Effect of Reynolds number and boundary layer thickness on the performance of V-cone flowmeter using CFD

The effect of Reynolds number and boundary layer thickness on the performance of V-cone flowmeter has been evaluated using computational fluid dynamics (CFD). The shear stress transport k-ω (SST k-ω) turbulence model has been adopted for closure. The performance of two V-cone flowmeters with different beta ratios (β) viz., 0.6 and 0.7 for a fixed vertex angle (ϕ) of 60° has been analysed as a function of Reynolds number (Re). The results show that the coefficient of discharge (C_d) increases with Reynolds number in the laminar and transition flow regimes whereas it is nearly constant in turbulent flow regime. From the results, it can be concluded that C_d is independent of Re for values equal to 4000 and beyond. Further, it is also seen that the performance of the V-cone flowmeter is not affected by the upstream boundary layer thickness if the velocity profiles having different boundary layer thickness are extracted from an axial distance of 10D and more are fed at 5D upstream of the meter. However, the meter is sensitive to the extracted velocity profile from an axial distance of 5D and uniform velocity profile being fed at 5D upstream. The value of C_d may be sensitive as a result of the pressure variation due to the obstruction.     

干扰对边界层风廓线的影响及对策

风廓线雷达是近年来发展起来的新型测风遥感设备,它在气象科学研究和预报业务上有着广泛的应用前景。它以晴空大气作为探测对象,利用晴空大气湍流对电磁波的散射,根据Doppler原理,能连续得到大气的风场信息。但风廓线探测的信号极其微弱,很可能被噪声淹没,边界层风廓线采用L波段工作,日常有很多电子设备工作在这个波段,因此很容易受各种电磁设备的干扰。本文分析了各种干扰情况下边界层风廓线接收的频谱,归纳了各种干扰的频谱类型,评估了各种干扰对风廓线生成产品的影响,并在信号处理、数据处理等方面提出了综合对策,并验证了这些对策的实际效果。     

Control of boundary layer flow transition via distributed

A reduced-order linear feedback controller, which is used to control the linear disturbance in two-dimensional plane Poiseuille flow, is applied to a boundary layer flow for stability control. Using model reduction and linear-quadratic-Gaussian/loop-transfer-recovery control synthesis, a distributed controller is designed from the linearized two-dimensional Navier-Stokes equations. This reduced-order controller, requiring only the wall-shear information, is shown to effectively suppress the linear disturbance in boundary layer flow under the uncertainty of Reynolds number. The controller also suppresses the nonlinear disturbance in the boundary layer flow, which would lead to unstable flow regime without control. The flow is relaminarized in the long run. Other effects of the controller on the flow are also discussed.     

Active control of flow noise sources in turbulent boundary layer on a flat-plate using piezoelectric bimorph film

The piezoelectric bimorph film, which, as an actuator, can generate more effective displacement than the usual PVDF film, is used to control the turbulent boundary-layer flow. The change of wall pressures inside the turbulent boundary layer is observed by using the multi-channel microphone array flush-mounted on the surface when actuation at the non-dimensional frequency f_b⁺=0.008 and 0.028 is applied to the turbulent boundary layer. The wall pressure characteristics by the actuation to produce local displacement are more dominantly influenced by the size of the actuator module than the actuation frequency. The movement of large-scale turbulent structures to the upper layer is found to be the main mechanism of the reduction in the wallpressure energy spectrum when the 700v/u_τ-long bimorph film is periodically actuated at the non-dimensional frequency f_b⁺ =0.008 and 0.028. The biomorph actuator is triggered with the time delay for the active forcing at a single frequency when a 1/8″ pressuretype, pin-holed microphone sensor detects the large-amplitude pressure event by the turbulent spot. The wall-pressure energy in the late-transitional boundary layer is partially reduced near the convection wavenumber by the open-loop control based on the large amplitude event.     

