Short Laminar Separation Bubble Research Articles

A data-driven analysis of short and long laminar separation bubbles

This work investigates the statistical response of short and long laminar separation bubbles to external flow parameters, such as Reynolds number, free-stream turbulence intensity and streamwise pressure gradient, known to govern bubble formation and characteristics. A parametric experimental campaign has been performed using particle image velocimetry on a flat plate to provide a comprehensive database for the characterization of separation-induced transition in both short and long separation bubbles. The proper orthogonal decomposition (POD) was applied to the data set of all dividing streamlines commonly used to identify a laminar separation bubble. This provides an optimal state-space basis for the data-driven classification of the state of a laminar separation bubble, with the leading modes capturing the change in length and height of the laminar separation bubble in response to changes in the flow parameters. When projected onto the POD subspace constituted by the first three leading modes, the normalized data from the present study and the results from prior investigations not used in the modal analysis collapse on the same trajectory in the low-dimensional space. The present POD basis can be therefore adopted for the description of the general response of the time-mean shape of a laminar separation bubble to changes in the main influencing parameters. A well-defined pattern was observed in the case of short laminar separation bubbles in the reduced-order space defined by the first three POD coefficients, whereas a higher dispersion in the long-bubble regime indicates an increased sensitivity of long bubbles to the external flow characteristics.

Dynamic Stall of a Finite-Aspect-Ratio Wing

An investigation of dynamic stall on a pitching NACA 0012 aspect ratio 4 wing is performed by means of high-fidelity wall-resolved large-eddy simulations. The flow parameters are freestream Mach number and chord Reynolds number . Three cycles of a sinusoidal pitching motion are considered with reduced frequency and minimum and maximum angles of attack of 4 and 22 deg, respectively. Details of the unsteady flow structure are elucidated to provide a model of three-dimensional dynamic stall at higher Reynolds number than previously studied using large-eddy simulation. The initial process involves the upstream movement of transition, the formation of a short laminar separation bubble (LSB), and the emergence of a dynamic stall vortex (DSV) following LSB bursting. The DSV is initially fairly regular in the spanwise direction; however, as it propagates downstream, it is distorted into a -type shape with legs pinned at the wing front corners. A marked kink appears in the DSV core outboard and near the wing surface. This induces a circulatory motion that self-intensifies, leading to the formation of an arch-type vortex. The legs of the arch vortex move downstream and inboard, leaving a distinct imprint on the instantaneous surface pressure. Eventually, through a reconnection process, the arch vortex transforms into a ringlike structure that is shed into the wake. The effect of a simulated endwall is also investigated because this arrangement is used in experiments. It is found that, during the early stages of the DSV, the endwall has no significant impact on the flow structure and loading; however, following the formation of the arch vortex, the presence of the wall has a pronounced effect.

A Simplified Model Predicting the Kelvin–Helmholtz Instability Frequency for Laminar Separated Flows

A semi-empirical model for the estimation of the Kelvin–Helmholtz (KH) instability frequency, in the case of short laminar separation bubbles over airfoils, has been developed. To this end, the Thwaites's pressure gradient parameter has been adopted to account for the effects induced by the aerodynamic loading distribution as well as by the Reynolds number on the separated shear layer thickness at separation. The most amplified frequency predicted by linear stability theory (LST) for a piecewise linear profile, which can be considered as the KH instability frequency, has been related to the shear layer thickness at separation, hence to the Reynolds number and the aerodynamic loading distribution through the Thwaites's pressure gradient parameter. This procedure allows the formulation of a functional dependency between the Strouhal number of the shedding frequency based on exit conditions and the dimensionless parameters. Experimental results obtained in different test cases, characterized by different Reynolds numbers and aerodynamic loading distributions, have been used to validate the model, as well as to identify the regression curve best fitting the data. The semi-empirical correlation here derived can be useful to set the activation frequency of active flow control devices for the optimization of boundary layer separation control strategies.

