Evaluation of an improved mixing plane interface for OpenFOAM

A mixing plane interface provides a circumferentially averaging rotor-statorcoupling interface, which is extremely useful in practical turbomachinery simulations. Itallows fundamentally transient problems to be studied in steady-state, using simplified meshcomponents having periodic properties, and with the help of a multiple reference frames(MRF) approach. An improved version of the mixing plane interface for the community-drivenversion of OpenFOAM is presented. This new version of the mixing plane introduces a perfield,user-selectable mixing option for the flow fields at the interface, including the possibilityto use a mass-flow averaging algorithm for the velocity field. We show that the quality of themass-flow transfer can be improved by a proper selection of the mixing options at theinterface. This paper focuses on the evaluation of the improved mixing plane interface forvarious steady-state simulations of incompressible flows, applied to a simple 2D validation testcase, and to more complex 3D turbomachinery cases

Complexity of viscous dissipation in turbulent thermal convection

Using direct numerical simulations of turbulent thermal convection for Rayleigh number ($\mathrm{Ra}$) between $10^6$ and $10^8$ and unit Prandtl number, we derive scaling relations for viscous dissipation in the bulk and in the boundary layers. We show that contrary to the general belief, the total viscous dissipation in the bulk is larger, albeit marginally, than that in the boundary layers. The bulk dissipation rate is similar to that in hydrodynamic turbulence with log-normal distribution, but it differs from $(U^3/d)$ by a factor of $\mathrm{Ra}^{-0.18}$. Viscous dissipation in the boundary layers are rarer but more intense with a stretched-exponential distribution.Comment: 5 pages, 4 figures, 1 supplemental materia

Numerical investigation of oil injection in a Roots blower operated as expander

The adoption of positive displacement machines as expanders in Organic Rankine Cycles (ORCs) is increasingly common, especially in the low to medium power range. At the same time, these devices often serve as compressor in Vapor-Compression Refrigeration Systems. In both cases, the application of Computation Fluid Dynamics (CFD) to optimize such machines has become an integrated tool in the design process. As a consequence, several challenges associated with the numerical simulation have to be taken into account. For example, the modeling of the gap represents a challenge for the stability of the numerical analysis. The dynamic of the process, combined with deformations of the clearances and of the working chamber has to be considered with extra care. To raise the efficiency of the machine, oil is typically injected. Its numerical modeling imply an extra challenge in the simulation of the actual operation of the machine. The present work is mainly focused on the multi-phase nature of the flow, with a particular analysis of the lubricant oil injected. In this work, a two-lobe Roots blower operated as expander has been simulated with the open-source software OpenFOAM-v1812, using the SCORG-V5.2.2. This analysis highlights the areas that are affected the most by the oil presence in order to highlight the sealing effect it provides

Eulerian Multi-Fluid Model for Polydisperse Flows

This work restricts the term multiphase only to disperse flows, where one of the phases is present in the form of particles, droplets or bubbles, which are suspended within the continuous phase. The dispersed elements can vary in size. The proposed method uses the classes method in the Euler-Euler framework to handle the flow's polydisperse nature. With this approach, every droplet/bubble/particle class is treated like a different phase in the calculation, i.e. every size class has its continuity and momentum equation. However, the pressure is shared among all phases. The derived model is tested for various polydisperse flows, which display the developed model's capability to predict such complex dynamic behaviour. These test cases include complex bubbly flows and dense spray (where droplet sizes vary significantly)

Numerical Capture and Validation of a Massively Separated Bluff-Body Wake

A flow over a bluff-body is numerically investigated and validated using a Detached-Eddy Simulation (DES) technique at Re=21,400. An incompressible solver that is nominally second-order accurate employing an implicit constant backward time-stepping scheme with blended upwind-central differencing spatial discretization is used to study the massively separated wake that is generated. Measurements are taken up to 6 downstream characteristic lengths, evaluating the wake time-averaged first- and second-moment statistics alongside near-wall boundary layer quantities and surface-force integrals. Results advocate the use of DES methods, which are found to be significantly more accurate for capturing wake statistics, compared to two different Reynolds-Averaged (RANS) models calibrated with an identical grid. Although comparative accuracy can be obtained with the RANS techniques for the boundary layer and surface-forces, these techniques are unsuitable for modeling wake statistics as they are inherently dissipative, evident through early velocity recovery when evaluated against experimental data

Implementation of integral viscoelastic constitutive models in OpenFOAM® computational library

This work reports the implementation and verification of a new so lver in OpenFOAM® open source computational library, able to cope with integral viscoelastic models based on the integral upper-convected Maxwell model. The code is verified through the comparison of its predictions with analytical solutions and numerical results obtained with the differential upper-convected Maxwell modelCAPES, FCT projects PEsT-C/CTM/LA0025/2013, PTDC/MAT/121185/2010 and FEDE