Narrow Results

Minimal Boltzmann kinetic models, such as lattice Boltzmann, are often used as an alternative to the discretization of the Navier–Stokes equations for hydrodynamic simulations. Recently, it was argued that modeling sub-grid scale phenomena at the kinetic level might provide an efficient tool for large scale simulations. Indeed, a particular variant of this approach, known as the entropic lattice Boltzmann method (ELBM), has shown that an efficient coarse-grained simulation of decaying turbulence is possible using these approaches. The present work investigates the efficiency of the entropic lattice Boltzmann in describing flows of engineering interest. In order to do so, we have chosen the flow past a square cylinder, which is a simple model of such flows. We will show that ELBM can quantitatively capture the variation of vortex shedding frequency as a function of Reynolds number in the low as well as the high Reynolds number regime, without any need for explicit sub-grid scale modeling. This extends the previous studies for this set-up, where experimental behavior ranging from Re~O(10) to Re≤1000 was predicted by a single simulation algorithm.^1–5

Accurate description of the aerodynamic and aerothermal environment is crucial to the integrated design and optimization for high performance hypersonic vehicles. In the simulation of aerothermal environment, the effect of viscosity is crucial. The turbulence modeling remains a major source of uncertainty in the computational prediction of aerodynamic forces and heating. In this paper, three turbulent models were studied: the one-equation eddy viscosity transport model of Spalart-Allmaras, the Wilcox k-ω model and the Menter SST model. For the k-ω model and SST model, the compressibility correction, press dilatation and low Reynolds number correction were considered. The influence of these corrections for flow properties were discussed by comparing with the results without corrections. In this paper the emphasis is on the assessment and evaluation of the turbulence models in prediction of heat transfer as applied to a range of hypersonic flows with comparison to experimental data. This will enable establishing factor of safety for the design of thermal protection systems of hypersonic vehicle.

Cavitating flows around a hydrofoil are simulated in this work by using a transport equation-based model. It is shown that the original Launder-Spalding k-ε model significantly over-predicts the viscosities. The viscosity-corrected models can simulate better the cavitation characteristics including the shedding process.

Eight turbulence models frequently used in aerodynamics have been employed in the detailed numerical investigations for transonic flows in the Sajben diffuser, to assess the predictive capabilities of the turbulence models for shock wave/turbulent boundary layer interactions (SWTBLI) in internal flows. The eight turbulence models include: the Spalart–Allmaras model, the standard $k-𝜀$ model, the RNG $k-𝜀$ model, the realizable $k-𝜀$ model, the standard $k-\omega$ model, the SST $k-\omega$ model, the $\overline{v^2}-f$ model and the Reynolds stress model. The performance of the different turbulence models adopted has been systematically assessed by comparing the numerical results with the available experimental data. The comparisons show that the predictive performance becomes worse as the shock wave becomes stronger. The $\overline{v^2}-f$ model and the SST $k-\omega$ model perform much better than other models, and the SST $k-\omega$ model predicts a little better than the $\overline{v^2}-f$ model for pressure on walls and velocity profile, whereas the $\overline{v^2}-f$ model predicts a little better than the SST $k-\omega$ model for separation location, reattachment location and separation length for strong shock case.

This work is devoted to perform systematic sensitivity analysis of different turbulence models and various inflow boundary conditions in predicting the wake flow behind a horizontal axis wind turbine represented by an actuator disc (AD). The tested turbulence models are the standard k–$𝜀$ model and the Reynolds Stress Model (RSM). A single wind turbine immersed in both uniform flows and in modeled atmospheric boundary layer (ABL) flows is studied. Simulation results are validated against the field experimental data in terms of wake velocity and turbulence intensity.

Unsteady cavitating turbulent flow around a NACA66 hydrofoil was simulated using a mass transfer cavitation model and a modified filter-based turbulence model in this paper. The modified filter-based turbulence model can accurately predict the pressure coefficient in midplane and shedding frequency of the unsteady cloud cavitation than standard $\kappa$–$𝜀$ model and filter-based turbulence model. The time evolution of transient cavitation cloud structure predicted by the three-turbulence model was compared. The result which was predicted by the modified filter-based turbulence model is in good agreement with the experimental results. The time evolution of re-entrant jet had been analyzed. The instantaneous wall-pressure evolution on the suction surface (SS) predicted by the modified filter-based turbulence model had been analyzed. The cavitation-vortex interaction had been analyzed in this study. The different effects on the cavitation-vortex interaction of the vortex stretching term, vortex dilatation term and baroclinic torque term in the transport equation of vorticity had been discussed.

