Suppressing a Laminar Flow Separation Zone by Spark Discharge at Mach Number M = 1.43

The influence of flow perturbations generated by an electric discharge on the region of interaction between a shock wave and laminar boundary layer in the flow on a flat plate at a Mach number of M = 1.43 has been experimentally studied. The oblique shock wave generated by a wedge mounted above the plate induced separation of the flow, while perturbations in the flow were introduced by a spark discharge on the model plate surface. It is established that the discharge leads to the formation of turbulent and thermal spots. The turbulent spot suppresses the separation zone, while the thermal spot leads to a local increase in the boundary layer thickness in the interaction zone.

     

Wind pressure features of large-span flat roof in different wind fields induced by conical vortex

The wind pressure features on a large-span flat roof in uniform flow field and turbulent field induced by conical vortex were studied, through wind tunnel tests. From the comparison of the mean and fluctuating wind pressure distributions on a flat roof in different wind fields induced by conical vortex, results indicate that the mean suction dominates in the smooth flow, whereas the fluctuating suction is more obvious in the turbulent flow. The probability density function for the pressure fluctuations under different approaching flows is analyzed. The two-peaked distribution, peculiar to turbulent flow field, is observed on the curve of probability density. The fluctuating pressures at reattachment points are larger under the turbulent flow. This indicates a more intense reattachment, which may cause overturning moment for roof-mounted items. Point vortex, RanKine vortex, and simplified Cook expression are applied to fit the pressure profiles beneath conical vortices, respectively. The results have shown that the RanKine vortex model and simplified Cook expression were applicable to forecast the wind pressure profiles beneath conical vortices, while point vortex underestimated the real wind suction. The wind pressure distributions in turbulent fields induced by different wind angles were contrasted, when the approaching flow is along the diagonal of the roof, the intensity of the vortex pairs is almost equal, with obvious reattachment. When the approaching flow deviate from the diagonal of the roof, the lateral turbulent component spins the vortex more quickly; this induces larger mean suctions beneath windward vortices. Smaller suctions are observed beneath the leeward vortex, due to less vorticity being converted to vortex motion from the freestream.     

湍流边界层激励下平板振动响应相似性研究

对流激平板的振动试验设计与响应预报作了分析与探讨。验证了湍流脉动压力自功率的归一化方法在不同流体介质间的适用性。对平板流激响应,考虑流体负载并采用模态叠加法进行计算。通过原始模型与缩比模型响应之间的对比,提出一种平板响应间的换算方法。换算结果表明,该方法可以实现同种介质湍流边界层激励下的平板响应之间的换算。对不同外流场介质的平板流激响应,用该方法可实现理论上的换算。该研究结果对水下结构物流激响应预报有实际意义。     

湍流边界层激励下加筋平板振声响应特性研究

船舶、汽车和飞机等高速运动时,其外壳受湍流边界层壁面脉动压力激励而产生的内场声辐射成为该类交通工具自噪声的重要成分。基于模态叠加法计算结构振动响应。采用湍流壁面脉动压力功率谱Corcos模型,计算了外侧气流或水流湍流边界层激励下简支平板振动及内场辐射声,计算值与解析解和试验值吻合良好,验证了算法的有效性。采用湍流壁面脉动压力功率谱改进型Corcos模型,研究了外侧水流湍流边界层激励下平板及板格的振声响应特性,结果表明:水流马赫数低,壁面脉动压力迁移波数大于平板结构弯曲波数,壁面脉动压力波数-频率谱的迁移脊对平板的激励作用可以忽略;横向或纵向加筋对板格振动速度自功率谱级基本无影响;减小板格宽度与长度之比,适当增大板格流向长度可使平板振动辐射声功率在2 000 Hz以上明显降低。     

Three-dimensional aspects of boundary-layer transition

A review is made in this paper of the three-dimensional nature of instability leading to transition in a two-dimensional flow on a flat plate (Blasius boundary layer) or between parallel plates (plane Poiseuille flow), with additional reference to the flow on a concave wall. Instability and transition in a three-dimensional boundary layer are then reviewed, with particular attention to the flow due to a rotating disk and the flow perturbed by a three-dimensional roughness element on a flat plate. The growth of a turbulent spot is discussed as a phenomenon exhibiting a similar feature of instability. Only the flow of an incompressible fluid is considered.     

大跨平屋盖风荷载特性及风压预测研究

在大气边界层风洞中开展了大跨平屋盖结构刚性模型试验,获得了屋盖表面测点的风压时程,分析了典型风向下屋盖表面平均风压与脉动风压特性.结合本征正交分解技术(POD)与BP神经网络法,提出了一种可用于大跨结构进行空间插值的机器学习法一POD-BPNN法,实现了对风压的高效预测.预测的平均风压系数、脉动风压系数、脉动风压的时域...     

