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## Full-Text Articles in Mechanical Engineering

High-Accuracy Large-Step Explicit Runge–Kutta (Hale-Rk) Schemes For Computational Aeroacoustics, Vasanth Allampalli, Ray Hixon, Raj Nallasamy, Scott Sawyer

#### High-Accuracy Large-Step Explicit Runge–Kutta (Hale-Rk) Schemes For Computational Aeroacoustics, Vasanth Allampalli, Ray Hixon, Raj Nallasamy, Scott Sawyer

*Dr. Scott Sawyer*

In many realistic calculations, the computational grid spacing required to resolve the mean flow gradients is much smaller than the grid spacing required to resolve the unsteady propagating waves of interest. Because of this, the high temporal resolution provided by existing optimized time marching schemes can be excessive due to the small time step required for stability in regions of clustered grid. In this work, explicit fourth-order accurate Runge–Kutta time marching schemes are optimized to increase the inviscid stability limit rather than the accuracy at large time steps. Single and multiple-step optimized schemes are developed and analyzed. The resulting ...

A Laplace Transform/Potential-Theoretic Method For Acoustic Propagation In Subsonic Flows, S. Hariharan, Scott Sawyer, D, Quinn

#### A Laplace Transform/Potential-Theoretic Method For Acoustic Propagation In Subsonic Flows, S. Hariharan, Scott Sawyer, D, Quinn

*Dr. Scott Sawyer*

This paper introduces a competitive computational approach for determining time-dependent far-field sound generated by subsonic flows around lifting airfoils. The procedure assumes the linearity of the sound field away from a bounded region surrounding the airfoil. It is assumed that the sound pressure on the boundary of this enclosed region (referred to as the Kirchhoff surface) is specified, possibly by another procedure such as solving the full Euler equations. Away from the Kirchhoff surface, the Euler equations are linearized about a uniform mean flow. It is well known that linearized Euler equations can be uncoupled into a scalar convective wave ...

Active Control Of Discrete-Frequency Noise Generated By Rotor-Stator Interactions, Scott Sawyer, Sanford Fleeter, John Simonich

#### Active Control Of Discrete-Frequency Noise Generated By Rotor-Stator Interactions, Scott Sawyer, Sanford Fleeter, John Simonich

*Dr. Scott Sawyer*

Discrete-frequency tones generated by unsteady blade row interactions are of particular concern in the design of advanced turbine engines...

Transform Potential-Theoretic Method For Acoustic Radiation From Structures, S. Hariharan, Scott Sawyer

#### Transform Potential-Theoretic Method For Acoustic Radiation From Structures, S. Hariharan, Scott Sawyer

*Dr. Scott Sawyer*

This paper presents further development in a new time-domain computational technique developed for the determination of far-field acoustic radiation. This method, referred to as the transform potential-theoretic (TPT) method, solves linear time-dependent wave propagation problems in an unbounded medium, combining techniques involving numerical transforms/inverse transforms and potential theory. The end result is a robust procedure that is accurate and computationally efficient. The TPT technique is meshless, yet it can handle arbitrary geometries for the acoustic source. The validity of the procedure is demonstrated for the case of an acoustic wave propagation from a two-dimensional bounded surface embedded in a ...

Validation Of A Computational Aeroacoustics Code For Nonlinear Flow About Complex Geometries Using Ringleb's Flow, Vinit Satav, Ray Hixon, M. Nallasamy, Scott Sawyer

#### Validation Of A Computational Aeroacoustics Code For Nonlinear Flow About Complex Geometries Using Ringleb's Flow, Vinit Satav, Ray Hixon, M. Nallasamy, Scott Sawyer

*Dr. Scott Sawyer*

This work is concerned with the validation of a high-accuracy Computational Aeroacoustics (CAA) code designed for use in the prediction of unsteady compressible flow about complex geometries, using Ringleb's analytic solution of the compressible steady Euler equations...

