A hybrid interface tracking – level set technique for multiphase flow with soluble surfactant

Shin, Seungwon,Chergui, Jalel,Juric, Damir,Kahouadji, Lyes,Matar, Omar K.,Craster, Richard V. Elsevier 2018 Journal of computational physics Vol.359 No.-

<P><B>Abstract</B></P> <P>A formulation for soluble surfactant transport in multiphase flows recently presented by Muradoglu and Tryggvason (JCP 274 (2014) 737–757) is adapted to the context of the Level Contour Reconstruction Method, LCRM, (Shin et al. IJNMF 60 (2009) 753–778, ) which is a hybrid method that combines the advantages of the Front-tracking and Level Set methods. Particularly close attention is paid to the formulation and numerical implementation of the surface gradients of surfactant concentration and surface tension. Various benchmark tests are performed to demonstrate the accuracy of different elements of the algorithm. To verify surfactant mass conservation, values for surfactant diffusion along the interface are compared with the exact solution for the problem of uniform expansion of a sphere. The numerical implementation of the discontinuous boundary condition for the source term in the bulk concentration is compared with the approximate solution. Surface tension forces are tested for Marangoni drop translation. Our numerical results for drop deformation in simple shear are compared with experiments and results from previous simulations. All benchmarking tests compare well with existing data thus providing confidence that the adapted LCRM formulation for surfactant advection and diffusion is accurate and effective in three-dimensional multiphase flows with a structured mesh. We also demonstrate that this approach applies easily to massively parallel simulations.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Extension of the LCRM Front-tracking method (Shin et al. IJNMF 60 (2009) 753–778) to flows with surfactant. </LI> <LI> Following Muradoglu and Tryggvason (JCP 274 (2014) 737–757) surfactant transport is solved on the interface and in the bulk. </LI> <LI> Accuracy demonstrated for mass conservation, surface advection and diffusion, bulk transport and Marangoni stresses. </LI> <LI> Large scale parallel calculations of two-phase annular film flow in the counter-current flow regime. </LI> </UL> </P>

A Generic Hybrid Model for the Simulation of Three-Dimensional Bulk Elastodynamics for Use in Nondestructive Evaluation

Choi, W.,Skelton, E. A.,Pettit, J.,Lowe, M. J. S.,Craster, R. V. IEEE 2016 and Frequency Control Vol.63 No.5

<P>A three-dimensional (3-D) generic hybrid model is developed for the simulation of elastic waves in applications in nondestructive evaluation (NDE) that efficiently links different solution strategies but, crucially, is independent of the particular schemes employed. This is an important step forward in facilitating rapid and accurate large-scale simulations, and this advances the two-dimensional (2-D) generic hybrid methodology recently developed by the authors. The hybrid model provides an efficient and effective tool for creating highly accurate simulations that model the wave propagation and scattering, enabling the interpretation of inspection data; the new methodology is verified against other numerical simulations. Furthermore, its deployment to simulate wave reflection from side-drilled holes (SDHs), comparing the results with experimental measurements, provides a realistic demonstration as well as further validation.</P>

The validity of Kirchhoff theory for scattering of elastic waves from rough surfaces

Shi, F.,Choi, W.,Lowe, M. J. S.,Skelton, E. A.,Craster, R. V. The Royal Society 2015 Proceedings, Mathematical, physical, and engineeri Vol.471 No.2178

<P> The Kirchhoff approximation (KA) for elastic wave scattering from two-dimensional (2D) and three-dimensional (3D) rough surfaces is critically examined using finite-element (FE) simulations capable of extracting highly accurate data while retaining a fine-scale rough surface. The FE approach efficiently couples a time domain FE solver with a boundary integration method to compute the scattered signals from specific realizations of rough surfaces. Multiple random rough surfaces whose profiles have Gaussian statistics are studied by both Kirchhoff and FE models and the results are compared; Monte Carlo simulations are used to assess the comparison statistically. The comparison focuses on the averaged peak amplitude of the scattered signals, as it is an important characteristic measured in experiments. Comparisons, in both two dimensions and three dimensions, determine the accuracy of Kirchhoff theory in terms of an empirically estimated parameter <I>σ</I><SUP>2</SUP> /λ 0 ( <I>σ</I> is the RMS value, and λ 0 is the correlation length, of the roughness), being considered accurate when this is less than some upper bound <I>c</I> , ( <I>σ</I><SUP>2</SUP> /λ 0 < <I>c</I> ). The incidence and scattering angles also play important roles in the validity of the Kirchhoff theory and it is found that for modest incidence angles of less than 30°, the accuracy of the KA is improved even when <I>σ</I><SUP>2</SUP> /λ 0 > <I>c</I> . In addition, the evaluation results are compared using 3D isotropic rough surfaces and 2D surfaces with the same surface parameters. </P>

