51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2013
To use a sector of annulus for turbomachinery fluid/structure interaction (FSI) simulation, a tim... more To use a sector of annulus for turbomachinery fluid/structure interaction (FSI) simulation, a time shifted phase lag (TSPL) is implemented at the circumferential boundaries where a phase lag condition exits based on a certain number of nodal diameters. A traveling wave initial condition to trigger the phase difference in blade vibration is also developed. For validation and comparison purpose for the phase-lag boundary conditions, full annulus flutter simulations of NASA Rotor 67 with backward traveling wave (BTW) of nodal diameter (ND) of 1 and 2 are conducted using a fully coupled FSI methodology, in which time accurate Reynolds averaged 3D Navier-Stokes equations are solved with a system of 5-decoupled structure modal equations in a fully coupled manner. The predicted blade vibration behavior from the single passage FSI using the TSPL shows good agreement with the full annulus FSI simulation. The traveling wave initial condition captures very well the effect of the phase angle difference for turbomachinery FSI simulation in the present study.
This paper uses the delayed detached eddy simulation (DDES) of turbulence to investigate the mech... more This paper uses the delayed detached eddy simulation (DDES) of turbulence to investigate the mechanism of non-synchronous vibration (NSV) of a multistage high speed axial compressor. DDES is a hybrid model for turbulence simulation, which uses RANS model within the wall boundary layer and uses large eddy simulation outside of the wall boundary layer. Time accurate Navier-Stokes equations are solved with a 3rd order WENO reconstruction for the inviscid flux and a 2nd order central differencing for the viscous terms. A fully conservative rotor/stator sliding BC is used to resolve the unsteady interaction between the rotor and the stationary blades. A 1/7th annulus sector is employed with the time shifted phase lag BC at the circumferential boundaries. The DDES shows that the NSV of the compressor occurs due to the rotating flow instability in the vicinity of the rotor tip at a stable operation condition. The tornado-like tip vortex causes the NSV of the rotor blades as it propagates to the next blade passage in the counter rotating direction above 80% rotor span. The tornado vortex travels fast on the suction surface of the blade and stays relatively longer at the passage outlet crossing to the next blade leading edge. Such a tornado vortex motion trajectory generates two low pressure regions due to the vortex core positions, one at the leading edge and one at the trailing edge, both are oscillating due to the vortex coming and leaving. These two low pressure regions create a pair of coupling forces that generates a torsion moment causing NSV.
50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012
Delayed Detached Eddy Simulation of supersonic flutter of a 3D wing is conducted at free stream M... more Delayed Detached Eddy Simulation of supersonic flutter of a 3D wing is conducted at free stream Mach number of 1.141 using a fully coupled fluid/structure interaction (FSI). Unsteady 3D compressible Navier-Stokes equations are solved with a system of 5 decoupled structure modal equations in a fully coupled manner. The low diffusion E-CUSP scheme with a 5th order WENO reconstruction for the inviscid flux and a set of 4th order central differencing for the viscous terms are used to accurately capture the shock wave/turbulent boundary layer interaction of the vibrating wing. The predicted flutter boundary at supersonic Mach number achieves excellent agreement with experiment. It appears to be the first time that a numerical prediction of supersonic flutter boundary matches with experiment accurately.
Volume 6: Fluids and Thermal Systems; Advances for Process Industries, Parts A and B, 2011
Multistage 3D steady and unsteady viscous computations are conducted for a transonic axial compre... more Multistage 3D steady and unsteady viscous computations are conducted for a transonic axial compressor with an IGV, rotor and stator blade row. A fully conservative sliding technique is developed with high order shock capturing schemes to better capture the interaction between stationary and rotating blade row. A 1/7th annulus is used for the unsteady simulation with a phase lag BC. For comparison purpose, a steady simulation of the same multistage compressor with single blade passage is performed using the mixing plane approach. It is shown that the sliding BC captures wake propagation very well in the interaction between blade rows. The unsteady multistage simulation using sliding BC and phase lag BC predicts almost identical loading distribution to the steady state simulation at the mid-span, but have significant loading distribution difference at tip section.
41st AIAA Fluid Dynamics Conference and Exhibit, 2011
This paper uses large eddy simulation (LES) to investigate Co-Flow Jet (CFJ) airfoil flows at hig... more This paper uses large eddy simulation (LES) to investigate Co-Flow Jet (CFJ) airfoil flows at high angle of attack (AOA). The standard Smagorinsky model with Van Driest damping is employed to resolve the subgrid scale stress. The 5th order WENO scheme is used for the reconstruction of the inviscid flux and the 4th order central differencing for the viscous flux. The momentum coefficient(C µ ) is chosen as a parameter to control the jet flow of the CFJ airfoil. LES simulations were carried out for the different momentum coefficients. The LES results at 30 o AOA are compared with the experiment to understand the flow structure of the jet mixing and flow separation. The numerically captured qualitative flow structures are very similar to those obtained in experiment. The quantitative prediction of lift and drag agree very well with experiment.
