Low Supply Pressure Research Articles

On the effects of pitch system faults on a wind turbine

In this study, the hydraulic pitch system of the DTU 10 MW reference wind turbine has been modelled, including different component-level faults, and the control algorithms have been modified to influence the effect of these faults on loads, production and robustness. The simulation results show that the performance of the pitch system model is similar to others in the literature, while at the same time simulating the hydraulic supply pressure variations. The results also show the importance of these variations for overall turbine performance. The number of safety events due to low supply pressure increases with the severity of internal hydraulic leaks. This can be mitigated with a control strategy which limits negative pitch rates based on accumulator pressure. The results show that this can preserve up to 100% of power production in the face of very severe pitch system faults, with as little as 0% impact on loads.

Dynamic Forced Performance of an O-Rings Sealed Squeeze Film Damper Lubricated With a Low Supply Pressure and a Simple Method to Quantify Air Ingestion

Abstract Contemporary squeeze film dampers (SFDs) in air-breathing engines are short in length to limit weight and part count and lubricated with a low feed pressure to reduce oil storage and pumping power. In SFDs, O-rings (ORs) restrict side leakage and increase the viscous damping while a adding a modest centering stiffness. Continuing a long-term project characterizing SFDs for aircraft engines and extending the original work (San Andrés and Rodríguez, 2021, “On the Experimental Dynamic Force Performance of a Squeeze Film Damper Supplied Through a Check Valve and Sealed With O-Rings,” ASME J. Eng. Gas Turb. Power, 143(11), p. 111011) the paper details measurements of the forced performance of an OR sealed damper (OR-SFD) with diameter D = 127 mm, land length L = 0.2 D, and radial clearance c = 0.0022 D. Lubricant ISO VG 2 supplied at 0.69 bar(g) fills an upstream oil plenum and flows into the middle of the land through a single orifice configured with a check valve. Measurements of applied single-frequency dynamic loads, along with the ensuing damper displacements and accelerations serve to identify the parameters of the test structure, ORs, and SFD. The tests encompass centered whirl motions with amplitude r = 0.05–0.45c, and a range of whirl frequencies, ω = 10–130 Hz. Note the squeeze velocity vs = rω reaches 102 mm/s. The ORs force coefficients are nearly invariant with frequency but do depend on the orbit amplitude. The ORs' stiffness (KOR) decreases by 75% as the motion amplitude increases, r → 0.45c, likely due to the large elastic deformations and slow recovery of the ORs material. For small amplitude motions (r/c = 0.05 and 0.10), the ORs damping coefficient (COR) is ∼10% of the overall coefficient for the lubricated system (CL), while for r/c > 0.25, COR ∼ 0.03CL. For small amplitudes of whirl (r ≤ 0.25c), the SFD experimental viscous damping (CSFD) and added mass (MSFD) coefficients, identified over a shorter frequency range (vs<30 mm/s) equal theoretical magnitudes for a fully sealed damper. As the orbit size grows to r = 0.45c MSFD drops nearly by 75% and CSFD decreases by ∼40% due to the onset of both lubricant cavitation and air ingestion occurring for squeeze velocities vs ≥ 24.5 mm/s, as also seen in the recorded dynamic pressures and video recordings of a bubbly mixture leaving the damper. A comprehensive flow model predicts CSFD and MSFD about 8% and 12% larger than the experimentally identified coefficients. A novel approach enables the estimation of the gas volume fraction (GVF) generated in the damper which rapidly increases as vs grows. The simple procedure draws into a deflated balloon the material contents in the squeeze film, weighs the sample, and identifies its volume to produce an estimate of the GVF. The procedure to quantify the GFV will assist in the validation of predictive tools.

