Transitional Separation Bubble Research Articles

Passive Manipulation of Separation-Bubble Transition Using Surface Modifications

Through experiments using two-dimensional particle-image velocimetry (PIV), this paper examines the nature of transition in a separation bubble and manipulations of the resultant breakdown to turbulence through passive means of control. An airfoil was used that provides minimal variation in the separation location over a wide operating range, with various two-dimensional modifications made to the surface for the purpose of manipulating the transition process. The study was conducted under low-freestream-turbulence conditions over a flow Reynolds number range of 28,000–101,000 based on airfoil chord. The spatial nature of the measurements has allowed identification of the dominant flow structures associated with transition in the separated shear layer and the manipulations introduced by the surface modifications. The Kelvin–Helmholtz (K-H) instability is identified as the dominant transition mechanism in the separated shear layer, leading to the roll-up of spanwise vorticity and subsequent breakdown into small-scale turbulence. Similarities with planar free-shear layers are noted, including the frequency of maximum amplification rate for the K-H instability and the vortex-pairing phenomenon initiated by a subharmonic instability. In some cases, secondary pairing events are observed and result in a laminar intervortex region consisting of freestream fluid entrained toward the surface due to the strong circulation of the large-scale vortices. Results of the surface-modification study show that different physical mechanisms can be manipulated to affect the separation, transition, and reattachment processes over the airfoil. These manipulations are also shown to affect the boundary-layer losses observed downstream of reattachment, with all surface-indentation configurations providing decreased losses at the three lowest Reynolds numbers and three of the five configurations providing decreased losses at the highest Reynolds number. The primary mechanisms that provide these manipulations include: suppression of the vortex-pairing phenomenon, which reduces both the shear-layer thickness and the levels of small-scale turbulence; the promotion of smaller-scale turbulence, resulting from the disturbances generated upstream of separation, which provides quicker transition and shorter separation bubbles; the elimination of the separation bubble with transition occurring in an attached boundary layer; and physical disturbance, downstream of separation, of the growing instability waves to manipulate the vortical structures and cause quicker reattachment.

Experimental analysis of the flow field over a novel owl based airfoil

The aerodynamics of a newly constructed wing model the geometry of which is related to the wing of a barn owl is experimentally investigated. Several barn owl wings are scanned to obtain three-dimensional surface models of natural wings. A rectangular wing model with the general geometry of the barn owl but without any owl-specific structure being the reference case for all subsequent measurements is investigated using pressure tabs, oil flow pattern technique, and particle-image velocimetry. The main flow feature of the clean wing is a transitional separation bubble on the suction side. The size of the bubble depends on the Reynolds number and the angle of attack, whereas the location is mainly influenced by the angle of attack. Next, a second model with a modified surface is considered and its influence on the flow field is analyzed. Applying a velvet onto the suction side drastically reduces the size of this separation at moderate angles of attack and higher Reynolds numbers.

Investigation of the vortex induced unsteadiness of a separation bubble via time-resolved and scanning PIV measurements

A transitional separation bubble on the suction side of an SD7003 airfoil is considered. The transition process that forces the separated shear layer to reattach seems to be governed by Kelvin–Helmholtz instabilities. Large scale vortices are formed due to this mechanism at the downstream end of the bubble. These vortices possess a three-dimensional structure and detach from the recirculation region, while other vortices are formed within the bubble. This separation of the vortex is a highly unsteady process, which leads to a bubble flapping. The structure of these vortices and the flapping of the separation bubble due to these vortices are temporally and spatially analyzed at angles of attack from 4° to 8° and chord-length based Reynolds numbers Rec = 20,000–60,000 using time-resolved PIV measurements in a 2D and a 3D set-up, i.e., stereo-scanning PIV measurements are done in the latter case. These measurements complete former studies at a Reynolds number of Rec = 20,000. The results of the time-resolved PIV measurements in a single light-sheet show the influence of the angle of attack and the Reynolds number. The characteristic parameters of the separation bubble are analyzed focusing on the unsteadiness of the separation bubble, e.g., the varying size of the main recirculation region, which characterizes the bubble flapping, and the corresponding Strouhal number are investigated. Furthermore, the impact of the freestream turbulence is investigated by juxtaposing the current and former results. The stereo-scanning PIV measurements at Reynolds numbers up to 60,000 elucidate the three-dimensional character of the vortical structures, which evolve at the downstream end of the separation bubble. It is shown that the same typical structures are formed, e.g., the c-shape vortex and the screwdriver vortex at each Reynolds number and angle of attack investigated and the occurrence of these patterns in relation to Λ-structures is discussed. To evidence the impact of the freestream turbulence, these results are compared with findings of former measurements.

