B. G. B. Klingmann

Experiments in a boundary layer subjected to free stream turbulence. Part 1. Boundary layer structure and receptivity

The modification of the mean and fluctuating characteristics of a flat-plate boundary layer subjected to nearly isotropic free stream turbulence (FST) is studied experimentally using hot-wire anemometry. The study is focussed on the region upstream of the transition onset, where the fluctuations inside the boundary layer are dominated by elongated flow structures which grow downstream both in amplitude and length. Their downstream development and scaling are investigated and the results are compared with those obtained by previous authors. This allows some conclusions about the parameters which are relevant for the modelling of the transition process. The mechanisms underlying the transition process and the relative importance of the Tollmien–Schlichting wave instability in this flow are treated in an accompanying paper (part 2 of the present report).

Experiments in a boundary layer subjected to free stream turbulence. Part 2. The role of TS-waves in the transition process

The natural occurrence of Tollmien-Schlichting (TS) waves has so far only been observed in boundary layers subjected to moderate levels of free stream turbulence (Tu<1 %), owing to the difficulty in detecting small-amplitude waves in highly perturbed boundary layers. By introducing controlled oscillations with a vibrating ribbon, it is possible to study small-amplitude waves using phase-selective filtering techniques. In the present work, the effect of TS-waves on the transition is studied at Tu=1.5%. It is demonstrated that TS-waves can exist and develop in a similar way as in an undisturbed boundary layer

Transition experiments in a boundary layer with embedded streamwise vortices

The stability of a flat plate boundary layer modulated by stationary streamwise vortices was studied experimentally in the T‐324 low speed wind tunnel in Novosibirsk. Vortices were generated inside the boundary layer by means of roughness elements arranged in a regular array along the spanwise (z‐) direction. Transition is not caused directly by these structures, but by the growth of small amplitude traveling waves riding on top of the steady vortices. This situation is analogous to the transition process in Gortler and cross‐flows. The waves were found to amplify up to a stage where higher harmonics are generated, leading to turbulent breakdown and disintegration of the spanwise boundary layer structure. For strong modulations, the observed instability is quite powerful, and can be excited ‘‘naturally’’ by small uncontrollable background disturbances. Controlled oscillations were then introduced by means of a vibrating ribbon, allowing a detailed investigation of the wave characteristics. The instability...

On transition due to three-dimensional disturbances in plane Poiseuille flow

The purpose of the present study is to characterize the process of laminar–turbulent transition at Reynolds numbers which are subcritical from the two-dimensional linear point of view. The development of a point-like disturbance was studied in an air flow channel with hot-wire anemometry at a Reynolds number of 1600. Localized disturbances were triggered at one of the walls and their development followed downstream by traversing the hot-wire probe in the streamwise direction over a distance of 90 half channel heights, as well as in the normal and spanwise directions. The disturbance evolved into elongated streaky structures with strong spanwise shear (i.e. normal vorticity) which grew in amplitude and streamwise extension and thereafter either decayed or gave rise to a turbulent spot. The results indicate that the mechanism underlying the initial growth is a linear one resulting from the coupling between the normal velocity and the normal vorticity, as described by the three-dimensional linear equations. The nonlinear development of the structure leads to the formation of intense normal shear layers and the appearance of oscillations and ‘spikes’, which multiply and form the rear or a turbulent spot.

The influence of riblets on a boundary layer with embedded streamwise vortices

The present study shows that surface manipulation makes it possible to suppress longitudinal vortex structures in a boundary layer, and thereby stabilize it with respect to high‐frequency traveling waves and delay the transition to turbulence. The results should be of interest for the control of transition in cross‐flow and Gortler flow.