Volume 6: Structures and Dynamics, Parts A and B, 2011
Detached eddy simulation of an aeroelastic self-excited instability, flutter in NASA Rotor 67 is ... more Detached eddy simulation of an aeroelastic self-excited instability, flutter in NASA Rotor 67 is conducted using a fully coupled fluid/structre interaction. Time accurate compressible 3D Navier-Stokes equations are solved with a system of 5 decoupled modal equations in a fully coupled manner. The 5th order WENO scheme for the inviscid flux and the 4th order central differencing for the viscous flux are used to accurately capture interactions between the flow and vibrating blades with the DES(detached eddy simulation) of turbulence. A moving mesh concept that can improve mesh quality over the rotor tip clearance was implemented. Flutter simulations were first conducted from choke to stall using 4 blade passages. Stall flutter initiated at rotating stall onset, grows dramatically with resonance. The frequency analysis shows that resonance occurs at the first mode of the rotor blade. Before stall, the predicted responses of rotor blades decayed with time, resulting in no flutter. Full annulus simulation at peak point verifies that one can use the multi-passage approach with periodic boundary for the flutter prediction.
This paper investigates the aerodynamic excitation mechanism of the rotor-tip flow instabilities ... more This paper investigates the aerodynamic excitation mechanism of the rotor-tip flow instabilities leading to nonsynchronous vibration in a high-speed multistage axial compressor. Numerical simulations for the one-seventh annulus periodic sector of 1-1/2 stage are performed using an unsteady Reynolds-averaged Navier-Stokes solver with a fully conservative, sliding interface, boundary condition to capture wake propagation between adjacent blade rows. The present numerical simulations for rigid blades demonstrate that the tip flow instability is the main cause of the compressor nonsynchronous vibration excitation and is generated by the circumferentially traveling vortices. The frequency of the vortex passing each blade in the counter-rotation direction is roughly equal to the nonsynchronous vibration excitation frequency. Three different rotor-tip clearance sizes and shapes are studied. The nonsynchronous vibration excitation frequency and amplitude vary slightly with the tip clearance size when the clearances have the same flat shape, but they change substantially when the tip clearance has a convex-type shape. The predicted nonsynchronous vibration excitation frequency is in excellent agreement with the rig testing.
49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2011
This paper presents a numerical methodology to study the mechanism of stall flutter at near stall... more This paper presents a numerical methodology to study the mechanism of stall flutter at near stall conditions using high fidelity detached eddy simulation (DES) in conjunction with a fully coupled fluidstructural interaction (FSI) model. At near-stall conditions, the flow is highly unsteady due to tip leakage vortex and may induce stall flutter. Such problems often involve shock wave-turbulent boundary layer interaction and flow separation due to rotating stall. In order to capture the possible flutter at near stall conditions, the DES is performed in a single passage domain with circumferential periodicity. The CFD techniques used include: a 5th order WENO scheme with a low diffusion Riemann solver for the inviscid fluxes, a fully conservative 4th order central differencing scheme for the viscous terms, a fully coupled fluid-structural interaction methodology, and a massive parallel procedure. The structural solver employs the efficient and accurate modal approach with five major mode shapes for the rotor blade. The work described herein will lay the foundation for the future work of simulating non-synchronous vibration. applications was made by Srinivasan et al. . Five widely-used turbulence models, namely the Baldwin-Lomax model , the Renormalization Group model [3], the half-equation Johnson-King model , and the one-equation models of Baldwin-Barth [6] and Spalart-Allmaras [7] are tested and evaluated for accuracy and robustness by using them to simulate the unsteady flows passing an oscillating rigid NACA 0015 airfoil. Their conclusion is that the one-equation models provide significant improvement over the algebraic and half-equation models. The work of Bohbot et al. provides a comparison between the Baldwin-Lomax solution and Spalart-Allmaras solution of the viscous flutter boundary prediction for a NACA64A010 airfoil. A significant difference can be observed between two predicted flutter boundaries from both turbulence models. Another example of 3-D aeroelastic computations for transonic flows has been given by Doi et al.[9]. They found that the k − ω model [10] was a suitable choice for prediction of flutter boundaries of turbomachinery components. Chen et al. have developed a fully coupled method for prediction of flutter boundaries of AGARD Wing 445.6 using the Baldwin-Lomax model. The Baldwin-Lomax model is found to be inadequate to predict complex flow field with massive unsteady separated zones [1] for aeroelastic simulations. Wang and zha successfully applied the DES to predict the LCO of NLR7301 airfoil [13] and [14] which use S-A model and predict the LCO as good as DES. An excellent quantitative agreement between the computation and experiement is achieved for both LCO frequency and amplitude. For a compressor rotor, the flow fields become more complicated since a flexible blade has many more degrees of freedom than a rigid body airfoil or a flexible wing. Therefore, the objective of this investigation is to apply the most recent turbulence closure method in the 3-D aeroelastic analysis.
