Sung-Hwa Jeung

Forced Coefficients for a Short Length, Open Ends Squeeze Film Damper With End Grooves: Experiments and Predictions

Squeeze film dampers (SFDs) are effective to ameliorate shaft vibration amplitudes and to suppress instabilities in rotor–bearing systems. Compact aero jet engines implement ultra-short length SFDs (L/D ≤ 0.2) to satisfy stringent weight and space demands with low parts count. This paper describes a test campaign to identify the dynamic forced response of an open ends SFD (L = 25.4 mm and D = 125.7 mm), single film land, and oil fed through three holes (120 deg apart), operating with similar conditions as in an aircraft engine. Two journals make for two SFD films with clearances cA = 0.129 mm and cB = 0.254 mm (small and large). The total oil-wetted length equals Ltot = 36.8 mm that includes deep end grooves, width and depth = 2.5 × 3.8 mm, for installation of end seals. In the current experiments, the end seals are not in place. A hydraulic static loader pulls the bearing cartridge (BC) to a preset static eccentricity (eS), and two electromagnetic shakers excite the BC with single frequency loads to create circular orbits, centered and off-centered, over a prescribed frequency range ω = 10–100 Hz. The whirl amplitudes range from r = 0.05cA–0.6cA and r = 0.15cB–0.75cB while the static eccentricity increases to eS = 0.5cA and eS = 0.75cB, respectively. Comparisons of force coefficients between the two identical dampers with differing clearances show that the small clearance damper (cA) provides ∼4 times more damping and ∼1.8 times the inertia coefficients than the damper with large clearance (cB). The test results demonstrate damping scales with ∼1/c3 and inertia with ∼1/c, as theory also showed. Analysis of the measured film land pressures evidence that the deep end grooves contribute to the generation of dynamic pressures enhancing the dynamic forced response of the test SFDs. A thin film flow model with an effective groove depth delivers predictions that closely match the test damping and inertia coefficients. Other predictions, based on the short length bearing model, use an effective length Leff ∼ 1.17L to deliver damping coefficients 15% larger than the experimental results; however, inertia coefficients are ½ of the identified magnitudes. The experiments and analysis complement earlier experimental work conducted with centrally grooved SFDs.

Orbit-Model Force Coefficients for Fluid Film Bearings: A Step Beyond Linearization

High performance turbomachinery demands high shaft speeds, increased rotor flexibility, tighter clearances in the flow passages, advanced materials, and increased tolerance to imbalances. Operation at high speeds induces severe dynamic loading with large amplitude shaft motions at the bearing supports. Typical rotordynamic models rely on linearized force coefficients (stiffness K, damping C, and inertia M) to model the reaction forces from fluid film bearings and seal elements. These true linear force coefficients are derived from infinitesimally small amplitude motions about an equilibrium position. Often, however, a rotor-bearing system does not reach an equilibrium position and displaces with motions amounting to a sizable portion of the film clearance; the most notable example being a squeeze film damper (SFD). Clearly, linearized force coefficients cannot be used in situations exceeding its basic formulation. Conversely, the current speed of computing permits to evaluate fluid film element reaction forces in real time for ready numerical integration of the transient response of complex rotor-bearing systems. This approach albeit fast does not help to gauge the importance of individual effects on system response. Presently, an orbit analysis method estimates force coefficients from numerical simulations of specified journal motions and predicted fluid film reaction forces. For identical operating conditions in static eccentricity and whirl amplitude and frequency as those in measurements, the computational physics model calculates instantaneous damper reaction forces during one full period of motion and performs a Fourier analysis to characterize the fundamental components of the dynamic forces. The procedure is repeated over a range of frequencies to accumulate sets of forces and displacements building mechanical transfer functions from which force coefficients are identified. These coefficients thus represent best fits to the motion over a frequency range and dissipate the same mechanical energy as the nonlinear mechanical element. More accurate than the true linearized coefficients, force coefficients from the orbit analysis correlate best with SFD test data, in particular for large amplitude motions, statically off-centered. The comparisons also reveal the fallacy in representing nonlinear systems as simple K-C-M models impervious to the kinematics of motion.Copyright

Experimental Performance of an Open Ends, Centrally Grooved, Squeeze Film Damper Operating With Large Amplitude Orbital Motions

