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Aerospace and Mechanical Engineering (252)

Graduate College (252)AuthorsFasel, Hermann F. (13)Heinrich, Juan C. (12)Madenci, Erdogan (12)Arabyan, Ara (11)Balsa, Thomas F. (11)Kececioglu, Dimitri B. (11)Nikravesh, Parviz E. (10)Pearlstein, Arne J. (10)Wirsching, Paul H. (10)Chen, Chuan F. (9)View MoreTypes
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ANALYSIS OF TWO-DIMENSIONAL VISCOUS FLOW OVER AN ELLIPTIC BODY IN UNSTEADY MOTION

Taslim, Mohammad E. (Mohammad Esmaail) (The University of Arizona., 1981)

The two-dimensional, viscous flow around an elliptic cylinder undergoing prescribed unsteady motions is analyzed. The fluid is taken to be incompressible. Departing from the conventional vorticity-stream function approach, the Biot-Savart law of induced velocities is utilized to account for the contribution to the velocity field of the different vorticity fields comprising the flow. These include the internal vorticity due to the rotation of the body, the free vorticity in the fluid surrounding the body, and the bound vorticity distributed along the body contour. In order to apply the method, the body must be assumed to be replaced by fluid of the same density as the undisturbed surroundings. The replacement fluid must have a rigid motion exactly the same as the actual body motion. This can be achieved by placing suitable distributed vorticity fields within and on the surface of the body. The bound vorticity on the body surface is in the form of a vortex sheet, and its distribution is governed by a Fredholm integral equation of the second kind. The equation is derived in detail. It is solved numerically. The motion of the free vorticity in the flow field is governed by the Navier-Stokes equations written in terms of vorticity. The descretized vorticity transport equation is derived for a control volume and is solved numerically using an explicit method with a forward-difference for the time derivative, and a central-difference for the diffusive terms. An upwind method is used for convection terms. The results obtained using the present method are compared with a number of special cases available in the literature. Viscous flows around a circular cylinder rotating in any arbitrary fashion possess an exact solution, as presented in Chapter 2. Two cases of this flow are chosen for comparison. In the first case the circular cylinder is initially given an impulsive twist such that it rotates with a constant velocity about its axis. In the second case, the angular velocity of the circular cylinder increases with time exponentially. For a Reynolds number of 100, based on the cylinder radius and the internal vorticity, the exact solutions are compared with the numerical results. Viscous flow around an elliptic cylinder of .0996 aspect ratio rotating with a constant angular velocity is another special case, available in the literature, which is chosen for comparison. For this case the Reynolds number, based on the cylinder semi-major-axis and internal vorticity is 202. The agreement in all above-mentioned cases is excellent. Finally, viscous flow around an elliptic cylinder of .25 aspect ratio undergoing a combined translation and pitching oscillation is presented. A Reynolds number of 500, based on the semi-major-axis and body translational velocity, is chosen for this case. No similar case has been reported until now. This case, however, is only one of the many cases that can be handled by the present method.

Influence of vane sweep on rotor-stator interaction noise.

Envia, Edmane. (The University of Arizona., 1988)

In this dissertation the influence of vane sweep on rotor-stator interaction noise is investigated. In an analytical approach, the interaction of a convected gust, representing the rotor viscous wake, with a cascade of finite span swept airfoils, representing the stator, is analyzed. The analysis is based on the solution of the exact linearized equations of motion. High-frequency convected gusts for which noise generation is concentrated near the leading edge of the airfoils are considered. In a preliminary study, the problem of an isolated finite span swept airfoil interacting with a convected gust is analyzed. Using Fourier transform methods and the Wiener-Hopf technique, an approximate solution for this problem is developed. Closed form expressions for the acoustic farfield are obtained and used in a parametric study to assess the effect of airfoil sweep on noise generation. Results indicate that sweep can substantially reduce the farfield noise levels for a single airfoil. Utilizing the single airfoil model, an approximate solution to the problem of noise radiation from a cascade of finite span swept airfoils interacting with a convected gust is derived. Only upstream radiated noise is considered. Neglecting the weak coupling between the adjacent leading edges at high frequencies, the cascade solution is constructed as a superposition of acoustic farfields emanating from an infinite number of isolated airfoils. A parametric study of noise generated by gust-cascade interaction is then carried out to assess the effectiveness of vane sweep in reducing rotor-stator interaction noise. The results of the parametric study show that, over a fairly wide range of conditions, sweep is beneficial in reducing noise levels. One conclusion of particular importance is that rotor wake twist or circumferential lean substantially influences the effectiveness of vane sweep. The orientation of the vane sweep must be chosen to enhance the natural phase lag caused by wake lean, in which case rather small sweep angles substantially reduce the noise levels.

