Guido Buresti

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.

The effect of surface roughness on the flow regime around circular cylinders

Abstract Experimental research has been carried out to study the influence of surface roughness on transitions between the subcritical, critical supercritical and postcritical regimes around isolated circular cylinders in cross-flow. The experiments were performed in an open-jet wind tunnel, which was modified to obtain satisfactorily bidimensional conditions, with Reynolds numbers ranging from ∼2.6 × 10 4 to ∼2.8 × 10 5 . The cylinders were covered with different types of standard commercial emery cloth, and the estimated relative roughness varied from ∼1.0 × 10 −3 to ∼12 × 10 −3 . The signal from a hot-wire probe placed near the cylinder was analysed in order to ascertain the presence of vortex shedding and to obtain the Strouhal number. The mean pressure distribution around most of the cylinders were also measured, and then the drag coefficients were obtained by integration. The boundaries between the various flow regimes as a function of the degree of roughness and of the Reynolds number were tentatively established, assuming the critical regime to be characterised by the absence of regular vortex shedding, and the postcritical one by Reynolds-number independence. The results confirmed the striking influence of roughness on the flow regime; in particular, the critical regime can be impressively reduced, and it may even disappear in the case of highly roughened cylinders. The boundaries between the various regimes, as well as the supercritical and postcritical drag coefficients and Strouhal numbers, seem to be a function of both the size and the type of surface roughness. This is confirmation, therefore, that it is not possible to characterise the flow regime only by means of a Reynolds number based on the size of the roughness. Finally, it was demonstrated that strong supercritical and postcritical vortex shedding can take place from highly, but uniformly, roughened circular cylinders.

Boundary layer diagnostics by means of an infrared scanning radiometer

A computerized infrared (IR) scanning radiometer is employed to characterize the boundary layer development over a model wing, having a Göttingen 797 cross-section, by measuring the temperature distribution over its heated surface. The Reynolds analogy is used to relate heat transfer measurements to skin friction. The results show that IR thermography is capable of rapidly detecting location and extent of transition and separation regions of the boundary layer over the whole surface of the tested model wing. Thus, the IR technique appears to be a suitable and effective diagnostic tool for aerodynamic research in wind tunnels.

Mean and fluctuating forces on a circular cylinder in cross-flow near a plane surface

Abstract An experimental investigation for the measurement of the mean and fluctuating forces acting on a circular cylinder in cross-flow, placed near a plane surface parallel to its axis, is described. Tests were carried out with three different types of boundary layer on the plane, having relative thickness δ/D from 0.1 to 1.1, at Reynolds numbers from 0.86 to 2.77 × 10 5 , a range in which the flow around an isolated cylinder is characterized by a progressive disappearance of regular vortex shedding. The results showed that at subcritical Reynolds numbers periodic oscillating lift forces induced by regular vortex shedding are present at least down to relative distances from the plane, G/D, of 0.4, irrespective of the thickness of the boundary layer. The mean lift coefficient was found to rapidly decrease by increasing the gap size, whereas the mean drag coefficient showed non-monotonic trends with G/D, clearly dependent on the Reynolds number and significantly influenced by the thickness of the boundary layer on the plane.

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.

On the analysis of fluctuating velocity signals through methods based on the wavelet and Hilbert transforms

Abstract In the present paper we describe signal processing procedures based on the wavelet and Hilbert transforms, which may be used to characterize the time variation of the amplitude and frequency modulations that are present in the fluctuating components of experimental velocity signals. The classical Hilbert demodulation technique is described together with the wavelet-ridge extraction procedure, and a new joint wavelet-Hilbert technique is presented. Methods to obtain the instantaneous contribution to the correlation between the components present in two signals through the wavelet and Hilbert transforms are also described. The procedures are then applied to the analysis of the velocity fluctuations in the wake of a bluff body, and it is shown that they allow a physical interpretation to be given of the connection between different fluctuating components present in the velocity signals and the dynamics of the vorticity structures in the wake.

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.

Influence of afterbody rounding on the pressure drag of an axisymmetrical bluff body

The effects of rounding the aft edges of a blunt-based axisymmetrical bluff body with streamlined forebody are studied by means of surface-pressure measurements, at Reynolds numbers (based on the model length) ReL = 1.15 x 106 and ReL = 2.88 x 106. The results show that for a certain radius of curvature a maximum in pressure drag occurs, connected with the variation of the pressures on the portion of the afterbody lying inside the separated wake, and with the contribution of the suction peaks before separation. However, the behaviour of the drag is found to be significantly dependent on Reynolds number, while the forcing of the boundary-layer transition at a fixed upstream position produces a substantial decrease of the drag only for the lower test velocity.

Vortex dynamics in evolutive flows: A weakly chaotic phenomenon

We make use of a wavelet method to extract, from experimental velocity signals obtained in an evolutive flow, the dominating velocity components generated by vortex dynamics. We characterize the resulting time series complexity by means of a joint use of data compression and of an entropy diffusion method. We assess that the time series emerging from the wavelet analysis of the vortex dynamics is a weakly chaotic process with a vanishing Kolmogorov-Sinai entropy and a power-law growth of the information content. To reproduce the Fourier spectrum of the experimental signal, we adopt a harmonic dependence on time with a fluctuating frequency, ruled by an inverse power-law distribution of random events. The complexity of these fluctuations is determined by studying the corresponding artificial sequences. We reproduce satisfactorily both spectral and complex properties of the experimental signal by locating the complexity of the fluctuating process at the border between the stationary and the nonstationary states.

Characterization Of Boundary Layer Conditions In Wind Tunnel Tests Through Ir Thermography Imaging

A computerized infrared (IR) imaging system is evaluated as a diagnostic tool for aerodynamic research by performing experimental tests in a subsonic wind tunnel. In particular, the IR technique is used to characterize the behaviour of the boundary layer on two wing models, having different cross-sections, by measuring the temperature distribution over their heated surfaces. The results show that IR thermography is capable of immediately yielding accurate information on the location and extent of the transition between laminar and turbulent flow and on the region of separation, for the whole boundary layer over the surface of a model wing.