Large Eddy Simulation of Hypersonic Turbulent Boundary Layers

The recent revival of interest in developing new hypersonic vehicles brings attention to the need for accurate prediction of hypersonic flows by computational methods. One of the challenges is prediction of aerothermodynamic loading over the surface of the vehicles. Reynolds Average Navier-Stokes (RANS) methods have not shown consistent accuracy in prediction of such flows. Therefore, new methods including Large Eddy Simulations (LES) should be investigated. In this paper, the LES method is used for prediction of the boundary layer over a flat plate. A new recycling-rescaling method is tested. The method uses total enthalpy and static pressure along with the velocity components to produce the best results for the Law of the Wall, turbulent statistics and turbulent Prandtl number.

Analysis of Applicability of CFD Numerical Studies Applied to Problem When Pump Working as Turbine

The present research depicts an analysis of the implementation of computational fluid dynamics (CFD) in the study of pumps such as turbines and PATs. To highlight the benefits of CFDs for PAT studies, results from both experimental tests have been compared to better understand the reproduction error phenomena. For this, data analysis used in successful models has been applied to determine variables and parameters, and to report a low relative error. The results show that most of the studies focused on fixed speed rotation with some cases of variable speed rotation. Furthermore, there is not enough information in the academic literature for PAT of axial and mixed flows with fixed and variable speed. Finally, turbulence models based on Reynolds average Navier–Stokes (RANS) have been used to simulate PATs with fixed speed rotation in most cases.

Comparative Study of Different Turbulence Models for Cavitational Flows around NACA0012 Hydrofoil

The present work focuses on the comparison of the numerical simulation of sheet/cloud cavitation with the Reynolds Average Navier-Stokes and Large Eddy Simulation(RANS and LES) methods around NACA0012 hydrofoil in water flow. Three kinds of turbulence models—SST k-ω, modified SST k-ω, and Smagorinsky’s model—were used in this paper. The unstable sheet cavity and periodic shedding of the sheet/cloud cavitation were predicted, and the simulation results, namelycavitation shape, shedding frequency, and the lift and the drag coefficients of those three turbulence models, were analyzed and compared with each other. The numerical results above were basically in accordance with experimental ones. It was found that the modified SST k-ω and Smagorinsky turbulence models performed better in the aspects of cavitation shape, shedding frequency, and capturing the unsteady cavitation vortex cluster in the developing and shedding period of the cavitation at the cavitation number σ = 0.8. At a small angle of attack, the modified SST k-ω model was more accurate and practical than the other two models. However, at a large angle of attack, the Smagorinsky model of the LES method was able to give specific information in the cavitation flow field, which RANS method could not give. Further study showed that the vortex structure of the wing is the main cause of cavitation shedding.

Numerical Assessment of Turbulence Effects on Water Entry of a Hemisphere

Water entry of a rigid hemisphere is simulated using the unsteady incompressible Reynolds-Average Navier-Stokes (RANS) equations and volume of fluid (VOF) method, which are implemented in the open-source library OpenFoam. The solver InterDyMFoam is applied and the algorithm PIMPLE which is a combination of PISO (Pressure Implicit with Splitting of Operators) and SIMPLE (Semi-Implicit Method for pressure-Linked Equations) algorithms are used in the simulations. A second-order backward difference scheme is applied for the temporal discretization. A convergence and uncertainty study is performed considering different resolutions and constant Courant number (CFL) using the procedures recommended by ITTC. The comparisons of slamming loads and motions between the CFD simulations are presented using both laminar and turbulence fluid models for the hemisphere entering the water at various speeds. Turbulence is modelled with a Reynolds averaged stress (RAS) k-ω two-equation model. The turbulence effects on the slamming loads will be assessed for the case with different entry velocities.

Investigation on Aircraft-store Compatibility Criteria of External Store Separation

The evaluation of aircraft-store compatibility on external store separation is a key issue in the separation system of vehicle design. Firstly, the aircraft-store compatibility criterion of an external store separation is put forward, and then the criterion is converted to an unequal relationship between velocity and acceleration in vertical displacement and pitch angle based on the constant force assumption, which is validated by the test result of wing pylon finned store model (WPFS). The three-dimensional compressible Reynolds average N-S equation and rigid body six-degree-of-freedom motion equation (6-DOF) are solved by using unstructured dynamic overlap grid technology, to obtain the kinematic parameters of the external separation. Finally, the most dangerous point M on the tail of the external store is selected to verify the aircraft-store separation criterion. The results show that the kinematic parameters of the most dangerous point M on the tail wing of the store fall in the safe separation area, which means that the complete separated process is safe.

