短螺旋型燃烧室旋流结构及气动边界发展规律数值模拟研究

短螺旋型燃烧室的头部轴线沿发动机周向与发动机轴线偏转一定角度,可以在保证燃烧效率不变的条件下有效缩短燃烧室轴向长度。短螺旋型燃烧室流场的最大特点是旋流流动单侧受限。为了研究头部安装角α变化对燃烧室旋流流动特性的影响,基于数值方法对短螺旋型燃烧室进行了计算分析。结果表明:随着α变化,旋流器下游旋涡依次出现对称环状、马蹄状、环状结构;随着α增加,气动边界逐渐出现并抑制旋流的周向扩张,导致流场出现不同的旋涡结构;不同α下的切向角动量随轴向距离增加而衰减,但α为35°和45°时,气动边界在非受限侧出现并对旋流产生约束,角动量衰减变慢;当α为0°,15°,35°,45°时,燃油液滴依次集中分布在旋流器下游两旋涡边缘、侧壁面和头部端壁、非受限旋涡边缘。本文研究了不同α下的旋涡结构及气动边界沿轴向的演化过程,为短螺旋型燃烧室进一步的设计与优化提供基础。

压气机叶片钝头前缘对边界层气动影响

由于加工误差、外物损伤等原因,压气机叶片前缘容易产生钝头变形。为分析其带来的影响,使用SST湍流模型并结合γ-θ转捩模型进行了数值模拟。通过叶片吸力面边界层形状因子、法向平均间歇因子和叶栅出口总压损失系数等参量,研究了钝头前缘对边界层发展所带来的影响。同时针对一种前缘曲率连续的改进叶型进行了研究。结果表明在很宽的攻角范围内,压力面边界层受钝头变形影响极小。吸力面边界层受影响程度则会随着攻角增大而提高,当攻角超过一定范围,钝头前缘会显著改变边界层发展,影响整个主流流场。

压气机叶栅非定常线性自回归气动降阶模型研究

为满足叶轮机械领域高效非定常气动计算的迫切需求,采用线性带外输入的自回归(ARX)理论建立了高亚声速压气机叶栅非定常气动性能降阶模型。该降阶模型构造简单,建立较为容易。通过逐渐增加输入/输出延迟阶数,可以获得较高精度的模型超参数。研究结果表明:降阶模型对线性非定常系统的气动参数响应具有较好的预测精度,但在部分无量纲总压一阶幅值比以及无量纲静压一阶相位角存在一定预测误差。非定常系统线性工况下,同一折合频率的气动参数的一阶幅值比和相位角基本不随攻角幅值的增加而变化。通过与具有泛化能力的非线性降阶模型比较,基于线性假设发展的ARX模型能获得更高精度的气动参数响应。

Wind Force Coefficients for Designing Porous Canopy Roofs

Wind force coefficients for designing porous canopy roofs have been investigated based on a series of wind tunnel experiments. Gable, troughed and mono-sloped roofs were tested. The roof models were made of 0.5 mm thick perforated duralumin plates, the porosity of which was changed from 0 to about 0.4. Overall aerodynamic forces and moments acting on the roof model were measured in a turbulent boundary layer with a six-component force balance for various wind directions. The results indicate that the wind loads on canopy roofs generally decrease with an increase in porosity of the roof. Assuming that the roof is rigid and supported by the four corner columns with no walls, the axial forces induced in the columns are regarded as the most important load effect for discussing the design wind loads. Two loading patterns causing the maximum tension and compression in the columns are considered. Based on a combination of the lift and moment coefficients, the design wind force coefficients on the windward and leeward halves of the roof are presented for the two loading patterns as a function of the roof pitch and porosity. The effect of porosity is taken into account as a reduction factor of the wind loads.

Effect of Porosity on the Wind Loads on a Hyperbolic Paraboloid Canopy Roof

A discussion is made of the wind force coefficients for designing the main wind force resisting systems of H.P. （Hyperbolic-Paraboid）-shaped porous canopy roofs on the basis of a wind tunnel experiment. Roof models with a number of small circular holes were made of nylon resin using laser lithography. The porosity was changed from 0 （solid） to 0.4. Besides the porosity, the geometric parameters of the models were the rise to span ratio and slope of the roof. The overall aerodynamic forces and moments acting on a model were measured by a six-component force balance in a turbulent boundary layer. The results indicate that the porosity significantly reduces the wind loads. The design wind force coefficients for porous canopy roofs can be provided by those for solid roofs with the same configuration multiplied by a reduction factor. The proposed wind force coefficients are verified by a comparison of the load effect predicted by the proposed wind force coefficients with the maximum load effect obtained from dynamic analyses using the time history of wind force and moment coefficients. The axial forces induced in the columns supporting the roof are regarded as the load effect for discussing the design wind loads.

