Proceedings of the Vertical Flight Society 78th Annual Forum
This paper presents the first use of integrated three-dimensional (3D) aeromechanics modeling, de... more This paper presents the first use of integrated three-dimensional (3D) aeromechanics modeling, defined as the coupling of 3D solid finite elements based comprehensive analysis (CA) with 3D Reynolds-Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD), to study modern lift offset coaxial rotors. The goal is to demonstrate the development of this methodology and assess its capabilities. The X3D structural dynamics solver is coupled with the UMD Mercury CFD framework for this analysis. Metaltail- a notional hingeless coaxial rotorcraft, developed as an open-access model for the U.S. Army / DoD's High Performance Computing Framework Helios is used as the test case here. The analysis is performed at a low speed transition flight and preliminary predictions of airloads and three-dimensional stresses are discussed. Modern coaxial rotors are compromised by heavy rotor hubs reducing their performance. This paper aims to demonstrate a high-fidelity capability that can help ove...
A Hybrid Navier-Stokes Free-Wake Computational Fluid Dynamics (CFD) methodology, coupled with a m... more A Hybrid Navier-Stokes Free-Wake Computational Fluid Dynamics (CFD) methodology, coupled with a multibody dynamics analysis code, has been applied to the UH-60A rotor to study the loads developed during selected revolutions of a severe diving turn maneuvers, flight counters 11680 and 11679. Loosely coupled CFD/CSD simulations were performed for selected revolutions of 11680 and 11679 characterized by maximum blade structural and pitch-link loads. Time histories of sectional normal loads and pitching moments, blade structural loads, and pitch-link loads for these revs have been examined. Harmonic content of structural loads for a representative revolution are presented and discussed. Results indicate that the current methodology gives an accurate prediction of harmonics 1P-3P, but under-predicts harmonics 4P-6P. This record was migrated from the OpenDepot repository service in June, 2017 before shutting down.
The accuracy deficiency of high-order conventional finite difference schemes in the high-frequenc... more The accuracy deficiency of high-order conventional finite difference schemes in the high-frequency range is attributed to the approximation property of a power polynomial interpolation over equispaced nodes. To improve the numerical representation of high-frequency solutions, a new family of high-order finite difference schemes is constructed by trigonometric polynomial interpolation. As expected, these new difference schemes can resolve a wider frequency range than conventional finite difference schemes with the same stencil. In particular, they remove the possibility of Runge's phenomenon for uniform mesh. The recently proposed optimized difference schemes are indeed types of mixed difference schemes which make a compromise between power and trigonometric polynomial interpolation.
An accuracy analysis of various high order schemes is performed from an interpolation point of vi... more An accuracy analysis of various high order schemes is performed from an interpolation point of view. The analysis indicates that classical high order finite difference schemes, which use polynomial interpolation, hold high accuracy only at nodes and are therefore not suitable for time-dependent problems. Thus, some schemes improve their numerical accuracy within grid cells by the near-minimax approximation method, but their practical significance is degraded by maintaining the same stencil as classical schemes. One-step methods in space discretization, which use piecewise polynomial interpolation and involve data at only two points, can generate a uniform accuracy over the whole grid cell and avoid spurious roots. As a result, they are more accurate and efficient than multistep methods. In particular, the Cubic-Interpolated Psuedoparticle (CIP) scheme is recommended for computational acoustics.
Comparisons of many analysis codes have been made during the course of the UH-60 Airloads Worksho... more Comparisons of many analysis codes have been made during the course of the UH-60 Airloads Workshops. However, most comparisons involve only a small number of codes, and modeling assumptions are not consistent among all researchers. The Improved Prediction of Rotor Loads TAJI project, funded by the Vertical Lift Consortium (VLC), was initiated in part to perform a rigorous comparison of coupled solutions with the most widely used CSD and CFD codes. Careful comparisons of the structural models were made, and small modeling differences were eliminated. Loosely coupled simulations were made for UH-60A flight and wind tunnel test cases, in addition to other industry helicopter configurations. Flap bending and blade torsion loads are well predicted for all of the cases studied, while edgewise bending and pushrod loads tend to contain higher harmonic load content, which is less well predicted by the analysis. The various code combinations studied yield very similar load predictions when id...
