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Andrea Sansica - One of the best experts on this subject based on the ideXlab platform.
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instability and low frequency unsteadiness in a shock induced laminar Separation bubble
Journal of Fluid Mechanics, 2016Co-Authors: Andrea Sansica, Neil D Sandham, Zhiwei HuAbstract: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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forced response of a laminar shock induced Separation bubble
Physics of Fluids, 2014Co-Authors: Andrea Sansica, Neil D Sandham, Zhiwei HuAbstract:The source of unsteadiness in shock-wave/boundary-layer interactions is currently disputed. This paper considers a two-dimensional Separation bubble induced by an oblique shock wave interacting with a laminar boundary layer at a free-stream Mach number of 1.5. The global response of the separated region to white noise forcing is analyzed for different interaction strengths, which generate small and large Separation bubbles. Forcing location and amplitude effects have been examined. For both interaction strengths and for forcing both upstream and inside the bubble, the wall-pressure spectra downstream of the Separation show a high-frequency peak that is demonstrated to be a Kelvin-Helmholtz instability. A low-frequency response at the Separation Point is also found when the Separation bubble is only forced internally, therefore with a disturbance-free upstream boundary layer. For low-amplitude internal forcing, the low-frequency response at the Separation Point and downstream of the bubble is linear. Howev...
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Forced response of a laminar shock-induced Separation bubble
Physics of Fluids, 2014Co-Authors: Andrea Sansica, Neil D SandhamAbstract:The source of unsteadiness in shock-wave/boundary-layer interactions is currently disputed. This paper considers a two-dimensional Separation bubble induced by an oblique shock wave interacting with a laminar boundary layer at a free-stream Mach number of 1.5. The global response of the separated region to white noise forcing is analyzed for different interaction strengths, which generate small and large Separation bubbles. Forcing location and amplitude effects have been examined. For both interaction strengths and for forcing both upstream and inside the bubble, the wall-pressure spectra downstream of the Separation show a high-frequency peak that is demonstrated to be a Kelvin-Helmholtz instability. A low-frequency response at the Separation Point is also found when the Separation bubble is only forced internally, therefore with a disturbance-free upstream boundary layer. For low-amplitude internal forcing, the low-frequency response at the Separation Point and downstream of the bubble is linear. However, when forced upstream the low-frequency unsteadiness of the large Separation bubble is found to be driven by nonlinearities coming from the downstream shedding. The same nonlinear behavior is found when the Separation bubble is internally forced over a narrow band around the shedding frequency, without low-frequency disturbances. This analysis for a laminar interaction is used to interpret the low-frequency unsteadiness found at the foot of the shock of turbulent interactions. Here, the low-frequency unsteadiness occurs in the absence of upstream disturbances and a linear relationship is found between the internal forcing and the response near the Separation Point. When low-frequencies are not present in the forcing they are generated from weak nonlinearities of the shear-layer instability modes.
Zhiwei Hu - One of the best experts on this subject based on the ideXlab platform.
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instability and low frequency unsteadiness in a shock induced laminar Separation bubble
Journal of Fluid Mechanics, 2016Co-Authors: Andrea Sansica, Neil D Sandham, Zhiwei HuAbstract: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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forced response of a laminar shock induced Separation bubble
Physics of Fluids, 2014Co-Authors: Andrea Sansica, Neil D Sandham, Zhiwei HuAbstract:The source of unsteadiness in shock-wave/boundary-layer interactions is currently disputed. This paper considers a two-dimensional Separation bubble induced by an oblique shock wave interacting with a laminar boundary layer at a free-stream Mach number of 1.5. The global response of the separated region to white noise forcing is analyzed for different interaction strengths, which generate small and large Separation bubbles. Forcing location and amplitude effects have been examined. For both interaction strengths and for forcing both upstream and inside the bubble, the wall-pressure spectra downstream of the Separation show a high-frequency peak that is demonstrated to be a Kelvin-Helmholtz instability. A low-frequency response at the Separation Point is also found when the Separation bubble is only forced internally, therefore with a disturbance-free upstream boundary layer. For low-amplitude internal forcing, the low-frequency response at the Separation Point and downstream of the bubble is linear. Howev...
Neil D Sandham - One of the best experts on this subject based on the ideXlab platform.
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instability and low frequency unsteadiness in a shock induced laminar Separation bubble
Journal of Fluid Mechanics, 2016Co-Authors: Andrea Sansica, Neil D Sandham, Zhiwei HuAbstract: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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forced response of a laminar shock induced Separation bubble
Physics of Fluids, 2014Co-Authors: Andrea Sansica, Neil D Sandham, Zhiwei HuAbstract:The source of unsteadiness in shock-wave/boundary-layer interactions is currently disputed. This paper considers a two-dimensional Separation bubble induced by an oblique shock wave interacting with a laminar boundary layer at a free-stream Mach number of 1.5. The global response of the separated region to white noise forcing is analyzed for different interaction strengths, which generate small and large Separation bubbles. Forcing location and amplitude effects have been examined. For both interaction strengths and for forcing both upstream and inside the bubble, the wall-pressure spectra downstream of the Separation show a high-frequency peak that is demonstrated to be a Kelvin-Helmholtz instability. A low-frequency response at the Separation Point is also found when the Separation bubble is only forced internally, therefore with a disturbance-free upstream boundary layer. For low-amplitude internal forcing, the low-frequency response at the Separation Point and downstream of the bubble is linear. Howev...
