James Tyacke

Large Eddy Simulation for Turbines: Methodologies, Cost and Future Outlooks

Flows throughout different zones of turbines have been investigated using large eddy simulation (LES) and hybrid Reynolds-averaged Navier–Stokes-LES (RANS-LES) methods and contrasted with RANS modeling, which is more typically used in the design environment. The studied cases include low and high-pressure turbine cascades, real surface roughness effects, internal cooling ducts, trailing edge cut-backs, and labyrinth and rim seals. Evidence is presented that shows that LES and hybrid RANS-LES produces higher quality data than RANS/URANS for a wide range of flows. The higher level of physics that is resolved allows for greater flow physics insight, which is valuable for improving designs and refining lower order models. Turbine zones are categorized by flow type to assist in choosing the appropriate eddy resolving method and to estimate the computational cost.

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.

LES for Turbines: Methodologies, Cost and Future Outlooks

Flows throughout different zones of turbines have been investigated using Large Eddy Simulation (LES) and hybrid Reynolds-Averaged Navier-Stokes-LES (RANS-LES) methods and contrasted with RANS modelling, more typically used in the design environment. Cases studied include low and high-pressure turbine cascades, real surface roughness effects, internal cooling ducts, trailing edge cut-backs and labyrinth and rim seals. Evidence is presented that shows that LES and hybrid RANS-LES produces higher quality data than RANS/URANS for a wide range of flows. The higher level of physics that is resolved allows greater flow physics insight which is valuable for improving designs and refining lower order models. Turbine zones are categorised by flow type, to assist in choosing the appropriate eddy resolving method and to estimate computational cost.© 2013 ASME

Predictive LES for jet aeroacoustics - current approach and industrial application

The major techniques for measuring jet noise have significant drawbacks, especially when including engine installation effects such as jet–flap interaction noise. Numerical methods including low order correlations and Reynolds-averaged Navier–Stokes (RANS) are known to be deficient for complex configurations and even simple jet flows. Using high fidelity numerical methods such as large eddy simulation (LES) allows conditions to be carefully controlled and quantified. LES methods are more practical and affordable than experimental campaigns. The potential to use LES methods to predict noise, identify noise risks, and thus modify designs before an engine or aircraft is built is a possibility in the near future. This is particularly true for applications at lower Reynolds numbers such as jet noise of business jets and jet-flap interaction noise for under-wing engine installations. Hence, we introduce our current approaches to predicting jet noise reliably and contrast the cost of RANS–numerical-LES (RANS–NLES) with traditional methods. Our own predictions and existing literature are used to provide a current guide, encompassing numerical aspects, meshing, and acoustics processing. Other approaches are also briefly considered. We also tackle the crucial issues of how codes can be validated and verified for acoustics and how LES-based methods can be introduced into industry. We consider that hybrid RANS–(N)LES is now of use to industry and contrast costs, indicating the clear advantages of eddy resolving methods.

LES of jet flow and noise with internal and external geometry features

The noise produced by aeroengines is a critical topic in engine design. Large-Eddy Simulation (LES) and hybrid Reynolds-Averaged Navier-Stokes (RANS)-LES, provides a method to increase understanding of influences on the noise produced and could lead to improved models for use in design. Use of Immersed Boundary (IB) and Body Force Methods (BFM) allows arbitrary geometry to be added rapidly and so this is explored to model internal geometry effects on jet flow and noise. This reduces grid complexity and broadens the accessible design space by reducing setup time and computational cost. Using LES and BFM/IB, many effects that are difficult to test experimentally can be assessed within useful time-frames. The introduction of internal geometry produces complex bypass duct exit flow and a more rapid development of the shear layers which may influence noise produced.

Unsteady CFD Modelling for Electronics Cooling

Numerical simulations of the relatively low Reynolds number flow (Re= 14,200, based on channel height) in a ribbed channel are made using hybrid methods originating from the aerospace industry. Based on Reynolds- averaged Navier Stokes (RANS) and Large Eddy Simulation (LES), hybrid methods are explored using RANS-LES and RANS-Implicit-LES (RANS-ILES) models. Using a hybrid method with simple models we see how coarse we can make the grid and obtain reasonable heat transfer predictions for this particular case. Results from using different LES and hybrid RANS-(I)LES approaches are in better agreement than those using different RANS models..