In this paper, we obtain normal dispersive $\mathcal{𝒩}\mathcal{ℒ}\mathcal{\mathscr{H}}\mathcal{𝒮}$ shock waves for $\varphi ({\mathcal{𝒥}}_1),\varphi ({\mathcal{𝒥}}_2),\varphi ({\mathcal{𝒥}}_3)$ dam-break intensity by the nonlinear heat system. Then, we obtain super-fluid normal dispersive $\mathcal{𝒩}\mathcal{ℒ}\mathcal{\mathscr{H}}\mathcal{𝒮}$ shock waves for $\varphi ({\mathcal{𝒥}}_1),\varphi ({\mathcal{𝒥}}_2),\varphi ({\mathcal{𝒥}}_3)$ dam-break microfluidics. Finally, we illustrate thermonormal $\mathcal{𝒩}\mathcal{ℒ}\mathcal{\mathscr{H}}\mathcal{𝒮}$ pressure of $\varphi ({\mathcal{𝒥}}_1),\varphi ({\mathcal{𝒥}}_2),\varphi ({\mathcal{𝒥}}_3)$ wave energy with normal dam-break in shallow water.

We consider the finite element discretization of the Navier–Stokes equations locally coupled with the equation for the turbulent kinetic energy through an eddy viscosity. We prove a posteriori error estimates which allow to automatically determine the zone where the turbulent kinetic energy must be inserted in the Navier–Stokes equations and also to perform mesh adaptivity in order to optimize the discretization of these equations. Numerical results confirm the interest of such an approach.

Computational fluid dynamics (CFD) has become an essential tool for designing and optimizing the structure of blood pumps. However, it is still questionable which turbulence model can better obtain the flow information for axial flow blood pump. In this study, the axial flow blood pump was used as the object, and the influence of the common turbulence models on simulation was compared. Six turbulence models (standard $k$–$\epsilon$ model, RNG $k$–$\epsilon$ model, standard $k$–$\omega$ model, SST $k$–$\omega$ model, Spalart–Allmaras model, SSG Reynolds stress model) were used to simulate the pressure difference and velocity field of the pump. In parallel, we designed a novel drive system of the axial flow blood pump, which allowed the camera to capture the internal flow field. Then we measured the flow field in the impeller region based on particle image velocimetry (PIV). Through the comparison of experiments and simulation results, the average errors of velocity field obtained by the above models are 30.97%, 19.40%, 24.25%, 15.28%, 28.51%, 23.00%, respectively. Since the SST $k$–$\omega$ model has the smallest error, and the streamline is consistent with the experimental results, it is recommended to use SST $k$–$\omega$ model for numerical analysis of the axial flow blood pump.

Hemolysis in blood-contacting devices severely affects the health of users, and computation fluid dynamics (CFD) simulation is a crucial method for hemolysis analysis. Medical equipment has high requirements for simulation accuracy. Modification of the turbulence model is one of the most effective ways to improve efficiency. In this study, we designed nozzle models to simulate hemolytic shear flow, varying the degree of shear flow through different nozzle orifice sizes. The study acquires microscopic flow results through Particle Image Velocimetry (PIV) experiments, and the Sparlart–Allmaras (S–A) model was modified based on the experimental results. In the study, we obtained the influence characteristics of the model coefficients on the simulation results and completed the accuracy correction. The results showed that the model coefficient $c_{b1}$ has the most significant effect on the simulation results. Correcting $c_{b1}$ to about 200% of the standard value can significantly improve the simulation accuracy, and the high shear flow intensity corresponds to a slightly lower correction value. The model modification eliminates the simulation error in the high-speed area, and the comparison results show that it is superior to the standard turbulence model.

The flow through the throat-type energy-dissipators is calculated by using an axis-symmetrical K ~ ε turbulence model. The velocity field, the pressure field and the distributions of turbulent energy and its dissipation rate are acquired. The energy dissipation through the throat-type energy-dissipators can be seen in detail. The calculated pressure distribution is compared with the measured and in good agreement. The results are useful to understand deeply the flow characteristics of the throat-type energy-dissipators.

To optimize the structure and make a reasonable choice for different field of coalescence plates, lateral mixing model based on turbulence flow of three-segment coalescence plates and the theoretical formula of separation efficiency were established. The relationship between separation efficiency and inlet velocity as well as pressure drop and inlet velocity are obtained by changing dip angle and plate spacing of coalescence plates in the experiment. It also explains the separation mechanism The experimental results agree with the theoretical results when inlet velocity is less than the critical rupture velocity (8m/s~10m/s), which are available to the design of gas-liquid separator. The separation performance is more excellent when the inlet velocity comes to 9m/s.