Recently, an asymmetric lift-offset compound helicopter has been conceptualized at the University... more Recently, an asymmetric lift-offset compound helicopter has been conceptualized at the University of Maryland with the objective of improving the overall performance of a medium-lift utility helicopter. The investigated form of lift-compounding incorporates an additional stubbed wing attached to the fuselage on the retreating side. This design alleviates rotor lift requirements and generates a roll moment that enables increased thrust potential on the advancing side in high-speed forward flight. In this study, a numerical model was developed based on the corresponding experimental test case. Threedimensional unsteady Reynolds-averaged Navier–Stokes equations were solved on overset grids with computational fluid dynamics–computational structural dynamics (CFD–CSD) coupling using the in-house CPU–GPU heterogeneous Mercury CFD framework. Simulations were performed at high-speed, high-thrust operating conditions and showed satisfactory agreement with the experimental measurements in ter...
To expand the cruise speed of a compound helicopter, alleviating the compressibility effects on t... more To expand the cruise speed of a compound helicopter, alleviating the compressibility effects on the advancing side with reduced rotor RPM is proved to be an effective design feature, which results in high advance ratio flight regime. To investigate the aerodynamic phenomena at high advance ratios and provide data for the validation of analytical tools, a series of wind tunnel tests were conducted progressively in the Glenn L. Martin Wind Tunnel with a 33.5-inch radius four-bladed articulated rotor. In a recent wind tunnel test, the rotor blades were instrumented with pressure sensors and strain gauges at 30% radius, and pressure data were acquired to calculate the sectional airloads by surface integration up to an advance ratio of 0.8. The experimental results of rotor performance, control angles, blade airloads, and structural loads were compared with the predictions of comprehensive analysis and computational fluid dynamics (CFD) analysis coupled with computational structural dyna...
International Journal of Computational Fluid Dynamics
ABSTRACT The multi-mesh, multi-solver paradigm makes use of multiple Computational Fluid Dynamics... more ABSTRACT The multi-mesh, multi-solver paradigm makes use of multiple Computational Fluid Dynamics (CFD) solvers in a single overset framework. A framework-level implementation of the Generalised Minimal Residual algorithm applied to the full implicit overset system is presented. The method requires only minimal changes to existing, Python-wrapped CFD solvers and demonstrates improved convergence compared to traditional methods. Preliminary validation for the method is performed using the Poisson equation on two overset grids. The multi-body, overset GMRES method is shown to be equivalent to a conventional implementation which allocates memory for data across all overset grids on a single processor. Results are shown for two time-accurate flow simulations. The first case analyses convergence of a 2D inviscid wedge and the second analyses convergence of a laminar 3D sphere. Both the wedge and sphere bodies shed unsteady structures into nested, background Cartesian grids. In both unsteady cases, the presented overset GMRES method demonstrates superior convergence over traditional methods.
The current γ−Reθt¯ Spalart–Allmaras (SA) laminar–turbulent transition model is evaluated through... more The current γ−Reθt¯ Spalart–Allmaras (SA) laminar–turbulent transition model is evaluated through the representative two- and three-dimensional test cases within the line-based Hamiltonian–Strand g...
The propagation characteristics of acoustic waves due to the interaction between a vortex and a h... more The propagation characteristics of acoustic waves due to the interaction between a vortex and a helicopter airfoil were computed for a wide variety of methods, ranging from linearized transonic small disturbance to thin-layer Navier-Stokes. The analysis of the data from these methods showed that the accurate calculation of the acoustics required a computational method that not only accurately calculated the
For the first time a computational fluid dynamics (CFD) method is used to calculate directly the ... more For the first time a computational fluid dynamics (CFD) method is used to calculate directly the high-speed impulsive (HSI) noise of a non-lifting hovering rotor blade out to a distance of over three rotor radii. In order to accurately propagate the acoustic wave in a stable and efficient manner, an implicit upwind-biased Euler method is solved on a grid with points clustered along the line of propagation. A detailed validation of the code is performed for a rectangular rotor blade at tip Mach numbers ranging from 0.88 to 0.92. The agreement with experiment is excellent at both the sonic cylinder and at 2.18 rotor radii. The agreement at 3.09 rotor radii is still very good, showing improvements over the results from the best previous method. Grid sensitivity studies indicate that with special attention to the location of the boundaries a grid with approximately 60,000 points is adequate. This results in a computational time of approximately 40 minutes on a Cray-XMP. The practicality of the method to calculate HSI noise is demonstrated by expanding the scope of the investigation to examine the rectangular blade as well as a highly swept and tapered blade over a tip Mach number range of 0.80 to 0.95. Comparisons with experimental data are excellent and the advantages of planform modifications are clearly evident. New insight is gained into the mechanisms of nonlinear propagation and the minimum distance at which a valid comparison of different rotors can be made: approximately two rotor radii from the center of rotation.
