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Serhiy Yarusevych - One of the best experts on this subject based on the ideXlab platform.
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The effects of three-dimensional forcing on flow development within a laminar Separation Bubble
2020Co-Authors: John Kurelek, Serhiy Yarusevych, Marios KotsonisAbstract:This work examines flow development in a laminar Separation Bubble (LSB) undergoing natural transition and transition controlled with two-dimensional and spanwise modulated disturbances. The investigation is carried out in a series of wind tunnel tests, with the Separation Bubble formed over a flat plate subjected to an adverse pressure gradient. Velocity field measurements are performed using time-resolved, two-component Particle Image Velocimetry (PIV). Disturbances are produced using surface-mounted plasma actuators in a novel configuration that allows for the introduction of controlled disturbances that are two-dimensional or of a prescribed spanwise wavelength. The natural transition process is dominated by shear layer vortex shedding which is characterized by significant spanwise deformations in the aft portion of the Bubble. When the flow is subjected to either two or three-dimensional forcing, vortex formation within the Separation Bubble is rendered two-dimensional. However, while the two-dimensionally forced perturbations remain largely two-dimensional until breakdown, a clear spanwise wavelength that matches the input wavelength of the forcing develops when the flow is subjected to the spanwise modulated forcing. The reported findings point to the presence of a secondary instability in the Separation Bubble, which leads to the amplification of the initially weak spanwise component of input disturbances, causing the shear layer vortices to develop significant spanwise undulations.
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Spanwise flow structures within a laminar Separation Bubble on an airfoil
2020Co-Authors: Theodoros Michelis, Marios Kotsonis, Serhiy YarusevychAbstract:The present study considers the development of a Laminar Separation Bubble on the suction side of a NACA0018 airfoil under natural and forced conditions. Deterministic forcing is applied by means of a two-dimensional plasma actuator installed on the airfoil surface. The spatiotemporal characteristics of the Bubble are measured using time-resolved, two-component Particle Image Velocimetry in streamwise and spanwise planes. Analysis of the results shows that while the time-average Bubble is strongly two-dimensional, the dominant coherent structures assume three dimensional organisation in the vicinity of laminarturbulent breakdown in both natural and forced conditions.
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Surface-Pressure-Based Estimation of the Velocity Field in a Separation Bubble
AIAA Journal, 2019Co-Authors: Burak Ahmet Tuna, John Kurelek, Serhiy YarusevychAbstract:The effectiveness in estimating the velocity field in a laminar Separation Bubble using surface-pressure-based stochastic estimation methods is examined. A Separation Bubble is formed over a NACA 0...
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transition in a Separation Bubble under tonal and broadband acoustic excitation
Journal of Fluid Mechanics, 2018Co-Authors: John Kurelek, Marios Kotsonis, Serhiy YarusevychAbstract:Transition and flow development in a Separation Bubble formed on an airfoil are studied experimentally. The effects of tonal and broadband acoustic excitation are considered since such acoustic emissions commonly result from airfoil self-noise and can influence flow development via a feedback loop. This interdependence is decoupled, and the problem is studied in a controlled manner through the use of an external acoustic source. The flow field is assessed using time-resolved, two-component particle image velocimetry, the results of which show that, for equivalent energy input levels, tonal and broadband excitation can produce equivalent changes in the mean Separation Bubble topology. These changes in topology result from the influence of excitation on transition and the subsequent development of coherent structures in the Bubble. Both tonal and broadband excitation lead to earlier shear layer roll-up and thus reduce the Bubble size and advance mean reattachment upstream, while the shed vortices tend to persist farther downstream of mean reattachment in the case of tonal excitation. Through a cross-examination of linear stability theory (LST) predictions and measured disturbance characteristics, nonlinear disturbance interactions are shown to play a crucial role in the transition process, leading to significantly different disturbance development for the tonal and broadband excited flows. Specifically, tonal excitation results in transition being dominated by the excited mode, which grows in strong accordance with linear theory and damps the growth of all other disturbances. On the other hand, disturbance amplitudes are more moderate for the natural and broadband excited flows, and so all unstable disturbances initially grow in accordance with LST. For all cases, a rapid redistribution of perturbation energy to a broad range of frequencies follows, with the phenomenon occurring earliest for the broadband excitation case. By taking nonlinear effects into consideration, important ramifications are made clear in regards to comparing LST predictions and experimental or numerical results, thus explaining the trends reported in recent investigations. These findings offer new insights into the influence of tonal and broadband noise emissions, resulting from airfoil self-noise or otherwise, on transition and flow development within a Separation Bubble.