Volume 7: Structures and Dynamics, Parts A and B, 2012
In this paper non-synchronous vibration (NSV) of a GE axial compressor is simulated using a fully... more In this paper non-synchronous vibration (NSV) of a GE axial compressor is simulated using a fully coupled fluid/strcuture interaction (FSI). Time accurate Navier-Stokes equations are solved with a system of 5 decoupled structure modal equations in a fully coupled manner. A 3rd order WENO scheme for the inviscid flux and a 2nd order central differencing for the viscous terms are used to resolve nonlinear interaction between vibrating blades and fluid flow. 1/7th annulus is used with a time shifted phaselag (TSPL) boundary condition to reduce computational efforts. A fully conservative rotor/stator sliding boundary condition is employed to accurately capture unsteady wake propagation between the rotor and stator blades. The predicted dominant frequencies using the blade tip response signals are not harmonic to the engine order, which is the NSV. The blade vibration is torsionally coupled with highly oscillating blade pressure and is not damped out during the NSV. No resonance to the blade natural frequencies is found. The instability of tornado vortices in the vicinity of the rotor tip due to the strong interaction of incoming flow, tip vortex and tip
This paper uses a fully coupled fluid/structure interaction (FSI) to investigate the flutter mech... more This paper uses a fully coupled fluid/structure interaction (FSI) to investigate the flutter mechanism of a modern transonic fan rotor with a forward travelling wave. To induce an initial travelling wave for the blade structure, an initial BC that can facilitate each blade to vibrate with a time lag by a given nodal diameter (ND) is implemented. Unsteady Reynolds-averaged Navier-Stokes (URANS) equations are solved with a system of structure modal equations in a fully coupled manner. The 5th order WENO scheme with a low diffusion E-CUSP Riemann solver is used for the inviscid fluxes and a 2nd order central differencing is used for the viscous terms. A half annulus sector is used for the flutter simulations with a time shifted phase lag boundary condition at the circumferential boundaries. The present FSI simulations show that the shock instability causes the flutter. When the detached normal shock moves further upstream in a direction normal to the blade chord, the interaction of the detached normal shock with tip leakage vortex creates more serious blockage to the blade passage that can introduce an aerodynamic instability to the blade structure due to the incoming flow disturbance, resulting in flutter. The flutter of the transonic fan observed in this study occurs at the 1st mode before the stall. The predicted flutter boundary agrees well with the experiment.
48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2010
This paper introduces a novel concept for supersonic airplane: supersonic bi-directional (SBiDir)... more This paper introduces a novel concept for supersonic airplane: supersonic bi-directional (SBiDir) flying wing (FW) concept, which is to achieve low sonic boom, low supersonic wave drag, and high subsonic performance. The SBiDir-FW planform is symmetric about both longitudinal and span axes. For supersonic flight, the planform will have low aspect ratio and high sweep angle to minimize wave drag. For subsonic mode, the airplane will rotate 90 • and the sweep angle will be reduced and the aspect ratio will be increased. To minimize sonic boom, the pressure surface of the flying wing will employ an isentropic compression surface. At zero angle of attack (AoA) as the example studied in this paper, a flat pressure surface achieves this purpose. The CFD simulation shows that it obtains low ground sonic boom overpressure of 0.3psf with L/D p = 5.3. Furthermore, the ground pressure signature is not the N shape wave with two strong shock wave pulses, but is in a smooth sin wave shape. The results show that it is possible to remove or achieve very low sonic boom using a supersonic bi-directional flying wing or blended wing body configuration. Future work will optimize the SBiDir-FW concept to achieve high aerodynamic efficiency and maintain low sonic boom.