Local wall shear stress and interface behavior of adiabatic air-water flows in rectangular ducts

Gottmann, Matthias, 1964- (The University of Arizona., 1997)

An experiment was designed and built to study vertical annular air-water flows in a channel with a rectangular cross section with no heat transfer. Flush-wire electrical conductivity probes were theoretically analyzed to demonstrate their potential to accurately measure liquid film thickness in the experiment with high temporal and spatial resolution. Flush-wire probes were then successfully implemented and film thickness measurements obtained. From theoretical analysis, the suitability of micromachined hot film and floating element wall shear stress sensors for measurements of wall shear stress in the annular flow was investigated. A microfabricated hot film wall shear stress sensor was subsequently packaged and installed in the experiment, where it provided wall shear stress measurements with high temporal and spatial resolution. After the implementation of these new measurement techniques, a large suite of test cases was run and data for film thickness and wall shear stress acquired. A statistical analysis of the film thickness data indicates the existence of two distinct wave regimes, ripple waves and disturbance waves, within the annular flow regime. Spectral decomposition of film thickness and wall shear stress data demonstrates the existence of dominant frequencies in the wave spectrum and an exponential decay of wave amplitudes at high frequencies indicative of a force balance between capillary and momentum forces. Wave velocities were determined from cross correlations which again provided evidence of different types of waves each with different wave velocities and spatial extensions. A semi-analytical model for wave velocities as a function of superficial Reynolds numbers was validated and improved. The improved model gives an accurate prediction for wave velocities and is based on physical arguments representing the appropriate length scales in annular flow. The experimental results and data analysis provide a new perspective of annular two-phase flows in a channel with a rectangular cross section.

Uncoupling of rigid-flexible multibody equations of motion using node annexation method

Park, Jungho, 1958- (The University of Arizona., 1997)

This study presents the node annexation method for modeling kinematic joints between rigid and flexible bodies of rigid-flexible multibody systems. Each node of a flexible body is assumed to have lumped mass and three translational degrees of freedom, resulting in a diagonal mass matrix. Based on the node annexation method, both the nodal- and the modal-coordinate formulations for rigid-flexible multibody dynamics are developed. Conventionally rigid-to-flexible-body joints are treated as kinematic constraints using the Lagrange multiplier method. The formulations based on kinematic constraint method yield coupled equations of motion which have the difficulties associated with modal truncation. On the other hand, the node annexation method transfers the inertia and force effect of connected nodes of a flexible body to the connected rigid body. The mass matrix of the resultant equations of motion consists of two different kind of sub-matrices: one is rigid-body sub-system matrix containing the inertia of both rigid bodies and connected nodes of the flexible body and another is flexible-body sub-system matrix containing the inertia of free nodes of the flexible body. Since there is no off-diagonal terms coupling the sub-matrices, the node annexation method allows the division of the equations of motion into smaller sub-system equations. The node annexation method not only provides computational efficiency but also fundamentally eliminates any kinematic error at rigid-to-flexible-body joints. In addition, the node annexation method preserves the uncoupled nature of modal coordinates, allowing a mathematically justified modal truncation. Computer simulations are performed using a vehicle model with a flexible car body. The simulation results show computational advantage over the kinematic constraint method.

Augmentation of heat transfer in a laminar wall jet by selective forcing

Quintana, Donald Larry (The University of Arizona., 1997)

In this investigation an attempt was made to understand the dominant mechanisms of heat and momentum transport in an externally excited wall jet. The mean and fluctuating characteristics of this flow were experimentally evaluated for a constant wall temperature boundary condition. Temperature and streamwise velocity profiles were obtained through simultaneous hot and cold wire measurements in air. Selective forcing of the flow at the most amplified frequencies produced profound effects on the temperature and velocity fields and hence the time-averaged wall heat transfer and shear stress. Large amplitude excitation of the flow (up to 2% of the velocity measured at the jet exit plane) at a high frequency resulted in a reduction in the maximum skin friction by as much as 60% with an increase in the maximum wall heat flux as high as 30%. The skin friction and wall heat flux were much less susceptible to low frequency excitation. These profound effects on the skin friction and heat transfer present a breakdown in the Reynolds analogy. The Reynolds analogy factor increased significantly relative to the unforced case by as much as 200% for high frequency forcing at a large excitation level. Thus, the ability to predict the heat transfer in a wall jet from a known hydrodynamic solution is restricted in the presence of large amplitude disturbances. The amplitude and phase distributions for the fluctuating streamwise velocity and temperature demonstrate, that for large excitation levels, a sub-harmonic wave experiences substantial growth in the measurement domain. Significant distortion of the sub-harmonic component of the fluctuating temperature provides evidence that these large scale structures are responsible for the significant widening of the boundary layer and the transport of energy and momentum away from the surface. This motion may also explain the increase in the temperature gradient near the surface since the unsteady upward coherent transport is increased compared to diffusive transport in this region. Temperature and velocity profiles were also acquired at different spanwise locations. Consistent with previous flow visualization studies, it was found that the transition process (including the coherent transport of the sub-harmonic wave) is two-dimensional in nature.

