Calibration of a γ-Re_θ transition model and its application in low-speed flows

The prediction of laminar-turbulent transition in boundary layer is very important for obtaining accurate aerodynamic characteristics with computational fluid dynamic(CFD)tools,because laminar-turbulent transition is directly related to complex flow phenomena in boundary layer and separated flow in space.Unfortunately,the transition effect isn’t included in today’s major CFD tools because of non-local calculations in transition modeling.In this paper,Menter’sγ-Re_θtransition model is calibrated and incorporated into a Reynolds-Averaged Navier-Stokes(RANS)code-Trisonic Platform(TRIP)developed in China Aerodynamic Research and Development Center(CARDC).Based on the experimental data of flat plate from the literature,the empirical correlations involved in the transition model are modified and calibrated numerically.Numerical simulation for low-speed flow of Trapezoidal Wing(Trap Wing)is performed and compared with the corresponding experimental data.It is indicated that theγ-Re_θtransition model can accurately predict the location of separation-induced transition and natural transition in the flow region with moderate pressure gradient.The transition model effectively imporves the simulation accuracy of the boundary layer and aerodynamic characteristics.     

Influence of porous-coating thickness on the stability and transition of flat-plate supersonic boundary layer

In the present study, we examined, both experimentally and theoretically, the influence of porouscoating thickness on the stability and laminar-turbulent transition of flat-plate supersonic boundary layer at free-stream Mach number M_∞ = 2. A qualitative agreement between the data calculated by the linear theory of stability and the experimental data on the transition obtained for models with different porous-coating thicknesses was established. We show that with decreasing (within a certain interval) the porouscoating thickness the boundary layer becomes more stable to perturbations, and the laminar-turbulent transition, more delayed.     

The mechanism of breakdown in laminar-turbulent transition of a supersonic boundary layer on a flat plate-temporal mode

Recently, more and more attentions have been paid to the transition and turbulence of compressible flows, especially supersonic flows, but progress is very slow. In general, attentions were paid to the evolution of disturbances in laminar flow region, and the mechanism of breakdown process in laminar-turbulent transition remains unclear. The conventional idea of what leads to laminar-turbulent transition is that the transi-tion starts from the amplification of disturbances, and when the distur…     

Laminar-turbulent transition in Hagen–Poiseuille flow of a real gas

The effect of gas non-ideality on the laminar-turbulent transition is studied experimentally as the flow in a long circular tube at room temperature. The gases SF₆ and Ar, differing significantly in the value of the second virial coefficient, were chosen for this study. Experiments were carried out by varying the pressure at the tube inlet (the maximum pressure of 10⁵ Pa) and at the tube outlet up to the chock flow (formation of a supersonic flow at the outlet). The difference between the critical Reynolds numbers in the flow of SF₆ and Ar was found. The largest difference was observed for the maximum pressures; with a decrease in pressure, the critical Reynolds numbers become closer. The conclusion is an effect of the non-ideal character of gas exists on the laminar-turbulent transition in Hagen–Poiseuille flow. Some experiments were suggested to confirm this conclusion.     

三维边界层内定常横流涡的感受性研究

层流向湍流转捩的预测与控制一直是研究的前沿热点问题之一,其中感受性阶段是转捩过程中的初始阶段,它决定着湍流产生或形成的物理过程.但是有关三维边界层内感受性问题的数值和理论研究都比较少;实际工程问题中大部分转捩过程都是发生在三维边界层流中,所以研究三维边界层中的感受性问题显得尤为重要.本文以典型的后掠角45?无限长平板为例,数值研究了在三维壁面局部粗糙作用下的三维边界层感受性问题,探讨了三维边界层感受性问题与三维壁面局部粗糙长、宽和高之间的关系;然后,考虑在后掠平板上设计不同的三维壁面局部粗糙的分布状态、几何形状、距离后掠平板前缘的位置以及流向和展向设计多个三维壁面局部粗糙对三维边界层感受性问题有何影响;最后,讨论两两三维壁面局部粗糙中心点之间的距离以及后掠角的改变对三维边界层感受性的物理过程将会发生何种影响等.这一问题的深入研究将为三维边界层流中层流向湍流转捩过程的认识和理解提供理论依据.     

Effect of unit Reynolds number on the laminar-turbulent transition on a swept wing in supersonic flow

An experimental study of the influence of unit Reynolds number on the position of laminar-turbulent transition in swept-wing boundary layer at supersonic flow velocities was carried out. In the experiments, a swept-wing model with 3-% circular arc airfoil and 45-deg gliding angle of wing edges was used. The position of the transition was iden-tified using a hot-wire anemometer. It was found that at М = 2 and 2.5, an increase in the unit Reynolds number (Re₁) leads to a transition delay. It was shown that an increase in freestream Mach number and in the level of flow pulsations in the wind-tunnel test section leads to a less pronounced influence of Re1 on the transition position. At a high noise level due to the growth of Mach number or due to the introduction of vortical disturbances, no effect due to unit Reyn-olds number on the transition position was observed.     

