The Experts below are selected from a list of 3777 Experts worldwide ranked by ideXlab platform
Peter J. Schmid - One of the best experts on this subject based on the ideXlab platform.
-
Global stability of swept flow around a Parabolic Body: the neutral curve
Journal of Fluid Mechanics, 2011Co-Authors: Christoph J. Mack, Peter J. SchmidAbstract:The onset of transition in the leading-edge region of a swept blunt Body depends crucially on the stability characteristics of the flow. Modelling this flow configuration by swept compressible flow around a Parabolic Body, a global approach is taken to extract pertinent stability information via a DNS-based iterative eigenvalue solver. Global modes combining features from boundary-layer and acoustic instabilities are presented. A parameter study, varying the spanwise disturbance wavenumber and the sweep Reynolds number, showed the existence of unstable boundary-layer and acoustic modes. The corresponding neutral curve displays two overlapping regions of exponential growth and two critical Reynolds numbers, one for boundary-layer instabilities and one for acoustic instabilities. The employed global approach establishes a first neutral curve, delineating stable from unstable parameter configurations, for the complex flow about a swept Parabolic Body with corresponding implications for swept leading-edge flow.
-
Global stability of swept flow around a Parabolic Body: features of the global spectrum
Journal of Fluid Mechanics, 2011Co-Authors: Christoph J. Mack, Peter J. SchmidAbstract:The global temporal stability of three-dimensional compressible flow about a yawed Parabolic Body of infinite span is investigated using an iterative eigenvalue technique in combination with direct numerical simulations. The computed global spectrum provides a comprehensive picture of the temporal perturbation dynamics of the flow, and a wide and rich variety of modes has been uncovered for the investigated parameter choices: stable and unstable boundary-layer modes, different types of stable and unstable acoustic modes, and stable wavepacket modes have been found. A parameter study varying the spanwise perturbation wavenumber and the sweep Reynolds number reproduced a preferred spanwise length scale and a critical Reynolds number for a boundary-layer or acoustic instability. Convex leading-edge curvature has been found to have a strongly stabilizing effect on boundary-layer modes but only a weakly stabilizing effect on acoustic modes. Furthermore, for certain parameter choices, the acoustic modes have been found to dominate the boundary-layer modes. © 2011 Cambridge University Press.
-
Direct numerical study of hypersonic flow about a swept Parabolic Body
Computers & Fluids, 2010Co-Authors: Christoph J. Mack, Peter J. SchmidAbstract:Direct numerical simulations (DNS) of hypersonic flow about a swept Parabolic Body have been performed to study the global stability of flow in the leading-edge region of a swept blunt Body. Previous stability investigations have been based on local models but have not fully succeeded in reproducing the established experimental findings. The current flow configuration represents a more realistic model and is thus expected to resolve some of the remaining questions. However, novel approaches like DNS-based global stability theory are necessary for such flow models and are employed in this study. As a result, boundary-layer modes have been identified by different but complementary techniques as the dominant instability mechanism. The DNS starting with small-amplitude white noise provide further evidence for the presence of non-modal effects which may be important in the subcritical regime. From a methodological point of view, the potential for quantitative flow analysis by combining numerical simulations with advanced iterative techniques represents a promising direction for investigating the governing physical processes of complex flows.
-
A preconditioned Krylov technique for global hydrodynamic stability analysis of large-scale compressible flows
Journal of Computational Physics, 2010Co-Authors: Christoph J. Mack, Peter J. SchmidAbstract:The combination of iterative Krylov-based eigenvalue algorithms and direct numerical simulations (DNS) has proven itself an effective and robust tool in solving complex global stability problems of compressible flows. A Cayley transformation is required to add flexibility to our stability solver and to allow access to specific parts of the full global spectrum which would be out of reach without such a transformation. In order to robustify the overall global stability solver an efficient ILU-based preconditioner has been implemented. With this Cayley-transformed DNS-based Krylov method two flow cases were successfully investigated: (i) a compressible mixing layer, a rather simple but well-known problem, which served as a test case and (ii) a supersonic flow about a swept Parabolic Body, a challenging large-scale flow configuration.
