Academic literature on the topic 'Wake-Induced Transition'

A theory for transition from laminar to turbulent flow as the result of unsteady, periodic passing of turbulent wakes in the free stream is developed using Emmons’ transition model. Comparisons made to flat plate boundary layer measurements and airfoil heat transfer measurements confirm the theory.

AbstractBoundary layer transition induced by the wake of a circular cylinder in the free stream has been investigated using the particle image velocimetry technique. Some differences between simulation and experimental studies have been reported in the literature, and these have motivated the present study. The appearance of spanwise vortices in the early stage is further confirmed here. A spanwise vortex appears to evolve into a $ \mrm{\Lambda} $/hairpin vortex; the flow statistics also confirm such vortices. With increasing Reynolds number, based on the cylinder diameter, and with decreasing cylinder height from the plate, the physical size of these hairpin-like structures is found to decrease. Some mean flow characteristics, including the streamwise growth of the disturbance energy, in a wake-induced transition resemble those in bypass transition induced by free stream turbulence. Streamwise velocity streaks that are eventually generated in the late stage often undergo sinuous-type oscillations. Similar to other transitional flows, an inclined shear layer in the wall-normal plane is often seen to oscillate and shed vortices. The normalized shedding frequency of these vortices, estimated from the spatial spacing and the convection velocity of these vortices, is found to be independent of the Reynolds number, similar to that in ribbon-induced transition. Although the nature of free stream disturbance in a wake-induced transition and that in a bypass transition are different, the late-stage features including the flow breakdown characteristics of these two transitions appear to be similar.

A study has been made of the process of laminar to turbulent transition induced by a von Karman vortex street wake, in the boundary layer on a flat plate. The boundary layer developed under zero pressure gradient conditions while the vortex street was generated by a cylinder positioned in the free stream. Hot-wire measurements over a range of Strouhal frequencies and free stream velocities were used for the identification of the transition onset. From the analysis of the experimental data, two different transition mechanisms known in the literature as ‘strong’ wake and ‘weak’ wake induced transition, could be identified. It was established that, the onset of the strong von Karman wake induced transition process was a function of the free stream velocity, the position of the cylinder with respect to the plate, the cylinder diameter, the drag coefficient and the minimum velocity in the developing wake at the streamwise position of the onset of the boundary layer transition. The end of the strong wake induced transition, was defined at the streamwise distance, where the wake of the cylinder met the wall. A correlation for the prediction of the onset of the transition was developed

The development of the unsteady suction side boundary layer of a highly loaded LP turbine blade has been investigated in a rectilinear cascade experiment. Upstream rotor wakes were simulated with a moving-bar wake generator. A variety of cases with different wake-passing frequencies, different wake strength, and different Reynolds numbers were tested. Boundary layer surveys have been obtained with a single hotwire probe. Wall shear stress has been investigated with surface-mounted hot-film gages. Losses have been measured. The suction surface boundary layer development of a modern highly loaded LP turbine blade is shown to be dominated by effects associated with unsteady wake-passing. Whereas without wakes the boundary layer features a large separation bubble at a typical cruise Reynolds number, the bubble was largely suppressed if subjected to unsteady wake-passing at a typical frequency and wake strength. Transitional patches and becalmed regions, induced by the wake, dominated the boundary layer development. The becalmed regions inhibited transition and separation and are shown to reduce the loss of the wake-affected boundary layer. An optimum wake-passing frequency exists at cruise Reynolds numbers. For a selected wake-passing frequency and wake strength, the profile loss is almost independent of Reynolds number. This demonstrates a potential to design highly loaded LP turbine profiles without suffering large losses at low Reynolds numbers.

Measurements of wake-affected heat transfer distributions on a flat plate are made by use of a wake generator that consists of a rotating disk and several types of circular cylinder. The main purpose of this study is to construct a wake-induced transition model in terms of an intermittency factor, considering the evolution of the wake-induced turbulent region, a so-called turbulent patch in a distance–time diagram. A comparison between the proposed transition model and the measured heat transfer data reveals that the transition model yields good agreement with the measured data of all test conditions in this study.

As the second part of the study, detailed hot-wire anemometry measurements of wake-affected boundary layers on the flat plate are made. These measurements are organized in order, first, to check the standpoint of the modeling of the wake-induced transition proposed in Part I, and second, to observe wake–boundary layer interaction in detail from a viewpoint of direct and indirect effect of the wake passage upon turbulent spot generation within the boundary layer, as described by Walker (1993). The validity of the presumed state of the wake-affected boundary layer in the distance–time domain, which constitutes the basis of the transition model, is confirmed to great extent. However, it is also found that the criterion for the onset of the wake-induced transition adopted in Part I should be reconsidered. Some successful attempts are therefore made to specify the transition onset.

