Publications

Unsteady Structure of Compressor Tip Leakage Flows

Direct numerical simulations (DNS) are performed of a cantilevered stator blade to identify the unsteady and turbulent flow structure within compressor tip flows. The simulations were performed with clearances of 1.6% and 3.2% of chord. The results show that the flow both within the gap and at the exit on the suction side highly unsteady phenomena controlled by fine-scale turbulent structures. The signature of the classical tip-leakage vortex is a consequence of time-averaging and does not exist in the true unsteady flow. Despite the complexity, we are able to replicate the flow within the tip gap using a validated quasi-three-dimensional (Q3D) model. This enables a wide range of Q3D DNS simulations to study the effects of blade tip corner radius and Reynolds number. Tip corner radius is found to radically alter the unsteady flow in the tip; it affects both separation bubble size and shape, as well as transition mechanisms in the tip flow. These effects can lead to variations in tip ma

Authors:

Maynard, JM ; Wheeler, APS ; Taylor, JV ; Wells R

Publication:

Journal of Turbomachinery

DOI:

DOI10.1115/1.4055769

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Importance of Nonequilibrium Modeling for Compressors

This paper investigates the importance of nonequilibrium boundary-layer modeling for three compressor blade geometries, using RANS and high-fidelity simulations. We find that capturing nonequilibrium effects in RANS is crucial to capturing the correct boundary-layer loss. This is because the production of turbulence within the nonequilibrium region affects both the loss generation in the nonequilibrium region, but also the final equilibrium state. We show that capturing the correct nonequilibrium behavior is possible by adapting industry standard models (in this case the k-omega SST model). We show that for the range of cases studied here, nonequilibrium effects can modify the trailing-edge momentum thickness by up to 40% and can change the trailing-edge shape factor from 1.8 to 2.1.

Authors:

Spencer, Robert ; Przytarski, Pawel; Adami, Paolo

Publication:

Journal of Turbomachinery

DOI:

DOI10.1115/1.4054813

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Desktop DNS : an open toolkit for turbomachinery aerodynamics

The prevailing view is that high fidelity simulation, particularly DNS (direct numerical simulation), is not something for the practical turbomachinery aerodynamicist — requiring too much computational and personal effort to make it worth it. The aim of the ‘Desktop-DNS’ toolkit described in this paper is to change this by greatly lowering the barrier to entry for running DNS. The paper shows how, using an efficient high-order Navier-Stokes computer code, it is becoming increasingly possible to solve testcases of industry relevance with high fidelity LES and DNS, making use of the latest advances in single compute node performance. This is achievable using both efficient algorithms and GPU acceleration. The paper will use a compressor blade testcase to illustrate how, in some cases, high-fidelity simulations can be performed at relatively low costs on a small number of computer nodes. This raises the possibility of a much more widespread use of DNS to inform early design choices, enhan

Authors:

Andrew P. S. Wheeler

Publication:

Proceedings of the ASME Turbo Expo 2023

DOI:

doi.org/10.1115/GT2023-102647

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Accurate Prediction of Loss Using High Fidelity Methods

Further improvements in aero-engine efficiencies require accurate prediction of flow physics and incurred loss. Currently, the computational requirements for capturing these are not known leading to inconsistent loss predictions even for scale-resolving simulations depending on the chosen convergence criteria. This work investigates two aspects of loss generation using high-fidelity simulation. In the first case study, we look at the effect of resolution on capturing entropy generation rate by simulating a Taylor-Green vortex canonical flow. The second case study focuses on the effect of resolution on flow physics and loss generation and uses a compressor cascade subjected to freestream turbulence. The results show that both resolving local entropy generation rate and capturing the inception and growth of instabilities are critical to accuracy of loss prediction. In particular, the interaction of free-stream turbulence at the leading-edge and development of instabilities in the laminar

Authors:

Przytarski, Pawel J. ; Wheeler, Andrew P. S.

Publication:

Journal of Turbomachinery

DOI:

DOI10.1115/1.4050115

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The Effect of Gapping on Compressor Performance

In this paper, we study the effect of rotor-stator axial gap on midspan compressor loss using high-fidelity scale-resolving simulations. For this purpose, we mimic the multi-stage environment using a new numerical method that recycles wake unsteadiness from a single blade passage back into the inlet of the computational domain. As a result, a type of repeating-passage simulation is obtained such as observed by an embedded blade-row. We find that freestream turbulence levels rise significantly as the size of the rotor-stator axial gap is reduced. This is because of the effect of axial gap on turbulence production, which becomes amplified at smaller axial gaps and drives increases in dissipation and loss. This effect is found to raise loss by between 5.5% and 9.5% over the range of conditions tested here. This effect significantly outweighs the beneficial effects of wake recovery on loss.

Authors:

Przytarski, Pawel J. ; Wheeler, Andrew P. S.

Publication:

Journal of Turbomachinery

DOI:

DOI10.1115/1.4047933

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The Effect of Non-Equilibrium Boundary Layers on Compressor Performance

The paper investigates the effect of non-equilibrium behaviour of boundary layers on the profile loss of a compressor. The investigation is undertaken using both direct numerical simulation (DNS) of a mid-height section of a compressor blade and a reduced order model, MISES. The solutions are validated using experimental measurements made in the embedded stage of a multistage low speed compressor. The paper shows that up to 35% of the suction surface boundary layer of the compressor blade exhibits non-equilibrium behaviour. The size of this region reduces as the Reynolds number is increased. The non-equilibrium behaviour was found to reduce profile loss in most cases, however, in a range of cases where transition occurs through a small separation the presence of non-equilibrium behaviour was found to increase profile loss.

Authors:

Andrew P. S. Wheeler, Anthony M. J. Dickens, Robert J. Miller

Publication:

Journal of Turbomachinery

DOI:

10.1115/1.4040094

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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 x 10(6) 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

Authors:

Andrew P. S. Wheeler et al

Publication:

Journal of Turbomachinery

DOI:

10.1115/1.4032435

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Numerical investigation of the flow over a model transonic turbine blade tip

Direct numerical simulations (DNS) are used to investigate the unsteady flow over a model turbine blade tip at engine-scale Reynolds and Mach numbers. The DNS arc performed with an in-house multiblock structured compressible Navier-Stokes solver. The particular case of a transonic tip flow is studied since previous work has suggested that compressibility has an important effect on the turbulent nature of the separation bubble at the inlet to the tip casing gap and subsequent flow reattachment. The flow is simulated over an idealized tip geometry where the tip gap is represented by a constant-area channel with a sharp inlet corner to represent the pressure side edge of the turbine blade. The effects of free-stream disturbances, cross-flow and the pressure side boundary layer on the tip flow aerodynamics and heat transfer are studied. For 'clean' inflow cases we find that even at engine scale Reynolds numbers the tip flow is intermittent in nature, i.e. neither laminar nor fully turbulen

Authors:

Wheeler, APS, Sandberg, RD

Publication:

Journal of Fluid Mechanics

DOI:

10.1017/jfm.2016.478