Hong Sik Im

Delayed Detached Eddy Simulation of Airfoil Stall Flows Using High-Order Schemes

An advanced hybrid Reynolds-Averaged Navier–Stokes/large eddy simulation (RANS/LES) turbulence model delayed detached eddy simulation (DDES) is conducted in thispaper to investigate the dynamic stall flows over 3D NACA0012 airfoil at 17 deg, 26 deg, 45 deg, and 60 deg angle of attack (AOA). The spatially filtered unsteady 3D Navier–Stokes equations are solved using a fifth-order weighted essentially nonoscillatory (WENO) reconstruction with a low diffusion E-CUSP (LDE) scheme for the inviscid fluxes and a conservative fourth-order central differencing for the viscous terms. An implicit second-order time marching scheme with dual time stepping is employed to achieve high stability and convergency rate. A 3D flat plate is validated for the DDES model. For quantitative prediction of lift and drag of the stalled NACA0012 airfoil flows, the detached eddy simulation (DES) and DDES achieve much more accurate results than the Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulation. In addition to the quantitative difference, the DES/DDES and URANS also obtain qualitatively very different unsteady stalled flows of NACA0012 airfoil with different vortical structures and frequencies. This may bring a significantly different prediction if those methods are used for fluid–structural interaction. For comparison purpose, a third-order WENO scheme with a second-order central differencing is also employed for the DDES stalled NACA0012 airfoil flows. Both the third- and fifth-order WENO schemes predict the stalled flow similarly for lift and drag at AOA less than 45 deg, while at AOA of 60 deg, the fifth-order WENO scheme shows better agreement with the experiment than the third-order WENO scheme. The high-order scheme of WENO 5 also resolves more small scales of flow structures than the second-order scheme. The prediction of the stalled airfoil flow using DDES with both the high-order scheme and second-order scheme is overall significantly more accurate than the URANS simulation.

Effects of rotor tip clearance on tip clearance flow potentially leading to NSV in an axial compressor

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. The instability of tornado like tip vortex oscillating in streamwise direction appears to be the main cause of the NSV observed in this study.Copyright

Supersonic bi-directional flying wing configuration with low sonic boom and high aerodynamic efficiency

In this paper, a parametric study is conducted to optimize a business jet using supersonic bi-directional (SBiDir) flying wing (FW)aiming at achieving high aerodynamic efficiency and low sonic boom. The SBiDir-FW concept has a symmetric planform about both longitudinal and span axes, allowing the plane to achieve high efficiency at both supersonic and subsonic by rotating by 90o in flight. With this parametric study, the L/Dp achieves 15 at M=1.6, 16 at M=2.0, whereas the sonic boom remains smooth without N-wave. The smooth peak over pressure value is 0.3 psf at M=1.6, 0.4 psf at M=2.0. It indicates that the conventional N-wave could be replaced by a strong acoustic wave, which generates a much less impulsive force and hence noise. The supersonic aspect ratio of the present configuration is 0.33 and the subsonic aspect ratio is 33, which ensures high performance at both supersonic and subsonic. The study shows that the sharp and long nose configuration with ultra-slender body is favorable to both high aerodynamic efficiency and low sonic boom. The numerical results demonstrate that the SbiDir-FW could be a very promising concept for supersonic flight. Further improvement can still be made by using systematic automated design optimization.

Prediction of Wing Flutter Boundary Using High Fidelity Delayed Detached Eddy Simulation

This paper conducts Delayed Detached Eddy Simulation(DDES) of a 3D wing flutter with free stream Mach number varied from subsonic to supersonic 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 2nd 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 boundaries at different free stream Mach numbers achieve very good agreement with experiment. It appears that the transonic dip phenomenon is due to the anticlimax contribution of the second mode, which is caused by the complicated shock oscillation on the wing.At the flutter boundary including at the sonic dip, no flow separation due to shock/boundary layer interaction is observed.

Detached Eddy Simulation of Unsteady Stall Flows of a Full Annulus Transonic Rotor

This paper uses the advanced Delayed-Detached Eddy Simulation (DDES) of turbulence to simulate rotating stall inception of NASA Rotor 67. The rotor is a low-aspect-ratio transonic axial-flow fan with a tip speed of 429 m/s and a pressure ratio of 1.63. A full annulus simulation was employed with the time accurate compressible Navier-Stokes code in order to accurately capture the the formation of long-length disturbance and a short-length inception (spike). The validation for all numerical methods used in this study was accomplished by the comparisons of the CFD solutions with the test data in advance of unsteady simulations. Self-induced rotating stall development is simulated holding the same back pressure at the near stall experiment without any throttling. Spike type rotating stall occurs and rotates at roughly 50% of rotor speed counter to the rotation. After spike onset, rotating stall fully develops approximately within 2 rotor revolutions. Two distinct characteristics that can advance the mechanism of spike type rotating stall are observed. First, the passage shock is fully detached from rotor and decays during the spike inception. Consequently the shifted sonic line at the upstream of rotor allows stalling flow to propagate to the neighboring passage. Second, the trailing edge back flow contributes to the build up of a fully developed stall cell by pushing tip clearance flow toward blade leading edge and inducing tip spillage flow. Tip vortex originated from the leading edge dies out during spike inception as the swirl angle of incoming tip flow decreases, while in the unstalled passages it develops without breakdown. DDES challenge for the complete blade row reflects well the sequence of rotating stall and its unsteady behavior.Copyright

