Dynamics Of Separation Bubble Research Articles

Investigations of shock–boundary layer interaction dynamics using high-bandwidth pressure field imaging

The large-scale pulsations of shock-induced separation with length scale that significantly exceeds the incoming boundary layer thickness are investigated. The shock–boundary layer interaction (SBLI) unit is generated by an inward-turning axisymmetric compression ramp at an inflow Mach number of 2.5. A substantial region surrounding the centre azimuth exhibited mean and dynamic flow features that are consistent with two-dimensional separation. Two-dimensional highly resolved maps of surface pressure field are obtained using fast-response pressure-sensitive paint fluorescence imaging at 40 kHz repetition rate. The measurement domain covered significant regions of the incoming boundary layer through the relaxing boundary layer downstream of the reattachment as well as over 25 boundary layer thicknesses in the azimuthal direction. These measurements provide new insights into the spanwise coupling of the SBLI unit in addition to its inherent dynamics. The power spectral density (PSD) of the centreline pressure exhibits very good agreement with theoretical models and complementary measurements using fast-response pressure transducers, which served to validate the pressure field measurements. Detailed examination of the PSD reveals strong agreement with the literature, which includes the peak Strouhal number of the separation and reattachment shock motions as well as the downward frequency shift along the separation bubble. Furthermore, the pressure fluctuation maps reveal streamwise-elongated structures just downstream of the ramp leading edge that persist well downstream of the reattachment. A time sequence of conditional average pressure fluctuation maps is constructed surrounding isolated pressure excursions in the intermittent region. This sequence, along with two-point cross correlation analysis, provides critical information about the flow processes that drive the separation bubble pulsations in the SBLI units with large separation scales. Overall, the imbalance in the mass within the separation bubble appears to be the critical mechanism that drives the separation bubble pulsations. Furthermore, the pressure perturbations originating at azimuthally offset locations are also observed to influence the separation bubble dynamics.

The effect of slip on the development of flow separation due to a bump in a channel

A numerical study on the effect of surface slip on the flow in a constricted channel is presented, with the aim of exploring the use of surface slip to control flow separation. Our focus is on two-dimensional flow in a channel over a bump, with a fixed aspect ratio, upon which a Robin-type slip boundary condition is imposed. When the channel walls are fully no-slip, such a flow is known to develop a region of separation behind the bump, at sufficiently large Reynolds numbers. The effect of slip on the separation bubble dynamics occurring behind the bump is investigated, for Reynolds numbers $2000$ and $4000$ . It is shown that surface slip (i) attenuates the intensity of separation as it diminishes the minimum of the streamwise velocity within the recirculation region; (ii) delays the onset of flow separation, shifting it downstream, along the bump, and (iii) reduces the dimensions of the separation bubble behind the bump, allowing the flow to reattach sooner. Ultimately, slip inhibits separation, with both the points of separation and reattachment coalescing, for a slip length $\lambda$ of approximately $0.2$ .

Buzz characteristics and separation bubble dynamics in supersonic intake

The flow field in an axisymmetric supersonic mixed-compression inlet is numerically solved to understand the separation bubble dynamics at different freestream Mach numbers of M∞ = 2.0, 3.0, 5.0, and throttling ratios of ψ = 0.515, 0.40, and 0.35. The two-dimensional compressible Reynolds Averaged Navier Stokes equations were solved in a finite volume framework. The computational results were validated with the wind tunnel experimental data for steady as well as unsteady operating conditions. Throttling conditions are simulated by varying the exit area of the model in the form of plug inserts. The analysis of the flow features in the complete computation domain indicates the formation of multiple separation bubbles near the exit plug as well as near the isolator entrance of the inlet. The frequency of unsteadiness (f) associated with the ramp surface pressure decreases with a decrease in throttling ratio ψ. The possibility of a common origin that is regardless of any buzz criteria is analyzed by considering separation and reattachment zones in the flow field in terms of wall shear stress variation. Inside the supersonic intake, the multiple shock waves interact with the wall boundary layer that can lead to a large separation bubble. This bubble is always unstable inside the intake flow passage and can generate the source of instability. The propagation of a pressure disturbance is analyzed by the temporal variation of the pressure on the ramp wall. The cross-correlation coefficient Cij(t) of the pressure signal on the ramp surface suggests the upstream propagation of the disturbance generated from the separation zone near the exit plug. The numerical Schlieren provides further information on the formation and propagation of the separation bubble near the exit plug.

