High Frequency Response Pressure Transducers Research Articles

Experimental investigation of expansion effect on shock wave boundary layer interaction near a compression ramp

The experiment is conducted to investigate the effect of expansion on the shock wave boundary layer interaction near a compression ramp. The small-angle expansion with an angle degree of 5° occurs at different positions in front of the compression ramp. The particle image velocimetry and flow visualization technology show the flow structures, velocity field, and velocity fluctuation near the compression ramp. The mean pressure distribution, pressure fluctuation, and power spectral density are measured by high-frequency response pressure transducers. The experimental results indicate that the expansion before the compression ramp position affects the shock wave boundary layer interaction to induce a large-scale separation. But the velocity fluctuation and pressure fluctuation are attenuated near the large-scale flow separation region. When the expansion occurs closer to the compression ramp, the expansion has a more significant impact on the flow. The fluctuation of velocity and pressure is significantly attenuated, and the wall pressure rise of the separation point is reduced obviously. And the characteristic low-frequency spectrum signal related to the unsteadiness of the shock wave boundary layer interaction is significantly suppressed. In addition, variation of the separation region scale at different compression angle degrees is distinctive with the effect of expansion.

Collapse behavior of carbon-fiber epoxy cylinders subjected to long-term seawater exposure at seafloor depth pressures

The effects of simulated long-term exposure of carbon-fiber/epoxy (CFE) hollow cylinders to high pressure seawater were investigated experimentally. Utilizing accelerated life testing informed by the Arrhenius methodology, specimens were aged in 41.1 MPa (6000 psi) pressurized 3.5% saline water for durations representing 0, 193.5, and 516 days of service at 3 °C. Hydrostatically and explosively initiated implosion experiments were performed in a large pressure vessel simulating free field conditions. The events were recorded with high-speed stereo photography and analyzed using 3D digital image correlation (DIC) techniques. High frequency response pressure transducers captured a detailed pressure history of each event. Analysis of the collapse phenomena suggests that exposure to the abyssal environment at the timescales investigated had negligible effects on the response of CFE structures subjected to dynamic instability.

Effect of bleed slots on turbocharger centrifugal compressor stability

Bleed slots located in the inducer region of centrifugal compressors have been demonstrated to extend the surge margin with minimal negative impact on performance. This paper describes the impact of bleed slots on surge margin enhancement in a turbocharger centrifugal compressor used for heavy duty diesel engines. The study included evaluation of pressure instabilities throughout the compressor map, characterization of dynamic phenomena occurring at low mass flow rates (mfr), and elucidation of the impacts of bleed slots on the compressor stability and surge line.A comparison of the compressor performance was performed between open and blocked bleed slots. Pressure instability levels were measured throughout the accessible compressor map using a high frequency response pressure transducer at the compressor outlet for the two cases. Static and dynamic pressure measurements were acquired within the diffuser using pressure taps and high frequency response pressure transducers.The compressor with open bleed slot showed lower instability levels at low mfr when not experiencing deep surge. Spectral analysis at low mfr showed that the bleed slot suppresses broadband frequencies linked to piping resonance and below the rpm frequency, thus improving the overall stability. When excited, these frequencies create periodic flow disturbances that originate from the low pressure region below the tongue, at low mfr. The low pressure perturbations are then convected by the flow and propagate around part of or the entire perimeter, depending on the compressor speed.

Elastic mismatch between ePTFE and PLLA vascular grafts in relation to femoral and carotid arteries in humans: in vivo, in vitro and in silico assessment