A high speed 1-1/2 axial compressor stage is simulated in this paper using an Unsteady Reynolds-A... more A high speed 1-1/2 axial compressor stage is simulated in this paper using an Unsteady Reynolds-Averaged Navier-Stokes (URANS) solver for a full-annulus configuration to capture its non-synchronous vibration (NSV) flow excitation. The simulation presented in this paper assumes rigid blades. A 3rd order WENO scheme for the inviscid flux and a 2nd order central differencing for the viscous terms are used to resolve nonlinear interaction between blades and fluid flow. A fully conservative rotor/stator sliding boundary condition is employed with multiple-processor capability for rotor/stator interface information exchange for parallel computing. The sliding BC accurately captures unsteady wake propagation between the rotor and stator blades while conserving fluxes across the rotor/stator interfaces. The predicted dominant frequencies using the blade tip response signals are not harmonic to the engine order, which is the NSV excitation. The simulation is based on a rotor blade with a 1.1% tip-chord clearance. Comparison to previous 1/7th annulus simulations show previous time-shifted phase-lag BCs are accurate. The NSV excitation frequency of the full annulus simulation is for the most part 3.3% lower than experimental and matches with the 1/7th annulus simulation, although some blades displayed slightly different NSV excitation frequencies. The full annulus simulation confirms that the instability of tornado vortices in the vicinity of the rotor tip due to the strong interaction of incoming flow, tip vortex and tip leakage flow is the main cause of the NSV excitation. This instability is present in all blades of the rotor annulus. While the time-shifted phase lag BCs can accurately capture the frequency of NSV excitation, phenomena related to flow separation with lower frequencies, including dual-vortex systems within blade passages, are not captured by the 1/7th annulus simulation, but are found in the full-annulus simulation.
Volume 7: Structures and Dynamics, Parts A and B, 2012
This paper investigates non-synchronous vibration (NSV) mechanism of a high-speed axial compresso... more This paper investigates non-synchronous vibration (NSV) mechanism of a high-speed axial compressor with three different rotor tip clearances. Numerical simulations for 1/7th annulus periodic sector are performed using an unsteady Reynolds-averaged Navier-Stokes(URANS) solver with a fully conservative sliding boundary condition to capture wake unsteadiness between the rotor and stator blades. The simulated NSV shows that the frequency and amplitude are strongly influenced by the tip clearance size and shape. The predicted NSV frequency is in good agreement with the experiment. The maximum amplitude of the NSV occurs at about 78% span of the rotor suction leading edge regardless of tip clearance due to a strong interaction of incoming flow, tip leakage flow and tip vortex.
Full-annulus detached-eddy simulation (DES) is conducted in this paper to investigate stall incep... more Full-annulus detached-eddy simulation (DES) is conducted in this paper to investigate stall inception for the axial transonic rotor, NASA rotor 67. A low-diffusion E-CUSP scheme with a third-order MUSCL reconstruction is used to discretize the inviscid fluxes, and second-order central differencing is used for the viscous terms. An implicit line Gauss-Seidel iteration with dual time-stepping method and second-order temporal accuracy is employed for time integration of the spatially filtered unsteady Navier-Stokes equations in a rotating frame. It is observed that the rotating stall is initiated by the local spike flow disturbance, which quickly induces the rotor to stall roughly over two rotor revolutions. The stall cell covering more than six blade tip passages propagates at 48% of rotor speed in the counter-rotor rotation direction. The process of rotating stall is captured by the full-annulus DES, which indicates that the blockage created by the low-energy vortical flow structure pushes the tip leakage flow to the adjacent blade behind the detached sonic boundary. The unsteady Reynolds-averaged Navier-Stokes (URANS) simulation is also performed for comparison. DES predicts the stall inception roughly one rotor revolution earlier than the URANS model. Overall, DES predicts the stall inception more realistically.
Large eddy simulation (LES) is conducted to investigate coflow jet (CFJ) airfoil flows at high an... more Large eddy simulation (LES) is conducted to investigate coflow jet (CFJ) airfoil flows at high angle of attack (AOA). The Smagorinsky model with Van Driest damping is employed to resolve the subgrid-scale stress. The fifth-order weighted essentially non-oscillatory (WENO) scheme is used for reconstruction of the inviscid flux and the fourthorder central differencing for the viscous flux. The LES results at an AOA of 0 deg, 12 deg, 25 deg, and 30 deg with momentum coefficients of C l ¼ 0.15 and 0.08 are compared with the experiment to understand the flow structure of the jet mixing and flow separation. The quantitative prediction of lift and drag and qualitative prediction of vortex structures are in good agreement with experiment.