Laminar separation bubble dynamics and its effects on thin airfoil performance during pitching-up motion

This article reports an investigation into dynamic characteristics of the laminar separation bubbles (LSBs) associated with aerodynamic loads unsteadiness of a cambered thin airfoil in pitching-up motions at low Reynolds number flows. Unsteady Reynolds-averaged Navier–Stokes (URANS) simulations were conducted for a 4%c cambered thin airfoil at Reynolds number of 30,000 and 60,000. The airfoil pitches up from 0° to 25°angles of attack at dimensionless pitch rate [Formula: see text] of 0.0398 and 0.0199. The [Formula: see text] SST [Formula: see text] turbulence transition model was used to account for the effect of transition on LSBs’ development. The LSBs are shown to evolve in their shape and size during the pitching motion. The influence of the LSBs on the airfoil upper surface during pitching motion continues to a higher incidence in comparison with that under static conditions before developing into a fully detached flow. Vortex merging is observed in the rear part of the LSBs in the turbulent portion for a Reynolds number of 30,000. At Reynolds number 60,000, the changing of the LSB length during pitching-up motion is similar to that of steady cases, except a delayed transition is observed as incidence increases. The results show further insight into the dynamic characteristics of the LSBs and their relation to the aerodynamic performance of the airfoil.

Passive flow control strategies for low Reynolds number airfoils

This study focuses on ameliorating the aerodynamic performance of an airfoil in low Reynolds number regime by implementing a passive flow control technique to suppress the effects created by the bubble bursting which holds significance in enhancing the aerodynamic performance of SUAVs which fall under this regime. A numerical study is conducted for the flow over E387 airfoil at a Reynolds number 105 to investigate the effects of the implementation of the passive flow control configuration on the laminar separation bubble dynamics. It is noted that orientation, thickness, inlet location, and configuration of the passive flow control passage had imposed significant effects on lift and drag coefficients generated by the airfoil. A significant drag reduction was achieved when the airfoil is implemented with a slot oriented at 30° that has a converging exit which resulted in the rise of the airfoil’s aerodynamic efficiency.

Spatio-temporal dynamics of turbulent separation bubbles

The spatio-temporal dynamics of separation bubbles induced to form in a fully-developed turbulent boundary layer (with Reynolds number based on momentum thickness of the boundary layer of 490) over a flat plate are studied via direct numerical simulations. Two different separation bubbles are examined: one induced by a suction-blowing velocity profile on the top boundary and the other, by a suction-only velocity profile. The latter condition allows reattachment to occur without an externally imposed favourable pressure gradient and leads to a separation bubble more representative of those occurring over airfoils and in diffusers. The suction-only separation bubble exhibits a range of clearly distinguishable modes including a high-frequency mode and a low-frequency "breathing" mode that has been observed in some previous experiments. The high-frequency mode is well characterized by classical frequency scalings for a plane mixing layer and is associated with the formation and shedding of spanwise oriented vortex rollers. The topology associated with the low-frequency motion is revealed by applying dynamic mode decomposition to the data from the simulations and is shown to be dominated by highly elongated structures in the streamwise direction. The possibility of G\"{o}rtler instability induced by the streamwise curvature on the upstream end of the separation bubble as the underlying mechanism for these structures and the associated low frequency is explored.

Flows over surface-mounted bluff bodies with different spanwise widths submerged in a deep turbulent boundary layer

The spatio-temporal dynamics of separation bubbles induced by surface-mounted bluff bodies with different spanwise widths and submerged in a thick turbulent boundary layer is experimentally investigated. The streamwise extent of the bluff bodies is fixed at 2.36 body heights and the spanwise aspect ratio ($AR$), defined as the ratio between the width and height, is increased from 1 to 20. The thickness of the upstream turbulent boundary layer is 4.8 body heights, and the dimensionless shear and turbulence intensity evaluated at the body height are 0.23 % and 15.8 %, respectively, while the Reynolds number based on the body height and upstream free-stream velocity is 12 300. For these upstream conditions and limited streamwise extent of the bluff bodies, two distinct and strongly interacting separation bubbles are formed over and behind the bluff bodies. A time-resolved particle image velocimetry is used to simultaneously measure the velocity field within these separation bubbles. Based on the dynamics of the mean separation bubbles over and behind the bluff bodies, the flow fields are categorized into three-dimensional, transitional and two-dimensional regimes. The results indicate that the low-frequency flapping motions of the separation bubble on top of the bluff body with $\mathit{AR}=1$ are primarily influenced by the vortex shedding motion, while those with larger aspect ratios are modulated by the large-scale streamwise elongated structures embedded in the oncoming turbulent boundary layer. For $\mathit{AR}=1$ and 20, the flapping motions in the wake region are strongly influenced by those on top of the bluff bodies but with a time delay that is dependent on the $AR$. Moreover, an expansion of the separation bubble on the top surface tends to lead to an expansion and contraction of separation bubbles in the wake of $\mathit{AR}=20$ and 1, respectively. As for the transitional case of $\mathit{AR}=8$, the separation bubbles over and behind the body are in phase over a wide range of time difference. The dynamics of the shear layer in the wake region of the transitional case is remarkably more complex than the limiting two-dimensional and three-dimensional configurations.

