HICFD – Highly Efficient Implementation of CFD Codes for HPC Many-Core Architectures

The objective of the German BMBF research project Highly Efficient Implementation of CFD Codes for HPC Many-Core Architectures (HICFD) is to develop new methods and tools for the analysis and optimization of the performance of parallel computational fluid dynamics (CFD) codes on high performance computer systems with many-core processors. In the work packages of the project it is investigated how the performance of parallel CFD codes written in C can be increased by the optimal use of all parallelism levels. On the highest level MPI is utilized. Furthermore, on the level of the many-core architecture, highly scaling, hybrid OpenMP/MPI methods are implemented. On the level of the processor cores the parallel SIMD units provided by modern CPUs are exploited

PIV Application for Investigation of the Rotor Blade Tip Interaction with a Casing Treatment in a Transonic Compressor Stage

This contribution describes the experimental investigation of the blade tip interaction with a casing treatment implemented to a transonic compressor stage using particle image velocimetry (PIV). The results obtained allowed for direct comparison with numerical simulations of the same compressor stage including the CT geometry, carried out using the DLR TRACE code following a new approach to efficiently perform time-accurate casing-treatment simulations. The single-stage transonic axial compressor was equipped with a casing treatment (CT), consisting of 3.5 axial slots per rotor pitch in order to investigate the predicted extension of the stall margin characteristics. Contrary to most other studies, the CT was designed especially accounting for an optimized optical access in the immediate vicinity of the CT, rather than giving maximum benefit in terms of stall margin extension. The nearly rectangular geometry of the CT cavities allowed one dividing bridge between two slots to be made of quartz glass with curvatures matching the casing. Thus the flow phenomena could be observed with essentially no disturbance caused by the optical access. Two periscope light sheet probes were specifically designed for this application to allow for precise alignment of the laser light sheet at three different radial positions in the rotor tip region (at 87.5%, 95% and 99% blade height). For the outermost radial position the light sheet probe was placed behind the rotor and aligned to pass the light sheet through the blade tip clearance. It was demonstrated that the PIV technique is capable of providing velocity information of high quality even in the tip clearance region of the rotor blades. Phase-constant measurements were carried out with a resolution of 8 phase angles per blade pitch in relation to the CT slots visible in the camera’s field of view. The chosen type of smoke-based seeding with very small particles (about 0.5 µm in diameter) supported data evaluation with high spatial resolution, resulting in a final grid size of 0.5 x 0.5 mm. The PIV data base established in this project forms the basis for further detailed evaluations of the flow phenomena present in the transonic compressor stage with CT and allows validation of accompanying CFD calculations using the DLR TRACE code. Based on the combined results of PIV measurements and CFD calculations of the same compressor and CT geometry a better understanding of the complex flow characteristics can be achieved

INVESTIGATION OF BLADE TIP INTERACTION WITH CASING TREATMENT IN A TRANSONIC COMPRESSOR – PART 2: NUMERICAL RESULTS

A single-stage transonic axial compressor was equipped with a casing treatment, consisting of 3.5 axial slots per rotor pitch in order to investigate its influence on stall margin characteristics as well as on the rotor near tip flowfield both numerically and experimentally. Contrary to most other studies a generic Casing Treatment was designed to provide optimal optical access in the immediate vicinity of the CT, rather than for maximum benefit in terms of stall margin extension. The second part of this two-part paper deals with the numerical developments, and their validation, carried out in order to efficiently perform time-accurate casing-treatment simulations. The numerical developments focus on the extension of an existing coupling algorithm in order to carry out unsteady calculations with any exterior geometry coupled to the main flow passage (in this case a single slot) having an arbitrary pitch. This extension is done by incorporating frequency domain, phase-lagged boundary conditions into this coupling procedure. Whereas the phaselag approach itself is well established and validated for standard rotor-stator calculations, its application to casing treatment simulations is new. Its capabilities and validation will be demonstrated on the given compressor configuration, making extensive use of the detailed PIV flowfield measurements near the rotor tip. Instantaneous data at all measurement planes will be compared for different rotor positions with respect to the stationary slots in order to evaluate the time-dependent interaction between the rotor and the casing treatment

Experimental Investigation of the Rotor / Casing Treatment Interaction in an Axial Single-Stage Transonic Compressor

In the design process of compressors for gas turbines, the application of casing treatments showed a substantial contribution to the enhancement of total pressure ratio and stability. The development of a treated casing endwall remains quite demanding due to the complicated flow features and interactions, particularly when applied to transonic rotors with strong shock systems. To achieve an insight into these flow mechanisms, the axial single-stage Darmstadt Transonic Compressor Test Rig was equipped with a casing treatment with 56 bend axial slots. Speedlines and exit plane measurements were made for design speed and for a completely subsonic flow field. Time resolved measurement techniques were chosen to investigate the rotorcasing treatment interaction. The casing treatment was designed to provide an optical access through two slots. In addition, the land between these slots was replaced with a glass insert. The assembly allowed phase-resolved particle image velocimetry investigations in the rotor tip area and inside the tip gap underneath one axial slot. Results showed an increase in stall margin for both operating conditions. Efficiency declined slightly. At design speed, a second shock structure develops when the casing treatment is applied. This additional shock front has considerable effect on the flow field. Vortices inside the slots account for the efficiency losses

