Squeeze film dampers (SFDs) in aircraft engines effectively aid to reduce rotor motion amplitudes, in particular when traversing a critical speed, and help to alleviate rotor whirl instabilities. The current work is a long term endeavor focused on quantifying the dynamic force performance of practical SFDs, exploring novel design damper configurations, and producing physically sound predictive SFD models validated by experimental data. Piston rings (PRs) and O-rings (ORs), commonly used as end seals in SFDs for commercial and military gas turbine engines, respectively, amplify viscous damping in a short physical length and while operating with a modicum of lubricant flow. This paper presents experimental force coefficients (damping and inertia) for two identical geometry SFDs with end seals, one configuration hosts PRs, and the other one ORs. The test rig comprises a stationary journal and bearing cartridge (BC) hosting the SFD and supported on four elastic rods to emulate a squirrel cage. The damper film land length, diameter and clearance are L = 25.4 mm, D = 5 L , and c = 0.373 mm ( D/c = 340), respectively. A supply feeds ISO VG 2 oil to the film land at its middle plane through either one hole or three holes, 2.5 mm in diameter, 120° apart. In the PR-SFD, the lubricant exits thru the slit opening at the ring butted ends. The O-rings suppress oil leakage; hence, lubricant evacuates through a 1 mm hole at ¼ L near one journal end. The O-rings when installed add significant stiffness and damping to the test structure. The ORSFD produces 20% more damping than the PR-SFD, whereas both sealed ends SFDs show similar size added mass. For oil supplied at 0.69 bar(g) through a single orifice produces larger damping, 60% to 80% more than when the damper operates with three oil feedholes. A computational model reproducing the test conditions delivers force coefficients in agreement with the test data. Archival literature calls for measurement of a single pressure signal to estimate SFD reaction forces. For circular centered orbits, the dynamic pressure field, in the absence of any geometrical asymmetry or feed/discharge oil condition, “rotates” around the bearing with a speed equal to the whirl frequency. The paper presents force coefficients estimated from (a) measurements of the applied forces and ensuing displacements, and (b) the dynamic pressure recorded at a fixed angular location and “integrated” over the journal surface. The first method delivers a damping coefficient that is large even with lubricant supplied at a low oil supply pressure whereas the inertia coefficient increases steadily with feed pressure. Predictions show good agreement with the test results, in particular the added mass. On the other hand, identified damping and inertia coefficients from dynamic pressures show a marked difference from one pressure sensor to another, and vastly disagreeing with test results from the first method or predictions. The rationale for the discrepancy relies on local distortions in the dynamic pressure fields that show zones of oil vapor cavitation at a near zero absolute pressure and/or with air ingestion producing high frequency spikes from bubble collapsing; both phenomena depend on the magnitude of the oil supply pressure. An increase in lubricant supply pressure suppresses both oil vapor cavitation and air ingestion which produces an increase of both damping and inertia force coefficients. No prior art compares the performance of a PR-SFD vis-à-vis that of an OR-SFD. Supplying lubricant with a large enough pressure (flow rate) is crucial to avoid the pervasiveness of air ingestion. Lastly, the discussion on force coefficients obtained from two distinct methods questions the use of an oversimplifying assumption; the dynamic pressure field is not invariant in a rotating coordinate frame.

The following work advances a new concept for a hermetically sealed squeeze film damper (HSFD), which does not require an open-flow lubrication system. The hermetically sealed concept utilizes a submersed plunger within a contained fluidic cavity filled with incompressible fluid and carefully controlled end plate clearances to generate high levels of viscous damping. Although the application space for a hermetic damper can be envisioned to be quite broad, the context here will target the use of this device as a rotordynamic bearing support damper in flexibly mounted gas bearing systems. The effort focused on identifying the stiffness and damping behavior of the damper while varying test parameters such as excitation frequency, vibration amplitude, and end plate clearance. To gain further insight to the damper behavior, key dynamic pressure measurements in the damper land were used for identifying the onset conditions for squeeze film cavitation. The HSFD performance is compared to existing gas bearing support dampers and a conventional open-flow squeeze film dampers (SFD) used in turbomachinery. The damper concept yields high viscous damping coefficients an order of magnitude larger than existing oil-free gas bearing supports dampers and shows comparable damping levels to current state of the art open-flow SFD. The force coefficients were shown to contribute frequency dependent stiffness and damping coefficients while exhibiting amplitude independent behavior within operating regimes without cavitation. Finally, using experimentally based force density calculations the data revealed threshold cavitation velocities, approximated for the three end seal clearance cases. To complement the experimental work, a Reynolds based fluid flow model was developed and is compared to the HSFD damping and stiffness results.

