Tip Clearance Vortex Oscillation, Vortex Shedding and Rotating Instabilities in an Axial Transonic Compressor Rotor

2008 ◽  
Author(s):  
Chunill Hah ◽  
Jo¨rg Bergner ◽  
Heinz-Peter Schiffer
Keyword(s):  
Flow Characteristics ◽  
Peak Frequency ◽  
Frequency Component ◽  
Dominant Frequency ◽  
Unsteady Motion ◽  
Wall Pressure ◽  
Tip Clearance ◽  
Fast Fourier Transformation ◽  
Transonic Compressor ◽  
Tip Clearance Vortex

Unsteady flow characteristics in a modern transonic axial compressor operating near stall are studied in detail. Measured data from high-response pressure probes show that the tip clearance vortex oscillates substantially near stall. Instantaneous flow structure varies substantially among different blade passages even with uniform inlet flow. Fast Fourier transformation of measured wall pressure shows a dominant frequency component that is between 30% and 40% of the rotor speed. To identify and analyze this phenomenon, computational studies based on a single passage and full annulus were carried out. The flow field in a transonic compressor near stall is heavily influenced by the unsteady motion of tip clearance vortices. Therefore, a Large Eddy Simulation (LES) was carried out to capture transient characteristics of the tip clearance vortex more realistically. The wall pressure spectrum from the current full annulus analysis also shows a dominant frequency when the rotor operates near stall. The calculated peak frequency is about 30% of the rotor frequency. The dominant frequency, which is non-synchronous with the rotor blade, is due to rotating flow instabilities. Flow interactions across blade passages due to synchronized tip clearance vortex oscillation seem to be the main cause.

Characteristics of Tip Clearance Flow Instability in a Transonic Compressor

2010 ◽  
Author(s):  
Chunill Hah ◽  
Melanie Voges ◽  
Martin Mueller ◽  
Heinz-Peter Schiffer
Keyword(s):  
Flow Field ◽  
Unsteady Flow ◽  
Flow Instability ◽  
Tip Clearance ◽  
Piv Measurements ◽  
Tip Clearance Flow ◽  
Transonic Compressor ◽  
Compressor Rotor ◽  
Clearance Flow ◽  
Tip Clearance Vortex

In the present study, unsteady flow phenomena due to tip clearance flow instability in a modern transonic axial compressor rotor are studied in detail. First, unsteady flow characteristics due the oscillating tip clearance vortex measured with the particle image velocimetry (PIV) and casing-mounted unsteady pressure transducers are analyzed and compared to numerical results with a large eddy simulation (LES). Then, measured characteristic frequencies of the unsteady flow near stall operation are investigated. The overall purpose of the study is to advance the current understanding of the unsteady flow field near the blade tip in an axial transonic compressor rotor near the stall operating condition. Flow interaction between the tip leakage vortex and the passage shock is inherently unsteady in a transonic compressor. The currently applied PIV measurements indicate that the flow near the tip region is unsteady even at the design condition. This self-induced unsteadiness increases significantly as the compressor operates toward the stall condition. PIV data show that the tip clearance vortex oscillates substantially near stall. The calculated unsteady characteristics from LES agree well with the PIV measurements. Calculated unsteady flow fields show that the formation of the tip clearance vortex is intermittent and the concept of vortex breakdown from steady flow analysis does not seem to apply in the current flow field. Fluid with low momentum near the pressure side of the blade close to the leading edge periodically spills over into the adjacent blade passage. The spectral analysis of measured end wall and blade surface pressure shows that there are two dominant frequencies near stall. One frequency is about 40–60% of the rotor rotation and the other dominant frequency is about 40–60% of the blade passing frequency (BPF). The first frequency represents the movement of a large blockage over several consecutive blade passages against the rotor rotation. The second frequency represents traditional tip flow instability, which has been widely observed in subsonic compressors. The LES simulations show that the second frequency is due to movement of the instability vortex.

