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| Numerical Investigation of Active Control for Low Pressure Turbine Blades |
| In a numerical and theoretical effort, we are investigating active control of separation for low-pressure turbine (LPT) blades. LPT stages are an integral part of many jet engines. Improved performance of the LPT stage would directly translate into an increased overall performance of modern propulsion systems. One limiting factor in the design of LPTs, however, is laminar separation on the suction side of the blades: For small blade dimensions (as found in modern UAVs) and/or at low ambient air density (as encountered during high altitude cruise), LPT operating Reynolds numbers can reach values as low as 25,000. Laminar flow separation can lead to a drastic reduction of the overall engine performance. Several years ago, a broadly based experimental research program was initiated at the Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base to counter such unfavorable conditions by means of active flow control (AFC). R. Rivir and co-workers systematically explored the potential benefits of steady and pulsed vortex generator jets (VGJs) for AFC of LPT blade separation. They were able to successfully reduce/eliminate separation losses on LPT blades for Reynolds numbers between 25,000 and 100,000. However, many of the underlying physical mechanisms responsible for these striking experimental results are still not well understood. Our goal is to uncover these fundamental mechanisms using CFD. Toward this end, we are taking a two-pronged approach: Simulations of the entire turbine cascade will help us understand the global LPT flow dynamics. Eventually, we will also include pulsed VGJs in our 3-D simulations. These simulations are, however, computationally very challenging. Secondly, we are performing detailed investigations of the jet-boundary layer interaction by means of highly resolved direct numerical simulations (108 - 109 grid points). |
Instantaneous iso-surface of the Q vortex identification criterion.
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Flow visualization for steady, angled VGJs: streamwise velocity with streamlines.
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| Funding agency: Air Force Office of Scientific Research (AFOSR) |
| People:
Wolfgang Balzer, Christoph Brehm, Dr. Andreas Gross
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| Publications: |
| D. Postl and H. F. Fasel; "Direct numerical simulation of turbulent flow separation from a wall-mounted hump", AIAA Journal, accepted for publication |
| A. Gross and H. F. Fasel, 2006; "Reduced Order Models for Closed-Loop Control of Time-Dependent Flows", AIAA paper 2006-1403, 45th AIAA Aerospace Sciences Meeting and Exhibit, 9-11 January 2006, Reno, NV |
| A. Gross and H. F. Fasel, 2005; "Numerical Investigation of Low-Pressure Turbine Blade Separation Control", AIAA Journal, Vol. 43, No. 12, pp. 2514-2526 |
| A. Gross and H. F. Fasel, 2005; "Turbulence modeling for low pressure turbine blades", AIAA paper 2005-5292, June 2005 |
| A. Gross and H. F. Fasel, 2005; "Simulation of Active Flow Control for a Low Pressure Turbine Blade Cascade", AIAA paper 2005-0869, 43rd AIAA Aerospace Sciences Meeting and Exhibit, 10-13 January 2005, Reno, NV |
| H. F. Fasel and D. Postl, 2005; "Interaction of separation and transition in
boundary layers: direct numerical simulations" (invited), Laminar-Turbulent Transition, Proceedings of the IUTAM Symposium, 13-17 December 2004, Bangalore, India, in print |
| H. F. Fasel, A. Gross, D. Postl, 2004; "Numerical simulation of active flow
control for low-pressure turbine blades" (invited), RTO AVT Specialist Meeting on Enhancement of NATO Military Flight Vehicle Performance, 4-7 October 2004, Prague, Czech Republic |
| H. F. Fasel, A. Gross, D. Postl, 2004; "Control of separation for low-pressure
turbine blades", Proceedings of the Minnowbrook IV - 2003 Workshop on Transition and Unsteady Aspects of Turbomachinery Flows, 17-20 August 2003, Blue Mountain Lake, NY |
| A. Gross, H. F. Fasel, 2004; "Active Control of Separation for Low-Pressure Turbine Blades", AIAA paper 2004-2203, June-July 2004 |
| D. Postl, A. Gross, H. F. Fasel, 2004; "Numerical investigation of active flow
control for low-pressure turbine blade separation", AIAA paper 2004-0750, 42nd AIAA Aerospace Sciences Meeting and Exhibit, 5-8 January 2004, Reno, NV |
| D. Postl, A. Gross, H. F. Fasel, 2003; "Numerical investigation of low-pressure turbine blade separation control", AIAA paper 2003-0614, 41st AIAA Aerospace Sciences Meeting and Exhibit, 6-9 January 2003, Reno, NV |
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| Numerical Investigation of Transition in Supersonic Boundary Layers Using DNS and LES |
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Reliable transition predictions are critically important for the design and safe operation of high-speed, advanced flight vehicles. The large increase in wall heat transfer due to transition to turbulence in supersonic boundary layers is one of the major difficulties in the design and operation of high-speed flight vehicles. In addition to these aero-thermodynamic loads, transition to turbulence has a large impact on the aerodynamic performance and flight characteristics of these vehicles as the skin friction is drastically increased.
