Modal Analysis of Axial Compressor Tip Rotating Instability under Varying Operating Conditions

The full annulus numerical research was performed on a low-speed compressor rotor to investigate the rotating instability in the tip region.The frequency spectra show the existence of rotating instability at narrow stable operating range.With the decrease of flow rate,31 cells of flow disturbance can be found in the instantaneous flow field.The distribution of vortex suggests that the circumferential propagation of the interaction between tip leakage vortex and adjacent blade brings about these cells.The dynamic mode decomposition(DMD)method and spatial discrete Fourier transform(SDFT)were applied to obtain the circumferential mode features,and the results indicate that the rotating instability is associated with the 31 cells of flow disturbance.Then the DMD method was further applied on the pressure data from a circle and an annulus domain,so as to extract different mode components with the corresponding spatial structures,frequencies and amplitudes.The results suggest that DMD modes can display the flow feature and explore the evolution of each instability source in the tip flow field.

Numerical Investigation on the Flow Mechanism of Multi-Peak Frequency Feature of Rotating Instability

In this paper,the rotating instability(RI)in an axial compressor has been investigated numerically in order to examine the capability of URANS method to simulate its typical characteristics of RI broadband humps with multi-peak frequencies(MPFs)and further to uncover the underlying flow mechanism.A full-annulus modeling solution has been adopted to fully capture the wide range of different length-scale flow disturbances that circumferentially propagating around the compressor rotor.During the transient computing process,long-term monitoring up to 50 revolutions has been carried out to achieve a fine frequency resolution,and that would be essential for resolving the MPFs with small frequency interval.It is shown that the MPFs feature of RI has been successfully captured by the full-annulus URANS approach,and also its frequency range and spectral feature agree well with the experimental results.Further,with a circumferential mode decomposition on the MPFs characteristics of RI,it has been found that the MPFs result from the interaction between long-and short-scale flow disturbances which circumferentially propagate around the compressor rotor near the clearance region.zDetailed examination on the numerical three dimensional flow field indicates that the short-scale disturbance is induced by the unsteady oscillation of tip clearance vortexes with inter-passage phase delay.The long-scale disturbance is caused by the mistuning of the wave number of the short-scale disturbance and the blade number within the whole annulus.

Experimental investigation of characteristics of instability evolution in a centrifugal compressor

The stable range of operation for the centrifugal compressor significantly influences the dynamic,economic,and environment-friendly characteristics of power systems.A deep understand-ing of the characteristics of instability evolution is necessary to improve the compressor stability.A centrifugal compressor equipped with a vaneless diffuser is experimentally investigated using high-response static-pressure measurements.The results obtained indicate that three typical rotational-speed ranges exist based on the different instability evolution characteristics,which reveals the var-ious impeller–diffuser matching behaviors over the entire speed range.At low-speed ranges((40%–75%)N_(max),N_(max)is the maximum rotational speed),the compressor exhibits stable,Rotating Insta-bility(RI),impeller stall(diffuser stall),and surge modes.The impeller stall mode is induced by RI and propagates downstream,resulting in the diffuser stall and compressor surge modes.In the medium-speed range((75%–85%)N_(max)),the compressor exhibits stable,diffuser stall,RI,and surge modes.In the high-speed range((85%–100%)N_(max)),the compressor exhibits stable,diffuser stall,and surge modes.The dominant instability position is shifted from the impeller to the diffuser as the rotational speed increases.Both the impeller and diffuser stall present an irregular sawtooth static-pressure wave and exhibit broadband frequency spectrum patterns.

Anatomy of the anterolateral ligament of the knee joint

Despite remarkable improvements in clinical outcomes after anterior cruciate ligament reconstruction,the residual rotational instability of knee joints remains a major concern.The anterolateral ligament(ALL)has recently gained attention as a distinct ligamentous structure on the anterolateral aspect of the knee joint.Numerous studies investigated the anatomy,function,and biomechanics of ALL to establish its potential role as a stabilizer for anterolateral rotational instability.However,controversies regarding its existence,prevalence,and femoral and tibial insertions need to be addressed.According to a recent consensus,ALL exists as a distinct ligamentous structure on the anterolateral aspect of the knee joint,with some anatomic variations.The aim of this article was to review the updated anatomy of ALL and present the most accepted findings among the existing controversies.Generally,ALL originates slightly proximal and posterior to the lateral epicondyle of the distal femur and has an anteroinferior course toward the tibial insertion between the tip of the fibular head and Gerdy’s tubercle below the lateral tibial plateau.

