Experimental and theoretical investigation of boundary layer perturbations on a low-aspect-ratio wing

Laminar-turbulent transition in a boundary layer of low-aspect-ratio wing was investigated. Experiments clarifying the flow structure, its mean and oscillatory characteristics were carried out accompanied by linear stability analysis of the wind tunnel data on the laminar flow velocity profiles. Theoretical results obtained in a parallel flow approximation are in a good agreement with the experimental data on disturbances evolution at the initial stage of transition to turbulence. The study was supported by the Ministry of Education and Science of the Russian Federation (Grant No. RNP 2.1.1.471) and Russian Foundation for Basic Research (Grant No. 03-01-06145)     

Obstacle influence on the flow structure and mass transfer in a boundary layer with ethanol combustion on horizontal surface

Experiments with ethanol combustion on horizontal surfaces revealed the most general properties of a boundary layer with chemical and phase transformations. The list of flow features includes development of large-scale structures and manifestation of volumetric forces, which impact the flow stability and heat and mass transfer. It was demonstrated that the range of velocities ensuring flame existence is wider for flow past a rib than for flow past a backward-facing step. The nature of mass transfer in a reactive flow past an obstacle is transient and remains of that kind until the flame blow-off. For a flow above a horizontal wall at Reynolds numbers Re < 5·10⁴, the intensity of mass transfer is twice higher than for combustion below the wall. When the combustion occurs below the wall, the surface temperature gradients are higher.     

Formation of a deep scattering layer in a stratified flow behind a two-dimensional obstacle

A stratified stationary flow around a circular cylinder is accompanied by the formation of a thin, acoustically contrasting layer extending along the horizontal axis of the flow. The theoretical and experimental studies show that the pressure differential and the backscattering coefficient of the layer can be quantitatively estimated in the framework of the problem on a stratified ideal incompressible liquid flow around a cylinder.     

Experimental investigation of flame spreading over pure methane hydrate in a laminar boundary layer

Flame spreading over pure methane hydrate in a laminar boundary layer is investigated experimentally. The free stream velocity (U_∞) was set constant at 0.4 m/s and the surface temperature of the hydrate at the ignition (T_s) was varied between ?10 and ?80 °C. Hydrate particle sizes were smaller than 0.5 mm. Two types of flame spreading were observed; “low speed flame spreading” and “high speed flame spreading”. The low speed flame spreading was observed at low temperature conditions (T_s = ?80 to ?60 °C) and temperatures in which anomalous self-preservation took place (T_s = ?30 to ?10 °C). In this case, the heat transfer from the leading flame edge to the hydrate surface plays a key role for flame spreading. The high speed flame spreading was observed when T_s = ?50 and ?40 °C. At these temperatures, the dissociation of hydrate took place and the methane gas was released from the hydrate to form a thin mixed layer of methane and air with a high concentration gradient over the hydrate. The leading flame edge spread in this premixed gas at a spread speed much higher than laminar burning velocity, mainly due to the effect of burnt gas expansion.     

激波与层流/湍流边界层相互作用实验研究

在超声速风洞中,分别对层流和湍流来流条件下的边界层和斜激波(激波强度足以引起流动分离)相互干扰进行了实验研究,利用纳米粒子示踪平面激光散射(NPLS)技术获得了两种条件下流场的精细结构图像;利用粒子图像测速(PIV)技术获得了两种条件下流场的速度场和涡量场;综合运用NPLS结果和PIV结果对比分析了两种流动的瞬时流动结构和时间相关性,实验结果表明:层流边界层内的分离区呈现出狭长的条状,而湍流边界层内分离区呈现出较规则的椭圆;在入射激波上游距入射点较远的位置,层流边界层外围拟序结构会诱导出一系列压缩波系,进而汇聚成空间位置不稳定的诱导激波,而湍流边界层则是在入射激波上游较近的地方直接形成较强且稳定的诱导激波;在入射激波下游,层流边界层内的膨胀区域较小且急促,膨胀后产生的再附激波很弱,而湍流边界层内的膨胀区域较大,膨胀后产生的激波较强。     