This paper analyzes the effects of air in the oil film of a tilting-pad journal bearing on oil-air distributions and characteristics. With a gaseous cavitation model and shear stress transport (SST) model with low-Re correction included, the air backflow from the outlet boundary is analyzed in numerical simulations of a titling-pad journal bearing at 3000 rpm rotation speed and under 180 kN load. The simulated bearing load, pressure and mechanical loss are in good accordance with the experimental data, indicating that the simulation results of the air backflow from the outlet boundary can catch the hydrodynamic characteristics accurately. Based on the analyses of simulated air volume fraction and shear stress, the shear stress of the high-pressure loaded area is mainly influenced by the velocity gradient in the normal direction to the rotor-side wall, not the air backflow and gaseous cavitation. In the unloaded area, the gaseous cavitation occurs around the center part, following the gaseous cavitation mechanisms. The backflow air flows into the low-pressure unloaded area from the outlet boundary and has a clear interval with the air from the gaseous cavitation. The air volume fraction increases with these two air sources and affects the mixture viscosity significantly, eventually influencing the shear stress on the rotor-side wall and bearing mechanical loss.

In this study, the complex modulus of double clearance squeeze film dampers (DCSFD) were experimentally determined under low amplitude unidirectional vibration. A DCSFD consists of two layers of squeeze film separated by an annular floating ring. Influences of the ring mass and clearances were examined. In our experiment, an increase in the damping coefficient of the DCFSD was observed when compared with that of a single clearance squeeze film damper (SCSFD) with the same total clearance as the DCSFD. Compared to theoretical values based on the classical theory of SCSFD, the damping coefficient of water-lubricated DCSFD was significantly higher at frequencies exceeding 700 Hz. Furthermore, DCSFD with larger clearances exhibited higher damping than those with smaller clearances. The origin of the increase in damping coefficient and the decrease in the inertia coefficient were discussed by considering the effect of the ring’s eccentricity, cavitation and turbulence in the film.

Nonlinear dynamic journal bearing modeling within rotordynamic analyses requires the calculation of the nonlinear bearing forces particularly depending on shaft eccentricity and velocity. The bearing forces can be calculated properly using Reynolds differential equation and mass conserving cavitation algorithms, based for example on Elrod’s cavitation algorithm. This approach achieves high model accuracy and allows the consideration of additional effects like misalignment, variable viscosity and transient local oil distribution in the lubricant film. However, despite rising calculating capacity dynamic bearing analyses are still very CPU-time consuming and, consequently, approximation methods are commonly applied in multibody or rotordynamic analyses, especially in day-to-day business. While many approximation procedures are limited to special bearing geometries Glienicke et al. describe a method which is flexible to model different journal bearing geometries, as well as to consider many additional effects like oil supply pressure or starved lubrication conditions in a time averaged manner. It can be applied for both fixed-pad and tilting-pad journal bearings and its characteristic data is included in an a priori calculated map enabling a time-efficient call up of characteristic parameters of the bearing forces from a look-up table in dynamic simulations. Further, the data can be transferred to any other bearing if the requirements of the theory of similarity are supposed to be valid. In this investigation, the method is first successfully extended by the authors to consider misalignment. Secondly, the general idea of the procedure is transferred and applied to thrust bearings in order to enable a six degree of freedom rotordynamic modeling. In case of a simply lateral movement and rotation-symmetric bearing design the procedure is simple, though, in case of tilting movements it becomes more complicated. A misaligned thrust bearing provides tilting and cross-coupling moments. Cross coupling moments are smaller than the main moments, but have similar orders of magnitude and should therefore be considered. Strategies are investigated for a proper approximation of the nonlinear thrust bearing main and cross-coupling forces and moments. All steps are verified using a direct solution of Reynolds differential equation based on an extended mass conserving algorithm adapted from Elrod’s numerical implementation for the stationary case. Finally, the whole procedure and its application to rotordynamic analysis is verified by comparisons with results gained using direct online solution of Reynolds equation in rotordynamic simulation. While good simulation quality of this approximation approach is documented for selected rotor-bearing-systems in literature the range of validity is not clearly defined. Here, the influences of different parameters on the simulation error are investigated conducting different variation calculations for an overhung rotor with documented vibrational behavior from literature. It is shown that the simulation quality depends on the cavitation zone and decreases with rising vibrational velocity. The root cause for this upcoming error and a possible modification for the elimination of this limitation are presented.

