Challenge Projects
 |
| Computed time-coded particle traces show the interaction of the unsteady synthetic jet with the projectile wake flow at Mach = 0.11 and zero-degree angle of attack. |
Time-accurate aerodynamics modeling of synthetic jets project
Dr. Jubaraj Sahu, Deputy Branch Chief, Aerodynamics Branch, WMRD
As part of a Department of Defense Challenge Project, advanced state-of-the-art, time-accurate computational fluid dynamics (CFD) techniques have been developed and applied to a new area of aerodynamic research on synthetic jets for control of subsonic projectiles.
Cutting-edge CFD technologies have been successfully applied in Micro Adaptive Flow Control (MAFC) to a joint Army Research Laboratory (ARL)/Defense Advanced Research Projects Agency (DARPA)/industry Self-Correcting Projectile for Infantry Operation (SCORPION) program and have demonstrated flight control authority on a 40 mm grenade for the M203 launcher.
This research has provided an increased fundamental understanding of the complex 3-D, time-dependent, aerodynamic interactions associated with micro-jet control for yawing spin-stabilized munitions. Detailed flow physics simulations have captured all the flow structures with high fidelity and successfully identified the locations of synthetic micro-jets for optimum aerodynamic interference and control authority. As demonstrated in the SCORPION program, this newly developed capability lays the foundation for significantly improving the accuracy of current and future small- and medium-caliber Army weapons.
ARL MSRC systems
Time-accurate unsteady simulations, even with a single synthetic jet, require a large amount of resources. The unsteady CFD modeling technique required about 600 time steps to resolve a full spin cycle (67 Hz). The unsteady synthetic jet operates at a high frequency of 1,000 Hz. Time-accurate CFD modeling of each jet cycle required more than 40 time steps. The actual computing time for one full spin cycle of the projectile was about 50 hours using 16 processors (800 processor-hours) on a SGI Origin 3K or an IBM SP P3 system for a mesh size about four million grid points. Multiple spin cycles and, hence, a large number of synthetic jet operations were required to reach the desired periodic time-accurate unsteady result. Some cases ran for as many as 60 spin cycles, requiring more than 48,000 processor hours of computer time for each case. These large-scale, time-accurate aerodynamic computations required more than 500,000 processors hours on the SGI Origin 3000, IBM SP P3, and IBM SP P4 systems at the ARL MSRC, primarily using CFD++ flow solver.
Many flow field solutions were saved at regular intermittent time-intervals to produce movies to gain insight into the physical phenomenon resulting from the synthetic jet interactions. This project required extensive use of HPC assets, and advanced visualization played a critical role in providing fundamental understanding of the fluid dynamic processes involved. Some of these unsteady time-dependent high fidelity simulations required more than 300 gigabytes (GB) of data and more than 1 terabyte (TB) of input/output (I/O) for advanced scientific visualization. This visualization work represents one of the largest datasets requiring possibly the largest use of HPC resources at the ARL MSRC.
Project description
Competent smart munitions that can more accurately hit a target can increase lethality and enhance survivability. A similar capability for small-scale munitions would increase the effectiveness of the infantry units and reduce the collateral damage and the weight of munitions that must be carried by soldiers. The Army is therefore seeking a new generation of autonomous, course-correcting, and gun-launched projectiles for infantry soldiers. Due to small projectile diameter (20 to 40 mm), control authority by canards and fins seem very unlikely.
An alternate and new evolving technology is the MAFC through synthetic jets. These very tiny (of the order of 0.3 mm) synthetic micro-jet actuators have been shown to successfully modify subsonic flow characteristics.
Recently, some studies have shown that tiny synthetic unsteady jets can significantly alter the flow field and pressure distributions for simple airfoils and cylinders. The synthetic jets (fluid being pumped in and out of the jet cavity at a high frequency of the order of 1,000 Hz) are control devices with zero net mass flux and are intended to produce the desired control of the flow field through momentum effects. Many parameters such as jet location, jet velocity, and jet actuator frequency can affect the flow control phenomenon.