Investigation of Blade Tip Interaction with Casing Treatment in a Transonic Compressor - Part 2: Numerical Results

A single-stage transonic axial compressor was equipped with a casing treatment, consisting of 3.5 axial slots per rotor pitch in order to investigate its influence on stall margin characteristics as well as on the rotor near tip flowfield both numerically and experimentally. Contrary to most other studies a generic Casing Treatment was designed to provide optimal optical access in the immediate vicinity of the CT, rather than for maximum benefit in terms of stall margin extension. The second part of this two-part paper deals with the numerical developments, and their validation, carried out in order to efficiently perform time-accurate casing-treatment simulations. The numerical developments focus on the extension of an existing coupling algorithm in order to carry out unsteady calculations with any exterior geometry coupled to the main flow passage (in this case a single slot) having an arbitrary pitch. This extension is done by incorporating frequency domain, phase-lagged boundary conditions into this coupling procedure. Whereas the phaselag approach itself is well established and validated for standard rotor-stator calculations, its application to casing treatment simulations is new. Its capabilities and validation will be demonstrated on the given compressor configuration, making extensive use of the detailed PIV flowfield measurements near the rotor tip. Instantaneous data at all measurement planes will be compared for different rotor positions with respect to the stationary slots in order to evaluate the time-dependent interaction between the rotor and the casing treatment

INVESTIGATION OF BLADE TIP INTERACTION WITH CASING TREATMENT IN A TRANSONIC COMPRESSOR – PART 2: NUMERICAL RESULTS

A single-stage transonic axial compressor was equipped with a casing treatment, consisting of 3.5 axial slots per rotor pitch in order to investigate its influence on stall margin characteristics as well as on the rotor near tip flowfield both numerically and experimentally. Contrary to most other studies a generic Casing Treatment was designed to provide optimal optical access in the immediate vicinity of the CT, rather than for maximum benefit in terms of stall margin extension. The second part of this two-part paper deals with the numerical developments, and their validation, carried out in order to efficiently perform time-accurate casing-treatment simulations. The numerical developments focus on the extension of an existing coupling algorithm in order to carry out unsteady calculations with any exterior geometry coupled to the main flow passage (in this case a single slot) having an arbitrary pitch. This extension is done by incorporating frequency domain, phase-lagged boundary conditions into this coupling procedure. Whereas the phaselag approach itself is well established and validated for standard rotor-stator calculations, its application to casing treatment simulations is new. Its capabilities and validation will be demonstrated on the given compressor configuration, making extensive use of the detailed PIV flowfield measurements near the rotor tip. Instantaneous data at all measurement planes will be compared for different rotor positions with respect to the stationary slots in order to evaluate the time-dependent interaction between the rotor and the casing treatment

INVESTIGATION OF BLADE TIP INTERACTION WITH CASING TREATMENT IN A TRANSONIC COMPRESSOR – PART 1: PARTICLE IMAGE VELOCIMETRY

A single-stage transonic axial compressor was equipped with a casing treatment (CT), consisting of 3.5 axial slots per rotor pitch in order to investigate the predicted extension of the stall margin characteristics both numerically and experimentally. Contrary to most other studies the CT was designed especially accounting for an optimized optical access in the immediate vicinity of the CT, rather than giving maximum benefit in terms of stall margin extension. Part 1 of this two-part contribution describes the experimental investigation of the blade tip interaction with casing treatment using Particle image velocimetry (PIV). The nearly rectangular geometry of the CT cavities allowed a portion of it to be made of quartz glass with curvatures matching the casing. Thus the flow phenomena could be observed with essentially no disturbance caused by the optical access. Two periscope light sheet probes were specifically designed for this application to allow for precise alignment of the laser light sheet at three different radial positions in the rotor passage (87.5%, 95% and 99%). For the outermost radial position the light sheet probe was placed behind the rotor and aligned to pass the light sheet through the blade tip clearance. It was demonstrated that the PIV technique is capable of providing velocity information of high quality even in the tip clearance region of the rotor blades. The chosen type of smoke-based seeding with very small particles (about 0.5 µm in diameter) supported data evaluation with high spatial resolution, resulting in a final grid size of 0.5 x 0.5 mm. The PIV data base established in this project forms the basis for further detailed evaluations of the flow phenomena present in the transonic compressor stage with CT and allows validation of accompanying CFD calculations using the TRACE code. Based on the combined results of PIV measurements and CFD calculations of the same compressor and CT geometry a better understanding of the complex flow characteristics can be achieved, as detailed in Part 2 of this paper