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effects of free stream turbulence intensity on transition in a laminar Separation Bubble formed over an airfoil
Experiments in Fluids, 2018Co-Authors: Mark S Istvan, Serhiy YarusevychAbstract:The laminar-to-turbulent transition process in a laminar Separation Bubble formed over a NACA 0018 airfoil is investigated experimentally. All experiments are performed for an angle of attack of 4\(^{\circ }\), chord Reynolds numbers of 80,000 and 125,000, and free-stream turbulence intensities between 0.06 and 1.99%. The results show that increasing the level of free-stream turbulence intensity leads to a decrease in Separation Bubble length, attributed to a downstream shift in mean Separation and an upstream shift in mean reattachment, the later ascribed to an upstream shift in mean transition. Maximum spatial amplification rates of disturbances in the separated shear layer decrease with increasing free-stream turbulence intensity, implying that the larger initial amplitudes of disturbances are solely responsible for the upstream shift in mean transition and as a result mean reattachment. At the baseline level of turbulence intensity, coherent structures forming in the aft portion of the Bubble are characterized by strong spanwise coherence at formation, and undergo spanwise deformations leading to localized breakup in the vicinity of mean reattachment. As the level of free-stream turbulence intensity is increased, the spanwise coherence of the shear layer rollers is reduced, and spanwise undulations in the vortex filaments start to take place at the mean location of roll-up. At the highest level of turbulence intensity investigated, streamwise streaks originating in the boundary layer upstream of the Separation Bubble are observed within the Bubble. These streaks signify an onset of bypass transition upstream of the Separation Bubble, which gives rise to a highly three-dimensional shear layer roll-up. A quantitative analysis of the associated changes in salient characteristics of the coherent structures is presented, connecting the effect of elevated free-stream turbulence intensity on the time-averaged and dynamic characteristics of the Separation Bubble.
Ulrich Rist - One of the best experts on this subject based on the ideXlab platform.
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DNS AND LES OF THE TRANSITION PROCESS IN A LAMINAR Separation Bubble
Direct and Large-Eddy Simulation V, 2020Co-Authors: Olaf Marxen, Ulrich RistAbstract:A region of strong local adverse pressure gradient acting on a laminar flat-plate boundary layer can produce a closed fully laminar Separation Bubble for sufficiently small pressure rise and Reynolds number. However, such a flow field is hydrodynamically highly unstable and transition will occur in the region of adverse pressure gradient. Due to an interaction with the potential flow, the transition process may even suppress laminar Separation completely.
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Active Control of a Laminar Separation Bubble
Aerodynamic Drag Reduction Technologies, 2020Co-Authors: Kai Augustin, Ulrich Rist, Siegfried WagnerAbstract:In the present paper an active control mechanism for the control of laminar Separation Bubbles on airfoils is investigated by means of direct numerical simulation and linear stability theory. Boundary layer instabilities excited by periodic oscillations are utilized to control the size and length of the Separation Bubble and to make it finally disappear when desired. Unlike traditional vortex generators a sensor-actuator system based on this method will be adaptive to the respective flow conditions and will cause no additional undesired drag.
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Numerical Simulation of Laminar Separation-Bubble Control
New Results in Numerical and Experimental Fluid Mechanics III, 2020Co-Authors: Ulrich Rist, Kai Augustin, Siegfried WagnerAbstract:In the present paper an active control mechanism for the control of laminar Separation Bubbles on airfoils is investigated by means of direct numerical simulation and linear stability theory. Boundary layer instabilities excited by periodic oscillations are utilized to control the size and length of the Separation Bubble and to make it finally disappear when desired. Unlike traditional vortex generators a sensor-actuator system based on this method will be adaptive to the respective flow conditions and will cause no additional undesired drag.
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Direct Numerical Simulation of Non-Linear Transitional Stages in an Experimentally Investigated Laminar Separation Bubble
High Performance Computing in Science and Engineering’ 05, 2020Co-Authors: Olaf Marxen, Ulrich RistAbstract:This paper details a joint numerical and experimental effort to investigate a transition process in a laminar Separation Bubble, with the emphasis being put on the numerical contribution. A laminar Separation Bubble is formed if a laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes transition, leading to a reattached turbulent boundary layer. Development of disturbances during the transition process in such a Separation Bubble is studied by means of direct numerical simulation with controlled disturbance input. Focus is put on the stage of non-linear development of these perturbations, for which a detailed comparison between numerical and experimental results is given. Beside physical phenomena like shear-layer roll-up and vortex shedding, computational aspects such as the performance of the numerical code on supercomputers are treated.
