Modification of Spalart–Allmaras model with consideration of turbulence energy backscatter using velocity helicity

The increase of the thrust/weight ratio of aircraft engines is extremely restricted by different 3-D flow loss mechanisms. One of them is the corner separation that can form at the junction between a blade suction side and a hub or shroud. In this paper, in order to further investigate the turbulent characteristics of corner separation, large-eddy simulation (LES) is conducted on a compressor cascade configuration using NACA65 blade profiles (chord based Reynolds number: 3.82 × 105), in comparison with the previous obtained experimental data. Using the shear-improved Smagorinsky model as subgrid-scale model, the LES gives a good description of the mean aerodynamics of the corner separation, especially for the blade surface static pressure coefficient and the total pressure losses. The turbulent dynamics is then analyzed in detail, in consideration of the turbulent structures, the one-point velocity spectra, and the turbulence anisotropy. Within the recirculation region, the energy appears to concentrate around the largest turbulent eddies, with fairly isotropic characteristics. Concerning the dynamics, an aperiodic shedding of hairpin vortices seems to induce an unsteadiness of the separation envelope.

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Large-eddy simulation of 3-D corner separation in a linear compressor cascade

In compression systems the stable operating range is limited by rotating stall and/or surge. Two distinct types of stall precursors can be observed prior to full scale instability: the development of long-wavelength modal waves or a short-wavelength, three-dimensional flow breakdown (so-called “spike” stall inception). The cause of the latter is not well understood; in axial machines it has been suggested that rotor blade-tip leakage flow plays an important role, but spikes have recently been observed in shrouded vaned diffusers of centrifugal compressors where these leakage flows are not present, suggesting an alternative mechanism may be at play. This paper investigates the onset of instability in a shrouded vaned diffuser from a highly loaded turbocharger centrifugal compressor and discusses the mechanisms thought to be responsible for the development of short-wavelength stall precursors. The approach combines unsteady 3D RANS simulations of an isolated vaned diffuser with previously obtained experimental results. The unsteady flow field simulation begins at the impeller exit radius, where flow is specified by a spanwise profile of flow angle and stagnation properties, derived from single-passage stage calculations but with flow pitchwise mixed. Through comparison with performance data from previous experiments and unsteady full-wheel simulations, it is shown that the diffuser is accurately matched to the impeller and the relevant flow features are well captured. Numerical forced response experiments are carried out to determine the diffuser dynamic behavior and point of instability onset. The unsteady simulations demonstrate the growth of short-wavelength precursors; the flow coefficient at which these occur, the rotation rate and circumferential extent agree with experimental measurements. Although the computational setup and domain limitations do not allow simulation of the fully developed spike nor full-scale instability, the model is sufficient to capture the onset of instability and allows the postulation of the following necessary conditions: (i) flow separation at the diffuser vane leading edge near the shroud endwall; (ii) radially reversed flow allowing vorticity shed from the leading edge to convect back into the vaneless space; and (iii) recirculation and accumulation of low stagnation pressure fluid in the vaneless space, increasing diffuser inlet blockage and leading to instability. Similarity exists with axial machines, where blade-tip leakage sets up endwall flow in the circumferential direction leading to flow breakdown and the inception of rotating stall. Rather than the tip leakage flows, the cause for circumferential endwall flow in the vaned diffuser is the combination of high swirl and the highly non-uniform spanwise flow profile at the impeller exit.Copyright © 2011 by ASME

An Investigation of Stall Inception in Centrifugal Compressor Vaned Diffuser1

The effects of two types of flow control devices, vortex generators (VGs) and Gurney flaps (GFs), on the power output performance of a multi-megawatt horizontal axis wind turbine is presented To that end, an improved blade element momentum (BEM)-based solver has been developed and BEM-based computations have been carried out on the National Renewable Energy Laboratory (NREL) 5 MW baseline wind turbine The results obtained from the clean wind turbine are compared with the ones obtained from the wind turbine equipped with the flow control devices A significant increase in the average wind turbine power output has been found for all of the flow control device configurations and for the wind speed realizations studied in the present work Furthermore, a best configuration case is proposed which has the largest increase of the average power output In that case, increments on the average power output of 104% and 35% have been found at two different wind speed realizations The thrust force and bending moment in the root of the blade have also been determined and compared with the values of the clean wind turbine A residual increase in the bending moment of less than 1% has been found

https://www.mdpi.com/1996-1073/10/6/742/pdf

Five Megawatt Wind Turbine Power Output Improvements by Passive Flow Control Devices

Turbulence characteristics in corner separation in a highly loaded linear compressor cascade

Six turbulence models frequently used in compressor aerodynamics were employed in the detailed numerical investigations of a low-speed large-scale axial compressor rotor, for which the tip flows were measured in detail with stereoscopic particle image velocimetry, to assess the predictive capabilities of the turbulence models for large-scale vortices in the tip region of the rotor The six turbulence models include: the mixing-length model, the Spalart-Allmaras model, the standard k - e model, the shear-stress transport k - w model, the v 2 - f model, and the Reynolds stress model Their results were carefully discussed and compared with the measurements both on velocity fields and turbulence stresses It was found that the Reynolds stress model is superior to the others in the prediction of the tip-leakage vortex at the design condition, whereas the standard k - e model shows the best results in the prediction of the corner vortex at the near-stall condition Although the simulation could predict the large-scale tip vortices well in the mean flowfield, the computed flow mechanism has large discrepancy with the reality

Yangwei Liu

Papers

Modification of Spalart–Allmaras model with consideration of turbulence energy backscatter using velocity helicity

Turbulence Models Assessment for Large-Scale Tip Vortices in an Axial Compressor Rotor

Turbulence characteristics in corner separation in a highly loaded linear compressor cascade

Investigation of Vortical Structures and Turbulence Characteristics in Corner Separation in a Linear Compressor Cascade Using DDES

Solidity effect on corner separation and its control in a high-speed low aspect ratio compressor cascade