Minnowbrook V: 2006 Workshop on Unsteady Flows in Turbomachinery

This volume contains materials presented at the Minnowbrook V 2006 Workshop on Unsteady Flows in Turbomachinery, held at the Syracuse University Minnowbrook Conference Center, New York, on August 20-23, 2006. The workshop organizers were John E. LaGraff (Syracuse University), Martin L.G. Oldfield (Oxford University), and J. Paul Gostelow (University of Leicester). The workshop followed the theme, venue, and informal format of four earlier workshops: Minnowbrook I (1993), Minnowbrook II (1997), Minnowbrook III (2000), and Minnowbrook IV (2003). The workshop was focused on physical understanding of unsteady flows in turbomachinery, with the specific goal of contributing to engineering application of improving design codes for turbomachinery. The workshop participants included academic researchers from the United States and abroad and representatives from the gas-turbine industry and U.S. Government laboratories. The physical mechanisms discussed were related to unsteady wakes, active flow control, turbulence, bypass and natural transition, separation bubbles and turbulent spots, modeling of turbulence and transition, heat transfer and cooling, surface roughness, unsteady CFD, and DNS. The workshop summary and the plenary discussion transcripts clearly highlight the need for continued vigorous research in the technologically important area of unsteady flows in turbomachines. This volume contains abstracts and copies of select viewgraphs organized according to the workshop sessions. Full-color viewgraphs and animations are included in the CD-ROM version only (Doc.ID 20070024781)

Mid-span losses in turbine blades at subsonic and supersonic speeds

The effects of compressibility are intrinsic to many axial flow turbomachines, is. Both subsonic and supersonic speed ranges are considered in this investigation. Subsonic surface base pressures, and wake energy separation, are a direct result of periodic von Kármán vortex shedding. This is the principal cause of both wake energy separation and the related subsonic base static pressure deficit. At high subsonic speeds a 17oC temperature difference across the wake was observed. This time-averaged temperature separation was a manifestation of the energy separation (Eckert-Weise) effect. At supersonic speeds the trailing edge base pressure, and the wake energy separation, exhibit different characteristics from the subsonic behavior. Shock waves from the trailing edge may impinge on the adjacent suction surface adversely affecting the downstream boundary layer. Supersonic flows usually cause shock and expansion waves and this may occur in steady flows. Other wake modes may also involve von Kármán vortex shedding from the confluence region of the wake. This is not the only form of shedding and anomalous, or exotic, shedding may also play an important role

Streamwise and crossflow vortical structures on turbine blades and swept cylinders

Flow visualization on a lengthy time-average basis on the suction surface of turbine blades showed robust and consistent streamwise streaks at subsonic and transonic speeds. The normal flow past a circular cylinder is a more canonical case and testing was undertaken at high speeds on a 37.23 mm diameter cylinder and at low speeds on a 152 mm diameter cylinder. The lateral spacing between streaks on cylinders had been predicted by Kestin and Wood and the present tests gave excellent agreement with their theory. Their work on unswept circular cylinders provides a good baseline model for understanding and predicting sweep effects on cylinders and turbomachinery blading. Experiments on a circular cylinder were performed over a range of sweep angles from zero to 61°, giving results for lateral spacing and angular orientation of the streaks. At high sweep angles, the results are consistent with those of Poll. Hot wire measurements away from the surface indicate a variable flow structure in the spanwise direction with a wavelength matching that of the surface traces. The streamwise disturbance was predominantly stationary in nature and resilient, often persisting from leading edge to trailing edge. Crossflow instability becomes more significant at high sweep angles. It grows aggressively and rapidly, being predominantly of a traveling nature. The observed streaks could be of particular concern for the thermal design of turbine blades

Base Pressure Measurements on a Circular Cylinder in Subsonic Cross Flow.

A circular cylinder was tested in cross flow over the subsonic speed range. Timeresolved pressure distributions give information on surface pressure fluctuations and the corresponding drag and base drag coefficients are provided. The Strouhal number variation is compared with the measurements of other authors. Flow changes at higher subsonic velocities and into the transonic range are described. At Mach numbers above 0.6 the changing strength of the vortices reduces the base drag coefficient up to a Mach number of 0.9, where the onset of sonic flow increases the drag. Time-resolved base pressure fluctuations at low Mach numbers are in agreement with the findings of other researchers with regard to the relative time spent in vortex formation and shedding. As the Mach number increases the time spent in vortex formation becomes equal to that spent in shedding. The paper concentrates on providing detailed base pressure data rather than attempting to produce universal correlations. Physical explanations have been given, where possible, to assist toward a more general modeling of the problem

Some examples of stability mode persistance and change in vortex structure formation

Three rather different physical cases have been studied. All represent very practical cases for which the modal behaviour of vortical structures is not completely understood. The work on these problems is on-going and it is hoped that physical confirmation, enhanced understanding and predictive capability for the vortical structures encountered will eventuate

Visualization of streamwise and crossflow instabilities on inclined circular cylinders