Gustiness and coherent structure under weak wind period in atmospheric boundary layer

Statistical analysis of turbulent and gusty characteristics in the atmospheric boundary layer under weak wind period has been carried out.The data used in the analysis were from the multilevel ultrasonic anemometer-thermometers at 47 m,120 m,and 280 m levels on Beijing 325 m meteorological tower.The time series of 3D atmospheric velocity were analyzed by using conventional Fourier spectral analysis and decompose into three parts:basic mean flow(period > 10 min),gusty disturbances(1 min < period < 10 min)and turbulence fluctuations(period < 1 min).The results show that under weak mean wind condition:1)the gusty disturbances are the most strong fluctuations,contribute about 60% kinetic energy of eddy kinetic energy and 80% downward flux of momentum,although both the eddy kinetic energy and momentum transport are small in comparison with those in strong mean wind condition;2)the gusty wind disturbances are anisotropic;3)the gusty wind disturbances have obviously coherent structure,and their horizontal and vertical component are negatively correlated and make downward transport of momentum more effectively;4)the friction velocities related to turbulence and gusty wind are approximately constant with height in the surface layer.

Recent Advances in the Theoretical and Applied Study on Compressible Turbulent Flow over Aerospace Vehicles

This paper focused on the fundamental and applied research of turbulent flows encountered in the hypersonic flight of aerospace vehicles,which take place in the boundary layer and mixing layer.As to the plate boundary layer,LES approach has been used to simulate the flows over compression corners and incident shock waves,revealing that turbulent flows would significantly inhibit the boundary layer separation caused by shock wave-boundary layer interaction(SWBLI).The boundary layer transition over a circular cone has been analyzed through stability analysis and wind-tunnel test,by which the angle-of-attack effect in case of small angle of attack has been studied.Non-linear evolution process and secondary instability structure in the supersonic mixing layer(Mc=0.5) were initially figured out through the study of mixing layer,and knowledge of the flow control mechanism of the boundary layer and mixing enhancement mechanism of the mixing layer has been obtained through this research.Artificial boundary-layer transition technique based on subharmonic resonance has been proposed and applied to the flow control in a scramjet inlet,inhibiting the flow separation of the boundary layer while improving the inlet performance.To guarantee the mixing of kerosene and supersonic airflow in the scramjet combustor,the mixing enhancement method based on subharmonic resonance has been adopted and a concept of combustor with smooth wall and low internal drag has been proposed for ignition and stable combustion.Finally,future turbulence research and technological development of aerospace vehicles is predicted.

A comparison study of the simulation accuracy between WRF and MM5 in simulating local atmospheric circulations over Greater Beijing

Several multi-scale numerical simulation experiments were carried out using the mesoscale modeling systems MM5V3.7 and WRFV2.2 for Greater Beijing to estimate the accuracy of WRF and MM5 in simulating the characteristics and variations of mesoscale local circulation in the atmospheric boundary layer of this area. We simulated the horizontal distribution and diurnal variations of temperature and wind fields near the ground and compared them with Automatic Weather System (AWS) data collected from 19 AWS stations in Beijing. Correlation and error analyses were also made. The modeling and statistical results showed that both WRF and MM5 model the temperature field near the ground significantly better than they model the wind field. The temperature field simulated by MM5 is more coincident than that of WRF with the AWS observation records, while WRF does better than MM5 in simulating the wind field, especially under the condition of gusty wind. Neither WRF nor MM5 can capture the fine structure of urban architectural complexity, which is the main error in the wind field simulation. Both models underestimate the land surface temperature at night and overestimate the temperature during the day. All the above results are supported by statistical analysis.

