MODELING MATEMATIS DAN SIMULASI DROPLET UNTUK PENDINGINAN ALAT-ALAT TEKNOLOGI INFORMASI DAN KOMPUTER DENGAN METODE LATTICE-BOLTZMANN

Abstrak : Kebutuhan inovasi skema pendinginan untuk pemeliharaan perangkat elektronik dengan suhu aman dibawah batas yang telah ditentukan oleh batasan material dan kendala realibilitas yang terkait pada miniaturisasi microchip yang agresif pada komponen elektronik. Pergeseran dari ketergantungan pada sistem berpendingin kipas menjadi ke skema pendinginan yang memanfaatkan pendingin cairan dielektrik menggunakan berbagai skema pendinginan fase tunggal. Perekayasa (engineer) sistem pendingin memusatkan perhatian pada skema pendinginan dua fase, untuk memanfaatkan kedua system pendingin. Sifat yang harus dimiliki perekayasa sistem pendingin ini yaitu konveksi fluida dan panas laten untuk memindahkan jumlah kalor yang jauh lebih besar dari pada skema fase tunggal, sambil mempertahankan suhu perangkat yang lebih rendah. Beberapa skema pendingin cairan dua fase telah direkomendasikan untuk menghilangkan fluks kalor tinggi dari perangkat yang digunakan diaplikasi. Momentum droplet memungkinkan cairan menembus penghalang uap yang dibuat oleh gelembung nukleasi dan secara lebih efektif mengisi kembali permukaan, keduanya sangat bermanfaat untuk pendinginan fluks tinggi. Pada model dan simulasi pengembangan droplet menggunakan metode LBM multi fase, parameter penting yang selalu didapatkan adalah arus semu maksimum (maximum spurious currents) yang menetukan stabilitas komputasi. Kata kunci : Modeling Matematis, Simulasi Droplet, Metode Latice-Boltzman Abstract: The need for innovative cooling schemes for maintaining electronic devices with safe temperatures below predetermined limits by material limitations and reliability constraints associated with aggressive microchip miniaturization of electronic components. Shifting from reliance on fan-cooled systems to cooling schemes that utilize dielectric liquid cooling using a variety of single-phase cooling schemes. The cooling system engineer focuses on two-phase cooling schemes, to take advantage of both cooling systems. Properties that these cooling system engineers must possess are fluid convection and latent heat to transfer a much greater amount of heat than a single-phase scheme, while maintaining a lower device temperature. Several two-phase liquid cooling schemes have been recommended to remove the high heat flux from the apparatus used in the application. The droplet momentum allows the liquid to penetrate the vapor barrier created by the nucleation bubbles and more effectively replenish the surface, both of which are very beneficial for high flux cooling. In droplet development models and simulations using the multi-phase LBM method, an important parameter that is always obtained is the maximum spurious currents which determine the computational stability. Keywords: Mathematical Modeling, Droplet Simulation, Latice-Boltzman Method

UAS current mapping: A wave-based heading and position correction

Our unmanned aerial system (UAS) current mapping is based on optical video data of the sea surface. We use three-dimensional fast Fourier transform and least-squares fitting to measure the surface waves’ phase velocities and the currents via the linear dispersion relationship. Our UAS is a low-cost off-the-shelf quadcopter with inaccurate camera position and attitude measurements, which may cause spurious currents as large as the signal. We present a novel wave-based UAS heading and position correction, improving the image rectification accuracy by a factor of ~3.5 and the current measurements’ temporal repeatability by factors of 1.8 to 4.8. This validation study maps the currents at high spatiotemporal resolution (5 m and 4 s) across the ~700 m wide tidally dominated Bear Cut channel in Miami, Florida. The UAS currents are compared to flotsam tracks, obtained through automated UAS video object detection and tracking, drifter tracks, and acoustic Doppler current profiler measurements. The root-mean-square errors of the cross- and along-channel currents are better than 0.03 m/s for the flotsam comparison and better than 0.06 m/s for the drifter comparison; the latter revealed a 0.06 m/s along-wind bias due to wind- and wave-driven vertical current shear. UAS current mapping could be used to monitor river discharge, buoyant pollutants, or submesoscale fronts and eddies; the proposed wave-based heading and position correction enables its use in areas without ground control points.

Exploring horizontal pressure gradient (HPG) schemes for general vertical coordinates.

