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Simulation of Turbulence and RoughnEss in Additive Manufactured parts

Periodic Reporting for period 1 - STREAM (Simulation of Turbulence and RoughnEss in Additive Manufactured parts)

Reporting period: 2019-10-01 to 2021-03-31

Additive manufacturing (AM) process offers tremendous gains over conventional subtractive manufacturing in heat exchanger design, key issue of thermal engine efficiency. The STREAM project aims at designing novel modeling strategies for the performance prediction of additive-manufactured heat exchangers. The consortium consists of two laboratories CNRS-CORIA and CNRS-LEGI, which have a long experience in high-fidelity multi-physics turbulent flow modeling and TEMISTh, a SME which develops customized solutions for heat exchangers. From the fluid dynamics point of view, AM often introduces important wall roughness, which depends strongly on the manufacturing process itself, and which impacts heat transfer and pressure loss across the device. It is therefore mandatory to design Computational Fluid Dynamics (CFD) models with a sufficient level of accuracy to predict the performances of heat exchangers. RANS (Reynolds-Averaged Navier-Stokes) and LES (Large-Eddy Simulation) are two complementary turbulence modeling approaches that are good candidates for such challenge. In these approaches, wall modeling often relies on statistical analysis, leading to law-of-the-wall models that are widely used in the prediction of internal flows. However, these models need to be extended and validated for wall roughness generated by additive manufacturing. To this aim, STREAM proposes to build a large database of high-fidelity roughness-resolving Large-Eddy Simulations that will be analyzed to derive well-parametrized statistical wall models. An original wall model parametrization inspired by combustion tabulation techniques will be used. Such approach has already been successfully adapted to heat transfers on a smooth turbine blade. The resulting statistical model, usable in both roughness-modeled RANS and LES approaches, will be extensively validated a priori by comparison with the high-fidelity database and a posteriori on classical heat exchanger applications: Fuel-Cooled Oil Cooler, Air-Cooled Oil Cooler, Surface Air-Cooled Oil Cooler.
During the reporting period, the project has progressed on the first objective dealing with the identification of the key physical parameters and the best methodology to model the turbulence especially in rough additive manufactured parts (like tubes, plate, fins...). The key physical statistical moments that define a rough surface have been identified and categorized from an extensive literature review. From this review, parameters such as the effective slope (ES) are very important for the modelling of the impact of roughness on the boundary layer. For the second objective, which deals with the improvement of rough-wall modelling based on resolved-rough unsteady simulations, a rough-surface generator (RSG) and analyzer, and a body-fitted mesh generator have been created and included in an automated simulation workflow to build a resolved-roughness (RR) Large-Eddy Simulation (LES) database. During the first reporting period, two sets of parameters have been added to the database. In parallel, all the RANS simulations with standard rough-wall models have been either finished or started. As soon as the novel statistical model derived from the RR-LES database, these simulations will be completed with the new model.

A public website has been created ( and updated regularly and communication material (brochures, flyers) have been set-up by the partners. Over the period, the main dissemination activity has been performed during a VKI course from 26 to 28 April 2021. This course on compact heat exchangers in additive manufacturing was organized by D. Laboureur, VKI, in the framework of the NATHENA project H2020-RIP-785520. D. Serret from TEMISTH and V. Moureau both gave a lecture of 1h30 each on the work they perform in the NATHENA and STREAM projects. This course was a good opportunity for dissemination due to the targeted audience.
The expected impact as planned in the description of work is listed below.
1. Definition of the predominant physical parameters impacting the predictive character of CFD simulations
2. Assessment of the existing wall models to predict pressure loss and heat flux for heart exchangers simulations for both RANS and LES approaches
3. Improvement of wall models to consider both roughness and complex geometries effects
4. Development of a roughness surface generator (RSG) and mesh generation tool for numerical LES-RANS benchmarks
5. Optimization of the architecture of the exchanger regarding the objective of dimensioning
6. Local optimization of the flow and heat transfer considering the orientation of the walls, and therefore their roughness
7. Adaption of the physical model to the type of heat exchanger

All these items are still relevant for the project. Items 1, 2, and 4 have been the focus of the first reporting period. Items 1 and 2 will be revisited in the next reporting period along with the other items. The mesh generator of item 4 has been integrated in the YALES2 platform and finds new applications (aeronautical injection systems).
Vortices created by wall roughness and turbulence.
Estimation of friction factor and Nusselt number from RANS
Resolved roughness surface generator and analyzer
Automatic unstructured mesh generation of rough surfaces