Locked-In Phenomenon Between Tip Clearance Flow Instabilities and Enforced Blade Motion in Axial Transonic Compressor Rotors

2020 ◽  
Author(s):  
Le Han ◽  
Dasheng Wei ◽  
Yanrong Wang ◽  
Xiaobo Zhang ◽  
Mingchang Fang
Keyword(s):  
Tip Clearance ◽  
Blade Vibration ◽  
Torsional Mode ◽  
Tip Clearance Flow ◽  
Transonic Compressor ◽  
Bending Mode ◽  
Clearance Flow ◽  
Tip Clearance Vortex ◽  
Transonic Rotor ◽  
Phase Differences

Abstract In this paper, tip clearance flow (TCF) instabilities and their relationship to blade motion are investigated numerically on a transonic transonic rotor with a large tip clearance. The numerical methods are verified by comparing with the experimental data of NACA0012 and show reliable results. It is found that the TCF instabilities are caused by the radial vortex formed in passage, which is induced by the interaction of tip clearance vortex (TCV) and main flow. When the blade is enforced vibrating with small amplitude, the results show that TCF instabilities are hardly affected by the blade vibration, and almost no phenomenon of locked-in is found. However, when the amplitude of blade vibration is increased, the interaction becomes stronger and the pressure fluctuation is enhanced. A wider locked-in region is observed. In addition, the simulation results show that the locked-in region is affected significantly by modal shapes. For the rotor here, it seems that the bending mode has a greater effect on the TCV instabilities than the torsional mode and causes a wider locked-in region. In locked-in region, the phase differences between TCV and the blade motion change with the flow conditions. In unlocked region, the period of TCF instabilities fluctuates over time, and the process is similar to that in the locked-in region.

Short Length-Scale Rotating Stall Inception in a Transonic Axial Compressor: Criteria and Mechanisms

2006 ◽  
Author(s):  
Chunill Hah ◽  
Jo¨rg Bergner ◽  
Heinz-Peter Schiffer
Keyword(s):  
Axial Compressor ◽  
Rotating Stall ◽  
Tip Clearance ◽  
Stall Inception ◽  
Tip Clearance Flow ◽  
Transonic Compressor ◽  
Eddy Simulation ◽  
Clearance Flow ◽  
Flow Mechanisms ◽  
Tip Clearance Vortex

The current paper reports on investigations aimed at advancing the understanding of the flow mechanism that leads to the onset of short-length scale rotating stall in a transonic axial compressor. Experimental data show large oscillation of the tip clearance vortex as the rotor operates near the stall condition. Inception of spike-type rotating stall is also measured in the current transonic compressor with high response pressure transducers. Computational studies of a single passage and the full annulus were carried out to identify flow mechanisms behind the spike-type stall inception in the current transonic compressor rotor. Steady and unsteady single passage flow simulations were performed, first to get insight into the interaction between the tip clearance vortex and the passage shock. The conventional Reynolds-averaged Navier-Stokes method with a standard turbulence closure scheme does not accurately reproduce tip clearance vortex oscillation and the measured unsteady pressure field. Consequently, a Large Eddy Simulation (LES) was carried out to capture more relevant physics in the computational simulation of the rotating stall inception. The unsteady random behavior of the tip clearance vortex and it’s interaction with the passage shock seem to be critical ingredients in the development of spike-type rotating stall in a transonic compressor. The Large Eddy Simulation was further extended to the full annulus to identify flow mechanisms behind the measured spike-type rotating stall inception. The current study shows that the spike-type rotating stall develops after the passage shock is fully detached from the blade passages. Interaction between the tip clearance vortex and the passage shock creates a low momentum area near the pressure side of the blade. As the mass flow rate decreases, this low momentum area moves further upstream and reversed tip clearance flow is initiated at the trailing edge plane. Eventually, the low momentum area near the pressure side reaches the leading edge and forward spillage of the tip clearance flow occurs. The flows in the affected blade passage or passages then stall. As the stalled blade passages are formed behind the passage shock, the stalled area rotates counter to the blade rotation just like the classical Emmon’s type rotating stall. Both the measurements and the computations show that the rotating stall cell covers one to two blade passage lengths and rotates at roughly 50% of the rotor speed.

Numerical Investigation of a Single Circumferential Groove Casing Treatment on Three Different Compressor Rotors

2011 ◽  
Author(s):  
F. Heinichen ◽  
V. Gu¨mmer ◽  
H.-P. Schiffer
Keyword(s):  
Aerodynamic Performance ◽  
Tip Clearance ◽  
Shock Interaction ◽  
Casing Treatment ◽  
Axial Compressors ◽  
Transonic Compressor ◽  
Circumferential Groove ◽  
Number Of Publications ◽  
Tip Clearance Vortex ◽  
Casing Treatments

In axial compressors, casing treatments represent a passive method to increase the working range without the need to modify the blade geometry. The majority of the open literature on the topic considers one or several casing treatment variants on the same compressor. The question how one casing treatment and its basic mechanisms can be transferred to a different compressor is only covered in a small number of publications. This paper tries to fill this gap by applying a single circumferential groove type casing treatment to three different transonic compressor rotors. It is demonstrated numerically that the casing treatment is able to improve the aerodynamic performance of all three rotors. Detailed investigation of the flow field near the rotor tip shows that the single circumferential groove works by influencing the interaction between the tip clearance vortex and the shock. Hence, this type of casing treatment can be generalized to transonic rotors with a stall mechanism that is based on tip clearance vortex-shock interaction.

