Obtaining absolute acoustic spectra in an aerodynamic wind tunnel

Cost effective methods for identifying and reducing sources of noise have become essential in the design of many modern transport vehicles. Whilst closed-section wind tunnels can readily evaluate aerodynamic performance, obtaining accurate acoustic spectra is often a major challenge because of the poor signal to noise ratios available. In this paper, methods of obtaining absolute spectra from the non-acoustically treated Markham wind tunnel at the University of Cambridge are discussed. Initial measurements with a small monopole source compare well with simulations and show that it is possible to obtain similar spectra from two nested acoustic arrays. However, a series of further experiments with simplified landing gear models show very different spectra from each array. By comparing measurements with simulations, it is shown that negative side lobes affect beamforming source maps. Measurements of an ‘empty tunnel’ cross spectral matrix allow the removal of sidelobes, providing much greater consistency between spectra. Finally, a dipole beamforming vector is used to account for the directivity of the landing gear noise, leading to good agreement between absolute spectra from the differently sized arrays. This analysis demonstrates that data from a phased array in a hard-walled, aerodynamic wind tunnel can provide meaningful acoustic spectra from low-noise models.     

机动车空气动力测试风洞的声学设计

测试、鉴定并研究高速机动车对环境及在车内所产生的噪声，必须有“安静”的风洞，抑制风洞内的气流噪声，将它的本底噪声降低到车辆噪声以下，测试大厅中有足够低的以上获得应有的自由声场范围，所有的声学处理措施不影响风洞的空气动力性能；吸声构件中不含矿物性纤维材料；具有相应的阻燃性能并易于清洁，本文介绍了满足以上所有要求的斯图加特大学空气动力实验室风洞的声学设计和所采用的新型吸声材料，给出了构造大样及实测结果     

Design and performance of an open jet wind tunnel for aero-acoustic measurement

This paper presents the design and performance of an open jet, blow down wind tunnel that was newly commissioned in the anechoic chamber at the ISVR, University of Southampton, UK. This wind tunnel is intended for the measurement of airfoil trailing edge self-noise but can be extended to other aeroacoustic applications. With the primary objectives of achieving acoustically quiet and low turbulence air jet up to 120 m/s through a 0.15 m × 0.45 m nozzle, several novel noise and flow control techniques were implemented in the design. Both the acoustical and aerodynamic performances of the open jet wind tunnel were examined in detail after its fabrication. It is found that the background noise of the facility is adequately low for a wide range of exit jet velocity. The potential core of the free jet is characterized by a low turbulence level of about 0.1%. Benchmark tests by submerging a NACA0012 airfoil with tripped and untripped boundary layers at 0° and 10° angles of attack respectively into the potential core of the free jet were carried out. It was confirmed that the radiating airfoil trailing edge self-noise has levels significantly above the rig noise over a wide range of frequencies. The low noise and low turbulence characteristics of this open jet wind tunnel are comparable to the best facilities in the world, and for its size it is believed to be the first of its kind in the UK.     

Application of the laser velocimeter in supersonic wind tunnel

This paper deals with the usability of laser velocimeters in supersonic flow field studies. Problems of the optical setup stability are considered as well as possibilities of data acquisition and evaluation. It is demonstrated that dust particles always present in the air are well suited as tracers for the flow velocity. The behaviour of these particles is studied in a rapidly decelerated flow.     

Building resolving large-eddy simulations and comparison with wind tunnel experiments

We perform large-eddy simulations (LES) of the flow past a scale model of a complex building. Calculations are accomplished using two different methods to represent the edifice. The first method employs the standard Gal-Chen and Somerville terrain-following coordinate transformation, common in mesoscale atmospheric simulations. The second method uses an immersed boundary approach, in which fictitious body forces in the equations of motion are used to represent the building by attenuating the flow to stagnation within a time comparable to the time step of the model. Both methods are implemented in the same hydrodynamical code (EULAG) using the same nonoscillatory forward-in-time (NFT) incompressible flow solver based on the multidimensional positive definite advection transport algorithms (MPDATA). The two solution methods are compared to wind tunnel data collected for neutral stratification. Profiles of the first- and second-order moments at various locations around the model building show good agreement with the wind tunnel data. Although both methods appear to be viable tools for LES of urban flows, the immersed boundary approach is computationally more efficient. The results of these simulations demonstrate that, contrary to popular opinion, continuous mappings such as the Gal-Chen and Somerville transformation are not inherently limited to gentle slopes. Calculations for a strongly stratified case are also presented to point out the substantial differences from the neutral boundary layer flows.     

