Unstructured Navier-Stokes Solver Research Articles

Effect of Counterflowing Jet on Supersonic Slender-Body Configurations: A Numerical Study

Previous studies have demonstrated that the use of counterflowing supersonic jets operating in the long penetration mode (LPM), directed from the nose of blunt bodies against a supersonic/hypersonic freestream, can greatly reduce the drag and heat loads on the vehicle. However, the benefits of LPM for slender bodies of revolution operating at low supersonic speeds, applicable to supersonic transport applications, remain largely unknown. This Paper conducts a systematic parametric study for the effects of a counterflowing jet on two slender body models (cone-cylinder and quartic body of revolution, respectively) operating at a freestream Mach number of 1.6, using time-accurate computations performed with an unstructured Navier–Stokes solver. Different ranges of jet flow rates, jet exit Mach numbers, nozzle exit diameters, and jet-to-base diameter ratios are examined systematically. The jet-to-base diameter ratio is determined to be the most important parameter to facilitate the establishment of the LPM regime. Depending on the geometry of the slender body, longer jet penetration (thus larger shock-standoff distance) associated with the LPM does not always result in larger drag reduction. Furthermore, current results indicate that inclusion of an LPM jet is likely to cause large oscillations of surface pressure and drag on these slender geometries.

Zonal Detached Eddy Simulation of a simplified nose landing-gear for flow and noise predictions using an unstructured Navier-Stokes solver

A Zonal Detached Eddy Simulation has been performed on the simplified LAGOON nose landing gear geometry using a Navier-Stokes solver on a fully unstructured grid. The attached boundary layers have been finely resolved using Y+ values in the order of unity, while the high curvature zones have been intensively meshed in order to accurately solving adverse pressure gradients present in these regions. The mean and fluctuating flow fields have been compared with the experimental results, proving that both the mean flow field and the spectral content recorded at the wall are accurately reproduced. Following these comparisons, a detailed analysis of the topology of the flow has been carried out through the analysis of the skin friction coefficient and friction lines, coupled with three dimensional visualizations of the landing gear wake. The far-field acoustics, computed through the Ffowcs-Williams and Hawkings equation from the computed pressure on the landing gear skin, has been compared with the experimental results, obtaining a very good agreement for the different microphones and directions. Finally, the CFD methodology presented in this study proves to be a moderate cost approach, enabling an accurate flow and noise prediction for bluff bodies such as landing gears.

Numerical study of effects of accommodation coefficients on slip phenomena

An unstructured mesh Navier-Stokes solver employing a Maxwell slip boundary condition was developed. The present flow solver was applied to the simulation of flows around an axisymmetric hollow cylinder in a Mach 10.4 freestream, known as Calspan-UB Research Center (CUBRC) Run 14 case, and the velocity slip and the temperature jump on the cylinder surface were investigated. The effect of tangential momentum and thermal accommodation coefficients used in the Maxwell condition was also investigated by adjusting their values. The results show that the reverse flow region is developed on the body surface due to the interaction between the shock and the boundary layer. Also, the shock impingement makes pressure high. The flow properties on the surface agree well with the experimental data, and the velocity slip and the temperature jump vary consistently with the local Knudsen number change. The accommodation coefficients affect the slip phenomena and the size of the flow region. The slip phenomena become larger when both tangential momentum and thermal accommodation coefficients are decreased. However, the range of the reverse flow region decreases when the momentum accommodation coefficient is decreased. The characteristics of the momentum and thermal accommodation coefficients also are overlapped when they are altered together.

Investigation of ELAC 1 Aerodynamics Using CFD in Supersonic Flow

The ELAC 1 (ELliptical Aerodynamic Configuration 1) is basically Hypersonic space vehicle and first stage of TSTO (Two Stage To Orbit) destined to fly up to altitude of 35 Km with M∞=7 (Free stream Mach No.). In this paper, the comparison between the experimental data available at RWTH, Aachen and Polyhedral Unstructured Navier-Stokes solver simulations are made. The different AOA (Angles of attack) 0̊, 6̊ and 10̊ are considered with Re = 3.6×106 for the supersonic flow simulations. The various coefficients such as Aerodynamic coefficients, Pressure coefficients are compared with the wind tunnel data and the physics of the flow field is investigated.