Flutter Stability Of A Detuned Cascade In Subsonic Compressible Flow, Scott Sawyer, Sanford Fleeter

#### Flutter Stability Of A Detuned Cascade In Subsonic Compressible Flow, Scott Sawyer, Sanford Fleeter

*Dr. Scott Sawyer*

A mathematical model is developed to predict the unsteady aerodynamics of a detuned two-dimensional flat plate cascade in subsonic compressible flow. Aerodynamic detuning is introduced by nonuniform circumferential spacing and chordwise offset. Combined aerodynamic-structural detuning is accomplished by replacing alternative airfoils with splitter blades...

Effect Of Grid Singularities On The Solution Accuracy Of A Caa Code, R. Hixon, M. Nallasamy, Scott Sawyer

#### Effect Of Grid Singularities On The Solution Accuracy Of A Caa Code, R. Hixon, M. Nallasamy, Scott Sawyer

*Dr. Scott Sawyer*

In this work, the effect of a grid singularities on the solution accuracy of a high-accuracy finite-difference Computational Aeroacoustics code is investigated by applying the NASA Glen Research Center ABS code to a simple benchmark flow.

Application Of A Nonlinear Computational Aeroacoustics Code To The Gust-Airfoil Problem, R. Hixon, V. Golubev, R. Mankbadi, J. Scott, Scott Sawyer, M. Nallasamy

#### Application Of A Nonlinear Computational Aeroacoustics Code To The Gust-Airfoil Problem, R. Hixon, V. Golubev, R. Mankbadi, J. Scott, Scott Sawyer, M. Nallasamy

*Dr. Scott Sawyer*

A time-domain solution of the gust-airfoil problem is obtained using a high-accuracy computational aeroacoustics code to solve the nonlinear Euler equations. For computational efficiency, the equations are cast in chain-rule curvilinear form, and a structured multiblock solver is used on a distributed-memory parallel computer cluster.

Bass Code Development, Scott Sawyer

#### Bass Code Development, Scott Sawyer

*Dr. Scott Sawyer*

The BASS computational aeroacoustic code solves the fully nonlinear Euler equations in the time domain in two-dimensions. The acoustic response of the stator is determined simultaneously for the first three harmonics of the convected vortical gust of the rotor. The spatial mode generation, propagation and decay characteristics are predicted by assuming the acoustic field away from the stator can be represented as a uniform flow with small harmonic perturbations superimposed. The computed field is then decomposed using a joint temporal-spatial transform to determine the wave amplitudes as a function of rotor harmonic and spatial mode order. This report details the ...

A Computational Aeroacoustic Prediction Of Discrete-Frequency Rotor-Stator Interaction Noise - A Linear Theory Analysis, Scott Sawyer, M. Nallasamy, R. Hixon, R. Dyson

#### A Computational Aeroacoustic Prediction Of Discrete-Frequency Rotor-Stator Interaction Noise - A Linear Theory Analysis, Scott Sawyer, M. Nallasamy, R. Hixon, R. Dyson

*Dr. Scott Sawyer*

The discrete-frequency noise generated by a rotor-stator interaction is computed by solving the fully nonlinear Euler equations in the time domain in two-dimensions. The acoustic response of the stator is determined simultaneously for the first three harmonics of the convected vertical gust of the rotor. The spatial mode generation, propagation and decay characteristics are predicted by assuming the acoustic field away from the stator can be represented as a uniform flow with small harmonic perturbations superimposed...

Solution Of Unsteady Euler Equations: Gust–Cascade Interaction Tones, Raj Nallasamy, Ray Hixon, Scott Sawyer

#### Solution Of Unsteady Euler Equations: Gust–Cascade Interaction Tones, Raj Nallasamy, Ray Hixon, Scott Sawyer

*Dr. Scott Sawyer*

The problem of interaction of a vortical gust with a two-dimensional cascade is considered. Full nonlinear time dependent Euler equations governing the flow are solved employing a 6th-order accurate spatial differencing scheme and a 4th-order accurate time marching technique. The vortical gust is represented by a Fourier series which includes three harmonics. The acoustic response of the cascade for single and multi frequency (vortical) excitations are calculated. The solutions show the generation and propagation of modes that are expected from the theory. It is demonstrated that at low amplitudes of excitation, the time domain analysis produces characteristics of the propagating ...