Research on the influence of a micro-groove-orifice structure and its layout form on the static characteristics of aerostatic journal bearings under a high gas supply pressure

To solve the problems of a low gas supply pressure and low load capacity of traditional orifice-compensated aerostatic journal bearings (OCAJBs), this paper presents the design of a double-row gas supply micro-groove-orifice aerostatic journal bearing (MGOAJB). By establishing new governing equations and introducing the SST k- ω turbulence model, the influence of the micro-groove-orifice (MGO) structure and its layout form on the static characteristics of aerostatic journal bearings under a high gas supply pressure are studied. It is found that under high gas supply pressure, the MGO structure can significantly improve the load capacity and stiffness and reduce the gas consumption of the MGOAJB compared with the OCAJB. With additional MGO structures in the eccentric direction of the spindle, the load capacity and stiffness of the bearing increase substantially, and the gas flow also increases. The layout direction of the MGO has a prominent influence on the static characteristics of the MGOAJB. Axial MGOs have a higher load capacity and stiffness and lower gas flow. The accuracy of the simulation method is verified by experiments. This study provides a theoretical basis and method for the design of aerostatic journal bearings under a high gas supply pressure.

Influencing factors of Legionella reproduction in secondary water supply operation and management

Objective: To analyze the pollution status and influencing factors of Legionella pneumophila in a secondary water supply facility in a city. Methods: From June to August 2020, a survey on the level of Legionella pneumophila in secondary water supply unit was carried out in a city in northern China, and 304 sets of secondary water supply facilities were included in the study. A total of 760 water samples were collected from the inlet and outlet water of the secondary water supply facilities and some water samples in the water tank were collected for the detection of Legionella pneumophila, standard plate-count bacteria and related physical and chemical indicators. Through questionnaire survey, the basic information of secondary water supply facilities and daily management of water quality were collected. Multivariate logistic regression model was used to analyze the influencing factors of Legionella pneumophila contamination. Results: Among 304 sets of secondary water supply facilities, most of them were located in residential buildings [57.24% (174/304)]. High and low water tank water supply, low water tank variable frequency conversion water supply and non-negative pressure water supply accounted for 26.6% (81/304), 36.8% (112/304) and 36.5% (111/304), respectively. About 25.7% of facilities (78/304) were positive for Legionella pneumophila. Among them, the positive rates of Legionella pneumophila in high and low water tank water supply, low water tank variable frequency conversion water supply and non-negative pressure water supply facilities were 38.3% (31/81), 29.5% (33/112) and 12.6% (14/111), respectively. The results of multivariate logistic regression model analysis showed that the disinfectant residue could reduce the risk of Legionella pneumophila contamination in water samples, and the OR (95%CI) value was 0.083 (0.022-0.317). The increase of the standard plate-count bacteria and conductivity might increase the risk of Legionella pneumophila contamination in water samples. The OR (95%CI) values were 3.160 (1.667-5.99) and 1.004 (1.001-1.006), respectively. Compared with the non-negative pressure water supply, the risk of Legionella pneumophila contamination of secondary water supply facilities was increased by water supply from high and low water tanks and variable frequency conversion water supply from low water tanks, with OR (95%CI) values of 4.296 (2.096-8.803) and 2.894 (1.449-5.782), respectively. Conclusion: The positive rate of Legionella pneumophila in secondary water supply in the study city is high. Disinfectant residue, conductivity and method of water supply are associated with the positive rate of Legionella pneumophila.

Thermodynamic analysis of a metal hydride hydrogen compressor with aluminium substituted LaNi5 hydrides

The operating conditions of a Metal Hydride Hydrogen Compressor (MHHC) is often dictated by the hydride used to build it. This research focuses on finding the appropriate operating conditions for MHHC to get the ideal throughput and efficiency using aluminium based hydrides. For this, hydrogen absorption and desorption characteristics of LaNi5-xAlx (where x is 0, 0.2 and 0.4) hydrides were studied for estimating the suitability of these materials for developing hydrogen compressor. From the Pressure Concentration Isotherms (PCIs) and van't Hoff plot, an estimate of the reversible hydrogen weight percent attainable, equilibrium pressure and absorption and desorption temperature range were collected and the effect of aluminium concentration on these properties was studied. Further thermodynamic analysis was made by calculating the performance parameters like thermal efficiency, isentropic work required, theoretical heat supplied and pressure ratio by varying the operating conditions like supply pressure (3–5 bar), absorption temperatures (20–40 °C), and desorption temperature (80–140 °C). It is observed that the higher aluminium content in the hydride allows the hydrogen compressor to be operated at a lower supply pressure. Whereas lower aluminium content in hydrides give higher thermal efficiency and discharge pressure for a particular desorption temperature, while requiring a higher supply pressure to perform efficiently. The maximum thermal efficiency within the studied operating conditions was found to be 0.24 for LaNi5 at 3 bar supply pressure, 30 °C absorption and 140 °C desorption temperature with a pressure ratio of 8.97.