尾缘厚度对压气机叶栅气动性能的影响研究

针对不同尾缘厚度对压气机叶栅气动性能影响的问题,采用尾缘对称加厚的方法,在保证弦长、最大厚度、最大厚度相对位置、前缘半径、安装节距和安装角等参数不变的条件下,使尾缘厚度相对于弦长在0.1%~8.0%范围变化。根据Baldwin-Lomax模型建立了压气机叶栅二维湍流流动模型,并对模型近壁面进行了网格加密处理,通过隐式多重网格法加速计算的收敛,利用NUMECA软件,对叶型原型和对称加厚尾缘叶型的总压损失系数、叶型气流转折角和升阻比进行了数值计算。研究结果表明,尾缘厚度在一定范围内增大时,气流转折角减小,升阻比呈先增后降的趋势,而尾缘厚度较大时,推迟了叶型失速的出现,但尾迹损失和总压损失系数较大。     

Effect of guide wall on jet impingement cooling in blade leading edge channel

The characteristics of fluid flow and heat transfer, which are affected by the guide wall in a jet impinged leading edge channel, have been investigated numerically using three-dimensional Reynolds-averaged Navier–Stokes analysis via the shear stress transport turbulence model and gamma theta transitional turbulence model. A constant wall heat flux condition has been applied to the leading edge surface. The jet-to-surface distance is constant, which is three times that of the jet diameter. The arrangement of the guide wall near the jet hole is set as a variable. Results presented in this study include the Nusselt number contour, velocity vector, streamline with velocity, and local Nusselt number distribution along the central line on the leading edge surface. The average Nusselt number and average pressure loss between jet nozzle and channel exit are calculated to assess the thermal performance. The application of the guide wall is aimed at improving heat transfer uniformity on the leading edge surface. Results indicated that the streamwise guide wall ensures the vertical jet impingement flow intensity and prevents the flow after impingement to reflux into jet flow. Thus, a combined rectangular guide wall benefits the average heat transfer, thermal performance and heat transfer distribution uniformity.     

Effect of boundary layer swirl on supersonic jet instabilities and thrust

This paper reports the effects of nozzle exit boundary layer swirl on the instability modes of underexpanded supersonic jets emerging from plane rectangular nozzles. The effects of boundary layer swirl at the nozzle exit on thrust and mixing of supersonic rectangular jets are also considered. The previous study was performed with a 30° boundary layer swirl (S=0.41) in a plane rectangular nozzle exit. At this study, a 45° boundary layer swirl (S=1.0) is applied in a plane rectangular nozzle exit. A three-dimensional unsteady compressible Reynolds-Averaged Navier-Stokes code with Baldwin-Lomax and Chien’sk-ε two-equation turbulence models was used for numerical simulation. A shock adaptive grid system was applied to enhance shock resolution. The nozzle aspect ratio used in this study was 5.0, and the fully-expanded jet Mach number was 1.526. The “flapping” and “pumping” oscillations were observed in the jet’s small dimension at frequencies of about 3,900Hz and 7,800Hz, respectively. In the jefs large dimension, “spanwise” oscillations at the same frequency as the small dimension’s “flapping“ oscillations were captured. As reported before with a 30° nozzle exit boundary layer swirl, the induction of 45° swirl to the nozzle exit boundary layer also strongly enhances jet mixing with the reduction of thrust by 10%.     

