Tip Flow Field Research Articles

Influence of Stator-Rotor Interaction on the Aerothermal Performance of Recess Blade Tips

This paper investigates the influence of stator-rotor interaction on the stage performance of three blade tip geometries. A reference flat tip is used to assess two different recess blade geometries. The study is made in the context of the realistic turbine stage configuration provided by the ETHZ 1.5-stage LISA turbine research facility. This numerical investigation describes the details of unsteady recess cavity flow structure and confirms the beneficial effects of the improved recess geometry over the flat tip and the nominal recess design both in terms of stage efficiency and tip heat load. The tip flow field obtained from the improved recess design combines the advantages of a nominal recess design (aerodynamic sealing) and the flat tip configuration. The turbine stage capacity is almost unchanged between the flat tip and the improved recess tip cases, which simplifies the design procedure when using the improved recess design. The overall heat load in the improved recess case is reduced by 26% compared with the flat tip and by 14% compared with the nominal recess. A key finding of this study is the difference in effects of the upstream stator wake on the recess cavity flow. Where cavity flow in the nominal design is only moderately influenced, the improved recess cavity flow shows enhanced flow unsteadiness. The tip Nusselt number from a purely steady-state prediction in the nominal recess case is nearly identical to the time-average prediction. The improved design shows a 6% difference between steady-state and time-average tip Nusselt number. This is due to the strong influence of the wake passing on the recess cavity flow. In fact, the wake enhances a small flow difference at the leading edge of the recess cavity between the nominal and improved recess cavities, which results in a completely different flow field further downstream in the recess cavity.

Investigation of Blade Tip Interaction With Casing Treatment in a Transonic Compressor—Part II: Numerical Results

A single stage transonic axial compressor was equipped with a casing treatment consisting of 3.5 axial slots per rotor pitch in order to investigate its influence on stall margin characteristics, as well as on the rotor near tip flow field, both numerically and experimentally. Contrary to most other studies, a generic casing treatment (CT) was designed to provide optimal optical access in the immediate vicinity of the CT, rather than for maximum benefit in terms of stall margin extension. The second part of this two-part paper deals with the numerical developments and their validation, carried out in order to efficiently perform time-accurate casing treatment simulations. The numerical developments focus on the extension of an existing coupling algorithm in order to carry out unsteady calculations with any exterior geometry coupled to the main flow passage (in this case a single slot), having an arbitrary pitch. This extension is done by incorporating frequency domain, phase-lagged boundary conditions into this coupling procedure. Whereas the phase lag approach itself is well established and validated for standard rotor-stator calculations, its application to casing treatment simulations is new. Its capabilities and validation will be demonstrated on the given compressor configuration, making extensive use of the detailed particle image velocimetry flow field measurements near the rotor tip. Instantaneous data at all measurement planes will be compared for different rotor positions with respect to the stationary slots in order to evaluate the time-dependent interaction between the rotor and the casing treatment.

J0501-2-1 軸流ファンにおける旋回不安定じょう乱の発生点付近での動翼先端流れ場([J0501-2]流体機械の研究開発におけるEFD/CFD(2))

J0501-2-1 軸流ファンにおける旋回不安定じょう乱の発生点付近での動翼先端流れ場([J0501-2]流体機械の研究開発におけるEFD/CFD(2))

軸流ファンにおけるモーダル波の発生点近傍での動翼先端流れ場(流体工学,流体機械)

軸流ファンにおけるモーダル波の発生点近傍での動翼先端流れ場(流体工学,流体機械)

軸流ファンにおける小スケール失速セルの発生点近傍での動翼先端流れ場(流体工学,流体機械)

Rotor tip flow-field in a low-speed axial-flow fan was investigated by steady numerical simulations to elucidate inception mechanisms of rotating stall initiated from a short length-scale stall cell. Tip leakage flow interacts with the incoming flow at the maximum pressure-rise point. Backflow from the trailing edge is initiated at the tip near the stall inception point and develops at the stall inception point. The interface between the tip leakage flow and the incoming flow becomes parallel to the leading edge plane near the stall inception point and moves upstream of the rotor leading edge at the stall inception point. Moreover, the tip leakage flow spills from the leading edge at the stall inception point. Tip-leakage vortex breakdown occurs and induces the three-dimensional separation on the suction surface of the rotor blade near the stall inception point. At the stall inception point, this separation grows into a three-dimensional separation vortex, which crosses the rotor passage near the trailing edge. It is concluded from the numerical results that the three-dimensional separation vortex and the spillage of tip leakage flow affect the inception of the short length-scale stall cell.