Parallel computational methods for constrained mechanical systems

Wu, Fei (The University of Arizona., 1997)

Two methods suitable for parallel computation in the study of mechanical systems with holonomic and nonholonomic constraints are presented: one is an explicit solution based on generalized inverse algebra; the second solves problems of this class through the direct application of Gauss' principle of least constraint and genetic algorithms. Algorithms for both methods are presented for sequential and parallel implementations. The method using generalized inverses is able to solve problems that involve redundant, degenerate and intermittent constraints, and can identify inconsistent constraint sets. It also allows a single program to perform pure kinematic and dynamic analyses. Its computational cost is among the lowest in comparison with other methods. In addition, constraint violation control methods are investigated to improve integration accuracy and further reduce computational cost. Constrained dynamics problems are also solved using optimization methods by applying Gauss' principle directly. An objective function that incorporates constraints is derived using a symmetric scheme, which is implemented using genetic algorithms in a parallel computing environment. It is shown that this method is capable of solving the same cases of constraints as the former method. Examples and numerical experiments demonstrating the applications of the two methods to constrained multiparticle and multibody systems are presented.

Three-dimensional pulsed disturbances in a plane mixing layer.

Gu, Xiaogang. (The University of Arizona., 1994)

The evolution of 3-D pulsed disturbances in a plane mixing layer is studied experimentally. The disturbance is effected via amplitude modulation of a spanwise uniform time-harmonic wave train which provides a clear phase reference for the phase-locked velocity measurements. The evolution of the pulsed disturbance depends critically on the time delay between the modulation pulse and the carrier wave train, the plane mixing layer is most receptive to pulsed excitation when the pulse appears in the braid region between adjacent primary vortices of the base flow. An amplitude demodulation technique is applied to decompose an isolated pulsed disturbance into a family of modal wave packets, and the evolution of the fundamental wave packet was studied in detail. The wave fronts of the wave packet in plane mixing layers are almost parallel to the span, in contrast to a boundary layer wave packet where wave fronts are bowed. The spanwise spreading speed of the wave packet is approximately equal to U(z) = 0.2U(c), while its growth in the streamwise direction is limited. The wave packet is non-dispersive, in agreement with the theoretical results. The effect of a pulse train having temporally and spatially periodic pattern is also studied. Wavelet transforms, both 1-D Morlet and 2-D Arc, are applied to study scales of the flow structures. The large-scale structure exhibits a staggered "chain-link"-like pattern in the streamwise and spanwise directions, whereas the small scale structures are initially generated half way between spanwise centers of pulses. The power spectra indicate that the energy at these small scales increases with increased x. This may suggest that pulsed disturbances may be used to enhance mixing.

A THEORETICAL STUDY OF THE DYNAMICS OF A VARIABLE MASS SYSTEM (APPLIED TOAEROBEE ROCKET)

Snyder, Virgil Ward (The University of Arizona., 1968)

THEORETICAL INVESTIGATION AND OPTIMIZATION OF AN AIRPLANE GUST ALLEVIATION SYSTEM

Polve, James Herschal, 1921- (The University of Arizona., 1966)

Dynamics of ceramic grinding: Regeneration and stability.

Kim, Hakin. (The University of Arizona., 1995)

In the present work, the process of grinding is represented through the dynamic interactions of the workpiece with the abrasive grits of the grinding wheel. A mathematical model (grinding dynamic model with wheel and workpiece-GDMWW) based on the resulting instantaneous depth of cut is then developed. The proposed model utilizes the governing equations of motions for both the wheel and the workpiece in a multi-grit grinding process. The cutting action is represented through the interactions of the wheel and the workpiece motions on each other. The parameters for the proposed multi-body dynamic model can be easily identified from simple experimentations (e.g., modal tests). Using the identified parameters, simulations with the proposed dynamic model are carried out for stable grinding processes. These simulation results are first verified against experimental observations obtained from grinding of soda-lime glass. The dynamic model is then utilized to investigate the regenerative effects, e.g., surface regeneration, mode coupling and velocity dependence. The effects of such effects on stability characteristics of ceramic grinding processes are investigated and possible remedies are suggested.

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