Helicopter rotor noise due to ingestion of atmospheric turbulence

A theoretical study was conducted to develop an analytical prediction method for helicopter main rotor noise due to the ingestion of atmospheric turbulence. This study incorporates an atmospheric turbulence model, a rotor mean flow contraction model and a rapid distortion turbulence model which together determine the statistics of the non-isotropic turbulence at the rotor plane. Inputs to the combined mean inflow and turbulence models are controlled by atmospheric wind characteristics and helicopter operating conditions. A generalized acoustic source model was used to predict the far field noise generated by the non-isotropic flow incident on the rotor. Absolute levels for acoustic spectra and directivity patterns were calculated for full scale helicopters, without the use of empirical or adjustable constants. Comparisons between isotropic and non-isotropic turbulence at the rotor face demonstrated pronounced differences in acoustic spectra. Turning and contraction of the flow for hover and low speed vertical ascent cases result in a 3 dB increase in the acoustic spectrum energy and a 10 dB increase in tone levels. Compared to trailing edge noise, turbulence ingestion noise is the dominant noise mechanism below approximately 30 rotor harmonics, while above 100 harmonics, trailing edge noise levels exceed turbulence ingestion noise by 25 dB

Potential field theory and its applications to classical mechanical problems

Advances in many scientific fields are expected to come from work in nanotechnology. Engineering at nano-scales presents novel problems that classical mechanics cannot solve. Many engineers are uncomfortable designing at this level because classical or continuum mechanics does not apply and quantum mechanics is said to apply in a tangible way. There are unique opportunities to contribute to the design, controls, and analysis of systems that are particularly suited to mechanical engineering. Within the derivations of classical mechanics are assumptions that limit its use to bulk engineering. These assumptions are examined to determine what principles can be extended to smaller scales. To allow engineers to do their job at these scales, it is necessary to understand strength and how changing scales affects the strength of material this leads directly to sets of variables necessary for engineering at any scale. Potential field theory is an old method that is experiencing a resurgence of interest. Potential fields are used to study quantum mechanics at the atomic scale, crack and dislocation mobility at the micro-scale, and even bulk analysis. It encompasses many problems that can be formulated using partial differential equations. These series solutions are well suited for computerized numerical approximation. Because of recent advances in computational abilities, potential field theory deserves a fresh look as a candidate for multiscale modeling and as the math that binds each level together

A compendium of computational fluid dynamics at the Langley Research Center

Through numerous summary examples, the scope and general nature of the computational fluid dynamics (CFD) effort at Langley is identified. These summaries will help inform researchers in CFD and line management at Langley of the overall effort. In addition to the inhouse efforts, out of house CFD work supported by Langley through industrial contracts and university grants are included. Researchers were encouraged to include summaries of work in preliminary and tentative states of development as well as current research approaching definitive results

Numerical simulation of separated flows

A new numerical method, based on the Vortex Method, for the simulation of two-dimensional separated flows, was developed and tested on a wide range of gases. The fluid is incompressible and the Reynolds number is high. A rigorous analytical basis for the representation of the Navier-Stokes equation in terms of the vorticity is used. An equation for the control of circulation around each body is included. An inviscid outer flow (computed by the Vortex Method) was coupled with a viscous boundary layer flow (computed by an Eulerian method). This version of the Vortex Method treats bodies of arbitrary shape, and accurately computes the pressure and shear stress at the solid boundary. These two quantities reflect the structure of the boundary layer. Several versions of the method are presented and applied to various problems, most of which have massive separation. Comparison of its results with other results, generally experimental, demonstrates the reliability and the general accuracy of the new method, with little dependence on empirical parameters. Many of the complex features of the flow past a circular cylinder, over a wide range of Reynolds numbers, are correctly reproduced

Application and extension of an analytical model of the confined acoustic beam generated by a transducer

Issued as Letter report and Final report, Project no. E-25-65

Sixth NASTRAN (R) Users' Colloquium

Papers are presented on NASTRAN programming, and substructuring methods, as well as on fluids and thermal applications. Specific applications and capabilities of NASTRAN were also delineated along with general auxiliary programs

