Andreas Gross

High-Order-Accurate Numerical Method for Complex Flows

A numerical method employing high-order-accurate (higher than third) upwind discretizations for solving the compressible Navier-Stokes equations on structured grids is discussed. The inviscid fluxes are computed by a procedure based on a weighted essentially nonoscillatory interpolation of the characteristic variables and the Roe scheme. Application of the numerical method to a number of test cases of increasing complexity, that are prototypical for several of the key aspects of practical flows, demonstrates the accuracy and robustness of the method even when computing on distorted curvilinear grids. Significant reductions in computer time are possible when a second-order-accurate implicit Adams-Moulton scheme is employed for time integration. The combination of implicit time integration and high-order-accurate spatial discretization is shown to lead to significant savings in compute time as the grid resolution requirement is lowered and the time step can be increased.

Numerical Investigation of Low-Pressure Turbine Blade Separation Control

Laminar separation on the suction side of low-pressure turbine (LPT) blades at low operating Reynolds numbers can degrade overall engine efficiency and impose limitations on the flight envelope. In wind-tunnel experiments it was shown that laminar separation can be controlled by pulsed vortex generator jets. This active-flow-control technology could be transferred to real flight hardware with more confidence if the physical mechanisms involved in the control were better understood. Here, calculations of a linear LPT cascade at a Reynolds number based on axial chord of 2.5 x 10 4 are presented

Multi-block Poisson grid generator for cascade simulations

High quality computational grids can greatly enhance the accuracy of turbine and compressor cascade simulations especially when time-dependent results are sought where vortical structures are convected through the computational domain. A technique for generating periodic structured grids for cascade simulations based on the Poisson equations is described. To allow for more complex geometries, the grid can be divided into individual zones or blocks. The grids are generated simultaneously in all blocks, assuring continuity of the grid lines and their slopes across the zonal boundaries. Simple geometric rules can be employed for enforcing orthogonality at block boundaries. The method results in grids with low grid distortion by allowing both, block boundaries and grid points on physical boundaries, to move freely. Results are presented for a linear turbine and a linear compressor cascade.

Active flow control for NACA 6-series airfoil at Re = 64,200

Earlier research for the NACA 64 3 -618 airfoil has shown that lift-curve and stall behavior change noticeably when the chord Reynolds number is reduced significantly below its design value. For the same airfoil and a Reynolds number based on chord of Re = 64, 200, separation control by pulsed vortex generator jets and a two-dimensional volume forcing that was motivated by a plasma actuator were investigated. Pulsed vortex generator jets with moderate jet velocities and the two-dimensional volume forcing were found to introduce coherent spanwise structures that are highly effective for separation control. A linear stability analysis of the time-averaged flow showed that spanwise disturbances are amplified by an instability mechanism, which explains why both control strategies are highly efficient. Angled pulsed vortex generator jets with large jet velocities were found to introduce streamwise vortices and to result in earlier transition. Although the control is highly effective, it is not very efficient, as streamwise vortices are not supported by the flow.

Numerical Simulation of Separated Cold Gas Nozzle Flows

A numerical method for the simulation of separated cold gas nozzle flows is presented and applied to six subscale nozzles. The nozzles considered were a truncated ideal nozzle, two thrust-optimized parabolic nozzles, and three dual bell nozzles. For the thrust-optimized parabolic contours, the occurrence of free and restricted shock separation and the cap shock pattern are discussed. Comparisons with experimental data are provided. Shortcomings of the numerical method are pointed out.