Steady and transient response of a laminar separation bubble to controlled disturbances

The steady and transient response of a laminar separation bubble to flow disturbances is examined experimentally. Wind tunnel experiments are performed on a NACA 0012 aerofoil at a chord Reynolds number of 130 000 and angle of attack of $2^{\circ }$. Under the investigated conditions, a laminar separation bubble forms on the suction side of the aerofoil in the unperturbed flow. Periodic disturbances are introduced into the boundary layer just upstream of separation by means of a surface-mounted dielectric barrier discharge plasma actuator. Two-component, time-resolved particle image velocimetry measurements are performed to characterise both quasi-steady and transient response of the flow to periodic disturbances. The results show that the dynamics of the laminar separation bubble is dominated by the periodic shedding of shear layer vortices, forming upstream of the mean reattachment location due to the amplification of unstable flow disturbances. Introducing the controlled perturbations leads to significant changes in separation bubble topology and the characteristics of the dominant coherent structures, with the effect dependent on both amplitude and frequency of disturbances. Linear stability analysis demonstrates that the induced changes to the mean bubble topology affect the stability characteristics, reducing the maximum growth rate and the frequency of the most amplified disturbances by 35 % and 20 %, respectively, when the bubble length is reduced by up to 40 %. The observed changes in stability characteristics are shown to correlate with the attendant variations in the shape factor. The transient response of the bubble is associated with significant changes in the separation bubble dynamics, with significant differences observed between the relative duration (${\approx}45\,\%$) of the transients flow response associated with the introduction and removal of the controlled disturbances. A detailed analysis of the results offers new insight into the response of laminar separation bubbles to changes in the disturbance environment.

Laminar separation bubble development on an airfoil emitting tonal noise

The subject of this experimental study is the feedback effects due to tonal noise emission in a laminar separation bubble (LSB) formed on the suction side of an airfoil in low Reynolds number flows. Experiments were performed on a NACA 0012 airfoil for a range of chord-based Reynolds numbers $0.65\times 10^{5}\leqslant \mathit{Re}_{c}\leqslant 4.5\times 10^{5}$ at angle of attack ${\it\alpha}=2^{\circ }$, where laminar boundary layer separation is encountered on both sides of the airfoil. Simultaneous time-resolved, two-component particle image velocimetry (PIV) measurements, unsteady surface pressure and far-field acoustic pressure measurements were employed to characterize flow development and acoustic emissions. Amplification of disturbances in separated shear layers on both the suction and pressure sides of the airfoil leads to shear layer roll-up and shedding of vortices from separation bubbles. When the vortices do not break up upstream of the trailing edge, the passage of these structures over the trailing edge generates tonal noise. Acoustic feedback between the trailing edge noise source and the upstream separation bubble narrows the frequency band of amplified disturbances, effectively locking onto a particular frequency. Acoustic excitation further results in notable changes to the overall separation bubble characteristics. Roll-up vortices forming on the pressure side, where the bubble is located closer to the trailing edge, are shown to define the characteristic frequency of pressure fluctuations, thereby affecting the disturbance spectrum on the suction side. However, when the bubble on the pressure side is suppressed via boundary layer tripping, a weaker feedback effect is also observed on the suction side. The results give a detailed quantitative description of the observed phenomenon and provide a new outlook on the role of coherent structures in separation bubble dynamics and trailing edge noise generation.

Characterization of Low-Frequency Oscillations in the Flowfield About an Iced Airfoil

Wind-tunnel measurements were used to identify and characterize the low-frequency oscillation present in the flowfield about an NACA 0012 airfoil with a horn-ice shape. This low-frequency mode was identified in the unsteady content of the airfoil lift coefficient and the corresponding Strouhal numbers compared well with those reported in the literature. To study the relationship between the ice-induced separation bubble dynamics and the airfoil circulation, the unsteady shear-layer reattachment location was determined using measurements from a surface-mounted hot-film array. At the low-frequency mode, the lift coefficient led the reattachment location by a phase of . Additional characterization of the relationship between the unsteady shear-layer reattachment location and the unsteady airfoil pressure distribution was also completed, along with an analysis of the convective qualities of the low-frequency oscillation throughout the airfoil pressure distribution.

