Investigation of the dynamics in separated turbulent flow

Le Centre pour la Communication Scientifique Directe - HAL - Inria, 2022

Experiments were conducted to study the transition and flow development in a laminar separation bubble (LSB) formed on an aerofoil. The effects of a wide range of freestream turbulence intensity (0.15% < Tu < 6.26%) and streamwise integral length scale (4.6mm < Λ u < 17.2mm) are considered. The coexistence of a modal instability due to the LSB and a non-modal instability caused by streaks generated by freestream turbulence is observed. The presence of streaks in the boundary layer modifies the mean flow topology of the bubble. These changes in the mean flow field result in the modification of the convective disturbance growth, where an increase in turbulence intensity is found to dampen the growth of the modal instability. For a relatively fixed level of Tu, the variation of Λ u has modest effects, however a slight advancement of the non-linear growth of disturbances and eventual breakdown with the decrease in Λ u is observed. The data shows that the streamwise growth of the disturbance energy is exponential for the lowest levels of freestream turbulence and gradually becomes algebraic as the level of freestream turbulence increases. Once a critical turbulence intensity is reached, there is enough energy in the boundary layer to suppress the LSB, which in turn, results in the non-modal instability to take over the transition process. Linear stability analysis is conducted in the fore position of the LSB, and accurately models unstable frequencies and eigenfunctions for configurations subjected to levels of turbulence intensity levels up to 3%. Increasing the Tu resulted in the Reynolds number dependence to increase, suggesting that a viscous, rather than an invicid formulation of the stability equations is appropriate for modeling modal instabilities in the fore portion of the studied LSB.

International journal of heat and fluid flow, 2001

The turbulence statistics of an unsteady separated¯ow was experimentally investigated. A backward-facing step¯ow at Re 3700 was chosen as the test case, where periodic perturbation was introduced from its step edge. A two-dimensional particle imaging velocimeter (PIV) was used in the¯ow-®eld measurement. The measured results showed that the reattachment length was reduced by the applied periodic perturbation. There existed an optimum frequency for the promotion of the reattachment. When perturbed at the optimum frequency, St 0:19, the reattachment length was reduced by 30%. The Reynolds stress components were increased by the perturbation, and their distribution varied with the perturbation frequency. When perturbation at the optimum frequency was applied, Reynolds stress markedly increased near the reattachment zone. This increase in Reynolds stress enhanced the momentum transfer across the shear layer, enabling the promotion of the reattachment. On the other hand, the region where the perturbation at higher frequency than the optimum frequency increases Reynolds stress was limited immediately behind the step. The perturbation at lower frequency than the optimum frequency increased Reynolds stress in the region downstream of the reattachment zone. Therefore, both low-and high-frequency perturbations have less eect on the promotion of the reattachment than optimum-frequency perturbation. Ó

Journal of Applied Mechanics, 2005

Experiments in Fluids, 2006

Laboratory measurements of wall pressure fluctuations and aerodynamic fields were made in separated flows over a forward facing step (h = 30, 40 and 50 mm with U e = 15-40 m/s). An array of 16 offset pressure probes extending in the streamwise and the spanwise directions was especially developed for sensing the wall pressure fluctuations. The flow field was also investigated by wall flow visualizations and PIV to analyze the flow topology in an open section wind tunnel. The results show a different behavior of the flow depending on the aspect ratio l/h and d/h for high Reynolds numbers. The space time correlations between the wall pressure and the velocity fields were highlighted. The results show that high levels of these correlations are located at the top of the recirculation bubble, mainly in the shear layer and are extended downstream of the re-attachment point. Indeed, the results indicate that the flapping motion at the separation is important in the flow organization at the reattachment point.

Flow, Turbulence and Combustion (formerly Applied Scientific Research), 2000

A laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes transition. Finally the turbulent boundary layer reattaches, forming a laminar separation bubble (LSB). Laminar-turbulent transition within such a LSB is investigated by means of Laser-Doppler-Anemometry (LDA), Particle Image Velocimetry (PIV), and direct numerical simulation (DNS). The transition mechanism occurring in the flow-field under consideration is discussed in detail. Observations for the development of small disturbances are compared to predictions from viscous linear instability theory (Tollmien-Schlichting instability). Non-linear development of these disturbances and their role in final breakdown to turbulence is analyzed.

In this study the focus is made on the properties of the bistable flow between two cylinder in tandem for a pitch ratio at a Reynolds number of . Time resolved particle image velocimetry (PIV) is used to measure the spatial distribution and time fluctuations of the velocity field in the wake of the upstream cylinder. From velocity spectra and statistics fields, we show the existence of two distinct seemingly stable flow patterns, with different spatial structures and Strouhal numbers.