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In our numerical effort, we are investigating the transition processes in high-speed boundary layers in detail. Although much is known about transition in incompressible boundary layers, the understanding of the transition mechanisms in high-speed boundary layers is still very limited. Two main reasons for this lack of understanding are: i) experimental investigations in supersonic wind tunnels are much more difficult and expensive than their low-speed counterparts, and ii) the physical mechanisms in supersonic transition are much more complex than in low-speed flows. Direct Numerical Simulations (DNS) - and especially the "spatial" model - provide an important tool to investigate the instability mechanisms in supersonic flows. In DNS, the complete, compressible, unsteady Navier-Stokes equations are solved directly by numerical integration in time with proper discretization of the spatial derivatives. In particular, in DNS, controlled periodic disturbances can be introduced to study the nonlinear development of disturbance waves as they travel downstream. This nonlinear development determines how rapidly the boundary layer transitions, and therefore is the focus of this research project. |
Oblique Breakdown over a flat plate at M = 3: Iso-surface of u-velocity with contour plots at z = 0 and at the outflow.
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The objective of our numerical effort is to investigate in detail possible routes to transition in supersonic boundary layers and to understand the underlying physical mechanisms. In particular, we are interested in the nonlinear, late stages of transition for a range of Mach numbers between M=2 and M=8. The geometries considered are a flat plate as well as circular and elliptical cones.
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| Funding agency: Air Force Office of Scientific Research (AFOSR) |
| People:
Frank Husmeier, Andreas Laible, Christian Mayer
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| Publications: |
| C. S. J. Mayer, H. F. Fasel, 2008; "Investigation of Asymmetric Subharmonic Resonance in a Supersonic Boundary Layer at Mach 2 Using DNS", AIAA-2008-0591, 46th AIAA Aerospace Sciences Meeting and Exhibit, January 7-10, 2008, Reno, NV |
| F. Husmeier, H. F. Fasel, 2007; "Numerical Investigations of Hypersonic Boundary Layer Transition for Circular Cones", AIAA-2007-3843, 18th AIAA Computational Fluid Dynamics Conference, June 25-28, 2007, Miami, FL |
| C. S. J. Mayer, S. Wernz, H. F. Fasel, 2007; "Investigation of Oblique Breakdown in a Supersonic Boundary Layer at Mach 2 using DNS", AIAA-2007-0949, 45th AIAA Aerospace Sciences Meeting and Exhibit, January 8-11, 2007, Reno, NV |
| F. Husmeier, C. S. J. Mayer, H. F. Fasel, 2005; "Investigation of Transition of Supersonic Boundary Layers at Mach 3 Using DNS", AIAA paper 2005-0095, 43rd AIAA Aerospace Sciences Meeting and Exhibit, 10-13 January 2005, Reno, NV |
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| Numerical Investigation of Coanda Flow Using DNS, LES, and URANS |
| The main goal of this project is to numerically investigate turbulent wall jet flows over convex surfaces (Coanda flows). Coanda wall jets may be applicable for maneuvering of submarines and other underwater vehicles. Special emphasis of this research is on uncovering the fundamental mechanisms responsible for the Coanda effect in the presence of convex walls. |
Coanda wall jets can give rise to centrifugal hydrodynamic instabilities. As a consequence of this instability, longitudinal vortices may develop. Of particular importance, therefore, is a fundamental understanding of the dynamical interaction of the spanwise coherent structures (caused by an inviscid instability due to an inflection point in the mean velocity profile) and the longitudinal vortices. This understanding is essential for possible future implementation of Coanda devices for Navy applications. Furthermore, the investigations will shed light on the turbulence generation mechanism for wall bounded flows with curvature when longitudinal vortices are present.