Wind loads on a residential solar water heater

This experimental study aims to address the effects of freestream turbulence intensity (TI) on wind loads on a residential solar water heater with and without a guide plate. Highly turbulent flow was produced by a turbulence generation grid at the inlet of the working section. Results show that the distributions of mean longitudinal and spanwise pressures are associated with tilt angle, guide plate, and freestream TI. Interactions between separation and corner vortices on the upper surface and impingement of shear layer from the tip of a guide plate on the lower surface would induce higher levels of fluctuating pressure, particularly in turbulent flow. A similarity parameter α/A* is also proposed to scale the uplift coefficients in both smooth uniform and turbulent flows.     

Turbulent boundary layer on a flat plate in a supersonic stream of equilibrium dissociated and ionized gas

A method is described for calculating the turbulent boundary layer on a flat plate in an equilibrium dissociated and ionized supersonic stream. An estimate is made of the effect of ionization on the drag and heat transfer in a stream of air.     

The non-linear evolution of modulated waves in a boundary layer

The series of experiments by Schubauer and Skramstad (1948) provided the first experimental evidence of the role that the instability of Tollmien-Schlichting waves played in the transition of a zero pressure-gradient flat plate boundary layer. The initial experiments studied the oscillations in the boundary layer excited by the freestream fluctuations. This was only possible after the background disturbances in the wind tunnel had been reduced to a very low level. The background wind tunnel environment excited a broad band of amplitude modulated disturbance waves that grew as they propagated downstream, eventually leading to the formation of turbulent spots. Further experiments used artificial two-dimensional harmonic excitation to produce regular wavetrains that could be directly compared with linear theory. Unfortunately, two-dimensional harmonic excitation of this type has also been used in many of the subsequent nonlinear transition investigations; the modulation of the disturbance waves, essential in nonlinear studies, has been largely ignored. Gaster and Grant (1975) used a short duration acoustic pulse to excite the boundary layer and found that the modulated wavepacket that was created admitted bursts of high frequency oscillations. These occurred at amplitudes that were insufficient to generate non-linear behaviour in purely periodic wavetrains. Gaster (1980) suggested that the modulation of the wavepacket played an important role in the non-linear region of transition. This investigation used computer generated deterministic white noise to excite the boundary layer on a flat plate through a single buried loudspeaker. This type of excitation produced amplitude modulated T-S waves at some point downstream from the source. By repeatedly exciting the boundary layer with the same white noise sequence it was possible to map the entire flow-field with a single hot-wire probe and so study the evolution of the modulated wavetrains and the eventual development of turbulent spots. The modulated wavetrains were found to grow initially according to linear theory. But downstream, departures from the linear pattern were observed at isolated time instants. The amplitude of the irregular portions of the signal increased rapidly with downstream distance until bursts of oscillations of frequencies five or six times the basic T-S frequency were observed. These regions developed even higher frequency bursts until a turbulent spot could be considered to have formed. Excitation signals of various amplitudes with different phase relations between the spectral components were used in these experiments. It was found that the phases between the Fourier components played an important role in the highly non-linear behaviour that is the precursor to a turbulent spot. Novel signal processing techniques, such as the wavelet transform and Singular Value Decomposition were used to investigate the fine structure and the propagation characteristics of the high frequency disturbances.     

基于大涡模拟的大气边界层湍流强度对低矮房屋风荷载特性影响研究

对德州理工大学(Texas tech university,TTU)低矮房屋标准模型,以已有现场实测以及缩尺模型风洞实验数据为验证对比,基于大涡模拟(Large-eddy simulation,LES)方法研究了大气边界层湍流强度对低矮房屋风荷载特征的影响机理。采用CDRFG (Consistent discretizing random flow generation) 人工合成湍流方法生成大气边界层湍流,研究了来流湍流度对低矮建筑表面的平均、脉动以及极小值风压分布以及风压非高斯特性的影响,并利用LES能提供非常场流动全流域信息的优势,结合瞬态湍流场结构对大气边界层湍流对低矮房屋风荷载特征的影响机理进行了阐释。结果表明:LES数值模拟得到的平均、脉动及极小值风压系数与实验以及实测结果一致,平均风压结果包络在实测误差范围以内,极小值风压系数最大误差小于10%,脉动风压系数最大误差小于20%且误差区域较小。在来流湍流度增大的过程中,低矮房屋屋面平均风压系数变化较小,脉动风压系数呈显著的线性增加;极小值风压系数变化规律相对复杂,呈现出非线性减小的趋势,风压系数极小值可达?5.0;屋面涡脱强度逐渐被抑制,锥形涡迹线与屋面迎风前缘的夹角由14.4°下降至8.7°。屋面风压非高斯特性主要与屋面形成的涡旋结构相关,表现出典型的右偏软化非高斯过程,且随着来流湍流度的增加风压非高斯特性逐渐减弱。从流场的角度来看,湍流度的增加抑制屋面迎风前缘柱状涡以及锥形涡的形成,加快流动分离的再附,减少分离泡尺度,同时提高了屋盖周围的湍流高频能量成分,从而使脉动风压增加,极小值风压减小以及风压非高斯特性减弱。该研究阐明了大气边界层湍流对低矮房屋风荷载特性的影响机理,有助于进一步理解低矮房屋风致破坏机理,并且为低矮房屋的抗风设计及抗风性能优化提供重要参考。     

管道内部湍流所引起的泄漏噪声之声强级估算

在管道通风系统中的湍流区内,震荡波动的阻力气压与推力气压之间的关系已被阐述。根据Nelson与Morfey的设定,在流体质元所受波动阻力和稳态阻力之比恒定的条件下,文中推广了一种基于气压值的计算方法,用来推算由管道内部因素与湍流作用所引起的管道壁泄漏噪声之声强级。     

Control of Velocity Profiles in a Boundary Layer

The control of average velocity in the turbulent boundary layer of a flat plate is investigated theoretically and experimentally. The control is exercised by injecting jets of air into the incident flow. Interpolation expressions are derived for the distribution of average velocities, dependent on the control parameters.     