Several recent applications (in the last five years) of Euler solvers in the computation of impul... more Several recent applications (in the last five years) of Euler solvers in the computation of impulsive noise from rotor blades emphasize their emerging role in complementing other methods and experimental work. In the area of high-speed impulsive noise the use of Euler solvers as research tools has become fairly mature with very favorable comparisons with experimental data, especially in hover. The grid sizes and resulting computational times are reasonable when compared to those required for accurate surface aerodynamics alone. Furthermore, Euler solvers have provided a rich database with the resolution and accuracy needed for input to Kirchhoff and acoustic analogy methods for predicting the far-field noise. On the other hand, the application of Euler solvers to calculate blade-vortex interaction noise is still far from mature. The computational resources required for accurate calculations away from the blade are much larger than for high-speed impulsive noise. Current calculations help improve the basic understanding of the phenomena involved, but to date no comparisons with experiment have been made. Fortunately, the use of coupled Euler solver/Kirchhoff methods seems to offer promise for a robust and efficient technique for predicting both high-speed impulsive noise and blade-vortex interaction noise. Finally, a simple model problem of an isolated vortex interacting with an arbitrarily prescribed pitching airfoil demonstrates the feasibility of using Euler solvers to examine noise reduction techniques. The use of simple aerodynamic quasi-static theory and the computed lift time history as feedback to determine the required pitching motion appears sufficient to significantly dampen the unsteady loading and subsequent acoustics by an order of magnitude within a few blade passages.
Smart Structures and Materials 1999: Smart Structures and Integrated Systems, 1999
The results of this feasibility study suggest that active blade twist technology is a viable mean... more The results of this feasibility study suggest that active blade twist technology is a viable means to reduce blade- vortex interaction (BVI) noise in rotorcraft systems. A linearized unsteady aerodynamics analysis was formulated and successfully validated with computation fluid dynamics analysis. A simple control scheme with three control points was found to be effective for active BVI noise reduction. Based on current-day actuation technology where 1 to 2 degrees of twist per blade activation span is expected, measurable noise reductions of 2 to 4 dB were predicted for the relatively strong, close vortex interactions. For weaker vortex interactions, reductions of 7 to 10 dB were predicted. The required twist actuation per blade span for complete unsteady loading cancellation, however, may be infeasible because of the large stroke and high frequency activation requirements.
50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012
Previous simulations of 2-D dynamic stall have demonstrated the need for transition modeling to c... more Previous simulations of 2-D dynamic stall have demonstrated the need for transition modeling to capture the boundary layer physics that will eventually influence the formation and departure of the leading-edge vortex. It is well known that the post-stall separated flow is highly three-dimensional and unsteady in nature. The present work investigates the suitability and effectiveness of hybrid RANS-LES methods to simulate this flow regime. Three-dimensional, time-accurate simulations of a sinusoidally pitching airfoil undergoing dynamic stall are performed using RANS, DES, and DDES methods. As expected, the RANS mode was found inadequate to capture the smaller structures and the spanwise variation present in the massively separated flow during downstroke. Both DES and DDES modes produced strong spanwise gradients as well as reduced turbulence levels in the vortical structures. As a result, the prediction of peak magnitudes of lift, drag, and moment at the end of the upstroke are improved compared to the RANS mode. However, DES caused early onset of stall and reduced upstroke lift at higher AoA due to the grid-induced separation phenomenon. Similarly, the lift recovery process was slower using DES compared to DDES due to an over-prediction of the trailing-edge separation region.