Up to now, the physics of this phenomenon have not been well understood and advanced numerical predictive capabilities or high-fidelity CFD design tools have not been developed or applied to 3-D simulation of these unsteady jets. However, the research effort described here has advanced the aerodynamic numerical capability to accurately predict and provide a crucial understanding of the complex flow physics associated the unsteady aerodynamics of this new class of tiny synthetic micro-jets for control of modern projectile configurations. High-performance CFD techniques were developed and applied for the design and analysis of these MAFC systems for steering a spinning projectile for infantry operations.
The control of the trajectory of a 40 mm spinning projectile is achieved by altering the pressure distribution on the projectile through forced asymmetric flow separation. Unsteady, time-accurate CFD modeling capabilities were developed and used to assist in the design of the projectile shape, the placement of the synthetic actuators and the prediction of the aerodynamic force and moments for these actuator configurations. Additionally, the advanced CFD capabilities provided a simpler way to explore various firing sequences of the actuator elements.
Time-accurate unsteady CFD computations were performed to predict and characterize the unsteady nature of the synthetic jet interaction flow field produced on the M203 grenade-launched projectile for various yaw and spin rates for fully viscous turbulent flow conditions. Turbulence was initially modeled using a traditional Reynolds-Averaged Navier-Stokes (RANS) approach. While this approach provided some detailed flow physics, it was found to be less accurate for this new class of unsteady flows associated with synthetic jets.
In order to improve the accuracy of the numerical simulation, the predictive capability was extended to include a higher order hybrid RANS/LES (Large Eddy Simulation) approach. This new approach computes the large eddies present in the turbulent flow structure and allowed the simulation to capture with high fidelity additional flow structures associated the synthetic jet interactions in a time-dependent fashion.
Modeling of azimuthally placed synthetic micro-jets required very fine grid resolution, highly specialized boundary conditions for the jet activation, and the use of advanced hybrid LES approach permitting local resolution of the unsteady turbulent flow with high fidelity. The addition of yaw and spin while the projectile is subjected to the pulsating micro-jets rendered predicting forces and moments created a major challenge. The DoD High Performance Computing Modernization Program Office (HPCMPO) chose this research as a Challenge Project and provided the computational resources required for these time-accurate simulations.
The new capability has been demonstrated and this technology has recently been successfully applied to the SCORPION program.
This capability has provided fundamental understanding of fluid dynamics mechanisms associated with the interaction of the unsteady synthetic jets and the projectile flow fields. Many flow field solutions resulting from the simulation of multiple spin cycles and, hence, a large number of synthetic jet operations were saved at regular intermittent time-intervals to produce movies to gain insight into the physical phenomenon resulting from the synthetic jet interactions. The unsteady jets were discovered to break up the shear layer coming over the step in front of the base of the projectile. It is this insight that was found to substantially alter the flow field (making it unsteady) both near the jet and in the wake region, which in turn produced the required forces and moments even at zero degree angle-of-attack (level flight).
Time-accurate velocity magnitude contours confirm the unsteady wake flow fields arising from the interaction of the synthetic jet with the incoming free stream flow at Mach =.24. More importantly, the break up of the shear layer is clearly evidenced by the particles clustered in regions of flow gradients or vorticity. Verification of this conclusion is provided by the excellent agreement between the predicted (solid line) and measured (solid symbols) values of the net lift force due to the jet.
The net lift force was determined from the actual time histories of the highly unsteady lift force resulting from the jet interaction (jet is on and off during spin cycle) at zero degree angle of attack and computed with the new hybrid RANS/LES turbulence approach. The computed lift force along with other aerodynamic forces and moments, directly resulting from the pulsating jet, were then used in a trajectory analysis. The synthetic micro-jet produced a substantial change in the cross range and thus, provided the desired course correction for the projectile to hit its target. The results showed the potential of CFD to provide insight into the jet interaction flow fields and provided guidance as to the locations and sizes of the jets to generate the maximum-control authority required to maneuver a spinning munition to its target with precision.
This research is at the forefront of technology in projectile aerodynamics area and represents a major increase in capability for determining the unsteady aerodynamics of munitions in a new area of flow control. This research has shown that MAFC with tiny synthetic jets can provide an affordable route to lethal precision-guided infantry weapons.