The interaction of shock waves with a boundary layer on a sharp plate and a blunted plate

The interaction of shock waves with a turbulent boundary layer on a sharp plate and a blunted plate is numerically investigated. The shock waves in the flow are generated by wedges installed on the flat plate. The flow is simulated by the dynamic equations of a viscous perfect gas. The effect of the blunting radius of the plate’s leading edge and the wedge angle on the flow field and the local aerodynamic coefficients is shown. The calculated results are in agreement with the experimental data.     

多风扇主动控制风洞类平板断面抖振力识别研究

针对目前一般的大气边界层物理风洞采用被动格栅产生的紊流脉动风场功率谱密度能量分布难以调整以及积分尺度过小的问题,借助于主动控制方法善于改变来流功率谱及积分尺度的特点,利用日本宫崎大学多风扇主动控制风洞调试出多种宽带紊流风场和谐波脉动风场,采用脉动风速和气动力同步采集监控装置,使用测量稳定性好、精度较高的节段模型高频天平测力方法识别类平板模型的抖振力,对影响模型抖振力的主要参数(积分尺度、紊流度、攻角等)进行分析。     

Modeling of frost growth and frost properties with airflow over a flat plate

A physical model of frost layer growth and frost properties with airflow over a flat plate at subfreezing temperature was developed. Frost roughness was measured, and an empirical correlation for the average frost roughness was suggested. Heat and mass transfer coefficients were calculated using the modified Prandtl mixing-length scheme containing the effects of both frost roughness and turbulent boundary layer thickness. Frost thermal conductivity was theoretically analyzed by solving the combined equations of air equivalent conductivity and thermal conductivity of the frost inner layer. Based on the present model, heat and mass transfer coefficient, frost thermal conductivity, frost thickness, frost mass concentration and frost density with time and space were estimated. The model showed good agreement with the basic trends of the test data taken from other literature. Spatial and temporal changes of heat flux and frost surface temperature were also investigated.     

Investigation of distribution of lithium atoms in the boundary layer of the flow of combustion products

The results are given of measurements of the profiles of temperature and concentration of lithium in the cross section of the boundary layer formed under conditions of flow of combustion products of propaneair flame past the surface of a flat metal plate. A model of chemically nonequilibrium laminar boundary layer is used to perform numerical simulation of various components of the combustion products. A comparison of the measured profiles of lithium concentration and profiles of hydrogen atoms obtained from the experimental data with the predicted results on a cooled and uncooled plate demonstrates that the suggested model of chemical kinetics describes the measurement results with good accuracy.     

Investigation of heat transfer in a region of gradient flow when a plane turbulent jet is incident on a flat plate normal to the flow

The boundary conditions are determined experimentally for a system of differential equations describing heat transfer in a region of gradient flow when a plane turbulent jet is incident on a flat plate normal to the flow. The analytical solution is compared with direct measurements.Translated from Inzhenerno-Fizicheskii Zhurnal, Vo. 18, No. 4, pp. 631–637, April, 1972.     

Investigation of the singularities of distribution of cesium atoms in the boundary layer of the flow of combustion products

The distribution of the temperature and concentration of cesium in the cross section of the boundary layer of combustion products formed on the surface of a flat metal plate is measured. A model of chemical kinetics is selected, and numerical simulation of the behavior of cesium in the boundary layer of combustion products is performed. A comparison of the results of numerical calculation with the results of measurements on a cooled and uncooled plate demonstrates that the suggested model of chemical kinetics describes the measurement results with good accuracy. The effect of different reactions, as well as of the processes of condensation and thermodiffusion, on the distribution of free atoms of cesium in the boundary layer is analyzed     

Interaction between a weak oblique shock wave and a turbulent boundary layer in purely supersonic flow

Summary The paper deals with the interaction of a weak oblique shock and a turbulent boundary layer at a flat plate at purely supersonic speeds. Except for a small region where nonlinear transonic effects are of importance analytical expressions for the pressure- and skin friction distributions are derived.With 6 Figures     

Convective heat and mass transfer in the nonadiabatic evaporation of volatile liquids from a porous plate

This paper describes the apparatus, the experimental procedure, and the calculation of the characteristics of convective heat and mass transfer in the nonadiabatic evap oration of volatile liquids into a boundary layer from a flat porous plate situated longitudinally in a turbulent flow of air.