Alessandro Talamelli

Experimental and theoretical investigation of the nonmodal growth of steady streaks in a flat plate boundary layer

An experimental and theoretical investigation aimed at describing the nonmodal growth of steady and spanwise periodic streamwise streaks in a flat plate boundary layer is presented. Stable laminar streaks are experimentally generated by means of a spanwise periodic array of small cylindrical roughness elements fixed on the plate. The streamwise evolution of the streaks is measured and it is proved that, except in a small region near the roughness elements, they obey the boundary layer scalings. The maximum achievable amplitude is mainly determined by the relative height of the roughness elements. Results are compared with numerical simulations of optimal and suboptimal boundary layer streaks. The theory is able to elucidate some of the discrepancies recently noticed between experimentally realizable nonmodal growth and optimal perturbation theory. The key factor is found to be the wall normal location and the extension of the laminar standing streamwise vortices inducing the streaks. The differences among previous experimental works can be explained by different dominating streak generation mechanisms which can be linked to the geometry and to the ratio between the roughness height and the boundary layer scale.

Experimental study of the stabilization of Tollmien–Schlichting waves by finite amplitude streaks

It has recently been found by using temporal and spatial numerical simulations that steady optimal streaks of moderate amplitude, i.e., sufficiently large but not exceeding the critical amplitude for the inflectional instability, are able to reduce the growth of Tollmien-Schlichting (TS) waves up to their complete suppression. This investigation aims at experimentally verifying this stabilizing effect by generating stable and symmetric, close to sinusoidal, streaks of moderate amplitudes (similar to 12% of the free-stream velocity) by means of a spanwise array of cylindrical roughness elements. The three-dimensional (3D) streaky base flow is then subjected to a secondary instability generated through a spanwise slot in the plate by means of regulated blowing and suction. In this study the stabilizing role of the streaks on TS waves is unambiguously confirmed and by increasing the height of the roughness elements, thus inducing larger amplitude streaks, we are also able to show that the stabilizing action on the TS waves increases with the streak amplitude. These results are the first to confirm the numerical predictions reported in earlier works. The full cross-stream plane has been measured at different downstream positions allowing a complete evaluation and comparison of the different amplitude measures used in previous experimental works. Furthermore, theoretical impulse response analysis and stability calculations are applied to the present experimental streaky base flow enabling a qualitative comparison of the 3D modulated TS wave distribution.

Experimental characterization of the velocity field of a coaxial jet configuration

Abstract The mean and fluctuating flow fields of a coaxial jet configuration, with ratios between inner and outer diameters and velocities D i /D o = 0.485 and U i /U o = 0.67 , turbulent exit boundary layers, and high turbulence levels in the exit cores are studied by means of LDA and hot-wire measurements, using an experimental facility designed for the purpose. The profiles of the mean axial velocity, of the axial and radial turbulence intensities, and of the shear stress are described for the initial and intermediate zones of the near field. The lenghts of the potential cores are in good agreement with available data obtained with lower exit turbulence levels and laminar boundary layers. It is shown that the prevailing frequencies of the flow fluctuations may be better recognized if the hot-wire spectra of both the axial and the radial velocities are analyzed. These frequencies show significant radial differences in the initial and intermediate mixing zones but become progressively more uniform with increasing distance from the exit. In the near-exit region of the inner mixing layer evidence is also found of a probable alternate vortex shedding from the inner duct wall. This phenomenon is shoen to exist for U i /U o ≥ 0.44 , with a constant Strouhal number St t = 0.24 , based on the thickness of the wall and on the average velocity of the two streams.

CICLoPE—a response to the need for high Reynolds number experiments

The invention is directed to a process for the preparation of a catalyst component for the polymerization of an olefin by:a) contacting metallic magnesium with an organic halide RX, where R is an organic group containing up to 20 carbon atoms and X is a halide, whereupon the dissolved reaction product I is separated from the solid residual products and whereafter,b) an alkoxy group or aryloxy group containing silane compound is added to the obtained reaction product I, whereupon the precipitate formed is purified to obtain reaction product II,c) which reaction product II is subsequently contacted with TiCl4 and the resulting product is purified to obtain the catalyst component.The invention is characterized in that in step b) the silane compound and reaction product I are introduced simultaneously to a mixing device.

Acoustic control of a coaxial jet

Measurements in the near field of a coaxial jet under unperturbed and controlled conditions are reported; the Reynolds number (defined with diameter and exit velocity of the inner-jet) is 5600, while the ratios of the velocities and of the diameters of the two jets are 1.8 and 2, respectively. The unperturbed flow, which issues in a nominally laminar state from the contraction nozzles, develops a shear-layer mode instability in the outer-shear layer of the outer-jet, followed by vortex pairing. A control perturbation, composed of sinusoidal acoustic waves at the frequency of the instability and its half, is applied to the flow. The control parameter is the phase difference Δ φ between the two sinusoidal waves, and is initially varied in the entire range of phases. Then, two values of Δ φ, providing the maximum attenuation and enhancement of the turbulence intensity, are chosen and, for these, detailed measurements in the axial and radial directions are performed. The data show that the control affects the...

