Depth-resolved numerical model of dam break mud flows with Herschel-Bulkley rheology

Mud flows are common phenomena in mountainous areas,which can threaten human safety and cause property losses under certain extreme circumstances.Studying the dynamic characteristics of mud flows,especially in the vertical direction,is helpful for risk reduction and hazard mitigation.In this study,a 2D depth-resolved numerical model based on Herschel-Bulkley rheology was developed to study the vertical structures of unsteady mud flows with a free-surface.The numerical model was solved by the projection method,and the free surface of mud flows was captured through the VOF method.To fully validate this new model,a series of laboratory experiments involving dam break mud flows were conducted,and the mud flow heights,bottom pressures and envelopes of mud residuum were measured.The numerical model proposed in this study was first validated by the steady-state solution for uniform flows of Herschel-Bulkley fluid on an inclined plane.Additionally,the simulated and measured mud flow heights,bottom pressures at different x locations and envelopes with different bed slopes showed good agreement.Furthermore,the numerical results for a Herschel-Bulkley fluid dam break flow were used to validate the proposed model,which further revealed good agreements.After that,the scenarios in which mud flows impact on a structure were numerically studied,and the vertical profiles of the front velocity and impact pressure on the structure were analyzed and discussed.The results show that a plug layer was formed in the mud flow under unsteady and nonuniform flow conditions,and the impact pressure on the structure was dominated by the dynamic pressure.In addition,the vertical position with the maximum impact pressure acting on the structure was not at the bottom or the surface of the mud flows,and the normalized vertical position rose as the yield stress and consistency coefficient increase for Herschel-Bulkley fluids.

Virtual-OCT:A simulated optical coherence tomography instrument

We report the virtual instrumentation of both time-domain(TD)and spectral-domain(SD)opticai coherence tomography(OCT)systems.With a virtual partial coherence source fromeither a simulated or measured spectrum,the OCT signals of both A-scan and B-scan weredemonstrated.The spectrometric detector's pixel number,dynamic range,noise,as well asspectral resolution can be simulated in the virtual spectral domain(SD-OCT).The virtual-OCT system provides an environment for parameter evaluation and algorithm optimization for ex-perimental OCT instrumentation,and promotes the understanding of OCT imaging and signal post-processing processes.The authors thank Dr.Thomas FitzGibbon forcomments on earlier drafts of the manuscript.

Adaptive-optical 3D microscopy for microfluidic multiphase flows

Measurements based on optical microscopy can be severely impaired if the access exhibits variations of the refractive index.In the case of fluctuating liquid-gas boundaries,refraction introduces dynamical aberrations that increase the measurement uncertainty.This is prevalent at multiphase flows(e.g.droplets,film flows)that occur in many technical applications as for example in coating and cleaning processes and the water management in fuel cells.In this paper,we present a novel approach based on adaptive optics for correcting the dynamical aberrations in real time and thus reducing the measurement uncertainty.The shape of the fluctuating water-air interface is sampled with a reflecting light beam(Fresnel Guide Star)and a Hartmann-Shack sensor which makes it possible to correct its influence with a deformable mirror in a closed loop.Three-dimensional flow measurements are achieved by using a double-helix point spread function.We measure the flow inside a sessile,oscillating 50-μl droplet on an opaque gas diffusion layer for fuel cells and show that the temporally varying refraction at the droplet surface causes a systematic underestimation of the flow field magnitude corresponding to the first droplet eigenmode which plays a major role in their detachment mechanism.We demonstrate that the adaptive optics correction is able to reduce this systematic error.Hence,the adaptive optics system can pave the way to a deeper understanding of water droplet formation and detachment which can help to improve the efficiency of fuels cells.

A novel needle probe for deeper photoacoustic viscoelasticity measurement

We present for the first time,to the best of our knowledge,a needle probe for photoacoustic viscoelasticity(PAVE)measurements at a depth of 1 cm below the sample surface.The probe uses a gradient index rod lens,encased within a side-facing needle(0.7 mm outer diameter),to direct excitation light(532 nm)and detection light(1325 nm)focused on the sample,collecting and directing the returned detection light in a spectral domain low coherence interferometry system,which allows for obtaining optical phase differences due to photoacoustic oscillations.The feasibility of needle probe for PAVE depth characterization was investigated on gelatin phantoms and in vitro biological tissues.The experimental results in an in vivo animal model predict the great potential of this technique for in vivo tumor boundary detection.