Syndesmotic Internal Brace^(TM) for anatomic distal tibiofibular ligament augmentation

Reconstruction of unstable syndesmotic injuries is not trivial, and there is no generally accepted treatment guidelines. Thus, there still remain considerable controversies regarding diagnosis, classification and treatment of syndesmotic injuries. Syndesmotic malreduction is the most common indication for early re-operation after ankle fracture surgery, and widening of the ankle mortise by only 1 mm decreases the contact area of the tibiotalar joint by 42%. Outcome of ankle fractures with syndesmosis injury is worse than without, even after surgical syndesmotic stabilization. This may be due to a high incidence of syndesmotic malreduction revealed by increasing postoperative computed tomography controls. Therefore, even open visualization of the syndesmosis during the reduction maneuver has been recommended. Thus, the most important clinical predictor of outcome is consistently reported as accuracy of anatomic reduction of the injured syndesmosis. In this context the Tight Rope~?system is reported to have advantages compared to classical syndesmotic screws. However, rotational instability of the distal fibula cannot be safely limited by use of 1 or even 2 Tight Ropes~?. Therefore, we developed a new syndesmotic Internal Brace^(TM) technique for improved anatomic distal tibiofibular ligament augmentation to protect healing of the injured native ligaments. The Internal Brace^(TM) technique was developed by Gordon Mackay from Scotland in 2012 using Swive Locks~? for knotless aperture fixation of a Fiber Tape~? at the anatomic footprints of the augmented ligaments, and augmentation of the anterior talofibular ligament, the deltoid ligament, the spring ligament and the medial collateral ligaments of the knee have been published so far. According to the individual injury pattern,patients can either be treated by the new syndesmotic Internal Brace^(TM) technique alone as a single anterior stabilization, or in combination with one posteriorly directed Tight Rope~? as a double stabilization, or in combination with one Tight Rope~? and a posterolateral malleolar screw fixation as a triple stabilization. Moreover,the syndesmotic Internal Brace^(TM) technique is suitable for anatomic refixation of displaced bony avulsion fragments too small for screw fixation and for indirect reduction of small posterolateral tibial avulsion fragments by anatomic reduction of the anterior syndesmosis with an Internal Brace^(TM) after osteosynthesis of the distal fibula. In this paper, comprehensively illustrated clinical examples show that anatomic reconstruction with rotational stabilization of the syndesmosis can be realized by use of our new syndesmotic Internal Brace^(TM) technique. A clinical trial for evaluation of the functional outcomes has been started at our hospital.

Experimental and Numerical Investigation of Non-synchronous Blade Vibration Excitation in a Transonic Axial Compressor

The complex flow phenomenon of rotating instability(RI) and its induced non-synchronous vibration(NSV) have become a significant challenge as they continuously increase aerodynamic load.This study aims to provide an understanding of the non-synchronous blade vibration phenomenon caused by the rotating instability of a transonic axial compressor rotor.In this case,blade vibrations and non-synchronous excitation are captured by strain gauges and unsteady wall pressure transducer sensors.Unsteady numerical simulations for a full-annulus configuration are used to obtain the non-synchronous flow excitation.The results show that the first-stage rotor blade exhibits an NSV close to the first bending mode;NSV is accompanied by a sharp increase in pressure pulsation;amplitude can reach 20%,and unsteady aerodynamic frequency will lock in a structural mode frequency when the blade vibrates in a large-amplitude motion.The predicted NSV frequency aligns well with the experimental results.The dominant mode of circumferential instability flow structure is approximately 47% of the number blades,and the cell size occupies 2-3 pitches in the circumferential direction.The full-annulus unsteady simulations demonstrate that the streamwise oscillation of the shedding and reattachment vortex structure is the main cause of NSV owing to the strong interaction between the tip leakage and separation vortices near the suction surface.

A high-order model of rotating stall in axial compressors with inlet distortion

In this paper,a high-order distortion model is proposed for analyzing the rotating stall inception process induced by inlet distortion in axial compressors.A distortion-generating screen in the compressor inlet is considered.By assuming a quadratic function for the local flow total pressure-drop,the existing Mansoux model is extended to include the effects of static inlet distortion,and a new high-order distortion model is derived.To illustrate the effectiveness of the distortion model,numerical simulations are performed on an eighteenth-order model.It is demonstrated that long length-scale disturbances emerge out of the distorted background flow,and further induce the onset of rotating stall in advance.In addition,the circumferential non-uniform distribution and time evolution of the axial flow are also shown to be consistent with the existing features.It is thus shown that the high-order distortion model is capable of describing the transient behavior of stall inception and will contribute further to stall detection under inlet distortion.