THREE-DIMENSIONAL BOUNDARY LAYER NEAR THE TIP OF HELICOPTER ROTOR

The three-dimensional viscous flow near the rotor tip is one of the most important and difficult problems in rotor dynamics. Most of theoretical investigations focused upon the tip flow field problem confined to inviscid analysis. The detailed study of the complex three-dimensional viscous flow near the tip region can play an important role in determining the generated rotor noise, rotor performances, and dynamic loading of the rotor blade. Through solving the boundary layer flow, the present work is to deeply investigate the viscous flow near the rotor tip. The solution of the boundary layer flow at that region is expected to explain many phenomena that arise in rotor dynamic.

Investigation of Pre-stall Mode and Pip Inception in High-Speed Compressors Through the Use of Correlation Integral

Five high-speed compressor configurations are used to identify pre-stall pressure signal activity under clean and distorted inlet conditions, and under steady injection and controlled injection conditions. Through the use of a nonlinear statistic called correlation integral, variations in the compressor dynamics are identified from the pre-stall pressure activity far before variations (modal or pip) are observed visually in the wall static pressure measurements. The correlation integral not only discerns changing dynamics of these compressors prior to stall, but is now used to measure the strength of the tip flow field for these five high-speed compressors. Results show that correlation integral value changes dramatically when the stall inception is modal; and it changes less severely when the stall inception is through pip disturbances. This algorithm can therefore distinguish from the pre-stall pressure traces when a machine is more likely to stall due to pips versus modes. When used in this manner, the correlation integral thus provides a measure of tip flow strength. The algorithm requires no predisposition about the expected behavior of the data in order to detect changing dynamics in the compressor; thus, no pre-filtering is necessary. However, by band-pass filtering the data, one can monitor changing dynamics in the tip flow field for various frequency regimes. An outcome of this is to associate changes in correlation integral value directly with frequency specific events occurring in the compressor, i.e., blade length scale events versus long length scale acoustic events. The correlation integral provides a potential advantage over linear spectral techniques because a single sensor is used for detection and analysis of the instabilities.

Flow Field Unsteadiness in the Tip Region of a Transonic Compressor Rotor

An experimental investigation concerning tip flow field unsteadiness was performed for a high-performance, state-of-the-art transonic compressor rotor. Casing-mounted high frequency response pressure transducers were used to indicate both the ensemble averaged and time varying flow structure present in the tip region of the rotor at four different operating points at design speed. The ensemble averaged information revealed the shock structure as it evolved from a dual shock system at open throttle to an attached shock at peak efficiency to a detached orientation at near stall. Steady three-dimensional Navier Stokes analysis reveals the dominant flow structures in the tip region in support of the ensemble averaged measurements. A tip leakage vortex is evident at all operating points as regions of low static pressure and appears in the same location as the vortex found in the numerical solution. An unsteadiness parameter was calculated to quantify the unsteadiness in the tip cascade plane. In general, regions of peak unsteadiness appear near shocks and in the area interpreted as the shock-tip leakage vortex interaction. Local peaks of unsteadiness appear in mid-passage downstream of the shock-vortex interaction. Flow field features not evident in the ensemble averaged data are examined via a Navier-Stokes solution obtained at the near stall operating point.

舶用プロペラの流れの可視化

The near flow field of rotating marine propellers were visualized by the dye injection method in the circulating water channel. The model propellers are a propeller with 2 small wings at blade tip (Bladelet propeller) and a conventional propeller (MAU type propeller). The Bladelet propeller has higher performance than the conventional propeller by the diffusion of tip vortex and the decrease of induced drag. The difference of tip vortex and flow field structure between Bladelet propeller and MAU type propeller made clear by flow visualization. And the instantaneous flow field of these marine model propellers were measured by Laser Doppler Velocimeter.