动失速型非定常分离涡结构的控制

本文对动失速型非定常分离涡结构的控制方法，在低速风洞中应用相平均测压技术进行了实验研究。在二元平板模型中部安装一作俯仰振荡的扰流板产生动失速型分离涡，在其上游安装另一用作控制的小扰流板。实验结果表明，应用前置的振荡小扰流板可影响并改变动失速分离涡的强度和对流特性。在最有利的控制相位下，涡吸力峰可降低４８％，涡对流时间可以推迟０．１９周期。对于间歇式振荡扰流板，采用相位提前控制方式比相位滞后控制方式更有效。     

Transient Characterization of the Reattachment of a Massively Separated Turbulent Boundary Layer Under Flow Control

The transient dynamics of a high Reynolds number separated flow over a two-dimensional ramp submitted to pulsed fluidic control is investigated. A spanwise array of 22 round jets, located upstream of the flap leading edge, is used as actuator to generate co-rotating vortical structures. Simultaneous measurements of wall friction using hot-film anemometry and phase-averaged velocity using 2D2C PIV are conducted. The PIV plane encompasses the incoming boundary layer upstream the flap leading edge, the separation bubble and the natural reattachment region. The dynamics of the separated flow is studied for successive sequences of pulsed actuation. Pockets of turbulence are periodically generated by the separation process and pushed downstream. After the transition period, the controlled flow shows large amplitude oscillations around a steady mean, particularly for the separation area. The transient dynamics of the flow at the actuation activation is also studied. The separated flow is strongly modified by the actuation from the first pulse. Characteristic times of the transient dynamics can be determined by fitting a first-order model with delay on the data. For the reattachment, the dimensionless characteristic rising times defined as \(\tau _{r}^{+} = \tau _{r} ~ U_{0} ~/~ L_{sep}\) of 11.7 for the friction gain, 4.8 for the separation length and 4.1 for the first mode of a Conditional Proper Orthogonal Decomposition analysis of the phase-averaged velocity fields were found. These values are in good agreement with previous studies and are of particular interest for modeling the transients and for further closed-loop control applications.     

Some Features of a Turbulent Separated Flow and Heat Transfer Behind a Step and a Rib. 2. Heat Transfer in a Separated Flow

Results of an experimental study of heat transfer in a separated flow behind a step and a rib are presented. The influence of the obstacle height (H = 6–30 mm) on heat and mass transfer and the structure of the thermal boundary layer is studied. The features of heat transfer in recirculation and relaxation zones of the separated flow are analyzed, and the effect of separation on intensification and suppression of turbulent heat transfer is determined.     

Advanced Turbulence Modelling of Separated Flow in a Diffuser

The paper describes an investigation into the predictive performance of linear and non-linear eddy-viscosity models and differential stress-transport closures for separated flow in a nominally two-dimensional, asymmetric diffuser. The test case forms part of a broader collaborative exercise between academic and industrial partners. It is demonstrated that advanced turbulence models using strain-dependent coefficients and anisotropy-resolving closure offer tangible advantages in predictive capability, although the quality of their performance can vary significantly, depending on the details of closure approximations adopted. Certain features of the flow defy resolution by any of the closures investigated. In particular, no model resolves correctly the flow near the diffuser's inclined wall immediately downstream of the inlet corner, which may reflect the presence of a “flapping” motion associated with a highly-localised process of unsteady separation and reattachment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Large Eddy Simulation of Separated Flow over a Two-dimensional Hump with and without Control by Means of a Synthetic Slot-jet

Large Eddy Simulations for two flows separating from a two-dimensional hump in a duct are reported and discussed. The flows differ through the presence or absence of a synthetic slot-jet injected in a sinusoidal manner, i.e. at zero net mass-flow rate, close to the location of separation and intended to reduce (“control”) the extent of the separated region. Results reported include instantaneous visualisations, pre-multiplied spectra, wall-pressure distributions, streamfunction fields and profiles of velocity and second moments. For both flows, agreement between the simulations and the experimental results is generally good, especially in respect of the overall control effectiveness of the synthetic jet, despite the use of an approximate wall treatment bridging the viscous sublayer. Proper Orthogonal Decomposition of the velocity field is used to study structural features, and this shows that the most energetic mode in the base flow is representative of large streamwise vortices in the separated region, while in the controlled flow, most of the energetically dominant modes are associated with large spanwise vortices.     