51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2013
To use a sector of annulus for turbomachinery fluid/structure interaction (FSI) simulation, a tim... more To use a sector of annulus for turbomachinery fluid/structure interaction (FSI) simulation, a time shifted phase lag (TSPL) is implemented at the circumferential boundaries where a phase lag condition exits based on a certain number of nodal diameters. A traveling wave initial condition to trigger the phase difference in blade vibration is also developed. For validation and comparison purpose for the phase-lag boundary conditions, full annulus flutter simulations of NASA Rotor 67 with backward traveling wave (BTW) of nodal diameter (ND) of 1 and 2 are conducted using a fully coupled FSI methodology, in which time accurate Reynolds averaged 3D Navier-Stokes equations are solved with a system of 5-decoupled structure modal equations in a fully coupled manner. The predicted blade vibration behavior from the single passage FSI using the TSPL shows good agreement with the full annulus FSI simulation. The traveling wave initial condition captures very well the effect of the phase angle difference for turbomachinery FSI simulation in the present study.
This paper uses the delayed detached eddy simulation (DDES) of turbulence to investigate the mech... more This paper uses the delayed detached eddy simulation (DDES) of turbulence to investigate the mechanism of non-synchronous vibration (NSV) of a multistage high speed axial compressor. DDES is a hybrid model for turbulence simulation, which uses RANS model within the wall boundary layer and uses large eddy simulation outside of the wall boundary layer. Time accurate Navier-Stokes equations are solved with a 3rd order WENO reconstruction for the inviscid flux and a 2nd order central differencing for the viscous terms. A fully conservative rotor/stator sliding BC is used to resolve the unsteady interaction between the rotor and the stationary blades. A 1/7th annulus sector is employed with the time shifted phase lag BC at the circumferential boundaries. The DDES shows that the NSV of the compressor occurs due to the rotating flow instability in the vicinity of the rotor tip at a stable operation condition. The tornado-like tip vortex causes the NSV of the rotor blades as it propagates to the next blade passage in the counter rotating direction above 80% rotor span. The tornado vortex travels fast on the suction surface of the blade and stays relatively longer at the passage outlet crossing to the next blade leading edge. Such a tornado vortex motion trajectory generates two low pressure regions due to the vortex core positions, one at the leading edge and one at the trailing edge, both are oscillating due to the vortex coming and leaving. These two low pressure regions create a pair of coupling forces that generates a torsion moment causing NSV.
50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012
Delayed Detached Eddy Simulation of supersonic flutter of a 3D wing is conducted at free stream M... more Delayed Detached Eddy Simulation of supersonic flutter of a 3D wing is conducted at free stream Mach number of 1.141 using a fully coupled fluid/structure interaction (FSI). Unsteady 3D compressible Navier-Stokes equations are solved with a system of 5 decoupled structure modal equations in a fully coupled manner. The low diffusion E-CUSP scheme with a 5th order WENO reconstruction for the inviscid flux and a set of 4th order central differencing for the viscous terms are used to accurately capture the shock wave/turbulent boundary layer interaction of the vibrating wing. The predicted flutter boundary at supersonic Mach number achieves excellent agreement with experiment. It appears to be the first time that a numerical prediction of supersonic flutter boundary matches with experiment accurately.
Volume 6: Fluids and Thermal Systems; Advances for Process Industries, Parts A and B, 2011
Multistage 3D steady and unsteady viscous computations are conducted for a transonic axial compre... more Multistage 3D steady and unsteady viscous computations are conducted for a transonic axial compressor with an IGV, rotor and stator blade row. A fully conservative sliding technique is developed with high order shock capturing schemes to better capture the interaction between stationary and rotating blade row. A 1/7th annulus is used for the unsteady simulation with a phase lag BC. For comparison purpose, a steady simulation of the same multistage compressor with single blade passage is performed using the mixing plane approach. It is shown that the sliding BC captures wake propagation very well in the interaction between blade rows. The unsteady multistage simulation using sliding BC and phase lag BC predicts almost identical loading distribution to the steady state simulation at the mid-span, but have significant loading distribution difference at tip section.
41st AIAA Fluid Dynamics Conference and Exhibit, 2011
This paper uses large eddy simulation (LES) to investigate Co-Flow Jet (CFJ) airfoil flows at hig... more This paper uses large eddy simulation (LES) to investigate Co-Flow Jet (CFJ) airfoil flows at high angle of attack (AOA). The standard Smagorinsky model with Van Driest damping is employed to resolve the subgrid scale stress. The 5th order WENO scheme is used for the reconstruction of the inviscid flux and the 4th order central differencing for the viscous flux. The momentum coefficient(C µ ) is chosen as a parameter to control the jet flow of the CFJ airfoil. LES simulations were carried out for the different momentum coefficients. The LES results at 30 o AOA are compared with the experiment to understand the flow structure of the jet mixing and flow separation. The numerically captured qualitative flow structures are very similar to those obtained in experiment. The quantitative prediction of lift and drag agree very well with experiment.
