Numerical Modeling of Local Scour Around Hydraulic Structure in Sandy Beds by Dynamic Mesh Method

Local scour, a non-negligible factor in hydraulic engineering, endangers the safety of hydraulic structures. In this work, a numerical model for simulating local scour was constructed, based on the open source code computational fluid dynamics model Open FOAM. We consider both the bedload and suspended load sediment transport in the scour model and adopt the dynamic mesh method to simulate the evolution of the bed elevation. We use the finite area method to project data between the three-dimensional flow model and the two-dimensional(2D) scour model. We also improved the 2D sand slide method and added it to the scour model to correct the bed bathymetry when the bed slope angle exceeds the angle of repose. Moreover, to validate our scour model, we conducted and compared the results of three experiments with those of the developed model. The validation results show that our developed model can reliably simulate local scour.

On the hydrodynamics of hydraulic machinery and flow control

Hydraulic machinery mainly includes turbine and pump, which is closely related to national economy and people＇s livelihood involving aerospace industry, marine engineering, hydropower engineering, petroleum industry, chemical industry, mining industry, biomedical engineering, environmental engineering, agricultural water-soil engineering, etc.. The internal flow of hydraulic machinery is extremely complex, and its characteristics can be summarized as high Reynolds number, multi-scales, inhomogeneous and vortex-dominant unsteady turbulence which interact with the rotating dynamic boundary（rotor blade）. Based on the analysis of the internal flow characteristics of hydraulic machinery, the author and his research team successively proposed a rotation correction model, a curvature corrected filter-based model, a scalable detached eddy simulation method, and a non-linear hybrid RANS/LES turbulence model to capture unsteady flow structures and then predict hydraulic performance and dynamic characteristics more accurately. According to the analysis on the internal flow, the corresponding flow control measures were put forward. It was verified by experiments that these methods could significantly improve the hydraulic performance, anti-cavitation performance and dynamic characteristics（pressure pulsation and vibration） of hydraulic machinery in a certain range of operating conditions. In addition, the mechanism how flow control measures influence internal flow was analyzed in depth, aiming at finding a feasible and effective way to improve hydraulic performance, anti-cavitation performance and dynamic characteristics of hydraulic machinery.

Flow regime and head loss in a drip emitter equipped with a labyrinth channel

Labyrinth channels are widely adopted in emitter designs to regulate the water flow.The flow regime and the head loss of labyrinth channels have significant impacts on the hydraulic performance of emitters.In this study,the flow behavior of water passing through an emitter channel is observed using the micro particle image velocimetry（PIV）,and the head loss during the flow is analyzed for an emitter with a triangular labyrinth channel.The results show that the flow regime is consistent with the classical theory of hydraulics governing straight channels,even when the cross-sectional area is very small（as small as 0.5 mm×0.5 mm）.The critical Reynolds number from laminar to turbulent flows in a labyrinth channel is approximately in a range between 43 and 94.The local head loss factor decreases as the Reynolds number increases for labyrinth channels with smaller cross-sectional areas,such as 0.5 mm×0.5 mm and 1.0 mm×1.0 mm.The local head loss factor is not related to the Reynolds number and is only a function of the boundary conditions of the labyrinth channel when the Reynolds number exceeds approximately 1 000（for cross-sectional areas of 1.5 mm×1.5 mm and 2.0 mm×2.0 mm）.The ratio of the local head loss to the total head loss total（ /）j fh h first increases and then remains nearly constant as the Reynolds number increases in the labyrinth channel.The head loss in the labyrinth channel is almost equal to the local head loss,and total（ h_j/h_（ftctal））is approximately 0.95 for cross-sectional areas of greater than 1.0 mm×1.0 mm.These results can be used for optimizing the design of emitter channels.

Free-surface air entrainment in open-channel flows

In hydraulic engineering,free-surface aeration is a natural phenomenon occurring in smooth channel flows.In self-aerated flows,a key aspect that has not yet been well understood is the formation mechanism of free-surface air entrainment.In this research,the process of free-surface entrapped deformation is analyzed theoretically and the critical radius of curvature for air entrainment is obtained,affected by flow mean velocity and depth.When the severity of local free-surface deformation exceeds the critical condition,the entrapped free surface encounters closure in the unstable deformation movement process,resulting in air entrainment.This inference agrees well with observed experimental results that are obtained from the processes of surface entrapped deformation and air entrainment captured by a high-speed camera-based data acquisition system.This agreement indicates that self-aeration occurs in low-velocity open-channel flows.It is also confirmed that free-surface turbulent deformation provides a mechanism for air entrainment.