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Experimental and Numerical Investigations on Transition in a Laminar Separation Bubble
New Results in Numerical and Experimental Fluid Mechanics III, 2020Co-Authors: M Lang, Olaf Marxen, Ulrich Rist, Siegfried WagnerAbstract:A laminar boundary layer separates in a region of adverse pressure gradient on a flat plate, undergoes transition, and finally the turbulent boundary layer reattaches. Laminar-turbulent transition within this laminar Separation Bubble (LSB) is investigated by means of measurements with a Laser-Doppler-Anemometer (LDA), flow visualization in water and direct numerical simulation (DNS). The role of unsteady disturbances with and without controlled spanwise variation in the occuring mechanism of transition are examined in detail.
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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stability and receptivity characteristics of a laminar Separation Bubble on an aerofoil
Journal of Fluid Mechanics, 2010Co-Authors: Lloyd E Jones, Richard D Sandberg, Neil D SandhamAbstract:Stability characteristics of aerofoil flows are investigated by linear stability analysis of time-averaged velocity profiles and by direct numerical simulations with timedependent forcing terms. First the wake behind an aerofoil is investigated, illustrating the feasibility of detecting absolute instability using these methods. The time-averaged flow around an NACA-0012 aerofoil at incidence is then investigated in terms of its response to very low-amplitude hydrodynamic and acoustic perturbations. Flow fields obtained from both two- and three-dimensional simulations are investigated, for which the aerofoil flow exhibits a laminar Separation Bubble. Convective stability characteristics are documented, and the Separation Bubble is found to exhibit no absolute instability in the classical sense; i.e. no growing disturbances with zero group velocity are observed. The flow is however found to be globally unstable via an acoustic-feedback loop involving the aerofoil trailing edge as a source of acoustic excitation and the aerofoil leading-edge region as a site of receptivity. Evidence suggests that the feedback loop may play an important role in frequency selection of the vortex shedding that occurs in two dimensions. Further simulations are presented to investigate the receptivity process by which acoustic waves generate hydrodynamic instabilities within the aerofoil boundary layer. The dependency of the receptivity process to both frequency and source location is quantified. It is found that the amplitude of trailing-edge noise in the fully developed simulation is sufficient to promote transition via leading-edge receptivity.
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strong interaction of a turbulent spot with a shock induced Separation Bubble
Physics of Fluids, 2007Co-Authors: L Krishnan, Neil D SandhamAbstract:Direct numerical simulations have been conducted to study the passage of a turbulent spot through a shock-induced Separation Bubble. Localized blowing is used to trip the boundary layer well upstream of the shock impingement, leading to mature turbulent spots at impingement, with a length comparable to the length of the Separation zone. Interactions are simulated at free stream Mach numbers of two and four, for isothermal (hot) wall boundary conditions. The core of the spot is seen to tunnel through the Separation Bubble, leading to a transient reattachment of the flow. Recovery times are long due to the influence of the calmed region behind the spot. The propagation speed of the trailing interface of the spot decreases during the interaction and a substantial increase in the lateral spreading of the spot was observed. A conceptual model based on the growth of the lateral shear layer near the wingtips of the spot is used to explain the change in lateral growth rate.
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On the response of shock-induced Separation Bubble to small amplitude disturbances
Modern Physics Letters B, 2005Co-Authors: L Krishnan, Neil D Sandham, Graham T. RobertsAbstract:Numerical simulations of an oblique shock interacting with a compressible laminar boundary layer are reported. The Mach number ranges from 2 to 6.85, while the Reynolds number based on the distance to the impingement location is fixed at 3 × 105. All the simulations are carried out with a constant wall temperature, equal to the adiabatic recovery temperature. At higher shock strength the evolved Separation Bubbles are taller and are biased towards the upstream side of the impingement location with an asymmetrical structure. Existing similarity scalings for the Bubble length need to be modified for the high Mach number range. Introduction of small amplitude disturbances upstream of the Separation Bubble resulted in the growth of organised streamwise structures downstream of the Bubble.