Transitional separation bubbles and unsteady aspects of aerofoil stall

AbstractA time-accurate solution method for the coupled potential flow and integral boundary-layer equations is used to study aerofoils near stall, where laboratory experiments have shown high-amplitude low-frequency oscillations. The laminar-turbulent transition model incorporates an absolute instability formulation, which allows the transition process in separation bubbles to be sustained in the absence of upstream disturbances, in agreement with recent direct numerical simulations. The method is demonstrated to capture large scale flow oscillations with Strouhal numbers and amplitudes comparable to experiments. The success of this particular physical model suggests that bubble bursting is primarily due to a potential-flow/boundary-layer interaction effect, in which relatively simple models of boundary layer transition and turbulence suffice to describe the key phenomena.

Numerical Study of Instability Mechanisms Leading to Transition in Separation Bubbles

In this paper, transition in a separation bubble is examined through numerical simulation. The flow Reynolds number and streamwise pressure distribution are typical of the conditions encountered on the suction side of low-pressure turbine blades of gas-turbine engines. The spatial and temporal resolutions utilized in the present computations correspond to a coarse direct numerical simulation, wherein the majority of turbulence scales, including the inertial subrange, are adequately resolved. The accuracy of the simulation results is demonstrated through favorable comparisons to experimental data corresponding to the same flow conditions. The results of the simulation show linear Tollmien-Schlichting (T-S) instability growth downstream of the point of separation, leading to the roll up of spanwise vorticity into discrete vortical structures, characteristic of Kelvin-Helmholtz (K-H) instability growth. The extent of cross-stream momentum exchange associated with packets of amplified T-S waves is examined, along with details of the time-periodic breakdown into turbulence occurring upon the development of the K-H instability. Reynolds-averaged properties of the separation bubble are presented and provide evidence of the strong three-dimensional nature of the reattachment process.

Time-resolved and volumetric PIV measurements of a transitional separation bubble on an SD7003 airfoil

To comprehensively understand the effects of Kelvin–Helmholtz instabilities on a transitional separation bubble on the suction side of an airfoil regarding as to flapping of the bubble and its impact on the airfoil performance, the temporal and spatial structure of the vortices occurring at the downstream end of the separation bubble is investigated. Since the bubble variation leads to a change of the pressure distribution, the investigation of the instantaneous velocity field is essential to understand the details of the overall airfoil performance. This vortex formation in the reattachment region on the upper surface of an SD7003 airfoil is analyzed in detail at different angles of attack. At a Reynolds number Re c 4°. Due to transition processes, turbulent reattachment of the separated shear layer occurs enclosing a locally confined recirculation region. To identify the location of the separation bubble and to describe the dynamics of the reattachment, a time-resolved PIV measurement in a single light-sheet is performed. To elucidate the spatial structure of the flow patterns in the reattachment region in time and space, a stereo scanning PIV set-up is applied. The flow field is recorded in at least ten successive light-sheet planes with two high-speed cameras enclosing a viewing angle of 65° to detect all three velocity components within a light-sheet leading to a time-resolved volumetric measurement due to a high scanning speed. The measurements evidence the development of quasi-periodic vortex structures. The temporal dynamics of the vortex roll-up, initialized by the Kelvin–Helmholtz (KH) instability, is shown as well as the spatial development of the vortex roll-up process. Based on these measurements a model for the evolving vortex structure consisting of the formation of c-shape vortices and their transformation into screwdriver vortices is introduced.