Experimental observations of streamwise and crossflow streaks on circular cylinders are presented over a range of cylinder inclinations. The spanwise wavelength, λ0, of an array of streamwise vortices on a normal circular cylinder was addressed by Kestin and Wood. That theory was for the limiting case of zero sweep and the results are here confirmed experimentally. These observations of streamwise vortices on unswept cylinders confirm the earlier predictions, providing a firm basis for referencing new measurements of vortical behaviour. The work on unswept cylinders provides an excellent benchmark for sweep effects. The principal published collection of results for medium to high sweep angles, Λ, is that of Poll, covering sweep angles between 55° and 71°. Surface flow visualization has been undertaken by the authors giving lateral spacing and streak orientation for sweep angles from zero to 61o. Results for lateral spacing are consistent with those of Poll, covering the range between the zero sweep results of Kestin and Wood and the medium to high sweep cases addressed by Poll

Minnowbrook VI: 2009 Workshop on Flow Physics and Control for Internal and External Aerodynamics

Topics covered include: Flow Physics and control for Internal and External Aerodynamics (not in TOC...starts on pg13); Breaking CFD Bottlenecks in Gas-Turbine Flow-Path Design; Streamwise Vortices on the Convex Surfaces of Circular Cylinders and Turbomachinery Blading; DNS and Embedded DNS as Tools for Investigating Unsteady Heat Transfer Phenomena in Turbines; Cavitation, Flow Structure and Turbulence in the Tip Region of a Rotor Blade; Development and Application of Plasma Actuators for Active Control of High-Speed and High Reynolds Number Flows; Active Flow Control of Lifting Surface With Flap-Current Activities and Future Directions; Closed-Loop Control of Vortex Formation in Separated Flows; Global Instability on Laminar Separation Bubbles-Revisited; Very Large-Scale Motions in Smooth and Rough Wall Boundary Layers; Instability of a Supersonic Boundary-Layer With Localized Roughness; Active Control of Open Cavities; Amplitude Scaling of Active Separation Control; U.S. Air Force Research Laboratory's Need for Flow Physics and Control With Applications Involving Aero-Optics and Weapon Bay Cavities; Some Issues Related to Integrating Active Flow Control With Flight Control; Active Flow Control Strategies Using Surface Pressure Measurements; Reduction of Unsteady Forcing in a Vaned, Contra-Rotating Transonic Turbine Configuration; Active Flow Control Stator With Coanda Surface; Controlling Separation in Turbomachines; Flow Control on Low-Pressure Turbine Airfoils Using Vortex Generator Jets; Reduced Order Modeling Incompressible Flows; Study and Control of Flow Past Disk, and Circular and Rectangular Cylinders Aligned in the Flow; Periodic Forcing of a Turbulent Axisymmetric Wake; Control of Vortex Breakdown in Critical Swirl Regime Using Azimuthal Forcing; External and Turbomachinery Flow Control Working Group; Boundary Layers, Transitions and Separation; Efficiency Considerations in Low Pressure Turbines; Summary of Conference; and Final Plenary Session Transcript

Mitigating secondary flows in a 1½ stage axial turbine by a guide groove casing

The interaction of secondary flows with the main passage flow in turbomachines generates entropy and aerodynamic loss. This loss source is most relevant to low aspect ratio blades. One approach for reducing this loss is by end-wall contouring. However, limited work has been reported on using non-axisymmetric end-walls at the stator casing and on its interaction with the tip leakage flow. In this paper, a non-axisymmetric end-wall design for the stator casing is achieved through a novel surface definition, towards mitigating secondary flow losses. The casing features a groove that follows the natural path of selected secondary flow structures. This design is tested on a three-dimensional axial turbine RANS model built in OpenFOAM Extend 3.2, with − SST turbulence closure. The stage isentropic efficiency is predicted to increase by 0.59% at the turbine design point and by 0.63% off-design, by using the new casing. This work also used an automated workflow process, linking surface definition in MATLAB, meshing in ICEM CFD, flow solving in OpenFOAM, and post-processing in Tecplot. This has generated a casing contouring design tool with a good portability to industry, to design and optimize new turbine blade passages. Stages designed with this new non-axisymmetric casing are likely to reduce the CO2 emissions from power generation, towards achieving the UNFCCC emissions goals

Encounters with Vortices in a Turbine Nozzle Passage

Experiments were conducted on the flow through a transonic turbine cascade. A wide range of vortices was encountered including horseshoe vortices, secondary flows, shock-induced passage vortices and streamwise vortices on the suction surface. In the separation region on the suction surface, a large roll-up of passage vorticity occurred. The blunt leading edge gave rise to strong horseshoe vortices and secondary flows. The suction surface had a strong convex curvature over the forward portion and was quite flat further downstream. Surface flow visualization was performed and this convex surface displayed coherent streamwise vorticity. At subsonic speeds strong von Kármán vortex shedding resulted in a substantial base pressure deficit. For these conditions time-resolved measurements were made of Eckert-Weise energy separation in the blade wake. At transonic speeds, exotic shedding modes were observed. These phenomena all occurred in experiments on the flow around one particular turbine nozzle vane in a linear cascade