Olaf Marxen - One of the best experts on this subject based on the ideXlab platform.
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DNS AND LES OF THE TRANSITION PROCESS IN A LAMINAR Separation Bubble
Direct and Large-Eddy Simulation V, 2020Co-Authors: Olaf Marxen, Ulrich RistAbstract:A region of strong local adverse pressure gradient acting on a laminar flat-plate boundary layer can produce a closed fully laminar Separation Bubble for sufficiently small pressure rise and Reynolds number. However, such a flow field is hydrodynamically highly unstable and transition will occur in the region of adverse pressure gradient. Due to an interaction with the potential flow, the transition process may even suppress laminar Separation completely.
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Direct Numerical Simulation of a Short Laminar Separation Bubble and Early Stages of the Bursting Process
Notes on Numerical Fluid Mechanics and Multidisciplinary Design (NNFM), 2020Co-Authors: Olaf Marxen, Dan S. HenningsonAbstract:Direct numerical simulation of a pressure-induced short laminar Separation Bubble developing on a flat plate has been carried out. Transition in this Bubble was triggered by small disturbance input with a fixed frequency and fixed spanwise wave number. The resulting short Bubble was shown to be converged in time to a statistically steady state, while possessing essential features of short laminar Separation Bubbles as reported in the literature. In the present case disturbance input is required to maintain a short Bubble. Switching off this disturbance input yields a growing Separation Bubble. This phenomenon is denoted as Bubble bursting, since indication is found that the Bubble develops towards a long-Bubble state.
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Direct Numerical Simulation of Non-Linear Transitional Stages in an Experimentally Investigated Laminar Separation Bubble
High Performance Computing in Science and Engineering’ 05, 2020Co-Authors: Olaf Marxen, Ulrich RistAbstract:This paper details a joint numerical and experimental effort to investigate a transition process in a laminar Separation Bubble, with the emphasis being put on the numerical contribution. A laminar Separation Bubble is formed if a laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes transition, leading to a reattached turbulent boundary layer. Development of disturbances during the transition process in such a Separation Bubble is studied by means of direct numerical simulation with controlled disturbance input. Focus is put on the stage of non-linear development of these perturbations, for which a detailed comparison between numerical and experimental results is given. Beside physical phenomena like shear-layer roll-up and vortex shedding, computational aspects such as the performance of the numerical code on supercomputers are treated.
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Experimental and Numerical Investigations on Transition in a Laminar Separation Bubble
New Results in Numerical and Experimental Fluid Mechanics III, 2020Co-Authors: M Lang, Olaf Marxen, Ulrich Rist, Siegfried WagnerAbstract:A laminar boundary layer separates in a region of adverse pressure gradient on a flat plate, undergoes transition, and finally the turbulent boundary layer reattaches. Laminar-turbulent transition within this laminar Separation Bubble (LSB) is investigated by means of measurements with a Laser-Doppler-Anemometer (LDA), flow visualization in water and direct numerical simulation (DNS). The role of unsteady disturbances with and without controlled spanwise variation in the occuring mechanism of transition are examined in detail.
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A Combined Numerical and Experimental Investigation of Transition in a Laminar Separation Bubble
Recent Results in Laminar-Turbulent Transition, 2020Co-Authors: M Lang, Olaf Marxen, Ulrich Rist, Siegfried WagnerAbstract:A laminar boundary layer separates in a region of adverse pressure gradient, undergoes transition, and finally the turbulent boundary layer reattaches, forming a laminar Separation Bubble (LSB). Laminar-turbulent transition within such a LSB is investigated by means of Laser-Doppler-Anemometry (LDA), Particle Image Velocimetry (mono PIV and stereoscopic PIV (SPIV)) and direct numerical simulation (DNS).
Metin I. Yaras - One of the best experts on this subject based on the ideXlab platform.
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interaction of viscous and inviscid instability modes in Separation Bubble transition
Physics of Fluids, 2011Co-Authors: Joshua R Brinkerhoff, Metin I. YarasAbstract:This paper describes numerical simulations that are used to examine the interaction of viscous and inviscid instability modes in laminar-to-turbulent transition in a Separation Bubble. The results of a direct numerical simulation are presented in which Separation of a laminar boundary-layer occurs in the presence of an adverse streamwise pressure gradient. The simulation is performed at low freestream-turbulence levels and at a flow Reynolds number and pressure distribution approximating those typically encountered on the suction side of low-pressure turbine blades in a gas-turbine engine. The simulation results reveal the development of a viscous instability upstream of the point of Separation which produces streamwise-oriented vortices in the attached laminar boundary layer. These vortices remain embedded in the flow downstream of Separation and are carried into the separated shear layer, where they are amplified by the local adverse pressure-gradient and contribute to the formation of coherent hairpin-...
