Numerical Investigation Into the Effect of Air Swirl on Non-Premixed Combustion

Endeavour is made to investigate the effect of swirled air on methane-air combustion in a Harwell combustor geometry. The inlet air swirl intensity on combustion characteristics such as temperature, pollutant formation, and flow dynamics is studied. The modeling of turbulent characteristics is performed with the standard K–ε model using ANSYS Fluent. Eddy dissipation model with one step reaction is used for modeling chemical reaction and P-I radiation model for radiation heat transfer. The swirl number is achieved in the range of 0.0 to 0.6, by varying the tangential velocity to the air inlet. With the increase in swirl intensity, the maximum flame temperature drops, and most of the flame formation shifts towards the inlet of the furnace. The change in the flow field is aided by the formation of recirculating bubbles. The swirl causes the flame to spread radially away from the axis, thereby increasing the heat transfer flux to the furnace wall. As a result, a significant reduction in the formation of NO pollutants is observed.

Turbulent Characteristics and Air Entrainment Patterns in Breaking Surge Waves

Breaking surge waves are highly turbulent three-dimensional (3D) flows, which occur when the water flow encounters a sudden change in depth or velocity. The 3D turbulent structures across a breaking surge are induced by the velocity gradient across the surge and phase discontinuity at the front. This paper examined the turbulent structures in breaking surge waves with Froude numbers of 1.71 and 2.13 by investigating the air entrainment and perturbation patterns across the surge front. A combination of the Volume Of Fluid (VOF) method and Large Eddy Simulation (LES) was utilized to capture air entrainment and turbulent structures simultaneously. The 3D nature of the vortical structures was simulated by implementing a spanwise periodic boundary. The water surface perturbation and air concentration profiles were extracted, and the averaged air concentration profiles obtained from the numerical simulations were consistent with laboratory observations reported in the literature. The linkage between turbulent kinetic energy distribution and air entrainment was also explored in this paper. Finally, using quadrant analysis and the Q-criterion, this paper examined the role of the spanwise perturbations in the development of turbulent structures in the surge front.

Influence of Upstream Disturbances on the Vortex Structure of Francis Turbine Based on the Criteria of Identification of Various Vortexes

The inter-blade passage vortex, the vortex rope of the draft tube, and the vortex in the guide apparatus are the characteristics of flow instability of the Francis turbine, which may lead to fatigue failure in serious cases. In the current study, in order to accurately capture the transient turbulent characteristics of flow under different conditions and fully understand the flow field and vortex structure, we conduct a simulation that adopts sliding grid technology and the large-eddy simulation (LES) method based on the wall-adapting local eddy viscosity (WALE) model. Using the pressure iso-surface method, the Q criterion, and the latest third-generation Liutex vortex identification method, this study analyzes and compares the inter-blade passage vortex, the vortex rope of the draft tube, and the outflow and vortex in the guide apparatus, focusing on the capture ability of flow field information by various vortex identification methods and the unique vortex structure under the condition of a small opening. The results indicate that the dependence of Liutex on the threshold is small, and the scale range of the flow direction vortex captured by Liutex is wider, but the ability of the spanwise vortex is relatively weak. The smaller the opening, the more disorderly the vortexes generated in each component and the more unstable the flow field. In the draft tube, the original shape of the vortex rope is destroyed due to the interaction between vortexes. Under the condition of a small opening, an inter-blade passage vortex is generated, affecting the efficient and stable operation of the turbine.

Atmospheric Optical Turbulence Characteristics over the Ocean Relevant to Astronomy and Atmospheric Physics

Due to the space and time constraints of turbulence measurement equipment and the experiment scene, it is difficult to obtain the atmosphere refractive index structure constant over the ocean. In this paper, the characteristics of atmospheric optical turbulence in offshore and open ocean conditions are summarized by analyzing the meteorological data obtained from two ocean atmospheric optical parameter field experiments. Because of the influence of land undersurface, the turbulence strength in offshore conditions is roughly the same as that on land and presents different characteristics in open ocean. Compared with the offshore area, the turbulence strength over the open ocean near-surface decreases during the day and increases at night, and the diurnal variation characteristics weaken. The turbulence strength profiles over the offshore area show different characteristics at different times, where the turbulence strength in the morning is higher than that in the evening. By retrieving the meteorological factors affecting the turbulence, it is found that the temperature gradient and wind shear are in good agreement with turbulence strength in both offshore and open ocean areas. Furthermore, the integrated parameters for astronomy and optical telecommunication are derived from profiles over the offshore and open ocean areas. It is of great significance to research the turbulent characteristics of ocean atmosphere for optical transmission and astronomical observations.