-
Global stability of swept flow around a Parabolic Body: connecting attachment-line and crossflow modes
Journal of Fluid Mechanics, 2008Co-Authors: Christoph J. Mack, Peter J. Schmid, Jörn SesterhennAbstract:The global linear stability of a three-dimensional compressible flow around a yawed Parabolic Body of infinite span is investigated using an iterative eigenvalue method in conjunction with direct numerical simulations. The computed global spectrum shows an unstable branch consisting of three-dimensional boundary layer modes whose amplitude distributions exhibit typical characteristics of both attachment-line and crossflow modes. In particular, global eigenfunctions with smaller phase velocities display a more pronounced structure near the stagnation line, reminiscent of attachment-line modes while still featuring strong crossflow vortices further downstream. This analysis establishes a link between the two prevailing instability mechanisms on a swept Parabolic Body which, so far, have only been studied separately and locally. A parameter study shows maximum modal growth for a spanwise wavenumber of β = 0.213, suggesting a preferred disturbance length scale in the sweep direction.
Christoph J. Mack - One of the best experts on this subject based on the ideXlab platform.
-
Global stability of swept flow around a Parabolic Body: the neutral curve
Journal of Fluid Mechanics, 2011Co-Authors: Christoph J. Mack, Peter J. SchmidAbstract:The onset of transition in the leading-edge region of a swept blunt Body depends crucially on the stability characteristics of the flow. Modelling this flow configuration by swept compressible flow around a Parabolic Body, a global approach is taken to extract pertinent stability information via a DNS-based iterative eigenvalue solver. Global modes combining features from boundary-layer and acoustic instabilities are presented. A parameter study, varying the spanwise disturbance wavenumber and the sweep Reynolds number, showed the existence of unstable boundary-layer and acoustic modes. The corresponding neutral curve displays two overlapping regions of exponential growth and two critical Reynolds numbers, one for boundary-layer instabilities and one for acoustic instabilities. The employed global approach establishes a first neutral curve, delineating stable from unstable parameter configurations, for the complex flow about a swept Parabolic Body with corresponding implications for swept leading-edge flow.
-
Global stability of swept flow around a Parabolic Body: features of the global spectrum
Journal of Fluid Mechanics, 2011Co-Authors: Christoph J. Mack, Peter J. SchmidAbstract:The global temporal stability of three-dimensional compressible flow about a yawed Parabolic Body of infinite span is investigated using an iterative eigenvalue technique in combination with direct numerical simulations. The computed global spectrum provides a comprehensive picture of the temporal perturbation dynamics of the flow, and a wide and rich variety of modes has been uncovered for the investigated parameter choices: stable and unstable boundary-layer modes, different types of stable and unstable acoustic modes, and stable wavepacket modes have been found. A parameter study varying the spanwise perturbation wavenumber and the sweep Reynolds number reproduced a preferred spanwise length scale and a critical Reynolds number for a boundary-layer or acoustic instability. Convex leading-edge curvature has been found to have a strongly stabilizing effect on boundary-layer modes but only a weakly stabilizing effect on acoustic modes. Furthermore, for certain parameter choices, the acoustic modes have been found to dominate the boundary-layer modes. © 2011 Cambridge University Press.
-
Direct numerical study of hypersonic flow about a swept Parabolic Body
Computers & Fluids, 2010Co-Authors: Christoph J. Mack, Peter J. SchmidAbstract:Direct numerical simulations (DNS) of hypersonic flow about a swept Parabolic Body have been performed to study the global stability of flow in the leading-edge region of a swept blunt Body. Previous stability investigations have been based on local models but have not fully succeeded in reproducing the established experimental findings. The current flow configuration represents a more realistic model and is thus expected to resolve some of the remaining questions. However, novel approaches like DNS-based global stability theory are necessary for such flow models and are employed in this study. As a result, boundary-layer modes have been identified by different but complementary techniques as the dominant instability mechanism. The DNS starting with small-amplitude white noise provide further evidence for the presence of non-modal effects which may be important in the subcritical regime. From a methodological point of view, the potential for quantitative flow analysis by combining numerical simulations with advanced iterative techniques represents a promising direction for investigating the governing physical processes of complex flows.