Volume 6: Structures and Dynamics, Parts A and B, 2011
Detached eddy simulation of an aeroelastic self-excited instability, flutter in NASA Rotor 67 is ... more Detached eddy simulation of an aeroelastic self-excited instability, flutter in NASA Rotor 67 is conducted using a fully coupled fluid/structre interaction. Time accurate compressible 3D Navier-Stokes equations are solved with a system of 5 decoupled modal equations in a fully coupled manner. The 5th order WENO scheme for the inviscid flux and the 4th order central differencing for the viscous flux are used to accurately capture interactions between the flow and vibrating blades with the DES(detached eddy simulation) of turbulence. A moving mesh concept that can improve mesh quality over the rotor tip clearance was implemented. Flutter simulations were first conducted from choke to stall using 4 blade passages. Stall flutter initiated at rotating stall onset, grows dramatically with resonance. The frequency analysis shows that resonance occurs at the first mode of the rotor blade. Before stall, the predicted responses of rotor blades decayed with time, resulting in no flutter. Full annulus simulation at peak point verifies that one can use the multi-passage approach with periodic boundary for the flutter prediction.
This paper investigates the aerodynamic excitation mechanism of the rotor-tip flow instabilities ... more This paper investigates the aerodynamic excitation mechanism of the rotor-tip flow instabilities leading to nonsynchronous vibration in a high-speed multistage axial compressor. Numerical simulations for the one-seventh annulus periodic sector of 1-1/2 stage are performed using an unsteady Reynolds-averaged Navier-Stokes solver with a fully conservative, sliding interface, boundary condition to capture wake propagation between adjacent blade rows. The present numerical simulations for rigid blades demonstrate that the tip flow instability is the main cause of the compressor nonsynchronous vibration excitation and is generated by the circumferentially traveling vortices. The frequency of the vortex passing each blade in the counter-rotation direction is roughly equal to the nonsynchronous vibration excitation frequency. Three different rotor-tip clearance sizes and shapes are studied. The nonsynchronous vibration excitation frequency and amplitude vary slightly with the tip clearance size when the clearances have the same flat shape, but they change substantially when the tip clearance has a convex-type shape. The predicted nonsynchronous vibration excitation frequency is in excellent agreement with the rig testing.
49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2011
This paper presents a numerical methodology to study the mechanism of stall flutter at near stall... more This paper presents a numerical methodology to study the mechanism of stall flutter at near stall conditions using high fidelity detached eddy simulation (DES) in conjunction with a fully coupled fluidstructural interaction (FSI) model. At near-stall conditions, the flow is highly unsteady due to tip leakage vortex and may induce stall flutter. Such problems often involve shock wave-turbulent boundary layer interaction and flow separation due to rotating stall. In order to capture the possible flutter at near stall conditions, the DES is performed in a single passage domain with circumferential periodicity. The CFD techniques used include: a 5th order WENO scheme with a low diffusion Riemann solver for the inviscid fluxes, a fully conservative 4th order central differencing scheme for the viscous terms, a fully coupled fluid-structural interaction methodology, and a massive parallel procedure. The structural solver employs the efficient and accurate modal approach with five major mode shapes for the rotor blade. The work described herein will lay the foundation for the future work of simulating non-synchronous vibration. applications was made by Srinivasan et al. . Five widely-used turbulence models, namely the Baldwin-Lomax model , the Renormalization Group model [3], the half-equation Johnson-King model , and the one-equation models of Baldwin-Barth [6] and Spalart-Allmaras [7] are tested and evaluated for accuracy and robustness by using them to simulate the unsteady flows passing an oscillating rigid NACA 0015 airfoil. Their conclusion is that the one-equation models provide significant improvement over the algebraic and half-equation models. The work of Bohbot et al. provides a comparison between the Baldwin-Lomax solution and Spalart-Allmaras solution of the viscous flutter boundary prediction for a NACA64A010 airfoil. A significant difference can be observed between two predicted flutter boundaries from both turbulence models. Another example of 3-D aeroelastic computations for transonic flows has been given by Doi et al.[9]. They found that the k − ω model [10] was a suitable choice for prediction of flutter boundaries of turbomachinery components. Chen et al. have developed a fully coupled method for prediction of flutter boundaries of AGARD Wing 445.6 using the Baldwin-Lomax model. The Baldwin-Lomax model is found to be inadequate to predict complex flow field with massive unsteady separated zones [1] for aeroelastic simulations. Wang and zha successfully applied the DES to predict the LCO of NLR7301 airfoil [13] and [14] which use S-A model and predict the LCO as good as DES. An excellent quantitative agreement between the computation and experiement is achieved for both LCO frequency and amplitude. For a compressor rotor, the flow fields become more complicated since a flexible blade has many more degrees of freedom than a rigid body airfoil or a flexible wing. Therefore, the objective of this investigation is to apply the most recent turbulence closure method in the 3-D aeroelastic analysis.