Asymptotic description of incipient separation bubble bursting

AbstractThe appearance of short laminar separation bubbles in high Reynolds number (Re) wall bounded flows due to appropriate adverse pressure gradient conditions is usually associated with minor effects on global flow properties (e.g. lift force). However, localized reverse flow regions are known to react very sensitively to perturbations and in further consequence may trigger the laminar‐turbulent transition process or even cause global separation. The present investigation of marginally separated boundary layer flows is based on an asymptotic approach Re → ∞. Special emphasis is placed on solutions of the corresponding model equations which blow up within finite time indicating the ejection of a vortical structure and the emergence of shorter spatio‐temporal scales reminiscent of the early transition scenario (‘ bubble bursting’ ). Within the framework of marginal separation theory, an alternative adjoint operator method is used to formulate evolution equations governing the viscous‐inviscid interaction process in leading and higher order correction required for the study of later stages of the flow development. Their blow up structure specifies the initial condition of and the match to the subsequent triple deck stage. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cauchy Problems and Breakdown in the Theory of Marginally Separated Flows

AbstractFlow separation can be viewed as one of the trigger events of laminar‐turbulent boundary layer transition. Thus, intensive investigations have been carried out as to when and how laminar boundary layers break down. In the special case of marginal separation, i.e. the formation of short laminar separation bubbles, high Reynolds number asymptotic theory yields integro‐differential equations governing the essential flow behavior. We will present further considerations of associated initial value problems (IVPs) in planar and three‐dimensional flow cases regarding the general ill‐posedness. As for regularization methods, higher order asymptotic terms comprising the streamline curvature in the boundary layer region are taken into account rendering the hence modified problem well‐posed, which can be seen from the dispersion relation and numerical solutions of the full problem. Even though the regularized IVPs admit limiting steady states in their time evolution, the phenomenon of finite time blow‐up, which can be interpreted as “bubble bursting”, is still present for certain initial conditions. The inevitable breakdown of the asymptotic description leads to the emergence of a new asymptotic structure of shorter spatio‐temporal scales associated with the unique self‐similar blow‐up structure. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical Investigation of Separation Control for Wing Sections

For Reynolds numbers that are typical for general aviation aircraft separation is typically turbulent. In this paper the question is addressed if flow control strategies that are successful at low-Reynolds number conditions remain effective at higher Reynolds numbers. Towards this end hybrid simulations based on a one-equation renormalization group model were carried out for a modified NACA643-618 airfoil at a chord Reynolds number of one million. For ten degrees angle of attack a short laminar separation bubble develops near the leading edge and the flow separates turbulent from the suction side downstream of the leading edge bubble. Active flow control by harmonic blowing through a spanwise slot was investigated. The control was found to be only mildly effective or even counterproductive. In particular, the disturbances that were introduced by the control were only weakly amplified or even dampened. The unresolved eddy viscosity in the separated boundary layer lowers the effective Reynolds number and ma...

Numerical Investigation to Forcing Frequency and Amplitude of Synthetic Jet Actuators

elimination of the laminar separation bubble in a boundary layer with an adverse pressure gradient. The numerical results of the synthetic jet actuator’s effectiveness were compared to identify the significance in changes made by varied forcing frequency at fixed forcing amplitude and by varied forcing amplitude at a fixed forcing frequency. Consistent with the wind-tunnel experiments, large eddy simulation results showed that the effectiveness of the synthetic jet actuator depends more on the forcing frequency than on the forcing amplitude. They support the inference from the experiments that instability due to adverse pressure gradient to seed the nonfrictional Kelvin– Helmholtz instability in a short laminar separation bubble be a source for enhancing the frictional Tollmien– Schlichting instability triggered by a synthetic jet actuator to resist the laminar separation. To make a micro synthetic jet actuator effective, all the instabilities in the base flow and generated by the actuators should be used. Understanding of the optimal uses of these instabilities is challenging.

On higher order effects in marginally separated flows

AbstractIf the angle of attack α of a slender airfoil reaches a critical value αs flow separation is known to occur at the upper s surface. Further increase of α initially leads to the formation of a short laminar separation bubble which has an extremely weak influence on the external flow field – a phenomenon known as marginal separation – but then rather rapidly causes a severe change of the flow behaviour, leading to leading edge stall. According to the asymptotic theory of marginal separation holding in the limit of large Reynolds numbers Re, the flow in the neighbourhood of the separation bubble is governed by an integro‐differential equation. This so‐called interaction equation contains a single controlling parameter which relates the angle of attack to the Reynolds number, with a value Γs corresponding to αs. Some recent results concerning higher order s s corrections to this theory and their effect on the stability of steady solutions will be presented. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of small-amplitude convective disturbances on the size and bursting of a laminar separation bubble