The investigations are carried out using DNS, LES, and unsteady RANS. Toward this end, a new methodology for simulations of turbulent flows is employed, the so-called Flow Simulation Methodology (FSM) which we developed with funding from ONR. The FSM employs a new subgrid-scale (SGS) turbulence model for time-dependent turbulent simulations, where the resolved scales are computed by solving the Navier-Stokes equations and the unresolved scales are modeled. The simulations will approach an unsteady Reynolds-Averaged Navier-Stokes (RANS) calculation when the grid resolution is decreased (and/or Reynolds number is increased) and consistently approaches a Direct Numerical Simulation (DNS) when the grid resolution is increased (and/or Reynolds number is decreased). In between these limits, we have a non-traditional Large-Eddy Simulation (LES) (with various degrees of modeling of the unresolved scales). The new methodology is non-traditional in the sense that the SGS model is far superior to models used in traditional LES (including the so-called dynamic model) which have significant shortcomings as they are based on the Smagorinsky model and are thus not really applicable to complex geometries. |
Turbulent Coanda flow. Q-criterion visualization obtained from an LES based on the FSM approach.
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Turbulent Coanda flow. Effect of focing on the development of longitudinal structures. Shown for three cases are iso-surfaces of time-averaged streamwise vorticity indicating an increase in size of dominant streamwise vortices
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| Funding agency: Office of Naval Research (ONR) |
| People:
Dr. Andreas Gross, Dr. Stefan Wernz
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| Publications: |
| A. Laible, P. Valsecchi, H. F. Fasel, 2006; "Numerical Investigation of Secondary Centrifugal Instabilities in the Coanda Wall Jet", AIAA paper 2006-0908, 44th AIAA Aerospace Sciences Meeting and Exhibit, 9 - 12 January 2006, Reno, NV |
| S. Wernz, A. Gross, H. F. Fasel, 2005; "Numerical Investigation of Coherent Structures in Plane and Curved Wall Jets", AIAA paper 2005-4911, June 2005 |
| A. Gross, H. F. Fasel, 2004; "Numerical Investigation of Streamwise Coherent Structures in a Turbulent Coanda Wall Jet", AIAA paper 2004-2346, 34th AIAA Fluid Dynamics Conference and Exhibit, 28 June - 1 July 2004, Portland, OR |
| S. Wernz, H. F. Fasel, 2004; "Numerical Investigation of Forced Coanda Wall Jet", AIAA paper 2004-2349, 34th AIAA Fluid Dynamics Conference and Exhibit, 28 June - 1 July 2004, Portland, OR |
| S. Wernz, P. Valsecchi, A. Gross, H. F. Fasel, 2003; "Numerical Investigation of Turbulent Wall Jets Over a Convex Surface", AIAA paper 2003-3727, June 2003 |
| A. Gross, S. Wernz, H. F. Fasel, 2003; "Numerical Investigation of Coherent Structures in a Turbulent Coanda Wall Jet", AIAA paper 2003-4020, June 2003 |
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| Numerical Simulation of Supersonic Axisymmetric Base Flows Using DNS, LES, and URANS |
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A comprehensive effort is being undertaken to investigate transitional and turbulent axisymmetric wakes behind cylindrical bodies aligned with the flow at supersonic speeds. Particular emphasis is on identifying and understanding the dynamical behavior of the large-scale vortical structures that control the flow behavior in a supersonic wake. Direct Numerical Simulations (DNS) and Large-Eddy Simulations (LES) are the main investigation tools.
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It is well known that for subsonic (incompressible) wakes, the dynamics of the large (coherent) structures play a dominant role in the local and global behavior of the flow. This evidence was found from both experimental investigations and numerical simulations (including ours), and was confirmed by theoretical studies. For supersonic speeds, on the other hand, very little is known about the dynamical behavior of turbulent flows. This is true for supersonic flows in general, and for axisymmetric wakes in particular. Thus, the question arises: Do large structures play a similarly important role for supersonic separated flows and, in particular, for supersonic axisymmetric wakes? The answer to this question is of fundamental relevance for applying flow control. There are few experimental investigations that have focused on this issue. However, when looking at flow visualization pictures of supersonic wake flows, distinct patterns with large-scale structures can be observed. For supersonic axisymmetric wakes, the mean flow structure of the near-wake region is characterized by the axisymmetric shear layer originating at the sharp corners of the blunt base.