The present paper reports the experimental characterization of the cavitation induced flow instabilities of a high head three-bladed inducer at design condition detected simultaneously by means of piezoelectric pressure transducers located at different axial and azimuthal stations on the casing of the pump and through strain gages mounted on the pressure side of each blade. The purpose of this experimental campaign is a deeper insight of the phenomena associated to the flow instabilities obtained by the simultaneous analysis performed in the stationary and rotating frames. Both the sensors set-ups proved to be able to detect the main flow instabilities such as synchronous and sub-synchronous rotating cavitation, cavitation surge and tip vortex instability. Moreover, the simultaneous analysis of the results supported by high speed movie has allowed for assessing the mutual relations between the instabilities.

Boosters are commonly used in liquid propellant rocket engines (LPRE) to allow lower propellant pressures in their storage tanks and, thus, smaller structural masses, contributing to cavitation free operation in the subsequent main turbopumps (TP). Boosters can be identified as key components for the overall performance of large engines, and if their operating requirements are stringent, they can operate under cavitation. Thus, effective design and performance tools are fundamental to design the components of these boosters considering this phenomenon. The simulation techniques based on turbulent and multiphase 3-D Computational Fluid Dynamics (CFD) were used in this work at steady state regime. The simulations were done using the commercial software CFX from ANSYS ® Workbench. The study was conducted analyzing the performance of the first stage of the hydraulic axial turbine of the liquid oxygen (LOX) booster of the Space Shuttle Main Engine (SSME), at various operation points under cavitation, considering 3.0% tip clearance relative to blade height. The results obtained for, the performance parameters of this stage were compared with those obtained through monophase simulation, and the multiphase technique showed results closer to the experimental ones around the design point (DP), with increased simulation times acceptable for the computational resources currently available. Moreover, the results from the current work show the importance of considering the effects of cavitation through multiphase flow in hydraulic turbines.

This study investigates the behavior of a turbopump assembly during critical cavitation of the propellant pumps in the upper rocket engine of the Korea Space Launch Vehicle-II. Turbopumps operate under conditions involving low pressure at the pump inlet and high rotational speeds to allow for a lightweight design. This severe environment can easily cause cavitation to occur in the pump. This cavitation can then cause the pump operation to fail. As the cavitation number in the pump decreases below the critical point, the pump fails to operate. There is concern regarding the behavior of the turbopump assembly arising from pump failure due to cavitation. It is necessary to verify the problems that may occur if the turbopump assembly operates under extreme conditions, such like the critical cavitation. This study performed tests to investigate the breakdown of pumps in the turbopump assembly. Tests were conducted with liquid nitrogen, water, and high-pressure air instead of the mediums used during actual operation of liquid oxygen, kerosene, and hot gas. The turbopump was tested at the design point of 27,000 rpm, while the inlet pressure of each pump was controlled to approach the critical cavitation number. The turbine power output was maintained during the tests. The results show that the breakdown point of the oxidizer pump using liquid nitrogen, which is a cryogenic medium, occurred at a lower cavitation number than during an individual component suction performance test using water. The fuel pump using water, meanwhile, experiences breakdown at similar cavitation numbers in both tests. As the breakdown of the pump occurs, the power required by that pump decreases, and the rotational speed of the turbopump increases. Compared with individual pump suction performance tests, this breakdown test can be used to determine the limit of the propellant inlet pressure of the turbopump and to characterize the behavior of the turbopump assembly when a breakdown occurs. Vibrations were also analyzed for tests at a high cavitation number and at the critical cavitation number. The vibration increased with breakdown and notable frequencies were analyzed.

