Turbulent boundary layer over a flat plate with uniform wall suction

A complete theory of turbulent boundary layer flow over a flat plate with uniform wall suction is proposed. The theory relies on an asymptotic analysis of the Reynolds equations and dimensional considerations and does not involve any special closure hypotheses. Characteristics of the turbulent boundary layer with suction are calculated for the entire range of flow parameters by using the known characteristics of a reference flow (turbulent boundary layer over an impermeable flat plate). The velocity and shear stress profiles, the distribution of skin friction along the plate, and integral flow characteristics are obtained by using only the known velocity profile in the reference flow. The normal Reynolds stresses are calculated by using analogous characteristics of the reference flow. Results are presented in terms of scaling variables.     

Effect of vertical large eddy breakup devices on the drag of a flat plate

Experiments are performed to study the possibility of decreasing the net drag of a flat plate with the use of streamwise-oriented vertical elements mounted normal to the surface in an incompressible equilibrium turbulent boundary layer. The Reynolds number based on the boundary-layer momentum thickness in the section where the vertical elements are placed is 820. It is demonstrated that vertical large eddy breakup elements with the geometry used do not reduce the drag of a flat plate in the major part of the range of Reynolds numbers Re _x examined. It is only at extremely low values of Re _x that a certain gain in the net drag is reached, as compared with the value for a non-modified flow.     

Experimental modeling of air blowing into a turbulent boundary layer using an external pressure flow

We have experimentally investigated the characteristics of an incompressible turbulent boundary layer on a plane plate upon the passive blowing of air through a fine-perforated surface and flushing it by supplying an external pressure flow through a wind tunnel using an intake device equipped with an attachment for draining the boundary layer on the inactive side of the plate. A stable decrease in the local values of the surface coefficient of friction, which reaches 80% at the end of the perforated region, has been detected over the length of the plate. The possibility of controlling surface friction by changing the velocity of the external flow and selecting the meshes and filters at the inlet to the flow passage has been demonstrated.     

高超声速平板边界层/圆柱粗糙元非定常干扰

以高超声速表面湍流控制为应用背景,平板/粗糙元干扰流动为模型,采用大涡模拟方法研究粗糙元流场干扰作用机理.分析粗糙元外形特征对于流动稳定性影响,给出其引起的流动表面参数的变化规律.结果显示超声速边界层在粗糙元作用下产生强逆压梯度并发生分离,粗糙元高度对高位自由剪切层失稳有明显影响,低粗糙元干扰下游流动稳定性,而高粗糙元剪切层发生流向失稳,形成涡串结构;同时粗糙元干扰导致下游摩阻和热流系数较平板略低,可能应用在进气道降热和减阻中.     

Turbulent boundary layer on the surface of a sea geophysical antenna

A theory is constructed that makes it possible to calculate the initial parameters necessary for calculating the hydrodynamic (turbulent) noise, which is a handicap to the operation of sea geophysical antennas. Algorithms are created for calculating the profile and defect of the average speed, displacement thickness, momentum thickness, and friction resistance in a turbulent boundary layer on a cylinder in its axial flow. Results of calculations using the developed theory are compared to experimental data. As the diameter of the cylinder tends to infinity, all relations of the theory pass to known relations for the boundary layer on a flat plate. The developed theory represents the initial stage of creating a method to calculate hydrodynamic noise, which is handicap to the operation of sea geophysical antennas.     

利用微气泡减小平板湍流摩阻实验研究

设计了一套实验装置研究微气泡减阻的效果.采用高速摄像机对二维平板微气泡湍流边界层进行了定量的可视化观察,用天平测量了平板的摩擦阻力,分析了不同通气量、来流速度、浮力对减阻性能的影响.结果表明,微气泡在高Reynolds数（Re=106）的流动中有效地减小了摩擦阻力,最大减阻率达到36%,证实了微气泡能显著降低平板摩擦阻力,实验结果也表明,随气体流量增加,减阻率增加.      

Simulation of a thick turbulent boundary layer via a rod grid

A possibility to simulate a thick Clauser-equilibrium incompressible turbulent boundary layer on a flat plate of finite length with the help of a grid formed by cylindrical rods was experimentally examined. A grid with rods oriented parallel to the streamlined surface proved to be an efficient tool enabling modification of the turbulent boundary layer. In most cases, at a distance of 600 rod diameters the time-average and fluctuation characteristics of the modified boundary layer exhibited values typical of a natural turbulent boundary layer. It is shown that the mean velocity profiles with artificially increased boundary-layer thickness can be represented, to a good accuracy, in terms of law-of-the-wall variables, and they can be generalized with a single dependence using an empirical velocity scale in the outside region. The use of a combined method for exerting an influence on the shear flow capable of improving the modeling procedure for turbulent velocity fluctuations in boundary layer is proposed.     