Comparison Of Numerical Schemes For A Realistic Computational Aeroacoustics Benchmark Problem, R. Hixon, M. Nallasamy, Scott Sawyer, R. Dyson

#### Comparison Of Numerical Schemes For A Realistic Computational Aeroacoustics Benchmark Problem, R. Hixon, M. Nallasamy, Scott Sawyer, R. Dyson

*Dr. Scott Sawyer*

In this work, a nonlinear block-structured CAA solver, the NASA Glenn Research Center BASS code, is tested on a realistic CAA benchmark problem in order to ascertain what effect the high-accuracy solution methods used in CAA have on a realistic test problem.

Simulating Nonlinear Stator Noise For Active Control, Rodgers Dyson, Ray Hixon, M. Nallasamy, Scott Sawyer

#### Simulating Nonlinear Stator Noise For Active Control, Rodgers Dyson, Ray Hixon, M. Nallasamy, Scott Sawyer

*Dr. Scott Sawyer*

No abstract provided.

Vortical Gust Boundary Condition For Realistic Rotor Wake/Stator Interaction Noise Prediction Using Computational Aeroacoustics, Ray Hixon, Adrian Sescu, Scott Sawyer

#### Vortical Gust Boundary Condition For Realistic Rotor Wake/Stator Interaction Noise Prediction Using Computational Aeroacoustics, Ray Hixon, Adrian Sescu, Scott Sawyer

*Dr. Scott Sawyer*

In this work, the NASA Glenn Research Center Broadband Aeroacoustic Stator Simulation (BASS) code is extended for use in the prediction of noise produced by realistic three-dimensional rotor wakes impinging on a downstream stator row. In order to accurately simulate such a flow using a nonlinear time-accurate solver, the inflow and outflow boundary conditions must simultaneously maintain the desired mean flow, allow outgoing vortical, entropic, and acoustic waves to cleanly exit the domain, and accurately impose the desired incoming flow disturbances. This work validates a new method for the acoustics-free imposition of three-dimensional vortical disturbances using benchmark test cases. *Vortical ...*

Modal Decomposition Of Rotor Wake-Stator Interaction Noise In A Swirling Shear Flow, Scott Sawyer, M. Nallasamy, R. Hixon

#### Modal Decomposition Of Rotor Wake-Stator Interaction Noise In A Swirling Shear Flow, Scott Sawyer, M. Nallasamy, R. Hixon

*Dr. Scott Sawyer*

Accurate prediction of turbomachinery noise is a continuing concern for aircraft engine designers. Although significant reductions in the noise generation have been achieved, future noise reductions will rely on a fundamental understanding of the noise generation process...

Mean Stator Loading Effect On The Acoustic Response Of A Rotating Cascade, Scott Sawyer, Sanford Fleeter

#### Mean Stator Loading Effect On The Acoustic Response Of A Rotating Cascade, Scott Sawyer, Sanford Fleeter

*Dr. Scott Sawyer*

Discrete-frequency tones generated by unsteady blade row interactions are of particular concern in the design of advanced turbine engines.With a rotor-stator mounted in a duct, only certain specific spatial modes are generated by the rotor-stator interaction, where the generated modes are a function of the number of rotor blades and stator vanes.

Passive Control Of Discrete-Frequency Tones Generated By Coupled Detuned Cascades, Scott Sawyer, S. Fleeter

#### Passive Control Of Discrete-Frequency Tones Generated By Coupled Detuned Cascades, Scott Sawyer, S. Fleeter

*Dr. Scott Sawyer*

Discrete-frequency tones generated by rotor–stator interactions are of particular concern in the design of fans and compressors. Classical theory considers an isolated flat-plate cascade of identical uniformly spaced airfoils. The current analysis extends this tuned isolated cascade theory to consider coupled aerodynamically detuned cascades where aerodynamic detuning is accomplished by changing the chord of alternate rotor blades and stator vanes. In a coupled cascade analysis, the configuration of the rotor influences the downstream acoustic response of the stator, and the stator configuration influences the upstream acoustic response of the rotor. This coupled detuned cascade unsteady aerodynamic model is first ...