Experimental investigation on drag reduction in a turbulent boundary layer with a submerged synthetic jet

This work investigates the active control of a fully developed turbulent boundary layer by a submerged synthetic jet actuator.The impacts of the control are explored by measuring the streamwise velocities using particle image velocimetry,and reduction of the skin-friction drag is observed in a certain range downstream of the orifice.The coherent structure is defined and extracted using a spatial two-point correlation function,and it is found that the synthetic jet can efficiently reduce the streamwise scale of the coherent structure.Proper orthogonal decomposition analysis reveals that large-scale turbulent kinetic energy is significantly attenuated with the introduction of a synthetic jet.The conditional averaging results show that the induction effect of the prograde vortex on the low-speed fluid in a large-scale fluctuation velocity field is deadened,thereby suppressing the bursting process near the wall.     

高超声速条件下7°直圆锥边界层转捩实验研究

在Ma=6低噪声风洞中开展了半锥角7?的直圆锥边界层转捩相关实验研究.利用响应频率达到MHz量级的高频压力传感器对圆锥壁面脉动压力进行了测量,研究了高超声速圆锥边界层中扰动波的发展过程.结果表明:高超声速圆锥边界层中第二模态扰动波产生的位置以及扰动波特征频率和波长等参数受雷诺数影响较大,当单位雷诺数从2×106m~(-1)增加到8×106m~(-1)时,第二模态波的特征频率从55 k Hz增加到226 k Hz;随着单位雷诺数增加,边界层中扰动增长速度加快,第二模态波出现在圆锥表面更靠近上游的位置;相同单位雷诺数条件下,随着第二模态波的向下游传播,其特征频率逐渐减小.通过对比发现自由来流湍流度对边界层中扰动波的发展同样有较大影响,自由来流湍流度降低,边界层中的第二模态波的特征频率明显减小.利用互相关分析得出第二模态扰动波在边界层中的传播速度大约为当地主流速度的0.8—0.9倍.在1?小攻角条件下,圆锥迎风面和背风面边界层发展呈现出明显的差异,背风面边界层中扰动发展提前,第二模态波出现在更靠近上游的位置,而迎风面中扰动发展受到抑制,第二模态波特征频率更大.     

Reply to the note on the flow behaviour of combustion gas in the boundary layer of an MHD generator

The temperature profile in Fig. 3 of Ref. 1 is better approximated by a relation for a turbulent boundary layer than a laminar boundary layer. The critical Reynolds number is expected to be lowered in the flow train of MHD combustion gas because of large turbulence in the combustor and by roughness along the duct.     

Roughness sub-layer investigation of a turbulent boundary layer

Particle image velocimetry (PIV) measurements at relatively low Reynolds numbers in a turbulent boundary layer over a three-dimensional roughened surface, consisting of pyramidal rows, have been presented. Measurements have been taken in a streamwise wall-normal plane intercepting the apex of a row of pyramids and the diagonal of the square base. The results shown in this paper point out the non-homogeneity of the flow in the roughness sub-layer. The different flow behaviour along the ascendant and the descendent part of the pyramids and in the region between two consecutive pyramids has been visualised. Low values of the streamwise component of the mean velocity and high values of the streamwise and wall-normal component of the Reynolds normal stresses, turbulent kinetic energy and Reynolds shear stresses are present in the downstream part of the pyramids, near their base. The mean representation of the flow shows swirling patterns in correspondence of the top of the pyramids. It is suggested that this representation is produced mainly by vortical structures travelling along the wall, retarded and intensified when interacting with the flow around the roughness element and by swirling patterns originating from the interaction of the incoming flow with the pyramids. In order to understand the origin of the high turbulence activity observed downstream the pyramids, near their base, a conditional analysis based on the quadrant method has been performed. It has been shown that sweep events are the major contributor to the very high values of the Reynolds shear stresses observed in this region and are due to the combination of the vortical flow at the top of the pyramid and a large scale (with respect to the roughness element) inflow motion.     