In this paper we present and validate a shape optimization framework for the design of splitter blades that extends the operative range under cavitation while maintaining the wetted performance of rocket engine turbopumps. For a target turbopump application, the optimization framework allows for independent changes to the blade angle distributions across the span and to the pitchwise position of the splitter blades while preserving the thickness distributions. The optimization is conducted with a surrogate-based gradient method. The geometry is optimized at a fixed cavitation number corresponding to a 5% head coefficient dropoff, while constraints are imposed on the wet pump performance. It is found that this approach, coupled with the optimal design points distribution provided by the Design of Experiment method, reduces the computational cost of the optimization process by minimizing the number of multiphase calculations. The numerical results suggest that the optimized splitter blades successfully increase the pump operative range by 2.2% and increase the head coefficient by 5.3% compared to the baseline case with non-optimized splitters. These results are corroborated by experiments conducted in a closed-loop water test facility. Several pump geometries are tested through rapid prototyping using additive manufacturing. The experimental data validate the optimization framework, demonstrating a 4.7% increase of pump operative range and a 7.6% increase in head coefficient. The calculations are used to gain insight in the physical mechanisms for the performance improvement. The analysis of the results indicates that the improved performance is due to the optimized position and shape of the splitter blades which increase the pump slip factor.

At present, advanced gas turbine are designed to work at increasingly higher turbine inlet temperatures (TIT) to obtain higher thermal efficiency and power. TIT of modern aircraft engine has exceeded temperature limit of high-temperature alloys application over 600C°. So turbine blade has to be cooled in order to work at an acceptable temperature environment. Double wall cooling technology with higher cooling efficiency which is developed by the RR and GE companies has become development trends of blade cooling. In this work, a flat double wall cooling structure is introduced. On basis of creep damage model considering interaction of cavitation and degradation, the creep behaviors of double wall cooling structure are investigated. Research shows that there are stress interferences among holes and pin fin. The higher stress region locates around holes. The lower stress region locates at pin fin. The distance of impingement hole and pin fin in the analysis model affect creep behavior seriously.

The recent growth of private options in launch vehicles has substantially raised price competition in the space launch market. This has increased the need to deliver reliable launch vehicles at reduced engine development cost, and has led to increased industrial interest in reduced order models. Large-scale liquid rocket engines require high-speed turbopumps to inject cryogenic propellants into the combustion chamber. These pumps can experience cavitation instabilities even when operating near design conditions. Of particular concern is rotating cavitation, which is characterized by an asymmetric cavity rotating at the pump inlet, which can cause severe vibration, breaking of the pump and loss of the mission. Despite much work in the field, there are limited guidelines to avoid rotating cavitation during design and its occurrence is often assessed through costly experimental testing. This paper presents a source term based model for stability assessment of rocket engine turbopumps. The approach utilizes mass and momentum source terms to model cavities and hydrodynamic blockage in inviscid, single-phase numerical calculations, reducing the computational cost of the calculations by an order of magnitude compared to traditional numerical methods. Comparison of the results from the model with experiments and high-fidelity calculations indicates agreement of the head coefficient and cavity blockage within 0.26% and 5% respectively. The computations capture rotating cavitation in a 2D inducer at the expected flow coefficient and cavitation number. The mechanism of formation and propagation of the instability is correctly reproduced.