Aircraft engines customarily implement squeeze film dampers (SFDs) to dissipate mechanical energy caused by rotor vibration and to isolate the rotor from its structural frame. The paper presents experimental results for the dynamic forced performance of an open ends SFD operating with large amplitude whirl motions, centered and off-centered. The test rig comprises of an elastically supported bearing with a damper section, 127 mm in diameter, having two parallel film lands separated by a central groove. Each film land is 25.4 mm long with radial clearance c = 0.251 mm. The central groove, 12.7 mm long, has a depth of 9.5 mm (38c). An ISO VG 2 lubricant flows into the groove via three 2.5 mm orifices, 120 deg apart, and then passes through the film lands to exit at ambient condition. Two orthogonally placed shakers apply dynamic loads on the bearing to induce circular orbit motions with whirl frequency ranging from 10 Hz to 100 Hz. A static loader, 45 deg away from each shaker, pulls the bearing to a static eccentricity (es). Measurements of dynamic loads and the ensuing bearing displacements and accelerations, as well as the film and groove dynamic pressures, were obtained for eight orbit amplitudes (r = 0.08c to ∼0.71c) and under four static eccentricities (es = 0.0c to ∼0.76c). The experimental damping coefficients increase quickly as the bearing offset increases (es/c → 0.76) while remaining impervious to the amplitude of whirl orbit (r/c → 0.51). The inertia coefficients decrease rapidly as the orbit amplitude grows large, r > 0.51c, but increase with the static eccentricity. A comparison with test results obtained with an identical damper but having a smaller clearance (cs = 0.141 mm) (San Andres, L., 2012, “Damping and Inertia Coefficients for Two Open Ends Squeeze Film Dampers With a Central Groove: Measurements and Predictions,” ASME J. Eng. Gas Turbines Power, 134(10), p. 102506), show the prior damping and inertia coefficients are larger, ∼5.0 and ∼2.2 times larger than the current ones. These magnitudes agree modestly with theoretical ratios for damping and inertia coefficients scaling as (c/cs)3 = 5.7 and (c/cs) = 1.8, respectively. In spite of the large difference in depths between a groove and a film land, the magnitudes of dynamic pressures recorded at the groove are similar to those in the lands. That is, the groove profoundly affects the dynamic forced response of the test damper. A computational physics model replicates the experimental whirl motions and predicts force coefficients spanning the same range of whirl frequencies, orbit radii, and static eccentricities. The model predictions reproduce with great fidelity the experimental force coefficients. The good agreement relies on the specification of an effective groove depth derived from one experiment.

Squeeze Film Dampers: A Further Experimental Appraisal Of Their Dynamic Forced Performance

Squeeze Film Dampers (SFDs) are effective means to ameliorate rotor vibration amplitudes and to suppress instabilities in rotor-bearing systems. A SFD is not an off-the-shelf mechanical element but tailored to a particular rotor-bearing system as its design must satisfy a desired damping ratio; if too low, the damper is ineffective, whereas if damping is too large, it locks the system aggravating the system response. In many cases, SFDs are also employed to control the placement of (rigid body) critical speeds displacing the machine operation into a speed range with effective structural isolation. Industry demands well-engineered SFDs with a low footprint to reduce cost, maintenance, weight, and space while pushing for higher operating shaft speeds to increase power output. Compact aero jet engines implement ultra-short length SFDs (L/D ≤ 0.2) to satisfy stringent weight and space demands with low parts count. A manufacturer, as part of a business plan to develop and commercialize energy efficient aircraft gas turbine engines, supported a multiple–year project to test novel SFD design spaces. In spite of the myriad of analyses and experimental results reported in the literature, there has not been to date a concerted effort to investigate the dynamic forced performance of a SFD through its many configurations: open ends vis-à-vis sealed ends conditions, and supply conditions with a fluid plenum or deep groove vis-à-vis feed holes directly impinging into the film land. This lecture presents experimental results obtained with a dedicated rig to evaluate short length SFDs operating under large dynamic loads (2.2 kN ≈ 500 lbf) that produced circular and elliptical whirl orbits of varying amplitude, centered and off-centered. The lecture first reviews how SFDs work, placing emphasis on certain effects largely overlooked by practitioners who often regard the SFD as a simple non-rotating journal bearing. These effects are namely fluid inertia amplification in the supply or discharge grooves, pervasive air ingestion at high whirl frequencies, and effective end sealing means to enhance damping. The bulk of the lecture presents for various SFD configurations comparisons of experimentally identified damping (C) and inertia or added mass (M) coefficients versus amplitude of motion (orbit size) and static eccentricity position, both ranging from small to large; as large as the film clearance! The experiments, conducted over six plus years of continued work give an answer to the following fundamental practitioners’ questions: (a) Dampers don’t have a stiffness (static centering capability), how come? (b) Why is there fluid inertia or added mass in a damper? Isn’t a damper a purely viscous element? (c) How much do the damping and added mass change when the film length is halved? What about increasing the clearance to twice its original magnitude? (d) How much more damping is available if the damper has end seals? (e) Is a damper with feed holes as effective as one containing a groove that ensures lubricant pools to fill the film? What if a hole plugs, is a damper still effective? (f) Does a flooded damper offer same force coefficients as one lubricated thru feed holes? (g) Do the amplitude and shape of whirl motion affect the damper force coefficients? (h) What happens if the damper operates largely off-centered; does its performance become nonlinear? (i) Is air ingestion a persistent issue with an open ends SFD? (j) How do predictions from accepted engineering practice SFD models correlate with the experimental record? Is an idealized SFD geometry representative of actual practice?