Validating the URANS shear stress transport γ −Reθ model for low‐Reynolds‐number external aerodynamics

SUMMARYA numerical investigation on low‐Reynolds‐number external aerodynamics was conducted using the transitional unsteady Reynolds‐averaged Navier–Stokes shear stress transport γ −Reθ model and the ANSYS‐CFX computational fluid dynamics suite. The NACA 0012 airfoil was exposed to chord‐based Reynolds numbers of 5.0 ×104, 1.0 ×105 and 2.5 ×105 at 0°, 4°and 8°angles of attack. Time‐averaged and instantaneous flow features were extracted and compared with fully turbulent shear stress transport results, XFLR5 panel e N method results, and published higher order numerical and experimental studies. The current model was shown to reproduce the complex flow phenomena, including the laminar separation bubble dynamics and aerodynamic performance, with a very good degree of accuracy. The sensitivity of the model to domain size, grid resolution and quality, timestepping scheme, and free‐stream turbulence intensity was also presented. In view of the results obtained, the proposed model is deemed appropriate for modelling low‐Reynolds‐number external aerodynamics and provides a framework for future studies for the better understanding of this complex flow regime. Copyright © 2011 John Wiley & Sons, Ltd.

Direct numerical simulation of flow separation behind a rounded leading edge: Study of curvature effects

The separation bubble formed over a 2D half-body is studied by direct numerical simulation. The aim of this work is to consider the physical influence of the shape of the body that can be viewed as a thick half-plate with a front edge more or less rounded. The present generic body geometry is defined with a unique parameter η = R / H corresponding to the ratio of the curvature radius R of the front edge over the body height H. In this paper, 18 calculations are presented depending on: (i) the value of η with η = 0.125 , 0.25 , 0.5 , 1 ; (ii) the 2D/3D nature of the computation; (iii) the inflow perturbations used to mimic residual turbulence in the free stream velocity U ∞ . Only one Reynolds number Re = U ∞ H / ν is used for every simulation, allowing us to focus on the curvature effects over the separation bubble dynamics. The value of the Reynolds number ( Re = 2000 ) combined with the resolution demand of the front edge (close to a sharp corner for the highest curvature case) requires to simulate the flow using up to 876 million mesh nodes. The curvature effects are found to deeply influence the separation bubble dynamics, with a significant expansion of the separated region size predicted by 3D computations. This expansion is driven by the separation angle rise combined with the reinforcement of turbulence levels as the curvature is increased. These trends are associated with a change of bubble sensitivity with respect to upstream/downstream perturbations that can be interpreted in terms of convective/absolute stability.

Three-dimensional simulation of the self-oscillating flow and side-loads in an over-expanded subscale rocket nozzle

Time-accurate three-dimensional Reynolds-averaged Navier-Stokes simulations have been performed to explore the flow unsteadiness of the subscale J-2S thrust optimized contour rocket nozzle in over-expansion at prescribed air supply conditions in the chamber. The results show the development of periodic low-frequency, high-amplitude, self-sustained oscillations at the simulated nozzle pressure ratios (NPRs). The cap-shock pattern is responsible for the two-coaxial jet topology that appears. The complex three-dimensional evolution of the flowfield is characterized by a periodical jet flapping, axial oscillations, and azimuthal fluctuations with or without swirl, depending on the NPR. According to the simulations, jet flapping quasi-restricted to a preferred plane may occur during the firings, being supported by the reported thrust and side-load measurements. The computed dominant frequencies corresponding to the longitudinal and transverse oscillation modes show an excellent agreement with the experimental counterpart. The analysis of the forces spectra and flowfields has identified that synchronization between side-load (transverse) and thrust (longitudinal) oscillations develops in consonance with the observations. Besides, their strong coupling is favoured by the response of the shocks to the pulsating pressure near the large recirculation bubble that arises behind the cap-shock. A discussion on the self-oscillating, high-coherent flow phenomenon is addressed. The present three-dimensional study has captured for the first time the phenomenon of cyclic transition between reattached flow separation mode and non-reattached mode, where the alternating bursting and formation of a separation bubble is accompanied by the large amplitude axial displacement of the shock pattern. Furthermore, the role of the separation bubble dynamics on triggering high side-load peaks at fixed NPRs is numerically confirmed.