压缩拐角激波/湍流边界层干扰特性分析

用大涡模拟方法对Mach数3.0下的压缩拐角激波/湍流边界层干扰问题进行数值研究.对拐角上游平板区域边界层转捩及湍流进行模拟,设定平板区域长度,使得转捩过程于平板区域发生并充分完成,从而在拐角处产生激波/湍流边界层相互干扰,研究激波/湍流边界层的作用机理.研究表明:流场能够在非定常扰动激励下迅速转捩,并于平板区发展为完全湍流;湍流边界层与激波相互作用过程中,拐角附近分离区较层流情况明显减小;展向不同区域分离区大小差异较大,局部区域分离现象消失.     

Effects of dimples on laminar boundary layers

A series of direct numerical simulations of the flow past a flat plate with two and eight rows of dimples in a staggered arrangement is carried out. The Reynolds number based on the boundary layer thickness and freestream velocity near the inflow plane is 1000 and the dimples are spherical with a depth to diameter ratio of 0.1. The incoming flow is laminar and the boundary layer thickness before the dimples is half the dimple depth. At this low Reynolds number the flow is expected to remain laminar over a smooth flat plate. The presence of the dimples triggers instabilities that cause significant momentum transport. It is shown that the shear layer that forms as the flow separates over the first two rows of dimple becomes unstable and sheds coherent vortex sheets. The vortex sheets become unstable and are transformed into packets of horseshoe vortices. When these vortices evolve over a flat plate or over a series of dimples the flow dynamics are very different with important changes in momentum transport across the boundary layer.     

Analysis of the turbulent boundary layer in the vicinity of a self-excited cylindrical Helmholtz resonator

This study investigates the changes in the structure of a turbulent boundary layer downstream of a flow-excited Helmholtz resonator. To this end, a fully developed turbulent boundary layer over a resonator mounted flush with a flat plate was simulated by implementing a large eddy simulation (LES). To assist in understanding the effect of the resonator on the flow structure, a sensitivity study was undertaken by changing the main geometrical parameters of the resonator. The results demonstrated that when the boundary layer thickness equals the orifice length, the cross-stream component of velocity fluctuations penetrates the boundary layer, resulting in a reduction of the turbulence intensity by up to 12%. Therefore, it is concluded that a Helmholtz resonator has the potential to reduce the instabilities within the boundary layer. These investigations also assist in identifying the optimal parameters to delay turbulence events within the grazing flow using Helmholtz resonators.     

高超声速平板边界层流动显示的试验研究

本文在高超声速脉冲式风洞内对基于纳米示踪的平面激光散射技术(nano-based planar laser scattering, NPLS)的应用进行了探索, 并在此基础上对平板边界层流动结构的精细测量进行了研究. 试验来流Ma=7.3, 总压4.8 MPa, 总温680 K. 通过时序的分析和调试, 对各分系统实现了高精度的同步控制; 定量的粒子注入及混合, 实现了粒子的均匀撒播, 对主流获得了均匀的显示效果; 对于边界层流动, 获得了精细的瞬态流动结构图像, 显示了层流到湍流的转捩过程, 并分析了其时空演化特性.     

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.     

DNS of compressible turbulent boundary layer around a sharp cone

Direct numerical simulation of the turbulent boundary layer over a sharp cone with 20° cone angle (or 10° half-cone angle) is performed by using the mixed seventh-order up-wind biased finite difference scheme and sixth-order central difference scheme. The free stream Mach number is 0.7 and free stream unit Reynolds number is 250000/inch. The characteristics of transition and turbulence of the sharp cone boundary layer are compared with those of the flat plate boundary layer. Statistics of fully developed turbulent flow agree well with the experimental and theoretical data for the turbulent flat-plate boundary layer flow. The near wall streak-like structure is shown and the average space between streaks (normalized by the local wall unit) keeps approximately invariable at different streamwise locations. The turbulent energy equation in the cylindrical coordinate is given and turbulent energy budget is studied. The computed results show that the effect of circumferential curvature on turbulence characteristics is not obvious.     

