RANS Turbulence Model Development using CFD-Driven Machine Learning

This paper presents a novel CFD-driven machine learning framework to develop Reynolds-averaged Navier-Stokes (RANS) models. The CFD-driven training is an extension of the gene expression programming method (Weatheritt and Sandberg, 2016), but crucially the fitness of candidate models is now evaluated by running RANS calculations in an integrated way, rather than using an algebraic function. Unlike other data-driven methods that fit the Reynolds stresses of trained models to high-fidelity data, the cost function for the CFD-driven training can be defined based on any flow feature from the CFD results. This extends the applicability of the method especially when the training data is limited. Furthermore, the resulting model, which is the one providing the most accurate CFD results at the end of the training, inherently shows good performance in RANS calculations. To demonstrate the potential of this new method, the CFD-driven machine learning approach is applied to model development for wake mixing in turbomachines. A new model is trained based on a high-pressure turbine case and then tested for three additional cases, all representative of modern turbine nozzles. Despite the geometric configurations and operating conditions being different among the cases, the predicted wake mixing profiles are significantly improved in all of these a posteriori tests. Moreover, the model equation is explicitly given and available for analysis, thus it could be deduced that the enhanced wake prediction is predominantly due to the extra diffusion introduced by the CFD-driven model.Comment: Accepted by Journal of Computational Physic

Rapport I.5 Pump design by viscous calculations

A viscous computer code for designing the meridional channels of high-performance pumps is presented. An averaging technique is used to reduce the three-dimensional flow to a two-dimensional model. The code, based upon an implicit finite difference method for steady two-dimensional incompressible flows, was validated in complex flow geometries prior to application in the design analysis of an actual pump. Viscous effects are taken into account by two different turbulence models. The Navier-Stokes solver is used in conjunction with a standard blade-to-blade calculation by means of an automatic graphic procedure that exchanges geometric and flowfleld data. Various flow calculations are presented and discussed in relation to physical evidence.Martelli Francesco, Michelassi Vittorio. Rapport I.5 Pump design by viscous calculations. In: Machines hydrauliques. Conception et exploitation. Développements récents et Applications aux différents secteurs industriels. Vingtièmes journées de l'hydraulique. Lyon, 4-6 avril 1989. Tome 1, 1989

Pipeline Stage Design For Increased Operating Range.

LecturePg. 45-54With the continuing increase in energy demand, pipeline compressors are being challenged to cope with both large flow rates and broad operating range flexibility, driven by the local seasonal markets. Overall, high efficiency still needs to be maintained, as compressor efficiency will significantly impact on gas transportation cost. Those joint requirements call for significant improvements in pipeline compressor stage design. This intends to highlight the experience gained in the development of these challenging stages in a unique effort combining experimental and computational fluid dynamics (CFD) studies. In particular, the study started with the detailed testing of a conventionally designed stage. Careful comparison of CFD versus test results was followed by intensive activity coordinating between CFD and testing. The intent of those simulations was to identify the limits of this initial design and to indicate the guidelines for improvement. The massive use of state-of-the-art CFD allowed investigation on a large number of impeller configurations. Analysis of the numerical results suggested the possibility of significant operating range improvement with minimal penalty on efficiency and choke limit. An intensive experimental test campaign confirmed the validity of the new impeller design for improved operating range improvement but also highlighted the balance between efficiency and operating range requirements. A final optimization on a statoric component is then performed to get back to original efficiency while retaining increased operating range

Kinetic Combustion Neural Modelling Integrated into Computational Fluid Dynamics

A methodology has been developed to replace traditional chemical kinetics model calculation with Neural Models and applied to air-methane combustion. The reacting flow field has been described taking a detailed chemistry reaction mechanism into account. Convective and turbulent diffusive transport of species have been taken into consideration by means of a well known finite volume CFD code. Two version of such a mechanism have been developed. The first one is based on traditional differential equations representing the specie production rates. Such equations are integrated over the time intervals related to the cell volumes and local volumetric flows.A reduced combustion mechanism involving twenty species and sixty-eight reactions has been developed both for traditional calculation as well as for the Neural Model ones. At the real end combustion flow fields are calculated with kinetics Neural Models with a CPU time forty-two times smaller than that of traditional procedures with a comparable solution accuracy of the combustion flow fields

