19,755 research outputs found
An experimental and analytical investigation of proprotor whirl flutter
The results of an experimental parametric investigation of whirl flutter are presented for a model consisting of a windmilling propeller-rotor, or proprotor, having blades with offset flapping hinges mounted on a rigid pylon with flexibility in pitch and yaw. The investigation was motivated by the need to establish a large data base from which to assess the predictability of whirl flutter for a proprotor since some question has been raised as to whether flutter in the forward whirl mode could be predicted with confidence. To provide the necessary data base, the parametric study included variation in the pylon pitch and yaw stiffnesses, flapping hinge offset, and blade kinematic pitch-flap coupling over a large range of advance ratios. Cases of forward whirl flutter and of backward whirl flutter are documented. Measured whirl flutter characteristics were shown to be in good agreement with predictions from two different linear stability analyses which employed simple, two dimensional, quasi-steady aerodynamics for the blade loading. On the basis of these results, it appears that proprotor whirl flutter, both forward and backward, can be predicted
Whirl and Stall Flutter Simulation Using CFD
This paper presents recent research on numerical methods for whirl and stall flutter using computational
fluid dynamics. The method involves coupling of the HMB3 CFD solver of the University of
Glasgow and a NASTRAN derived structural model. Based upon a literature survey, a significant amount
of research has been conducted on the numerical investigation of tiltrotors, with a focus on the XV-15
and V-22 aircraft. Within this paper, the coupling procedure is presented along with a steady CFD computation
to highlight the accuracy of the high-fidelity method. In addition to this, a simple method is used
to investigate the whirl flutter boundary of a standard propeller and the XV-15 blade
Prediction of destabilizing blade tip forces for shrouded and unshrouded turbines
The effect of a nonuniform flow field on the Alford force calculation is investigated. The ideas used here are based on those developed by Horlock and Greitzer. It is shown that the nonuniformity of the flow field does contribute to the Alford force calculation. An attempt is also made to include the effect of whirl speed. The values predicted by the model are compared with those obtained experimentally by Urlicks and Wohlrab. The possibility of using existing turbine tip loss correlations to predict beta is also exploited. The nonuniform flow field induced by the tip clearnance variation tends to increase the resultant destabilizing force over and above what would be predicted on the basis of the local variation of efficiency. On the one hand, the pressure force due to the nonuniform inlet and exit pressure also plays a part even for unshrouded blades, and this counteracts the flow field effects, so that the simple Alford prediction remains a reasonable approximation. Once the efficiency variation with clearance is known, the presented model gives a slightly overpredicted, but reasonably accurate destabilizing force. In the absence of efficiency vs. clearance data, an empirical tip loss coefficient can be used to give a reasonable prediction of destabilizing force. To a first approximation, the whirl does have a damping effect, but only of small magnitude, and thus it can be ignored for some purposes
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An efficient model of drillstring dynamics
This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.jsv.2015.06.033High amplitude vibration regimes can cause significant damage to oilwell drillstrings: torsional stick-slip oscillation, forward whirl and backward whirl are each associated with different kinds of damage. There is a need for models of drillstring dynamics that can predict this variety of phenomena that are: efficient enough to carry out parametric studies; simple enough to provide insight into the underlying physics, and which retain sufficient detail to correlate to real drillstrings. The modelling strategy presented in this paper attempts to balance these requirements. It includes the dynamics of the full length of the drillstring over a wide bandwidth but assumes that the main nonlinear effects are due to spatially localised regions of strong nonlinearity, for example at the drillbit cutting interface and at stabilisers where the borehole wall clearance is smallest. The equations of motion can be formed in terms of this reduced set of degrees of freedom, coupled to the nonlinear contact laws and solved by time-domain integration. Two implementations of this approach are presented, using (1) digital filters and (2) a Finite Element model to describe the linear dynamics. Choosing a sampling period that is less than the group delay between nonlinear degrees of freedom results in a decoupled set of equations that can be solved very efficiently. Several cases are presented which demonstrate a variety of phenomena, including stick-slip oscillation; forward whirl and backward whirl. Parametric studies are shown which reveal the conditions which lead to high amplitude vibration regimes, and an analytic regime boundary is derived for torsional stick-slip oscillation. The digital filter and Finite Element models are shown to be in good agreement and are similarly computationally efficient. The digital filter approach has the advantage of more intuitive interpretation, while the Finite Element model is more readily implemented using existing software packages.The authors are grateful to industrial support and permission to publish this work, and would like to thank Prof Jim Woodhouse for technical contributions, and Dr Louis Kovalevski for assistance with the Finite Element model
Observing complete gravitational wave signals from dynamical capture binaries
We assess the detectability of the gravitational wave signals from highly
eccentric compact binaries. We use a simple model for the inspiral, merger, and
ringdown of these systems. The model is based on mapping the binary to an
effective single black hole system described by a Kerr metric, thereby
