Navier-Stokes CFD Code Research Articles

Comparing the effect of three transition models on the CFD predictions of a NACA0012 airfoil aerodynamics

The ability to accurately predict transition is important for flows around airfoils, especially at low Reynolds numbers. Transition from laminar to turbulent flow strongly influences the flow separation and the skin friction, thus affecting the airfoil aerodynamic characteristics. In the present work, three models for predicting flow transition were implemented in a 2D curvilinear, aerodynamic Navier–Stokes CFD code. These are Michel's empirical model, the eN model, and a newly proposed transition model (K–V model). The last two models are based on linear stability analysis employing the Orr-Sommerfeld equation to determine the amplification of spatial flow perturbations. The effect of the transition models on the airfoil aerodynamic characteristics at different Reynolds number and incidence angle are studied numerically. Both these parameters, when increased, promote the growth of flow perturbations. The test case is a 2D incompressible, low turbulence air flow around a smooth NACA0012 airfoil. The results of the new K–V transition model are in better agreement with existing experimental data.

Numerical comparison of hydrogen-air reaction mechanisms for unsteady shock-induced combustion applications

An unsteady shock-induced combustion (SIC) is characterized by the regularly oscillating combustion phenomenon behind the shock wave supported by the blunt projectile flying around the speed of Chapman-Jouguet detonation wave. The SIC is the coupling phenomenon between the hypersonic flow and the chemical kinetics, but the effects of chemical kinetics have been rarely reported. We compared hydrogen-air reaction mechanisms for the shock-induced combustion to demonstrate the importance of considering the reaction mechanisms for such complex flows. Seven hydrogen-air reaction mechanisms were considered, those available publically and used in other researches. As a first step in the comparison of the hydrogen combustion, ignition delay time of hydrogen-oxygen mixtures was compared at various initial conditions. Laminar premixed flame speed was also compared with available experimental data and at high pressure conditions. In addition, half-reaction length of ZND (Zeldovich-Neumann-Doring) detonation structure accounts for the length scale in SIC phenomena. Oscillation frequency of the SIC is compared by running the time-accurate 3rd-order Navier-Stokes CFD code fully coupled with the detailed chemistry by using four levels of grid resolutions.

Multiple threads and parallel challenges for large simulations to accelerate a general Navier–Stokes CFD code on massively parallel systems

SUMMARYComputational fluid dynamics is an increasingly important application domain for computational scientists. In this paper, we propose and analyze optimizations necessary to run CFD simulations consisting of multibillion‐cell mesh models on large processor systems. Our investigation leverages the general industrial Navier–Stokes CFD application, Code_Saturne, developed by Electricité de France for incompressible and nearly compressible flows. In this paper, we outline the main bottlenecks and challenges for massively parallel systems and emerging processor features such as many‐core, transactional memory, and thread level speculation. We also present an approach based on an octree search algorithm to facilitate the joining of mesh parts and to build complex larger unstructured meshes of several billion grid cells. We describe two parallel strategies of an algebraic multigrid solver and we detail how to introduce new levels of parallelism based on compiler directives with OpenMP, transactional memory and thread level speculation, for finite volume cell‐centered formulation and face‐based loops. A renumbering scheme for mesh faces is proposed to enhance thread‐level parallelism. Copyright © 2012 John Wiley & Sons, Ltd.

CFD 해석 및 최적화 기법을 이용한 피토관 형상설계

Accurate measurement of speed and altitude of flying vehicles in air data system remains a critical technical issue. A highly reliable Pitot-static probe is required to obtain air data such as total pressure and static pressure. In this study, an analysis of the characteristics of flowfield around the Pitot-static probe was performed by using a Navier-Stokes CFD code. In addition, for the purpose of finding an optimal configuration, a technique based on the response surface method is applied to the problem with design parameters including shape of the nose section and cone angle. It is shown that the optimal configuration fulfills the MIL specification in wider range of high angles of attack.

Investigation of computational aeroacoustic tools for noise predictions of wind turbine aerofoils

In this work trailing edge noise levels of a research aerofoil have been computed and compared to aeroacoustic measurements using two different approaches.On the other hand, aerodynamic and aeroacoustic calculations were performed with the full Navier-Stokes CFD code Fluent [Fluent Inc 2005 Fluent 6.2 Users Guide, Lebanon, NH, USA] on the basis of a steady RANS simulation. Aerodynamic characteristics were computed by the aid of various turbulence models. By the combined usage of implemented broadband noise source models, it was tried to isolate and determine the trailing edge noise level. Throughout this work two methods of different computational cost have been tested and quantitative and qualitative results obtained.On the one hand, the semi-empirical noise prediction tool NAFNoise [Moriarty P 2005 NAFNoise User's Guide. Golden, Colorado, July. http://wind.nrel.gov/designcodes/ simulators/NAFNoise] was used to directly predict trailing edge noise by taking into consideration the nature of the experiments.