Nitrite reduction using a membrane biofilm reactor (MBfR) in a hypoxic environment with dilute methane under low pressures

Methane-based membrane biofilm reactors (MBfRs) can be an effective solution for nitrogen control in wastewater, but there is limited information on nitrite reduction for dilute wastewater (e.g., municipal wastewater) in hypoxic MBfRs. This study assessed the impacts of dilute (20 %), low-pressure methane (0.35–2.41 kPa) applied to MBfRs at hydraulic retention times (HRTs) of 2–12 h on nitrite removals, dissolved methane concentrations, and the resulting changes in the microbial community. High nitrite flux along with rapid and virtually complete (>99 %) nitrite removals were observed at methane pressures of 1.03–2.41 kPa at HRTs above 4 h, despite the use of diluted methane gas for the MBfR. The lowest methane pressure (0.35 kPa) was also able to achieve up to 98 % nitrite removals but required HRTs of up to 12 h. All scenarios had low dissolved methane concentrations (<10 mg/L), indicating that dilute methane at low supply pressures can effectively remove nitrite while meeting dissolved methane guidelines in treated effluent. Methylococcus genus was the key bacterium in MBfR biofilm grown at different HRTs and methane pressures, along with Methylocystis and other heterotrophic denitrifiers (Terrimonas and Hyphomicrobium). This study indicates that methane-based denitrification MBfRs can be a valuable tool to meet nitrogen limits for dilute wastewater coupled to partial nitrification, while limiting the release of methane to the environment.

English

Many rural households do not have access to treated drinking water. However, some households may be in regions where a water treatment plant has excess capacity to supply some additional households. In such circumstances, small diameter low-pressure supply systems can be connected to major transmission lines to give these households access to water. These typically occur in locations that do not belong to recognized municipal or other local government. Such supply systems, commonly referred to as &ldquo;trickle fill,&rdquo; have been implemented in South America, Africa and Canada. This paper explains the concept of trickle fill supply systems. A user-friendly decision-making framework is proposed to assist the decision maker to determine whether the implementation of a trickle fill system is economically feasible for a given rural area. The decision-making assessment template (DMAT) takes into projects the economic, energy and carbon footprint changes. A case study for a rural municipality in the province of Alberta, Canada is presented to illustrate the value of the DMAT and the feasibility of trickle fill water supply as a solution. In this case study, a trickle fill water supply solution is found to be technically and economically viable and would reduce energy consumption and the carbon footprint. It is shown that trickle fill solution can be implemented, resulting in an average cost of water to be $165 per month per household. This is higher than the affordability threshold of 2% of the median household income, but comparable to the cost that the end users currently pay for drinking water in the area. A capital grant from a higher level of government, or low interest rate financing, would reduce the end user costs to the level of the affordability criterion mentioned here. Hence, a trickle fill solution is worth investigating for some rural residents. Key words: Rural water systems; economic feasibility; carbon footprint, small diameter water distribution pipelines.

An Analytical Two-Phase Flow Model for Prediction of Leakage in Wet Gas Labyrinth Seals and Pocket Damper Seals. Is Simplicity Still Desired?