Squeeze Film Dampers: An Experimental Appraisal of Their Dynamic Performance

Luis San Andrés performs research in the fields of fluid film lubrication and rotordynamics, having advanced the technologies of hydrostatic bearings for primary power cryogenic turbo pumps, squeeze film dampers for aircraft jet engines, and gas foil bearings for oil-free micro turbomachinery. Luis is a Fellow of ASME and STLE, and a member of the Industrial Advisory Committees for the Texas A&M Turbomachinery Symposia. Dr. San Andrés has educated numerous graduate students who serve the profession with distinction. Dr. San Andrés earned a MS in ME from the University of Pittsburgh and a PhD in ME from Texas A&M University. Luis has published over 150 peer reviewed papers in various journals (ASME Journal of Tribology and ASME Journal of Gas Turbines and Power). Several papers are recognized as best in various international conferences.

Dynamic Forced Performance of Short Length Open-Ends Squeeze Film Damper with End Grooves

Squeeze Film Dampers (SFDs) are effective means to reduce shaft vibration and eliminate instabilities in high performance rotating machinery. Presently there is a need to characterize the performance of ultra-short length SFDs for aero jet engines where overall weight and space are at a premium. The paper presents force coefficients and dynamic film pressures measured in an open ends SFD with slenderness ratio L/D = 0.2 and for two film clearances c A = 0.129 mm and c B = 0.254 mm. The film land length L = 25.4 mm and diameter D = 125.7 mm. ISO VG2 lubricant flows into the axial mid-plane of the film land through three orifices spaced 120o. The journal has end grooves (width and depth = 2.5 × 3.8 mm) for the installation of piston rings, and hence the total wetted length L tot = 36.8 mm. A static loader pulls the bearing cartridge (BC) to a set static eccentricity (e s ), and two shakers, orthogonally positioned, exert dynamic loads on the BC to create circular orbits of amplitude (r) over a range of whirl frequencies (ω). In the current tests, the end seals are not in place. Comparing the dynamic forced performance of the open ends SFDs, the small clearance damper generates about four times more damping than the one with a larger clearance, whereas the inertia coefficients are approximately twice as large. The test results modestly agree with the theoretical ratios, where damping scales with ~1/c 3 and inertia with ~1/c. The measurements also evidence significant dynamic pressures at the end grooves, which amplify the test elements’ inertia coefficients. The test results continue to demonstrate the paramount effect of grooves on enhancing the dynamic forced response of SFDs.

Forced Coefficients for a Short Length, Open-Ends Squeeze Film Damper With End Grooves: Experiments and Predictions

Squeeze Film Dampers (SFDs) are effective to ameliorate shaft vibration amplitudes and to suppress instabilities in rotor-bearing systems. Compact aero jet engines implement ultra-short length SFDs (L/D ≤ 0.2) to satisfy stringent weight and space demands with low parts count. This paper describes a test campaign to identify the dynamic forced response of an open ends SFD (L=25.4 mm, D=125.7 mm), single film land and oil fed through three holes (120° apart), operating with similar conditions as in an aircraft engine. Two journals make for two SFD films with clearances cA=0.129 mm and cB=0.254 mm (small and large). The total oil wetted length equals Ltot=36.8 mm that includes deep end grooves, width and depth = 2.5 × 3.8 mm, for installation of end seals. In the current experiments, the end seals are not in place. A hydraulic static loader pulls the bearing cartridge (BC) to a preset static eccentricity (eS) and two electromagnetic shakers excite the BC with single frequency loads to create circular orbits, centered and off-centered, over a prescribed frequency range ω=10–100Hz. The whirl amplitudes range from r=0.05cA–0.6cA and r=0.15cB–0.75cB while the static eccentricity increases to eS=0.5cA and eS=0.75cB, respectively. Comparisons of force coefficients between the two identical dampers with differing clearances show that the small clearance damper (cA) provides ∼4 times more damping and ∼1.8 times the inertia coefficients than the damper with large clearance (cB). The test results demonstrate damping scales with ∼1/c3 and inertia with ∼1/c, as theory also shows. Analysis of the measured film land pressures evidence that the deep end grooves contribute to the generation of dynamic pressures enhancing the dynamic forced response of the test SFDs. A thin film flow model with an effective groove depth delivers predictions that closely match the test damping and inertia coefficients. Other predictions, based on the short length bearing model, use an effective length Leff ∼1.17L to deliver damping coefficients 15% larger than the experimental results; however, inertia coefficients are ½ of the identified magnitudes. The experiments and analysis complement earlier experimental work conducted with centrally grooved SFDs.© 2015 ASME