Turbulence and mixing across gravitationally unstable interface by tank-overturn experiment

Turbulence and mixing across gravitationally unstable interface were studied in the laboratory by overturning a tank of two-layer fluids initially of stable stratification. As the dense fluid fell under gravity to mix with the lighter fluid from below, highly unsteady exchange of fluids across the unstable interface was produced by the buoyancy force. The exchange was captured in the experiment by a video camera using dye in the dense fluid as tracer. Absorption of light by the dye determined the excess mass at every pixel of the digitized images. The position of the excess-mass center and the speed of the center were computed from the excess-mass profiles as parameters to characterize the mixing across the unstable interface. With positive feedback by the buoyancy force, mixing across the interface rapidly intensified. It increased linearly from zero to a maximum in an acceleration regime and then asymptotically toward a terminal state, as the total buoyancy in the layer stayed constant. In the terminal state, the excess-mass center advanced at a terminal speed in proportion to the square root of the layer's total buoyancy.     

Numerical simulation of three-dimensional internal flow field in the spinning channel on rotor spinning unit北大核心CSCD

view of the complex airflow variation in spinning channel on air-extraction rotor, an unstructured fluid calculation domain with single phase and steady slate was built by ICKM CFI) and FLUENT software, based on RNG k-e turbulence physical model.The pressure-velocity coupling of I he internal How field in the spinning channel was solved through SIMPLE algorithm, and the mass flow difference between inlet and outlet Am =0.001 4 g/s was obtained through calculation.The numerical simulation satisfied the convergence condition of residual.The results showed that there was an obvious pressure gradient in the rotor, and the static and dynamic pressure gradually increased from central area of the rotor to the edge of condensing groove.Besides, there was turbulence velocity at the outlet of fil>er pipeline, and the air velocity(342 - 428.68 m/s) on the slip surface and flow area in the condensing groove was relatively high.The trajectory and boundary of two airflow lines in the spinning channel was clear and the flow direction was regular, the line characteristics conformed to the analysis expectation of the pressure field and velocity field.Our work strongly supports research on the flow field characteristics and spinning mechanism of high-speed rotor. © 2021 China Silk Association. All rights reserved.     

Numerical evaluation of wind effects on a tall steel building by CFD

A comprehensive numerical study of wind effects on the Commonwealth Advisory Aeronautical Council (CAARC) standard tall building is presented in this paper. The techniques of Computational Fluid Dynamics (CFD), such as Large Eddy Simulation (LES), Reynolds Averaged Navier-Stokes Equations (RANS) Model etc., were adopted in this study to predict wind loads on and wind flows around the building. The main objective of this study is to explore an effective and reliable approach for evaluation of wind effects on tall buildings by CFD techniques. The computed results were compared with extensive experimental data which were obtained at seven wind tunnels. The reasons to cause the discrepancies of the numerical predictions and experimental results were identified and discussed. It was found through the comparison that the LES with a dynamic subgrid-scale (SGS) model can give satisfactory predictions for mean and dynamic wind loads on the tall building, while the RANS model with modifications can yield encouraging results in most cases and has the advantage of providing rapid solutions. Furthermore, it was observed that typical features of the flow fields around such a surface-mounted bluff body standing in atmospheric boundary layers can be captured numerically. It was found that the velocity profile of the approaching wind flow mainly influences the mean pressure coefficients on the building and the incident turbulence intensity profile has a significant effect on the fluctuating wind forces. Therefore, it is necessary to correctly simulate both the incident wind velocity profile and turbulence intensity profile in CFD computations to accurately predict wind effects on tall buildings. The recommended CFD techniques and associated numerical treatments provide an effective way for designers to assess wind effects on a tall building and the need for a detailed wind tunnel test.     

Influence of the mass flow rate of secondary air on the gas/particle flow characteristics in the near-burner region of a double swirl flow burner

The influence of the mass flow rate of secondary air on the gas/particle flow characteristics of a double swirl flow burner, in the near-burner region, was measured by a three-component particle-dynamics anemometer, in conjunction with a gas/particle two-phase test facility. Velocities, particle volume flux profiles, and normalized particle number concentrations were obtained. The relationship between the gas/particle flows and the combustion characteristics of the burners was discussed. For different mass flow rates of secondary air, annular recirculation zones formed only in the region of r/d=0.3–0.6 at x/d=0.1–0.3. With an increasing mass flow rate of secondary air, the peaks of the root mean square (RMS) axial fluctuating velocities, radial mean velocities, RMS radial fluctuating velocities, and tangential velocities all increased, while the recirculation increased slightly. There was a low particle volume flux in the central zone of the burner. At x/d=0.1–0.7, the profiles of particle volume flux had two peaks in the secondary air flow zone near the wall. With an increasing mass flow rate of secondary air, the peak of particle volume flux in the secondary air flow zone decreased, but the peak of particle volume flux near the wall increased. In section x/d=0.1–0.5, the particle diameter in the central zone of the burner was always less than the particle diameter at other locations.     