Proceedings of the Vertical Flight Society 78th Annual Forum
This paper presents the first use of integrated three-dimensional (3D) aeromechanics modeling, de... more This paper presents the first use of integrated three-dimensional (3D) aeromechanics modeling, defined as the coupling of 3D solid finite elements based comprehensive analysis (CA) with 3D Reynolds-Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD), to study modern lift offset coaxial rotors. The goal is to demonstrate the development of this methodology and assess its capabilities. The X3D structural dynamics solver is coupled with the UMD Mercury CFD framework for this analysis. Metaltail- a notional hingeless coaxial rotorcraft, developed as an open-access model for the U.S. Army / DoD's High Performance Computing Framework Helios is used as the test case here. The analysis is performed at a low speed transition flight and preliminary predictions of airloads and three-dimensional stresses are discussed. Modern coaxial rotors are compromised by heavy rotor hubs reducing their performance. This paper aims to demonstrate a high-fidelity capability that can help ove...
A Hybrid Navier-Stokes Free-Wake Computational Fluid Dynamics (CFD) methodology, coupled with a m... more A Hybrid Navier-Stokes Free-Wake Computational Fluid Dynamics (CFD) methodology, coupled with a multibody dynamics analysis code, has been applied to the UH-60A rotor to study the loads developed during selected revolutions of a severe diving turn maneuvers, flight counters 11680 and 11679. Loosely coupled CFD/CSD simulations were performed for selected revolutions of 11680 and 11679 characterized by maximum blade structural and pitch-link loads. Time histories of sectional normal loads and pitching moments, blade structural loads, and pitch-link loads for these revs have been examined. Harmonic content of structural loads for a representative revolution are presented and discussed. Results indicate that the current methodology gives an accurate prediction of harmonics 1P-3P, but under-predicts harmonics 4P-6P. This record was migrated from the OpenDepot repository service in June, 2017 before shutting down.
The accuracy deficiency of high-order conventional finite difference schemes in the high-frequenc... more The accuracy deficiency of high-order conventional finite difference schemes in the high-frequency range is attributed to the approximation property of a power polynomial interpolation over equispaced nodes. To improve the numerical representation of high-frequency solutions, a new family of high-order finite difference schemes is constructed by trigonometric polynomial interpolation. As expected, these new difference schemes can resolve a wider frequency range than conventional finite difference schemes with the same stencil. In particular, they remove the possibility of Runge's phenomenon for uniform mesh. The recently proposed optimized difference schemes are indeed types of mixed difference schemes which make a compromise between power and trigonometric polynomial interpolation.
An accuracy analysis of various high order schemes is performed from an interpolation point of vi... more An accuracy analysis of various high order schemes is performed from an interpolation point of view. The analysis indicates that classical high order finite difference schemes, which use polynomial interpolation, hold high accuracy only at nodes and are therefore not suitable for time-dependent problems. Thus, some schemes improve their numerical accuracy within grid cells by the near-minimax approximation method, but their practical significance is degraded by maintaining the same stencil as classical schemes. One-step methods in space discretization, which use piecewise polynomial interpolation and involve data at only two points, can generate a uniform accuracy over the whole grid cell and avoid spurious roots. As a result, they are more accurate and efficient than multistep methods. In particular, the Cubic-Interpolated Psuedoparticle (CIP) scheme is recommended for computational acoustics.
Comparisons of many analysis codes have been made during the course of the UH-60 Airloads Worksho... more Comparisons of many analysis codes have been made during the course of the UH-60 Airloads Workshops. However, most comparisons involve only a small number of codes, and modeling assumptions are not consistent among all researchers. The Improved Prediction of Rotor Loads TAJI project, funded by the Vertical Lift Consortium (VLC), was initiated in part to perform a rigorous comparison of coupled solutions with the most widely used CSD and CFD codes. Careful comparisons of the structural models were made, and small modeling differences were eliminated. Loosely coupled simulations were made for UH-60A flight and wind tunnel test cases, in addition to other industry helicopter configurations. Flap bending and blade torsion loads are well predicted for all of the cases studied, while edgewise bending and pushrod loads tend to contain higher harmonic load content, which is less well predicted by the analysis. The various code combinations studied yield very similar load predictions when id...
