Reynolds stresses and turbulent heat fluxes in fan-shaped and cylindrical film cooling holes

Large eddy simulation (LES) is carried out to predict the thermal performance, unsteady flow patterns, and turbulence characteristics of the fan-shaped hole and the cylindrical hole cooling films. Velocity fields, Reynolds stresses, and turbulent heat flux distributions are the factors that ultimately determine the film cooling effectiveness by deciding the concentration of coolant and heat near the surface. Accurate prediction of these processes can provide a reliable data set for the design of film cooling systems. It can also help solve difficulties of improving RANS-based simulations in terms of their inaccuracies in matching measured flow rates for a given pressure drop across the film and the mixing process once the coolant is ejected from the film. Credible estimates and modest costs of the current LES originate from the multiblock hexahedra meshing, the turbulence inflow generator, and the numerical schemes implemented. Multiblock hexahedra meshes are adopted and generated by a quasi-automatic mesh generation algorithm utilizing template-based multi-block construction followed by elliptic smoothing. Computational resources are utilized in a way that mesh refinement exists only near cooling decorations of small size and wall boundaries, without the need for associated clustering in the far field which is commonly used in commercial packages. The high orthogonality and continuous smoothness throughout the computational domain are maintained to facilitate turbulence capturing. The synthetic inflow generator produces a random field matching a realistic set of two-point statistics based on eigen-reconstruction in a computationally inexpensive processing step. To prove that the current LES simulations of modest cost can reproduce with high accuracy, the discharge characteristics, velocity, and turbulence intensity profiles of cylindrical and fan-shaped cooling films are compared with published measured data on the same flow configurations. Results reveal that the set of modeling methodologies is valid for calculating film-cooling performance within an acceptable range of accuracy. The energy-carrying turbulence structures in and near the cooling holes are shown and their behaviors are linked to turbulent Reynolds stresses. Reynolds stresses and turbulent heat fluxes are compared for different blowing ratios and two types of cooling holes in the parametric study. Similarity patterns of velocity profiles, non-dimensional temperature profiles, Reynolds stress and turbulent heat flux profiles are discussed

Simulation of multistage compressor at off-design conditions

Computational fluid dynamics (CFD) has been widely used for compressor design, yet the prediction of performance and stage matching for multistage, high-speed machines remains challenging. This paper presents the authors' effort to improve the reliability of CFD in multistage compressor simulations. The endwall features (e.g., blade filet and shape of the platform edge) are meshed with minimal approximations. Turbulence models with linear and nonlinear eddy viscosity models are assessed. The nonlinear eddy viscosity model predicts a higher production of turbulent kinetic energy in the passages, especially close to the endwall region. This results in a more accurate prediction of the choked mass flow and the shape of total pressure profiles close to the hub. The nonlinear viscosity model generally shows an improvement on its linear counterparts based on the comparisons with the rig data. For geometrical details, truncated filet leads to thicker boundary layer on the filet and reduced mass flow and efficiency. Shroud cavities are found to be essential to predict the right blockage and the flow details close to the hub. At the part speed, the computations without the shroud cavities fail to predict the major flow features in the passage, and this leads to inaccurate predictions of mass flow and shapes of the compressor characteristic. The paper demonstrates that an accurate representation of the endwall geometry and an effective turbulence model, together with a good quality and sufficiently refined grid, result in a credible prediction of compressor matching and performance with steady-state mixing planes.</p

LASTA 2.0: validation of a reverse time integration method

Shock tube experiments provide critical insight into the thermochemical processes that occur in the shock layer of hypersonic flight vehicles and are used to validate many chemical-kinetic and radiative models for vehicle design. Shock tube flows exhibit a number of non-ideal behaviours that must be accounted for when interpreting experimental data. Previous work has shown that variations in shock speed and boundary layer growth along the length of the shock tube have a strong effect on the test slug properties. The LAgrangian Shock Tube Analysis code (LASTA) is an a posteriori tool that successfully addressed this problem, allowing reconstruction of the test slug from an experimentally obtained shock trajectory. LASTA 2.0 is presented here, which further constrains the test slug properties using an additional experimental pressure boundary condition whose effects are included via a backwards time integration scheme. The tool is validated against ideal gas cases following accelerating and decelerating shock trajectories, each with a tube-end Mach number of 6.5 and a fill pressure of 66.66 Pa. Agreement between the method and results from a viscous, axisymmetric Navier-Stokes solution is found to be within 1% in pressure and temperature in the majority of cases. Improved agreement with experimental data is evident when compared to the previous version of LASTA, particularly where there is strong shock speed non-uniformity

