Bypass transition and spot nucleation in boundary layers

The spatio-temporal aspects of the transition to turbulence are considered in the case of a boundary layer flow developing above a flat plate exposed to free-stream turbulence. Combining results on the receptivity to free-stream turbulence with the nonlinear concept of a transition threshold, a physically motivated model suggests a spatial distribution of spot nucleation events. To describe the evolution of turbulent spots a probabilistic cellular automaton is introduced, with all parameters directly fitted from numerical simulations of the boundary layer. The nucleation rates are then combined with the cellular automaton model, yielding excellent quantitative agreement with the statistical characteristics for different free-stream turbulence levels. We thus show how the recent theoretical progress on transitional wall-bounded flows can be extended to the much wider class of spatially developing boundary-layer flows

Turbulence transition in the asymptotic suction boundary layer

We study the transition to turbulence in the asymptotic suction boundary layer (ASBL) by direct numerical simulation. Tracking the motion of trajectories intermediate between laminar and turbulent states we can identify the invariant object inside the laminar-turbulent boundary, the edge state. In small domains, the flow behaves like a travelling wave over short time intervals. On longer times one notes that the energy shows strong bursts at regular time intervals. During the bursts the streak structure is lost, but it reforms, translated in the spanwise direction by half the domain size. Varying the suction velocity allows to embed the flow into a family of flows that interpolate between plane Couette flow and the ASBL. Near the plane Couette limit, the edge state is a travelling wave. Increasing the suction, the travelling wave and a symmetry-related copy of it undergo a saddle-node infinite-period (SNIPER) bifurcation that leads to bursting and discrete-symmetry shifts. In wider domains, the structures localize in the spanwise direction, and the flow in the active region is similar to the one in small domains. There are still periodic bursts at which the flow structures are shifted, but the shift-distance is no longer connected to a discrete symmetry of the flow geometry. Two different states are found by edge tracking techniques, one where structures are shifted to the same side at every burst and one where they are alternatingly shifted to the left and to the right.Comment: Conference TSFP8, Poitiers 2013. TSFP-8 conference proceedings 2013, http://www.tsfp-conference.org/proceedings

Transition to turbulence in the asymptotic suction boundary layer

The focus of this thesis is on the numerical study of subcritical transition to turbulence in the asymptotic suction boundary layer (ASBL). Applying constant homogeneous suction prevents the spatial growth of the boundary layer, granting access to the asymptotic dynamics. This enables research approaches which are not feasible in the spatially growing case. In a first part, the laminar–turbulent separatrix of the ASBL is investigated numerically by means of an edge-tracking algorithm. The consideration of spanwise-extended domains allows for the robust localisation of the attracting flow structures on this separatrix. The active part of the identified edge states consists of a pair of low- and high-speed streaks, which experience calm phases followed by high energy bursts. During these bursts the structure is destroyed and re-created with a shift in the spanwise direction. Depending on the streamwise extent of the domain, these shifts are either regular in direction and distance, and periodic in time, or irregular in space and erratic in time. In all cases, the same clear regeneration mechanism of streaks and vor- tices is identified, bearing strong similarities with the classical self-sustaining cycle in near-wall turbulence. Bifurcations from periodic to chaotic regimes are studied by varying the streamwise length of the (periodic) domain. The resulting bifurcation diagram contains a number of phenomena, e.g. multistability, intermittency and period doubling, usually investigated in the context of low-dimensional systems. The second part is concerned with spatio–temporal aspects of turbulent ASBL in large domains near the onset of sustained turbulence. Adiabatically decreasing the Reynolds number, starting from a fully turbulent state, we study low-Re turbulence and events leading to laminarisation. Furthermore, a robust quantitative estimate for the lowest Reynolds number at which turbulence is sustained is obtained at Re  270.QC 20140213</p

Edge states and transition to turbulence in boundary layers

The focus of this thesis is the numerical study of subcritical transition to turbulence in boundary-layer flows. For the most part, boundary layers with uniform suction are considered. Constant homogeneous suction counteracts the spatial growth of the boundary layer, rendering the flow parallel. This enables research approaches which are not feasible in the context of spatially developing flows. In the first part, the laminar–turbulent separatrix of the asymptotic suction boundary layer (ASBL) is investigated numerically by means of an edge-tracking algorithm. The obtained edge states experience recurrent dynamics, going through calm and bursting phases. The self-sustaining mechanism bears many similarities with the classical regeneration cycle of near-wall turbulence. The recurrent simple structure active during calm phases is compared to the nucleation of turbulence events in bypass transition originating from delocalised initial conditions. The implications on the understanding of the bypass-transition process and the edge state's role are discussed. Based on this understanding, a model is constructed which predicts the position of the nucleation of turbulent spots during free-stream turbulence induced transition in spatially developing boundary-layer flow. This model is used together with a probabilistic cellular automaton (PCA), which captures the spatial spreading of the spots, correctly reproducing the main statistical characteristics of the transition process. The last part of the thesis is concerned with the spatio-temporal aspects of turbulent ASBL in extended numerical domains near the onset of sustained turbulence. The different behaviour observed in ASBL, i.e. absence of sustained laminar–turbulent patterns, which have been reported in other wall-bounded flows, is associated with different character of the large-scale flow. In addition, an accurate quantitative estimate for the lowest Reynolds number with sustained turbulence is obtainedQC 20160429</p

Turbulence and laminarisation in the spatially extended asymptotic suction boundary layer