Experimental investigation on the turbulent near-field of coaxial jets

Abstract The flow field of a coaxial jet configuration having inner to outer diameter ratio Di/Do≅0.5 is studied for two values of the velocity ratio, Ui/Uo=0.30 and Ui/Uo=0.67, both with a 5 mm thick and with a sharp inner duct wall. LDA and hot-wire measurements are used to obtain the statistical moments up to the fourth order and the time histories of the axial and radial velocity components. As expected, the inner jet core length is seen to decrease with decreasing velocity ratio, and for Ui/Uo=0.30 it is comparable to that of the outer jet, a condition that gives rise to large fluctuations and to a high level of mixing between the two streams. The sharpening of the inner duct wall produces a reduction in the radial fluctuations and Reynolds stresses in the near field, particularly for the case Ui/Uo=0.67, at which a regular vortex shedding occurs with the 5 mm thick duct; however, the differences in the velocity moments between the two configurations rapidly decrease, and become negligible after the end of both potential cores. The prevailing frequencies of the fluctuating flow field, estimated from wavelet spectra of the time histories of both velocity components, decrease regularly moving downstream from the jet outlet, and seem to confirm the dominance of the stronger vortices of the outer shear layer. Finally, the trends of the higher order moments may be interpreted in terms of plausible physical schemes of the mixing processes of the developing shear layers.

A study of swirling turbulent pipe and jet flows

Axially rotating turbulent pipe flow is an example in which the rotation strongly affects the turbulence, which then also influences the mean flow properties. For instance, in the fully developed flow as well, the fluid is not in solid body rotation due to the influence of the cross-stream Reynolds stress. The present paper reports new measurements from a rotating pipe flow and the streamwise mean velocity distribution is compared with recent scaling ideas of Oberlack [J. Fluid Mech. 379, 1 (1999)] and good agreement is found. A second part of the paper deals with the initial stages when the flow leaves the pipe and forms a swirling jet. The measurements in the jet show that at some distance downstream (approximately five jet diameters) the central part of the jet actually rotates in the opposite direction as compared to the rotation of the pipe. This effect is explained by the influence of the cross-stream Reynolds shear stress.

Transition delay by means of a passive mechanism

The laminar breakdown of a three-dimensional flat-plate boundary-layer flow with favourable and ensuing adverse pressure gradient (APG), generic for an infinite swept wing, is investigated in detail by means of spatial direct numerical simulations. Emphasis is on transition mechanisms in the region of adverse pressure gradient where the undistorted laminar base flow also becomes unstable with respect to Tollmien– Schlichting (TS) waves that are most amplified if two-dimensional with respect to the local streamwise direction. The influence of finite-amplitude crossflow vortices, coming from the region of favourable pressure gradient, on the TS instability properties is investigated. It turns out that crossflow-vortex-induced secondary instabilities are the most amplified disturbances even for low-amplitude vortex modes. The TS waves act as generators of fundamental low-frequency secondary modes, but are neither important for their growth nor for breakdown. As for active disturbance control, any method aiming at attenuating two-dimensional TS waves must fail. On the other hand, the passive upstream-flow-deformation technique also delays transition in the APG region.

Effect of the spatial filtering and alignment error of hot-wire probes in a wall-bounded turbulent flow

The effort to describe velocity fluctuation distributions in wall-bounded turbulent flows has raised different questions concerning the accuracy of hot-wire measurement techniques close to the wall and more specifically the effect of spatial averaging resulting from the finite size of the wire. Here, an analytical model which describes the effect of the spatial filtering and misalignment of hot-wire probes on the main statistical moments in turbulent wall-bounded flows is presented. The model, which is based on the two-point velocity correlation function, shows that the filtering is directly related to the transverse Taylor micro-scale. By means of turbulent channel flow DNS data, the capacity of the model to accurately describe the probe response is established. At the same time, the filtering effect is appraised for different wire lengths and for a range of misalignment angles which can be expected from good experimental practice. Effects of the second-order terms in the model equations are also taken into account and discussed. In order to use the model in a practical situation, the Taylor micro-scale distribution at least should be provided. A simple scaling law based on classic turbulence theory is therefore introduced and finally employed to estimate the filtering effect for different wire lengths.

Low aspect-ratio triangular prisms in cross-flow: measurements of the wake fluctuating velocity field

Abstract An experimental investigation is carried out to study the characteristics of the fluctuating wake flow field of finite-length prismatic bodies in cross-flow, placed vertically on a plane and having isosceles triangular cross-section with 60° or 90° apex angle orientated in the downstream direction. The wake flow field is studied with hot-wire anemometry for aspect ratios h/w =1.0, 1.5, 2.0 and 3.0 at Re= wU/ν ≅1.2×10 5 , and the velocity signals are analysed by means of procedures based on the wavelet transform. Velocity fluctuations with a clear dominating frequency are found immediately outside the wake along all the models, even if their magnitude decreases at a height corresponding to the free end of the bodies. The wavelet analysis of the phase difference between signals acquired at opposite sides of the wake shows that alternate vortex shedding occurs for all the models. An increase in the phase shift between the fluctuations on the same side of the wake with increasing vertical distance between the probes suggests that the shed vortices are curved. No evidence of symmetrical shedding or of the presence of arch-type vortices is found, even for the models with the smaller aspect ratios.