非流线体分离流动态载荷的数值模拟方法研究

以圆柱绕流为研究对象,针对圆形边界,采用O型网格对流场进行离散,用二阶精度的中心差分有限体积法作空间离散,用二阶精度的中心差分处理时间问题,用双时间方法求解了二维非定常Navier-Stokes方程,系统研究了计算方法对收敛精度、时间步长和网格数量的依赖性.计算结果表明,对于长时间历程的非定常问题,虽然双时间方法收敛性很好,但对于分离流而言,时间步长的选取并非没有限制;每一步伪时间的推进中,收敛精度也有要求;而要模拟圆柱分离流的非线性气动力现象,计算网格至少要达到260×80的数量.     

Prediction of Turbulent Separated Flow in a Turnaround Duct Using Wall Functions and Adaptivity

This paper presents an application of adaptive remeshing to the prediction of turbulent separated flows. The paper shows that the κ - ε model with wall functions can predict separated flows along smooth curved surfaces. Success is achieved if the wall functions exhibit values of y⁺ close to 30, and if meshes are fine enough to guarantee that wall function boundary conditions are grid converged. Adaptive remeshing proves to be a very cost effective tool in this context. The methodology is demonstrated on a problem possessing a closed form solution to establish the performance and reliability of the proposed approach. The method is then applied to prediction of turbulent flow in an annular, axisymmetric turnaround duct (TAD). Predictions from two computational models of the TAD are compared with experimental measurements. The importance of appropriate meshes to achieve grid independent solutions is demonstrated in both cases. Better agreement with measurements is obtained when partially developed profiles of u, κ, and ε are specified at the TAD inlet.     

Some Features of a Turbulent Separated Flow and Heat Transfer Behind a Step and a Rib. 1. Flow Structure

The influence of the shape and size of the obstacle on separated flow and heat transfer is studied experimentally. Results of investigation and comparative analysis of the hydrodynamic structure of a separated flow behind a step and a rib are presented. A principally different character of transfer processes in the separated flow behind obstacles of these types is demonstrated. The flow structure in the secondary vortex region is considered.     

Experimental Investigation of Separated Shear Flow under Subharmonic Perturbations over a Backward-Facing Step

Subharmonic-perturbed shear flow downstream of a two-dimensional backward-facing step was experimentally investigated. The Reynolds number was Re_h = 2.0 ×10⁴, based on free-stream velocity and step height. Planar 2D-2C particle image velocimetry was employed to measure the separating and reattaching flow in the horizontal-vertical plane in the center position. The subharmonic perturbations were generated by an oscillating flap which was implemented over the step edge and driven by periodic Ampere force. The subharmonic frequency was 55 Hz as the half of the fundamental frequency of the turbulent shear layer. As a result of the subharmonic perturbations, the size of recirculation region behind the backward-facing step is reduced and the time-averaged reattachment length is 31.0% shorter than that of the natural flow. The evolution of vortices, including vortex roll-up, growth and breakdown process, is analyzed by using phase-averaging, cross-correlation function and proper orthogonal decomposition. It is found that Reynolds shear stress is considerably increased in which the vortices roll up and then break down further downstream. In particular, rapid growth of vortices based on the “step mode” occurs at approximate half of the recirculation region, caused by in interaction between the shear layer and the recirculation region. Furthermore, the coherent structures, which are represented by a phase-correlated POD mode pair, are reconstructed in phases in order to show regular patterns of the subharmonic-perturbed coherent structures.     

Wing Buffeting Control at Transonic Flight Velocities with the Use of Ejector-Type Pulse Thermal Actuators

A new type of ducted pulse thermal actuators with a high pulse repetition frequency is proposed to control wing buffeting at transonic flight velocities. Ducted pulse thermal actuators can operate up to frequencies of about 1 kHz, which is sufficient for controlling the majority of aerodynamic processes at high subsonic flow velocities. As the use of a pulse thermal actuator in the regime of tangential injection of a jet is less efficient from the energy viewpoint than in the regime of boundary layer suction, an ejector-type pulse thermal actuator is proposed for implementation of the suction regime.     