We study numerically transitional coherent structures in a boundary-layer flow with homogeneous suction at the wall (the so-called asymptotic suction boundary layer ASBL). The dynamics restricted to the laminar-turbulent separatrix is investigated in a spanwise-extended domain that allows for robust localisation of all edge states. We work at fixed Reynolds number and study the edge states as a function of the streamwise period. We demonstrate the complex spatio-temporal dynamics of these localised states, which exhibits multistability and undergoes complex bifurcations leading from periodic to chaotic regimes. It is argued that in all regimes the dynamics restricted to the edge is essentially low-dimensional and non-extensive.QC 20140213</p

Screening for Diabetic Retinopathy in Optometric Practice

Maģistra darbs uzrakstīts latviešu valodā uz 46 lapām, satur 21 attēlu, 12 tabulas, 2 pielikumus un 105 atsauces uz literatūras avotiem. Darba mērķis: novērtēt optometristu un automātiskās skrīninga programmatūras prasmes atpazīt diabētiskās retinopātijas pazīmes. Pētījuma dalībnieki: pētījumā piedalījās 2 oftalmologi, 14 optometristi un automatizētā skrīninga programmatūra AI Avenue. Metode: 196 tīklenes attēliem tika veikts skrīninga tests atbilstoši starptautiskās diabētiskās retinopātijas klasifikācijai. Rezultāti: Rekomendēto testa standartu – jutība un specifiskums ≥ 80% sasniedza oftalmologs un 2 optometristi. AI Avenue sasniedza vienīgi rekomendēto testa specifiskumu. Secinājumi: AI Avenue ļoti labi atpazīst gadījumus, kad nav saslimšana, savukārt optometristiem ir tendence kļūdaini pozitīvi diagnosticēt slimību. Atslēgas vārdi: cukura diabēts, diabētiskā retinopātija, diabētiskā makulas tūska, automatizēta skrīninga programmatūra.Master thesis is written in Latvian. It contains 46 pages, 21 images, 12 tables, 105 references and 2 attachments. Purpose: To evaluate the skills of optometrists and automatic diabetic retinopathy analysis software to recognize the signs of diabetic retinopathy. Participants: The study included 17 participants – 2 ophthalmologists, 14 optometrists and 1 automatic diabetic retinopathy screening software AI Avenue. Methods: The grading according International Diabetic Retinopathy Classification of 196 retinal images provided by participants. Results: The recommended test standard – sensitivity and specificity are more or equal to 80% was reached by 3 participants – ophthalmologist and 2 optometrists. AI Avenue reached only recommended test specificity ≥ 80.00%. Conclusions: AI Avenue recognizes cases of non-disease very well, while optometrists tend to misdiagnose the grade of disease. Key words: diabetes mellitus, diabetic retinopathy, diabetic macular oedema, automated detection of diabetic retinopathy

Turbulence collapse in a suction boundary layer

International audienceurbulence in the asymptotic suction boundary layer is investigated numerically at the verge of laminarisation using direct numerical simulation. Following an adiabatic protocol, the Reynolds number Re is decreased in small steps starting from a fully turbulent state until laminarisation is observed. Computations in a large numerical domain allow in principle for the possible coexistence of laminar and turbulent regions. However, contrary to other subcritical shear flows, no laminar–turbulent coexistence is observed, even near the onset of sustained turbulence. High-resolution computations suggest a critical Reynolds number Reg&#8776;270, below which turbulence collapses, based on observation times of O(105) inertial time units. During the laminarisation process, the turbulent flow fragments into a series of transient streamwise-elongated structures, whose interfaces do not display the characteristic obliqueness of classical laminar–turbulent patterns. The law of the wall, i.e. logarithmic scaling of the velocity profile, is retained down to Reg, suggesting a large-scale wall-normal transport absent in internal shear flows close to the onset. In order to test the effect of these large-scale structures on the near-wall region, an artificial volume force is added to damp spanwise and wall-normal fluctuations above y+=100, in viscous units. Once the largest eddies have been suppressed by the forcing, and thus turbulence is confined to the near-wall region, oblique laminar–turbulent interfaces do emerge as in other wall-bounded flows, however only transiently. These results suggest that oblique stripes at the onset are a prevalent feature of internal shear flows, but will not occur in canonical boundary layers, including the spatially growing ones

Localised edge states in the asymptotic suction boundary layer

International audienceThe dynamics on the laminar–turbulent separatrix is investigated numerically for boundary-layer flows in the subcritical regime. Constant homogeneous suction is applied at the wall, resulting in a parallel asymptotic suction boundary layer (ASBL).When the numerical domain is sufficiently extended in the spanwise direction, the coherent structures found by edge tracking are invariably localized and their dynamics shows bursts that drive a remarkable regular or irregular spanwise dynamics. Depending on the parameters, the asymptotic dynamics on the edge can be either periodic in time or chaotic. A clear mechanism for the regeneration of streaks and streamwise vortices emerges in all cases and is investigated in detail

Localized edge states in the asymptotic suction boundary layer

The dynamics on the laminar-turbulent separatrix is investigated numerically for boundary-layer flows in the subcritical regime. Constant homogeneous suction is applied at the wall, resulting in a parallel asymptotic suction boundary layer (ASBL). When the numerical domain is sufficiently extended in the spanwise direction, the coherent structures found by edge tracking are invariably localized and their dynamics shows bursts that drive a remarkable regular or irregular spanwise dynamics. Depending on the parameters, the asymptotic dynamics on the edge can be either periodic in time or chaotic. A clear mechanism for the regeneration of streaks and streamwise vortices emerges in all cases and is investigated in detail. © 2013 Cambridge University Press