Catálogo de publicaciones - libros

Numerical simulations are used to model the disturbance field associated with a spanwise periodic array of circular disks in a laminar boundary layer. Besides capturing the major trends from the recent wind tunnel measurements by White and Ergin (2003), the simulations reveal the intricate effects of disturbance nonlinearity and roughness geometry on the transient growth characteristics in the wake of the roughness array and provide a numerical database for validating detailed features of optimal growth theory in the context of roughness induced stationary disturbances.

Linear feedback control has been applied to transitional boundary layer flows. Information from wall-mounted sensors is used to estimate the flow state. The estimated state is then used to compute the optimal feedback control which is applied as blowing and suction with zero net mass-flux through the wall. The performance of the controller is tested in direct numerical simulations of a spatially growing Falkner–Skan– Cooke boundary layer where an inflectional instability is triggered. The extension to spatial boundary layer flows is an important step towards real applications.

Flow past a cylinder becomes unstable at ~47 and vortex shedding ensues. A cylinder mounted on lightly damped springs can undergo vibrations as result of the unsteady forces experienced by it due to the vortex shedding. In this paper we examine the effect of elastic supports on the critical Reynolds number at which the vortex shedding sets in. It is found that for a cylinder with elastic supports, vortex shedding can be observed for Reynolds number as low as 22.

Aero-Train is a new concept of environment friendly high speed transport system invented by our group. In order to realize this concept, improvement of the aerodynamic performance is essential. Our goal is to attain the overall lift to drag ration up to 25. Presently it is around 13 to 15. Some of the possible reasons why the value is low are in the interference drag and induced drag around lifting and guide wing junction. Flow separates largely at suction surfaces of the wing section enhanced by the existence of horizontal and vertical walls. Separation control study is conducted in present investigation using CFD and EFD approaches. Drastic improvement of lift to drag ratio was achieved by introducing several devices at the wing section.

The temporal and spatial development of natural Tollmien-Schlichting (TS) instabilities and artificially generated disturbances was investigated comprehensively in-flight as well as in a wind tunnel. The experiments were performed on a laminar wing glove for a sailplane using different surface sensor arrays (surface hot-wire sensors, piezo-sensor arrays). Two-dimensional TS waves dominate in the early linear stage of the boundary layer flow, but single three-dimensionally (3D) dominated wave packets, observed in the late stage of TS development, can be measured in the early stage as well. Furthermore, in the in-flight measurements, the 3D-instabilities occur earlier than in the wind tunnel. For the experiments on controlled transition, an array of spanwise distributed harmonic point sources which induced mono and multifrequency disturbances was used in order to compare the development of natural and controlled instabilities.

Abstract The conventional idea of Laminar-turbulent transition is that the transition starts from the ampli.cation of disturbances, and when the disturbances become larger, higher harmonics will be generated due to non-linear effect, making the flow more and more complicated, and finally becomes turbulent. Though the scenario seems is clear, yet there is a missing link, that is, what happens in the breakdown process. Here we show by analyzing the results from direct numerical simulations that the change of stability characteristics of the mean flow profile plays a key role in the breakdown process.

The unsteady boundary layer transition over an airfoil is investigated experimentally for several Strouhal numbers. Continuous wavelet analysis is used to highlight the key mechanism of unsteady boundary layer transition. A quasisteady approach is used to compute wave amplifications which are compared successfully with experimental amplifications.

Crossflow–dominated laminar–turbulent transition is studied by nonlinear nonlocal theory (PSE) for the DLR swept–flat plate experiment. The strongly nonlinear stages of the transition process, i.e. the formation of high–frequency secondary instabilities, their linear and nonlinear growth as well as the subsequent disintegration of the secondary disturbance structures which is accompanied by a degradation of the stationary crossflow vortices are discussed.

The present experiment focused on revealing effect of scale and directional component of free stream turbulence on boundary layer transition, especially growth rate of disturbance energy in the boundary layer. The free stream turbulence generated by a turbulence grid mounted upstream of the contraction has typical character of axisymmetric turbulence with strong anisotropy. The experimental results with anisotropic free stream turbulence show the non-modal growth disturbance even at 0.7 % turbulence intensity. The correlation between the filtered free stream fluctuation and growth rates of the disturbance in the boundary layer suggests that the wall-normal velocity fluctuation in the free stream dominates the disturbance growth.

In the present work we investigate receptivity and disturbance amplification behind a circular roughness element in a zero-pressure-gradient flat-plate boundary layer with the aim to identify and understand the basic mechanisms at work. The low disturbance background and the high repeatability of the direct numerical simulations allow to evaluate the different contributions of acoustic-roughness receptivity and of local disturbance amplification. A distinct feature of the boundary-layer flow with roughness element are streamwise streaks which develop from the spanwise edges of the roughness. Small-scale three-dimensional disturbances develop and amplify within these streaks both by instability and by receptivity. For the present small amplitude unsteady forcing however, they remain confined to these areas. In contrast to this, the far-field receptivity result is similar to a wave train generated by periodic suction and blowing at the wall.