Dispersion simulation of a wind tunnel experiment with Lagrangian particle models

Summary A particle model LAMDA suitable for dealing with the atmospheric dispersion is presented. The reliability of the model is tested comparing the results of its simulations to the wind tunnel measurements by Khurshudyanet al. (1981). Two versions of the model, both based on the Langevin equation and askewed distribution of the vertical wind velocity fluctuations, are considered. To develop the second version of the model we derived a proper scheme to produce skewed distributions of particle velocities consistent with the observed first and second moments of turbulent fluctuating velocities, with their horizontal and vertical derivatives, and their cross-correlations. The 2D geometry of the obstacle (in the wind tunnel experiment considered) allowed some simplifications. Due to the lack of some input data (such as the vertical profiles of crosswind standard deviation of wind velocity fluctuations and of the Lagrangian time scales) we looked for three different parametrizations. It was found that particular combinations of measured and parametrized data could give rise to critical vertical regions in which the derived scheme for the generation of random vertical velocity fluctuations cannot be applied. The best vertical distributions of the Lagrangian time scales (third parametrization) were estimated by fitting simple formulations for the average plume height and lateral variances to the measured data. The main results of this work were the model performance in simulating dispersion in shear flow over flat terrain, and its sensivity to the shape of the crosscorrelation term .     

Electron-phonon multiplier involving a cascade of NIS tunnel junctions and hot-electron microcalorimeters

A device for amplifying weak electric signals arising in a normal-or a superconducting-metal absorber upon the absorption of photons or phonons is proposed. The device consists of a cascade of normal-metal-insulator-superconductor (NIS) tunnel junctions, where the normal metal is used as an electrode and as a hot-electron microcalorimeter. Tunneling through the barrier, a particle transports energy of about Δ (energy gap in the superconductor) from one microcalorimeter to another and amplifies the current through the next NIS junction. The device operates as a solid-state electron multiplier at low temperatures.     

High-enthalpy hot-shot wind tunnel with combined heating and stabilization of parameters

In the present paper, we consider instrumentation and the experimental procedure for conducting tests in a highenthalpy short-duration wind-tunnel facility, namely, a hypersonic hot-shot wind tunnel. We consider operation of the hot-shot wind tunnel with the test gas (TG) parameters kept constant during the regime and also operation of the tunnel as a traditional shock tube with a decay of the TG parameters that occurs as the TG leaves a constant-volume settling chamber. Stabilization of the TG parameters is achieved by using a pressure multiplier installed coaxially with the settling chamber, the configuration presenting a linear arrangement of the two components. Unloading of pressure multiplier dynamic component is achieved by using an equalizer whose piston moves in the opposite direction to the multiplier piston system. Several modes of wind tunnel operation with various combinations of different TG heating methods (electric arc, chemical energy, adiabatic compression, or heating in an external with respect to the settling chamber heat source) are possible. The design of a device responsible for diaphragm breakdown delay is considered. The design and dimensions of the wind tunnel provide for its normal operation under the following conditions: range of Mach numbers M = 4–20, range of settling-chamber temperatures T _ch1 = 600–4000 K, and settling-chamber pressure p _ch1 up to 200 MPa (in operation with a double settling chamber, the stagnation pressure p _0n = p _ch2 can be varied from 1 to 200 MPa). The settling chamber volume (80–100 dm³) is sufficiently large, allowing obtaining a 1-m diameter hypersonic stream in the test section during ～ 100 ms (in combination with a second settling chamber).     

Enhanced visualization methods for interpretation of wind tunnel data

Journal of Visualization -     

Evolution of downsized crescent-shaped dune in wind tunnel experiment

The migration of a downsized crescent-shaped dune was investigated in a wind tunnel experiment.Quantified upwind influx and vertical oscillation of the sand bed were introduced to modulate the saturation level of the sand flux above the dune surface to affect dune evolution.The evolution was recorded by top-view photography and then abstracted as the evolution of self-defined characteristic quantities using a digital image processing algorithm.The results showed that,in contrast to the case for spanwise quantities,the evolution of streamwise quantities corresponds to a linear increase in the modulation magnitude more positively and in a monotonic and convergent manner.In contrast with quantities on the windward face,the changes in quantities with respect to the horns were nonmonotonic with time and almost uncorrelated with the variation in modulation strength,which reveals the distinctiveness of leeside evolution.     

Design and performance of a small-scale aeroacoustic wind tunnel

The D5 aeroacoustic wind tunnel at Beihang University is a newly commissioned small-scale closed-circuit wind tunnel with low turbulence intensity and low background noise. The wind tunnel is built to study both aerodynamic and aeroacoustic performance of aircraft components or scaled models. The wind tunnel has two types of test sections, the closed type test section is used for aerodynamic tests while the open type test section is mainly used for aeroacoustic experiments. Both types of test section are 1 m in height and 1 m in width, and the maximum wind velocity in the test section can be up to 80 m/s. An anechoic chamber is built surrounding the test section to provide the non-reflecting condition. This paper provides an overview of design criteria and performance of the small-scale wind tunnel. The layout of the wind tunnel and some critical design treatments to improve aerodynamic and acoustic performance are discussed in detail. Some experiments are conducted to verify the performance of D5 wind tunnel, results confirm that the turbulence intensity is less than 0.08% in the core of test section and the background noise is comparable with other aeroacoustic wind tunnels. A scaled simplified nose landing gear model is also measured as a benchmark test, results reveal that noise radiated from the model is adequately higher than the background noise for a wide frequency range and remarkably consistent with other results from literatures.     