Abstract Current and upcoming two-phase pump and compression systems in subsea production facilities must demonstrate long-term operation and continuous availability. Annular pressure seals, limiting secondary flow, also influence the dynamic stability of turbomachinery. Hence, it becomes paramount to quantify the leakage and dynamic force coefficients of annular seals operating with two-phase flow, a liquid in gas mixture or wet gas. Until now, a simple model for labyrinth seals (LSs) and the more modern pocket damper seals (PDSs) is not available, though these seal types find wide applications in subsea machinery. The paper develops a simple analytical model predicting the leakage and cavity pressures for LSs and PDSs operating with two-phase flow. The model adapts Neumann’s leakage equation for use with the physical properties of a homogeneous two-phase flow mixture. Predictions of leakage for a four-blade, eight-pocket, fully partitioned PDS operating under a low supply pressure (PS = 2.3 bar and 3.2 bar) and a low rotor speed equal to 5250 rpm (surface speed = 35 m/s) agree well with experimental results procured for both a pure gas and a wet gas conditions (2.2% in liquid volume). Predicted leakage and cavity pressures also agree with those found by a multimillion node computational fluid dynamics (CFD) model. For an eight-blade, sixteen-pocket PDS supplied with air at PS = 62.1 bar, discharge pressure Pa = 31.1 bar, and rotor speed of 15 krpm (surface speed = 91 m/s), the analytical model predicts leakage that is just 2% larger than a published CFD prediction. For the PDS supplied with an oil in gas mixture having gas volume fraction βS = 0.92–0.98, the simple model delivers leakage that is up to ∼6% lower than published CFD results. An analysis of the two-phase leakage predictions via a modified flow factor reveals a loss coefficient (cd) impervious to the range of supply and discharge pressures considered and growing in proportion to the liquid volume fraction (LVF). Throughout the life of an oil well that sees radical changes in gas and liquid composition as well as pressure conditions, the expedient model, quick and accurate to estimate leakage in wet gases seals, can be readily integrated into an engineering routine or practice.

Integral Sliding-Mode Tracking Control for Heavy Vehicle Electrohydraulic Power Steering System

Since heavy vehicles need all-terrain adaptability and large load drive capability, the dual hydraulic cylinders symmetrically arranged and connected in series are widely used in their steering system, which is quite different from the traditional electrohydraulic power steering systems. Accurate steering control for this dual hydraulic system is challenged due to its complicated structure and a wide range of steering loads. To solve this issue, an improved integral sliding-mode control (ISMC) is presented in this article. Specifically, a hyperbolic tangent function is integrated into the controller to mitigate the chattering caused by the sliding mode. A stability analysis is then conducted based on the finite-time Lyapunov stability theory, revealing the proposed controller's capability of achieving the asymptotic convergence of the tracking error within a finite time. A test bench is also established to experimentally validate the controller's effectiveness through multiple test scenarios, including various tire loads, supply pressures, and command signals frequencies. The results show that the tracking error of ISMC is kept within 0.5°, which is significantly superior to PID control. In addition, the ISMC can achieve high-precision control under lower supply pressure, showing the energy-saving potential of nonlinear control for the steering system in heavy vehicles.

Effect of outlet diameter on atomization characteristics and dust reduction performance of X-swirl pressure nozzle

In this study, experimental studies on the atomization characteristics and dust reduction performance of six different X-swirl nozzles with different outlet diameters were performed. The results of the nozzle atomization characteristics shows that, as the diameter of the nozzle increased, the flow rate of the nozzle gradually increased, while the flow coefficient kept decreasing almost linearly. Meanwhile, with the increase of the outlet diameter, the nozzle range and droplet velocity decreased continuously, while atomization angle and droplet size exhibited an increasing trend in general. From results of the dust-reduction experiment, considering the dust-reduction efficiency and water consumption, it is more appropriate to select a nozzle with an outlet diameter of 1.2 mm when the water supply pressure is higher than 4.0 MPa. When the water supply pressure was low, as the diameter of the outlet increased, the dust-reduction efficiencies of the nozzle for both the total dust and the respirable dust increased significantly. Therefore, under low water supply pressure, when the water consumption at the engineering application site is not limited, using a nozzle with a larger outlet diameter can achieve higher dust-reduction efficiency.

Investigation on influences of herringbone grooves for the aerostatic journal bearings applied to ultra-high-speed spindles

Aerostatic bearings are the core parts of ultra-high-speed spindles with maximum running speed greater than 100,000 r/min. In this paper, the influences of herringbone grooves on the performance of aerostatic journal bearings are studied to design spindles with the features of higher precision and higher speed. Parametric studies and sensitivity analyses are executed in terms of the improved finite element method, which is specially developed to solve the compressible Reynolds equation for herringbone grooved air bearings. The calculated results indicate that herringbone grooves significantly improve the performance of aerostatic journal bearings under the conditions of ultra-high speeds and low supply pressures. Groove parameters are nonlinear dependent, and groove depth and length are the dominant influential factors for the load capacity at speed of 200,000 r/min. Experiments are designed and conducted to verify the improved finite element method, which show that the improved finite element method can be used to analyze the influences of herringbone grooves on aerostatic journal bearings and also manifest that suitable herringbone groove geometrical parameters can obviously decrease spindle radial runouts at ultra-high speeds.