The effects of axial casing grooves on the performance and flow structures in the tip region of an axial low speed fan rotor have been studied experimentally in the JHU refractive index-matched liquid facility. The four-per-passage semicircular grooves are skewed by 45° in the positive circumferential direction, and have a diameter of 65% of the rotor blade axial chord length. A third of the groove overlaps with the blade front, and the rest extends upstream. These grooves have a dramatic effect on the machine performance, reducing the stall flow rate by 40% compared to the same machine with a smooth endwall. However, they reduce the pressure rise at high flow rates. The flow characterization consists of qualitative visualizations of vortical structures using cavitation, as well as stereo-PIV (SPIV) measurements in several meridional and ( z,θ ) planes covering the tip region and interior of the casing grooves. The experiments are performed at a flow rate corresponding to pre-stall conditions for the untreated machine. They show that the flow into the downstream sides of the grooves and the outflow from their upstream sides vary periodically. The inflow peaks when the downstream end is aligned with the pressure side (PS) of the blade, and decreases, but does not vanish, when this end is located near the suction side (SS). These periodic variations have three primary effects: First, substantial fractions of the leakage flow and the tip leakage vortex (TLV) are entrained periodically into the groove. Consequently, in contrast to the untreated flow, The TLV remnants remain confined to the vicinity of the entrance to the groove, and the TLV strength diminishes starting from the mid-chord. Second, the grooves prevent the formation of large scale backflow vortices (BFVs), which are associated with the TLV, propagate from one blade passage to the next, and play a key role in the onset of rotating stall in the untreated fan. Third, the flow exiting from the grooves causes periodic variations of about 10° in the relative flow angle around the blade leading edge, presumably affecting the blade loading. The distributions of turbulent kinetic energy provide statistical evidence that in contrast to the untreated casing, very little turbulence originating from a previous TLV, including the BFVs, propagates from the PS to the SS of the blade. Hence, the TLV-related turbulence remain confined to the entrance to groove. Elevated, but lower turbulence is also generated as the outflow from the groove jets into the passage.

Cavitation is the formation of vapor bubbles within a liquid where flow dynamics cause the local static pressure to drop below the saturation pressure. Compared to the water, cavitation in cryogens is more complex due to the thermodynamic effects of cryogens. Existing Computational fluid dynamics (CFD) methods for water may not simulate the cryogens accurately. This paper presents the steady CFD results of cavitation in water and liquid nitrogen flow through a rotating inducer with four different cavitation models, and the experimental results are displayed to compare with the CFD results to verify the cavitation models’ applicability in water and the liquid nitrogen. The four cavitation models could simulate the cavitation flow in water, and the simulation result presents a consistent trend, which is stable in the high σ regime and drop down suddenly due to the blockage in blade passages caused by vapor as the cavitation number decreases. The “Zwart-Gerber-Belamri model” and the “Schnerr & Sauer model” have better performance on simulating the cavitation flow in water compared with the other two models, and the simulation results are close to the experimental one. The full cavitation model is applicable for cavitation prediction in liquid nitrogen, and the results of other three models are not satisfactory in liquid nitrogen.

SFD (short for squeeze film damper) is a kind of passive vibration isolator widely used in rotor supporting structures of aero-engines for stabilization and vibration control. However, the conventional SFDs are highly nonlinear in terms of damping coefficient, which lead to complex response such as bitable state. In this paper, numerical simulations are carried out to investigate a new kind of SFD, elastic ring squeeze film damper (ERSFD). The elastic ring is modeled by FEM and the film is analyzed by CFD, the orifices on the ring is also included. An FSI approach is introduced to account for the influence of elastic ring’s deformation on oil film thickness. The Zwart-Gerber-Belamri model is included to account for air ingestion and cavitation in the damper land. The characteristics such as pressure distribution, oil film force and the deformation of the ring are obtained and compared with the results without FSI to reveal the self-adaptive mechanism of film thickness. The force coefficients for ERSFD are derived and gained by the FFT method. The dynamic coefficients for ERSFD versus whirl frequency are obtained and compared with corresponding air volume fraction.

The present work advances experimental results and analytical predictions on the dynamic performance of an integral squeeze film damper (ISFD) for application in a high-speed super-critical CO 2 (sCO 2 ) expander. The test campaign focused on conducting controlled orbital motion mechanical impedance testing aimed at extracting stiffness and damping coefficients for varying end seal clearances, excitation frequencies, and vibration amplitudes. In addition to the measurement of stiffness and damping; the testing revealed the onset of cavitation for the ISFD. Results show damping behavior that is constant with vibratory velocity for each end seal clearance case until the onset of cavitation/air ingestion, while the direct stiffness measurement was shown to be linear. Measurable added inertia coefficients were also identified. The predictive model uses an isothermal finite element method to solve for dynamic pressures for an incompressible fluid using a modified Reynolds equation accounting for fluid inertia effects. The predictions revealed good correlation for experimentally measured direct damping, but resulted in grossly overpredicted inertia coefficients when compared to experiments.