The efficiency of Italian pension funds: costs, membership, assets

The scope of the supplementary pension funds is to provide workers with a satisfactory standard of living at retirement. An efficient and affordable system of pension funds is therefore an important factor to realize the workers’ aims of maximizing the value of their pension wealth. A rationalization of the industry structure, leading to the creation of bigger pension funds, that should be better able to take advantage of economies of scale, might contain the costs sustained by participants. In this paper, and for the first time (to the best of our knowledge), we attempt to carry out an econometric study of the principal factors which determine the costs level and the efficiency of Italian pension funds. Based on an original dataset of Italian closed pension funds in the 2007-2013 period, this work runs a panel estimate of the impact of dimension (the number of participants) on administrative costs. Our results highlight the existence of important overall economies of scale and that in those funds characterized by the outsourcing of some activities, the administrative costs result smaller. We adopt the same dataset also for the open pension funds, in order to evaluate the link between financial costs and the sum of resources under management. The estimates do not confirm the existence of particular economies of scale, probably due to the distinctive traits of the complementary pension funds industry in Italy. The commission fees of the financial management of pension funds, in particular of closed type, are much lower than those relative to other financial services and also to other types of foreign pension funds. This situation, fuelled by competition among financial managers, has gone on for some time, thus further limiting the ways in which savings can be made through an increase in the volume of the assets managed

Generating high-efficiency swimming kinematics using hydrodynamic eigenmode decomposition

This paper explores the use of hydrodynamic eigenmode decomposition as a means of generating optimal swimming kinematics of slender three-dimensional bodies. The eigenvectors of the unsteady hydrodynamic system are used as basis functions for the response to external forcing, such as perturbations generated by the deformation of the body. Exploiting the orthogonality of the modes, we show that swimming according to a single appropriately selected hydrodynamic eigenmode results in high-efficiency swimming. To demonstrate this result, we use an inviscid three-dimensional vortex lattice model to investigate the hydrodynamic eigenmodes of a selection of geometries. We find that for all of the body geometries tested, hydrodynamic efficiency far exceeding that of pure heaving or pitching can be achieved. All eigenmodes tested produce high-efficiency motion, as long as the beat frequency is higher than the mode's “cut-in” frequency for thrust generation. The eigenmodes show qualitative similarity to swimming patterns observed in nature and also correspond well to the existing classifications of undulatory and oscillatory swimming. This study demonstrates that the hydrodynamic eigenmode analysis can generate high-efficiency swimming kinematics based only on information about the body and wake geometry, and as such, this method has significant potential for further development and application to autonomous underwater vehicle design

The efficiency of Italian pension funds: costs, membership, assets

The scope of the supplementary pension funds is to provide workers with a satisfactory standard of living at retirement. An efficient and affordable system of pension funds is therefore an important factor to realize the workers’ aims of maximizing the value of their pension wealth. A rationalization of the industry structure, leading to the creation of bigger pension funds, that should be better able to take advantage of economies of scale, might contain the costs sustained by participants. In this paper, and for the first time (to the best of our knowledge), we attempt to carry out an econometric study of the principal factors which determine the costs level and the efficiency of Italian pension funds. Based on an original dataset of Italian closed pension funds in the 2007-2013 period, this work runs a panel estimate of the impact of dimension (the number of participants) on administrative costs. Our results highlight the existence of important overall economies of scale and that in those funds characterized by the outsourcing of some activities, the administrative costs result smaller. We adopt the same dataset also for the open pension funds, in order to evaluate the link between financial costs and the sum of resources under management. The estimates do not confirm the existence of particular economies of scale, probably due to the distinctive traits of the complementary pension funds industry in Italy. The commission fees of the financial management of pension funds, in particular of closed type, are much lower than those relative to other financial services and also to other types of foreign pension funds. This situation, fuelled by competition among financial managers, has gone on for some time, thus further limiting the ways in which savings can be made through an increase in the volume of the assets managed

Numerical simulation of transpiration cooling on stagnation line in thermochemical non-equilibrium