<p>Terrain following coordinates allow for better representation of physics at the sea-bed than traditional z-coordinates but result in numerical discretisation errors in the calculation of the horizontal pressure gradient (HPG) which manifest as spurious currents.  As of NEMO r4.0.4, there were two HPG schemes available for use with terrain following coordinates, the traditional 2<sup>nd</sup> order sco scheme and the 3<sup>rd</sup> order prj scheme.  The prj scheme, while highly accurate in the ocean interior, shows unphysical behaviour at the sea-bed for steeply sloping bathymetry.  A task in the IMMERSE project was set up to identify, implement and test promising HPG schemes suitable for general vertical coordinates that are accurate, robust and physically consistent.  As part of this task, the 3<sup>rd</sup>-order accurate density Jacobian scheme (djc) as proposed by Shchepetkin and McWilliams (2003) has now been implemented in the NEMO trunk (as a rewrite of the previously existing but non-operational djc scheme).  Idealised testing has shown this scheme to be significantly more accurate than the sco scheme, and more robust than the prj scheme in coping with steeply sloping bathymetry.  Initial results from applying the djc scheme in a challenging realistic configuration (the AMM7 with hybrid s-z-coordinates and non-uniform vertical discretisation) show a reduction in spurious currents with respect to the sco scheme.  The prj scheme is highly sensitive to the rmax (maximum permitted slope) criterion.  In cases where the bathymetry is so steep that a velocity-point may lie multiple levels below one of its neighbouring tracer-points, the nature of the prj near-bed HPG calculation leads to sudden spin-ups of spurious velocities which can exceed those of the djc scheme in the longer-term.  Performance-wise, the djc scheme is 3 times slower than the sco scheme, but less expensive than the prj.  Further work is planned to reduce the memory footprint.  In addition to continued testing of the djc scheme, further work will look at alternative formulation (finite volume) HPG schemes, and high order variants.</p><p>This work is distributed under the Creative Commons Attribution 4.0 License. This licence does not affect the Crown copyright work, which is re-usable under the Open Government Licence (OGL). The Creative Commons Attribution 4.0 License and the OGL are interoperable and do not conflict with, reduce or limit each other.</p>

On methods to reduce spurious currents within VOF solver frameworks. Part 1: a review of the static bubble/droplet

In two-phase flows in which the Capillary number is low, errors in the computation of the surface tension force at the interface cause Front-Capturing methods such as Volume of Fluid (VOF) and Level-Set (LS) to develop interfacial spurious currents. To better solve low Capillary number flows, special treatment is required to reduce such spurious currents. Smoothing the phase indicator field to more accurately compute the curvature or adding interfacial artificial viscosity are techniques that can treat this problem. This study explores OpenFOAM, Fluent and StarCCM+ VOF solvers for the classical case of a static bubble/droplet immersed in a continuous aqueous phase, with the focus on the ability of these solvers to adequately reduce spurious currents. The results are expected to be helpful for practicing chemical engineers who use multiphase CFD solvers in their work.

Numerical Study of Single Taylor Bubble Movement Through a Microchannel Using Different CFD Packages

A Computation Fluid Dynamics (CFD) study for micro-scale gas–liquid flow was performed by using two different software packages: OpenFOAM® and ANSYS Fluent®. The numerical results were compared to assess the capability of both options to accurately predict the hydrodynamics of this kind of system. The focus was to test different methods to solve the gas–liquid interface, namely the Volume of Fluid (VOF) + Piecewise Linear Interface Calculation (PLIC) (ANSYS Fluent®) and MULES/isoAdvector (OpenFOAM®). For that, a single Taylor bubble flowing in a circular tube was studied for different co-current flow conditions (0.01 < CaB < 2.0 and 0.01 < ReB < 700), creating representative cases that exemplify the different sub-patterns already identified in micro-scale slug flow. The results show that for systems with high Capillary numbers (CaB > 0.8) each software correctly predicts the main characteristics of the flow. However, for small Capillary numbers (CaB < 0.03), spurious currents appear along the interface for the cases solved using OpenFOAM®. The results of this work suggest that ANSYS Fluent® VOF+PLIC is indeed a good option to solve biphasic flows at a micro-scale for a wide range of scenarios becoming more relevant for cases with low Capillary numbers where the use of the solvers from OpenFoam® are not the best option. Alternatively, improvements and/or extra functionalities should be implemented in the OpenFOAM® solvers available in the installation package.