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In the present research, a possible generation mechanism of low-frequency buffeting phenomenon based on a 1:15 open jet automotive wind tunnel was investigated. Evolution of vortex structures and pressure field in the plenum chamber have been visualized and analyzed by Large-eddy simulation (LES). It is shown that the low frequency pressure fluctuation is caused by the large-scale structures and their interaction. Multiple proper orthogonal decomposition was adopted to analyze the flow field in the plenum chamber. The characteristic frequencies of the vortex-rings after pairing is the same as the dominant resonance of buffeting at this wind velocity.

In this study, the Taylor-Couette flow was disturbed by incorporating annular fins over the inner rotating surface. The finned surface had three parameters: height, width, and length between fins. In this work, seven different fin configurations, in which only the fin height varied, were examined and compared using experimental and numerical techniques. We found that annular fins disturbed the flow behavior by reducing the smooth critical Taylor number (Ta=57.18), but more important, we noticed a vortex enlargement induced by the incorporation of a relatively modest perturbation (b´<0.5) and this change remained in a wider range (0.57<T<14.18). On the other hand, it was identified the appearance of smaller secondary vortices just over the fins, it is as a result of an increment in fin height. The relevance of this finding lies on the field of micro-mixing processes.

The pantograph monitoring device on high-speed trains bears not only its own strength but also the aerodynamic load applied by the air flow when the train is running at high speed. A well designed shape of the pantograph monitoring device on high-speed trains reduces the loads and pressure fluctuations acting on it, and therefore, increases its function stability and life cycle. In this paper, we present an aerodynamic shape design method for such device. Firstly, an efficient and reliable numerical simulation approach is established for the evaluation of the aerodynamic loads acting on the device. According to the numerical computations, a basic shape for the monitoring device is formed, with which the minimum functional space of the device is reserved. Then, the corners of the basic shape are smoothed out with three types of continuous transitions. By comparing the numerical results of the three smoothed shapes, we obtain an optimal aerodynamic shape for the pantograph monitoring device. The design method is not limited to the monitoring device studied in this manuscript. The aerodynamic shape of other small functional devices on high-speed trains can also be generated or optimized with the method presented herein.

This research paper deals with the use of a dam break system to generate a surge model to study tsunami run-up, run-down and scouring around a vertical cylinder. The dam break system was provided with one or two gates to store water at a predefined depth; the water could then be quickly released by opening the gates to create a tsunami surge that runs up on land. In addition, numerical simulations of dam break surges resulting from various lengths of reservoirs were conducted to obtain more findings for further analysis regarding the characteristics of the dam break surges. A vertical cylinder model was installed on the beach at 6 meter downstream of the main gate to study the scour caused by the tsunami surge. The bed material was fine sand with a 0.19 mm diameter. The results were compared with existing experimental results. The comparison indicated that the dam break surge can be used to simulate tsunami surges by adjusting the reservoir length, the reservoir depth and the water depth downstream of the gate. The ratio of the difference between the upstream and downstream water depth on one side to the length of the reservoir on the other affect the run-up height and duration. Thus, this ratio should be considered when simulating tsunami based on dam break systems. Although different in magnitude, the shape of both the surge mareogram and the velocity time history of the tsunami surge generated using dam break system was comparable with the tsunami surge induced by a solitary wave generated using a long flume. For the relatively large cylinder located at the bore location, the separation flow was strongly directed to the wall which produced significantly unsymmetrical scour result.

This article discusses the importance of using different turbulence modulation models in simulation of evaporating sprays. An in-house CFD code has been modified to take into account the effect of considering turbulence modulation by standard or consistent models. These models may predict an augmentation (consistent model) or a reduction (standard model) in the turbulence kinetic energy of continuous phase. Calculations are done in a Eulerian-Lagrangian framework and the effect of injected droplets on turbulent kinetic energy and its rate of dissipation is included in the equations of the continuous phase. Results are shown to be valid by comparing them to Sandia spray A configuration experimental data. Results show that considering the effect of existing droplets in a turbulent combustion chamber can play a major role in having a more accurate CFD simulation. These models can alter the velocity field drastically when droplets are injected into the chamber with a high velocity. As a result, spray characteristics such as evaporation rate is also altered. It can be concluded that modulation models should be used in the simulation of evaporating sprays in order to attain more accurate and realistic results.

