David Car

An Investigation of Wake-Shock Interactions in a Transonic Compressor With Digital Particle Image Velocimetry and Time-Accurate Computational Fluid Dynamics

The effects of varying axial gap on the unsteady flow field between the stator and rotor of a transonic compressor stage are important because they can result in significant changes in stage mass flow rate, pressure rise, and efficiency. Some of these effects are analyzed with measurements using digital particle image velocimetry (DPIV) and with time-accurate simulations using the 3D unsteady Navier-Stokes computational fluid dynamics solver TURBO. Generally there is excellent agreement between the measurements and simulations, instilling confidence in both. Strong vortices of the wake can break up the rotor bow shock and contribute to loss. At close spacing vortices are shed from the trailing edge of the upstream stationary blade row in response to the unsteady, discontinuous pressure field generated by the downstream rotor bow shock. Shed vortices increase in size and strength and generate more loss as spacing decreases, a consequence of the effective increase in rotor bow shock strength at the stationary blade row trailing edge. A relationship for the change in shed vorticity as a function of rotor bow shock strength is presented that predicts the difference between close and far spacing TURBO simulations.

Radial Migration of Shed Vortices in a Transonic Rotor Following a Wake Generator: A Comparison Between Time Accurate and Average Passage Approaches

This paper shows a comparison of an unsteady simulation using turbo and an average passage simulation for a two blade row configuration consisting of a wake generator followed by a transonic rotor. Two spacings were simulated, both close and far. The unsteady results compare well with experiment especially for the profile of efficiency difference between close and far. An analysis of results helps to explain the unusual profile seen experimentally that is due to the radial migration of wake generator shed vortices with negative radial velocities near the tip. In addition, different components of the average passage body forces (deterministic stresses) are explored that shows the main terms are the axial momentum and the metal blockage.

An Investigation of Wake-Shock Interactions in a Transonic Compressor With DPIV and Time-Accurate CFD

The effects of varying axial gap on the unsteady flow field between the stator and rotor of a transonic compressor stage are important because they can result in significant changes in stage mass flow rate, pressure rise and efficiency. Some of these effects are analyzed with measurements using Digital Particle Image Velocimetry (DPIV) and with time-accurate simulations using the 3D unsteady Navier-Stokes CFD solver TURBO. Generally there is excellent agreement between the measurements and simulations, instilling confidence in both. Strong vortices of the wake can break up the rotor bow shock and contribute to loss. At close spacing vortices are shed from the trailing edge of the upstream stationary blade row in response to the unsteady, discontinuous pressure field generated by the downstream rotor bow shock. Shed vortices increase in size and strength and generate more loss as spacing decreases, a consequence of the effective increase in rotor bow shock strength at the stationary blade row trailing edge. A relationship for the change in shed vorticity as a function of rotor bow shock strength is presented that predicts the difference between close and far spacing TURBO simulations.Copyright

PyF95++: a templating capability for the Fortran 95/2003 language

This article outlines a framework that was used to create a templating capability in the Fortran 95/2003 language (hereafter simply referenced as Fortran) as well as additional useful features which provide a native feel for integration into the language. Recent comparisons of Fortran with C++ have cited the lack of templating in Fortran as a major deficiency in the language [1]. Templating is a very powerful capability which allows the programmer to easily create and maintain more complex code than would be desirable to do manually. There are some very good C++ resources on templating for those not familiar with the subject [4] and are worth reading to gain a more thorough appreciation for the concept than what will be discussed here.

A fortran unit-testing framework utilizing templating and the PyF95++ toolset

A simple unit testing framework has been developed utilizing a templating capability and Python based preprocessor for Fortran. The implementation of this framework and its use for testing serial and parallel components is discussed. The capability was successfully applied to the development of a Fortran Standard Template Library and associated toolsets.

Streamwise Vorticity Effects in a Curved Diffuser with Slot Jet Flow Control

Abstract : A slot jet is used to increase the diffusion level of a curved, diffusing wind tunnel passage. The passage entrance coincides with the tunnel throat, which has respective Mach and Reynolds numbers of 0.7 and 187000 (based on throat height) and jet Reynolds numbers ranging from 10000 to 22000. Each of four presented configurations uses linear slot jet flow control at the same relative location. Three configurations include co- or counter-rotating vortex generator (VG) fins of varying heights upstream of the slot jet, while the other configuration has no VGs. The configurations are tested for varying flow control input and evaluated primarily on the basis of passage exit total pressure surveys. At baseline conditions (minimum slot jet flow), the shorter VG designs were found to provide some diffusion enhancement and loss reduction compared to the non-VG configuration, whereas the tall counter-rotating VGs exhibit an increase in loss. With the addition of slot jet flow, all configurations showed an initial decline followed by a net increase in performance of similar magnitude, with choked slot jet flows tending to lead to flow instabilities. Upon further increases in slot jet flow, all configurations eventually achieved a stable flow pattern. A noteworthy change in the flow pattern, with substantially improved uniformity, is documented near 6% flow control input and will be the subject of more detailed investigation.

