1,720,989 research outputs found
On the error sensitivity of calibration procedures for normal hot-wire probes
No full textThe results of an investigation on the sensitivity of various hot-wire calibration procedures to errors in probe positioning are described. The analysis was carried out by means of a computer simulation procedure and regarded, in particular, methods for the evaluation of the direction sensitivity coefficients. It is shown that the classical calibration procedures are characterized by a high sensitivity to experimental errors, and this may explain both the large scatter usually found in the values of the direction sensitivity coefficients and, at least partially, their variation with yaw and pitch angle. New calibration procedures are also described which appear to be significantly less sensitive to errors in probe positioning and velocity setting. The practical application of the various procedures to the calibration of a typical commercial probe seems to confirm the results of the computer simulation analysis
Bluff-body aerodynamics: Research challenges from wind engineering
No full textThe evaluation of wind-induced loads poses challenging problems of bluff-body aerodynamics because, from an aerodynamic point of view, almost all civil structures are bluff bodies. In order to devise physically sound design procedures, considerable research is still necessary as regards many important topics, a few of which are briefly reviewed in the present paper by mainly focusing on recent developments. Vortex shedding response and galloping are first considered, as well as their possible interaction, which is a complex and open issue. The interference between bluff bodies and its ensuing effects on the aerodynamic loads and on the possible aeroelastic phenomena are then considered, pointing out the still unsatisfactory level of the relevant present information. Finally, the problem of the prediction of the loads acting on bodies subjected to accelerating flows, like those that may often occur in thunderstorms, is tackled. In particular, the present available procedures to evaluate acceleration-induced forces are critically described, together with a discussion on their potential importance in wind engineering
On the analysis of fluctuating velocity signals through methods based on the wavelet and Hilbert transform
No full textSeparation delay through contoured transverse grooves on a 2D boat-tailed bluff body: Effects on drag reduction and wake flow features
No full textThe effectiveness of properly contoured transverse grooves in delaying the flow separation occurring on a two-dimensional boat-tailed bluff body is assessed through numerical simulations. The body has a cross-section with a 3:1 elliptical forebody and a rectangular main part followed by a circular-arc boat tail. Three-dimensional Variational Multiscale Large Eddy Simulations are carried out at Re=Du∞∕ν=9.6×104, using a mixed finite-volume/finite-element method. The introduction of one contoured groove on each of the boat-tail lateral surfaces produces a significant delay of flow separation and a consequent increase of the base pressure, with a global drag reduction of the order of 9.7%. The wake dynamical structure remains qualitatively similar to the one typical of blunt-based two-dimensional bodies, with quantitative variations that are consistent with the reduction in wake width caused by boat tailing and by the grooves. The introduction of the grooves leads also to a regularization of the vortex shedding downstream of the body, which is more correlated in the spanwise direction. Finally, a few supplementary simulations show that the effect of the grooves is also robust to the variation of the geometrical parameters defining their location and shape
Experimental variation on the turbulent near-field of coaxial jets
No full textThe 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 and both velocity componnents, 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
Experimental characterization of the velocity field of a coaxial jet configuration
No full textThe mean and fluctuating flow fields of a coaxial jet configuration, with ratios between inner and outer diameters and velocities Di/Do = 0.485 and Ui/Uo = 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 Ui/Uo ≥ 0.44, with a constant Strouhal number Stt = 0.24, based on the thickness of the wall and on the average velocity of the two streams. © 1994
Low aspect-ratio triangular prisms in cross-flow: measurements of the wake fluctuating velocity field
No full textAn 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/v≅1.2×105, and the velocity signals are analyzed 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
Effects of Spanwise-Discontinuous Contoured Transverse Grooves on Flow Separation and Vortex Shedding
No full textThe delay of boundary layer separation over curved solid surfaces is of great importance in many engineering applications. The proposed idea is to introduce small and suitably shaped grooves transverse to the flow (i.e., contoured transverse grooves) to passively generate local steady flow recirculations
Constant-frequency time cells in the vortex-shedding from a square cylinder in accelerating flows
No full textAn extensive experimental campaign has been carried out in a multiple-fan wind tunnel to study the effects of flow acceleration on a sectional model of a sharp-edged square cylinder. Different levels of positive and negative acceleration are reproduced, which are compatible with those induced by full-scale thunderstorm outflows. Various initial and final conditions are also explored and, in all conditions, multiple test repeats are carried out in order to obtain satisfactory ensemble averages. Particular attention is devoted to the acquisition of signals associated with vortex-shedding, for which tailored time-frequency analyses, based on the continuous wavelet and Hilbert transforms, are introduced. Sensitivity analyses are carried out on a selection of the relevant pa-rameters that better allow the tracing of the temporal variation of the shedding frequency. Time intervals in which the shedding frequency is constant, separated by discontinuities, are found during the transients. The number and extent of such constant-frequency time cells and discontinuities seem to be connected with the flow acceleration, but are not strictly repetitive. For higher levels of acceleration the ensemble mean of the Strouhal number is found to be comparable with or moderately lower than the steady-flow value corresponding to the instantaneous velocity
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