Volume 7: Structures and Dynamics, Parts A and B, 2012
In this paper non-synchronous vibration (NSV) of a GE axial compressor is simulated using a fully... more In this paper non-synchronous vibration (NSV) of a GE axial compressor is simulated using a fully coupled fluid/strcuture interaction (FSI). Time accurate Navier-Stokes equations are solved with a system of 5 decoupled structure modal equations in a fully coupled manner. A 3rd order WENO scheme for the inviscid flux and a 2nd order central differencing for the viscous terms are used to resolve nonlinear interaction between vibrating blades and fluid flow. 1/7th annulus is used with a time shifted phaselag (TSPL) boundary condition to reduce computational efforts. A fully conservative rotor/stator sliding boundary condition is employed to accurately capture unsteady wake propagation between the rotor and stator blades. The predicted dominant frequencies using the blade tip response signals are not harmonic to the engine order, which is the NSV. The blade vibration is torsionally coupled with highly oscillating blade pressure and is not damped out during the NSV. No resonance to the blade natural frequencies is found. The instability of tornado vortices in the vicinity of the rotor tip due to the strong interaction of incoming flow, tip vortex and tip
This paper uses a fully coupled fluid/structure interaction (FSI) to investigate the flutter mech... more This paper uses a fully coupled fluid/structure interaction (FSI) to investigate the flutter mechanism of a modern transonic fan rotor with a forward travelling wave. To induce an initial travelling wave for the blade structure, an initial BC that can facilitate each blade to vibrate with a time lag by a given nodal diameter (ND) is implemented. Unsteady Reynolds-averaged Navier-Stokes (URANS) equations are solved with a system of structure modal equations in a fully coupled manner. The 5th order WENO scheme with a low diffusion E-CUSP Riemann solver is used for the inviscid fluxes and a 2nd order central differencing is used for the viscous terms. A half annulus sector is used for the flutter simulations with a time shifted phase lag boundary condition at the circumferential boundaries. The present FSI simulations show that the shock instability causes the flutter. When the detached normal shock moves further upstream in a direction normal to the blade chord, the interaction of the detached normal shock with tip leakage vortex creates more serious blockage to the blade passage that can introduce an aerodynamic instability to the blade structure due to the incoming flow disturbance, resulting in flutter. The flutter of the transonic fan observed in this study occurs at the 1st mode before the stall. The predicted flutter boundary agrees well with the experiment.
48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2010
This paper introduces a novel concept for supersonic airplane: supersonic bi-directional (SBiDir)... more This paper introduces a novel concept for supersonic airplane: supersonic bi-directional (SBiDir) flying wing (FW) concept, which is to achieve low sonic boom, low supersonic wave drag, and high subsonic performance. The SBiDir-FW planform is symmetric about both longitudinal and span axes. For supersonic flight, the planform will have low aspect ratio and high sweep angle to minimize wave drag. For subsonic mode, the airplane will rotate 90 • and the sweep angle will be reduced and the aspect ratio will be increased. To minimize sonic boom, the pressure surface of the flying wing will employ an isentropic compression surface. At zero angle of attack (AoA) as the example studied in this paper, a flat pressure surface achieves this purpose. The CFD simulation shows that it obtains low ground sonic boom overpressure of 0.3psf with L/D p = 5.3. Furthermore, the ground pressure signature is not the N shape wave with two strong shock wave pulses, but is in a smooth sin wave shape. The results show that it is possible to remove or achieve very low sonic boom using a supersonic bi-directional flying wing or blended wing body configuration. Future work will optimize the SBiDir-FW concept to achieve high aerodynamic efficiency and maintain low sonic boom.