气动光学效应湍流脉动模型系数修正

以光学窗口的气动光学效应研究为背景,研究影响光学传输预测的流场湍流脉动量预测.选取平板、压缩拐角、凹腔流动模型,采用大涡模拟（LES）方法研究超声速湍流脉动工程预测模型的系数修正.结果表明：LES方法能够获得无激波干扰、强激波干扰及底部大分离条件下,湍流密度脉动的定量分布,并据此给出脉动工程模型的系数修正,结果已经应用于型号飞行器的光学窗口气动光学效应预测.     

Hot-wire anemometer measurement of local skin friction coefficient

Based on fine measurement of time-averaged data of hot-wire anemometer we have developed the method determining local values of skin-friction coefficient in the vicinity of the wall on the planar porous surface at the flow past which the noncanonical turbulent boundary layer is formed. The method may be applied at investigation of the efficiency of various effects on the incompressible boundary layer, in particular, with microblowing of air where usage of direct methods of friction determination is complicated.     

Scaling of turbulent wall pressure fluctuations downstream of a rearward facing step

The turbulent boundary layer that forms downstream of a surface discontinuity is locally inhomogeneous. Here, we consider the boundary layer that occurs downstream of a rearward facing step discontinuity on a flat plate that is exposed to a zero incidence, uniform subsonic flow. The wall pressure "point" spectra are measured by small, flush-mounted pressure transducers located at various locations downstream of the step. A new form of the non-dimensional point wall pressure auto-frequency spectrum is proposed that includes the statistical variations of the reattachment location. This form is shown to collapse quite well all of the spectra measured slightly upstream, within, and downstream of the flow reattachment location.     

Spatial hot-wire visualization of the Λ-structure transformation into the turbulent spot on the smooth flat plate surface and riblet effect on this process

Abstract  

Evolution of the Λ-vortex and its transformation into the turbulent spot in a flat plate boundary layer are investigated experimentally. Extensive measurements visualizing the Λ-structure transformation into the turbulent spot on the smooth and ribbed surfaces of the flat plate are presented. The flow behavior in the course of spatial evolution of the Λ-structure and turbulent spot is discussed. Specific features of the downstream evolution of Λ-structure and turbulent spot on the smooth and ribbed surfaces are demonstrated, such as suppression by riblets of the Λ-vortex transformation into the turbulent spot, appearance of the coherent structures of Λ-vortex within ensemble-averaged turbulent spot, and oblique waves generation both by the Λ-vortex and turbulent spot.     

The evolution of synthesised vortices in turbulent boundary layer

Particle image velocimetry technique has been used to investigate the evolution of synthesised vortical structures in the turbulent boundary layer. Synthetic jet actuator is implemented on the flat plate surface to synthesise various vortical structures by operating the actuator at varying operating parameters. The vortices are issued into the boundary layer and their evolution and subsequent interaction with the relatively less energetic near wall fluid is studied. The investigation is based on the quantitative measurements that are made both on the central and parallel lateral planes. Finally, the enhancement of the wall shear stress resulted in by the passing vortices is measured to evaluate the effectiveness of the actuator towards flow separation control.     

二维高超声速后台阶表面传热特性实验研究

高超声速后台阶流动是大气层内高速飞行器发动机设计、表面热防护以及高超声速拦截器红外成像窗口气动光学效应校正等诸多先进高超声速技术研发过程中所涉及的一类基础流动问题. 研究高超声速后台阶流动特性对有效提升飞行器综合性能, 进一步掌握高超声速流动机理具有重大基础 意义. 本文以二维高超声速后台阶流动为研究对象, 在KD-01高超声速激波风洞中测量了二维后台阶模型表面传热系数和表面静压, 并将实测台阶下游表面传热系数分布同采用高超声速边界层理论所得估计值进行了比较. 为进一步验证实验结果, 使用NPLS技术测量了其中一种实验状态下台阶周围流动结构. 研究发现, 对于二维高超声速后台阶流动, 台阶下游表面传热分布受台阶处边界层外缘流动特性的直接影响; 在台阶下游分离区和再附区内, 气体黏性占主导作用; 在台阶下游远场区域, 边界层流动特性趋同于平板边界层; 下游边界层基本结构取决于台阶处边界层相对厚度. 对高超声速后台阶流动, 若使用数值模拟方法研究气动热问题, 应当使用湍流模型.