多压电膜式零质量射流激励器的设计与性能研究

设计了一种多压电膜式零质量射流激励器,并利用热线风速仪,得到了对应于射流中心线上固定位置处速度最大时的最佳频率.在该频率下,测量了零质量射流在不同孔径时沿中心线的速度分布;测量、分析和比较了不同激励膜数目对射流出口速度分布的影响.表明:在其他条件完全相同的情况下,一片振动膜时的激励器出口中心线上的平均速度和峰值速度,都较5片膜的情况小一半左右,而3片和5片压电膜时的激励器性能差别却不大.也就是说,虽然压电膜的数目增加能使激励器出口射流的能量增加,但这两者之间并非呈线性关系.此外还用PIV测量方法,直观地显示了射流出口的涡结构.

几何因素对多压电膜式零质量射流激励器性能的影响

利用热线风速仪,在最佳激振频率f=560 Hz时,研究了不同孔径、不同孔深、不同小孔出口几何形状等对多压电膜式零质量射流激励器性能的影响。实验结果表明:所用激励器的最佳孔径为2.5 mm,最佳孔深为2 mm,最佳深径比为0.8。激励器射流出口为锥孔要比为直孔好得多。锥孔在流场中心线上距离激励器出口5<y/d<18范围内,射流速度几乎不变。最后利用CFD数值模拟直观地显示了射流出口的涡结构,说明了几何尺寸产生影响的根源。

Characteristics of array of distributed synthetic jets and effect on turbulent boundary layer

An array of distributed round synthetic jets was used to control a fully developed turbulent boundary layer.The study focused on the related skin friction drag reduction and mechanisms involved.The control effects were analyzed by measuring the streamwise velocities using a hot-wire anemometer downstream of the array.A reduction in the skin friction was observed both in the regions downstream of the orifices and in the regions between two adjacent orifices.A statistical analysis with the variable-interval time-averaging(VITA)technique demonstrated a weakened bursting intensity with synthetic jet in the near-wall region.The streamwise vortices were lifted by the upwash effect caused by synthetic jet and induced less low-speed streaks.The control mechanism acted in a way to suppress the dynamic interaction between the streamwise vortices and low-speed streaks and to attenuate the turbulence production in the near-wall region.The forcing frequency was found to be a more relevant parameter when synthetic jet was applied in turbulent boundary layer flow control.A higher forcing frequency induced a higher reduction in the skin friction.The power spectral density and autocorrelation of the fluctuating velocities showed that the synthetic jets gradually decayed in the streamwise direction,having an effect as far as 34.5 times the displacement thickness that was on the trailing edge of the distributed synthetic jets array.

Experimental Investigation on Mixing Enhancement Mechanism of Turbulent Jet Flow with Tabbed Nozzle

Flow visualization and hot-wire measurement techniques were combined to investigate the influence of the size and number of tabs on jet flow field and vortex structure generation mechanism. Streamwise vortices generated by the tabs of different sizes and numbers were observed from the flow visualization images. Combined with flow visualization, hot-wire measurement gave a quantitative insight of the effect of various tabbed jet flows. Instantaneous two-component velocity signals (longitudinal and transverse velocity components) at different cross sections along radius direction and streamwise direction with different tabbed jet nozzles were measured using hot-wire anemometer. Average flow field parameters of tabbed jet flow such as mean velocity, tur-bulence intensity, vorticity were analyzed and the effects of tabs with different sizes and numbers were compared with that of circular no-tab jet flow. It is revealed that the generation of a series of counter-rotating quasi-streamwise vortices, azimuthal vortices and double-row azi-muthal vortex are the reasons for mixing enhancement of tabbed turbulent jet flow.

Periodic Wall Blow/Suction Perturbation Evolution in Turbulent Boundary Layer

Time sequence signals of instantaneous longitudinal and normal velocity components at different longitudinal and normal positions in a turbulent boundary layer have been finely measured simultaneously by IFA300 constant temperature anemometer and double-sensor hot-wire probe with sampling resolution higher than the frequency that corresponds to the smallest time scale of Kolmogorov dissipation scale before/after introducing artificial periodic blow/suction perturbation. The period-phase-average technique is applied to extract the periodic waveforms of artificial perturbation from instantaneous time sequence signals of longitudinal and normal turbulence background. Experimental investigation is carried out on the attenuation characteristics of periodic perturbation wave with different frequency along longitudinal direction and normal direction in a turbulent boundary layer. The amplitude distributions of longitudinal and normal disturbing velocity component for different perturbation frequencies are measured at different downstream and normal positions in turbulent boundary layer. The amplitude growth rate of artificial periodic perturbation wave is calculated according to flow instability theory. The experimental results are compared and in consistent with the theoretical and numerical results.