41st AIAA Fluid Dynamics Conference and Exhibit, 2011

Wide field of view stereo PIV measurements were conducted in the wall-normal-spanwise plane of fully-developed turbulent channel flow. These experiments represent a significant challenge to the stereo PIV method as the dominant velocity component was oriented normal to the PIV measurement plane while the in-plane components were predominantly much weaker turbulent fluctuations. Nevertheless, these measurements agreed well with DNS results at a similar Reynolds number, both in the mean streamwise velocity as well as the Reynolds normal and shear stresses, which validated the experimental methodology employed. Inspection of instantaneous velocity fields in this cross plane uncovered the spatial signatures of low-and high-momentum regions, the former of which have been previously linked to hairpin vortex packets in the outer layer of wall turbulence. POD was employed to study the spatial characteristics of the large-scale motions further and revealed a significant spanwise coherence in the form of alternating low-and high-momentum regions well beyond that reflected in two-point velocity correlations. Finally, similar crossplane stereo PIV measurements were conducted in a higher-Reynolds-number smooth-wall turbulent boundary layer and a similar level of spanwise coherence of the large-scale motions was noted. Rough-wall measurements, however, indicated weaker coherence of the larger spatial scales.

Journal of Fluid Mechanics, 1987

Detailed measurements within the separated shear layer behind a flat plate normal to an airflow are reported. A long, central splitter plate in the wake prevented vortex shedding and led to an extensive region of separated flow with mean reattachment some ten plate heights downstream. The Reynolds number based on plate height was in excess of 2 × 1044.Extensive use of pulsed-wire anemometry allowed measurements of all the Reynolds stresses throughout the flow, along with some velocity autocorrelations and integral timescale data. The latter help to substantiate the results of other workers obtained in separated flows of related geometry, particularly in the identification of a very low-frequency motion with a timescale much longer than that associated with the large eddies in the shear layer. Wall-skin-friction measurements are consistent with the few similar data previously published and indicate that the thin boundary layer developing beneath the separated region has some ‘laminar...

Flow, Turbulence and Combustion (formerly Applied Scientific Research), 2000

A three-dimensional Direct Numerical Simulation (DNS) of a laminar separation bubble in the presence of oscillating flow is performed. The oscillating flow induces a streamwise pressure gradient varying in time. The special shape of the upper boundary of the computational domain, together with the oscillating pressure gradient causes the boundary layer flow to alternately separate and re-attach. When the inflow decelerates, the shear layer starts to separate and rolls up. Simultaneously the flow becomes 3D. After a transient period, the phase-averaged reverse flow inside the separation bubble reaches speeds ranging from 20 up to 150% of the free-stream velocity. During these phases, the flow is absolutely unstable and self-sustained turbulence can exist. When the inflow starts to accelerate, a spanwise roll of turbulent flow is shed from the shear layer. Shortly after this, the remainder of the separation bubble moves downstream and rejoins with the shed turbulent roll. During the flow-acceleration phase, a patch of laminar boundary layer flow is obtained. Along the flat plate, a series of turbulent patches of flow travelling downstream, separated by laminar flow can be observed, reminiscent of boundary layer flow in a turbine cascade with periodically appearing free-stream disturbances.

The effects of non-normality and nonlinearity of the two-dimensional Navier–Stokes differential operator on the dynamics of a large laminar separation bubble over a flat plate have been studied in both subcritical and slightly supercritical conditions. The global eigenvalue analysis and direct numerical simulations have been employed in order to investigate the linear and nonlinear stability of the flow. The steady-state solutions of the Navier–Stokes equations at supercritical and slightly subcritical Reynolds numbers have been computed by means of a continuation procedure. Topological flow changes on the base flow have been found to occur close to transition, supporting the hypothesis of some authors that unsteadiness of separated flows could be due to structural changes within the bubble. The global eigenvalue analysis and numerical simulations initialized with small amplitude perturbations have shown that the non-normality of convective modes allows the bubble to act as a strong amplifier of small disturbances. For subcritical conditions, nonlinear effects have been found to induce saturation of such an amplification, originating a wave-packet cycle similar to the one established in supercritical conditions, but which is eventually damped. A transient amplification of finite amplitude perturbations has been observed even in the attached region due to the high sensitivity of the flow to external forcing, as assessed by a linear sensitivity analysis. For supercritical conditions, the non-normality of the modes has been found to generate low-frequency oscillations (flapping) at large times. The dependence of such frequencies on the Reynolds number has been investigated and a scaling law based on a physical interpretation of the phenomenon has been provided, which is able to explain the onset of a secondary flapping frequency close to transition.