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Instability and Transition in a Separation Bubble Under a Three-Dimensional Freestream Pressure Distribution
Journal of Turbomachinery-transactions of The Asme, 2011Co-Authors: Metin I. YarasAbstract:This paper presents measurements of the instability and transition processes in Separation Bubbles under a three-dimensional freestream pressure distribution. The measurements are performed on a flat plate on which a pressure distribution is imposed by a contoured surface facing the flat test-surface. The three-dimensional pressure distribution that is established on the test-surface approximates the pressure distributions encountered on swept blades. This type of pressure field produces crossflows in the laminar boundary layer upstream of the Separation and within the Separation Bubble. The effects of these crossflows on the instability of the upstream boundary layer and on the instability, transition onset, and transition rate within the separated shear-layer are examined. The measurements are performed at two flow-Reynolds numbers and relatively low level of freestream turbulence. The results of this experimental study show that the three-dimensional freestream pressure field and the corresponding redistribution of the freestream flow can cause significant spanwise variation in the Separation-Bubble structure. It is demonstrated that the instability and transition processes in the modified Separation Bubble develop on the basis of the same fundamentals as in two-dimensional Separation Bubbles and can be predicted with the same level of accuracy using models that have been developed for two-dimensional Separation Bubbles.
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Instability and Transition in a Separation Bubble Under a Three-Dimensional Freestream Pressure Distribution
Volume 4: Heat Transfer Parts A and B, 2008Co-Authors: Metin I. YarasAbstract:This paper presents measurements of the instability and transition processes in Separation Bubbles under a three-dimensional freestream pressure distribution. Measurements are performed on a flat plate upon which a pressure distribution is imposed by a contoured surface facing the flat test surface. The three-dimensional pressure distribution that is established on the test surface approximates the pressure distributions encountered on swept blades. This type of pressure field produces crossflows in the laminar boundary layer upstream of Separation and within the Separation Bubble. The effects of these crossflows on the instability of the upstream boundary layer and on the instability, transition onset and transition rate within the separated shear layer are examined. The measurements are performed at two flow Reynolds numbers and relatively low level of freestream turbulence. The results of this experimental study show that the three-dimensional freestream pressure field and the corresponding redistribution of the freestream flow cause significant spanwise variation of the Separation-Bubble structure. It is demonstrated that the instability and transition processes in the modified Separation Bubble develop on the basis of the same fundamentals as in two-dimensional Separation Bubbles, and can be predicted with the same level of accuracy using models that have been developed for two-dimensional Separation Bubbles.Copyright © 2008 by ASME
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large eddy simulation of transition in a Separation Bubble
Journal of Fluids Engineering-transactions of The Asme, 2006Co-Authors: Stephen K Roberts, Metin I. YarasAbstract:In this paper, large-eddy simulation of the transition process in a Separation Bubble is compared to experimental results. The measurements and simulations are conducted under low freestream turbulence conditions over a flat plate with a streamwise pressure distribution typical of those encountered on the suction side of turbine airfoils. The computational grid is refined to the extent that the simulation qualifies as a "coarse" direct numerical simulation. The simulations are shown to accurately capture the transition process in the separated shear layer. The results of these simulations are used to gain further insight into the breakdown mechanisms in transitioning Separation Bubbles.
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large eddy simulation of transition in a Separation Bubble
ASME Turbo Expo 2005: Power for Land Sea and Air, 2005Co-Authors: Stephen K Roberts, Metin I. YarasAbstract:In this paper, large-eddy simulation of the transition process in a Separation Bubble is compared to experimental results. The measurements and simulations are conducted under low free-stream turbulence conditions over a flat plate with a streamwise pressure distribution typical of those encountered on the suction side of turbine airfoils. The computational grid is sufficiently refined that the effects of sub-grid scale turbulence are adequately represented by the numerical dissipation of the computational algorithm. The large-eddy simulations are shown to accurately capture the transition process in the separated shear layer. The results of these simulations are used to gain further insight into the breakdown mechanisms in transitioning Separation Bubbles.Copyright © 2005 by ASME