On the flow behaviour of combustion gas in the boundary layer of MHD generators

The flow behaviour of combustion gas in the boundary layer of a small-scale MHD generator has been reviewed in the light of empirical conclusions drawn by Onda, Kaga, and Kato.     

Experimental study of mean and pulsation flow characteristics in the 2D/3D supersonic boundary layer behind flat roughness elements

For the first time, detailed experiments aimed at evaluation of the influence of flat microroughness elements (adhesive tape stickers attached to the streamlined surface) on the mean and pulsation flow characteristics in flat-plate and swept-wing boundary layers were carried out under comparable dimensional conditions. At Mach number M = 2, the sticker shape was found to have almost no influence on the amplitude of mass-flow pulsations in the wake of sticker. The relative receptivity of 3D boundary layer to stationary disturbances was obtained to be approximately 1.5 times higher than the receptivity of 2D boundary layer. It was found that the presence of a sticker at the line of the source of controlled disturbance changes the pulsation spectra in 2D and 3D boundary layers in comparison with unperturbed flow. In the wake behind the center of a sticker, the flow can be most unstable.     

Heat exchange in boundary layer on permeable plate at injection and combustion

The peculiarities of the heat and mass exchange in a laminar boundary layer with combustion at the injection of the fuel mixture H²/N² through the permeable surface are considered. It is shown that at a certain value of the injection parameter, the value of the heat flux into the wall averaged over the length has a maximum. An analytic estimate is proposed for determining the maximum heat flux at the combustion depending on the injection intensity. The obtained relations agree with the results of experimental studies and numerical modelling.     

Chaotic motion in an oscillatory boundary layer

The chaotic time oscillations in an incompressible fluid driven into motion by a harmonic time-varying pressure gradient is examined. Special attention is given to centrifugal destabilization of the viscous boundary layer. The basic flow is shown to be linearly unstable. For increasing modulation amplitude, the flow exhibits chaotic oscillations. The energy exchange between subharmonics and superharmonics of the least-stable spanwise wave number is considered. The presence of subharmonic Fourier modes are shown to accelerate the transition to temporally chaotic motion. (c) 1996 American Institute of Physics.     

障碍物后周期性Bottlebeam的自重建

本文分析了由两束贝塞尔（Bessel）光相干产生的周期性局域空心光束（Bottlebeam）遇到障碍物后的自重建特性．由汉克尔波理论分析了自重建的原理,利用衍射积分理论数值模拟了轴上圆形障碍物对光束传输的影响．结果表明,周期性Bottlebeam遇到障碍物遮挡时具有自重建特性,重建后的光束依然保持着障碍物前的光强分布特性．研究结果对于利用周期性Bottlebeam实现微粒的多层面操控具有特殊意义．     

Study of the current layer at the boundary between the plasma and a magnetic obstacle in a laboratory experiment

The behavior of the plasma and magnetic and electric fields in the current layer that separates oppositely-directed magnetic fields is studied experimentally. The layer is formed by the interaction of a magnetic dipole with a magnetized plasma flow at Mach numbers lower than 10.     

Effect of boundary layer thickness before the flow separation on aerodynamic characteristics and heat transfer behind an abrupt expansion in a round tube

Results of numerical investigation of the boundary layer thickness on turbulent separation and heat transfer in a tube with an abrupt expansion are shown. The Menter turbulence model of shear stress transfer implemented in Fluent package was used for calculations. The range of Reynolds numbers was from 5·10³ to 105. The air was used as the working fluid. A degree of tube expansion was (D ₂/D ₁)² = 1.78. A significant effect of thickness of the separated boundary layer both on dynamic and thermal characteristics of the flow is shown. In particular, it was found that with an increase in the boundary layer thickness the recirculation zone increases, and the maximum heat transfer coefficient decreases. The work was financially supported by the Russian Foundation for Basic Research (project codes 07-08-00025 and 06-08-00300).     

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.