Acoustic excitation of stationary streamwise structures in a separation region on a straight wing

An effect of sound on the flow with a transitional separation bubble on a wing model at low subsonic freestream velocity was experimentally investigated. Using hot-wire measurements and liquid-crystal visualization, stationary streamwise structures were found to appear in the reattachment region. Quantitative characteristics of the disturbances upcoming during the breakdown to turbulence were measured.

Boundary-Layer Transition Affected by Surface Roughness and Free-Stream Turbulence

This paper presents experimental results documenting the effects of surface roughness and free-stream turbulence on boundary-layer transition. The experiments were conducted on a flat surface, upon which a pressure distribution similar to those prevailing on the suction side of low-pressure turbine blades was imposed. The test matrix consists of five variations in the roughness conditions, at each of three free-stream turbulence intensities (approximately 0.5%, 2.5%, and 4.5%), and two flow Reynolds numbers of 350,000 and 470,000. The ranges of these parameters considered in the study, which are typical of low-pressure turbines, resulted in both attached-flow and separation-bubble transition. The focus of the paper is on separation-bubble transition, but the few attached-flow test cases that occurred under high roughness and free-stream turbulence conditions are also presented for completeness of the test matrix. Based on the experimental results, the effects of surface roughness on the location of transition onset and the rate of transition are quantified, and the sensitivity of these effects to free-stream turbulence is established. The Tollmien–Schlichting instability mechanism is shown to be responsible for transition in each of the test cases presented. The root-mean-square height of the surface roughness elements, their planform size and spacing, and the skewness (bias towards depression or protrusion roughness) of the roughness distribution are shown to be relevant to quantifying the effects of roughness on the transition process.

Effects of Surface-Roughness Geometry on Separation-Bubble Transition

This paper presents measurements of separation-bubble transition over a range of surfaces with randomly distributed roughness elements. The tested roughness patterns represent the typical range of roughness conditions encountered on in-service turbine blades. Through these measurements, the effects of size and spacing of the roughness elements, and the tendency of the roughness pattern toward protrusions or depressions (skewness), on the inception location and rate of transition are evaluated. Increased roughness height, increased spacing of the roughness elements, and a tendency of the roughness pattern toward depressions (negative skewness) are observed to promote earlier transition inception. The observed effects of roughness spacing and skewness are found to be small in comparison to that of the roughness height. Variation in the dominant mode of instability in the separated shear layer is achieved through adjustment of the streamwise pressure distribution. The results provide examples for the extent of interaction between viscous and inviscid stability mechanisms.

Characteristic Regimes of Transitional Separation Bubbles in Unsteady Flow

This experimental investigation deals with transition phenomena of a separated boundary layer under unsteady inlet flow conditions. The main purpose of this investigation is to understand the influence of the rotor-stator interaction in turbomachinery on the subsequent, highly loaded boundary layer. The research project is divided into two phases. In the first phase, which has been completed recently, only the variation of mean velocity caused by upstream blades was simulated in the experiments while the free-stream turbulence intensity was retained at a constant low level. The experiments are carried out in an Eifel-type wind tunnel to investigate the laminar separated boundary layer of a flat plate under oscillating inlet conditions. The adverse pressure gradient, similar to that of turbomachines, is generated by the contoured upper wall. The unsteadiness is produced by a rotating flap located downstream of the test section. The reduced frequency, the amplitude and the mean Reynolds number are varied to simulate the conditions prevailing in turbomachines. In addition to the Kelvin–Helmholtz instability of the separated shear layer, a lower frequency instability was observed. This is frequently referred to as `free shear layer flapping' and results in two distinctly different ways of re-attachment, depending primarily on the Reynolds number. For low momentum thickness Reynolds numbers at the separation point, large-scale vortices locked to the frequency of the unsteady main flow are identified. They originate nearly at the top of the separation bubble and are ejected downstream. A fully turbulent boundary layer develops after these vortices mix out. For higher Reynolds numbers, transition is completed within a short length of the free shear layer and there-attachment region. The characteristic momentum thickness Reynolds number separating these two regimes in unsteady flow is about 125. The Strouhal number (reduced frequency) does not appear to have any significant effect. Based on the experimental results, this behaviour is discussed in some detail.