Many high-speed, rotating machines across a wide range of industrial applications depend on fluid film bearings to provide both static support of the rotor and to introduce stabilizing damping forces into the system through a developed hydrodynamic film wedge. Reduced oil supply flow rate to the bearings can cause cavitation, or a lack of a fully developed film layer, at the leading edge of the bearing pads. Reducing oil flow has the well-documented effects of higher bearing operating temperatures and decreased power losses due to shear forces. While machine efficiency may be improved with reduced lubricant flow, little experimental data on its effects on system stability and performance can be found in the literature. This study looks at overall system performance of a test rig operating under reduced oil supply flow rates by observing steady-state bearing performance indicators and baseline vibrational response of the shaft. The test rig used in this study was designed to be dynamically similar to a high-speed industrial compressor. It consists of a 1.55 m long, flexible rotor supported by two tilting pad bearings with a nominal diameter of 70 mm and a span of 1.2 m. The first bending mode is located at approximately 5,000 rpm. The tiling-pad bearings consist of five pads in a vintage, flooded bearing housing with a length to diameter ratio of 0.75, preload of 0.3, and a load-between-pad configuration. Tests were conducted over a number of operating speeds, ranging from 8,000 to 12,000 rpm, and bearing loads, while systematically reducing the oil supply flow rates provided to the bearings under each condition. For nearly all operating conditions, a low amplitude, broadband subsynchronous vibration pattern was observed in the frequency domain from approximately 0–75 Hz. When the test rig was operated at running speeds above its first bending mode, a distinctive subsynchronous peak emerged from the broadband pattern at approximately half of the running speed and at the first bending mode of the shaft. This vibration signature is often considered a classic sign of rotordynamic instability attributed to oil whip and shaft whirl phenomena. For low and moderate load conditions, the amplitude of this 0.5x subsynchronous peak increased with decreasing oil supply flow rate at all operating speeds. Under the high load condition, the subsynchronous peak was largely attenuated. A discussion on the possible sources of this subsynchronous vibration including self-excited instability and pad flutter forced vibration is provided with supporting evidence from thermoelastohydrodynamic (TEHD) bearing modeling results. Implications of reduced oil supply flow rate on system stability and operational limits are also discussed.

To explore the impact of hydrofoil surface water injection on cavitation suppression, the unsteady cavitation flow around the NACA0066 hydrofoil was simulated by the modified RNG k-ε turbulence model combined with the full-cavitation model. The influence of water injection position and jet angle on cavitation suppression were analyzed. A detailed analysis shows that cavitation suppression effect is closely related to water injection position and jet angle. It is noted that the optimal position is just the vertex position on the upper surface of hydrofoil. Water injection in an appropriate position will increase the velocity gradient in the boundary layer and enhanced the ability of anti reverse pressure gradient, thus weakening the intensity of separation flow. The recirculation zone thickness and the velocity of re-entrant jet are all significantly reduced. For the study of jet angle, the cavitation suppression effect is obvious with jet angles from 145° to 155°. This may due to the high pressure produced by the mixing of jet with the main flow, and the high-pressure zone limit the spread of low pressure from the leading edge to the trailing edge of hydrofoil. This can also explain why the suppression effect is significant at the vertex position on the upper surface.