A theory for transition from laminar to turbulent flow with an unsteady, periodic passing of turbulent wakes in the free stream has recently been presented by the authors. The theory considers a time-averaged transitional flow caused by the formation and propagation of turbulent strips along the surface. To apply the theory, however, both the origin and a quantity related to the production rate of these turbulent strips must be known. In this paper, after a brief review of the theory, a dimensional analysis of the problem is presented and data from experiments reexamined in light of the result. From this, an expression for the time-averaged intermittency is obtained, which may be used to calculate the time-averaged distributions of various boundary layer quantities for wake-induced transitional flow.

Aerodynamic and heat transfer investigations were performed on a constant curvature curved plate in a subsonic wind tunnel facility for various wake passing frequencies under zero pressure gradient conditions. Steady and unsteady boundary layer transition measurements were taken on the concave surface at different wake passing frequencies in which a rotating squirrel cage was used to generate the unsteady wake flow. The data were analyzed using time-averaged and ensemble averaged techniques to provide insight into the growth of the boundary layer and transition. Ensemble averaged turbulence intensity contours in the temporal spatial domain showed that transition was induced for increasing wake passing frequency and structure. The local heat transfer coefficient distributions for the concave and convex surfaces were determined for each wake passing frequency using a liquid crystal heat transfer measurement technique. Aerodynamic and heat transfer investigations showed that higher wake passing frequencies caused earlier transition on the concave surface. Local Stanton numbers were calculated on the concave surface and compared to Stanton numbers predicted using a boundary layer and heat transfer calculation method. On the convex side, no effect of wake passing on heat transfer was observed, due to a separation bubble that induced transition.

Aerodynamic and heat transfer investigations were done on a constant curvature curved plate in a subsonic wind tunnel facility for various wake passing frequencies and zero pressure gradient conditions. Steady and unsteady boundary layer transition measurements were taken on the concave surface of the curved plate at different wake passing frequencies where a rotating squirrel-cage generated the unsteady wake flow. The data were analyzed using timeaveraged and ensemble-averaged techniques to provide insight into the growth of the boundary layer and transition. Ensemble-averaged turbulence intensity contours in the temporal spatial domain showed that transition was induced for increasing wake passing frequency and structure. The local heat transfer coefficient distribution for the concave and convex surface was determined at those wake passing frequencies using a liquid crystal heat transfer measurement technique. Detailed aerodynamic and heat transfer investigations showed that higher wake passing frequency caused transition to occur earlier on the concave surface. Local Stanton numbers were also calculated on the concave surface and compared with Stanton numbers predicted using a differential boundary layer and heat transfer calculation method. On the convex side, no effect of wake passing frequency on heat transfer was observed due to a separation bubble that induced transition.

The concentrated use of one-dimensional hot-wire anemometry has shown leading edge boundary layer disturbances induced under each passing wake, which grow steadily via by-pass and natural transition methods into turbulent strips that convect with the flow. These disturbances are of such strength that the separated region is resisted and effectively swept away by the passing turbulence, momentarily giving rise to a wholly attached laminar boundary layer across the entire flat plate surface.;Propagation rates have shown leading edge speeds in excess of freestream values, a combination of boundary layer destabilisation and the negative jet effect of each wake. Trailing edge values are of typically 50% freestream.;Controlling the chordwise proximity of neighbouring wakes allowed for the investigation of the effect and extent of the calmed region behind each induced turbulent strip. Measurements have shown that although there is no slowing of the advancing turbulence by the calmed flow, a strong suppression of velocity fluctuations is seen, related to the proximity of the turbulent strips. Turbulence level reductions of up to 40% have been demonstrated as wake spacing is reduced.;The use of microphones to measure surface pressure fluctuations revealed the amplification of instabilities in the separated shear layer. These have been shown to be viscous Tollmien-Schlichting vortices, originating from fluctuations in the attached laminar boundary layer, and are responsible for the natural development of turbulent flow between wakes.

