Lower bound for transient growth of inclined buoyancy layer

The relationship between stabilities of the buoyancy boundary layers along an inclined plate and a vertical plate immersed in a stratified medium is studied theoretically and numerically. The eigenvalue problem of energy stability is solved with the method of descending exponentials. The disturbance energy is found to be able to grow to 11.62 times as large as the initial disturbance energy for Pr = 0.72 when the Grashof number is between the critical Grashof numbers of the energy stability and the linear stability. We prove that, with a weighted energy method, the basic flow of the vertical buoyancy boundary layer is stable to finite-amplitude streamwise-independent disturbances.

Identification, characterization and evolution of non-local quasi-Lagrangian structures in turbulence

The recent progress on non-local Lagrangian and quasi-Lagrangian structures in turbulence is reviewed.The quasi-Lagrangian structures, e.g., vortex surfaces in viscous flow, gas-liquid interfaces in multi-phase flow, and flame fronts in premixed combustion, can show essential Lagrangian following properties, but they are able to have topological changes in the temporal evolution. In addition,they can represent or influence the turbulent flow field. The challenges for the investigation of the non-local structures include their identification, characterization, and evolution.The improving understanding of the quasi-Lagrangian structures is expected to be helpful to elucidate crucial dynamics and develop structure-based predictive models in turbulence.

Viscous Rayleigh-Taylor instability with and without diffusion effect

The approximate but analytical solution of the viscous Rayleigh-Taylor insta- bility （RTI） has been widely used recently in theoretical and numerical investigations due to its clarity. In this paper, a modified analytical solution of the growth rate for the viscous RTI of incompressible fluids is obtained based on an approximate method. Its accuracy is verified numerically to be significantly improved in comparison with the previous one in the whole wave number range for different viscosity ratios and Atwood numbers. Fur- thermore, this solution is expanded for viscous RTI including the concentration-diffusion effect.

Droplet impact on wetted structured surfaces

In this study,we numerically investigate the droplet impact onto a thin liquid film deposited on a structured surface with square pillars and cavities.The time evolution of crown geometry is strongly affected by the surface structure.When the thickness of the liquid film is larger than the structure height,the expanding speed of the crown base radius is independent of the structure width.However,if the liquid film thickness is equal to the structure height,the crown base expands slower as the structure width increases.Surface structures have strong effects on the crown height and radius,and can prevent ejected filament from breaking into satellite droplets for certain cases.For the liquid film with the thickness equal to the pillar height,both the crown height and the radius exhibit non-monotonic behaviors as the pillar width increases.There exists one pillar width which produces the smallest crown height and the largest crown radius.

Deflection Reduction of GaN Wafer Bowing by Coating or Cutting Grooves in the Substrates

GaN films on sapphire substrates are obtained using the metal-organic chemical vapor deposition growth technique.We present two methods to reduce the GaN wafer bowing caused by the mismatch of the thermal expansion coefficients(TECs)between the film and the substrate.The first method is to use coating materials on the back side of the substrate whose TECs are smaller than that of the GaN films.The second is to cut grooves on the back side of the sapphire substrate and filling the grooves with appropriate materials(e.g.,tungsten,silicon nitride).For each method,we minimize wafer bowing and even reduce it to zero.Moreover,the two methods can reduce stress concentration and suppress the propagation of cracks in the GaN/sapphire structure.

Review on laminar flame speed and its measurement from outwardly propagatingspherical flames

The laminar flame speed is one of the most fundamental properties of a fuel/air mixture.It determines the fuel burning rate in combustion engines and is an important target used to validate chemical models.Due to its importance,accurate measurement of the laminar flame speed receives great attention and different experimental methods have been developed.This review first introduces the laminar flame speed as well as its significance and dependence on different factors.Then,different experimental methods for the laminar flame speed measurement are described.Since the outwardly propagating spherical flame method is currently the most popular method for laminar flame speed measurement,its challenges and recent advances are reviewed.Both the constant pressure method and constant volume method using propagating spherical flames are discussed.

A dislocation-based model for cyclic plastic response of lath martensitic steels

Softening behavior of lath martensitic steels is related to the coarsening of laths and dislocation evolution during cyclic deformation.Involving the physical mechanism,we developed a dislocation-based model to study the cyclic plastic response for lath martensitic steels.For a block,we proposed an interfacial dislocation evolution model to physically present the interaction between mobile dislocations in the block and interfacial dislocations by considering the coarsening mechanism of the laths.Moreover,the evolution behavior of backstress caused by dislocation pile up at the block boundary has been considered.Then,a hierarchical model based on the elastic-viscoplastic self-consistent(EVPSC)theory is developed,which can realize the scale transition among representative volume element(RVE),prior austenite grains(PAGs)and blocks.According to the proposed model,the effective mechanical responses including the cyclic hysteretic loop and peak stress at different cycles for lath martensitic steel have been theoretically predicted and investigated.