压力梯度对边界层两相流动稳定性的影响

分析了有压力梯度的边界层两相流动稳定性,推导出类似于Saffman理论的修正的稳定性方程,数值计算采用高精度的谱方法。结果说明,压力梯度对边界层两相流动稳定性有显著的影响,顺压梯度增强流动稳定性,而逆压梯度则促进流动失稳。在不同的压力梯度和浓度下,Stokes数对流动稳定性的影响是一致的,存在一个临界Stokes数,小Stokes数促进流动失稳,而大Stokes数则提高临界雷诺数,抑制流动失稳的最佳Stokes数为10的量级。     

Receptivity of Hypersonic Boundary Layer to Wall Disturbances

Theoretical analysis of hypersonic boundary-layer receptivity to wall disturbances is conducted using a combination of asymptotic and numerical methods. Excitation of the second mode by distributed and local forcing on a flat-plate surface is studied under adiabatic and cooled wall conditions. Analysis addresses receptivity to wall vibrations, periodic suction/blowing, and temperature disturbances. A strong excitation occurs in local regions where forcing is in resonance with normal waves. It is shown that the receptivity function tends to infinity as the resonance point tends to the branch point of the discrete spectrum that is typical for boundary layers on cool surfaces. Asymptotic analysis resolves this singularity and provides the receptivity coefficient in the branch-point vicinity. Numerical results indicate extremely high receptivity to vibrations and suction/blowing in the vicinity of the branch point located near the lower neutral branch of the Mack second mode. Received 5 September 2000 and accepted 7 September 2001     

Receptivity of the Flat-Plate Boundary Layer to Free-Stream Turbulence

The linear problem of generation of perturbations of a flat-plate boundary layer by external turbulence is solved. The turbulence is represented in the form of a set of space- and time-periodic vortex modes. It is shown that the boundary layer is most receptive to low-frequency longitudinal vorticity modes. The mean-square velocity fluctuations in the boundary layer and their spectrum are found for isotropic turbulence with a spectrum satisfying the Kolmogorov-Obukhov law.     

Flat-Plate Boundary Layer Receptivity to a Steady Free-Stream Vortex Disturbance

A single trailing vortex developed behind a micro-wing immersed in a free stream was used to study the vortex receptivity of the boundary layer on a flat plate. As a result of the interaction, in the boundary layer there develop longitudinal-velocity disturbances which grow almost linearly in the longitudinal coordinate. The parameters of the excited steady disturbances agree with the data of previous experiments performed under natural conditions and dealing with an indirect scenario of laminar-turbulent transition at high free-stream turbulence. It is shown that the leading edge of the plate does not play a decisive role in the mechanism of growth of disturbances of this kind and the receptivity is non-local in nature.     

Swept-Wing Boundary Layer Receptivity to a Steady Free-Stream Vortex Disturbance

A single trailing vortex developed behind a micro-wing immersed in a free stream was used to study the vortex receptivity of a swept-wing boundary layer. As a result of the interaction, longitudinal-velocity disturbances develop in the boundary layer. On the swept wing, disturbance transformation occurs near the leading edge and is accompanied by the formation of a wave packet consisting of waves typical of cross-flow instability. Disturbances with other characteristics are also detected. These disturbances may be attributable to distributed boundary-layer receptivity to the free-stream vortex disturbance considered.     

Receptivity of the Blunt-Leading-Edge Plate Boundary Layer to Unsteady Vortex Perturbations

The response of the boundary layer on a plate with a blunt leading edge to frozen-in vortex perturbations whose vorticity is normal to the plate surface is found. It is shown that these vortices generate an inhomogeneity of the streamwise velocity component in the boundary layer. This inhomogeneity is analogous to the streaky structure developing as the degree of free-stream turbulence increases. The dependence of the amplitude and shape of the boundary layer inhomogeneity on the distance from the leading edge, the streamwise and spanwise scales, and other parameters is found for periodic and local initial perturbations. It is shown that the receptivity of the boundary layer decreases with increase in the frequency and with decrease in the streamwise perturbation scale.     

Receptivity of the Swept Wing Boundary Layer to a Steady Flow Inhomogeneity

The boundary layer on a plate with an inclined blunt leading edge is investigated for a free-stream flow with a small span-periodic velocity inhomogeneity. This flow simulates the penetration of the outer turbulence into the swept wing boundary layer. It is shown that the boundary layer perturbations generated by the inhomogeneity generally have a streamwise velocity component significantly greater than the initial inhomogeneity amplitude. The dependence of the perturbations on the distance from the leading edge and the spanwise inhomogeneity period is found. It is shown that the swept wing boundary layer is less sensitive to the perturbation type in question than the straight wing boundary layer.     