Recently, an asymmetric lift-offset compound helicopter has been conceptualized at the University... more Recently, an asymmetric lift-offset compound helicopter has been conceptualized at the University of Maryland with the objective of improving the overall performance of a medium-lift utility helicopter. The investigated form of lift-compounding incorporates an additional stubbed wing attached to the fuselage on the retreating side. This design alleviates rotor lift requirements and generates a roll moment that enables increased thrust potential on the advancing side in high-speed forward flight. In this study, a numerical model was developed based on the corresponding experimental test case. Threedimensional unsteady Reynolds-averaged Navier–Stokes equations were solved on overset grids with computational fluid dynamics–computational structural dynamics (CFD–CSD) coupling using the in-house CPU–GPU heterogeneous Mercury CFD framework. Simulations were performed at high-speed, high-thrust operating conditions and showed satisfactory agreement with the experimental measurements in ter...
To expand the cruise speed of a compound helicopter, alleviating the compressibility effects on t... more To expand the cruise speed of a compound helicopter, alleviating the compressibility effects on the advancing side with reduced rotor RPM is proved to be an effective design feature, which results in high advance ratio flight regime. To investigate the aerodynamic phenomena at high advance ratios and provide data for the validation of analytical tools, a series of wind tunnel tests were conducted progressively in the Glenn L. Martin Wind Tunnel with a 33.5-inch radius four-bladed articulated rotor. In a recent wind tunnel test, the rotor blades were instrumented with pressure sensors and strain gauges at 30% radius, and pressure data were acquired to calculate the sectional airloads by surface integration up to an advance ratio of 0.8. The experimental results of rotor performance, control angles, blade airloads, and structural loads were compared with the predictions of comprehensive analysis and computational fluid dynamics (CFD) analysis coupled with computational structural dyna...
International Journal of Computational Fluid Dynamics
ABSTRACT The multi-mesh, multi-solver paradigm makes use of multiple Computational Fluid Dynamics... more ABSTRACT The multi-mesh, multi-solver paradigm makes use of multiple Computational Fluid Dynamics (CFD) solvers in a single overset framework. A framework-level implementation of the Generalised Minimal Residual algorithm applied to the full implicit overset system is presented. The method requires only minimal changes to existing, Python-wrapped CFD solvers and demonstrates improved convergence compared to traditional methods. Preliminary validation for the method is performed using the Poisson equation on two overset grids. The multi-body, overset GMRES method is shown to be equivalent to a conventional implementation which allocates memory for data across all overset grids on a single processor. Results are shown for two time-accurate flow simulations. The first case analyses convergence of a 2D inviscid wedge and the second analyses convergence of a laminar 3D sphere. Both the wedge and sphere bodies shed unsteady structures into nested, background Cartesian grids. In both unsteady cases, the presented overset GMRES method demonstrates superior convergence over traditional methods.
The current γ−Reθt¯ Spalart–Allmaras (SA) laminar–turbulent transition model is evaluated through... more The current γ−Reθt¯ Spalart–Allmaras (SA) laminar–turbulent transition model is evaluated through the representative two- and three-dimensional test cases within the line-based Hamiltonian–Strand g...
The propagation characteristics of acoustic waves due to the interaction between a vortex and a h... more The propagation characteristics of acoustic waves due to the interaction between a vortex and a helicopter airfoil were computed for a wide variety of methods, ranging from linearized transonic small disturbance to thin-layer Navier-Stokes. The analysis of the data from these methods showed that the accurate calculation of the acoustics required a computational method that not only accurately calculated the
For the first time a computational fluid dynamics (CFD) method is used to calculate directly the ... more For the first time a computational fluid dynamics (CFD) method is used to calculate directly the high-speed impulsive (HSI) noise of a non-lifting hovering rotor blade out to a distance of over three rotor radii. In order to accurately propagate the acoustic wave in a stable and efficient manner, an implicit upwind-biased Euler method is solved on a grid with points clustered along the line of propagation. A detailed validation of the code is performed for a rectangular rotor blade at tip Mach numbers ranging from 0.88 to 0.92. The agreement with experiment is excellent at both the sonic cylinder and at 2.18 rotor radii. The agreement at 3.09 rotor radii is still very good, showing improvements over the results from the best previous method. Grid sensitivity studies indicate that with special attention to the location of the boundaries a grid with approximately 60,000 points is adequate. This results in a computational time of approximately 40 minutes on a Cray-XMP. The practicality of the method to calculate HSI noise is demonstrated by expanding the scope of the investigation to examine the rectangular blade as well as a highly swept and tapered blade over a tip Mach number range of 0.80 to 0.95. Comparisons with experimental data are excellent and the advantages of planform modifications are clearly evident. New insight is gained into the mechanisms of nonlinear propagation and the minimum distance at which a valid comparison of different rotors can be made: approximately two rotor radii from the center of rotation.