Direct Numerical Simulations of a High-Pressure Turbine Vane

In this paper, we establish a benchmark data set of a generic high-pressure (HP) turbine vane generated by direct numerical simulation (DNS) to resolve fully the flow. The test conditions for this case are a Reynolds number of 0.57 106 and an exit Mach number of 0.9, which is representative of a modern transonic HP turbine vane. In this study, we first compare the simulation results with previously published experimental data. We then investigate how turbulence affects the surface flow physics and heat transfer. An analysis of the development of loss through the vane passage is also performed. The results indicate that freestream turbulence tends to induce streaks within the near-wall flow, which augment the surface heat transfer. Turbulent breakdown is observed over the late suction surface, and this occurs via the growth of two-dimensional Kelvin–Helmholtz spanwise roll-ups, which then develop into lambda vortices creating large local peaks in the surface heat transfer. Turbulent dissipation is found to significantly increase losses within the trailing-edge region of the vane. [DOI: 10.1115/1.4032435]Partnership for Advanced Computing in Europe (PRACE) and the UK Turbulence Consortium funded by the EPSRC under Grant No. EP/L000261/

Analysis of Vaneless Diffuser Stall Instability in a Centrifugal Compressor

Numerical simulations based on the large eddy simulation approach were conducted with the aim to explore vaneless diffuser rotating stall instability in a centrifugal compressor. The effect of the impeller blade passage was included as an inlet boundary condition with sufficiently low flow angle relative to the tangent to provoke the instability and cause circulation in the diffuser core flow. Flow quantities, velocity and pressure, were extracted to accumulate statistics for calculating mean velocity and mean Reynolds stresses in the wall-to-wall direction. The paper focuses on the assessment of the complex response of the system to the velocity perturbations imposed, the resulting pressure gradient and flow curvature effects.QC 20171211</p

Analysis of Vaneless Diffuser Stall Instability in a Centrifugal Compressor

Numerical simulations based on the large eddy simulation approach were conducted with the aim to explore vaneless diffuser rotating stall instability in a centrifugal compressor. The effect of the impeller blade passage was included as an inlet boundary condition with sufficiently low flow angle relative to the tangent to provoke the instability and cause circulation in the diffuser core flow. Flow quantities, velocity and pressure, were extracted to accumulate statistics for calculating mean velocity and mean Reynolds stresses in the wall-to-wall direction. The paper focuses on the assessment of the complex response of the system to the velocity perturbations imposed, the resulting pressure gradient and flow curvature effects

A NEW SLIP FACTOR CORRELATION FOR CENTRIFUGAL IMPELLERS IN A WIDE RANGE OF FLOW COEFFICIENTS AND PERIPHERAL MACH NUMBERS

ABSTRACT The preliminary design of new centrifugal stages often relies on one-dimensional codes implementing the concept of slip factor. This parameter plays a primary role in the stage design process since it directly affects the calculation of the impeller work coefficient and hence of the components situated downstream. Classical slip factor correlations may not always provide a satisfactory accuracy and generally they fail while attempting at covering a design space in a wide range of flow coefficients and peripheral Mach numbers. In that case the preliminary design has to be refined with more advanced tools, such as computational fluid dynamics (CFD). Often this process needs to be repeated several times before the design cycle ends. In order to predict more effectively the work coefficient as well as to reduce the number of iterations between 1D/CFD codes during the design activity, a new correlation has been developed, which is based on a large number of historical data from both CFD and experimental results. Accurate statistical analyses have shown that slip factor can be strongly linked to significant flow and geometry parameters by means of the outlet deviation angle. As the available calibration dataset gets more and more populated, the presence of specific constants in the structure of the correlation allows the designer to improve the accuracy of predictions