高超音速流场中锥体边界层转捩图象的纹影显示及处理

本文介绍了一种大画幅纹影成像技本,应用该技术,显示了高超音速流场中锥体边界层转捩。将所得到的纹影图象在显微密度计图象处理系统中进行处理与测量,并结合纹影法原理进行了分析计算,得到了边界层诸截面相对密度变化曲线以及边界层转捩位置等实验结果,该结果与有关气动理论和实践结果进行了比较。     

Stability of time-periodic flow with laminar boundary-layer separation

The influence of the low-frequency modulation of flow behind a rectangular backward-facing step on the amplitude characteristics of disturbances in the separated laminar boundary layer has been studied. The experimental data were obtained by the method of hot-wire anemometry in a wind tunnel at a low subsonic velocity. Response of the separated flow to the long-wave oscillations generated by a local source of disturbances on the surface of the experimental model was clarified. The low-frequency nonstationarity of the separation region leads to a growth of velocity fluctuations in the separated boundary layer, which dominate the laminar-turbulent transition and the state of the flow in the near-wall region.     

Numerical simulation of the evolution of unstable disturbances of various modes and initial stages of the laminar-turbulent transition in the boundary layer at the freestream Mach number М = 6

Direct numerical simulations of linear and nonlinear stages of the evolution of unstable disturbances of various modes and initial stages of the laminar-turbulent transition in the boundary layer on a flat plate at the freestream Mach number M = 6 are performed on the basis of full unsteady Navier–Stokes equations for a compressible gas. A considerable effect of three-dimensional unstable disturbances on initiation of the laminar-turbulent transition is demonstrated.     

Formation of the turbulent boundary layer at air blowing through a wall with an abrupt change in boundary conditions

Development of an incompressible turbulent boundary layer with air blowing through a finely perforated flat surface, consisting of a permeable region and impermeable region behind, was studied experimentally. The mass flow rate of injected air Q per an area unit was varied from 0 to 0.2 (kg/s)/m². Detailed data about the internal structure of the boundary layer in the flow region, characterized by an abrupt change in the flow conditions at the boundary of permeable and impermeable regions, were obtained. A consistent decrease in the local values of skin friction coefficient along a permeable sample and with an increase in the values of Q, reaching 90% at maximal Q, is shown. The role of the flow region behind the zone with an abrupt change in the boundary conditions, essential from the viewpoint of skin friction reduction, is revealed.     

Numerical simulation of transitional flow on a wind turbine airfoil with RANS-based transition model

This paper presents a numerical investigation of transitional flow on the wind turbine airfoil DU91-W2-250 with chord-based Reynolds number Re_c = 1.0 × 10⁶. The Reynolds-averaged Navier–Stokes based transition model using laminar kinetic energy concept, namely the k ? k_L ? ω model, is employed to resolve the boundary layer transition. Some ambiguities for this model are discussed and it is further implemented into OpenFOAM-2.1.1. The k ? k_L ? ω model is first validated through the chosen wind turbine airfoil at the angle of attack (AoA) of 6.24° against wind tunnel measurement, where lift and drag coefficients, surface pressure distribution and transition location are compared. In order to reveal the transitional flow on the airfoil, the mean boundary layer profiles in three zones, namely the laminar, transitional and fully turbulent regimes, are investigated. Observation of flow at the transition location identifies the laminar separation bubble. The AoA effect on boundary layer transition over wind turbine airfoil is also studied. Increasing the AoA from ?3° to 10°, the laminar separation bubble moves upstream and reduces in size, which is in close agreement with wind tunnel measurement.     

Heat transfer prediction on transonic turbine blades

The design and optimization of turbine blades submitted to high-temperature flows require the prediction of aerodynamic and thermal flow characteristics. A computation method for aerothermal viscous flows has been developed. The method is based on a compressible boundary layer approach. Tests were performed on turbine blade configurations. These tests include most difficulties that can be encountered in reality: laminar-turbulent transition, separation bubbles, strong accelerations, shock waves. Predictions of the wall heat transfer prove to be satisfactory.     

Control of the Laminar-Turbulent Transition in a Three-Dimensional Boundary Layer under Elevated External Turbulence Using Dielectric Barrier Discharge

Doklady Physics - The results of experimental study of the effect the dielectric barrier discharge (DBR) actuator on the laminar-turbulent transition in a three-dimensional boundary layer under the...     

Breakdown of the chain of ring vortices in a transitional boundary layer

A large number of experiments have been devoted to the study of the generation and development of the high-frequency structures, i.e. the chain of ring-like vortices and breakdown of the chain of ring-llke vortices, to the investi-gation of their relevance to laminar-turbulent transition in a boundary layer flow; however, the origins of turbulence via the process of flow randomization are still not well understood. For this reason a detailed experiment was performed to document the important shape and configuration of the late-stage transitional structures and to investigate some of the still insufficiently understood mechanisms of flow randomization.     

