Kiran Ramesh

Large Eddy Simulation of High-Speed, Premixed Ethylene Combustion

A large-eddy simulation / Reynolds-averaged Navier-Stokes (LES/RANS) methodology is used to simulate premixed ethylene-air combustion in a model scramjet designed for dual mode operation and equipped with a cavity for flameholding. A 22-species reduced mechanism for ethylene-air combustion is employed, and the calculations are performed on a mesh containing 93 million cells. Fuel plumes injected at the isolator entrance are processed by the isolator shock train, yielding a premixed fuel-air mixture at an equivalence ratio of 0.42 at the cavity entrance plane. A premixed flame is anchored within the cavity and propagates toward the opposite wall. Near complete combustion of ethylene is obtained. The combustor is highly dynamic, exhibiting a large-scale oscillation in global heat release and mass flow rate with a period of about 2.8 ms. Maximum heat release occurs when the flame front reaches its most downstream extent, as the flame surface area is larger. Minimum heat release is associated with flame propagation toward the cavity and occurs through a reduction in core flow velocity that is correlated with an upstream movement of the shock train. Reasonable agreement between simulation results and available wall pressure, particle image velocimetry, and OH-PLIF data is obtained, but it is not yet clear whether the system-level oscillations seen in the calculations are actually present in the experiment.

Augmentation of Inviscid Airfoil Theory to Predict and Model 2D Unsteady Vortex Dominated Flows

A criterion for predicting flow separation and reattachment at the leading-edge using a Leading-Edge Suction Parameter (LESP) is presented. Stemming from inviscid theory, the LESP serves to predict the onset of separation or reattachment at the leading-edge for any unsteady motion using a critical value which is defined by the airfoil shape and Reynolds number of operation. This criterion is applied to a flat plate and an SD7003 airfoil undergoing various pitch and plunge motions and the critical LESP values for these airfoils are seen to predict the onset of separation or reattachment at the leading-edge, on both upper and lower surfaces. Separation at the leading-edge and subsequent formation of a leading-edge vortex (LEV) is accompanied by a loss in the leading-edge suction force resulting in increased drag. However, formation of LEV also serves to prevent separation at the trailing-edge and keeps the bound circulation intact, thus yielding high lift even at large angles of attack where the flow would have been fully separated otherwise. Using the criterion presented in this paper, it is possible to design kinematics which promote or suppress LEV formation by moving the LESP above or below its critical value.

Theoretical, Computational and Experimental Studies of a Flat Plate Undergoing High-Amplitude Pitching Motion

A pitch-up, hold, pitch-down motion for a flat plate is studied using theoretical, computational (immersed boundary method), and experimental (water-tunnel) methods. This motion is one of several canonical pitch motions introduced by the AIAA Fluid Dynamics Technical Committee’s Low Reynolds Number Discussion Group. An inviscid theoretical method that is applicable to non-periodic motions and that accounts for large amplitudes and nonplanar wakes is employed. Results from theory are compared against those from computation and experiment which are also compared with each other. The variation of circulatory and apparent-mass loads as a function of pivot location for this motion is examined. The flow phenomena leading up to leading edge vortex shedding and the limit of validity of the inviscid theory in the face of vortex dominated flows is investigated. Also, the effect on pitch amplitude on leading edge vortex shedding is examined and two distinctly different vortex dominated flows are studied using dye flow visualizations from and experiment and vorticity plots from computation.

Theoretical modeling of leading edge vortices using the leading edge suction parameter

A theoretical low-order model to predict the forces and flow evolution on airfoils and flat plates undergoing arbitrary 2D unsteady maneuvers is presented. An inviscid airfoil theory with discrete vortex shedding from the trailing edge is augmented with a Leading Edge Suction Parameter (LESP), which is a criterion to predict leading edge separation. The LESP is used to mediate intermittent discrete vortex shedding from the leading edge. This model is validated against CFD and experiment for motions in recent unsteady airfoil literature. In addition, the influences of leading and trailing edge vortices on the force histories are studied in detail by examining the various constituents of forces on the airfoil from theory. Overall, the model is seen to be successful in predicting forces and flow evolution for unsteady motions.