Simulation of stall inception of a high speed axial compressor with rotor-stator interaction

This paper solves unsteady Reynolds-averaged Navier-Stokes (URANS) equations to simulate stall inception of NASA compressor stage 35 with rotor-stator interaction. A full annulus of the rotor-stator stage is simulated with an interpolation sliding boundary condition (BC) to resolve the rotor-stator interaction. The tip clearance is fully gridded to accurately resolve tip vortices and its effect on stall inception. The unsteady simulation indicates that the inception of rotating stall in Stage 35 is spike inception. The stall cells grow quickly and brings the rotor to full stall within roughly 1.2 revolutions. The stall cell propagates at about 90% of rotor speed in the counter rotor rotation direction in relative the frame.

Full-Annulus Simulation of Nonsynchronous Blade Vibration Excitation of an Axial Compressor

A high speed 1-1/2 axial compressor stage is simulated in thi s paper using an Unsteady Reynolds-Averaged Navier-Stokes (URANS) solver for a full-annulus configurat ion o capture its non-synchronous vibration (NSV) flow excitation. The simulation presented in this paper assu mes 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 /s ator sliding boundary condition is employed with multiple-processor capability for rotor/stator interfac e information exchange for parallel computing. The sliding BC accurately captures unsteady wake propagation between t h rotor and stator blades while conserving fluxes across the rotor/stator interfaces. The predicted dominan t frequencies using the blade tip response signals are not harmonic to the engine order, which is the NSV excitation. Th e 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 annul us 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 confir ms 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 bla des of the rotor annulus. While the time-shifted phase lag BCs can accurately capture the frequency of NSV excitati on, phenomena related to flow separation with lower frequencies, including dual-vortex systems within blade p assages, are not captured by the 1/7th annulus simulation, but are found in the full-annulus simulation. Nomenclature e total energy per unit mass L∞ blade chord at hub NB number of blade ND number of nodal diameter p static pressure Ro Rossby number, ΩL∞ U∞ r radius T period for one nodal diameter Ph.D. Student †Ph.D., Currently an engineer at Honeywell ‡Professor. 1 D ow nl oa de d by G ec he ng Z ha o n Ju ne 6 , 2 01 4 | h ttp :// ar c. ai aa .o rg | D O I: 1 0. 25 14 /6 .2 01 407 90 52nd Aerospace Sciences Meeting 13-17 January 2014, National Harbor, Maryland AIAA 2014-0790 Copyright

Stall Flutter Simulation of a Transonic Axial Compressor Stage Using a Fully Coupled Fluid-Structure Interaction

In this paper, numerical simulation of stall flutter for full annulus NASA Stage 35 is conducted using a fully coupled fluid/structure interaction. Time accurate compressible 3D Navier-Stokes equations with Spalart-Allmaras turbulence model are solved with a system of 5 decoupled structural modal equations in a fully coupled manner. The 3rd order WENO scheme for the inviscid flux and 2nd order central difference for the viscous terms are used to accurately capture the interactions of the fluid and structure. Delayed detached eddy simulation is also applied to predict the flow induced vibration at near and stall conditions for comparison. The mechanism and aerodynamic damping behavior causing the stall flutter are analyzed. The effect of rotor stator interaction on the onset of flutter is studied. The fully coupled FSI simulation shows that Stage 35 has stall flutter at rotating stall.

Investigation of a Compressor Rotor Non-Synchronous Vibration With and Without Fluid-Structure Interaction

This paper study the non-synchronous vibration (NSV) of a high speed multistage axial compressor using rigid blade and vibrating blade with fluid-structural interaction(FSI). The unsteady Reynolds-averaged Navier-Stokes (URANS) equations and mode based structural dynamic equations are solved. A low diffusion E-CUSP Reimann solver with a 3rd order WENO scheme for the inviscid fluxes and a 2nd order central differencing for the viscous terms are employed. A 1/7th annulus sector of IGV-rotor-stator is used with a time shifted phase lag BC at circumferential boundaries. An interpolation sliding boundary condition is used for the rotorstator interaction. The URANS simulation for rigid blades

Investigation of non-synchronous vibration mechanism for a high speed axial compressor using delayed DES

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.