The flows in the tip regions of two rotors with blades of similar geometry but different tip clearance are studied experimentally to determine the effect of gap on the flow structure at different operating conditions. The experiments have been performed in the JHU optically index-matched facility, where the refractive index of the fluid is matched with that of the acrylic rotor blades and casing, facilitating unobstructed Stereo Particle Image Velocimetry (SPIV) measurements. The blade geometries are based on the first one and a half stages of the Low Speed Axial Compressor (LSAC) facility at NASA Glenn. The tip gap sizes are 0.49% and 2.3% of the blade chordlength, and measurements are performed for two flow rates, the lower of which is just above stall conditions. The presence and trajectories of the tip leakage vortex (TLV) and secondary structures are visualized by recording high speed movies of cavitation at lower pressures. The results consist of performance curves, distributions of velocity, circumferential vorticity and turbulent kinetic energy, as well as the strength and trajectory of vortices. Increasing the tip gap reduces the static-to-static pressure coefficient for all flow conditions. For the higher flow rate, a wider tip gap has several effects: ( i ) It delays the rollup of the TLV and its detachment from the suction side (SS) corner of the blade, presumably due to the larger distance from the endwall casing and the ‘image vortex’. ( ii ) It alters the blade loading and reduces the circulation shed from the blade. ( iii ) It delays the onset of TLV bursting in the aft part of the rotor passage. ( iv ) For both gaps, the endwall boundary layer separates at the point where the leakage flow meets the opposite-direction main passage flow. For the wide gap, the separated layer with opposite sign vorticity remains above the TLV; while for the narrow gap, the TLV entrains this layer around itself. And ( v ) consistent with the major differences in flow structure, the spatial distributions and magnitudes of all the turbulence intensity are also very different. Trends and flow structure are quite different at pre-stall conditions. Most notably, TLV rollup is still delayed for the wide gap, but vortex bursting and associated arrival of multiple secondary structures to the pressure side (PS) of the next blade occur earlier. Consequently, the turbulence level on both sides of the blade tip is substantially higher, and remnants of the previous TLV are ingested into the next tip gap.

Flow visualizations and stereoscopic PIV (SPIV) measurements are carried out to study the flow phenomena developing in the rotor passage of an axial compressor at the onset of stall. Experiments have been performed in the JHU optically index-matched facility, using acrylic blades and liquid that have the same optical refractive index. The blade geometries are based on the first one and a half stages of the Low Speed Axial Compressor (LSAC) facility at NASA Glenn. The SPIV measurements provide detailed snapshots and ensemble statistics on the flow in a series of meridional planes. Data recorded in closely spaced planes enable us to obtain ensemble averaged 3D vorticity distributions. High speed imaging of cavitation, performed at low pressure, is used to qualitatively visualize the vortical structures within the rotor passage. The observations are performed just above and at stall conditions. At pre-stall condition, shortly after it rolled up, the tip leakage vortex (TLV) breaks up into widely distributed intermittent vortical structures. In particular, interaction of the backward tip leakage flow with the nearly opposite direction main passage flow under (radially inward) it results in periodic generation of large scale vortices that extend upstream, from the suction side (SS) of one blade to the pressure side (PS) or even near the leading edge of the next blade. When these structures penetrate to the next passage, they trigger formation of a similar phenomenon there, initiating a process that sustains itself. Once they form, these vortices rotate with the blade, indicating little through flow in the tip region. The 3D velocity and vorticity distributions confirm the presence of these large flow structures at the transition between the high circumferential velocity region below the TLV center and the main flow deeper in the passage. Further reduction in flow rate into the stall range caused a rapid increase in the number and scale of these vortices, demonstrating that their formation and proliferation plays a key role in the onset of stall.

A single stage cryogenic liquid turbine expander is developed as a replacement for traditional Joule-Thomson valves used in the large-scale internal compression air-separation unit for the purpose of energy saving. Similar to the conventional hydraulic turbine, detrimental swirling and cavitation flow is also encountered at turbine expander impeller exit and its successive diffuser tube, but due to significant thermodynamic effect of cryogenic fluid flow, it is much more complicated than the conventional hydraulic turbine. In the present study, cavitating flow mechanism of the turbine expander is investigated first with a combination of the homogenous multiphase mixture model and the Rayleigh-Plesset model, where the former treats liquid and gas as a continuum mixture and the latter depicts the bubble dynamics. Then sensitivity study is conducted for the impeller fairing cone geometry on suppression of cavitating flow. The following are demonstrated: with a use of the fairing cone, flow behavior near and downstream the impeller exit is significantly improved, where the low static pressure region is reduced and the local temperature rise decreases, subsequently the cavitating flow is effectively suppressed. The cavitating flow is sensitive to a tuning of the fairing cone geometry, and an optimal design of the cone geometry is essential.