The initial slow viscous growth of the Tollmein-Schlichting wave in a canonical boundary layer transition is absent in bypass and wake-induced transitions. Although there have been a great deal of studies pertaining to bypass transition in boundary layers, the underlying breakdown mechanism is not clearly understood and it continues to be a subject of interest. Similarly, a wake-induced transition caused by Karman wake in the freestream remains poorly understood. The breakdown in this case is caused by anisotropic disturbances containing large scale unsteadiness in the freestream. Differing view points among workers on the transition process have also added to the complexities. In this thesis, bypass and wake-induced boundary layer transitions studied experimentally towards understanding various flow breakdown features are reported. The measurements were made on a flat plate boundary layer in a low-speed wind tunnel. The particle image velocimetry (PIV) technique was extensively used. Various grids were used to generate nearly isotropic freestream turbulence. A circular cylinder was placed at different heights from the plate leading edge to generate Karman wake in the freestream. Two cylinders of different diameters were used to vary the Reynolds number(based on the cylinder diameter). The PIV measurements being simultaneous over a large spatial domain enabled to assess various spatial transitional flow structures. In the case of bypass transition, the streamwise velocity fluctuation, u, is found to exhibit some organized negative and positive fluctuations that dominate the flow during transition, and confirm the simulation results reported in the literature. These positive and negative u fluctuations are found to be associated with the streak unsteadiness. By conditional sampling of these positive and negative u fluctuations, we find that urms (root-mean-squaredof u)can be expressed as a linear combination of urms,f and urms,b,i.e. urms = a(urms,f + urms,b); ais constant, and the subscripts fand bdenote the positive and nega-tive ufluctuations, respectively. Both urms,f and urms,b arefoundto follow the non-modal growth distribution. The wall-normal results clearly show that an inclined shear layer is often associated with an organized structure of negative ufluctuations and an inflectional in-stantaneous velocity profile. These inclined shear layers appear to be similar to those in ribbon-induced transition. The turbulent spot precursor appears to be the vortex shedding from an oscillating in-clined shear layer. Interestingly, the normalized vortex shedding fre-quency is found to be Reynolds number invariant, as in the case of ribbon-induced transition. The present study also confirms the sim-ulated turbulent spot features, including a thin log-law at the break-down stage. The spanwise plane PIV results reveal the signature of streak secondary instability in the flow in terms of symmetric and anti-symmetric streaks oscillations. The initial growth of streak amplitude is followed by a slow decay. The maximum streak amplitude is well above30% of the freestream velocity. These two aspects provide support to the streak instability analysis reported in the literature. While the present wake-induced transition study provides some sup-port to the available numerical simulation and experimental results, some new results have also emerged. The measured sharp rise in the disturbance energy during transition is found to be closer to the simulated result, compared to the difference reported in the literature. The spanwise vortices in the early stage, as also seen in other experimental studies, deform leading to the formation of lambda structures, the signature of which is found by the linear stochastic analysis. With increased Reynolds number and decreased cylinder height from the plate, the physical size of the lambda structure is found to decrease. These lambda structures are often found to appear in a staggered manner in the spanwise plane, as in the case of sub-harmonic boundary layer transition. Although a sub-harmonic peak in the frequency spectra is reported in the literature, as also in the present study, the clear staggered pattern went unnoticed. Streamwise streaks are subsequently generated due to the mean shear stretching of these lambda vortices. The spanwise spacing of these streamwise streaks is found to be comparable with the recent simulation results. Also, these streaks are found to undergo somewhat sinuous-like oscillations, compared to the only varicose type oscillations reported in the literature. The streak amplitude is found to saturate at about 35% of the freestream speed. Here again an inclined shear layer in the wall-normal plane is associated with organized negative u fluctuations and an inflectional instantaneous velocity profile. The movement of the peak urms towards the wall is found to be due to the positive u fluctuation, which follows a hairpin-like structure. The inclined shear layers herein are associated with the lambda or a hairpin-like structure. As in a by-pass transition, an inclined shear layer, vortex shedding from it, the imprint of which is also found in the linear stochastic analysis are present. The normalized high frequency shed vortices is found to be Reynolds number invariant in the present wake-induced transition, as in ribbon-induced and bypass transitions. Compared to the re-cent suggestion that the parent-offspring mechanism is the governing self-sustaining mechanism in the boundary layer, the present study suggests that streak-instability mechanism is also present. The proper orthogonal decomposition(POD) analysis of the experimental data was carried out with an emphasis on the bypass transition case studied. The first few energetic POD modes are found to capture the dominant flow structures, i.e. the organized positive and negative u fluctuations. In the case of bypass transition, the first two energetic POD modes are self-similar, i.e. independent of the freestream turbulent intensity and the Reynolds number. An attempt is also made to construct a low-dimensional model with the POD eigenfunction modes to predict the qualitative dynamics of bypass transition. This has revealed the existence of a traveling disturbance in the bypass transition. On the whole, the present study shows some similar breakdown features in bypass and wake-induced transitions, although more studies in this regard are essential.