Experimental Study on the Influence Factors of Flow Velocity Structure and Turbulent Characteristics in Open Channel with Biomimetic Grass

The problem of suspension treatment of subsea oil-gas pipelines has been highly concerned by engineering construction units and researchers. The current research indicates that the bionic sea grass can effectively reduce the flow rate, promote sediment deposition, and control the development of the pipeline suspension area. The velocity distribution of open channel flow with bionic grass is very complex. The height and laying space of bionic grass will affect the flow velocity distribution. At present, the flow velocity in open channels with bionic grass is mainly studied by measuring the velocity variation at the front, middle, and back of bionic grass. Few effective measurements are made for the full velocity field. The velocity field distribution of bionic aquatic grass along the vertical plane is measured by using standard particle image velocimetry (PIV). The effects of height and laying space of bionic grass on probability density distribution, spatial correlation of pulsating velocity, turbulence intensity, Reynolds stress and turbulent kinetic energy in the open channel after the protection section of bionic grass are further analyzed.

Performance of Unsteady Reynolds-Averaged Navier-Stokes and Hybrid Scale-Resolving Simulation Approaches in Simulating a Low-Speed Axial Compressor Rotor

Aerodynamics phenomena in compressors are highly unsteady and turbulent. Selecting a proper turbulence-modeling method is significant to reveal the complex flow mechanism in turbomachines. In the current paper, the shear stress transport (SST) model as an unsteady Reynolds-averaged Navier-Stokes (RANS) method, the scale-adaptive simulation (SAS) model, and the zonal wall-modeled large eddy simulation (zonal-LES) as two hybrid scale-resolving simulation approaches have been compared. These turbulence-modeling methods were employed to simulate a single rotor of a low-speed research compressor featuring a tip clearance of 1.3% of chord length. Comparisons were made between the simulation results and the experimental data at three operating points, and the flow fields at the design point have been specifically discussed in detail. The results show that the advantage of the zonal-LES model becomes obvious as the compressor throttles. The zonal-LES model brings a significant improvement over both the SST model and the SAS model in capturing the experimental data, especially the velocity distribution in the low-span region, as well as the loss near the endwalls. The SAS model as a scale-resolving method presents no benefits in predicting the relevant flow compared with the SST model, as the activation of the SAS source term is limited for this test case. For the loss prediction, the variation in the upper half-span region is mainly due to the different results in modeling turbulent characteristics of the tip leakage flow, whereas the mechanism behind the higher loss at the lower half-span predicted by the zonal-LES model is a consequence of the complex topology of the corner separation and the intensive mixing.

Numerical Study of the Influence of the Inlet Turbulence Length Scale on the Turbulent Boundary Layer

In the past half century, large eddy simulations (LESs) have played an important role in turbulent flow simulation and improving the performance of computing technology. To generate a fully developed turbulent boundary layer in the channel domain using LES, suitable inflow conditions along with turbulent characteristics are required. This study aimed to clarify the effect of the integral length scale on the generation of turbulent boundary layers. To accomplish this, an artificially created boundary layer was imposed on the inlet section, which gradually evolved into a fully developed turbulent boundary layer flow inside the numerical domain. In this study, the synthetic inflow method, which is a commonly employed technique, was used by imposing the spatial and temporal correlation between two different points on the inlet section. In addition, we conducted parametric length scale studies on the inlet section and compared our results with existing data. The results showed that the larger length scales in the spanwise direction were not only effective in achieving the target shape of a fully developed turbulent boundary layer, but also developed it faster than the smaller length scales.