Computation of Free Surface Flows around Box-Shaped Ships by an Unstructured Navier-Stokes Solver

A novel ship concept which is called ultra large block coefficient ship (ULBS) to enable efficient environmentconscious sea transportation is under investigation at Yokohama National University. Since ULBS is supposed to have a very blunt hull, flow field analysis around a ship is crucial for a design of hull forms with better hydrodynamic performance. Computational Fluid Dynamics (CFD) is expected to be an efficient design tool for unconventional hull forms such as ULBS. However, it is desirable to examine applicability of the CFD method before the actual design application. Thus, free-surface flow computations of two box-shaped ships which can be considered as the extreme cases of ULBS are carried out. Grid convergence study is performed with respect to resistance for the verification of the results. Total resistance coefficients are compared with each other and also with available experimental data. The pressure and velocity distributions of the two ships are compared with each other. The flow structures with large separations are observed and the influences of the box geometry to the flow fields are discussed.

AEROELASTIC ANALYSIS OF ROTOR BLADES USING CFD/CSD COUPLING IN HOVER MODE

A computational fluid dynamics (CFD) is coupled with a computational structural dynamics (CSD) to simulate the unsteady rotor flow with aeroelasticity effects. An unstructured upwind Navier-Stokes solver was developed for this simulation, with 2nd order time-accurate dual-time stepping method for temporal discretization and low Mach number preconditioning method. For turbulent flows, both the Spalart-Allmaras and Menter's SST model are available. Mesh deformation is achieved through a fast dynamic grid method called Delaunay graph map method for unsteady flow simulation. The rotor blades are modeled as Hodges & Dowell's nonlinear beams coupled flap-lag-torsion. The rotorcraft computational structural dynamics code employs the 15-dof beam finite element formulation for modeling. The structure code was validated by comparing the natural frequencies of a rotor model with UMARC. The flow and structure codes are coupled tightly with information exchange several times at every time step. A rotor blade model's unsteady flow field in the hover mode is simulated using the coupling method. Effect of blade elasticity with aerodynamic loads was compared with rigid blade.

Error Quantification for Computational Aerodynamics Using an Error Transport Equation

Error quantification in computational fluid dynamics is a subject of increasing interest and research. Although solution verification is conventionally performed using systematic grid refinement, Richardson extrapolation can be restrictive in its applicability, possibly requiring more than three grids to establish monotonic behavior. Ultimately, a single grid error estimation method may prove of greater utility for practical application in computational aerodynamics. In this work, an error transport equation is implemented in a three-dimensional upwind unstructured Navier-Stokes solver. An approach for deriving errors in related quantities of interest from the solution of the error transport equation is presented. Error quantification is demonstrated in two and three dimensions for aerodynamic flows where experimental data are available for comparison. Predicted error bars are found to contain fine grid solutions, test data, and the results of Richardson extrapolation. Furthermore, the error transport equation provides meaningful error quantification for aerodynamic coefficients in cases where Richardson extrapolation cannot be applied.