A time-domain finite element boundary integration method for ultrasonic nondestructive evaluation

Fan Shi,Wonjae Choi,Skelton, Elizabeth A.,Lowe, Michael J. S.,Craster, Richard V. IEEE 2014 and Frequency Control Vol.61 No.12

<P>A 2-D and 3-D numerical modeling approach for calculating the elastic wave scattering signals from complex stress-free defects is evaluated. In this method, efficient boundary integration across the complex boundary of the defect is coupled with a time-domain finite element (FE) solver. The model is designed to simulate time-domain ultrasonic nondestructive evaluation in bulk media. This approach makes use of the hybrid concept of linking a local numerical model to compute the near-field scattering behavior and theoretical mathematical formulas for postprocessing to calculate the received signals. It minimizes the number of monitoring signals from the FE calculation so that the computation effort in postprocessing decreases significantly. In addition, by neglecting the conventional regular monitoring box, the region for FE calculation can be made smaller. In this paper, the boundary integral method is implemented in a commercial FE code, and it is validated by comparing the scattering signals with results from corresponding full FE models. The coupled method is then implemented in real inspection scenarios in both 2-D and 3-D, and the accuracy and the efficiency are demonstrated. The limitations of the proposed model and future works are also discussed.</P>

Rough surface reconstruction of real surfaces for numerical simulations of ultrasonic wave scattering

Choi, Wonjae,Shi, Fan,Lowe, Michael J.S.,Skelton, Elizabeth A.,Craster, Richard V.,Daniels, William L. Elsevier 2018 NDT & E international Vol.98 No.-

<P><B>Abstract</B></P> <P>The scattering of waves by rough surfaces plays a significant role in many fields of physical sciences including ultrasonics where failure surfaces are often rough and their accurate identification is critical. The prediction of the strength of scattering can be hampered when the roughness is not adequately characterised and this is a particular issue when the surface roughness is within an order of the incident wavelength. Here we develop a methodology to reconstruct, and accurately represent, rough surfaces using an AutoRegressive (AR) process that then allows for rapid numerical simulations of ultrasonic wave rough surface scattering in three dimensions. Gaussian, exponential and AR surfaces are reconstructed based on real surface data and the statistics of the surfaces are compared with each other. The statistics from the AR surfaces agree well with those from actual rough surfaces, taken from experimental samples, in terms of the heights as well as the gradients, which are the two main factors in accurately predicting the wave scattering intensities. Ultrasonic rough surface scattering is simulated numerically using the Kirchhoff approximation, and comparisons with Gaussian, exponential, AR and real sample surfaces are performed; scattering intensities found using AR surfaces show the best agreement with the real sample surfaces.</P>

Simulation of immiscible liquid–liquid flows in complex microchannel geometries using a front-tracking scheme

Kahouadji, Lyes,Nowak, Emilia,Kovalchuk, Nina,Chergui, Jalel,Juric, Damir,Shin, Seungwon,Simmons, Mark J. H.,Craster, Richard V.,Matar, Omar K. Springer Berlin Heidelberg 2018 MICROFLUIDICS AND NANOFLUIDICS Vol.22 No.11

<P>The three-dimensional two-phase flow dynamics inside a microfluidic device of complex geometry is simulated using a parallel, hybrid front-tracking/level-set solver. The numerical framework employed circumvents numerous meshing issues normally associated with constructing complex geometries within typical computational fluid dynamics packages. The device considered in the present work is constructed via a module that defines solid objects by means of a static distance function. The construction combines primitive objects, such as a cylinder, a plane, and a torus, for instance, using simple geometrical operations. The numerical solutions predicted encompass dripping and jetting, and transitions in flow patterns are observed featuring the formation of drops, ‘pancakes’, plugs, and jets, over a wide range of flow rate ratios. We demonstrate the fact that vortex formation accompanies the development of certain flow patterns, and elucidate its role in their underlying mechanisms. Experimental visualisation with a high-speed imaging are also carried out. The numerical predictions are in excellent agreement with the experimental data.</P><P><B>Electronic supplementary material</B></P><P>The online version of this article (doi:10.1007/s10404-018-2149-y) contains supplementary material, which is available to authorized users.</P>