Wake-Induced Bypass Transition over a Flat Plate under Favorable and Adverse Pressure Gradients

The present study investigates wake-induced bypass transition of boundary layers on a flat plate subjected to favorable and adverse pressure gradients. The aim is to exploit unsteady effects in order to design an aft-loaded turbine blade without increasing the profile loss, as has been achieved for low pressure turbine blades. First of all, this fundamental study is to reveal the effect of the Strouhal number, which changed by using different numbers of wake generating bars. Detailed boundary layer measurements were conducted using two hot-wire probes. A passage-contouring device was employed to generate a pressure gradient on the test model, which was typical to that generated by an aft-loaded turbine blade. A spoked-wheel-type wake generator was used to create periodic wakes in front of the flat plate. It was found that the wake passage induces a significant change in the flow structure downstream of the flow acceleration region.

Experimental Study of Corner Stall in a Linear Compressor Cascade

In order to gain a better knowledge of the mechanisms and to calibrate computational fluid dynamics （CFD） tools including both Reynolds-averaged Navier-Stokes （RANS） and large eddy simulation （LES）, a detailed and accurate experimental study of comer stall in a linear compressor cascade has been carried out. Data are taken at a Reynolds number of 382 000 based on blade chord and inlet velocity. At first, inlet flow boundary layer is surveyed using hot-wire anemometry. Then in order to investigate the effects of incidence, measurements are acquired at five incidences, including static pressures on both blade and endwall sur- faces measured by pressure taps and the total pressure losses of outlet flow measured by a five-hole pressure probe. The maxi- mum losses as well as the extent of losses of the comer stall are presented as a function of the investigated incidences.

NEAR WAKE OF A MODEL HORIZONTAL-AXIS WIND TURBINE

An experimental investigation on the properties of the near wake behind the rotor of a Horizontal-Axis Wind Turbine （HAWT） was carried out at model scale. Measurements were made with a stationary slanted hot-wire anemometer using the technique of phase-locked averaging. The primary aim is to study the formation and development of the three-dimensional wake. Five axial locations were chosen within four chord lengths of the blades over a range of tip speed ratios. The results show that during the downstream developmerit of the wake, the wake centre traces a helical curve with its rotation direction opposite to that of the rotor. The distribution of mean velocity behind the HAWT rotor reveals an expansion and a decay of the three-dimensional wake. The shapes of the mean velocity distribution are similar along the blades span at the same downstream axial location. It is shown that the turbulence levels in the wake are higher than those in the non-wake region. The circumferential component and the radial component of the turbulence intensity are higher than the axial component. Our study offers some food of thought for better understanding of the physical features of the flow field as well as the performance of HAWT.

Investigation of the Influence of Free Convection on the Heat Transfer of Cylinders with Different Aspect Ratios

The heat transfer from an electrically heated cylinder (wire) of finite length to the surrounding fluid can be divided into heat radiation, conduction and convection. A technical application of these cylinders with typical dimensions of 1-2 mm length and a few micrometers diameter is the Hot-Wire-Anemometry.This systematic study should clarify the influence of the convection to three dimensional heat transfer of cylinders. For this aim it is planned to investigate Reynolds numbers below Re = 1 (creeping flow). For this reason measurements should be done under 1g in the earth laboratory and also under microgravity (μg) conditions. Comparisons of these measurements under otherwise same conditions allows to distinguish between the pure convection heat transfer and the contributions due to conduction and other effects. For measarements under μg the Drop Tower Bremen can be used as research facility. Due to the fast response of convection to changes in the gravity conditions the Drop Tower is an ideal and cost efficient experimental tool. The experimental setup is build to operate at velocity range of 0-l m/s which includes the whole range of convection from pure free convection at 0 m/s over mixed convection up to pure forced convection at velocities above about 0.15 m/s. This velocity region corresponds to a range of the Reynolds number of Re = 0 - 0.18 for a cylinder of 5 μ m diameter at Tf= 140℃ in air at an ambient temperature of about 21℃.

Experimental Analysis of the Aerodynamic Performance of an Innovative Low Pressure Turbine Rotor

In the present work the aerodynamic performances of an innovative rotor blade row have been experimentally investigated. Measurements have been carried out in a large scale low speed single stage cold flow facility at a Reynolds number typical of aeroengine cruise, under nominal and off-design conditions. The time-mean blade aerodynamic loadings have been measured at three radial positions along the blade height through a pressure transducer installed inside the hollow shaft, by delivering the signal to the stationary frame with a slip ring. The time mean aerodynamic flow fields upstream and downstream of the rotor have been measured by means of a five-hole probe to investigate the losses associated with the rotor. The investigations in the single stage research turbine allow the reproduction of both wake-boundary layer interaction as well as vortex-vortex interaction. The detail of the present results clearly highlights the strong dissipative effects induced by the blade tip vortex and by the momentum defect as well as the turbulence production, which is generated during the migration of the stator wake in the rotor passage. Phase-locked hot-wire investigations have been also performed to analyze the time-varying flow during the wake passing period. In particular the interaction between stator and rotor structures has been investigated also under off-design conditions to further explain the mechanisms contributing to the loss generation for the different conditions.