-
A preconditioned Krylov technique for global hydrodynamic stability analysis of large-scale compressible flows
Journal of Computational Physics, 2010Co-Authors: Christoph J. Mack, Peter J. SchmidAbstract:The combination of iterative Krylov-based eigenvalue algorithms and direct numerical simulations (DNS) has proven itself an effective and robust tool in solving complex global stability problems of compressible flows. A Cayley transformation is required to add flexibility to our stability solver and to allow access to specific parts of the full global spectrum which would be out of reach without such a transformation. In order to robustify the overall global stability solver an efficient ILU-based preconditioner has been implemented. With this Cayley-transformed DNS-based Krylov method two flow cases were successfully investigated: (i) a compressible mixing layer, a rather simple but well-known problem, which served as a test case and (ii) a supersonic flow about a swept Parabolic Body, a challenging large-scale flow configuration.
-
Global stability of swept flow around a Parabolic Body: connecting attachment-line and crossflow modes
Journal of Fluid Mechanics, 2008Co-Authors: Christoph J. Mack, Peter J. Schmid, Jörn SesterhennAbstract:The global linear stability of a three-dimensional compressible flow around a yawed Parabolic Body of infinite span is investigated using an iterative eigenvalue method in conjunction with direct numerical simulations. The computed global spectrum shows an unstable branch consisting of three-dimensional boundary layer modes whose amplitude distributions exhibit typical characteristics of both attachment-line and crossflow modes. In particular, global eigenfunctions with smaller phase velocities display a more pronounced structure near the stagnation line, reminiscent of attachment-line modes while still featuring strong crossflow vortices further downstream. This analysis establishes a link between the two prevailing instability mechanisms on a swept Parabolic Body which, so far, have only been studied separately and locally. A parameter study shows maximum modal growth for a spanwise wavenumber of β = 0.213, suggesting a preferred disturbance length scale in the sweep direction.
Peter Schmid - One of the best experts on this subject based on the ideXlab platform.
-
Global stability of swept flow around a Parabolic Body: The neutral curve
Journal of Fluid Mechanics, 2011Co-Authors: Christoph Mack, Peter SchmidAbstract:The onset of transition in the leading-edge region of a swept blunt Body depends crucially on the stability characteristics of the flow. Modelling this flow configuration by swept compressible flow around a Parabolic Body, a global approach is taken to extract pertinent stability information via a DNS-based iterative eigenvalue solver. Global modes combining features from boundary-layer and acoustic instabilities are presented. A parameter study, varying the spanwise disturbance wavenumber and the sweep Reynolds number, showed the existence of unstable boundary-layer and acoustic modes. The corresponding neutral curve displays two overlapping regions of exponential growth and two critical Reynolds numbers, one for boundary-layer instabilities and one for acoustic instabilities. The employed global approach establishes a first neutral curve, delineating stable from unstable parameter configurations, for the complex flow about a swept Parabolic Body with corresponding implications for swept leading-edge flow. © 2011 Cambridge University Press.
-
A preconditioned Krylov technique for global hydrodynamic stability analysis of large-scale compressible flows
Journal of Computational Physics, 2010Co-Authors: C.j. Mack, Peter SchmidAbstract:The combination of iterative Krylov-based eigenvalue algorithms and direct numerical simulations (DNS) has proven itself an effective and robust tool in solving complex global stability problems of compressible flows. A Cayley transformation is required to add flexibility to our stability solver and to allow access to specific parts of the full global spectrum which would be out of reach without such a transformation. In order to robustify the overall global stability solver an efficient ILU-based preconditioner has been implemented. With this Cayley-transformed DNS-based Krylov method two flow cases were successfully investigated: (i) a compressible mixing layer, a rather simple but well-known problem, which served as a test case and (ii) a supersonic flow about a swept Parabolic Body, a challenging large-scale flow configuration. © 2009 Elsevier Inc. All rights reserved.
-
Global stability of swept flow around a Parabolic Body: Connecting attachment-line and crossflow modes
Journal of Fluid Mechanics, 2008Co-Authors: Christoph J. Mack, Peter Schmid, Jörn SesterhennAbstract:The global linear stability of a three-dimensional compressible flow around a yawed Parabolic Body of infinite span is investigated using an iterative eigenvalue method in conjunction with direct numerical simulations. The computed global spectrum shows an unstable branch consisting of three-dimensional boundary layer modes whose amplitude distributions exhibit typical characteristics of both attachment-line and crossflow modes. In particular, global eigenfunctions with smaller phase velocities display a more pronounced structure near the stagnation line, reminiscent of attachment-line modes while still featuring strong crossflow vortices further downstream. This analysis establishes a link between the two prevailing instability mechanisms on a swept Parabolic Body which, so far, have only been studied separately and locally. A parameter study shows maximum modal growth for a spanwise wavenumber of ß = 0.213, suggesting a preferred disturbance length scale in the sweep direction. © 2008 Cambridge University Press.