Transonic flow studies

Major emphasis was on the design of shock free airfoils with applications to general aviation. Unsteady flow, transonic flow, and shock wave formation were examined

Research in structural and solid mechanics, 1982

Advances in structural and solid mechanics, including solution procedures and the physical investigation of structural responses are discussed

The influence of intraglottal vortices upon the dynamics of the vocal folds

Intelligibility of speech is of critical importance in many respects. Diminished speech intelligibility causes part or all of the intended message to be lost to listeners; the outcome is frustration of speakers and listeners in the least, but impaired communication may be dangerous in some situations. Consequently, improved understanding of the factors which contribute to intelligibility of speech is of great interest in the speech community. For example, the rapid closing of the glottis in the closing phase of the phonatory cycle is generally understood to contribute favourably to the intelligibility of speech by increasing the high frequency content of the resulting speech signal. Recent experimental and numerical studies have suggested that the presence of intraglottal vortices in the closing phase of the phonatory cycle might promote rapid closing of the glottis due to the pressure gradients which arise in the presence of the vortices. To date, computational studies to assess the impact of vortex shedding within the glottis which incorporate a dynamical model of the vocal fold tissues together with a vortex advection scheme are not prevalent. In a recent computational study, an ad hoc pressure condition, of magnitude on the order of the disturbances observed in experimental work, was superimposed upon the medial vocal fold surfaces to simulate the effect of a perturbation of the pressure field. However, this approach, while it is able to quantify the effect of a perturbation of the pressure field, is not wholly satisfactory because the temporal or spatial evolution of the perturbation of the pressure field is not a consequence of a modelled physical effect or mechanism which is fundamentally related to the physics of the fluid or the fluid-structure interaction. In the present study, a two-dimensional ideal potential flow model is developed and coupled to a low-order lumped-element dynamical model of the vocal folds. Irrotational vortices are superimposed upon the glottal flow and allowed to advect through the glottis from an upstream station at a rate which ensures that they will arrive at the superior portion of the glottis in the closing phase of the phonatory cycle, when the glottis obtains its diverging configuration. This is to emulate the roll-up and shedding of intraglottal vortices occurring in the closing phase of the phonatory cycle. The vortices may be removed to compare the dynamical response of the vocal fold tissue model in the absence or presence of the intraglottal vortices. The extension of the glottal flow model to two-dimensions is important in general, not merely because it allows for the inclusion of effects which require higher spatial dimension for their description, such as advecting vortices, but the two-dimensional glottal flow model captures the salient physics of glottal flow with improved fidelity over the standard one-dimensional Bernoulli flow models which have typically been employed in studies of phonation. Additionally, the pressure field, which is unsteady in glottal flow, is determined with the unsteady Bernoulli equation; the unsteady term is found to be significant, thus, again, the model improves upon potential flow models employed to date. The surface pressure on the medial surface of the vocal folds exhibits strong deviation near the inferior and superior margins of the medial surfaces, and, because these entail longer moment arms, larger pitching moments are obtained. It is demonstrated that the mucosal wave, in the transverse motion of the vocal folds, as the cover tissues pitch about their respective nodal points, a greater amplitude of angular displacement is observed with the two-dimensional model. The resulting glottal area waveform obtains a more skewed appearance which causes it to qualitatively appear more closely akin to clinically obtained glottal waveforms despite that the simulation model is uncoupled from the acoustics of the upper vocal tract. In comparing the simulation results, the effect of the vortices is seen to be ephemeral; the vortices rapidly advect into the supraglottal space whence they impart little upstream influence. The vortex strength determines two competing effects, which entails that the vortices should have little effect upon the dynamics of the vocal folds. In particular, as the strength of the vortices increases, the magnitude of the pressure perturbation becomes more significant, however, for a given vortex spacing, the vortices will advect more rapidly into the supraglottal region thus rapidly reducing their effect; because the perturbation of the pressure field is brief, it does not impart sufficient impulse to overcome the inertia of the vocal fold tissues. Alternatively, as vortex strength decreases, the intraglottal vortices dwell in the glottal space longer, but the magnitude of the pressure perturbation is significantly diminished. Again, the modified pressure field does not impart sufficient impulse to overcome the inertia of the vocal folds, and their behaviour remains relatively unperturbed. That the glottal area waveform in its closing phase is essentially unaffected by the presence of intraglottal vortices is demonstrated within the proposed modelling framework