Non-Parallel Acoustic Receptivity of a Blasius Boundary Layer Using an Adjoint Approach

Localized and non-localized acoustic receptivity for a Blasius boundary layer is investigated using the adjoint Parabolized Stability Equations. The scattering of an acoustic wave onto a hump, a rectangular roughness or a wall steady blowing and suction is described. Comparisons with local approaches, triple deck theory, direct numerical simulations and experiments are successfully shown. Non-parallel effects are discussed. For comparable parameters, the non-localized receptivity problem produces amplitudes one order of magnitude larger than for the case of localized receptivity. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Flow Structure Produced by Pulsed-jet Vortex Generators in a Turbulent Boundary Layer in an Adverse Pressure Gradient

The effect of pulsed jet vortex generators on the structure of an adverse pressure gradient turbulent boundary layer flow was investigated. Two geometrically optimised vortex generator configurations were used, co-rotating and counter-rotating. The duty cycle and pulse frequency were both varied and measurements of the skin friction (using hot films) and flow structure (using stereo PIV) were performed downstream of the actuators. The augmentation of the mean wall shear stress was found to be dependent on the net mass flow injected by the actuators. A quasi steady flow structure was found to develop far downstream of the injection location for the highest pulse frequency tested. The actuator near field flow structure was observed to respond very quickly to variations in the jet exit velocity.     

Permissible Height of the Surface Roughness in a Turbulent Boundary Layer with a Streamwise Pressure Gradient

The results of an experimental investigation of the effect of the streamwise pressure gradient in a turbulent boundary layer on the permissible height of the surface roughness of bodies in an incompressible fluid flow are presented. The permissible roughness Reynolds number for which the characteristics of the turbulent boundary layer remain the same as in the case of flow past a smooth surface is determined.     

Transition to Turbulence in the Boundary Layer on a Plate in the Presence of a Negative Free-Stream Pressure Gradient

Transition to turbulence in the boundary layer on a flat plate is investigated numerically for an incompressible fluid flow with a given negative free-stream pressure gradient. The transition is investigated using the three-parameter turbulence model developed by the authors. The calculation results are compared with the available experimental data.     

The Effect of Wall Normal Actuation on a Turbulent Boundary Layer

In this work, a series of direct numerical simulations are conducted to study the effect of wall normal spanwise homogeneous wall actuation on a turbulent boundary layer. The moving boundary is represented by a boundary data immersion technique. A parametric study was performed, varying the actuator length, the wall normal actuation amplitude and the actuation frequency. It was found that localized actuation, relying only on wall motion instead of requiring a plenum in the case of synthetic jets, generated a net momentum flux jet affecting the flow not only in the immediate vicinity of the actuator but also for a significant distance downstream. The cases with an actuator velocity of \( u^{+}_{act}=?20.1 \) showed a particularly pronounced effect on the boundary layer and resulted in a recirculation region.     

含尘离心风机中边界层对叶轮侵蚀的影响

本文采用二维可压缩紊流边界层计算模型,计算了离心风机内考虑边界层影响后的粒子轨迹和粒子与叶片表面的碰撞角、碰撞速度、磨损率沿叶片(相对弧长)的变化规律,计算结果表明:对于小直径的粒子,无粘流动假设是不合适的,计及边界层的影响,对于准确预测风机内的粒子轨迹和磨损是必不可少的。     

考虑边界层影响时溢流坝动水压强分布规律的研究

根据溢流坝反弧段水流运动的基本方程,考虑水流边界层的影响,在流线曲率同心圆假设条件下,得到了溢流坝反弧段水流压强计算的表达式。通过引入近壁薄流层要领和缓和过渡流线假设,在水工模型试验的基础上,得到了溢流坝反弧段水流流线曲率半径的计算公式,使反弧段离心力影响范围内的水流流线曲率光滑连续,从而得到反弧段水流压强分布规律的统一表达式,该表达式能够反映水流压强沿反弧法线方向及切线方向的变化规律。     

Interaction of a Traveling Pressure Perturbation with a Boundary Layer

The interaction between a traveling pressure perturbation and a laminar compressible boundary layer is investigated for a perturbation level higher than that needed to initiate steady-state flow separation. It is found that if the velocity of the pressure perturbation is fairly high the flow may remain unseparated and its direction of motion determines the nature of the perturbation propagation in the boundary layer. It is shown that even in the linear approximation the perturbations are mainly induced by the linear wall layer and not by the critical layer, which always remains nonlinear. It is also found that in the unsteady case shortwave perturbation oscillations are damped with time while the longwave ones grow and that the growth of the perturbations with time amplifies their damping along the streamwise coordinate while damping reduces it.     

