112 research outputs found
Bayesian quantification of thermodynamic uncertainties in dense gas flows
A Bayesian inference methodology is developed for calibrating complex equations of state used in numerical fluid flow solvers. Precisely, the input parameters of three equations of state commonly used for modeling the thermodynamic behavior of so-called dense gas flows, â i.e. flows of gases characterized by high molecular weights and complex molecules, working in thermodynamic conditions close to the liquid-vapor saturation curveâ, are calibrated by means of Bayesian inference from reference aerodynamic data for a dense gas flow over a wing section. Flow thermodynamic conditions are such that the gas thermodynamic behavior strongly deviates from that of a perfect gas. In the aim of assessing the proposed methodology, synthetic calibration data âspecifically, wall pressure dataâ are generated by running the numerical solver with a more complex and accurate thermodynamic model. The statistical model used to build the likelihood function includes a model-form inadequacy term, accounting for the gap between the model output associated to the best-fit parameters, and the rue phenomenon. Results show that, for all of the relatively simple models under investigation, calibrations lead to informative posterior probability density distributions of the input parameters and improve the predictive distribution significantly. Nevertheless, calibrated parameters strongly differ from their expected physical values. The relationship between this behavior and model-form inadequacy is discussed.ANR-11-MONU-008-00
Predictive RANS simulations via Bayesian Model-Scenario Averaging
The turbulence closure model is the dominant source of error in most Reynolds-Averaged NavierâStokes simulations, yet no reliable estimators for this error component currently exist. Here we develop a stochastic, a posteriori error estimate, calibrated to specific classes of flow. It is based on variability in model closure coefficients across multiple flow scenarios, for multiple closure models. The variability is estimated using Bayesian calibration against experimental data for each scenario, and Bayesian Model-Scenario Averaging (BMSA) is used to collate the resulting posteriors, to obtain a stochastic estimate of a Quantity of Interest (QoI) in an unmeasured (prediction) scenario. The scenario probabilities in BMSA are chosen using a sensor which automatically weights those scenarios in the calibration set which are similar to the prediction scenario. The methodology is applied to the class of turbulent boundary-layers subject to various pressure gradients. For all considered prediction scenarios the standard-deviation of the stochastic estimate is consistent with the measurement ground truth. Furthermore, the mean of the estimate is more consistently accurate than the individual model predictions.ANR UF
Toward improved simulation tools for compressible turbomachinery: assessment of Residual-Based Compact schemes for the transonic compressor NASA Rotor 37
Residual-based-compact schemes (RBC) of 2nd and 3rd-order accuracy are applied to a challenging 3D ow through a transonic compressor. The proposed schemes provide almost mesh-converged solutions in good agreement with experimental data on relatively coarse grids, which allows achieving a given accuracy level with computational cost reductions by a factor between 2 and 4 with respect to standard solvers.FP7- Projet IDIHO
Sensitivity of Supersonic ORC Turbine Injector Designs to Fluctuating Operating Conditions
The design of an efficient organic rankine cycle (ORC) expander needs to take properly into account strong real gas effects that may occur in given ranges of operating conditions, which can also be highly variable. In this work, we first design ORC turbine geometries by means of a fast 2-D design procedure based on the method of characteristics (MOC) for supersonic nozzles characterized by strong real gas effects. Thanks to a geometric post-processing procedure, the resulting nozzle shape is then adapted to generate an axial ORC blade vane geometry. Subsequently, the impact of uncertain operating conditions on turbine design is investigated by coupling the MOC algorithm with a Probabilistic Collocation Method (PCM) algorithm. Besides, the injector geometry generated at nominal operating conditions is simulated by means of an in-house CFD solver. The code is coupled to the PCM algorithm and a performance sensitivity analysis, in terms of adiabatic efficiency and power output, to variations of the operating conditions is carried out
Sensitivity of Supersonic ORC Turbine Injector Designs to Fluctuating Operating Conditions
