speed boundary layer with direct numerical simulations

The influence of local cooling/heating on two types of nonlinear instabilities of the high-speed boundary layer,namely,the First and Second Mode Oblique Breakdown(FMOB and SMOB),is studied using direct numerical simulations.Local cooling and heating are performed at the weak and strong nonlinear stages of the two types of nonlinear instabilities.It is found that for the FMOB,local cooling at the weak nonlinear region will suppress the increase of the fundamental mode,leading to transition delay.Opposite to local cooling,local heating at the weak nonlinear region of the FMOB will promote the growth of the fundamental mode,resulting in the occurrence of more upstream transition onset.However,if local cooling and heating are performed at the strong nonlinear region,the influence of both local cooling and heating on the FMOB can be neglected.Remarkably,both local heating and cooling can delay the SMOB for different mechanisms.Performing local cooling at the weak nonlinear region of the SMOB,the low amplitude of higher spanwise wavenumber steady mode caused by local cooling lies behind transition delay.When local cooling is set at the strong nonlinear region,the low amplitude of harmonic modes around the cooling area can cause transition delay.Additionally,local heating will suppress the SMOB for the slowing amplification rate of various modes caused by the local heating at both the weak and strong nonlinear stages of the SMOB.

Liutex(vorex)cores in transitional boundary layer with spanwise-wall oscillation

The vortex core detection method based on the Liutex vector is utilized to investigate the alternation of vortical structures on a boundary layer transition subjected to spanwise-wall oscillation.Compared with iso-surface based methods,the Liutex core line method is shown to be precise,free of threshold and capable to capture both strong and weak vortices simultaneously.Tollmien-Schlichting(T-S)waves in the linear growth region,A-and hairpin vortices in the transition region and twisted vortices in the turbulent region are all well captured by Liutex core lines.The cyclic wall movement accelerates the transition process while reducing the turbulent drag by 21.8%with selected parameters.For the wall oscillation case,the development from T・S wave to Avortex is advanced about one T・S wave length in the stream wise direction.In the transition region,the A-vortex and legs of hairpin vortex are shortened in the wall oscillation case,and the symmetry of the vortical structures is lost in the late transition region since the introduction of asymmetry disturbances by the cyclic wall movement.Extrusions of weak vortices at the edge of boundary layer are found in the turbulent section which is often omitted by iso-surface based vortex identification method.Thus,it is demonstrated that for the transitional boundary layer the Liutex core line method provides a systematic and thresholdvortex definition,which could serve as a powerful tool to understand and guide flow control.

QUADRANT ANALYSIS OF BUBBLE INDUCED VELOCITY FLUCTUATIONS IN A TRANSITIONAL BOUNDARY LAYER

Our previous study showed that the frictional drag decreases with increasing void fraction at Re〉1300, while it increases at Re 〈 1000. Decomposition of the Reynolds shear stress also implied that bubbles induce isotropy of turbulence. In order to confirm our previous analysis and to further investigate flow fields in the vicinity of bubbles, we analyze velocity fluctuations on the quadrant space in the streamwise and transverse directions （u′-v′ plane）. Here, we focus on two specific Reynolds numbers （at Re≈900 and ≈1410, which are close to the laminar-to-turbulent transition regime） and discuss bubble effects on sweep （u′〉 0, v′〈 0 ） and ejection （u′〈 0, v′〉 0） events as a function of the Reynolds number. We also illustrate velocity fluctuations in the vicinity of an individual bubble and a swarm of bubbles on the u′- v′ coordinates. The results show that a bubble swarm suppresses the velocity fluctuations at Re≈1410.

Entropy generation in bypass transitional boundary layer flows

The primary objective of this study is to evaluate the accuracy of using computational fluid dynamics （CFD） turbulence models to predict entropy generation rates in bypass transitional boundary layers flows under zero and adverse pressure gradients. Entropy generation rates in such flows are evaluated employing the commercial CFD software, ANSYS FLUENT. Various turbulence and transitional models are assessed by comparing their results with the direct numerical simulation （DNS） data and two recent CFD studies. A solution verification study is conducted on three systematically refined meshes. The factor of safety method is used to estimate the numerical error and grid uncertainties. Monotonic convergence is achieved for all simulations. The Reynolds number based on momentum thickness, Reo, skin-friction coefficient, Cf, approximate entropy generation rates, S, dissipation coefficient, Cd , and the intermittency, y, are calculated for bypass transition simulations. All Reynolds averaged Navier-Stokes （RANS） turbulence and transitional models show improvement over previous CFD results in predicting onset of transition. The transition SST k - ω 4 equation model shows closest agreement with DNS data for all flow conditions in this study due to a much finer grid and more accurate inlet boundary conditions. The other RANS models predict an early onset of transition and higher boundary layer entropy generation rates than the DNS shows.

Characteristics of Transient Vortices in the Boundary Layer on a Rotating Disk under Orbital Motion

The objective of this study is to experimentally examine the characteristics of transient vortices in the boundary layer on a disk undergoing both rotation and orbital motion. The velocity fluctuations on a rotating, orbiting disk （disk radius equal to orbital radius） are measured by the hot-wire method, and the effects of orbital motion on the transient vortices in the boundary layer are examined. When the ratio of the orbital speed to the speed of rotation is i-0.025, the interval of transient vortices depends on only the orbital radius, regardless of the directions of rota- tion and orbital motion. The rate of low-frequency disturbances increases as the orbital speed increases, and the vortices induced by these low-frequency disturbances travel over the disk and then develop in the region of in- creased velocity. Consequently, no vortices generated on a rotating disk under orbital motion are stationary rela- tive to the disk.