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Supersonic wake behind axisymmetric projectile wit blunt base. Instantaneous iso-surfaces of Q for 3D DNS. ReD=100,000, M=2.46.
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Supersonic axisymmetric wakes are extremely difficult to investigate experimentally. Wind tunnel interference and interference from model support strongly affect the mean flow behavior. Therefore, numerical simulations represent a new alternative for investigating the complicated unsteady flow phenomena in the supersonic wake. DNS using the complete Navier-Stokes equations are restricted to somewhat low-to-moderate Reynolds numbers because of the rapidly increasing demands on computing power as the Reynolds number increases. Because of the Reynolds number limitations on Direct Numerical Simulations, we are also performing Large-Eddy Simulations (LES) using subgrid-scale turbulence models. However, even with current LES models, reproducing the large Reynolds numbers seen in experiments requires a massive computational effort. For this reason, a new LES methodology has been implemented, called the Flow Simulation Methodology (FSM). It allows the simulations to be pushed to considerably larger Reynolds numbers than traditional LES, and thus closer to the flow conditions seen in experiments, while at the same time capturing the large (coherent) structures that, in all likelihood, have a central importance to the flow and are not captured with a Reynolds-averaged simulation.
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Supersonic wake behind axisymmetric projectile wit blunt base. Iso-contours of instantaneous Q=0.2 (yellow) and Q=2 (orange) for 3D-FSM. ReD=3,300,000, M=2.46.
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| Funding agency: Army Research Office (ARO) |
| People:
Jayahar Sivasubramanian
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| Publications: |
| R. D. Sandberg and H. F. Fasel; "Numerical Investigation of Transitional Supersonic Axisymmetric Wakes", accepted for publication in JFM. |
| R. D. Sandberg and H. F. Fasel; "Direct Numerical Simulations of Transitional Supersonic Base Flows", accepted for publication in AIAA Journal |
| J. Sivasubramanian, R. D. Sandberg, D. A. von Terzi and H. F. Fasel, 2006; "Numerical investigation of flow control mechanisms for drag reduction in supersonic base-flows", AIAA paper 2006-902, 44th AIAA Aerospace Sciences Meeting and Exhibit, 9-12 January 2006, Reno, NV |
| J. Sivasubramanian, R. D. Sandberg, D. A. von Terzi and H. F. Fasel, 2006; "Numerical investigation of transitional supersonic base flows with flow control", AIAA paper 2006-479, 44th AIAA Aerospace Sciences Meeting and Exhibit, 9-12 January 2006, Reno, NV |
| D. A. von Terzi, R. D. Sandberg, J. Sivasubramanian and H. F. Fasel, 2005; "High accuracy DNS and LES of high Reynolds number, supersonic base flows and passive control of the near wake", 2005 DoD HPC Users Group Conference, Nashville, TN,27-30 June 2005, published in IEEE Proceedings of the HPCMP Users Group Conference 2005 - ISBN 0-7695-2496-6 |
| R. D. Sandberg and H. F. Fasel, 2005; "Direct Numerical Simulations of Transitional Supersonic Base Flows", AIAA Paper 2005-0098, 43rd AIAA Aerospace Sciences Meeting and Exhibit, 10-13 January 2005, Reno, NV |
| R. D. Sandberg and H. F. Fasel, 2004; "High-Accuracy DNS of Supersonic Base Flows and Control of the Near Wake", 2004 DoD HPC Users Group Conference, Williamsburg, Virginia, 7-11 June 2004, published in IEEE Proceedings of the Users Group Conference 2004 - ISBN 0-7695-2259-9 |
| R. D. Sandberg and H. F. Fasel, 2004; "Application of a new Flow Simulation Methodology for Supersonic Axisymmetric Wakes", AIAA Paper 2004-0067, AIAA 42nd Aerospace Sciences Meeting and Exhibit, 5-8 January 2004, Reno, NV |
| R. D. Sandberg and H. F. Fasel, 2004; "Instability Mechanisms in Supersonic Base Flows", AIAA Paper 2004-0593, AIAA 42nd Aerospace Sciences Meeting and Exhibit, 5-8 January 2004, Reno, NV |
| R. D. Sandberg and H. F. Fasel, 2003; "A Flow Simulation Methodology for Compressible Turbulent Axisymmetric Wakes", AIAA Paper 2003-0267, AIAA 41st Aerospace Sciences Meeting and Exhibit, 6-9 January 2003, Reno, NV |
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