Christoph Mack - One of the best experts on this subject based on the ideXlab platform.
-
Global stability of swept flow around a Parabolic Body: The neutral curve
Journal of Fluid Mechanics, 2011Co-Authors: Christoph Mack, Peter SchmidAbstract:The onset of transition in the leading-edge region of a swept blunt Body depends crucially on the stability characteristics of the flow. Modelling this flow configuration by swept compressible flow around a Parabolic Body, a global approach is taken to extract pertinent stability information via a DNS-based iterative eigenvalue solver. Global modes combining features from boundary-layer and acoustic instabilities are presented. A parameter study, varying the spanwise disturbance wavenumber and the sweep Reynolds number, showed the existence of unstable boundary-layer and acoustic modes. The corresponding neutral curve displays two overlapping regions of exponential growth and two critical Reynolds numbers, one for boundary-layer instabilities and one for acoustic instabilities. The employed global approach establishes a first neutral curve, delineating stable from unstable parameter configurations, for the complex flow about a swept Parabolic Body with corresponding implications for swept leading-edge flow. © 2011 Cambridge University Press.
-
Global stability of compressible flow about a swept Parabolic Body
2009Co-Authors: Christoph MackAbstract:The present thesis is concerned with the global stability of compressible flow in the leading-edge region of a swept blunt Body, a flow situation which can be found in many engineering applications. It is the goal of the present study to investigate a more comprehensive model and to thus gain new insight into the complex stability characteristics of this flow. To this end, two objectives have been pursued: (i) a powerful DNS-based global stability solver has been developed and (ii) this stability algorithm has then been employed to extract stability information from our flow model. The former objective has been accomplished by combining direct numerical simulations (DNS) and Krylov methods using a matrix-free implementation; furthermore, a Cayley transformation as well as preconditioning techniques are used to add, on the one hand, flexibility and, on the other hand, robustness and efficiency to our algorithm. The developed stability algorithm has then been employed to study the global stability of compressible flow about a swept Parabolic Body. The computed spectrum provides a comprehensive picture of the temporal perturbation dynamics of this flow and, as a result, a wide and rich variety of global modes has been uncovered: boundary-layer modes, different types of acoustic modes and wave packet modes have been found. Furthermore, boundary-layer modes connecting attachment-line and crossflow instabilities as well as composite global modes featuring both the structure of boundary-layer and acoustic instabilities have been computed. Moreover, the neutral curve for boundary-layer and acoustic modes has been presented.
P. W. Hammerton - One of the best experts on this subject based on the ideXlab platform.
-
Analysis of the unstable Tollmien-Schlichting mode on bodies with a rounded leading edge using the parabolized stability equation
Journal of Fluid Mechanics, 2009Co-Authors: M. R. Turner, P. W. HammertonAbstract:The interaction between free-stream disturbances and the boundary layer on a Body with a rounded leading edge is considered in this paper. A method which incorporates calculations using the parabolized stability equation in the Orr-Sommerfeld region, along with an upstream boundary condition derived from asymptotic theory in the vicinity of the leading edge, is generalized to bodies with an inviscid slip velocity which tends to a constant far downstream. We present results for the position of the lower branch neutral stability point and the magnitude of the unstable Tollmien-Schlichting (T-S) mode at this point for both a Parabolic Body and the Rankine Body. For the Rankine Body, which has an adverse pressure gradient along its surface far from the nose, we find a double maximum in the T-S wave amplitude for sufficiently large Reynolds numbers.
-
Effect of Nose Bluntness on Leading-Edge Receptivity
Instability Transition and Turbulence, 1992Co-Authors: P. W. Hammerton, E. J. KerschenAbstract:The effect of the curvature of the leading edge on boundary layer receptivity is analyzed using asymptotic methods supplemented by numerical results. The case of free-stream acoustic waves, propagating parallel to a symmetric mean flow past a Parabolic Body, is considered. The Body nose radius, r n , enters the theory through a Strouhal number, S = ωr n /U∞, where ω is the frequency of the acoustic wave and U∞ is the mean flow speed. The finite nose radius dramatically reduces the receptivity level, with the amplitude of the instability waves in the boundary layer being decreased by an order of magnitude when the value of S is only 0.3.