Many factors have been held responsible for the elevated failure rates of vascular substitutes. Several studies have shown that the lower the mismatches regarding mechanical or functional properties between the native vessel and its vascular substitute, the lower the hemodynamic alterations. From a theoretical point of view, the best substitutes have similar characteristics to the tissue or organ to be replaced. In the present work, samples of electrospun nanofibrous Poly(L-lactic acid) (PLLA) and expanded polytetrafluoroethylene (ePTFE) vascular grafts were subjected to pulsated pressure conditions using an experimental setup. Graft dynamic response was obtained from experimental measurements applying an ultrasonic gold standard method (to measure graft diameter) and a high frequency response pressure transducer (to obtain intraluminal pressure). Similarly, femoral and carotid arteries of normotensive male control subjects and ambulatory male patients with mild to moderate hypertension were explored in vivo in order to relate their mechanical performance to that observed in the synthetic grafts. The obtained results verified the existence of important biomechanical differences between vascular grafts and human arteries. Under normal physiological conditions, the pressure-strain modulus (106dyn/cm2) of ePTFE and PLLA (1095 ± 21.5 and 12.4 ± 1.47, respectively) resulted significantly higher (p < 0.05) than the carotid and femoral modulus values (4.47 ± 1.36 and 5.9 ± 1.42, respectively). During the hypertensive condition (higher pressure levels) ePTFE and PLLA increased their elasticity values to 1136 ± 21.6 and 15.97 ± 2.6, respectively, with respect to an increase of 9.27 ± 3.59 and 15.70 ± 5.96 of carotid and femoral arteries. Additionally, the elastic mismatch generated by the interposition of both grafts between native arteries (carotid or femoral) was quantified by means of computational simulations. PLLA tubular structures manifested a mechanical behavior that mimics the response to pulsatile hypertensive pressure regimes (the lowest reflection coefficient was observed) which constitutes a highly desired property for vascular tissue-engineered scaffold production.

Blade-Row Interaction Effects on Unsteady Stator Loading in an Embedded Compressor Stage

Blade-row interactions drive unsteady blade forces in compressors. This paper presents a perspective on understanding how the pitchwise variations in the flow at the exit of the rotor affect the surface pressures on the downstream vane. The rotor wakes and tip leakage flows are the primary unsteady flow features that drive unsteady lift on the vane. However, those rotor flow features are affected by their interaction with the wakes from upstream vanes. Thus, the interaction with stator 1 and rotor 2 must be understood to adequately characterize the interaction between rotor 2 and stator 2. This paper uses vane clocking, or the circumferential shift in successive vane rows of similar counts, to illuminate these blade-row interaction effects on the downstream vane surface pressure distribution. To accomplish this, experiments are performed in a three-stage axial compressor where high-frequency response pressure transducers are flush-mounted in the stator 2 pressure and suction surfaces are flush-mounted at 50 and 80% spans. Results show that rotor 1–rotor 2 interactions contribute significantly to the changes in unsteady stator surface pressure over the course of a rotor revolution. Also, the rotor 1–rotor 2 interaction levels change in the pitchwise direction downstream of the rotor; thus, the changes in surface pressure are affected by vane clocking. Furthermore, the rotor tip leakage flow is an additional contributor to the unsteady stator surface pressures measured at 80% span, providing an additional pressure peak per blade passing, and thus more high-frequency content (twice the blade-passing frequency).

Experimental Study on the Cavitation Characteristics of an Oil Jet Pump With Multiple Nozzles for the Lubrication System in a Steam Turbine

Cavitation characteristics of oil jet pumps with multiple nozzles were studied using high frequency response pressure transducers with Mobile DTE Light Oil in experiment, which has an environment the same as that in the lubrication system of the steam turbine in a power plant. The influence of working oil pressure, pressure ratio, and area ratio on cavitation characteristics were studied with the area ratio ranging from 4 to 9, and the working oil pressure ranging from 1.8 to 2.8 MPa. Results show that the cavitation erosion on the throat surface is caused by the intense shear layer of high-speed jet in the throat, which leads to the collapse of vortex cavitation bubbles near the throat surface in an oil jet pump with multiple nozzles. What is more, the vortex cavitation is difficult to eradicate in an oil jet pump with multiple nozzles for the lubrication system of a steam turbine. However, there is a working point with low cavitation intensity, which is also the high efficiency point that ensures both relative safety and high efficiency. This study provides quantitative relation for the determination of working oil pressure, area ratio, and pressure ratio of an oil jet pump with multiple nozzles, which is significant for engineering.