Efecto del refinamiento de la descripción de la rugosidad en una aproximación 2D para un río de montaña: un caso de estudio

La predicción de niveles de agua en ríos es importante para prevenir pérdidas económicas así como de vidas humanas causadas por inundaciones. Los modelos hidráulicos son comúnmente usados para predecir estos niveles de agua y tomar acciones para mitigar el daño debido a inundaciones. En la presente investigación, se analizó una aproximación 2D para resolver las ecuaciones promediadas en profundidad de Reynolds Average Navier Stokes (RANS), llamado Conveyance Estimation System (CES), para explorar sus capacidades predictivas. Este artículo presenta una ampliación del estudio realizado por Knight et al. (2009). De igual forma, en esta investigación se explora una caracterización más detallada del parámetro de rugosidad y del número de zonas de rugosidad produciendo diversos escenarios. Se evaluó el desempeño de cada escenario mediante diferentes funciones de ajuste usando curvas de descarga para comparación. La investigación muestra que el uso de una adecuada descripción de la rugosidad, como un factor de rugosidad calibrado para toda la sección transversal o un modelo de rugosidad para cantos rodados calibrado para el lecho junto con valores de rugosidad obtenidos en valores sugeridos por el CES para los bancos, produce resultados del modelo óptimos en un río de montaña.

Numerical Modeling of the Ladle Flow by a LES-Based Eulerian–Lagrange Approach: A Systematic Survey

To account for increasing economic and ecological pressure, the steel industry is obligated to continuously optimize all processes. An important optimization approach is numerical modeling although its potential is limited by the accuracy of the mathematical models. In a previous work, a validation database was created and a validation score was derived from this data which allows a comprehensive qualitative accuracy assessment for those models. Here, this system is employed for a systematic optimization of the isothermal flow in the casting ladle. For that, different submodels, namely the turbulence models, subgrid turbulence models, bubble-induced turbulence and interfacial closure models as well as influencing factors, such as the grid resolution or the initial bubble size, are analyzed. It is shown that the large eddy turbulence model is more accurate than the Reynolds-average approach because it is able to reproduce the anisotropy of turbulence in the bubble region. In accordance with the literature, a grid dependency of the lift force is found which can be reduced using an averaged shear field as an additional variable. For the interfacial closure models, the combination of the Tomiyama drag model for fully contaminated systems and the Tomiyama lift correlation showed the best agreement with the experimental data. The results of the survey are summarized to a best-practice guideline with which the validation score can be increased from 38.7 with the Reynolds-average approach to 85.1 on a coarse grid respectively, and 87.8 on a fine grid. However, some upscaling problems of the numerical system from the water model to the real ladle are revealed. There is a need to find accurate yet efficient grid resolutions which make the large eddy turbulence model affordable with the current computational resources. Furthermore, alloying elements or non-metallic inclusions might alter the interfacial forces considerably. However, no studies on their effect have been published yet.

3D-CFD Study of Oblique Streamlining Membranes on the Roof-Top of a Low-Rise Building, Targeting Pedestrian Comfort-A Case Study

Considering the cubic nature of the most frequent geometries among the urban elements and their configurations, the manipulation of the free stream wind flow around the enormous objects, as well as within relatively narrow corridors, such as street canyons is significant in many regards. One of its instances could be mentioned as pedestrian comfort. In this case study, the implementation of an Oblique Streamlining Membrane (OSM) has been studied on a sub-branch of a low rise, but bulky, building, over which a roof-top outdoor food court area has been primarily designed. The OSM serves as the streamlining mechanism, preventing the roof-top area from the intense fluctuations of the wind flow after the flow is separated from the inlet corner side of the building in question. The optimization of sub-geometries and the proof of the concept for the OSM flow controlling mechanism were the subject of the current manuscript, for which a 3D numerical Reynolds Average Navier-Stokes (RANS) scheme has been used. An unstructured computational grid has been applied around the 3D geometry of the entire building and its sub-branching details, which is in contrast with most studies on full-scale geometries, tending to have simplifications on the body to reach a more generalized set of results. All cases have been numerically tested with and without the presence of the OSM. This comparison has been the grounds for proving the effectivity of the said flow controlling mechanism to eliminate high gradient fluctuations of the separated flow off the roof corners. This has led to pedestrian/resident comfort on the roof-top food court area located on top of the building.