Whole-annulus aeroelasticity analysis of a 17-bladerow WRF compressor using an unstructured Navier–Stokes solver

This paper describes a large-scale aeroelasticity computation for an aero-engine core compressor. The computational domain includes all 17 bladerows, resulting in a mesh with over 68 million points. The Favre-averaged Navier–Stokes equations are used to represent the flow in a non-linear time-accurate fashion on unstructured meshes of mixed elements. The structural model of the first two rotor bladerows is based on a standard finite element representation. The fluid mesh is moved at each time step according to the structural motion so that changes in blade aerodynamic damping and flow unsteadiness can be accommodated automatically. An efficient domain decomposition technique, where special care was taken to balance the memory requirement across processors, was developed as part of the work. The calculation was conducted in parallel mode on 128 CPUs of an SGI Origin 3000. Ten vibration cycles were obtained using over 2.2 CPU years, though the elapsed time was a week only. Steady-state flow measurements and predictions were found to be in good agreement. A comparison of the averaged unsteady flow and the steady-state flow revealed some discrepancies. It was concluded that, in due course, the methodology would be adopted by industry to perform routine numerical simulations of the unsteady flow through entire compressor assemblies with vibrating blades not only to minimise engine and rig tests but also to improve performance predictions.

A fully distributed unstructured Navier-Stokes solver for large-scale aeroelasticity computations

AbstractThis paper presents the development and validation of a parallel unsteady flow and aeroelasticity code for large-scale numerical models used in turbo machinery applications. The work is based on an existing unstructured Navier-Stokes solver developed over the past ten years by the Aeroelasticity Research Group at Imperial College Vibration University Technology Centre. The single-process multiple-data paradigm was adopted for the parallelisation of the solver and several validation cases were considered. The computational mesh was divided into several sub-sections using a domain decomposition technique. The performance and numerical accuracy of the parallel solver was validated across several computer platforms for various problem sizes. In cases where the solution could be obtained on a single CPU, the serial and parallel versions of the code were found to produce identical results. Studies on up to 32 CPUs showed varying levels of parallelisation efficiency, an almost linear speed-up being obtained in some cases. Finally, an industrial configuration, a 17 blade row turbine with a 47 million point mesh, was discussed to illustrate the potential of the proposed large-scale modelling methodology.

Formula 1 car wheel aerodynamics

A detailed project has been carried out to investigate the aerodynamic performance of a Formula 1 car front wheel. Experimental drag measurements were carried out on a 40% scale rig representing the front right-hand quarter of a generic Formula 1 car, with features such as the front wing and car body modelled accurately to generate a suitable flowfield around the wheel. Smoke flow visualization gave valuable insights into the major flow features and enabled understanding of how the wheel aerodynamic perform- ance might be improved. Further visualization was carried out using a fully 3D CFD (Computational Fluid Dynamics) analysis of the test rig geometry, using state-of-the-art in-house geometry handling and CFD facilities based around an unstructured Navier–Stokes solver. Drag measurements were successfully obtained for a large range of typical car configurations, including front wing endplate variations. The experimental and computational flow visualization enabled a new front wing geometry to be designed, which gave a significant reduction in the drag of the wheel.

A Non-Linear Aeroelasticity Analysis of a Fan Blade Using Unstructured Dynamic Meshes

The main objective of this paper is to present a methodology for the three-dimensional aeroelasticity analysis of turbomachinery blades using an unstructured compressible Navier-Stokes solver for the fluid and a modal model for the structure. The basic fluid solver is constructed in the form of a central difference scheme with explicitly added artificial dissipation which is based upon the fourth- and second-order differences of the solution. The temporal discretization uses an implicit time integration scheme based on a Jacobi relaxation procedure. The structural modes of vibration are determined via a finite element model and the mode shapes are interpolated on to the fluid mesh in a manner that is consistent with general unstructured tetrahedral grids. A spring analogy algorithm that can move the mesh according to the instantaneous shape of a deforming blade has been developed for the accurate tracking of the solid boundaries without creating excessive grid distortions. The performance of the resulting system was examined by considering the aeroelastic behaviour of NASA Rotor 67 fan blade and predictions were compared to experimental results wherever possible. Using a three-dimensional cyclic symmetry model, the tip leading edge time histories were predicted under peak-efficiency and near-stall conditions, and the corresponding aeroelastic natural frequencies and aerodynamic damping values were determined. The blade was found to be stable in all cases considered.