Modification of Ninth-Order Weighted Essentially Nonoscillatory Scheme for Mixed Subsonic/Supersonic Flow

T HIS Note describes a modification to the high-order-accurate weighted essentially nonoscillatory (WENO) scheme implementation byGross and Fasel [1,2]. Themotivation behind this effort was to have a high-order-accurate discretization for the convective fluxes that works equallywell for subsonic and supersonicflows. The schemes described in this Note are based on schemes by Jiang and Wu [3] and Balsara and Shu [4]. In [2], we discussed a procedure for constructing high-orderaccurate upwind schemes. We suggested reconstructing the flow variables at the left (L) and right side (R) of the i 1 2 interface using high-order-accurate WENO approximations [3,4] and to employ the Roe scheme [5],

Aerodynamic Scaling of General Aviation Airfoil for Low Reynolds Number Application

Meaningful scaled model flight research requires models that are dynamically and aerodynamically scaled. While dynamic scaling assures that the flight dynamics scale (such as turn rates etc.), aerodynamic scaling assures that the scaled airplane, especially the airfoil, has the same aerodynamic properties (such as aerodynamic coefficients, derivatives, stall behavior etc.) as the full size airplane or airfoil despite the change in Reynolds number associated with the change in length scales, air speeds, and air properties. The present study is concerned with the Aeromot 200S Super Ximango motor glider for which a dynamically scaled 1:5 scale model was built. We investigated aerodynamic scaling for a two-dimensional section of its wing which has a modified NACA 643-618 geometry. For this airfoil, wind tunnel data and airfoil analysis code predictions were found to change noticeably when going from the full size cruise to the model cruise conditions. We investigated two different approaches for obtaining aerodynamic similarity between the 1:5 scale and the full size airfoil at cruise conditions: i) Forced transition with a trip wire and ii) modifying the airfoil geometry.

1/5 Scale Model of Aeromot 200S SuperXimango for Scaled Flight Research

Within the framework of a NASA STTR program, we designed and developed a 1/5 dynamically scaled model of the Aeromot (AMT) 200S motor glider (“SuperXimango”). This model is one element in a “tool box” for scaled model flight research that also includes wind/water tunnel experiments, aerodynamic design tools, and computational fluid dynamics. This 1/5 dynamically scaled model of the Aeromot 200S SuperXimango is awaiting flight testing which will allow for a validation and verification of the scaling by comparison with full scale flight test data. An important criterion in the design and development of the flight model is its ability to serve as a versatile platform for developing and testing of novel technologies. Therefore, great emphasis was placed on a modular design and construction so that certain modules can be easily exchanged.

Investigation of Low-Pressure Turbine Separation Control

Low-pressure turbines are a common element of many modern jet engines. Flow separation from the suction side of the constituent blades at low Reynolds number conditions can noticeably deteriorate overall engine performance. Two separation control strategies for an aggressive low-pressure turbine blade that was designed for an integrated o w control were investigated numerically using a hybrid turbulence modeling approach: Pulsed vortex generator jets are shown to result in an earlier transitioning of the o w and successful separation control. An even more ecien t separation control can be accomplished by harmonic blowing through a slot. The astounding eectiv eness of the latter control scheme is attributed to the suppression of three-dimensional structures. Instead, the o w is dominated by strong spanwise coherent structures that very eectiv ely reduce separation.

Numerical Investigation of Low-Pressure Turbine Endwall Flows

Highly-loaded low-pressure turbine blades promise overall turbine performance improvements and lower overall costs but suffer from unacceptable endwall losses. The endwall flow physics have to better understood to develop effective flow control strategies aimed at a reduction of the endwall losses. The Air Force Research Laboratory is carrying out highly resolved particle image velocimetry measurements and oil-flow flow visualizations for the L2F airfoil at Re=100,000. To complement the wind tunnel experiments, implicit large-eddy simulations for the same airfoil are being performed for Re=100,000 and 5,000. The simulation for Re=100,000 is in good agreement with the experiment with regard to skin friction line patterns and total pressure loss coefficient. Instantaneous flow visualizations reveal a strong passage vortex that is intermittently loosing its coherence. A linear stability theory analysis of the endwall boundary layer upstream of the cascade indicates cross-flow instability. For Re=5,000 the passage vortex is steady and the suction side separation on the airfoil is much increased. As a result, the total pressure loss coefficient is significantly larger than for Re=100,000.