Short laminar separation bubbles can develop on a flat plate due to an externally imposed pressure gradient. Here, these bubbles are computed by means of direct numerical simulations. Laminar–turbulent transition occurs in the bubble, triggered by small disturbance input with fixed frequency, but varying amplitude, to keep the bubbles short. The forcing amplitudes span a range of two orders of magnitude. All resulting bubbles differ with respect to their mean flow, linear-stability characteristics and distance between transition and mean reattachment locations. Mechanisms responsible for these differences are analysed in detail. Switching off the disturbance input or reducing it below a certain, very small threshold causes the short bubble to grow continuously. Eventually, it no longer exhibits typical characteristics of a short laminar separation bubble. Instead, it is argued that bursting has occurred and the bubble displays characteristics of a long-bubble state, even though this state was not a statistically steady state. This hypothesis is backed by a comparison of numerical results with measurements. For long bubbles, the transition to turbulence is not able to reattach the flow immediately. This effect can lead to the bursting of a short bubble, which remains short only when sufficiently large disturbances are convected into the bubble. Large-scale spanwise-oriented vortices at transition are observed for short but not for long bubbles. The failure of the transition process to reattach the flow in the long-bubble case is ascribed to this difference in transitional vortical structures.

On the effects of compressibility in marginally separated flows

AbstractIf the angle of attack α of a slender airfoil reaches a critical value αs flow separation is known to occur at the upper surface. Further increase of α initially leads to the formation of a short laminar separation bubble which has an extremely weak influence on the external flow field — a phenomenon known as marginal separation — but then rather rapidly causes a severe change of the flow behaviour (leading edge stall). According to the asymptotic theory of marginal separation holding in the limit of large Reynolds numbers t→∞ the flow in the neighbourhood of the separation bubble is governed by an integro–differential equation exhibiting a single controlling parameter Γ which relates the angle of attack to the Reynolds number, with a value Γs corresponding to αs. Numerical investigations have shown that solutions to this equation exist up to a critical value Γc>Γs only and this has been taken as an indication that a substantial change of the flow field must take place if Γ exceeds Γc. In the case of unsteady three–dimensional perturbations of a two–dimensional steady marginally separated boundary layer in the limit Γ→Γc, the (generalized form of the) integro–differential equation reduces to a nonlinear diffusion equation of Fisher type. This asymptotic description serves as a basis for the investigations concerning compressibility effects, also in respect of the mechanism through which the hydromechanic motion inside the boundary layer could be transformed into aerodynamic sound. A modified interaction equation is obtained, which in addition to Γ, contains the Mach number at the point of separation as a second controlling parameter. The unsteady changes of the boundary layer displacement thickness in the limit ΔΓ=Γc−Γ→0, again governed by Fisher's equation, then are identified to act as a quadrupole source for sound radiation. The most surprising outcome is the fact that in the three–dimensional case, the second derivative with respect to the lateral direction contained in the Fisher equation changes its sign and becomes negative if the Mach number exceeds a certain value. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Turbulence Amplification with Incidence at the Leading Edge of a Compressor Cascade

Detailed measurements, with a two‐component laser‐Doppler velocimeter and a thermal anemometer were made near the suction surface leading edge of controlled‐diffusion airfoils in cascade. The Reynolds number was near 700,000, Mach number equal to 0.25, and freestream turbulence was at 1.5% ahead of the cascade.It was found that there was a localized region of high turbulence near the suction surface leading edge at high incidence. This turbulence amplification is thought to be due to the interaction of the free‐shear layer with the freestream inlet turbulence. The presence of the local high turbulence affects the development of the short laminar separation bubble that forms very near the suction side leading edge of these blades. Calculations indicate that the local high levels of turbulence can cause rapid transition in the laminar bubble allowing it to reattach as a short “non‐burst” type.The high turbulence, which can reach point values greater than 25% at high incidence, is the reason that leading edge laminar separation bubbles can reattach in the high pressure gradient regions near the leading edge. Two variations for inlet turbulence intensity were measured for this cascade. The first is the variation ofmaximum inlet turbulence with respect to inlet‐flow angle; and the second is the variation of leading edge turbulence with respect to upstream distance from the leading edge of the blades.

マイクロアクチュエータによる柱状物体後流の周波数制御

The "lock-in" phenomenon has been investigated experimentally by vibrating cylindrical bluff body surfaces locally using piezoelectric actuators. The maximum amplitude of oscillation is of the order of 20 pm (nearly 1.0 X 10-3 times the body thickness), and the excitation frequency covers below and above a natural vortex shedding frequency from the body. The Reynolds number renges from 5.25 X 103 to 2.31 x 104. Substantial entrainment of vortex shedding frequency has been observed by exciting the upstream surface of sidewalls, where a short laminar separation bubble exists. Vortex shedding becomes more periodical and the total pressure in the wake becomes high by the local excitation of body surfaces. Possibilities for application to the active control of vibration and noise due to vortex shedding, as well as to drag reduction are discussed.