自适应粒子成像测速场仪（PIV）

The far wake trajectory of the tumbling tip vortex effect of a reduced-scale, 1 m diame- ter, four-bladed rotor during hover is studied using vortex methods combined with a center of mass analysis approach. Measurements of all three components of the velocity field are acquired using a stereo PIV system synchronized to capture up to 800 wake age of the vortex with 10 offsets during hover conditions. The nominal operating condition of the rotor is at a rotational speed of 1520RPM, corresponding to Rec = 248,000 with a chord length of 58.5mm. The rotor was operated with a pitch of 7.2± 0.5 and a CT / of 0.029. The far wake vortex tumbling phenomenon is captured and described. It is shown that tip vortices from two blades tumble through approximately 180 of rotation before they coalesce. It is seen that the parent vortices are stronger than the daughter vortex, but due to vortex stretching the daughter vortex has a smaller radius with greater swirl strength. An accurate characterization and prediction of the trajectory of the far wake vortex tum- bling can enhance the ability to predict and alleviate the resuspension of particles during brownout as well as provide a database for far wake validation of CFD codes.

The mass balance of orchard air-blast sprayers has historically been assessed using an array of samplers to capture airborne particles. However, these methods only provide an idea of flux with no other information which is pertinent to understand the movement of droplets and their potential to drift. While droplet analysis for agricultural sprayers has always been conducted in a laboratory setting with the use of laser devices, a new phase Doppler approach is being explored to assess droplet spectra, velocity, and flux in outdoor field conditions. Therefore it is the objective of this study to develop a methodology and the potential limitations for using a phase Doppler system while in a laboratory setting. Due to the expected variability of field conditions as well as the turbulence of orchard sprayers, a computational approach was sought to assess flux from a single scan of a conical spray plume's diameter. Using a constant scanning speed of 0.0079 m/s, a disc core (D1/DC33) hollow cone nozzle was examined at 310, 410, and 520 kPa pressure at five different heights (10, 20, 30, 40, and 50 cm). Computational flux was then compared to the actual flow rate, finding a
3.3% average error with a range of
16.9% and 4.7% illustrating a small underestimation of mass with the phase Doppler which was related to distance and droplet frequency. Further, comparisons were also assessed including pattern/symmetry, droplet spectra, velocity, and the overall number of samples. The proposed methodology indicates potential for the use of phase Doppler technology for in situ measurements of spray equipment using a conical-type spray nozzle, such as that of the orchard air-blast sprayer.

The three-dimensional behavior of flow transition in circular and 6-chevron jets at Re?000 is investigated with experiments conducted on a free water jet by time-resolved tomographic particle image velocimetry. The emphasis is on the unsteady organization of coherent flow structures, which play a role in the generation of acoustic noise. Shedding and pairing of vortices are the most pronounced phenomena observed in the near field of the circular jet. The first and second pairing amplify the axial pulsatile motion in the jet column and lead to the growth of azimuthal waves culminating in the breakup of the vortex ring. Streamwise vortices of axial and radial vorticity are observed in the outer region and move inward and outward under the effect of the vortex rings. In the jet with chevrons, the axisymmetric ring-like coherence of the circular jet is not encountered. Instead, streamwise flow structures of azimuthal vorticity emanate from the chevron apices, and counter-rotating streamwise vortices of axial and radial vorticity develop from the chevron notches. The decay of streamwise vortices is accompanied by the formation of C-shaped structures. The three-dimensional analysis allows quantifying the vortex stretching and tilting activity, which, for the circular jet exit, is related to the azimuthal instabilities and the streamwise vortices connecting the vortex rings. In the chevron jet, stretching and tilting peak during the formation of C-structures. Following Powell抯 aeroacoustic analogy, the spatial distribution of the source term is mapped, evaluating the temporal derivative of the Lamb vector. The spatio-temporal evolution of such source term is visualized revealing that the events of highest activity are associated with the processes of vortex-ring pairing and vortex-ring disruption for the circular jet, and with the decay of streamwise instabilities and the formation of C-shaped structures for the chevron case.