Ein nichtlineares Modell zur kombinierten Berechnung von Wind- und Wellenlasten auf Offshore-Windenergieanlagen

This thesis presents a numerical model capable of simulating offshore wind turbines exposed to extreme loading conditions. External condition-based extreme responses are reproduced by coupling a fully nonlinear wave kinematic solver with a hydro-aero-elastic simulator. First, a two-dimensional fully nonlinear wave simulator is developed. The transient nonlinear free surface problem is formulated assuming the potential theory and a higher-order boundary element method (HOBEM) is implemented to discretize Laplace's equation. For temporal evolution a second-order Taylor series expansion is used. The code, after validation with experimental data, is successfully adopted to simulate overturning plunging breakers which give rise to dangerous impact loads when they break against wind turbine substructures. The impact force is quantified by means of an analytical model and the total hydrodynamic action is finally obtained by adding the impulsive term to the drag and inertial ones. In the second main core of the thesis, emphasis is placed on the random nature of the waves. Indeed, a global simulation framework embedding the numerical wave simulator into a more general stochastic environment is developed. Namely, first a linear irregular sea is generated by the spectral approach, then, only on critical space-time sub-domains, the fully nonlinear solver is invoked for a more refined simulation. The space-time sub-domains are defined as the wind turbine near field (space) times the time interval in which wave impacts are expected (time). Such a domain decomposition approach permits systematically accounting for dangerous effects on the structural response (which would be totally missed by adopting linear or weakly nonlinear wave theories alone) without penalizing the computational effort normally required. At the end of the work the attention is moved to the consequences that the proposed model would have in the quantification of the structural risk.In dieser Arbeit wird ein numerisches Modell zur Simulation von Offshore-Windenergieanlagen unter extremen Lasteinwirkungen entwickelt. Dazu wird ein vollständig kinematisch nichtlineares Wellenmodell mit einem hydroaeroelastischen Modell kombiniert. Zunächst wird das instationäre nichtlineare Problem der freien Wasseroberfläche unter Verwendung der zweidimensionalen Potentialtheorie beschrieben. Die sich ergebende Laplace-Gleichung wird mit einer Randelementmethode höherer Ordnung räumlich diskretisiert. Für die zeitliche Entwicklung wird eine Taylor Reihe zweiter Ordnung verwendet. Nach Abgleichung mit experimentellen Daten wird der entwickelte Algorithmus angewendet, um die für die Stoßbelastung von Windkraftanlagen ursächlichen überschlagenden brechenden Wellen zu simulieren. Die gesamte hydrodynamische Last wird schließlich durch ein analytisches Modell beschrieben, bei dem ein Term, der die Stoßwirkung der Wellen berücksichtigt, zu den Längs- und Trägheitskräften hinzugefügt wird. Im zweiten Teil der Arbeit wird das Wellenmodell in eine Simulationsumgebung eingebettet, welche die stochastischen Natur des Wellengangs erfasst. Hierbei wird zuerst ein linear beschriebener breitbandiger Seegang mithilfe des Spektralansatzes erzeugt. Beschränkt auf kritische Bereiche in der räumlichen und zeitlichen Simulation wird im Anschluss das vollständig nichtlineare hydrodynamische Modell für eine genauere Lösung herangezogen. Die kritischen Bereiche sind auf die nähere Umgebung der Windenergieanlage beim Eintreffen der brechenden Welle begrenzt. Diese Substrukturtechnik erlaubt es, für die Strukturantwort maßgebende Effekte systematisch zu erfassen, die bei einer Verwendung von linearen oder schwach nichtlinearen Wellentheorien komplett vernachlässigt werden, ohne dabei den herkömmlichen Rechenaufwand substantiell zu erhöhen. Zum Abschluss der Arbeit wird diskutiert, wie sich das vorgestellte Modell auf die Quantifizierung des Risikos der Struktur auswirkt