A high speed 1-1/2 axial compressor stage is simulated in this paper using an Unsteady Reynolds-A... more A high speed 1-1/2 axial compressor stage is simulated in this paper using an Unsteady Reynolds-Averaged Navier-Stokes (URANS) solver for a full-annulus configuration to capture its non-synchronous vibration (NSV) flow excitation. The simulation presented in this paper assumes rigid blades. A 3rd order WENO scheme for the inviscid flux and a 2nd order central differencing for the viscous terms are used to resolve nonlinear interaction between blades and fluid flow. A fully conservative rotor/stator sliding boundary condition is employed with multiple-processor capability for rotor/stator interface information exchange for parallel computing. The sliding BC accurately captures unsteady wake propagation between the rotor and stator blades while conserving fluxes across the rotor/stator interfaces. The predicted dominant frequencies using the blade tip response signals are not harmonic to the engine order, which is the NSV excitation. The simulation is based on a rotor blade with a 1.1% tip-chord clearance. Comparison to previous 1/7th annulus simulations show previous time-shifted phase-lag BCs are accurate. The NSV excitation frequency of the full annulus simulation is for the most part 3.3% lower than experimental and matches with the 1/7th annulus simulation, although some blades displayed slightly different NSV excitation frequencies. The full annulus simulation confirms that the instability of tornado vortices in the vicinity of the rotor tip due to the strong interaction of incoming flow, tip vortex and tip leakage flow is the main cause of the NSV excitation. This instability is present in all blades of the rotor annulus. While the time-shifted phase lag BCs can accurately capture the frequency of NSV excitation, phenomena related to flow separation with lower frequencies, including dual-vortex systems within blade passages, are not captured by the 1/7th annulus simulation, but are found in the full-annulus simulation.
Volume 7: Structures and Dynamics, Parts A and B, 2012
This paper investigates non-synchronous vibration (NSV) mechanism of a high-speed axial compresso... more This paper investigates non-synchronous vibration (NSV) mechanism of a high-speed axial compressor with three different rotor tip clearances. Numerical simulations for 1/7th annulus periodic sector are performed using an unsteady Reynolds-averaged Navier-Stokes(URANS) solver with a fully conservative sliding boundary condition to capture wake unsteadiness between the rotor and stator blades. The simulated NSV shows that the frequency and amplitude are strongly influenced by the tip clearance size and shape. The predicted NSV frequency is in good agreement with the experiment. The maximum amplitude of the NSV occurs at about 78% span of the rotor suction leading edge regardless of tip clearance due to a strong interaction of incoming flow, tip leakage flow and tip vortex.
Full-annulus detached-eddy simulation (DES) is conducted in this paper to investigate stall incep... more Full-annulus detached-eddy simulation (DES) is conducted in this paper to investigate stall inception for the axial transonic rotor, NASA rotor 67. A low-diffusion E-CUSP scheme with a third-order MUSCL reconstruction is used to discretize the inviscid fluxes, and second-order central differencing is used for the viscous terms. An implicit line Gauss-Seidel iteration with dual time-stepping method and second-order temporal accuracy is employed for time integration of the spatially filtered unsteady Navier-Stokes equations in a rotating frame. It is observed that the rotating stall is initiated by the local spike flow disturbance, which quickly induces the rotor to stall roughly over two rotor revolutions. The stall cell covering more than six blade tip passages propagates at 48% of rotor speed in the counter-rotor rotation direction. The process of rotating stall is captured by the full-annulus DES, which indicates that the blockage created by the low-energy vortical flow structure pushes the tip leakage flow to the adjacent blade behind the detached sonic boundary. The unsteady Reynolds-averaged Navier-Stokes (URANS) simulation is also performed for comparison. DES predicts the stall inception roughly one rotor revolution earlier than the URANS model. Overall, DES predicts the stall inception more realistically.
Large eddy simulation (LES) is conducted to investigate coflow jet (CFJ) airfoil flows at high an... more Large eddy simulation (LES) is conducted to investigate coflow jet (CFJ) airfoil flows at high angle of attack (AOA). The Smagorinsky model with Van Driest damping is employed to resolve the subgrid-scale stress. The fifth-order weighted essentially non-oscillatory (WENO) scheme is used for reconstruction of the inviscid flux and the fourthorder central differencing for the viscous flux. The LES results at an AOA of 0 deg, 12 deg, 25 deg, and 30 deg with momentum coefficients of C l ¼ 0.15 and 0.08 are compared with the experiment to understand the flow structure of the jet mixing and flow separation. The quantitative prediction of lift and drag and qualitative prediction of vortex structures are in good agreement with experiment.
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Papers by Hongsik Im