Investigation of Anisotropy-Resolving Turbulence Models by Reference to Highly-Resolved LES Data for Separated Flow

The predictive properties of several non-linear eddy-viscosity models are investigated by reference to highly-resolved LES data obtained by the authors for an internal flow featuring massive separation from a curved surface. The test geometry is a periodic segment of a channel constricted by two-dimensional (2D) `hills' on the lower wall. The mean-flow Reynolds number is 21560. Periodic boundary conditions are applied in the streamwise and spanwise directions. This makes the statistical properties of the simulated flow genuinely 2D and independent from boundary conditions, except at the walls. The simulation was performed on a high-quality, 5M-node grid. The focus of the study is on the exploitation of the LES data for the mean-flow, Reynolds stresses and macro-length-scale. Model solutions are first compared with the LES data, and selected models are then subjected to a-priori studies designed to elucidate the role of specific model fragments in the non-linear stress-strain/vorticity relation and their contribution to observed defects in the mean-flow and turbulence fields. The role of the equation governing the length-scale, via different surrogate variables, is also investigated. It is shown that, while most non-linear models overestimate the separation region, due mainly to model defects that result in insufficient shear stress in the separated shear layer, model forms can be derived which provide a satisfactory representation of the flow. One such model is identified. This combines a particular quadratic constitutive relation with a wall-anisotropy term, a high-normal-strain correction and a new form of the equation for the specific dissipation ω = ∈/k. This revised version was published online in July 2006 with corrections to the Cover Date.     

An Experimental Study of the Rotational Effects on Separated Turbulent Flow During Stall Delay

Three-dimensional velocity fields were measured using tomographic particle image velocimetry (Tomo-PIV) on a model of the blade of a small-scale horizontal axis wind turbine (HAWT) to study the effects of rotation on separated turbulent flows during stall delay at a global tip speed ratio (TSR) of 3 and a Reynolds number of 4800. The flow fields on a static airfoil were also measured at a similar angle-of-attack (AOA) and Reynolds number for comparison. It was observed that the blade’s rotation in the streamwise direction significantly affected both the mean flow and the turbulence statistics over the suction surface. The mean velocity fields revealed that, different from the airfoil flow at large AOA, the recirculation region with reversed flow did not exist on the suction surface of the blade and the flow was rather attached. Mean spanwise flow from blade’s root to its tip was also generated by the rotation. The mean vorticity vector of the blade flow was found to be tilted in the rotational direction of the blade, as well as in the wall-normal direction. Of particular effects of the rotation on Reynolds stresses were the enhancement of 〈w ²〉 and the creation of strong 〈v w〉. The production of Reynolds stresses was also affected by blade’s rotation directly through the rotational production terms and indirectly by dramatically changing the fluctuating velocity fields. The distribution of enstrophy was observed to be modified by rotation, too.     

The Effect of Sweep-Angle Variation on the Turbulence Structure in a Separated,Three-Dimensional Flow

A three-dimensional separated flow behind a swept, backward-facing step is investigated by means of DNS for Re _H = C _∞ H/ν = 3000 with the purpose to identify changes in the statistical turbulence structure due to a variation of the sweep angle α from 0° up to 60°. With increasing sweep angle, the near-wall turbulence structure inside the separation bubble and downstream of reattachment changes due to the presence of an edge-parallel mean flow component W. Turbulence production due to the spanwise shear ∂W/∂y at the wall becomes significant and competes with the processes caused by impingement of the separated shear-layer. Changes due to a sweep angle variation can be interpreted in terms of two competing velocity scales which control the global budget of turbulent kinetic energy: the step-normal component U _∞ = C _∞cosα throughout the separated flow region and the velocity difference C _∞ across the entire shear-layer downstream of reattachment. As a consequence, the significance of history effects for the development into a two-dimensional boundary layer decreases with increasing sweep angle. For α ≥50°, near-wall streaks tend to form inside the separated flow region. Received 7 November 2000 and accepted 9 July 2002 Published online 3 December 2002 RID="*" ID="*" Part of this work was funded by the Deutsche Forschungsgemeinschaft within Sfb 557. Computer time was provided by the Konrad-Zuse Zentrum (ZIB), Berlin. Communicated by R.D. Moser     