Frequency effects of upstream wake and blade interaction on the unsteady boundary layer flow

Effects of the reduced frequency of upstream wake on downstream unsteady boundary layer flow were simulated by using a Navier-Stokes code. The Navier-Stokes code is based on an unstructured finite volume method and uses a low Reynolds number turbulence model to close the momentum equations. The geometry used in this paper is the MIT flapping foil experimental set-up and the reduced frequency of the upstream wake is varied in the range of 0.91 to 10.86 to study its effect on the unsteady boundary layer flow. Numerical solutions show that they can be divided into two categories. One is so called the low frequency solution, and behaves quite similar to a Stokes layer. Its characteristics is found to be quite similar to those due to either a temporal or spatial wave. The low frequency solutions are observed clearly when the reduced frequency is smaller than 3.26. The other one is the high frequency solution. It is observed for the reduced frequency larger than 7.24. It shows a sudden shift of the phase angle of the unsteady velocity around the edge of the boundary layer. The shift of phase angle is about 180 degree, and leads to separation of the boundary layer flow from corresponding outer flow. The high frequency solution shows the characteristics of a temporal wave whose wave length is half of the upstream frequency. This characteristics of the high frequency solution is found to be caused by the strong interaction between unsteady vortices. This strong interaction also leads to destroy of the upstream wake strips inside the viscous sublayer as well as the buffer layer.     

Experimental investigation of the effects of leading edge bluntness on supersonic flow over a double compression ramp

Understanding the flow characteristics over a double compression ramp is crucial for high-speed vehicle design. Leading edge bluntness is a key factor influencing the formation of a separation region on a double compression ramp flow. In the present study, the effect of bluntness on a double compression ramp is investigated experimentally at a nominal Mach number of 4. The test model has 13° and 40° inclinations with respect to the freestream. Five different levels of leading-edge radius, varying from 0.0 to 2.0 mm, were subjected to supersonic wind tunnel tests. Shadowgraph and infrared thermography techniques were employed to visualize the flow features of the double ramp model. Measurements of surface heat-transfer along the centerline of the test model were obtained from the acquired infrared images. It is shown that the leading-edge radius alters the separation characteristics as well as the surface heat-transfer. Possible reasons for such flow characteristics are discussed.

     

Effect of compound dimple on tribological performances of journal bearing

The effect of the compound dimple on the tribological performances of a journal bearing is studied. In doing so, the tribological performances for the bearing with the compound dimple and simple dimple are studied using a fluid structure interaction (FSI) method and compared further. Numerical results show that the compound dimple can supply the larger load-carrying capacity and lower friction coefficient due to its twice hydrodynamic action in comparison with the simple dimple. Moreover, the above improvement depends on the geometry sizes of the compound dimple, the dimple interval, and working parameters of the bearing.     

Effect of turbulent boundary layer on the surface pressure around trench cavities

This paper presents a parametric study on steady incompressible flow past cavities. Numerical calculations were performed around two- and three-dimensional trench cavities. Numerical and experimental results were compared to understand the fluid dynamics mechanism of vortex generation and diffusion in shear and mixing layers around cavities. Using the commercial computational fluid dynamics software FLUENT, the standard k-? and k-ω shear stress transport (SST) models were applied to solve the Reynolds-averaged Navier-Stokes (RANS) equation of turbulent wind flow. The calculations were performed using a Reynolds number of 1.6 × 10⁴ based on the free-stream velocity U∞ and the length of the cavity L. Computational meshes were carefully designed to be dense on the cavity surface and to be coarse in far-field to obtain an appropriate solution for the RANS equation for cavity flow because these measures result in decreased computational cost and more rapid convergence. The standard k-? model produces an almost similar distribution regardless of whether the grid is two- or three-dimensional, whereas the k-ω SST model has different values of velocity, surface pressure, and Reynolds stress. The three-dimensional grid shows better prediction of surface pressure around the cavity compared with the two-dimensional grid.     

风洞侧壁边界层控制算法优化及仿真分析研究

为提高风洞侧壁边界层控制系统性能,文中通过介绍NF-3大型低速翼型风洞多喷嘴级联吹气侧壁边界层控制系统的结构和原理,分析并优化侧壁边界层控制系统的控制算法,利用MATLAB软件实现了对优化前与优化后的控制算法的仿真,并对仿真结果进行了分析。仿真结果表明：神经网络自适应PSD控制算法可提高控制系统的稳定性、准确性和响应速度,在一定程度上能够减小阀门的时滞性和非线性对控制结果的影响。     

Numerical investigation of boundary layer suction control positions on airfoils for vertical-axis wind turbine

Journal of Mechanical Science and Technology - Flow separation is a common phenomenon on vertical-axis wind turbine (VAWT), delaying or restraining flow separation is very beneficial to improve the...     