ABSTRACT Widespread episodes of major contractional orogenesis correlate commonly with ages of high-pressure eclogitic rocks formed during bottom-driven, induced subduction of crustal terranes. Rapid exhumation of the deeply emplaced crust has led to the development of the concept of a “tectonic dunk.” The dunk process is a hallmark component of a suite of linked tectonic, magmatic, metamorphic, and sedimentologic processes that systematically follow plate interactions, including collision, coupling, and capture resulting in plate reconfiguration and changes of movement. Plate capture, which takes place during mechanical connection of plates within a “clutch” zone, is followed generally by an abrupt transition to plate stretching in response to drag or plate spin. Plate stretch, which is accommodated during drag by a network of complementary strike-slip and normal faults or during spin by regional domains of transtension, is recorded by “postorogenic,” back-arc extension, basin formation, and magmatism, extensive domains of which comprise large igneous provinces. As a captured continental plate is dragged or rotates, ductile mantle is disrupted and displaced by protuberances, such as a slab coupled against the base of an overriding plate and/or orogenic roots extending down from a cratonic core. The mantle turbulence resembles a wave-like ship’s wake with tsunami-like movement, albeit below crust. The arrival of a moving mantle bulge or wave is inferred to be focused along continental plate margins where subduction is induced, as recorded by magmatism and eclogitic rocks that form during deep emplacement of crustal terranes. Concurrent shortening of crust in the vicinity of the plate margin is inferred from inversion and uplift of marginal rift basins, obduction, and development of fold-and-thrust belts. As the mantle wave passes beneath plate interiors, tens to hundreds of meters of uplift, recorded by oceanic atolls, continental stream incision, regional unconformities, and local transitions to evaporite within shelf settings, record epeirogeny. After passage of the wave, common development of sheet-like bodies of quartzose sandstone, especially during the early Paleozoic, suggest postwave, regional subsidence. Resumption and re-invigoration of extension are recorded by eduction of dunked crust and conspicuous, widespread, volcanic eruptions recorded by tuffaceous layers intercalated with carbonaceous black shale within broad basins developed above thickened crust.

A theory for transition from laminar to turbulent flow as the result of unsteady, periodic passing of turbulent wakes in the free stream is developed using Emmons’ transition model. Comparisons made to flat plate boundary layer measurements and airfoil heat transfer measurements confirm the theory.

The paper presents an experimental and numerical analysis of the interaction between wakes and boundary layers on aerodynamic blade profiles. The experiment revealed that incoming wakes interact with boundary layers and cause the significant increase of velocity fluctuations in the boundary layer and in consequence shift the transition zone towards the leading edge. The full time evolution of periodic wake induced transition was reproduced from measurements. The numerical simulation of the flow around the blade profile has been performed with the use of the adaptive grid viscous flow unNEWT PUIM solver with a prescribed unsteady intermittency method (PUIM) developed at Cambridge University, UK. The results obtained give evidence that the turbulence transported within the wake is mainly responsible for the transition process. The applied CFD solver was able to reproduce some essential flow features related to the bypass and wake-induced transitions and the simulations reveal good agreement with the experimental results in terms of localisation and extent of wake-induced transition.

A theory for transition from laminar to turbulent flow with an unsteady, periodic passing of turbulent wakes in the free stream has recently been presented by the authors. The theory considers a time-averaged transitional flow caused by the formation and propagation of turbulent strips along the surface. To apply the theory, however, both the origin and a quantity related to the production rate of these turbulent strips must be known. In this paper, after a brief review of the theory, a dimensional analysis of the problem is presented and data from experiments re-examined in light of the result. From this, an expression for the time-averaged intermittency is obtained which may be used to calculate the time-averaged distributions of various boundary layer quantities for wake-induced transitional flow.

This study aims at deepening the understanding of wake-induced bypass transition process of a flat-plate boundary layer using two types of wake generating objects, which are small spheres and thin wires. Main focus is on emergence of isolated turbulent spots from the influence of the wake passage over the boundary layer. Precursors of the wake-induced turbulent spot, which have not been observed in an explicit manner in any other previous studies, are also of concern in this study. It is expected that wakes from the wires are so weak that an isolated turbulent spot may be induced by the wire wake, while the position of the spot emergence varies randomly along the wire. A multi-channel sensor with 7 hot-wire probes acquires the velocity data of the flow over the flat plate subjected to the wake passage. These velocity data reveal the spot shape and spot generation rate. In addition, the existence of Klebanoff mode in this wake-affected boundary layer is examined.

