Low Frequency Oscillation of Laminar Separation Bubble Near Stall-Discussion on Turbulent Energy Production-

High-fidelity large eddy simulation is carried out for the flow field around a NACA-0012 aerofoil at Reynolds number of [Formula: see text], Mach number of 0.4, and various angles of attack around the onset of stall. The laminar separation bubble is formed on the suction surface of the aerofoil and is constituted by the reattached shear layer. At these conditions, the laminar separation bubble is unstable and switches between a short bubble and an open bubble. The instability of the laminar separation bubble triggers a low-frequency flow oscillation. The aerodynamic coefficients oscillate accordingly at a low frequency. The lift and the drag coefficients compare very well to recent high-accuracy experimental data, and the lift leads the drag by a phase shift of [Formula: see text]. The mean lift coefficient peaks at the angle of attack of [Formula: see text], in total agreement with the experimental data. The spectra of the lift coefficient does not show a significant low-frequency peak at angles of attack lower than or equal the stall angle of attack ([Formula: see text]). At higher angles of attack, the spectra show two low-frequency peaks and the low-frequency flow oscillation is fully developed at the angle of attack of [Formula: see text]. The behaviour of the flow-field and changes in the turbulent kinetic energy over one low-frequency flow oscillation cycle are described qualitatively.

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Instability and low-frequency unsteadiness in a shock-induced laminar separation bubble

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.297 ◽

2016 ◽

Vol 798 ◽

pp. 5-26 ◽

Cited By ~ 16

Author(s):

Andrea Sansica ◽

Neil D. Sandham ◽

Zhiwei Hu

Keyword(s):

Boundary Layer ◽

Linear Stability ◽

Flow Instability ◽

Separation Bubble ◽

Low Frequency ◽

Boundary Layer Transition ◽

Separation Point ◽

Oblique Shock Wave ◽

Laminar Separation Bubble ◽

Laminar Separation

Three-dimensional direct numerical simulations (DNS) of a shock-induced laminar separation bubble are carried out to investigate the flow instability and origin of any low-frequency unsteadiness. A laminar boundary layer interacting with an oblique shock wave at $M=1.5$ is forced at the inlet with a pair of monochromatic oblique unstable modes, selected according to local linear stability theory (LST) performed within the separation bubble. Linear stability analysis is applied to cases with marginal and large separation, and compared to DNS. While the parabolized stability equations approach accurately reproduces the growth of unstable modes, LST performs less well for strong interactions. When the modes predicted by LST are used to force the separated boundary layer, transition to deterministic turbulence occurs near the reattachment point via an oblique-mode breakdown. Despite the clean upstream condition, broadband low-frequency unsteadiness is found near the separation point with a peak at a Strouhal number of $0.04$, based on the separation bubble length. The appearance of the low-frequency unsteadiness is found to be due to the breakdown of the deterministic turbulence, filling up the spectrum and leading to broadband disturbances that travel upstream in the subsonic region of the boundary layer, with a strong response near the separation point. The existence of the unsteadiness is supported by sensitivity studies on grid resolution and domain size that also identify the region of deterministic breakdown as the source of white noise disturbances. The present contribution confirms the presence of low-frequency response for laminar flows, similarly to that found in fully turbulent interactions.

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Experimental Investigation on the Time–Space Evolution of a Laminar Separation Bubble by Proper Orthogonal Decomposition and Dynamic Mode Decomposition

Journal of Turbomachinery ◽

10.1115/1.4034917 ◽

2016 ◽

Vol 139 (3) ◽

Cited By ~ 10

Author(s):

D. Lengani ◽

D. Simoni ◽

M. Ubaldi ◽

P. Zunino ◽

F. Bertini

Keyword(s):

Separation Bubble ◽

Low Frequency ◽

Dynamic Mode Decomposition ◽

Dynamic Mode ◽

Laminar Separation Bubble ◽

Laminar Separation ◽

Time Space ◽

Mode Decomposition ◽

Low Frequency Motion ◽

Proper Orthogonal

A time-resolved particle image velocimetry (TR-PIV) system has been employed to investigate a laminar separation bubble which is induced by a strong adverse pressure gradient typical of ultrahigh-lift low-pressure turbine (LPT) blades. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) are described and applied within this paper. These techniques allow reducing the degrees-of-freedom of complex systems producing a low-order model ranked by the energy content (POD) or by the modal contribution to the dynamics of the system itself (DMD), useful to highlight the dominant dynamics. The time–space evolution of the laminar separation bubble is characterized by rollup vortices shed in the surrounding of the bubble maximum displacement as a consequence of the Kelvin–Helmholtz (KH) instability process as well as by a low-frequency motion of the separated shear layer. The decomposition techniques proposed allow the identification of these coherent structures and the characterization of their modal properties (e.g., temporal frequency, spatial wavelength, and growth rate). The POD separates the different dynamics that induce velocity fluctuations at different frequencies and wavelength looking at their contribution to the overall kinetic energy. The DMD provides complementary information: the unstable spatial frequencies are identified with their growth (or decay) rates. DMD modes associated with the Kelvin–Helmholtz instability and the corresponding vortex shedding phenomenon clearly dominate the unsteady behavior of the laminar separation bubble, being characterized by the highest growth rate. Modes with longer wavelength describe the low-frequency motion of the laminar separation bubble and are neutrally stable. Results reported in this paper prove the ability of the present methods in extracting the dominant dynamics from a large dataset, providing robust and rapid tools for the in depth analysis of transition and separation processes.

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Experimental Investigation on the Time-Space Evolution of a Laminar Separation Bubble by POD and DMD

Volume 2B: Turbomachinery ◽

10.1115/gt2016-57581 ◽

2016 ◽

Author(s):

D. Lengani ◽

D. Simoni ◽

M. Ubaldi ◽

P. Zunino ◽

F. Bertini

Keyword(s):

Growth Rate ◽

Separation Bubble ◽

Temporal Frequency ◽

Low Frequency ◽

Dynamic Mode ◽

Laminar Separation Bubble ◽

Helmholtz Instability ◽

Laminar Separation ◽

Time Space ◽

Low Frequency Motion

A time resolved Particle Image Velocimetry (TR-PIV) system has been employed to investigate a laminar separation bubble which is induced by a strong adverse pressure gradient typical of Ultra-High-Lift lo pressure turbine blades. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are described and applied within the paper. These techniques allow reducing the degrees of freedom of complex systems producing a low order model ranked by the energy content (POD) or by the modal contribution to the dynamics of the system itself (DMD), useful to highlight the dominant dynamics. The time-space evolution of the laminar separation bubble is characterized by rollup vortices shed in the surround of the bubble maximum displacement as a consequence of the Kelvin-Helmholtz instability process as well as by a low frequency motion of the separated shear layer. The decomposition techniques proposed allow the identification of these coherent structures and the characterization of their modal properties (e.g. temporal frequency, spatial wavelength and growth rate). The POD separates the different dynamics that induce velocity fluctuations at different frequencies and wavelength looking at their contribution to the overall kinetic energy. The DMD provides complementary information: the unstable spatial frequencies are identified with their growth (or decay) rates. DMD modes associated with the Kelvin-Helmholtz instability and the corresponding vortex shedding phenomenon clearly dominate the unsteady behavior of the laminar separation bubble, being characterized by the highest growth rate. Modes with longer wavelength describe the low frequency motion of the laminar separation bubble and are neutrally stable. Results reported in the paper prove the ability of the present methods in extracting the dominant dynamics from a large dataset, providing robust and rapid tools for the in dept analysis of transition and separation processes.

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