Effects of Fuel Distribution on Detonation Tube Performance

A pulse detonation engine (PDE) uses a series of high frequency intermittent detonation tubes to generate thrust. The process of filling the detonation tube with fuel and air for each cycle may yield non-uniform mixtures. Lack of mixture uniformity is commonly ignored when calculating detonation tube thrust performance. In this study, detonation cycles featuring idealized non-uniform H2/air mixtures were analyzed using the SPARK two-dimensional Navier-Stokes CFD code with 7-step H2/air reaction mechanism. Mixture non-uniformities examined included axial equivalence ratio gradients, transverse equivalence ratio gradients, and partially fueled tubes. Three different average test section equivalence ratios (phi), stoichiometric (phi = 1.00), fuel lean (phi = 0.90), and fuel rich (phi = 1.10), were studied. All mixtures were detonable throughout the detonation tube. It was found that various mixtures representing the same test section equivalence ratio had specific impulses within 1 percent of each other, indicating that good fuel/air mixing is not a prerequisite for optimal detonation tube performance.

Numerical aspects of simulating the flow-induced oscillations of a rectangular bluff body

The flow-induced oscillatory behavior of a rectangular body with a length-to-height ratio of L/ H=2 in incompressible, turbulent flow is numerically investigated at different Reynolds numbers based on the body length in a range between 2×10 4 and 12×10 4 at zero incidence. The body has one degree of freedom perpendicular to the mean-flow direction with a linear spring and linear damping. To compute the flow, a finite-volume based Navier–Stokes CFD-code is used, and a finite-difference-based algorithm is employed to solve the differential equation for the body vibration. The goals are the numerical simulation of an incident flow velocity at which resonance occurs, the exact determination of the physical mechanisms of the resonance, especially in the flowing medium, and a discussion of the numerical requirements to simulate the phenomenon. Key parameters of the overall model which exert influence on the quality of the simulation, e.g. turbulence modelling, time step and grid resolution, are examined. To achieve this aim, simulations with both steady and oscillating bodies are compared with experimental data and discrepancies are analyzed. Finally, an outlook on suggested future steps is given.

Numerical study on flow characteristics at blade passage and tip clearance in a linear cascade of high performance turbine blade

A numerical analysis has been conducted in order to simulate the characteristics of complex flow through linear cascades of high performance turbine blade with/without tip clearance by using a pressure-correction based, generalized 3D incompressible Navier-Stokes CFD code. The development and generation of horseshoe vortex, passage vortex, leakage vortex, tip vortex within tip clearance, etc. are clearly identified through the present simulation which uses the RNG k-e turbulent model with wall function method and a second-order linear upwind scheme for convective terms. The present simulation results are consistent with the generally known tendency that occurs in the blade passage and tip clearance. A 3D model for secondary and leakage flows through turbine cascades with/without tip clearance is also suggested from the present simulation results, including the effects of tip clearance height.

Numerical Simulation of Tip Leakage Flows in Axial Flow Turbines, With Emphasis on Flow Physics: Part I—Effect of Tip Clearance Height

A pressure-correction based, 3D Navier-Stokes CFD code was used to simulate the effects of turbine parameters on the tip leakage flow and vortex in a linear turbine cascade to understand the detailed flow physics. A baseline case simulation of a cascade was first conducted in order to validate the numerical procedure with experimental measurements. The effects of realistic tip clearance spacing, inlet conditions, and relative endwall motion were then sequentially simulated, while maintaining previously modified parameters. With each additional simulation, a detailed comparison of the leakage flow’s direction, pressure gradient, and mass flow, as well as the leakage vortex and its roll-up, size, losses, location, and interaction with other flow features, was conducted. Part I of this two-part paper focuses on the effect of reduced tip clearance height on the leakage flow and vortex. Reduced tip clearance results in less mass flow through the gap, a smaller leakage vortex, and less aerothermal losses in both the gap and the vortex. The shearing of the leakage jet and passage flow to which leakage vortex roll-up is usually attributed to is not observed in any of the simulations. Alternative explanations of the leakage vortex’s roll-up are presented. Additional secondary flows that are seen near the casing are also discussed.

Numerical Simulation of Tip Leakage Flows in Axial Flow Turbines, With Emphasis on Flow Physics: Part II—Effect of Outer Casing Relative Motion

A pressure-correction based, 3D Navier-Stokes CFD code was used to simulate the effects of turbine parameters on the tip leakage flow and vortex in a linear turbine cascade to understand the detailed flow physics. A baseline case simulation of a cascade was first conducted in order to validate the numerical procedure with experimental measurements. The effects of realistic tip clearance spacing, inlet conditions, and relative endwall motion were then sequentially simulated, while maintaining previously modified parameters. With each additional simulation, a detailed comparison of the leakage flow’s direction, pressure gradient, and mass flow, as well as the leakage vortex and its roll-up, size, losses, location, and interaction with other flow features, was conducted. Part II of this two-part paper series focuses on the effect of relative motion of the outer casing on the leakage flow and vortex development. Casing relative motion results in less mass flow through the gap and a smaller leakage vortex. The structure of the aerothermal losses in the passage change dramatically when the outer casing motion was incorporated, but the total losses in the passage remained very similar. Additional secondary flows that are seen near the casing are also discussed.