scholarly journals Influence of Tip Clearance on Flow Characteristics of Axial Compressor

Processes ◽  
2020 ◽  
Vol 8 (11) ◽  
pp. 1445
Author(s):  
Moru Song ◽  
Hong Xie ◽  
Bo Yang ◽  
Shuyi Zhang
Keyword(s):  
Flow Field ◽  
Flow Characteristics ◽  
Axial Compressor ◽  
Suction Side ◽  
Rotating Stall ◽  
Tip Clearance ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Tip Leakage ◽  
Tip Clearance Vortex

This paper studies the influence of tip clearance on the flow characteristics related to the performance. Based on full-passage numerical simulation with experimental validation, several clearance models are established and the performance curves are obtained. It is found that there exists an optimum clearance for the stable working range. By analyzing the flow field in tip region, the role of the tip leakage flow is illustrated. In the zero-clearance model, the separation and blockage along the suction side is the main reason for rotating stall. As the tip clearance is increased to the optimum value, the separation is suppressed by the tip leakage flow. However, with the continuing increasing of the tip clearance, the scale and strength of the tip clearance vortex is increased correspondingly. When the tip clearance is larger than the optimum value, the tip clearance vortex gradually dominates the flow field in the tip region, which can increase the unsteadiness in the tip region and trigger forward spillage in stall onset.

Role of Tip-Leakage Vortices and Passage Shock in Stall Inception in a Swept Transonic Compressor Rotor

2004 ◽  
Author(s):  
Chunill Hah ◽  
Douglas C. Rabe ◽  
Aspi R. Wadia
Keyword(s):  
Boundary Layer ◽  
Flow Fields ◽  
Operating Conditions ◽  
Tip Clearance ◽  
Stall Inception ◽  
Tip Clearance Flow ◽  
Transonic Compressor ◽  
Compressor Rotor ◽  
Clearance Flow ◽  
Tip Clearance Vortex

The current paper reports on investigations aimed at advancing the understanding of the flow field near the casing of a forward-swept transonic compressor rotor. The role of tip clearance flow and its interaction with the passage shock on stall inception are analyzed in detail. Steady and unsteady three-dimensional viscous flow calculations are applied to obtain flow fields at various operating conditions. The numerical results are first compared with available measured data. Then, the numerically obtained flow fields are interrogated to identify the roles of flow interactions between the tip clearance flow, the passage shock, and the blade/endwall boundary layers. In addition to the flow field with nominal tip clearance, two more flow fields are analyzed in order to identify the mechanisms of blockage generation: one with zero tip clearance, and one with nominal tip clearance on the forward portion of the blade and zero clearance on the aft portion. The current study shows that the tip clearance vortex does not break down, even when the rotor operates in a stalled condition. Interaction between the shock and the suction surface boundary layer causes the shock, and therefore the tip clearance vortex, to oscillate. However, for the currently investigated transonic compressor rotor, so-called breakdown of the tip clearance vortex does not occur during stall inception. The tip clearance vortex originates near the leading edge tip, but moves downward in the spanwise direction inside the blade passage. A low momentum region develops above the tip clearance vortex from flow originating from the casing boundary layer. The low momentum area builds up immediately downstream of the passage shock and above the core vortex. This area migrates toward the pressure side of the blade passage as the flow rate is decreased. The low momentum area prevents incoming flow from passing through the pressure side of the passage and initiates stall inception. It is well known that inviscid effects dominate tip clearance flow. However, complex viscous flow structures develop inside the casing boundary layer at operating conditions near stall.

EEG Frequency Characteristics in Healthy Advanced Elderly

2018 ◽  
Vol 32 (3) ◽  
pp. 131-139 ◽  
Author(s):  
Kimiko Mizukami ◽  
Akiyoshi Katada
Keyword(s):  
Peak Frequency ◽  
Frequency Component ◽  
Dominant Frequency ◽  
Significant Shift ◽  
Cross Sectional ◽  
Appreciable Change ◽  
Eeg Activity ◽  
Elderly Individuals ◽  
Healthy Elderly ◽  
Power Curves

Abstract. Changes in electroencephalographic (EEG) activity over the course of the normal aging process demonstrate that elderly individuals exhibit an increase in slow wave activity. In this study, we reexamined findings that EEG activity in healthy elderly individuals aged over 85 years (range 85–94) was the same or different compared to individuals younger than 84 years (range 65–84). We performed EEG frequency analysis to investigate the characteristics of resting state frequency components in a cross-sectional study. In individuals aged 85 and older, the dominant frequency component demonstrated a significant shift toward the slower alpha frequency in the left temporal and occipital regions. However, among the oldest individuals, a spectral peak appeared at 9.0–10.0 Hz, and power curves at peak frequency exhibited a sharp rise. Interestingly, healthy individuals older than 85 did not exhibit an appreciable change in EEG, suggesting that “slowing” is not a characteristic sign of aging.