Experimental Force Coefficients for Two Sealed Ends Squeeze Film Dampers (Piston Rings and O-Rings): An Assessment of Their Similarities and Differences

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 = 5L, 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 deg apart. In the PRSFD, the lubricant exits through the slit opening at the ring butted ends. The ORs suppress oil leakage; hence, lubricant evacuates through a 1 mm hole at ¼ L near one journal end. The ORs when installed add significant stiffness and damping to the test structure. The ORSFD produces 20% more damping than the PRSFD, 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, 60–80% 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 (CCOs), 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 from measured forces and displacements, 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 PRSFD vis-à-vis that of an ORSFD. Supplying lubricant with a large enough pressure (flow rate) is crucial to avoid the pervasiveness of air ingestion. Last, 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.

Investigation on combustion process and emissions characteristic in direct injection diesel engine powered by wet ethanol using blend mode

This paper presents results of investigation of the engine powered by diesel–ethanol fuel blends with wide range of ethanol fraction. Two supply systems of fuel pump are investigated. It is used two supply systems of fuel pump: low pressure 0.1bar and high pressure 1bar. Researches are conducted on 1-cylinder direct injection compression ignition engine with constant rotational speed 1500rpm. Thermodynamic parameters and exhaust emission of engine powered by fuels blend are taken into account. Heat release rate and pressure rise have been analyzed as well. Test results confirmed that the combustion process occurring in the diesel engine powered by blend takes place in a shorter time than in the typical diesel engine. High pressure supply system (HPSS) allows to combustion blends with higher ethanol fraction, up to 45%. With high pressure supply system the value of indicated mean effective pressure (IMEP) up to 40% of ethanol fuel fraction kept almost constant with value near to 6bar. For 15% ethanol fuel fraction in HPSS case the COVIMEP was 2.5-times lower then in low pressure supply system (LPSS) case. In both supply systems emission of THC increased or was on near a constant level. With the increase of ethanol fraction in blend the NOx increased as far as combustion process started to deteriorate.

Numerical analysis and experimental study on the microvibration of an aerostatic thrust bearing with a pocketed orifice-type restrictor

Large eddy simulation was used to analyze the time-variation flow field in order to study the microvibration of an aerostatic thrust bearing with a pocketed orifice-type restrictor. It was found that the vortex shedding caused air pressure fluctuation, which was directly related to the occurrence of the vibration of the bearing. Furthermore, the influence of bearing parameters on the vibration was studied through numerical analysis and experiments. The results indicated that large eddy simulation could predict the vibration by calculating the air pressure fluctuation inside the bearing. Moreover, the vibration could be reduced if the bearing was featured with a small orifice diameter, a large bearing diameter, a shallow air chamber or a low supply pressure.

Experimental and Numerical Studies of Cavitation Effects in a Tapered Land Thrust Bearing

In thrust bearings, cavitation may occur at high rotational speeds or low lubricant supply pressures, and it will influence the bearing performances. In this paper, a hydrodynamic tapered land thrust bearing has been studied both experimentally and numerically, with concentration on the cavitation phenomenon and its effects on the bearing performances. Evident cavitation regions have been observed in the experiments at higher rotational speeds. Traditional Reynolds equation and 3D Navier–Stokes equation (3D NSE) with a cavitation model have been used for numerical simulation, and the predicted results are examined against the experimental results. Compared with Reynolds equation, 3D NSE with Rayleigh–Plesset model provides better predictions of both oil–film pressure profile and cavitation area. Furthermore, the effects of the cavitation phenomenon on the thrust bearing performances are studied by parametric studies involving various rotational speeds and oil feeding pressures, using 3D NSE. It is found that the load capacity decreases at higher speeds because of enlargement of the cavitation area. And the negative effects of cavitation can be reduced at smaller film thickness and higher oil supply pressure. Conclusively, the above results show that the cavitation phenomenon has significant influences on the bearing performances at higher speeds, and 3D NSE provides an effective tool for analyzing the cavitation effects in thrust bearings.