Aerothermal characteristics of bleed slot in hypersonic flows

Two types of flow configurations with bleed their aerodynamic thermal loads and related in two-dimensional hypersonic flows flow structures at choked conditions. are numerically examined to investigate One is a turbulent boundary layer flow without shock impingement where the effects of the slot angle are discussed, and the other is shock wave boundary layer in- teractions where the effects of slot angle and slot location relative to shock impingement point are surveyed. A key separation is induced by bleed barrier shock on the upstream slot wall, resulting in a localized maximum heat flux at the reattachment point. For slanted slots, the dominating flow patterns are not much affected by the change in slot angle, but vary dramatically with slot location relative to the shock impingement point. Different flow structures are found in the case of normal slot, such as a flow pattern similar to typical Laval nozzle flow, the largest separation bubble which is almost independent of the shock position. Its larger detached distance results in 20% lower stagnation heat flux on the downstream slot corner, but with much wider area suffering from severe thermal loads. In spite of the complexity of the flow patterns, it is clearly revealed that the heat flux generally rises with the slot location moving downstream, and an increase in slot angle from 20° to 40° reduces 50% the heat flux peak at the reattachment point in the slot passage. The results further indicate that the bleed does not raise the heat flux around the slot for all cases except for the area around the downstream slot corner. Among all bleed configurations, the slot angle of 40° located slightly upstream of the incident shock is regarded as the best.

The Effect of Variable Stator on Performance of a Highly Loaded Tandem Axial Flow Compressor Stage

Increasing the aerodynamic load on compressor blades helps to obtain a higher pressure ratio in lower rotational speeds. Considering the high aerodynamic load effects and structural concerns in the design process, it is possible to obtain higher pressure ratios compared to conventional compressors. However, it must be noted that imposing higher aerodynamic loads results in higher loss coemcients and deteriorates the overall performance. To avoid the loss increase, the boundary layer quality must be studied carefully over the blade suction surface. Employment of advanced shaped airfoils （like CDAs）, slotted blades or other boundary layer control methods has helped the de- signers to use higher aerodynamic loads on compressor blades. Tandem cascade is a passive boundary layer control method, which is based on using the flow momentum to control the boundary layer on the suction surface and also to avoid the probable separation caused by higher aerodynamic loads. In fact, the front pressure side flow momentum helps to compensate the positive pressure gradient over the aft blade＇s suction side. Also, in compari- son to the single blade stators, tandem variable stators have more degrees of freedom, and this issue increases the possibility of finding enhanced conditions in the compressor off-design performance. In the current study, a 3D design procedure for an axial flow tandem compressor stage has been applied to design a highly loaded stage. Following, this design is numerically investigated using a CFD code and the stage characteristic map is reported. Also, the effect of various stator stagger angles on the compressor performance and especially on the compressor surge margin has been discussed. To validate the CFD method, another known compressor stage is presented and its performance is numerically investigated and the results are compared with available experimental results.

Shock wave-boundary layer interactions control by plasma aerodynamic actuation

This study demonstrates the potential for shock wave-boundary layer interaction control in air by plasma aerodynamic actuation.Experimental investigations on shock wave-boundary layer interactions control by plasma aerodynamic actuation are conducted in a Mach 3 in-draft air tunnel.Schlieren imaging shows that the discharges cause the oblique shock to move forward.Schlieren imaging and static pressure probes also show that separation phenomenon shifts backward and the size of separation is enlarged when plasma aerodynamic actuation is applied.The intensity of shock wave is weakened through wall pressure probe.Furthermore,numerical investigations on shock wave-boundary layer interactions control are conducted with plasma aerodynamic actuation.The discharge is modeled as a steady volumetric heat source which is integrated into the energy equation.The input energy level is about 7 kW through discharge process.Results show that the separation phenomenon shifts backward and the intensity of shock is reduced with plasma actuation.These numerical results are consistent with the experimental results.

Calculations of the bounds on limit cycle oscillations in nonlinear aeroelastic systems based on equivalent linearization

The nonlinear aeroelastic system of an airfoil with an external store was investigated,with emphasis on the bounds of limit cycle oscillations(LCOs).Based on the equivalent linearization,an approach was proposed to calculate the bounds on LCOs over the full flight envelope.The bounds are determined directly without solving LCOs one by one as the flow speed varies.The presented approach can provide us with the maximal LCO amplitudes and the lower threshold for flow speed beyond which LCOs may arise.Numerical examples show that the obtained bounds are in nice agreement with numerical simulation results.The speed threshold can be predicted to a relative error less than 0.1%,and the maximal LCO amplitude to about 3%.The influences of the system parameters on the speed threshold for speed were investigated efficiently by the proposed approach.