A closely-spaced array of hot-film gages fully covering both suction and pressure surfaces on the outlet stator of a 1.5-stage axial compressor was used to obtain dynamic measurements of wall shear stress. Observations were made over a range of Reynolds numbers at an incidence close to the design value. Various methods of presnting the data, including time-space contour plots of ensemble-average intermittency from the film gages are analyzed: related problems of interpretation are discussed. Extensive regions of laminar flow were identified on the suction surface: at the highest Reynolds number, small laminar patches were still evident at 85% chord and transitional flow covered up to 70% of suction surface length. The influence of passing rotor wakes on transition varied markedly with Reynolds number. The behavior of wake-induced transitional strips on the suction and pressure surfaces of the compressor blade differed significantly; their propagation characteristics also varied in some respects from those observed on turbine airfoils.

Wake-induced bypass transition of boundary layers on a flat plate subjected to favorable and adverse pressure gradients was investigated. Detailed boundary layer measurements were conducted using two hot-wire probes. A spoked-wheel-type wake generator was used to create periodic wakes in front of the flat plate. The main focus of this study was to reveal the effect of the Strouhal number, which changed by using different numbers of wake-generating bars, on the turbulence intensity distribution and the transition onset position of the boundary layer on the flat plate using two hot-wire probes.

Recent experimental work has documented the importance of wake passing on the behavior of transitional boundary layers on the suction surface of axial compressor blades. This paper documents computational fluid dynamics (CFD) simulations using a commercially-available general-purpose CFD solver, performed on a representative case with unsteady transitional behavior. The study implements a new, advanced version of a three-equation eddy-viscosity model previously developed and documented by the authors, which is capable of resolving boundary-layer transition. It is applied to the test cases of steady and unsteady boundary-layer transition on a 2-D flat plate geometry with a freestream velocity distribution representative of the suction side of a compressor airfoil. The CFD results are analyzed and compared to a similar experimental test case from the open literature. Results with the new model show a dramatic improvement over more typical RANS-based modeling approaches, and highlight the importance of resolving transition in both steady and unsteady compressor aero simulations.

Measurements of transitional flow in regions of strong adverse pressure gradient on an axial compressor stator are reported. The range of observations covers separating laminar flow at transition onset, and reattachment of intermittently turbulent periodically separated shear layers. Transition was characterised by the regular appearance of turbulent spots in association with the rotor blade wake disturbances. However, the initial breakdown did not coincide with the wake passage as has usually been observed by other workers. The spots rather evolved from the growth of instability wave packets which lagged the wake passage. Data presented from the compressor blade measurements include: mean and ensemble-average velocities and associated integral parameters; distributions of total, periodic and random disturbance components; typical individual velocity fluctuation records; contours of ensemble-average random disturbance level; and boundary layer intermittency distributions. Measurements of turbulent intermittency showed a significant fall in this quantity near the wall in the reattaching flow. This has significant implications for the interpretation of transition data from surface film gage observations.

The development of the unsteady suction side boundary layer of a highly loaded LP turbine blade has been investigated in a rectilinear cascade experiment. Upstream rotor wakes were simulated with a moving-bar wake generator. A variety of cases with different wake-passing frequencies, different wake strength and different Reynolds-numbers were tested. Boundary layer surveys have been obtained with a single hot-wire probe. Wall shear stress has been investigated with surface-mounted hot-film gauges. Losses have been measured. The suction surface boundary layer development of a modern highly loaded LP turbine blade is shown to be dominated by effects associated with unsteady wake-passing. Whereas without wakes the boundary layer features a large separation bubble at a typical cruise Reynolds-number, the bubble was largely suppressed if subjected to unsteady wake-passing at a typical frequency and wake strength. Transitional patches and becalmed regions, induced by the wake, dominated the boundary layer development. The becalmed regions inhibited transition and separation and are shown to reduce the loss of the wake-affected boundary layer. An optimum wake-passing frequency exists at cruise Reynolds-numbers. For a selected wake-passing frequency and wake-strength, the profile loss is almost independent of Reynolds-number. This demonstrates a potential to design highly loaded LP turbine profiles without suffering large losses at low Reynolds-numbers.

A transition model for describing wake-induced transition is presented based on the SST turbulence model by Menter and two dynamic equations for intermittency: one for near-wall intermittency and one for free-stream intermittency. In the Navier-Stokes equations, the total intermittency factor, which is the sum of the two, multiplies the turbulent viscosity computed by the turbulence model. The quality of the transition model is illustrated on the T106A test cascade for different levels of inlet free-stream turbulence intensity. The unsteady results are presented in space-time diagrams of shape factor, wall shear stress, momentum thickness and intermittency on the suction side. Results show the capability of the model to capture the physics of unsteady transition. Inevitable shortcomings are also revealed.