前缘曲率对三维边界层内被激发出非定常横流模态的影响研究

三维边界层感受性问题是三维边界层层流向湍流转捩的初始阶段,是实现三维边界层转捩预测与控制的关键环节.在高湍流度的环境下,非定常横流模态的失稳是导致三维边界层流动转捩的主要原因;但是,前缘曲率对三维边界层感受性机制作用的研究也是十分重要的课题之一.因此,本文采用直接数值模拟方法研究在自由来流湍流作用下具有不同椭圆形前缘三维（后掠翼平板）边界层内被激发出非定常横流模态的感受性机制;揭示不同椭圆形前缘曲率对三维边界层内被激发出非定常横流模态的扰动波波包传播速度、传播方向、分布规律、感受性系数以及分别提取获得一组扰动波的幅值、色散关系和增长率等关键因素的影响;建立在不同椭圆形前缘曲率情况下,三维边界层内被激发出非定常横流模态的感受性问题与自由来流湍流的强度和运动方向变化之间的内在联系;详细分析了不同强度各向异性的自由来流湍流在激发三维边界层感受性机制的物理过程中起着何种作用等.通过上述研究将有益于拓展和完善流动稳定性理论,为三维边界层内层流向湍流转捩的预测与控制提供依据.     

The influence of surface porosity on the stability and transition of supersonic boundary layer on a flat plate

In the present study, we examined, both experimentally and theoretically, the influence of surface permeability on the stability and laminar-turbulent transition of supersonic boundary layer at free-stream Mach number M_∞ = 2. A satisfactory agreement was obtained between the data calculated by the linear theory of stability and the data obtained in experiments with natural disturbances performed on models with different porous inserts.     

Evolution of kinematic structure of the flow behind a transverse rib for transitional flow regimes

Results are presented on visualization of a separated flow behind two kinds of transverse ribs in a channel for a range of Reynolds numbers covering the different stages of laminar-turbulent flow transition. The data was obtained on dynamics of kinematic structure of flow and on evolution of large-scale transverse vortex structures which were generated in the mixing layer during late stages of laminar-turbulent transition. The qualitative estimates were obtained for the vortex generation frequency and velocity of their convective transfer. The features of flow structure were identified for flow behind tested shapes of ribs.     

Transition prediction of a hypersonic boundary layer over a cone at small angle of attack—with the improvement of e<Superscript><Emphasis Type="Italic">N</Emphasis></Superscript> method

The problem of transition prediction for hypersonic boundary layers over a sharp cone has been studied in this work. The Mach number of the oncoming flow is 6, the cone half-angle is 5°, and the angle of attack is 1°. The conventional e ^N method is used, but the transition location so obtained is obviously incorrect. The reason is that in the conventional method, only the amplifying waves are taken into account, while in fact, for different meridians the decay processes of the disturbances before they begin to grow are different. Based on our own previous work, new interpretation and essential improvement for the e ^N method are proposed. Not only the amplification process but also the decay process is considered. The location, where by linear stability theory, the amplitude of disturbance wave is amplified from its initial small value to 1%, is considered to be the transition location. The new result for transition prediction thus obtained is found to be fairly satisfactory. It is also indicated that for the calculation of base flow, boundary layer equations can be used for a small angle of attack. Its computational cost is much smaller than those for DNS. Supported by the National Natural Science Foundation of China (Grant Nos. 10632050 and 90716007), the Special Foundation for the Authors of National Excellent Doctoral Dissertations (Grant No. 200328), and the Foundation of Liu-Hui Center of Applied Mathematics of Nankai University and Tianjin University     

三维边界层内诱导横流失稳模态的感受性机理

边界层感受性问题是层流向湍流转捩的初始阶段,在转捩过程中起关键性作用,尤其是三维边界层流动.因此,研究三维边界层感受性问题对进一步理解层流向湍流转捩机理以及湍流成因具有重要的理论意义.采用数值方法研究自由来流湍流与三维壁面局部粗糙相互作用下三维边界层的感受性问题,确定是否能在三维边界层内寻找一种新的横流失稳模态;确定在何种条件下三维边界层内能诱导出定常、非定常的横流失稳模态;探索自由来流湍流的强度、展向波数和法向波数以及三维壁面局部粗糙的大小和结构类型等因素在自由来流湍流与三维壁面局部粗糙作用下三维边界层内被激发出的感受性过程中有何影响,并确定何种横流失稳模态在三维边界层感受性过程中占据何种地位.对自由来流湍流与三维壁面局部粗糙作用激发三维边界层内感受性问题的深入研究,将有助于完善流动稳定性与湍流理论,为层流向湍流转捩过程的预测与控制提供合理的理论依据.     

Wind-tunnel simulation of thick turbulent boundary layer

An experimental study aimed at revealing the possibility of simulation, in a subsonic wind tunnel, of enhanced Reynolds numbers Re^** via modeling a thick flat-plate boundary layer possessing the properties of a Clauser-equilibrium shear flow is reported. We show that turbulators prepared in the form of variable-height cylinders of height h and diameter d = 3 mm and installed in two rows along the normal to the streamlined wall offer rather an efficient means for modification of turbulent boundary layer in solving the problem. In the majority of cases, mean and fluctuating characteristics of the boundary layer exhibit values typical of naturally developing turbulent boundary layers at a distance of 530 cylinder diameters. The profiles of mean velocity with artificially enhanced boundary-layer thickness can be well approximated, in the law-of-the-wall variables, with the well-known distribution of velocities for canonical boundary layer.