A numerical and experimental study of a transitional separation bubble

A combined numerical and experimental study of a two-dimensional transitional separation bubble due to an adverse pressure gradient is reported. The experiments have been performed in the MTL wind tunnel with a contoured wall imposing an adverse pressure gradient on the flow over a flat plate. The separated shear-layer is highly unstable and transition to turbulence occurs in the flow. The experimental separation bubble flow is modelled numerically using two-dimensional direct numerical simulations (DNS). Prescribing free stream boundary conditions in the wall normal velocity the experimental bubble is reproduced. The development of artificially forced two-dimensional instability waves is investigated and good agreement is found between experiments, simulations and linear stability theory (LST). The performance of several engineering transition prediction methods applied on the present separation bubble is presented and compared. Methods based on simplifications of the e n -method yield predictions in accordance with experiments and DNS.

Embedded direct numerical simulation for aeronautical CFD

AbstractA method is proposed by which a direct numerical simulation of the compressible Navier-Stokes equations may be embedded within a more general aeronautical CFD code. The method may be applied to any code which solves the Euler equations or the Favre-averaged Navier-Stokes equations. A formal decomposition of the flowfield is used to derive modified equations for use with direct numerical simulation solvers. Some preliminary applications for model flows with transitional separation bubbles are given.

Direct numerical simulation of flow separation behind a swept, rearward-facing step at ReH=3000

The effect of sweep on a transitional separation bubble behind a backward-facing step is investigated using direct numerical simulation (DNS). The Reynolds number based on step height and free stream component normal to the step, Reα=(HC∞ cos α)/ν, is kept constant at 3000 for sweep angles α between 0° and 60°. Results agree well with two experimental investigations. Up to α=40°, the mean flow follows the sweep-independence principle. For higher sweep angles, the size of the separated flow region is shortened considerably which is mainly due to an upstream shift of transition in the laminar shear layer emanating from the step and—to a lesser degree—to an increase in turbulent momentum flux uv¯. Fluctuations of wall-stress and wall pressure are largest near the reattachment location and their magnitude scales with (C∞ cos α)2 for all sweep angles. Transition is dominated by a Kelvin–Helmholtz-type instability of the free shear layer. The influence of skewing on the growth rate of instability waves is weak in accordance with linear stability theory predictions of Lu and Lele [J. Fluid Mech. 249, 441 (1993)]. The upstream shift of transition for α⩾50° is related to the way how disturbances spread sidewise in a wedge-shaped region inside of the shear layer.

Refined Interaction Method for Direct Numerical Simulation of Transition in Separation Bubbles

In direct numerical simulations (DNS) of laminar-turbulent transition in laminar separation bubbles, the definition of a well-posed freestream boundary condition is crucial. Different, partially contradicting properties are required: e rst, an adverse pressure gradient has to be imposed to force separation. Moreover, oscillations at the freestream boundary caused by disturbance waves, which extend into the potential e ow, have to be treated accurately. Finally, displacement effects of the separation bubble on the surrounding potential e ow by the so-called viscous-inviscid boundary-layer interaction have to be captured. Usually, either the integration domain has to be sufe ciently high, or a state-of-the-art boundary-layer interaction model based on the theory of thin airfoils can be applied. At a high Reynolds number at separation, neither possibility is applicable. Therefore, an improved model has been developed that meets the just-mentioned requirements. The method is validated by variations of the height of the integration domain. It is shown that boundary-layer interaction has not only quantitative but even qualitative impact on the separation bubble. The comparison of a highly resolved DNS with an experiment proves the applicability of the present method for separation bubbles on airfoils. Nomenclature Cu, cu,ij = matrix and its coefe cients; Eqs. (15) and (16) Cv, cv,ij = matrix and its coefe cients; Eqs. (12) and (13) c = chord length ci = blending function in interaction model fc = polynomial function (interaction model )