including certain relativistic effects such as zoom-whirl-type behavior. The
resultant geodesics source quadrupolar radiation and, in turn, are evolved
under its dissipative effects. At the light ring, we attach a merger model that
was previously developed for quasicircular mergers but also performs well for
eccentric mergers with little modification. We apply this model to determine
the detectability of these sources for initial, Enhanced, and Advanced LIGO
across the parameter space of nonspinning close capture compact binaries. We
conclude that, should these systems exist in nature, the vast majority will be
missed by conventional burst searches or by quasicircular waveform templates in
the advanced detector era. Other methods, such as eccentric templates or, more
practically, a stacked excess power search, must be developed to avoid losing
these sources. These systems would also have been missed frequently in the
initial LIGO data analysis. Thus, previous null coincidence results with
detected gamma-ray bursts cannot exclude the possibility of coincident
gravitational wave signals from eccentric binaries.Comment: 18 pages, 18 figures; revised to match accepted version, PRD in pres
Model predictions and experimental results for the rotordynamic characteristics of leakage flows in centrifugal pumps
The role played by fluid forces in determining the rotordynamic
stability and characteristics of a centrifugal pump is gaining
increasing attention. The present research investigates the contributions
to the rotordynamic forces from the discharge-to-suction
leakage flows between the front shroud of the rotating impeller and
the stationary pump casing. An experiment was designed to measure
the rotordynamic shroud forces due to simulated leakage
flows for different parameters such as flowrate, shroud clearance,
face seal clearance, and eccentricity. The functional dependence
on the ratio of whirl frequency to rotating frequency (termed the
whirl ratio) is very similar to that measured in experiments and
similar to that predicted by the theoretical work of Childs [1].
Childs' bulk flow model yielded some unusual results including
peaks in the rotordynamic forces at particular positive whirl ratios,
a phenomenon which Childs tentatively described as a "resonance"
of the leakage flow. This unexpected phenomenon developed
at small positive whirl ratios when the inlet swirl velocity
ratio exceeds about 0.5. Childs points out that a typical swirl
velocity ratio at inlet (pump discharge) would be about 0.5 and
may not, therefore, be large enough for the resonance to be
manifest. To explore whether this effect occurs, an inlet guide vane
was constructed which introduced a known amount of swirl into
the flow upstream of the leakage flow inlet. A detailed comparison
of model predictions with the present experimental program is
presented. The experimental results showed no evidence of the
"resonances," even at much larger swirl inlet velocities than
explored by Childs
Fluid-induced Rotordynamic Forces and Instabilities
In the late 1970s, the authors began a collaboration with our colleague Tom Caughey that helped define a new set of fluid-structure interaction phenomena in turbomachines, namely fluid-induced rotordynamic forces and instabilities. That collaboration and the 31 joint ABC papers it produced epitomized Tom Caughey's genius and we reprise it here in his honor.
The design of the space shuttle main engine (SSME) pushed beyond the boundaries of many known technologies. In particular, the rotating speeds and operating conditions of the high speed liquid oxygen and liquid hydrogen turbopumps were extreme and early testing revealed a whirl instability whose magnitude exceeded expectations and allowable limits. It was suspected and later proven that fluid-induced rotordynamic effects were a contributing factor and yet very little was known of such phenomena. As one of the efforts seeking understanding, we constructed a facility to measure fluid-induced rotordynamic forces. This was subsequently used in a broad range of investigations. Initially, the effort was directed to understanding the source and parametric variations of destabilizing fluid forces. Later various components of the flow in a high speed turbopump were investigated. And finally, some ameliorative measures and their effectiveness were examined. This paper reviews this body of knowledge and the lessons learnt along the way
High-resolution broadband spectroscopy using externally dispersed interferometry at the Hale telescope: Part 1, data analysis and results
High-resolution broadband spectroscopy at near-infrared wavelengths (950 to 2450 nm) has been performed using externally dispersed interferometry (EDI) at the Hale telescope at Mt. Palomar. Observations of stars were performed with the “TEDI” interferometer mounted within the central hole of the 200-in. primary mirror in series with the comounted TripleSpec near-infrared echelle spectrograph. These are the first multidelay EDI demonstrations on starlight, as earlier measurements used a single delay or laboratory sources. We demonstrate very high (10×) resolution boost, from original 2700 to 27,000 with current set of delays (up to 3 cm), well beyond the classical limits enforced by the slit width and detector pixel Nyquist limit. Significantly, the EDI used with multiple delays rather than a single delay as used previously yields an order of magnitude or more improvement in the stability against native spectrograph point spread function (PSF) drifts along the dispersion direction. We observe a dramatic (20×) reduction in sensitivity to PSF shift using our standard processing. A recently realized method of further reducing the PSF shift sensitivity to zero is described theoretically and demonstrated in a simple simulation which produces a 350× times reduction. We demonstrate superb rejection of fixed pattern noise due to bad detector pixels—EDI only responds to changes in pixel intensity synchronous to applied dithering. This part 1 describes data analysis, results, and instrument noise. A section on theoretical photon limited sensitivity is in a companion paper, part 2
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