North Carolina State University
Advanced Instrumentation
Laser holographic Interferometry (LHI) and laser holographic focusing schlieren (LHFS) methods have recently been developed to study 2D and 3D flows in short-duration ground test facilities. The LHI techniques was applied to yield quantitative flow field data, and to determine a method to identify flow separation and reattachment. LHFS method was developed to yield quantitative data in 3D flow fields. The necessary system design parameters were developed, and a pilot system evaluated. An iterative numerical method was used in conjunction with image process techniques to study high-speed flows. Presently the technique is being applied in the study of high-speed boundary layer stability and transition.
Experimental Combustion Research
Experimental combustion research is continuing in both premixed and diffusion laboratory-scale flames at NCSU. The effects of unsteady, three-dimensional strain fields on premixed flames near the lean limit has been studied to gain an understanding of the relevant scales in turbulent combustion. Non-unity Lewis numbers are used to assess the effect of thermo-diffusive instabilities on the quenching process at the lean limit. A counterflow diffusion flame burner is currently being used to understand the effects of strain on an idealized 1-dimensional flamelet. Temperature and OH mole fractions are being measured to gain insight into the heat release and hydrodynamic-chemical kinetics interaction. This burner will be modified in the near future to allow the investigation of transient phenomena associated with and unsteady strain rate.
Combustion Modeling Research
Research in turbulent combustion modeling has been going on for over ten years under the direction of H. A. Hassan. PDF methods have been developed to allow for combustion turbulence interaction. Moreover, a three dimensional Navier-Stokes code has been developed for calculating complex combustor geometries. The code incorporates a variety of turbulence models and detailed kinetic models.
Turbulence Research
A new approach for calculating turbulent flows has been developed. This approach differs from other approaches in that it is based on the exact equations governing the turbulence fluctuations. The model developed was able to reproduce the growth rates of all free shear flows using the same set of model constants. Moreover, the same set of model constants is used to calculate wall bounded shear flows.
Correct prediction of heat transfer at high temperatures, which is the case encountered in gas turbines, requires the use of a variable turbulent Prandtl number. The approach is being extended to address these issues.
Issues pertaining to the role of unfavorable pressure gradients which result in separation and loss of efficiency in turbomachines are addressed in the present model.
Two additional efforts in turbulence modeling are also underway. The first study seeks to use the helicity density as a means of modeling the effects of large-scale vortical structures on turbulence production in complex, three-dimensional flows. The second effort is focused on the development of a cost-effective conditional moment approach for modeling the effects of turbulent fluctuations on chemical reaction rates. This technique will be combined with the finite-rate reacting flow algorithms to yield a tool for predicting No x and CO emissions levels in practical combustors.
Dynamic solution Adaptive Grids for 2D and 3D Unsteady Flows
The flow in turbomachinery is inherently unsteady. Since no a priori knowledge is usually available to determine local mesh resolution requirements as the solution changes, near global grid refinement is used to ensure resolution under all conditions. A dynamic solution adaptive grid algorithm DSAGA3D has been developed to resolve chosen flow features as they evolve and translate. Comparison of full Navier Stokes calculations using this algorithm applied to a supersonic self-excited oscillatory flow reveal excellent agreement with the experimental Fourier waveform decomposition up to 25KHz, the maximum available in the experiment. The number of mesh cells can be reduced by a factor of 2 to 3 in each coordinate direction while still achieving increased spatial accuracy through adaptation. This algorithm is being applied presently in multi-block form to transonic airfoils and, in collaboration with Duke University , to turbomachinery blading. Illustrative videotape 2D and 3D animations of unsteady flow results obtained with DSAGA3D are available.
Algorithm Development
Efforts are underway to develop a viable multilevel approach for accelerating the convergence of Navier-Stokes simulations of finite-rate combustion processes. Initial emphasis in on hydrogen and propane combustion at low and high speeds. Both two- and three-dimensional codes are under development. The algorithms are designed for use within an overset-grid domain-decomposition framework, thus allowing an accurate representation of complex combustor geometries.
Research Facilities
Subsonic wind tunnel, which is optically accessible from three sides. Instrumentation includes: pressure measurement system; probe traverse system; and flow visualization.
High-speed wind tunnel, equipped with pressure measurement system and boundary layer traverse system. Several optical diagnostic methods available include: shadowgraph, conventional schlieren, focusing schlieren, laser holographic focusing schlieren, and laser holographic interferometry.
Combustion laboratory, currently equipped with a counterflow diffusion flame burner. Laser based optical diagnostics include planar laser induced fluorescence and degenerate four wave mixing for radical mole fraction and temperature measurements, and LDV and particle image velocimetry for velocity and strain field measurements.
Research workstation facility, 18 high level graphics workstations and computational engines for visualization, pre and post processing and access to supercomputers. (Current equipment inventory of approx. $600,000).
NCSU/AFOSR unsteady flow animation facility, an SGI 440 VGX with 25 Gbytes of disk space and auxiliary professional video equipment (Commodore Miga/video toaster, professional VCR's, etc.) to perform rapid frame accurate true animation and analysis of unsteady flow computational results.
North Carolina Supercomputing Center , a CRAY Y-MP with attached CRAY T3D (currently in installation). 40% of CPU time on this facility is granted to NC academic institutions for research support. The NCSU MAE department is the largest user of this facility and has historically been granted 2 < 000 to 5,000 CPU hours yearly for use in faculty and student research. |