Several recent applications (in the last five years) of Euler solvers in the computation of impul... more Several recent applications (in the last five years) of Euler solvers in the computation of impulsive noise from rotor blades emphasize their emerging role in complementing other methods and experimental work. In the area of high-speed impulsive noise the use of Euler solvers as research tools has become fairly mature with very favorable comparisons with experimental data, especially in hover. The grid sizes and resulting computational times are reasonable when compared to those required for accurate surface aerodynamics alone. Furthermore, Euler solvers have provided a rich database with the resolution and accuracy needed for input to Kirchhoff and acoustic analogy methods for predicting the far-field noise. On the other hand, the application of Euler solvers to calculate blade-vortex interaction noise is still far from mature. The computational resources required for accurate calculations away from the blade are much larger than for high-speed impulsive noise. Current calculations help improve the basic understanding of the phenomena involved, but to date no comparisons with experiment have been made. Fortunately, the use of coupled Euler solver/Kirchhoff methods seems to offer promise for a robust and efficient technique for predicting both high-speed impulsive noise and blade-vortex interaction noise. Finally, a simple model problem of an isolated vortex interacting with an arbitrarily prescribed pitching airfoil demonstrates the feasibility of using Euler solvers to examine noise reduction techniques. The use of simple aerodynamic quasi-static theory and the computed lift time history as feedback to determine the required pitching motion appears sufficient to significantly dampen the unsteady loading and subsequent acoustics by an order of magnitude within a few blade passages.
Smart Structures and Materials 1999: Smart Structures and Integrated Systems, 1999
The results of this feasibility study suggest that active blade twist technology is a viable mean... more The results of this feasibility study suggest that active blade twist technology is a viable means to reduce blade- vortex interaction (BVI) noise in rotorcraft systems. A linearized unsteady aerodynamics analysis was formulated and successfully validated with computation fluid dynamics analysis. A simple control scheme with three control points was found to be effective for active BVI noise reduction. Based on current-day actuation technology where 1 to 2 degrees of twist per blade activation span is expected, measurable noise reductions of 2 to 4 dB were predicted for the relatively strong, close vortex interactions. For weaker vortex interactions, reductions of 7 to 10 dB were predicted. The required twist actuation per blade span for complete unsteady loading cancellation, however, may be infeasible because of the large stroke and high frequency activation requirements.
50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012
Previous simulations of 2-D dynamic stall have demonstrated the need for transition modeling to c... more Previous simulations of 2-D dynamic stall have demonstrated the need for transition modeling to capture the boundary layer physics that will eventually influence the formation and departure of the leading-edge vortex. It is well known that the post-stall separated flow is highly three-dimensional and unsteady in nature. The present work investigates the suitability and effectiveness of hybrid RANS-LES methods to simulate this flow regime. Three-dimensional, time-accurate simulations of a sinusoidally pitching airfoil undergoing dynamic stall are performed using RANS, DES, and DDES methods. As expected, the RANS mode was found inadequate to capture the smaller structures and the spanwise variation present in the massively separated flow during downstroke. Both DES and DDES modes produced strong spanwise gradients as well as reduced turbulence levels in the vortical structures. As a result, the prediction of peak magnitudes of lift, drag, and moment at the end of the upstroke are improved compared to the RANS mode. However, DES caused early onset of stall and reduced upstroke lift at higher AoA due to the grid-induced separation phenomenon. Similarly, the lift recovery process was slower using DES compared to DDES due to an over-prediction of the trailing-edge separation region.
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Papers by James Baeder