High resolution experimental measurement of turbulent flow field in a high pressure homogenizer model and its implications on turbulent drop fragmentation

Particle image velocimetry is performed on a model of a high pressure homogenizer, scaled for qualitative similarity of the one phase turbulent flow field in a production scale homogenizer. Flow fields in gap entrance, gap and gap outlet chamber are obtained with high resolution. The measurements show gap flow development and formation of a turbulent wall adherent jet when exiting into the outlet chamber. Turbulent kinetic energy spectra show how the turbulent energy available for fragmentation is transported over distance along the jet centre axis.The high resolution images are also used together with a Kolmogorov–Hinze theory framework for discussing drop fragmentation together with a direct evaluation of disruptive stresses from measurements. For the turbulent inertial mechanism large drops experience high fragmenting force close to eight gap heights downstream of the gap exit whereas this occurs closer to 20 gap heights for smaller drops. The turbulent viscous mechanism is most efficient at a downstream distance of eight gap heights into the outlet chamber for all drops sizes.     

Accuracy analysis of the window overlapping technique for velocity measurements using artificial and real signals

The feasibility of using window overlapping processing technique together with cross-correlation method was investigated for measuring instantaneous particle velocities using a fiber optic system. A pair of artificial signal pulses was generated to yield a known velocity field when they were cross-correlated. A parametric analysis was conducted to determine whether the window overlapping technique enhanced the velocity sampling frequency of the fiber optic probe system.The window overlapping technique was found to be effective in improving the accuracy of the instantaneous velocity calculations for relatively shorter data segments (windows). As the length of the window size increased, the efficiency of the window overlapping method diminished. The window overlapping technique was found to be effective only for window sizes up to four times longer than the minimum window size. The minimum window size contains sufficient number of data points that corresponds to the maximum velocity sampling frequency according to the Nyquist Theory. That is, the window overlapping technique could be beneficial for enhancing the velocity sampling frequency only if the velocity data (or the window size used in the cross-correlation function) were sampled at a frequency of four times lower (or better) than the highest frequency in the flow.Due to the hardware limitations, it is not possible to sample data at frequencies high enough to capture the highest frequency velocity fluctuations in a turbulent flow. Therefore, in this study, it was investigated that the velocity sampling frequency of a fiber optic probe could be enhanced by window overlapping technique using data collected from a highly turbulent particle laden pipe flow. The results indicated that the window overlapping technique slightly improved the accuracy of the instantaneous particle velocity measurements. This is believed to be due to the larger window sizes (or low sampling frequencies) relative to the high frequency velocity fluctuations in the flow. Therefore, the particle velocity fluctuations may not be computed accurately using a fiber optic probe due to the nature of the cross-correlation method which filters the velocity variations during the time period of ΔT. In order to obtain reasonably accurate results for measuring the particle velocity fluctuations in real life applications, the time interval (window size) must be sufficiently small such that it corresponds to the frequencies, at most, four times lower than the highest frequency occurring in the flow.     

Flow simulation in Electro-Static-Precipitator (ESP) ducts with turning vanes

Flow simulation in inlet ducts along with several turning vanes used in electrostatic precipitator (ESP) are analysed to understand the flow pattern at its exit locations. The geometry of inlet duct has been extracted from the Plant Design Manufacturing System (PDMS) and refined with turning vanes placed at several locations. The domain of duct geometry around turning vanes are decomposed with several volumes and filled with hexahedral elements with the help of stat-of-art mesh generator - Hypermesh. The resulting computational grid has been used in TASCflow solver to predict its flow pattern in the duct. Simulation for the specified conditions predicts uneven flow distribution in the ESP inlet duct. Due to large flow recirculation and turbulent losses in the duct, non-uniform averaged mass flow rates are noticed at duct exit locations. Simulation results suggest that the improvement of flow distribution in the duct through optimization can be tried by placing more turning/splitter vanes in the inlet duct. In order to ensure that the results obtained from TASCflow are meaningful and in right direction, in the absence of measurement data, simulation was benchmarked with other industry standard commercial flow solvers. The observations made from these popular solvers confirm the findings obtained using the TASCflow solver. The analysis with multiple solvers indicates that Fluent provides quick results, while better visualization can be made using CFX solver. The Star-CD solver, which captures the turbulent losses accurately takes more time for convergence provides alternatives.     

增强燃烧室内空气紊流降低柴油机油耗,噪声及有害排放物的研究

柴油机燃烧室中空气紊流运动对促进燃烧有重要作用。为改善Ｘ４１０５Ｂ柴油机性能，作采用了高紊流的“Ｑｕａｄｒａｍ”型燃烧室。通过合理选择进气涡流强度、供油速率和供油提前角，取得了良好的效果。不仅改善了燃油经济性，减小了烟度，而且降低了其排放物和噪声。     

Volume of Fluid Model for Turbulence Numerical Simulation of Stepped Spillway Overflow

The stepped spillway has increasingly become an effective energy dissipator. When the hydraulic performance of the overflow is clearly known, the energy dissipation could be increased. However, the study of stepped spillway overflow has been based only on model tests until now. In this paper, the k–? turbulence model is used to simulate the complex turbulence overflow. The unstructured grid is used to fit the irregular boundaries and the volume of fluid method is introduced to solve the complex free-surface problem. The free surface, velocities, and pressures on the stepped spillway are obtained by the turbulence numerical simulation. Furthermore, the simulation results compare well with measured data. The study indicates that the turbulence numerical simulation is an efficient and useful method for the complex stepped spillway overflow.