Görtler Vortices with System Rotation

The steady primary instability of Görtler vortices developing along a curved Blasius boundary layer subject to spanwise system rotation is analysed through linear and nonlinear approaches, to clarify issues of vortex growth and wavelength selection, and to pave the way to further secondary instability studies.A linear marching stability analysis is carried out for a range of rotation numbers, to yield the (predictable) result that positive rotation, that is rotation in the sense of the basic flow, enhances the vortex development, while negative rotation dampens the vortices. Comparisons are also made with local, nonparallel linear stability results (Zebib and Bottaro, 1993) to demonstrate how the local theory overestimates vortex growth. The linear marching code is then used as a tool to predict wavelength selection of vortices, based on a criterion of maximum linear amplification.Nonlinear finite volume numerical simulations are performed for a series of spanwise wave numbers and rotation numbers. It is shown that energy growths of linear marching solutions coincide with those of nonlinear spatially developing flows up to fairly large disturbance amplitudes. The perturbation energy saturates at some downstream position at a level which seems to be independent of rotation, but that increases with the spanwise wavelength. Nonlinear simulations performed in a long (along the span) cross section, under conditions of random inflow disturbances, demonstrate that: (i) vortices are randomly spaced and at different stages of growth in each cross section; (ii) “upright” vortices are the exception in a universe of irregular structures; (iii) the average nonlinear wavelengths for different inlet random noises are close to those of maximum growth from the linear theory; (iv) perturbation energies decrease initially in a linear filtering phase (which does not depend on rotation, but is a function of the inlet noise distribution) until coherent patches of vorticity near the wall emerge and can be amplified by the instability mechanism; (v) the linear filter represents the receptivity of the flow: any random noise, no matter how strong, organizes itself linearly before subsequent growth can take place; (vi) the Görtler number, by itself, is not sufficient to define the state of development of a vortical flow, but should be coupled to a receptivity parameter; (vii) randomly excited Görtler vortices resemble and scale like coherent structures of turbulent boundary layers.

TURBULENT SPOTS IN PLANE POISEUILLE FLOW : MEASUREMENTS OF THE VELOCITY FIELD

An experimental study on the development of turbulent spots in plane Poiseuille flow at a Reynolds number of 1600 has been carried out with the aim of achieving a better understanding of the transition to and maintenance of turbulence at low Reynolds numbers. Spots were triggered by a loudspeaker‐induced jet of high velocity. The initial disturbance was found to undergo a first stage of rapid expansion, in which sharp internal shear layers form at locations away from the symmetry plane and precede the transition to turbulence. After this initial stage, a nearly self‐similar structure develops with the typical features of a turbulent spot. The general features, turbulent properties, and spanwise spreading of the spot were investigated and compared both to previous experimental data and to numerical simulations. High‐frequency fluctuations are absent at the front of the spot, whereas the turbulence is apparently self‐sustained at the rear and displays features similar to fully turbulent Poiseuille flow at m...

Experiments on the Evolution of a Point-like Disturbance in Plane Poiseuille Flow into a Turbulent Spot

The streamwise velocity field resulting from a point-like disturbance in plane channel flow in the transition regime was studied using hot-wire anemometry. Elongated structures associated with strong spanwise shear (i.e. normal vorticity) were observed preceding the appearance of ‘spikes’ and the subsequent formation of a turbulent spot. The streaks were found to have properties similar to those resulting from the solution of the 3-dimensional Orr-Sommerfeld problem for normal vorticity waves which are elongated in the streamwise direction. Although they eventually decay, the initial growth of these structures appears to have a destabilizing effect on the flow.

On the Development of Turbulent Spots in Plane Poiseuille Flow

Results from an experimental study of the development of turbulent spots in plane Poiseuille flow is presented. Spots were triggered at a Reynolds number of 1600 by a loudspeaker-induced jet of high velocity. The initial disturbance was found to undergo a first stage of rapid expansion, in which sharp internal shear layers form, and subsequently break down to turbulence. This was first observed at locations away from the symmetry plane. After an initial development phase the spot gets a self-similar shape. Oblique Tollmien-Schlichting waves were found at the wing-tips of the spot, where they reach amplitudes of more than 15% of the undisturbed centreline velocity.

An Experimental Study of the Velocity Field of Turbulent Spots in Plane Poiseuille Flow

The streamwise velocity field of a turbulent spot in plane channel flow has been measured and is compared with that of a numerical simulation of a spot. The comparison shows in general good agreement, however, it seems that the numerical spot develops somewhat faster than the physical one.