Forces on flapping or rotating wings, like flapping wings of micro air vehicles or blades of wind turbines are of great interest to engineers. To investigate the ways birds and insects fly, forces created by flapping wings are of importance to biologists. The pressure field, combined with the velocity field, gives a complete description of the (incompressible) flow dynamics. Furthermore the pressure field is the main contributor to the aerodynamic loading of bodies immersed in the fluid. Traditional techniques to determine pressure and forces rely on the determination of surface pressure and integral loads by point pressure and force balance measurements. In situations where it is difficult (or impossible) to instrument the body, using particle image velocimetry (PIV) velocity data to determine forces and pressure poses an interesting alternative to the existing approaches to determine sectional loading.

Turbulence of the round jet has been assessed using invariants of the velocity gradient tensor. Experimental data, obtained using Tomographic Particle Image Velocimetry (Tomo-PIV), using four PCO-4000 cameras with 11 megapixel resolution, is presented for a seeded free air jet, operating in the turbulent regime and the Re number based on the diameter of the nozzle is 10000. Using the acquired 3-D velocity fields, the local statistical and geometrical structure of three-dimensional turbulent flow can be described by properties of the velocity gradient tensor. The invariants of the velocity gradient (R and Q), rate-of-strain ( Rs andQs ), and rate-of-rotation (Qw ) tensors are analyzed across the turbulent expanding regions at different distances from the nozzle outlet. More specifically, the JPDF of invariants is computed, which allows a detailed statistical characterization of the dynamics, geometry and topology of the flow during the entrainment process. It should be noted that the results obtained are indicative of the preliminary work in this area.

Experimental velocity measurements of a low-speed jet, performed using time-resolved tomographic PIV, are used to study the dynamics of large-scale structures and their sound radiation. The experimental results show the roll-up of axisymmetric vortices that pair downstream, and subsequently lose their azimuthal coherence. Models of linear instability waves using both steady laminar and mean-field base flows flow are applied. While good agreement can be obtained for the vortex roll-up frequency in the near-nozzle region using the laminar base flow, non-linear effects must be included, via the mean field, in order to capture the downstream evolution of both the fundamental and subharmonic (vortex pairing). The velocity fluctuations for both frequencies have a wave-packet structure with some jitter in the form of modulations of the spatial extent and amplitude of the envelope. The sound radiation is modelled using a jittering wave-packet model, an shows agreement with the exponential directivity shape of Laufer and Yen (J. Fluid Mech. 134, 1983).

Energy supply is one of the primary building blocks of our society. Worldwide, and especially in rapidly developing countries, e.g. China and India, the demands of energy for housing, industry, transportation and communication are constantly increasing [83]. The global energy supply is currently dominated by combustion of fossil fuels. In 2008, 85.1% of the global primary energy supply was produced from the combustion of oil (34.6%), coal (28.4%) and natural gas (22.1%). From the 12.4% of renewable energy sources, 82% (in total 10.2%) was bioenergy based on burning renewable rescources [45]. Recent reports confirm that a dramatic shift towards CO2 neutral energy supply is required to stop the current trend of global warming [135]. Nevertheless, the slow developement of renewable energies over the last decades as well as the absence of large technological advancements in nuclear fusion technology indicate that this structure will not substantially change in the coming decades. Besides emissions of CO2, further pollutants are formed during the combustion of fossil or biofuels. From these emissions, nitric oxides (NOx) have been identified as a major problem of practical combustion processes [214]. In the recent decades nitric oxides became a significant contributor to photochemical smog and ozone in urban air [180]. It further participates in the chain reaction removing ozone form the stratosphere with the consequence of increased ultraviolet radiation reaching earth’s surface [90]. NOx is generally formed during the oxidation process of nitrogen containing fuels such as coal and oil. However, the gross NOx emissions are created at high combustion temperatures from nitrogen in the air. These are typical for hard coal plants, internal combustion engines and gas turbines. Hence, it is essential to understand the physico-chemical processes of NOx formation and concepts to reduce or even eliminate NOx emissions.

全场测试。测试对象为辉光放电中的等离子体。和理论计算模拟相对比。对于电子设备散热，器件热传导规律的认识有借鉴和参照意义。