Simon Eastwood

Developing large eddy simulation for turbomachinery applications

For jets, large eddy resolving simulations are compared for a range of numerical schemes with no subgrid scale (SGS) model and for a range of SGS models with the same scheme. There is little variation in results for the different SGS models, and it is shown that, for schemes which tend towards having dissipative elements, the SGS model can be abandoned, giving what can be termed numerical large eddy simulation (NLES). More complex geometries are investigated, including coaxial and chevron nozzle jets. A near-wall Reynolds-averaged Navier–Stokes (RANS) model is used to cover over streak-like structures that cannot be resolved. Compressor and turbine flows are also successfully computed using a similar NLES–RANS strategy. Upstream of the compressor leading edge, the RANS layer is helpful in preventing premature separation. Capturing the correct flow over the turbine is particularly challenging, but nonetheless the RANS layer is helpful. In relation to the SGS model, for the flows considered, evidence suggests issues such as inflow conditions, problem definition and transition are more influential.

Hybrid LES - RANS of complex geometry jets

Large eddy simulation (LES) type studies are made of a realistic geometry coaxial nozzle with a pylon. For the LES, since the solver being used tends towards having dissipative qualities, the subgrid scale (SGS) model is omitted, giving Numerical LES (NLES). To overcome near wall streak resolution problems a near wall RANS (Reynolds averaged Navier Stokes) model is used giving a hybrid NLES-RANS approach. The pylon is shown to influence the flow development, having a significant impact on peak turbulence levels and spreading rates. The results show that real geometry effects are influential and should be taken into account when moving towards real engine simulations. If their effects are ignored then, based on the studies here, key turbulence parameters will have significant error.

Large-Eddy Simulations and Measurements of a Small-Scale High-Speed Coflowing Jet

Measurements and predictions are made of a short-cowl coflowing jet with a bypass ratio of 8:1. The Reynolds number is 300,000, and the inlet Mach numbers are representative of aeroengine conditions. The low Reynolds number of the measurements makes the case well suited to the assessment of large-eddy-simulation-related strategies. The nozzle concentricity is carefully controlled to deal with the emerging metastability issues of jets with coflow. Measurements of mean quantities and turbulence statistics are made using both laser Doppler anemometry and particle image velocimetry. The simulations are completed on 6 x 10(6), 12 x 10(6), and 50 x 10(6) cell meshes. To overcome near-wall modeling problems, a hybrid large-eddy-simulation–Reynolds-averaged-Navier–Stokesrelated method is used. The near-wall Reynolds-averaged-Navier–Stokes layer is helpful in preventing nonphysical separation from the nozzle wall.

Towards Jet Flow LES of Conceptual Nozzles for Acoustic Predictions

Progress in simulating chevron nozzle jet flows usi ng ILES/RANS-ILES approaches and using the Ffowcs Williams and Hawkings (FW-H) surface integral method to predict the radiated far field sound is presented in this paper . With the focus on the realistic chevron geometries, SMC001 and SMC006, coarse and fine meshes are generated in the range of 3~13 million mesh cells. Throughout this work, to minimize numerical dissipation introduced by mesh quality issues, the hexahedral c ell type is used. Numerical simulations are then carried out with cell-vertex and cell-cent ered codes. Despite the modest grids, mean velocities and turbulent statistics are found to be in reasonable accord with measurements. Also, far field sound levels predicted by the FW -H post processor are encouraging.

Hybrid LES Approach for Practical Turbomachinery Flows—Part II: Further Applications

A hybrid large eddy simulation (LES) related technique is used to explore some key turbomachinery relevant flows. Near wall Reynolds-averaged Navier-Stokes (RANS) modeling is used to cover over especially small scales, the LES resolution of which is generally intractable with current computational power. Away from walls, large eddy type simulation is used but with no LES model (numerical LES (NLES)). Linking of the two model zones through a Hamilton–Jacobi equation is explored. The hybrid strategy is used to predict turbine and compressor end wall flows, flow around a fan blade section, jet flows, and a cutback trailing edge. Also, application of NLES to the flow in an idealized high pressure compressor drum cavity is considered. Generally, encouraging results are found. However, challenges remain, especially for flows where transition modeling is important.