Experimentally tuned dual stage hydrogen compressor for improved compression ratio

An experiment-driven design procedure for optimizing the combination of stages of a dual stage hydrogen compressor with enhanced compression ratio is presented herein. Three different combinations of reactors were used using LaNi5, Ca0.6Mm0.4Ni5 and Ca0.2Mm0.8Ni5 as hydrogen storage materials. Compression ratios were found to be similar for low supply pressure conditions, which improved significantly for high supply pressure conditions in single stage experiments. A dual stage compressor system with LaNi5 in first stage and Ca0.2Mm0.8Ni5 in the second stage was proposed based on single stage results, which was found to be very effective for enhancing compression ratio. Results show that 53% higher compression ratio can be attained by selecting appropriate storage materials for stages, compared to LaNi5 based economic dual stage system.

Experimental study of metal hydride-based hydrogen storage tank at constant supply pressure

Metal hydride-based hydrogen storage tank is tested using 1 kg of AB5 alloy, namely LaNi5. The hydrogen tank is of annular cylindrical with inner and outer heat exchangers. The inner one is a finned spiral heat exchanger and the outer one is a conventional jacket. Performance (storage capacity and storage time) studies are carried out by varying the supply pressure and the cooling temperature of the hydride bed. At any given cooling temperature, hydrogen storage rate is found to increase with supply pressure. Cooling temperature is found to have a significant effect on hydrogen storage capacity at lower supply pressures.

A Numerical Analysis of Optimum Air Journal Bearings

The responses to changes of bearing length-to-diameter ratio and supply pressure of hybrid air journal bearings are investigated numerically for rotor dynamic instability. Different types of external pressure compensations, including multi-array of 1, 2, 3, 4, or 5-row orifice bearings and porous bearings, are analyzed to obtain more insight to optimum designs of journal bearings to improve the problem of whirl instability of rotor mass. The results show that the long porous bearings in the higher rotation speeds have higher threshold load capacities W before the onset of whirl instability and thus are more stable than orifice bearings. On the contrary, the short 5-row orifice bearings are more stable than the porous bearing in the lower rotation speeds. The results also show that the change of supply pressure from makes no difference to orifice bearing with whirl instability of rotor but that the porous bearing is the most stable for the lower supply pressure and becomes unstable as the supply pressure increases in the higher rotation speeds.

Microjet-Based Active Flow Control on a Fixed Wing UAV

A fixed wing, remote controlled, unmanned aerial vehicle (UAV), has been retrofitted with a system of microjet-based actuators to test microjet efficacy for flow control during flight. This allows one to evaluate the flow control system in a more complex and uncontrolled environment relative to the “clean” laboratory flowfield. The system is composed of off-the-shelf components and provides some appreciation of the challenges associated with implementing such an active control scheme in more “practical” configurations. A wing section with actuators was first tested in a low-speed wind tunnel to characterize microjet control efficacy in the laboratory using surface flow visualizations and particle image velocimetry (PIV). The laboratory results show that the microjet actuators are an effective means of controlling separation with fairly low supply pressures and flow rates. Only relatively robust and straightforward diagnostics can be used to determine the flow conditions on the UAV during flight. As such, tufts are installed on the aircraft’s wing to serve as a qualitative way of measuring control efficacy. Results from the flight tests confirm that this flow control system is capable of delaying flow separation in the complex flows occurring during flight.

A Numerical Analysis of Optimum Air Journal Bearings

The responses to changes of bearing length-to-diameter ratio and supply pressure of hybrid air journal bearings are investigated numerically for rotor dynamic instability. Different types of external pressure compensations, including multi-array of 1, 2, 3, 4, or 5-row orifice bearings and porous bearings, are analyzed to obtain more insight to optimum designs of journal bearings to improve the problem of whirl instability of rotor mass. The results show that the long porous bearings (L/D>1.0) in the higher rotation speeds (Λ>0.5) have higher threshold load capacities W before the onset of whirl instability and thus are more stable than orifice bearings. On the contrary, the short 5-row orifice bearings (L/D ≤ 1.0) are more stable than the porous bearing in the lower rotation speeds (0.1 ≤ Λ ≤ 0.5). The results also show that the change of supply pressure from Ps=2.0 to Ps=8.0 makes no difference to orifice bearing with whirl instability of rotor but that the porous bearing is the most stable for the lower supply pressure Ps=2.0 and becomes unstable as the supply pressure Ps increases in the higher rotation speeds (Λ>0.5).