An experimental study of the natural noise in the Transit-M hypersonic wind tunnel

Tests were performed at Mach number M = 6. Observations of flow pulsations were performed with the help of high-frequency measurements of pressure and heat fluxes and with the help of hot-wire anemometry. Measurements were carried out in a broad range of Re₁ numbers at various orientations and locations of sensors with respect to the nozzle exit plane. It is shown that the Transit-M noise varies weakly over the longitudinal coordinate, whereas on moving along the radial coordinate, some increase of the fluctuation level is observed due to approaching the mixing layer. Also, the distribution of axial asymmetry of the noise level was obtained.     

激波风洞设施中的等离子体包覆目标电磁散射实验研究

利用JF10高焓激波风洞设施, 进行了等离子鞘包覆目标的电磁散射测量实验. 基于矢量网络仪的步进扫频体制, 在C波段进行实验, 观测到等离子鞘对目标雷达散射截面(radar cross section, RCS)的影响. 并且, 目标散射测量值中直接体现了激波风洞的高速气流状态信息: 气流前段会造成散射回波的剧烈变化且稳定性差, 持续0.5–1 ms; 激发的等离子鞘有效持续时间仅约为2 ms, 衰减了目标RCS回波.     

Comparisons of wind tunnel experiments and computational fluid dynamics simulations

This paper describes the architecture of an image and data level comparative visualization system and experiences using it to study computational fluid dynamics data and experimental wind tunnel data. We illustrate how the system can be used to compare data sets from different sources, data sets with different resolutions and data sets computed by using different mathematical models of fluid flow. We include recent improvements including use of the comparison of vector fields, ability to interactively interrogate the comparisons, addition of new metrics, the latest FEL2.0 and VisTech libraries, and GUI for a calculator module.     

Influence of wind tunnel freestream turbulence level on boundary-layer separation

An investigation of aerodynamic performance for micro air vehicle (MAV) wing is performed. It is shown, what is a reason of significant wing performance difference between tests, which were carried out in different wind tunnels. New method for estimation of wind tunnel condition influence on experimental results is suggested.     

Research on six-degree-of-freedom calibration system for wind tunnel balances with a collimated laser beam

A newly-developed six-degree-of-freedom calibration system for the wind tunnel balances is introduced. The frame of the system, the functions and the operating principle of different parts are presented in detail. The system is composed of four parts: the automatically loading subsystem, the automatically resetting subsystem, the data-acquisition subsystem and the measurement subsystem. The results of some cell experiments proved that the system can meet the needs of the present calibration task of the balance. Through further improvement, the system can be also used to calibrate other devices with multi degree-of-freedom and measure the minute shifts, such as the guide rail of machine tool and the assembling of large parts and so on.     

隧道围岩压力监测的非线性回归分析研究

隧道围岩压力的变化是导致公路隧道岩块错动、掉块、甚至塌方的重要因素。结合田心隧道工程,将一种可以把压力转变成光纤Bragg光栅波长移位的FBG土压力传感器应用于隧道围岩压力的监测;将40支土压力传感器安装在20个代表性断面中。采用非线性回归分析统计方法对隧道的量测数据进行分析,比较了监测数据的处理分析方法,选取最优的非线性回归分析模型。监测数据与回归拟合数据分析表明,非线性回归分析预测方法不仅可以反映隧道围岩变形和压力变化过程,还能根据其变化趋势进行结构异常判断。     

Operating safety of a hot-shot wind tunnel with combined test gas heating in stabilization mode

In the present paper, we analyze emergency situations typical of short-duration wind tunnels with electric-arc or combined test-gas heating in the presence of stabilization and diaphragm-rupturing systems, which occur in the case of no discharge initiation in the settling chamber, with the capacitor battery having remained charged during the start of wind-tunnel systems. For avoiding such emergency situations, some additional changes based on using feedback elements are introduced into the wind-tunnel design: the piston of the fast-response valve is made hollow for increasing the volume of the shutoff cavity and for making the release of pressure from this cavity unnecessary; the high-pressure channel, which connects the piston and the piston rod with the settling-chamber cavity, is filled with a liquid and is closed from the side of the settling chamber with a piston; the device for controlled diaphragm breakdown is provided with an external electric circuit intended to control the diaphragm-rupturing process. Those modifications allow subsequent functioning of the wind-tunnel systems only in the presence of heat-supply-induced pressure growth in the settling chamber of the wind tunnel.