Implementation of a Roughness Element to Trip Transition in Large-eddy Simulation

In aerodynamics, the laminar or turbulent regime of a boundary layer has a strong influence on friction or heat transfer. In practical applications, it is sometimes necessary to trip the transition to turbulent, and a common way is by use of a roughness element （e.g. a step） on the wall. The present paper is concerned with the numerical im- plementation of such a trip in large-eddy simulations. The study is carried out on a flat-plate boundary layer con- figuration, with Reynolds number Rex=l.3x 106. First, this work brings the opportunity to introduce a practical methodology to assess convergence in large-eddy simulations. Second, concerning the trip implementation, a volume source term is proposed and is shown to yield a smoother and faster transition than a grid step. Moreover, it is easier to implement and more adaptable. Finally, two subgrid-scale models are tested： the WALE model of Nic0ud and Ducros （Flow Turbul. Combust., vol. 62, 1999） and the shear-improved Smagorinsky model of Ldv^que et al. （J. Fluid Mech., vol. 570, 2007）. Both models allow transition, but the former appears to yield a faster transition and a better prediction of friction in the turbulent regime.

Study for the Gas Flow through a Critical Nozzle

In the present study, computational work using the axisymmetric, compressible, Navier-Stokes equations is carried out to predict the discharge coefficient and critical pressure ratio of gas flow through a critical nozzle. The Reynolds number effects are investigated with several nozzles with different throat diameter. Diffuser angle is varied to investigate the effects on the discharge coefficient and critical pressure ratio. The computational results are compared with the previous experimental ones. It is known that the discharge coefficient and critical pressure ratio are given by functions of the Reynolds number and boundary layer integral properties. It is also found that diffuser angle affects the critical pressure ratio.

Exploration in Optimal Design of an Airfoil with a Leading Edge Rotating Cylinder

Based on the theory of moving surface boundary layer control(MSBC),a concept of an airfoil having a rotating cylinder at the leading edge has been developed and experimentally proven to have good aerodynamic performance even at large angles of attack.Thus,this research aims to give guidance on optimizing the design of this kind of airfoil with high lift coefficients.Using computational fluid dynamics(CFD)technique,the CFD simulation results have been compared with the experimental results available in the literature,and then the SST two-equation model is selected as the appropriate turbulence model.At a given cylinder surface velocity ratio,the cylinder diameter d,the drop height of trailing edgeδand the curvatures of the pressure and suction surfaces of the airfoil are regarded as the optimal design parameters and the airfoil lift coefficient is considered as the optimization objective function.Therefore,using orthogonal optimization method,we herein develop a new design of airfoil favorable for having a rotating leading edge.It has been numerically proven that the resulting airfoil has good capability of achieving a substantially superior performance when compared to the airfoils of the prior art.

Effects of rarefaction on the characteristics of micro gas journal bearings

Given the definition of the reference Knudsen number for micro gas journal bearings,the range in the number is related to the viscosity of air at different temperatures. A modified Reynolds equation for micro gas journal bearings based on Burgdorfer's first-order slip boundary condition is proposed that takes into account the gas rarefaction effect. The finite difference method (FDM) is adopted to solve the modified Reynolds equation to obtain the pressure profiles,load capacities and attitude angles for micro gas journal bearings at different reference Knudsen numbers,bearing numbers and journal eccentricity ratios. Numerical analysis shows that pressure profiles and non-dimensional load capacities decrease markedly as gas rarefaction in-creases. Attitude angles change conversely,and when the eccentricity ratio is less than 0.6,the attitude angles rise slightly and the influence of the reference Knudsen number is not marked. In addition,the effect of gas rarefaction on the non-dimensional load capacity and attitude angle decreases with smaller bearing numbers.

Influence of volute distortion on the performance of turbocharger centrifugal compressor with vane diffuser

As the geometry of the volute of turbocharger compressor is non-axisymmetric,it causes a distortion at the outlet of the diffuser and influences the upstream components.A distortion model in which a pressure distortion was applied as outlet boundary condition was established to simulate the distortion induced by the volute.It turned out to be sufficient to impose a circumferentially asymmetric pressure distribution at the outlet of the diffuser to replace the volute.Based on the distortion model which was verified,the influence of the amplitude of the distortion on the performance of centrifugal compressor was studied in detail.The results show that the distortion severely harms aerodynamic stability of the investigated compressor.The larger the amplitude of the distortion,the worse the performance of the compressor.The distortion induced by asymmetric volute propagates to upstream components and causes local flow separation at part of diffuser and impeller,and then causes the compressor surge.When the amplitude of the volute distortion is 10%,the stable flow range of the centrifugal compressor decreases to near zero.To authors’knowledge,the relationship between the compressor performance and distortion amplitude is first obtained quantitatively,which provides evidence to improve the performance of turbocharger compressor by decreasing the distortion induced by asymmetric volute.