The role of variable pitch in motion blades and variable rotational speed in an industrial fan stall

Stall-induced vibration places fundamental limitations on industrial fan performance and remains a persistent problem in the development of industrial fans and compressors. In this paper, we present an experimental programme that utilised two strategies to study rotating stall recovery patterns in an industrial fan. The experimental facility incorporated a fan with both variable pitch in motion blades and a variable speed drive. The facility enabled us first, to study the evolution of unsteady pressure signals on the fan casing whilst varying fan blade pitch and then to examine varying fan rotational speed. Consequently, we were able to recover a fan from stall either by varying blade pitch at constant speed, or by varying speed at constant blade pitch. We also studied the physical flow phenomena associated with stall recovery by cross-correlating signals from circumferentially offset high frequency response pressure transducers and then analysed the cross-spectra. This enabled us to gain insight into the transient fluid flow behaviour associated with stall recovery in the studied industrial fan class. The transient fluid flow behaviour associated with stall recovery by varying blade pitch was indicative of both a mild and progressive transition. In contrast, stall recovery by varying fan speed occurred more suddenly. In comparison with the variable pitch transition, the variable speed transition was more indicative of recovery from mild surge. From this we may conclude that stall recovery via variable speed resulted in significantly higher unsteady mechanical stress induced in fan rotating components than recovery via variable pitch. Higher bending stresses in the fan blades are more likely to result in mechanical failure, and therefore we may conclude that the studied fan is more ‘stall tolerant’ when one operates it as a variable pitch fan rather than as a variable speed fan.

Comparison of Harmonic and Time Marching Unsteady Computational Fluid Dynamics Solutions With Measurements for a Single-Stage High-Pressure Turbine

The unsteady aerodynamics of a single-stage high-pressure turbine has been the subject of a study involving detailed measurements and computations. Data and predictions for this experiment have been presented previously, but the current study compares predictions obtained using the nonlinear harmonic simulation method to results obtained using a time-marching simulation with phase-lag boundary conditions. The experimental configuration consisted of a single-stage high-pressure turbine (HPT) and the adjacent, downstream, low-pressure turbine nozzle row (LPV) with an aerodynamic design that is typical to that of a commercial high-pressure ratio HPT and LPV. The flow path geometry was equivalent to engine hardware and operated at the proper design-corrected conditions to match cruise conditions. The high-pressure vane and blade were uncooled for these comparisons. All three blade rows are instrumented with flush-mounted, high-frequency response pressure transducers on the airfoil surfaces and the inner and outer flow path surfaces, which include the rotating blade platform and the stationary shroud above the rotating blade. Predictions of the time-dependent flow field for the turbine flow path were obtained using a three-dimensional, Reynolds-averaged Navier–Stokes computational fluid dynamics (CFD) code. Using a two blade row computational model of the turbine flow path, the unsteady surface pressure for the high-pressure vane and rotor was calculated using both unsteady methods. The two sets of predictions are then compared to the measurements looking at both time-averaged and time-accurate results, which show good correlation between the two methods and the measurements. This paper concentrates on the similarities and differences between the two unsteady methods, and how the predictions compare with the measurements since the faster harmonic solution could allow turbomachinery designers to incorporate unsteady calculations in the design process without sacrificing accuracy when compared to the phase-lag method.

Characterization of Cavitation Fields From Measured Pressure Signals of Cavitating Jets and Ultrasonic Horns

Cavitation pressure fields under a cavitating jet and an ultrasonic horn were recorded for different conditions using high frequency response pressure transducers. This was aimed at characterizing the impulsive pressures generated by cavitation at different intensities. The pressure signals were analyzed and statistics of the amplitudes and widths of the impulsive pressure peaks were extracted. Plots of number densities and cumulative numbers of peaks as functions of peak amplitude, peak width, and the power of the ultrasonic horn or the jet were generated. The analysis revealed the dominance of pulses with smaller amplitudes and larger durations at lower cavitation intensities and the increase of the amplitudes and reduction of the pulse widths at higher intensities. The ratio of the most probable peak amplitude to peak width was computed. A representative Gaussian curve was then generated for each signal using a characteristic peak amplitude and the corresponding most probable peak duration/width. This resulted in a proposed statistical representation of a cavitation field, useful to characterize cavitation fields of various intensities.