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Forced response of a laminar shock-induced Separation bubble
Physics of Fluids, 2014Co-Authors: Andrea Sansica, Neil D SandhamAbstract:The source of unsteadiness in shock-wave/boundary-layer interactions is currently disputed. This paper considers a two-dimensional Separation bubble induced by an oblique shock wave interacting with a laminar boundary layer at a free-stream Mach number of 1.5. The global response of the separated region to white noise forcing is analyzed for different interaction strengths, which generate small and large Separation bubbles. Forcing location and amplitude effects have been examined. For both interaction strengths and for forcing both upstream and inside the bubble, the wall-pressure spectra downstream of the Separation show a high-frequency peak that is demonstrated to be a Kelvin-Helmholtz instability. A low-frequency response at the Separation Point is also found when the Separation bubble is only forced internally, therefore with a disturbance-free upstream boundary layer. For low-amplitude internal forcing, the low-frequency response at the Separation Point and downstream of the bubble is linear. However, when forced upstream the low-frequency unsteadiness of the large Separation bubble is found to be driven by nonlinearities coming from the downstream shedding. The same nonlinear behavior is found when the Separation bubble is internally forced over a narrow band around the shedding frequency, without low-frequency disturbances. This analysis for a laminar interaction is used to interpret the low-frequency unsteadiness found at the foot of the shock of turbulent interactions. Here, the low-frequency unsteadiness occurs in the absence of upstream disturbances and a linear relationship is found between the internal forcing and the response near the Separation Point. When low-frequencies are not present in the forcing they are generated from weak nonlinearities of the shear-layer instability modes.
A I Ruban - One of the best experts on this subject based on the ideXlab platform.
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on unsteady boundary layer Separation in supersonic flow part 1 upstream moving Separation Point
Journal of Fluid Mechanics, 2011Co-Authors: A I Ruban, D Araki, R Yapalparvi, J S B GajjarAbstract:This study is concerned with the boundary-layer Separation from a rigid body surface in unsteady two-dimensional laminar supersonic flow. The Separation is assumed to be provoked by a shock wave impinging upon the boundary layer at a Point that moves with speed V sh along the body surface. The strength of the shock and its speed V sh are allowed to vary with time t, but not too fast, namely, we assume that the characteristic time scale t « Re -1/2 / V 2 w . Here Re denotes the Reynolds number, and V w =-V sh is wall velocity referred to the gas velocity V ∞ in the free stream. We show that under this assumption the flow in the region of interaction between the shock and boundary layer may be treated as quasi-steady if it is considered in the coordinate frame moving with the shock. We start with the flow regime when V w = O(Re -1/8 ). In this case, the interaction between the shock and boundary layer is described by classical triple-deck theory. The main modification to the usual triple-deck formulation is that in the moving frame the body surface is no longer stationary; it moves with the speed V w =-V sh . The corresponding solutions of the triple-deck equations have been constructed numerically. For this purpose, we use a numerical technique based on finite differencing along the streamwise direction and Chebyshev collocation in the direction normal to the body surface. In the second part of the paper, we assume that 1 » V w » O(Re -1/8 ), and concentrate our attention on the self-induced Separation of the boundary layer. Assuming, as before, that the Reynolds number, Re, is large, the method of matched asymptotic expansions is used to construct the corresponding solutions of the Navier-Stokes equations in a vicinity of the Separation Point.
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on laminar Separation at a corner Point in transonic flow
Journal of Fluid Mechanics, 2000Co-Authors: A I Ruban, Ibrahim TurkyilmazAbstract:The Separation of the laminar boundary layer from a convex corner on a rigid body contour in transonic flow is studied based on the asymptotic analysis of the Navier-Stokes equations at large values of the Reynolds number. It is shown that the flow in a small vicinity of the Separation Point is governed, as usual, by strong interaction between the boundary layer and the inviscid part of the flow. Outside the interaction region the Karman-Guderley equation describing transonic inviscid flow admits a self-similar solution with the pressure on the body surface being proportional to the cubic root of the distance from the Separation Point. Analysis of the boundary layer driven by this pressure shows that as the interaction region is approached the boundary layer splits into two parts: the near-wall viscous sublayer and the main body of the boundary layer where the flow is locally inviscid. It is interesting that contrary to what happens in subsonic and supersonic flows, the displacement effect of the boundary layer is primarily due to the inviscid part. The contribution of the viscous sublayer proves to be negligible to the leading order. Consequently, the flow in the interaction region is governed by the inviscid-inviscid interaction
H Babinsky - One of the best experts on this subject based on the ideXlab platform.
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boundary layer vortex sheet evolution around an accelerating and rotating cylinder
Journal of Fluid Mechanics, 2021Co-Authors: Pascal Gehlert, H BabinskyAbstract:The evolution of the boundary layer vortex sheet on a rotating and translating accelerating circular cylinder at Reynolds numbers of 10 000 and 20 000 is investigated using planar particle image velocimetry. The vortex sheet is decomposed into contributions resulting from translation and rotation as well as from local and far-field vorticity. Their individual development is explored to understand the overall time history of the boundary layer as well as its evolution at the unsteady Separation Point. The boundary layer vortex sheet distribution changes considerably throughout the motion as well as between different kinematic cases. The same is observed for the vortex sheet strength at the unsteady Separation Point. A non-dimensional parameter is proposed which removes the effect of rotation rate, instantaneous velocity and shed vorticity accumulating in the far field. It was found that this was successful at collapsing the vortex sheet strength at the unsteady Separation Point during cylinder motion as well as for the individual kinematic test cases investigated. This confirms that cylinder kinematics and far-field vorticity are driving factors contributing to the development of the unsteady boundary layer and its strength at the Separation Point.