Effect of airfoil shape and Reynolds number on leading edge vortex shedding in unsteady flows

Leading edge vortex (LEV) formation, which is initiated by separation at the leading edge, depends on airfoil shape, Reynolds number, rate of translation/rotation, amplitude of motion and the pitch axis location. The Leading Edge Suction Parameter (LESP) derived from inviscid theory, serves to predict the onset LEV formation for any unsteady motion using a critical value that depends on the airfoil shape and Reynolds number of operation. In this paper, the critical LESP value is determined for different airfoils over a range of Reynolds numbers. The SD7003 airfoil was used as a baseline and modified airfoils were derived by altering the leading edge radius, keeping maximum thickness unchanged. Increasing the leading edge radius resulted in higher critical LESP values, demonstrating that separation is delayed for flow over a more rounded leading edge since it can support more suction. The effect of Reynolds number was more complicated, with the critical LESP decreasing from Re = 5,000 to Re = 100,000, and increasing after. Hence conditions which promote and suppress leading edge vortex formation were identified, and the critical LESP values were made available over a broad range of parameters, which can be used for low-order prediction and modeling of complicated, vortex-dominated flows.

Theoretical analysis of perching and hovering maneuvers

Unsteady aerodynamic phenomena are encountered in a large number of modern aerospace and non-aerospace applications. Leading edge vortices (LEVs) are of particular interest because of their large impact on the forces and performance. In rotorcraft applications, they cause large vibrations and torsional loads (dynamic stall), affecting the performance adversely. In insect flight however, they contribute positively by enabling high-lift flight. Identifying the conditions that result in LEV formation and modeling their effects on the flow is an important ongoing challenge. Perching (airfoil decelerates to rest) and hovering (zero freestream velocity) maneuvers are of special interest. In earlier work by the authors, a Leading Edge Suction Parameter (LESP) was developed to predict LEV formation for airfoils undergoing arbitrary variation in pitch and plunge at a constant freestream velocity. In this research, the LESP criterion is extended to situations where the freestream velocity is varying or zero. A point-vortex model based on this criterion is developed and results from the model are compared against those from a computational fluid dynamics (CFD) method. Abstractions of perching and hovering maneuvers are used to validate the low-order models performance in highly unsteady vortex-dominated flows, where the time-varying freestream/translational velocity is small in magnitude compared to the other contributions to the velocity experienced by the leading edge region of the airfoil. Time instants of LEV formation, flow topologies and force coefficient histories for the various motion kinematics from the low-order model and CFD are obtained and compared. The LESP criterion is seen to be successful in predicting the start of LEV formation and the point-vortex method is effective in modeling the flow development and forces on the airfoil. Typical run-times for the low-order method are between 30-40 seconds, making it a potentially convenient tool for control/design applications.

Model Reduction in Discrete Vortex Methods for 2D Unsteady Aerodynamic Flows

In this paper, we propose a method for model reduction in discrete-vortex methods. Discrete vortex methods have been successfully employed to model separated and unsteady airfoil flows. Earlier research revealed that a parameter called the Leading Edge Suction Parameter (LESP) can be used to model leading-edge vortex (LEV) shedding in unsteady flows. The LESP is a measure of suction developed at the leading edge, and whenever the LESP exceeds a critical value, a discrete vortex is released from the leading edge so as to keep the LESP at the critical value. Though the method was successful in predicting the forces on and the flow field around an airfoil in unsteady vortex-dominated flows,it was necessary to track a large number of discrete vortices in order to obtain the solution. The current study focuses on obtaining a model with a reduced number of leading-edge vortices, thus improving the computation time. Vortex shedding from the leading edge is modelled by a shear layer that comprises of a few discrete vortices, and a single concentrated vortex whose strength varies with time. The single vortex at the end of the shear layer accounts for the concentrated vortical structure that comprises several discrete vortex elements in conventional vortex methods. A merging algorithm is initiated when the edge of the shear layer starts rolling up. Suitable discrete vortices are identified using a kinematic criterion, and are merged to the growing vortex at every time step. The reduced order method is seen to bring down the number of discrete vortices shed from the leading edge significantly.