Study of Near-Stall Flow Behavior in a Modern Transonic Fan With Compound Sweep

Detailed flow behavior in a modern transonic fan with a compound sweep is investigated in this paper. Both unsteady Reynolds-averaged Navier-Stokes (URANS) and large eddy simulation (LES) methods are applied to investigate the flow field over a wide operating range. The calculated flow fields are compared with the data from an array of high-frequency response pressure transducers embedded in the fan casing. The current study shows that a relatively fine computational grid is required to resolve the flow field adequately and to calculate the pressure rise across the fan correctly. The calculated flow field shows detailed flow structure near the fan rotor tip region. Due to the introduction of compound sweep toward the rotor tip, the flow structure at the rotor tip is much more stable compared to that of the conventional blade design. The passage shock stays very close to the leading edge at the rotor tip even at the throttle limit. On the other hand, the passage shock becomes stronger and detaches earlier from the blade passage at the radius where the blade sweep is in the opposite direction. The interaction between the tip clearance vortex and the passage shock becomes intense as the fan operates toward the stall limit, and tip clearance vortex breakdown occurs at near-stall operation. URANS calculates the time-averaged flow field fairly well. Details of measured rms static pressure are not calculated with sufficient accuracy with URANS. On the other hand, LES calculates details of the measured unsteady flow features in the current transonic fan with compound sweep fairly well and reveals the flow mechanism behind the measured unsteady flow field.

A study of the unsteady tip flow field of a transonic compressor

An experimental investigation on the unsteady tip flow field of a transonic compressor rotor has been performed. The casing-mounted high frequency response pressure transducers were arranged along both the blade chord and the blade pitch. The chord-wise ones were used to indicate both the ensemble averaged and time varying flow structure of the tip region of the rotor at different operating points under 95% design speed and 60% design speed. The pitch-wise circumferential transducers were mainly used to analyze the unsteadiness frequency of the tip leakage flow in the rotor frame at the near stall condition. The contours of casing wall pressure show that there were two clear low pressure regions in blade passages, one along the chord direction, caused by the leakage flow and the other along the tangential direction, maybe caused by the forward swept leading edge. Both low pressure regions were originated from the leading edge and formed a scissor-like flow pattern. At 95% design speed condition, the shock wave interacted with the low pressure region and made the flow field unsteady. With the mass flow reduced, the two low pressure regions gradually contracted to the leading edge and then a spike disturbance emerged.

Measurement of Decompression Wave Speed in Rich Gas Mixtures at High Pressures (370 bars) Using a Specialized Rupture Tube

Measurements of decompression wave speed in conventional and rich natural gas mixtures following rupture of a high-pressure pipe have been conducted. A high-pressure stainless steel rupture tube (internal diameter=38.1 mm and 42 m long) has been constructed and instrumented with 16 high frequency-response pressure transducers mounted very close to the rupture end and along the length of the tube to capture the pressure-time traces of the decompression wave. Tests were conducted for initial pressures of 33–37 MPa-a and a temperature range of 21–68°C. The experimentally determined decompression wave speeds were compared with both GASDECOM and PIPEDECOM predictions with and without nonequilibrium condensation delays at phase crossing. The interception points of the isentropes representing the decompression process with the corresponding phase envelope of each mixture were correlated with the respective plateaus observed in the decompression wave speed profiles. Additionally, speeds of sound in the undisturbed gas mixtures at the initial pressures and temperatures were compared with predictions by five equations of state, namely, BWRS, AGA-8, Peng–Robinson, Soave–Redlich–Kwong, and Groupe Européen de Recherches Gaziéres. The measured gas decompression curves were used to predict the fracture arrest toughness needed to assure fracture control in natural gas pipelines. The rupture tube test results have shown that the Charpy fracture arrest values predicted using GASEDCOM are within +7% (conservative) and −11% (nonconservative) of the rupture tube predicted values. Similarly, PIPEDECOM with no temperature delay provides fracture arrest values that are within +13% and −20% of the rupture tube predicted values, while PIPEDECOM with a 1°C temperature delay provides fracture arrest values that are within 0% and −20% of the rupture tube predicted values. Ideally, it would be better if the predicted values by the equations of state were above the rupture tube predicted values to make the predictions conservative but that was not always the case.

Pressure behaviour in riser tube of a short airlift pump

The pressure behaviour in riser tubes of a short airlift pump was investigated for various submergence ratios, air discharges and riser tube lengths. Two high-frequency response pressure transducers were placed at various positions along the riser tube. The analysis of the recorded pressure signals gave the mean local pressure at specified vertical positions for various submergence ratios, air discharges and riser tube lengths. The experimental data show a non-linear pressure behaviour near the exit of the air ejector due to the increased friction losses in this region. The pressure behaviour for increasing air discharge at a position along the riser tube is related to the corresponding two-phase flow pattern in the riser tube. The pressure distributions along the riser tube for various submergence ratios were compared with the predictions of the Stenning–Martin model.