Sharp leading edges offer drag and maneuverability advantages for hypersonic applications such as powered flight and some gliding trajectories. The reduction in standoff distance resulting from the adoption of sharp edges, however, results in increased surface convective heating that needs to be managed. Ablative layers are unfeasible as a cooling strategy for sharp edges because they lead to a change in surface curvature radius and render systems non-reusable. Among possible strategies, transpiration cooling has been presented as an attractive option in terms of cooling effectiveness and system complexity. In this study, a method is presented for the numerical simulation of transpiration cooling in proximity of the stagnation point of sharp leading edges or tips. The method is suited to both equilibrium and non-equilibrium flight regimes and is based on stagnation line theory presented by Cheng. Transport properties are determined through rigorous Chapman-Enskog theory. Non-equilibrium cases are handled with Park’s two temperature model. The solver explicitly represents coolant flow through a porous medium. Detailed temperature profiles and mole fractions in the shock layer and within the porous medium can be evaluated, with variations in in these profiles computed as functions of flight altitude, speed, leading edge radius, and coolant flow rate and composition. Argon, helium, and nitrogen are tested for their efficacy. The ability of different coolant mixtures to limit the transport of catalytic species to the surface is studied

Numerical simulations of carbon contaminants in T6 shock tube tests

The influence of carbon contamination on a range of synthetic air and pure nitrogen shock tube experiments conducted in Oxford’s T6 Stalker Tunnel is investigated using a numerical model designed for thermochemically reacting flows. Experimental conditions range from 6 to 7 km/s with fill pressures between 18 and 100 Pa. The addition of carbon was found to significantly improve agreement between the numerical model and experimental data, especially after the non-equilibrium peak and during relaxation towards equilibrium. For the chosen thermochemistry set and test conditions, minimal affect on the chemical kinetics of the original test gas was found especially for the neutral species, with minor changes for ion and electron number densities. The performance of the chosen thermochemistry model in radiance regions corresponding to NO and non-equilibrium atomic oxygen was poor, with improvements also required for the parameters governing translational-vibrational relaxation

Fan similarity model for the fan-intake interaction problem

Very high bypass ratio turbofans with large fan tip diameter are an effective way of improving the propulsive efficiency of civil aero-engines. Such engines, however, require larger and heavier nacelles, which partially offset any gains in specific fuel consumptions. This drawback can be mitigated by adopting thinner walls for the nacelle and by shortening the intake section. This binds the success of very high bypass ratio technologies to the problem of designing an intake with thin lips and short diffuser section, which is well matched to a low speed fan. Consequently, the prediction of the mutual influence between the fan and the intake flow represents a crucial step in the design process. Considerable effort has been devoted in recent years to the study of models for the effects of the fan on the lip stall characteristics and the operability of the whole installation. The study of such models is motivated by the wish to avoid the costs incurred by full, three-dimensional (3D) computational fluid dynamics (CFD) computations. The present contribution documents a fan model for fan–intake computations based on the solution of the double linearization problem for unsteady, transonic flow past a cascade of aerofoils with finite mean load. The computation of the flow in the intake is reduced to a steady problem, whereas the computation of the flow in the fan is reduced to one steady problem and a set of solutions of the linearized model in the frequency domain. The nature of the approximations introduced in the fan representation is such that numerical solutions can be computed inexpensively, while the main feature of the flow in the fan passage, namely the shock system and an approximation of the unsteady flow encountered by the fan are retained. The model is applied to a well-documented test case and compares favorably with much more expensive 3D, time-domain computations

Radiative heat transfer measurements of Titan atmospheric entry in a shock tube

Measurements were performed in the T6 Stalker facility operating in Aluminum Shock Tube mode for conditions relevant to Titan entry. Spatially and spectrally resolved radiation emitted from a high-temperature test gas behind a normal shock was recorded by means of emission spectroscopy. For Titan atmospheric entry, the main radiator of interest is cyano radical, formed in the nonequilibrium region behind the shock. The tests reported in this work measured radiation at velocities from 3.1 to 8.5 km/s and freestream pressures of 13, 20, and 133 Pa at a nominal composition of 98% N2 and 2% CH4. These shock layer radiation experiments employed an optical emission spectroscopy system on each side of the facility to allow two spectral regions to be measured simultaneously for each test, covering the spectral range of 200–900 nm. The present work provides a new, comprehensive benchmark set of data relevant to Titan entry studies