Comparison of les to LDA and PIV measurements of a small scale high speed coflowing jet

Measurements and predictions are made of a short cowl co-flowing jet with a bypass ratio of 8:1. The Reynolds number for computations and measurements are matched at 300,000 and the Mach numbers representative of realistic jet conditions with core and co flow velocities of 240m/s and 216m/s respectively. The low Reynolds number of the measurements makes the case well suited to the assessment of large eddy resolving computational strategies. Also, the nozzle concentricity was carefully controlled to deal with the emerging metastability issues of jets with coflow. Measurements of mean quantities and turbulence statistics are made using both two dimensional coincident LDA and PIV systems. The computational simulations are completed on a modest 12×106 mesh. The simulation is now being run on a 50×106 mesh using hybrid RANSNLES (Numerical Large Eddy Simulation). Close to the nozzle wall a k-l RANS model is used. For an axisymmetric jet, comparison is made between simulations which use NLES, RANSNLES and also a simple imposed velocity profile where the nozzle is not modeled. The use of a near wall RANS model is shown to be beneficial. When compared with the measurements the NLES results are encouraging. Copyright

Contrasting Code Performances for Computational Aeroacoustics of Jets

The numerical propagation of subcritical Tollmein-Schlichting (T-S), inviscid vortical and cut-on acoustic waves is explored. For the former case, the performances of the very different NEAT, NTS, HYDRA, FLUXp and OSMIS3D codes is studied. A modest/coarse hexahedral computational grid that starkly shows differences between the different codes and schemes used in them is employed. For the same order of discretization the five codes show similar results. The unstructured codes are found to propagate vortical and acoustic waves well on triangular cell meshes but not the T-S wave. The above code contrasting exercise is then carried out using implicit LES or Smagorinsky LES for and Ma = 0.9 plane jet on modest 0.5 million cell grids moving to circa 5 million cell grids. For this case, even on the coarse grid, for all codes results were generally encouraging. In general, the spread in computational results is less than the spread of the measurements. Interestingly, the finer grid turbulence intensity levels are slightly more under-predicted than those of the coarse grid. This difference is attributed to the numerical dispersion error having a favourable coarse grid influence. For a non-isothermal jet, HYDRA and NTS also give encouraging results. Peak turbulence values along the jet centreline are in better agreement with measurements than for the isothermal jets. Copyright

Large-Eddy Simulation of Complex Geometry Jets

Computations are made for chevron and coflowing jet nozzles. The latter has a bypass ratio of 6:1. Also, unlike the chevron nozzle, the core flow is heated, making the inlet conditions reminiscent of those for a real engine. A large-eddy resolving approach is used with circa 12x10(6) cell meshes. Because the codes being used tend toward being dissipative the subgrid scale model is abandoned, giving what can be termed numerical large-eddy simulation. To overcome near-wall modeling problems a hybrid numerical large-eddy simulation–Reynolds-averaged Navier–Stokes related method is used. For y 60 a Reynolds-averaged Navier–Stokes model is used. Blending between the two regions makes use of the differential Hamilton–Jabobi equation, an extension of the eikonal equation. For both nozzles, results show encouraging agreement with measurements of other workers. The eikonal equation is also used for ray tracing to explore the effect of the mean flow on acoustic ray trajectories, thus yielding a coherent solution strategy.

Computational modeling of jets with co-flow

An integrated solution approach using Hamilton-Jacobi and Eikonal equations for providing turbulence length scales, sponge, numerical order and also blending control is shown to be promising. For all these aspects essentially distance functions from surfaces can be needed. Although, use of these integrated features focuses on jet noise problems and structured grids perhaps the greater potential is for more complex geometry problems with dense unstructured solution adaptive grids suited for say aircraft landing gear noise modeling. Studies are made exploring optimal scheme options in the high order Large Eddy Simulation (LES) code. Since, for jets, being able to model the initial shear layer instability is important, the propagation of a Tollmien-Schlichting (TS) wave is considered. Comparison with an exact analytical solution of the Orr-Sommerfeld equation for the TS wave is made. On relatively coarse grids (intended to most strongly test the discretization) the code gave excellent agreement with the analytical TS data. The code is also found to propagate well the TS wave through distorted embedded, overset grid sections. A new hybrid Implicit LES (ILES) – Reynolds Averaged Navier Stokes (RANS) method making use of the HamiltonJacobi equation is discussed and applied to jets with co-flow. Using the hybrid ILES-RANS method, the influence of swirl, synthetic chevrons, jet eccentricity, jet external surface taper and width of the co-flow region is explored. Jet centre line velocity decay predictions show encouraging agreement with established data. Results show the co-flow region thickness has a more startling influence on potential core region length than jet eccentricity. As expected, predictions show that with co-flow the acoustic waves are directed more in the downstream direction. Predictions suggest a thick co-flow region has a strong acoustic influence and for a thin co-flow jet eccentricity has a diminished effect on the acoustic field. Mild co-flow angle (<30 and produced by nozzle external surface taper) has little effect on centerline velocity decay.

High-Performance Computing in Jet Aerodynamics

Reducing the noise generated by the propulsive jet of an aircraft engine is of great environmental importance. The ‘jet noise’ is generated by complex turbulent interactions that are demanding to capture numerically, requiring fine spatial and temporal resolution. The use of high-performance computing facilities is essential, allowing detailed flow studies to be carried out that help to disentangle the effects of numerics from flow physics. The scalability and efficiency of algorithms and different codes are also important and are considered in the context of the physical problem being investigated.