Measurement in a Zero-Pressure Gradient Turbulent Boundary Layer with Forced Thermal Convection

A subsonic zero-pressure gradient turbulent boundary layer developing on a uniformly heated surface at a Reynolds number in the range of 3, 560?≤?Re _θ ?≤ 5,360 was investigated. Particle-image velocimetry measurements were performed at various positions in the streamwise direction for several wind-tunnel speeds and for different wall excess temperatures to show the thermal convection effects to expand the boundary-layer thickness δ _0.99 and to enlarge the turbulence intensities in the log-law and wake region. The mean velocity profiles are found to be self-preserving. The inclination of large-scale ramp-like vortex packets increases to higher characteristic angles, i.e., the mean angles are enlarged by approximately 5–10°. Hairpin-like vortex structures originating from the near-wall region seem to undergo higher climbing rates in the wall-normal direction causing the above mentioned significant changes in the boundary-layer thickness δ _0.99 and the strongly increased distributions of turbulence intensities in the wake region of the boundary layer. Changes in the distributions of the skewness and flatness of the probability density function (PDF) of the streamwise fluctuations corroborate these findings. The two-point correlation distribution of the streamwise velocity fluctuations R _uu is increased for wall distances y/δ _0.99?=?0.1 to y/δ _0.99?=?0.75 indicating the existence of coherent structures in higher regions of the boundary layer.     

Effect of a Short Roughness Strip on a Turbulent Boundary Layer

The response of a turbulent boundary layer to a short roughness strip is investigated using laser Doppler velocimetry (LDV) and laser induced fluorescence (LIF). Skin friction coefficients are inferred from accurate near-wall measurements. There is an undershoot in , where is the undisturbed smooth wall skin friction coefficient, immediately after the strip. Downstream of the strip, overshoots before relaxing back to unity in an oscillatory manner. The roughness strip has a major effect on the turbulent stresses ; these quantities increase, relative to the undisturbed smooth wall, in the region between the two internal layers originating at the upstream and downstream edges of the strip. The increase in the ratio suggests a decrease in near-wall anisotropy. From the flow visualizations, it is inferred that streamwise vortical structures are weakened immediately downstream of the strip. Consistently, streamwise length scales are also reduced; direct support for this is provided by measured two-point velocity correlations.     

Modeling the Effect of Freestream Turbulence on Unsteady Boundary Layer Flow

We present a new version of the two-equation turbulence model, which makes it possible to calculate continuously the entire flow range from laminar to turbulent, including transition, in the case of a time-periodic, high-turbulence-level freestream. The influence of the parameters characterizing the harmonic fluctuations of the external velocity and the freestream turbulence intensity and scale on the parameters of the flat-plate flow are analyzed. A comparison of the numerical solutions with the experimental and theoretical data indicates the possibility of describing the wall flow properties on the basis of a quasi-stationary turbulence model, as the Reynolds number varies from low to high values.     

Effect of Strong External Turbulence on a Wall Jet Boundary Layer

The initial stage of the development of a wall jet under the influence of strong external turbulence has been studied in a novel shear-flow mixing-box experiment. A fully developed channel flow of depth h (40 mm) enters along the top wall of a cuboidal box of height 11 h in which a combination of oscillatory and turbulent velocity fluctuations are generated by a vertical oscillating grid at the midplane 5 h below the wall. When the ratio of the rms grid-generated velocity fluctuations, , to the local mean velocity inside the wall jet layer, u, is greater than about 0.1, significant changes are observed in the mean shear profile and in the eddy structure of the wall jet. The wall jet thickness increases by approximately 25% but the maximum velocity decreases by less than 10% compared to the case without the external turbulence. Fluctuations of the streamwise velocity component increase as expected in the outer part of the wall jet, but the most significant result is the increase by 70% of the fluctuations in the boundary layer close to the wall. CFD simulations using the k-ɛ RNG of the FLUENT CFD Code do not properly model the effect of the large scale external turbulence in this experiment. However, an artificial method, which introduces a series of small inlet/outlet jets to represent external turbulence, approximately simulates the overall effects of the oscillating grid on the wall jet, but does not simulate the amplification of the near wall turbulence. F. T. M. Nieuwstadt: Rest in peace (1946–2005).     

Effect of Gas Diffusion Layer Properties on Breakthrough Time and Pressure

The gas diffusion layer (GDL) plays an important role in the removal of product water from the catalyst layer to the flow plate in a fuel cell. Numerous studies have reported water management, especially in the GDL, as the limiting factor hindering convective and diffusive transport of reactants which results in lowering power density. In this paper, an experimental technique is presented to study the GDL water transport properties associated with the breakthrough conditions which are critical to overall water management. Fluorescence microscopy technique is used to measure the pressure and time required for water to penetrate and break through the surface of the GDL. The results obtained for GDLs produced by different manufacturers confirm that the breakthrough time and pressure are larger for PTFE treated hydrophobic GDLs. The results are analyzed in terms of the contact angle, thickness, and SEM images to see the effects of different structural properties. The changes in morphology due to compression are also presented. In addition, the changes in breakthrough conditions when samples are reused are presented. The results provide basic insights into the water transport properties of the GDL, leading to the design of new materials with enhanced water management.