Nonaxisymmetric Turbine End Wall Design: Part I— Three-Dimensional Linear Design System

A linear design system, already in use for the forward and inverse design of three-dimensional turbine aerofoils, has been extended for the design of their end walls. This paper shows how this method has been applied to the design of a nonaxisymmetric end wall for a turbine rotor blade in linear cascade. The calculations show that nonaxisymmetric end wall profiling is a powerful tool for reducing secondary flows, in particular the secondary kinetic energy and exit angle deviations. Simple end wall profiling is shown to be at least as beneficial aerodynamically as the now standard techniques of differentially skewing aerofoil sections up the span, and (compound) leaning of the aerofoil. A design is presented that combines a number of end wall features aimed at reducing secondary loss and flow deviation. The experimental study of this geometry, aimed at validating the design method, is the subject of the second part of this paper. The effects of end wall perturbations on the flow field are calculated using a three-dimensional pressure correction based Reynolds-averaged Navier–Stokes CFD code. These calculations are normally performed overnight on a cluster of work stations. The design system then calculates the relationships between perturbations in the end wall and resulting changes in the flow field. With these available, linear superposition theory is used to enable the designer to investigate quickly the effect on the flow field of many combinations of end wall shapes (a matter of minutes for each shape). [S0889-504X(00)00902-8]

Application of an automatic differentiation method to a 2D Navier-Stokes CFD code

The ADIFOR2.0 automatic differentiator is applied to the FLOMGEM flow solver along with the grid generator HYPERG2. The FLOMGEM is an advanced Navier-Stokes CFD code for airfoil computations, which employs multigrid method. The HYPERG2 is a two-dimensional hyperbolic grid generator for generating a C-mesh. Both non-geometric and geometric sensitivity derivative codes are obtained. The sensitivity derivatives via automatic differentiation are presented and compared with divided difference generated derivatives. The study shows that an automatic differentiation method gives accurate derivative values in an efficient manner.

Computation of shock-induced combustion using a detailed methane-air mechanism

The shock-induced combustion of methane-air mixtures in hypersonic flows is investigated using a new reaction mechanism consisting of 19 reacting species and 52 elementary reactions. This reduced model is derived from a full kinetic mechanism via the detailed reduction technique. Zero-dimensional computations of several shocktube experiments are presented first. The computed values for ignition delay and flame speed are in close agreement with experimental data and with results obtained using a full mechanism. The new reaction mechanism is then combined with a fully implicit Navier-Stokes CFD code to simulate two-dimensional and axisymmetric shock-induced combustion experiments of stoichiometric methane-air mixtures at a Mach number of M = 6.61. Good agreement with the experiments is obtained. Furthermore, it is shown that previous calculations were unable to accurately predict this type of flow due to their use of severely limited reduced chemical mechanisms. Finally, applications to the ram accelerator concept are also presented, based on a novel double-ramp configuration.

Computational analysis of off-design waveriders

Comprehensive inviscid and viscous numerical simulations of hypersonic flow past nonconical rounded-nose waveriders are presented. The flow fields and aerodynamic forces at off-design conditions are determined inviscidly by a space-marching CFD code with the initial-data plane provided by a time-marching Navier-Stokes CFD code. Off-design conditions include off-design Mach numbers, angles of attack, and rounded leading edges. A wide range of waverider configurations is investigated and compared. These calculations show the effects of viscous interactions, which are influential near the leading edges, and determine the viscous drag. The inviscid calculations show that L/D decreases as freestream M increases (with alpha = 0). At the on-design Mach numbers, the maximum L/D may occur at slight positive or negative alpha, depending on the shape of the waverider, and zero lift occurs at a negative alpha approximately equal to half of the body thickness. The effects of slight leading-edge blunting produce only local effects in the flow field and small losses in L/D.

Numerical simulation of gas flow in pneumatic components

The flow through pneumatic components is characterized by very complex flow phenomina. In general the flow is viscous, transonic (0≦M≦4) and turbulent. Small geometrical dimensions of pneumatic components make flow measurement difficult or sometimes impossible. Hence the accurate numerical prediction of the flow field becomes of great importance. In this paper we present the theoretical framework and the numerical capabicities of the commercial Navier-Stokes CFD code TASC flow. We solve some test problems which reflect many features of the numerical flow simulation in pneumatic components. For the test cases considered here, TASC flow was found to be an excellent tool for fast and accurate flow simulation.