Refined interaction method for direct numerical simulation of transition in separation bubbles

Refined interaction method for direct numerical simulation of transition in separation bubbles

Two-equation turbulence modelling of a transitional separation bubble

Calculations of the Reynolds averaged equations using two different turbulence models have been compared with direct numerical simulation of a transitional separation bubble. Three methods of transition modelling were investigated. The first had no transition adjustment, the second involved fixing the transition point at the location observed in the simulation and the third was a direct transformation of a method proposed by Wilcox [1] which involved sensitising the eddy viscosity and transport equations to the local turbulent Reynolds number. The models captured the general features of the flow but were unable to show the recovery behaviour of the flow behind the bubble. Reasons for the failure are discussed using apriori analysis of terms in the model equations.

Two- and three-dimensional large-eddy simulations of a transitional separation bubble

In the present study we use two- and three-dimensional large-eddy simulations to examine the role of small-scale turbulence within a transitional separation bubble studied experimentally by Gaster (AGARD Conference Proceedings No. 4, 1966, pp. 813–854). In addition, several large-eddy simulation parameters and models are studied to show their effect on the computations. The inclusion of a small-scale turbulence model in the two-dimensional computations leads to an increase in the time-averaged separation bubble length and a slight reduction in the peak of the pressure coefficient distribution near reattachment. Increasing the filter width or increasing the Smagorinsky coefficient reduces the peak in the pressure coefficient distribution but also decreases the pressure coefficient within the pressure plateau. The two-dimensional LES accurately predicts the time-averaged bubble length of Gaster but does not accurately describe the experimental wall pressure distribution within the bubble. Three-dimensional LES computations allow the generation of vortex shedding and Görtler vortices within the separated region. A computation without a subgrid scale model allows the Görtler vortices to grow in strength and eliminate the boundary layer separation. The application of a subgrid scale model reduces the strength of Görtler vortices and spanwise vortex shedding. This produces a bubble size and time-averaged wall pressure distribution which compare favorably with experiment. Little difference is seen between the results using the constant coefficient and dynamic coefficient models.

Transition in a Separation Bubble

In the interest of being able to predict separating–reattaching flows, it is necessary to have an accurate model of transition in separation bubbles. An experimental investigation of the process of turbulence development in a separation bubble shows that transition occurs within the separated shear layer. A comparison of simultaneous velocity traces from comparison of simultaneous velocity traces from probes separated in the lateral direction suggests that Kelvin–Helmholtz waves, which originate in the laminar shear layer, do not break down to turbulence simultaneously across their span when they proceed to agglomerate. The streamwise development of intermittency in this region can be characterized by turbulent spot theory with a high dimensionless spot production rate. Moreover, the progression of intermittency along the centerline of the shear layer is similar to that in attached boundary layer transition. The transverse development of intermittency is also remarkably similar to that in attached boundary layers. The parameters obtained from these measurements agree with correlations previously deduced from turbulence intensity measurements.

Prediction of separation bubbles using improved transition criterion with two-equation turbulence model

The two-layer κ-e turbulence models has been incorporated successfully in the Navier-Stokes procedure developed earlier to solve the transitional separation bubble on an airfoil. On the basis of the transition points deduced from a series of calculations with this improved procedure, a new transition criterion for leading-edge separation bubbles that correlates the transition Reynolds number with the Reynolds number at separation and the Thwaites parameter is established. Sample calculations demonstrate that this correlation performs excellently for all the cases examined.

On the Scaling of Separation Bubbles.

A simple analysis, based on the scaling of the Navier-Stokes equations, is presented for general flow situations involving separation bubbles. Three possible functional relations between the separation bubble length L and Reynolds number Re are derived : (i) L∝Re, (ii) L∝1/Re, and (iii) L=constant. These relations are shown to correspond to the experimentally observed behavior of laminar, transitional and turbulent separation bubbles, respectively. The results of the analysis are discussed in the context of several internal and external separated flows in various geometries, and the corresponding empirical constants are evaluated from available experimental and numerical data.