International audienceThe design of an efficient organic rankine cycle (ORC) expander needs to take properly into account strong real gas effects that may occur in given ranges of operating conditions, which can also be highly variable. In this work, we first design ORC turbine geometries by means of a fast 2-D design procedure based on the method of characteristics (MOC) for supersonic nozzles characterized by strong real gas effects. Thanks to a geometric post-processing procedure, the resulting nozzle shape is then adapted to generate an axial ORC blade vane geometry. Subsequently, the impact of uncertain operating conditions on turbine design is investigated by coupling the MOC algorithm with a Probabilistic Collocation Method (PCM) algorithm. Besides, the injector geometry generated at nominal operating conditions is simulated by means of an in-house CFD solver. The code is coupled to the PCM algorithm and a performance sensitivity analysis, in terms of adiabatic efficiency and power output, to variations of the operating conditions is carried out
Finite-rate chemistry effects in turbulent hypersonic boundary layers: a direct numerical simulation study
The influence of high-enthalpy effects on hypersonic turbulent boundary
layers is investigated by means of direct numerical simulations (DNS). A
quasi-adiabatic flat-plate air flow at free-stream Mach number equal to 10 is
simulated up to fully-developed turbulent conditions using a five-species,
chemically-reacting model. A companion DNS based on a frozen-chemistry
assumption is also carried out, in order to isolate the effect of finite-rate
chemical reactions and assess their influence on turbulent quantities. In order
to reduce uncertainties associated with turbulence generation at the inlet of
the computational domain, both simulations are initiated in the laminar flow
region and the flow is let to evolve up to the fully turbulent regime. Modal
forcing by means of localized suction and blowing is used to trigger
laminar-to-turbulent transition. The high temperatures reached in the near wall
region including the viscous and buffer sublayers activate significant
dissociation of both oxygen and nitrogen. This modifies in turn the
thermodynamic and transport properties of the reacting mixture, affecting the
first-order statistics of thermodynamic quantities. Due to the endothermic
nature of the chemical reactions in the forward direction, temperature and
density fluctuations in the reacting layer are smaller than in the
frozen-chemistry flow. However, the first- and second-order statistics of the
velocity field are found to be little affected by the chemical reactions under
a scaling that accounts for the modified fluid properties. We also observed
that the Strong Reynolds Analogy (SRA) remains well respected despite the
severe hypersonic conditions and that the computed skin friction coefficient
distributions match well the results of the Renard-Deck decomposition extended
to compressible flows
Assessment of a high-order shock-capturing central-difference scheme for hypersonic turbulent flow simulations
High-speed turbulent flows are encountered in most space-related applications
(including exploration, tourism and defense fields) and represent a subject of
growing interest in the last decades. A major challenge in performing
high-fidelity simulations of such flows resides in the stringent requirements
for the numerical schemes to be used. These must be robust enough to handle
strong, unsteady discontinuities, while ensuring low amounts of intrinsic
dissipation in smooth flow regions. Furthermore, the wide range of temporal and
spatial active scales leads to concurrent needs for numerical stabilization and
accurate representation of the smallest resolved flow scales in cases of
under-resolved configurations. In this paper, we present a finite-difference
high-order shock-capturing technique based on Jameson's artificial diffusivity
methodology. The resulting scheme is ninth-order-accurate far from
discontinuities and relies on the addition of artificial dissipation close to
large gradients. The shock detector is slightly revised to enhance its
selectivity and avoid spurious activations of the shock-capturing term. A suite
of test cases ranging from 1D to 3D configurations (namely, shock tubes,
Shu-Osher problem, isentropic vortex advection, under-expanded jet,
compressible Taylor-Green Vortex, supersonic and hypersonic turbulent boundary
layers) is analysed in order to test the capability of the proposed numerical
strategy to handle a large variety of problems, ranging from
calorically-perfect air to multi-species reactive flows. Results obtained on
under-resolved grids are also considered to test the applicability of the
proposed strategy in the context of implicit Large-Eddy Simulations
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