The Effect of a Bend-Slot Casing Treatment on the Blade Tip Flow Field of a Transonic Compressor Rotor

2013 ◽  
Author(s):  
M. Voges ◽  
C. Willert ◽  
R. Mönig ◽  
H.-P. Schiffer
Keyword(s):  
Flow Field ◽  
Numerical Simulations ◽  
Operating Conditions ◽  
Tip Clearance ◽  
Fluid Exchange ◽  
Effective Measure ◽  
Transonic Compressor ◽  
Compressor Rotor ◽  
Blade Tip ◽  
Tip Clearance Vortex

The application of casing treatments (CT) is an effective measure to increase the stable operating range of modern aero engine or gas turbine compressors. As the development and design process of optimized CT geometries is primarily based on numerical simulations, the need for accurate experimental flow field data for related code validation is increasing with the number of applications. While the stall margin enhancement and other stage characteristics can be verified using conventional measurement techniques such as pressure and temperature probes, a deeper insight to the aerodynamic effect of the CT on the rotor flow field can only be provided using non-intrusive, laser-based flow field diagnostics, given that optical access to the compressor stage can be established. The investigation presented herein involved particle image velocimetry (PIV) measurements at high spatial resolution in the blade tip region of the Darmstadt Transonic Compressor Rotor-1 under the influence of a bend-slot CT. Tangential PIV measurement planes were placed at 95% span as well as in the tip gap of the rotor. The investigation included operating conditions at the aerodynamic design point (peak efficiency) and near stall conditions at 100% rpm. Additional reference measurements were performed with the untreated, smooth casing. The experimental study was complimented by numerical simulations of the same compressor and CT geometry using the DLR TRACE code. Based on the combination of both, experimental and numerical flow field results, a detailed analysis of the shock structures and the tip clearance vortex under the influence of the CT was performed. Under the influence of the CT, the fluid exchange between rotor passage and CT slots — driven by the pressure gradient over the blade tip and the leading edge bow shock, respectively — induces secondary flow structures in the tip vortex regime. At near stall conditions the periodical injection of energized fluid out of the CT cavities was identified to be one of the major effects stabilizing the tip clearance vortex and hence delaying the onset of rotating stall.

Experimental Investigation of the Flow in a Forward Swept Transonic Compressor Rotor at Stall Inception

2007 ◽  
Author(s):  
Jo¨rg Bergner ◽  
Heinz-Peter Schiffer
Keyword(s):  
Flow Field ◽  
Stability Limit ◽  
Leading Edge ◽  
Rotating Stall ◽  
Tip Clearance ◽  
Numerical Work ◽  
Stall Inception ◽  
Transonic Compressor ◽  
Compressor Rotor ◽  
Tip Clearance Vortex

Three-dimensional laser-2-focus measurements complemented by measurements of the instantaneous static wall pressure in the casing above the rotor are used to investigate short length-scale rotating stall inception in an axial transonic compressor rotor. The data was collected at the Darmstadt Transonic Compressor using the forward swept “Rotor-3”. Detailed analysis of the experimental data reveals that in this configuration with pronounced forward sweep stall is not directly caused by the blockage created by the shock vortex interaction. Due to the reduced aerodynamic loading, the tip clearance vortex passes the shock without significant deceleration but shows some great fluctuation in terms of vortex strength. As the compressor is throttled to near stall, the tip clearance vortex eventually reaches the leading edge of the adjacent blade. It can be suggested that as an result, spill forward and so-called “self-induced vortex-oscillation” occurs. A phase-lock of both of these phenomena might be the trigger for a spike-type disturbance of the flow-field. The investigation underpins the great importance of the unsteady flow phenomena at near stall. For a thorough understanding of the flow features at the stability limit of a compressor, which is the basis of any effort to increase the operation range, special attention has to be paid to the unsteadiness of the flow in both experimental and numerical work. To study the mechanism of stall inception it might even be necessary to analyze the flow field around the whole annulus, as there appears to be significant interaction of the flow between neighboring passages.

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