Tool and Process Improvement for High-Fidelity Compressor Simulations

Compressors for modern gas turbine engines are challenging to simulate. Disparate length and time scales exist in an aggressive adverse pressure gradient environment amongst a wide array of physical phenomena requiring refinement in both space and time. The resulting mesh sizes and CPU time required to complete time-accurate simulations have become staggering, though they will only continue to increase as the simulation strategy switches from Unsteady Reynolds-Averaged Navier-Stokes (URANS) to Detached Eddy Simulation (DES) and Large Eddy Simulation (LES). For the complex compressor flows, this transition has long been necessary. In order to more effectively simulate compressor flows, several tool developments have taken place, which result in better process and reduced engineer effort. Utilizing the Air Force Research Laboratory Department of Defense (DoD) Supercomputing Resource Center (AFRL DSRC) at Wright-Patterson AFB, improvements in geometry handling, grid generation methodologies, and solver features have reduced workload while benefiting simulation quality. Available applications such as Doxygen, Python, VTK, and Subversion created a productive collaboration environment suitable for both development and testing.

Slot Jet Flow Control for Diusion Enhancement: Endwall Observations and Improvements

wind tunnel passage. The entrance Mach number to the passage is 0.7 with jet Reynolds numbers in the 600012000 range and tunnel Reynolds number based on throat height of 188000. The throat aspect ratio (throat height to span ratio) is approximately 7:1 and the diusing section of the passage has an approximate radius of curvature of 5.08 cm for the convex surface. Three slot jet configurations are presented, each one introducing the jet in a dierent spanwise manner. The linear slot jet is first presented where the jet is introduced at a constant streamwise position across the entire span of the passage. It is observed that this jet has eective midspan control, but poor endwall performance as determined by total pressure exit traverse measurements. The endwall flowfield results in blockage similar and more explosive in nature than the uncontrolled flowfield. This is observed with Particle Image Velocimetry (PIV) and corroborated with exit total pressure data. The hypothesis is that this is a result breakdown of the turbulent shear layer due to the abrupt termination at the sidewalls. Other researchers have observed this phenomenon with laminar planar slot jets with jet Reynolds numbers of approximately 3000 where an unraveling of the vortex filament formed by the shear layer occurs due to rapid turbulent formation at the jet ends which quickly encompasses the planar slot jet. Interestingly, this phenomenon occurs with and without sidewalls. It is suggested that the abrupt termination of the shear layer is the cause of the breakdown. Therefore, two dierent slot jet configurations are presented whereby the slot jet is introduced in a parabolic fashion. One configuration introduces the jet earlier at the midspan than at the sidewalls, while the other introduces the jet at the sidewalls first and midspan later. These configurations are labeled rearward and forward swept respectively. Both configurations are compared with the linear slot jet configuration.

A polymorphic reference counting implementation in Fortran 2003

This paper outlines a revised reference counting programming pattern for derived types in Fortran 2003. It extends the method described by Car [1] to work with polymorphic types (i.e. using the class keyword). This reference counting pattern reduces the possibility of memory leaks by allowing the object itself to manage allocations and deallocations. It follows the pattern found in many modern object oriented languages like Java [2,3] and Python[4,5] and the auto_ptr template class in C++ [6]. The reference counting pattern described here is being used in conjunction with the PyF95++ pre-processor [7,8] and the accompanying Standard Template Library (STL). All the containers in that project are reference counted and employ the original pattern for derived types. Some of the intricacies of working with polymorphic types in Fortran are described as the implementation is discussed.

Use of Time-Accurate CFD for Inlet-Fan Interaction

The Compressor Aerodynamics Research Laboratory at Wright-Patterson Air Force Base is currently investigating, experimentally and numerically, the effects of inlet distortion on transonic fan performance. The experimental investigation will quantify the flowfield for a cold-tested diffuser-fan with distortion screens, and a coupled diffuser-fan system. Each of these experiments will be mirrored numerically using the solver TURBO. The primary research goal of the work is to quantify the physical mechanisms for distortion-transfer and develop reduced-order models to account for performance and stall-margin loss in engine design. As inlet geometries become more aggressive, namely having smaller length to diameter ratios, the secondary flow physics induce potentially-harmful distortion at the engine fan face. The effects of distortion are typically only investigated experimentally, and at too low a temporal and spatial resolution to adequately quantify the mechanisms which attenuate or amplify total pressure and total temperature non-uniformities. Because of the imbalance of operating conditions between different passages of the turbo-machine, adverse effects have been noted on stall-margin which cannot be explained with traditional design methods. Recently large-scale simulations have been used to investigate total pressure distortion patterns on several full-annulus, multi-stage fan configurations. The current effort simulates a distortion-producing inlet coupled with a single-stage fan. This allows distortion of total pressure, total temperature, and flow angularity to be investigated. Preliminary numerical results have been obtained on the US Air Force Research Laboratory DoD Supercomputing Resource Center (AFRL DSRC) SGI Altix 4700 system for the coupled diffuser-fan system. Total pressure and temperature exhibit both a distinct circumferential variation and a counter-rotation shift. As the simulations are analyzed, a greater understanding of performance detriment, stall-inception, and distortion- transfer in an installed aircraft system will be gained. Finally, a brief discussion of continued efforts investigating blade-row interactions will be presented.