Catálogo de publicaciones - libros

The phenomenon of leading edge stall is associated with the “bursting” of leading edge separation bubbles from a short form, where the length is roughly 100 momentum thicknesses, to a long form that maybe 1000 or more momentum thicknesses long. The paper reports experiments and theoretical discussions of work carried out by the author 50 years ago during his PhD study on bubbles. Detailed measurements of the flow within bubbles are shown together with the oscillogram traces of the velocity fluctuations present. A linear model of the stability of separated shear layers was developed that suggested that the disturbances were spatially evolving waves described by modes with complex wavenumbers and not the temporal modes usually used in stability studies. It was noted that some modes appeared to have a very small group velocity. Although at the time the full implications of this were not properly understood, the conjecture was put forward that a true instability (or absolute instability as it is now called) could therefore exist. A change in the sign of the group velocity could dramatically change the transition process and thus explain the bursting phenomenon.

Transient growth associated with non-normal stability operators are discussed. Results for both temporal and spatial transient growth are presented. These results allow for an understanding of the type of bypass transition found when laminar boundary layers are subjected to free-stream turbulence. Direct numerical simulations of such a transition scenario show that precursors to turbulent spots are optimal streaks which undergo secondary instability. Finally aspects of the receptivity of the streaks are discussed.

Transition to turbulence in parallel or nearly parallel flows is known to be critically influenced by ambient perturbations. At relatively low intensity level, external perturbations initiate transition through receptivity process, i.e. by exciting instability waves, but without affecting linear and subsequent nonlinear amplification rate. At moderate to high intensity level, however, external disturbances may directly alter the instability properties. The present paper reports some recent theoretical progresses in understanding the role of external disturbances in transition. These include (a) a self-consistent theory for the vortical receptivity, which predicts accurately the experimental measurements, (b) the demonstration that the streaks formed due to the free stream vortical fluctuations modify the viscous Tollmien-Schlichting (T-S) waves, and indeed may even induce stronger intermittent inviscid instability, and (c) a theoretical model, which links the critical Reynolds number to the external perturbations, and shows, for reduced by even a minimal level of external perturbations. Several problems which require further studies are highlighted. the case of channel flow, that the critical Reynolds number can be substantially

An experimental and computational study is conducted to develop a model for the effects of surface steps on transition to turbulence in boundary layers. The step effects are captured within the framework of a variable n-factor method, where the step results in a reduction in the TS-wave transition n-factor Δ. Data is presented for NTS for favorable and adverse pressure gradients. Backward-facing steps result in significantly larger reductions in the transition n-factor when compared to forward-facing steps. The results show that step effects can be accounted for by using a Δ for step heights up to 1.5 times the local boundary-layer displacement thickness.

We report the results of an experimental investigation of the transition to turbulence of Poiseuille flow in a long pipe. Our findings confirm that the recently established scaling law for the finite amplitude perturbation required to cause transition is ). New results are presented concerning the decay of disturbances injected into the flow field at values of where the flow is known to be globally stable. Exponential decay and critical behaviour is observed and these are consistent with observations in other shear flows. This new approach has enabled us to uncover a sharp cut off at the lower limit of the stability threshold.

We propose an experimental method to study the instability of thin unsteady separation bubbles, i.e. of unsteady boundary layers with reverse flow. The unsteady boundary layer is created by controlled temporal and spatial variations of the velocity external to the boundary layer. We present results of the evolution of instability in different temporally varying flows in a shallow angle diffuser. Depending on the extent of reverse flow in the boundary we observe that instability can be spatially localised.

The role of hydrodynamic instability mechanisms in the presence of laminar boundary layer separation is investigated by means of Direct Numerical Simulations. In a series of simulations involving generic laminar separation bubbles we show that the “natural” onset of unsteadiness (i.e. the development of visible vortex shedding) is not necessarily caused by an absolute/global instability. Our results indicate that the entrainment of high-momentum fluid required to “close” the separation bubble is primarily provided by 2-D or “2-D coherent” structures, which are a consequence of the (inviscid) hydrodynamic instability of the separated shear layer. In a series of highly resolved simulations for a flat-plate boundary layer subjected to low-pressure turbine blade conditions, we demonstrate that this natural instability mechanism (with respect to two-dimensional disturbances) can be exploited for effective control of separation using pulsed vortex generator jets.

The instability of nominally laminar steady two-dimensional closed separation bubbles is investigated using direct numerical simulations and BiGlobal instability analysis. The canonical flat-plate case is studied in some detail. We demonstrate that large steady two-dimensional bubbles may become unsteady and shortened in the mean upon applying periodic forcing. Using BiGlobal instability analysis we demonstrate, for the first time, the generation of Kelvin- Helmholtz instabilities as solutions of the pertinent partial-derivative eigenvalue problem, without resorting to the simplifying assumptions on the form of the underlying basic state. Finally, we employ appropriate instability analysis to study the effect of periodic forcing as means of active control of separation on a trailing-edge geometry.

The present paper deals with the stability analysis of the flow induced As the basic flow is strongly nonparallel, the modal form gives amplistability problem is thus described by a PDE system whose results seem by wall injection either in a rectangular duct or in a cylindrical pipe. tude functions which are dependent on two space variables. The linear to be in good agreement with the experiments.

Wind-tunnel experiments on longitudinal structures developing in laminar boundary layers on a straight wing have been carried out. High-frequency perturbations, that is, “forerunners” occurring near the fronts of the streamwise disturbances were detected. Their characteristics and dynamics were clarified revealing similarities of the disturbances with boundary-layer instability waves.