Density and compressibility effects in turbulent subsonic jets part 1: mean velocity field

The behavior of compressible jets originated from initially turbulent pipe flows issuing in still air has been investigated at three different subsonic Mach numbers, 0.3, 0.6 and 0.9. Helium, nitrogen and krypton gases were used to generate the jet flows and investigate the additional effects of density on the flow structure. Particle image velocimetry, high-frequency response pressure transducers and thermocouples were used to obtain velocity, Mach number and total temperature measurements inside the flow field. The jets were formed at the exit of an adiabatic compressible frictional turbulent pipe flow, which was developing toward its corresponding sonic conditions inside the pipe, and continued to expand within the first four diameters distance after it exited the pipe. Theoretical considerations based on flow self-similarity were used to obtain the decay of Mach number along the centerline of the jets for the first time. It was found that this decay depends on two contributions, one from the velocity field which is inversely proportional to the distance from the exit and one from the thermal field which is proportional to this distance. As a result, a small non-linearity in the variation of the inverse Mach number with downstream distance was found. The decay of the Mach number at the centerline of the axisymmetric jets increases by increasing the initial Mach number at the exit of the flow for all jets. The decay of mean velocity at the centerline of the jets is also higher at higher exit Mach numbers. However, the velocity non-dimensionalized by the exit velocity seems to decrease faster at low exit Mach numbers, suggesting a reduced mixing with increasing exit flow Mach numbers. Helium jets were found to have the largest spreading rate among the three different gas jets used in the present investigation, while krypton jets had the lowest spreading rate. The spreading rate of each gas decreases with increasing its kinetic energy relatively to its internal energy.

Shock wave impact on monolithic and composite material plates: The preferential aeroelastic response

An experimental investigation was carried out to determine the aeroelastic response of thin flat plates during face-on impact with planar shock waves. The experiments were performed in a large-scale shock tube research facility, which had a working section of 12″ in diameter and a length of 80 ft. One aluminum plate, one stainless steal plate and several composite plates were tested in the present investigation. Miniature semi-conductor strain-gauges of high-frequency response were employed to measure locally the strain on the exterior side of the plates and high-frequency response pressure transducers were used to measure time-dependent wall and total pressure. Due to the elastic deformation of the plates and their reverberation, strong acoustic waves were generated on the external side of the impact which carry a significant signature of the plates’ properties. Composite plates were found to suppress several of the modes of the wave patterns while metallic ones demonstrate a rich variety of interacting modes. The amplitude of the excited acoustic waves, however, was higher in the case of composite plates than in the case of steel plates. The frequency content of the strain signals on the surface of composite plates was not always the same with the content of the surface acceleration measured in free vibration experiments. Calculations by using a coupled system of equations between the fluid and solid phases of monolithic materials provided predictions in good agreement with the measured values of modal frequencies. These theoretical results were also in agreement with the classical modal analysis results by using the Poisson–Kirchoff theory for thin plates under axisymmetric or non-axisymmetric conditions.

Determination of Decompression Wave Speed in Rich Gas Mixtures

AbstractExperimental data on the decompression wave speed of a conventional gas mixture and three other rich gas mixtures with varying C1 from 70 to 82% have been collected. An NPS 2 stainless steel expansion tube test rig has been constructed at TCPL Gas Dynamic Test Facility (GDTF) in Didsbury, AB, Canada, specifically for this task. Decompression wave was generated by a rupture disc at the end of the tube, which was instrumented with eight high frequency response pressure transducers located along the tube. The facility has the following three distinct capabilities: high initial pressures of up to 22 MPa, low initial temperature down to ‐20°C, and flexibility in testing any rich gas mixtures. The decompression wave speed data are presented in terms of the decompression expansion pressure ratio for all four gas mixtures and three initial line pressure of ∼ 10, 14 and 20 MPa. Comparison between experimentally obtained decompression wave speeds and those predicted by GASDECOM is also made for each test.

Intermittent Flow Regimes in a Transonic Fan Airfoil Cascade

A study was conducted in the NASA Glenn Research Center linear cascade on the intermittent flow on the suction surface of an airfoil section from the tip region of a modern low aspect ratio fan blade. Experimental results revealed that, at a large incidence angle, a range of transonic inlet Mach numbers exist where the leading-edge shock-wave pattern was unstable. Flush mounted high frequency response pressure transducers indicated large local jumps in the pressure in the leading edge area, which generates large intermittent loading on the blade leading edge. These measurements suggest that for an inlet Mach number between 0.9 and 1.0 the flow is bi-stable, randomly switching between subsonic and supersonic flows. Hence, it appears that the change in overall flow conditions in the transonic region is based on the frequency of switching between two stable flow states rather than on the continuous increase of the flow velocity. To date, this flow behavior has only been observed in a linear transonic cascade. Further research is necessary to confirm this phenomenon occurs in actual transonic fans and is not the byproduct of an endwall restricted linear cascade.