Shear stress is a parameter of high significance. Through knowledge of this parameter, assessment of scour or sedimentations at different points of bed is made viable. Therefore, this paper investigated alterations in shear stress along the bend, specifically around a bridge pier, under the influence of applying submerged vanes at the upstream side of the bridge pier. With the aim of modeling submerged vanes, vanes of Plexiglas with a thickness of 20% of the pier diameter, a length of 1.5 times the pier diameter, and submergence ratio of 75% were utilized. The vanes were installed at a distance equal to 5 times the pier diameter from the pier center at a distance of 40 to 60% of the channel width from the inner bank at the upstream side of the bridge pier. Acoustic-Doppler Velocity velocimeter device was utilized for measuring three-dimensional velocity components. The experiments were conducted in a 1-meter-wide flume with a degree of curvature of 180. The results of the study suggested that upon reaching the bend apex, the maximum flow turbulence rate occurred in a transverse direction in the case of installing submerged vanes at a distance of 40% of the channel width from the inner bank towards the inner wall; while in the case of installing submerged vanes at a distance of 60% of the channel width from the inner bank, it occurred towards the outer wall, and it could be observed that the maximum longitudinal and vertical components of turbulence rate increased by 16 and 5.5% respectively upon increase in the distance of submerged vanes from the inner bank. Furthermore, the values of and turbulence shear stresses at the outer bank in the case of installing the vanes at a distance of 40% of the channel width from the inner bank were smaller than those in the case of installing the submerged vanes at a distance of 60% of the channel width from the inner bank.

In this study, the synthetic jet position was optimized to obtain the maximum rate of heat transfer and the best state of temperature uniformity on a heated surface in micro-channels. Based on micro-channel length, several cases were simulated to investigate the effects of synthetic jet position on the heat transfer rate and temperature uniformity. After that, the synthetic jet position was optimized using the CFD results and the GMDH-MOGA optimization code. The obtained results show that the synthetic jet placement in all longitudinal positions of micro-channel increases the heat transfer rate, although the improvement of temperature uniformity of heated surface decreases at some positions as compared to the micro-channel without synthetic jet. The optimization results show that for obtaining the maximum value of heat transfer and the best state of temperature uniformity on the heated surface, the dimensionless longitudinal position of synthetic should be between 0.45 and 0.65. The maximum rate of heat transfer and the best state of temperature uniformity have been observed in the vicinity of lower and upper bounds of this range, respectively.

An experimental study was carried out to investigate the effects of recess length and mixture ratio on the discharge coefficient of bi-swirl coaxial injectors with inner closed-type and outer open-type swirl injectors. Ten bi-swirl coaxial injectors were classified into two groups with different recess lengths. By independently varying the mass flow rates through the inner and outer injectors, the discharge coefficients of the injectors were obtained. Single-injection cold-flow tests indicated that the discharge coefficients of both the inner and outer swirl injectors were only marginally affected by the recess length and mass flow rate. Bi-injection cold-flow tests showed that the discharge coefficients of the inner swirl injectors were also almost constant, regardless of the recess length and mixture ratio. On the other hand, those of the outer swirl injectors in the tip-mixing and internal-mixing bi-swirl coaxial injectors with long recess lengths had significantly decreased with the increase in mixture ratio. A novel empirical equation for the discharge coefficient of the outer swirl injector in the internal-mixing bi-swirl coaxial injector is suggested through a linear regression analysis of the present test data. It was found that the present empirical equation could accurately predict the experimental data.

This paper investigates the accuracy of Reynolds-averaged Navier-Stokes (RANS) turbulence modelling applied to complex industrial applications. In the context of the increasing instability of the energy market, hydropower plants are frequently working at off-design parameters. Such operation conditions have a strong impact on the efficiency and life span of hydraulic turbines. Therefore, research is currently focused on improving the design and increasing the operating range of the turbines. Numerical simulations represent an accessible and cost efficient alternative to model testing. The presented test case is the Porjus U9 Kaplan turbine model operated at best efficiency point (BEP). Both steady and unsteady numerical simulations are carried out using different turbulence models: k-epsilon, RNG k-epsilon and k-omega Shear Stress Transport (SST). The curvature correction method applied to the SST turbulence model is also evaluated showing nearly no sensitivity to the different values of the production correction coefficient Cscale. The simulations are validated against measurements performed in the turbine runner and draft tube. The numerical results are in good agreement with the experimental time-dependent velocity profiles. The advantages and limitations of RANS modelling are discussed. The most accurate results were provided by the simulations using the k-epsilon and the SST-CC turbulence models but very small differences were obtained between the different tested models. The precision of the numerical simulations decreased towards the outlet of the computational domain. In a companion paper, the pressure profiles obtained numerically are investigated and compared to experimental data.