具有溢流帽结构的旋流器流场特征及分离性能研究

针对普通旋流器运行过程中存在短路流导致溢流跑粗的问题,提出一种溢流帽式旋流器,并对其进行了数值模拟和试验研究.模拟结果表明,相比于普通旋流器,溢流帽式旋流器内空气柱直径减小,能耗降低,离心力场强度更大,流体切向速度增大,轴向速度降低,颗粒参与分离时间长,分离更充分,有利于改善溢流跑粗,提高分离效率.试验结果显示,溢流帽...     

Effect of oxide layer roughness and thickness on spectral dependence of optical parameters of copper mirrors

Two copper mirrors with different surface roughness are studied by means of spectroscopic ellipsometry. It is established that spectral dependences of optical conductivity are qualitatively similar, but shifted along the ordinate, and some of their peaks may be related to light absorption by the surface oxide layer.     

Mathematical modeling of pressure characteristics of the deflector-jet pilot stage considering boundary layer flow

The performance of a deflector-jet servo valve significantly depends on the pressure characteristics of its pilot stage. The complex flow field within the deflector-jet pilot stage presents enormous difficulty and challenge in establishing its mathematical model. In this work, according to the energy conversion characteristics of the flow field, the flow process within the pilot stage is divided into five phases for modeling: inside-pipe flow, first jet, first pressure recovery, secondary jet, and secondary pressure recovery. To better reveal the intrinsic operating mechanism of a deflector-jet pilot stage, the boundary layer is introduced into the model. Computational fluid dynamic (CFD) simulation and experiment are conducted to validate the developed mathematical model at different supply pressures. The result demonstrates that the velocity distribution of the jets and the pressure characteristics of the pilot stage are significantly influenced by the boundary layer. The boundary layer flow within the V-groove has a greater impact on the pressure characteristics than that within the nozzle. Verified by the CFD simulation and experiment, the developed model can accurately predict the velocity distribution of the jets and pressure characteristics. At different supply pressures, the maximum relative error between the theoretical and experimental results of the dimensionless pressure characteristic is 5.36%.     

Effect of variable viscosity on thermal boundary layer over a permeable flat plate with radiation and a convective surface boundary condition

In this paper, the combined effects of radiation, temperature dependent viscosity, suction and injection on thermal boundary layer over a permeable flat plate with a convective heat exchange at the surface are investigated. By taking suitable similarity variables, the governing boundary layer equations are transformed into a boundary value problem of coupled nonlinear ordinary differential equations and solved numerically using the shooting technique with sixth-order Runge-Kutta integration scheme. The solutions for the velocity and temperature distributions together with the skin friction coefficient and Nusselt number depend on six parameters; Prandtl number Pr, Brinkmann number Br, the radiation parameter Ra, the viscosity variation parameter a, suction/injection parameter f _w and convection Biot number Bi. Numerical results are presented both in tabular and graphical forms illustrating the effects of these parameters on thermal boundary layer. The thermal boundary layer thickens with a rise in the local temperature as the viscous dissipation, wall injection, and convective heating each intensifies, but decreases with increasing suction and thermal radiation. For fixed Pr, Ra, Br and Bi, both the skin friction coefficient and the Nusselt number increase with a decrease in fluid viscosity and an increase in suction. A comparison with previously published results on special case of the problem shows excellent agreement.     

Effect of edge preparation technologies on cutting edge properties and tool performance

The International Journal of Advanced Manufacturing Technology - Edge preparation has gained widespread use due to its low cost and high impact. Various edge preparation methods are reported in the...