LES-RANS of installed ultra-high bypass-ratio coaxial jet aeroacoustics with a finite span wing-flap geometry and flight stream-part 1: Round nozzle

© 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. The jet noise produced by aeroengines is a critical topic in engine design. Large-Eddy Simulation (LES) and hybrid LES-Reynolds-Averaged Navier-Stokes (LES-RANS), provides a method to increase understanding of influences on the noise produced. Installed jet noise modelling has received less attention than isolated jet noise yet is becoming more important for design. Here, using LES-RANS, a coaxial nozzle with an ultra-high bypass-ratio of 15 is studied with and without a wing-flap geometry. The bypass ratio leads the nozzle to become extremely close to the wing-flap geometry introducing strong installation effects. Two different flap deflections of 8 and 14 degrees are contrasted with an isolated round nozzle. A flight stream is applied and an FWH surface placement procedure for installed jets is proposed. The installed cases generate more directional noise at mid-low frequencies as the presence of the flap trailing edge produces a strong dipole source. Second order space-time correlations reveal length and time scales in the flow. Fourth order space-time correlations indicate increasing magnitudes of the dominant noise source components with in increasing flap angle and may lead to improved acoustics models.

Predictive LES for Jet Aeroacoustics: Current Approach and Industrial Application

© 2016 by ASME. The major techniques for measuring jet noise have significant drawbacks, especially when including engine installation effects such as jet-flap interaction noise. Numerical methods including low order correlations and Reynolds-Averaged Navier- Stokes (RANS) are known to be deficient for complex configurations and even simple jet flows. Using high fidelity numerical methods such as Large Eddy Simulation (LES) allow conditions to be carefully controlled and quantified. LES methods are more practical and affordable than experimental campaigns. The potential to use LES methods to predict noise, identify noise risks and thus modify designs before an engine or aircraft is built is a possibility in the near future. This is particularly true for applications at lower Reynolds numbers such as jet noise of business jets and jet-flap interaction noise for under-wing engine installations. Hence, we introduce our current approaches to predicting jet noise reliably and contrast the cost of RANS-Numerical-LES (RANS-NLES) with traditional methods. Our own predictions and existing literature are used to provide a current guide, encompassing numerical aspects, meshing and acoustics processing. Other approaches are also briefly considered. We also tackle the crucial issues of how codes can be validated and verified for acoustics and how LES based methods can be introduced into industry. We consider that hybrid RANS-(N)LES is now of use to industry and contrast costs, indicating the clear advantages of eddy resolving methods.

On LES Methods Applied to Seal Geometries

Nine Large Eddy Simulation (LES) methods are used to simulate flow through two labyrinth seal geometries and are compared with a wide range of Reynolds-Averaged Navier-Stokes (RANS) solutions. These involve one-equation, two-equation and Reynolds Stress RANS models. Also applied are linear and nonlinear pure LES models, hybrid RANS-Numerical-LES (RANS-NLES) and Numerical-LES (NLES). RANS is found to have a maximum error and a scatter of 20%. A similar level of scatter is also found among the same turbulence model implemented in different codes. In a design context, this makes RANS unusable as a final solution. Results show that LES and RANS-NLES is capable of accurately predicting flow behaviour of two seals with a scatter of less than 5%. The complex flow physics gives rise to both laminar and turbulent zones making most LES models inappropriate. Nonetheless, this is found to have minimal tangible results impact. In accord with experimental observations, the ability of LES to find multiple solutions due to solution non-uniqueness is also observed.Copyright

Hybrid LES Approach for Practical Turbomachinery Flows: Part 1—Hierarchy and Example Simulations

Unlike Reynolds Averaged Navier Stokes (RANS) models which need calibration for different flow classes, LES (where larger turbulent structures are resolved by the grid and smaller modeled in a fashion reminiscent of RANS) offers the opportunity to resolve geometry dependent turbulence as found in complex internal flows — albeit at substantially higher computational cost. Based on the results for a broad range of studies involving different numerical schemes, LES models and grid topologies an LES hierarchy and hybrid LES related approach is proposed. With the latter, away from walls, no LES model is used, giving what can be termed Numerical LES (NLES). This is relatively computationally efficient and makes use of the dissipation present in practical industrial CFD programs. Near walls, RANS modeling is used to cover over numerous small structures, the LES resolution of which is generally intractable with current computational power. The linking of the RANS and NLES zones through a Hamilton-Jacobi equation is advocated. The RANS-NLES hybridization makes further sense for compressible flow solvers, where, as the Mach number tends to zero at walls, excessive dissipation can occur. The hybrid strategy is used to predict flow over a rib roughened surface and a jet impinging on a convex surface. These cases are important for blade cooling and show encouraging results. Further results are presented in a companion paper.Copyright