Averaged and Time-Dependent Aerodynamics of a High Pressure Turbine Blade Tip Cavity and Stationary Shroud: Comparison of Computational and Experimental Results

The unsteady aero-dynamics of a single-stage high-pressure turbine blade operating at design corrected conditions has been the subject of a thorough study involving detailed measurements and computations. The experimental configuration consisted of a single-stage high-pressure turbine and the adjacent, downstream, low-pressure turbine nozzle row. All three blade-rows were instrumented at three spanwise locations with flush-mounted, high-frequency response pressure transducers. The rotor was also instrumented with the same transducers on the blade tip and platform and the stationary shroud was instrumented with pressure transducers at specific locations above the rotating blade. Predictions of the time-dependent flow field around the rotor were obtained using MSU-TURBO, a three-dimensional (3D), nonlinear, computational fluid dynamics (CFD) code. Using an isolated blade-row unsteady analysis method, the unsteady surface pressure for the high-pressure turbine rotor due to the upstream high-pressure turbine nozzle was calculated. The predicted unsteady pressure on the rotor surface was compared to the measurements at selected spanwise locations on the blade, in the recessed cavity, and on the shroud. The rig and computational models included a flat and recessed blade tip geometry and were used for the comparisons presented in the paper. Comparisons of the measured and predicted static pressure loading on the blade surface show excellent correlation from both a time-average and time-accurate standpoint. This paper concentrates on the tip and shroud comparisons between the experiments and the predictions and these results also show good correlation with the time-resolved data. These data comparisons provide confidence in the CFD modeling and its ability to capture unsteady flow physics on the blade surface, in the flat and recessed tip regions of the blade, and on the stationary shroud.

Experimental Methods Applied in a Study of Stall Flutter in an Axial Flow Fan

Flutter testing is an integral part of aircraft gas turbine engine development. In typical flutter testing blade mounted sensors in the form of strain gages and casing mounted sensors in the form of light probes (NSMS) are used. Casing mounted sensors have the advantage of being non-intrusive and can detect the vibratory response of each rotating blade. Other types of casing mounted sensors can also be used to detect flutter of rotating blades. In this investigation casing mounted high frequency response pressure transducers are used to characterize the part-speed stall flutter response of a single stage unshrouded axial-flow fan. These dynamic pressure transducers are evenly spaced around the circumference at a constant axial location upstream of the fan blade leading edge plane. The pre-recorded experimental data at 70% corrected speed is analyzed for the case where the fan is back-pressured into the stall flutter zone. The experimental data is analyzed using two probe and multi-probe techniques. The analysis techniques for each method are presented. Results from these two analysis methods indicate that flutter occurred at a frequency of 411 Hz with a dominant nodal diameter of 2. The multi-probe analysis technique is a valuable method that can be used to investigate the initiation of flutter in turbomachines.

Intermittent Flow Regimes in a Transonic Fan Airfoil Cascade

A study was conducted in the NASA Glenn Research Center (NASA-GRC) linear cascade on the intermittent flow on the suction surface of an airfoil section from the tip region of a modern low aspect ratio fan blade. Experimental results revealed that, at a large incidence angle, a range of transonic inlet Mach numbers exist where the leading-edge shock-wave pattern was unstable. Flush-mounted, high-frequency response pressure transducers indicated large local jumps in the pressure in the leading edge area, which generates large intermittent loading on the blade leading edge. These measurements suggest that for an inlet Mach number between 0.9 and 1.0, the flow is bi-stable, randomly switching between subsonic and supersonic flows. Hence, it appears that the change in overall flow conditions in the transonic region is based on the rate of switching between two stable flow states rather than on the continuous increase of the flow velocity.To date, this flow behavior has only been observed in a linear transonic cascade. Further research is necessary to confirm this phenomenon occurs in actual transonic fans and is not the by-product of an endwall restricted linear cascade.