The aim of the paper is to investigate the limitations of unsteady Reynolds-averaged Navier-Stokes (RANS) simulations of the flow in an axial turbine. The study is focused on modelling the pressure pulsations monitored on the runner blades. The scanned blade geometry renders the meshing process more difficult. As the pressure monitor points are defined on the blade surface the simulation relies on the wall functions to capture the flow and the pressure oscillations. In addition to the classical turbulence models, a curvature correction model is evaluated aiming to better capture the rotating flow near curved, concave wall boundaries. Given the limitations of Reynolds-averaged Navier-Stokes models to predict pressure fluctuations, the Scale Adaptive Simulation-Shear Stress Transport (SAS-SST) turbulence model is employed as well. The considered test case is the Porjus U9, a Kaplan turbine model, for which pressure measurements are available in the rotating and stationary frames of reference. The simulations are validated against time-dependent experimental data. Despite the frequencies of the pressure fluctuations recorded on the runner blades being accurately captured, the amplitudes are considerably underestimated. All turbulence models estimate the correct mean wall pressure recovery coefficient in the upper part of the draft tube.

In this study the dynamical characteristics of the salinity front between the Persian Gulf inflow and outflow were studied using the HYCOM numerical model. This model was integrated for 5 years from the beginning of 2011 to the end of 2015and the results of 2015 were discussed. The results of the model clearly showed seasonal variations in the salinity front in which the intrusion of the salinity front extends much farther into the Persian Gulf in summer. The salinity front appears to be prone to baroclinic instability with maximum intensity in spring and summer months (with a strong density stratification), forming cyclonic eddies (saline center) and anticyclones (sweeter center), that peaks in August. Results showed that some anti-cascade processes occur in mesoscale eddy activity, in agreement with the quasi-two-dimensional turbulence behavior. Spectral analysis of salinity time series in the front showed eddies with time scales ranging from a few hours to about 3 months. The result also showed that there was a reasonable relation between mixed layer depth and the formation of mesoscale eddies, so that mesoscale eddies disappeared when the thickness of mixed layer was increased in winter.

The aerodynamic characteristics are concerned with the fuel consumption rate and the stability of a high speed vehicle. The current research aims at studying the aerodynamic behavior of a typical SUV vehicle model mounted with the vortex generator (VG) at various linear positions with reference to its rear roof edge. The flow field around the vehicle model was observed at different wind speed conditions. It had been determined that at the instance of lower wind speed, the VG had minimal effects of aerodynamic drag on the vehicle body. However, at the instance of higher wind speed conditions the magnitude of the drag force decreased significantly. Vehicles move at higher speeds in the highways, location of the VG varied towards the upstream of the vehicle due to early flow separation. Therefore test were conducted at different wind speeds and locations of VG. The numerical simulation conduced in this study provides flow characteristics around the vehicle model for different wind speeds. The realizable k−ε model was used to simulate and validate the empirical results in an effective manner. By using experimental data, the drag was reduced by 9.04 % at the optimized VG location. The results revealed that the induced aerodynamic drag would determine the best car shape. This paper provides a better understanding of VG positioning for enhanced flow separation control.

Pigging is a routine operation in the oil and gas industry. In this paper, the governing equation of pig speed was combined with the gas flow equations. The transient equations of gas flow are solved by the method of characteristics (MOC). An experiment was carried out to test the proposed pigging model. The measured speed of the pig coincides with the calculated speed well. The process of a pig carrying a brake unit to pass over a hilly gas pipeline is simulated. The results indicate that the brake unit would lead to a sharp increase of the pressure on the tail of the pig, because the pig is dragged by the brake unit and thus prevented to accelerate together with the gas column in a downhill gas pipeline. This way, the pig speed in a downhill gas pipeline is much lower by using a brake unit, but the speed of pig still can hardly be controlled in the desired range. Furthermore, response surface methodology (RSM) is used to study the maximum speed of pig with/without a brake unit in downhill gas pipeline. Based on the results of the RSM simulations, two equations are present to predict the maximum speed of a pig in a downhill gas pipeline.

In this numerical study, a laminar separation bubble is simulated by imposition of suction to create an adverse pressure gradient. The DNS elucidates the entire transition process over the separation bubble leading to turbulence. Several important conclusions are drawn from the simulations regarding the origins of transition and evolution of turbulence. Break down to turbulence, preceded by three-dimensional motions and non-linear interactions, occurs in the second half of the mean bubble length. Two topological structures of the bubble causing vortex shedding are suggested; one for the normal shedding and the other for the low frequency flapping. The normal shedding frequency can be attributed to the regular shedding of smaller vortices while shedding of large vortices formed due to coalescence of smaller vortices results in the low-frequency flapping. Due to the shedding of bigger vortices, the instantaneous reattachment point varies greatly resulting in large variation in the instantaneous bubble length. Break down of longitudinal streaks, appearing via Λ-vortices and vortex stretching mechanism, characterizes the transition process. Low values of reverse flow suggest that a convective instability is involved. The instability analysis indicates that the initial amplification of disturbances is due to T-S mechanism while the roll-up of the shear layer takes place due to Kelvin-Helmholtz instability.