Friction and Heat Transfer in a Laminar Separated Flow behind a Rectangular Step with Porous Injection or Suction

Results of a numerical study of a laminar separated flow behind a rectangular step on a porous surface with uniform injection or suction are described. Two cases are considered: an unconfined flow past a step and flow evolution in a confined channel (duct). It is shown that mass transfer on the surface causes strong changes in the flow structure and substantially affects the position of the reattachment point, as well as friction and heat transfer. More intense injection leads first to an increase in the separation-zone length and then to its rapid vanishing due to boundary-layer displacement. Vice versa, suction at high Reynolds numbers Re _s > 100 reduces the separation-zone length. The duct flow has a complicated distribution of friction and heat-transfer coefficients along the porous surface owing to the coupled effect of the transverse flow of the substance and changes in the main flow velocity due to mass transfer. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 1, pp. 18–28, January–February, 2006.     

Separated nozzle flow

A separated turbulent flow in an axisymmetrical nozzle is studied numerically. Two configurations nozzle are investigated. The first one is the truncated ideal contour nozzle, DLR-TIC, is fed with nitrogen. The second configuration is called the thrust optimized contour nozzle or TOC type, ONERA-TOC, where the operating gas is a hot air. The classical pattern of a free shock separation is obtained for different values of the nozzle pressure ratio. The results are compared and validated using experimental data.     

大跨悬索桥抖振内力响应分析

基于虚拟激励法和有限元法,在频域建立了一种新的桥梁抖振内力响应分析的随机振动方法。该方法与传统随机振动方法相比具有如下两个特点:(1)单元抖振内力响应同时考虑了保留模态多模态耦合产生的动力效应和保留模态外高频模态产生的拟静力效应;(2)单元抖振内力响应同时考虑了单元杆端位移产生的单元杆端力和单元上分布荷载产生的单元固端力。以香港青马悬索桥为例,分析了保留模态多模态耦合产生的动力效应、高频模态拟静力效应、单元上分布荷载产生的单元固端力及主缆上的抖振荷载等因素对主梁抖振内力响应的贡献。结果表明:保留模态多模态耦合产生的动力效应对主梁抖振内力响应占据主导地位,高频模态拟静力效应、单元上分布荷载产生的单元固端力等因素对主梁抖振内力响应均有一定的影响,主缆上的抖振荷载对主梁侧向抖振内力响应有较大贡献。     

Mesh Adaptation for Stationary Flow Control

We consider local mesh adaptation for stationary flow control problems. The state equation is given by the incompressible Navier-Stokes equations with Neumann boundary control. As an example, the functional to be minimized is the drag coefficient of an immersed body. We use finite elements on locally refined meshes to discretize the first order necessary conditions based on the Lagrangian. An a posteriori error estimate is derived which is directly related to the control problem. It is used to successively enrich the finite element space until the computed solution satisfies prescribed accuracy requirements.     

大口径天线电性能风振响应的时域分析

建立天线风荷动力的计算方法,运用谐波合成技术,对天线受风空间的脉动风进行时程数值模拟,结合平均风速,获得天线反射面各结点的风速时程值,进而计算各结点的风荷动力;利用有限元法进行天线结构的风振响应计算,求得反射面各结点的位移响应;依据位移响应,进行电性能响应的计算;以40m口径天线为例进行了数值模拟,获得了该天线电性能风振响应的特性,结果表明脉动风的脉动对电性能的影响较为突出,其中,天线处于低仰角时更为严重.算例说明本文的时域分析思路与计算方法对大天线电性能风振响应的掌握是可行和有效的.     

Separated flow modelling and interfacial transport phenomena

A physical explanation of the different types of interfacial waves that appear in stratified and annular gas-liquid flows is presented. The role of waves in affecting process performance is discussed. Particular attention is paid to interfacial drag, gas absorption, the initiation of slug flow and atomization.This paper was presented at the Shell Conference on Computational Fluid Dynamics in Apeldoorn, December, 1989.