Split Stream Flow Past a Blunt Trailing Edge with Application to Combustion Instabilities

In shear coaxial injectors, commonly used for cryogenic liquid rocket engines, propellants traveling at different velocities are separated by the inner jet post before they come into contact with each other, mix, and combust. Knowing how the fluids mix and how susceptible they are to hydrodynamic instabilities is paramount for a successful liquid rocket engine. In this study, the wake behind a blunt trailing edge of a long plate, similar to an unwrapped coaxial injector, was studied in a water tunnel. Two fluid streams of different velocities were introduced on opposite sides of the plate. PIV was used to visualize and determine the influence of the velocity ratio of the split stream on the wake behavior. Measurements of the vortex shedding frequency were taken at various velocity ratios and compared with well characterized cases with a uniform free stream. Operating conditions ranged from Reynolds number 6,000 to 22,000 and velocity ratios 0.30 to 1.00

Connection between base drag, separating boundary layer characteristics and wake mean recirculation length of an axisymmetric blunt-based body

The variation of the base drag of an axisymmetric bluff body caused by modifications of the boundary-layer separating at the sharp-edged contour of its base is analysed through different numerical simulations, and the results are compared with those of a previous experimental investigation. Variational MultiScale Large-Eddy Simulations (VMS-LES) are first carried out on the same nominal geometry and at the same Reynolds number of the experiments. Subsequently, Direct Numerical Simulations (DNS) are performed at Reynolds numbers that are roughly two orders of magnitude lower, in order to investigate on the sensitivity of the main findings to the Reynolds number. The results of experiments, VMS-LES and DNS simulations show that an increase of the base pressure - and thus a decrease of the base drag - may be obtained by increasing the boundary layer thickness before separation, which causes a proportional increase of the length of the mean recirculation region behind the body. In spite of the different setups, Reynolds numbers and turbulence levels in the experiments and numerical simulations, in all cases the base pressure is found to be directly proportional to the length of the mean recirculation region, which is thus a key index of the base drag value. In turn, the recirculation length seems to be connected with the location of the incipient instability of the detaching shear layers, which can be moved downstream by an increase of the thickness of the separating boundary layer and upstream by an increase of the turbulence level

Control of the turbulent flow in a plane diffuser through optimized contoured cavities

A passive control strategy, which consists in introducing contoured cavities in solid walls, is applied to a plane asymmetric diffuser at a Reynolds number that implies fully-turbulent flow upstream of the diffuser divergent part. The analysed reference configuration, for which experimental and numerical data were available, is characterized by an area ratio of 4.7 and a divergence angle of 10 degrees. A large zone of steady flow separation is present in the diffuser without the introduction of the control. One and two subsequent contoured cavities are introduced in the divergent wall of the diffuser and a numerical optimization procedure is carried out to obtain the cavity geometry that maximizes the pressure recovery in the diffuser and minimizes the flow separation extent. The introduction of one optimized cavity leads to an increase in pressure recovery of the order of 6.9% and to a significant reduction of the separation extent, and further improvement (9.6%) is obtained by introducing two subsequent cavities in the divergent wall. The most important geometrical parameters are also identified, and the robustness of the solution to small changes in their values and in the Reynolds number is assessed. The present results show that the proposed control strategy, previously tested in the laminar regime, is effective also for turbulent flows at higher Reynolds numbers. As already found for laminar flow, the success of the control is due both to a virtual geometry modification of the diffuser and to a favourable effect of the cavities in reducing the momentum losses near the wall

The appearance of constant-frequency time cells during vortex-shedding from a square cylinder in accelerating flows

A wind tunnel test campaign is conducted to trace the temporal variation of the shedding frequency of a square cylinder tested under the action of accelerating flows. These are characterized by flow accelerations which are consistent with those typical of thunderstorm outflows. Time intervals in which the shedding frequency is constant are found. The presence of these constant-frequency time cells reflects a local violation of the Strouhal law, and it seems to be connected with the flow acceleration. Furthermore, the ensemble mean of the Strouhal number is seen to decrease for higher levels of acceleration

Constant-frequency time cells in the vortex-shedding from a square cylinder in accelerating flows

An 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

Separation control and efficiency improvement in a 2D diffuser by means of contoured cavities

The performance of a passive control method aimed at reducing and, possibly, eliminating boundary layer separation is evaluated by means of numerical simulation. The passive control, which consists of introducing appropriately-shaped cavities in solid walls, is applied to a plane diffuser. The Reynolds number is such that the turbulence can be neglected (Re = 500, based on the diffuser half-width at the inlet section and the inlet velocity on the axis). A configuration characterized by an area ratio of 2 and a divergence angle of 7 degrees is chosen, so that, without the introduction of the control, the flow is characterized by a large zone of steady asymmetrical boundary layer separation. In order to reduce the separated zone and to increase the efficiency of the diffuser, a couple of symmetric contoured cavities is introduced in the diverging walls. An optimization procedure is developed to obtain the cavity geometry that maximizes the pressure recovery in the diffuser and minimizes the boundary layer separation extent. The introduction of the optimal cavities leads to an increase in pressure recovery of the order of 13% and to a strong reduction of the separation extent. This result is due to a favourable modification of the velocity and vorticity fields in the near-wall region. The most important geometrical parameters are also identified and the robustness of the control to small changes in their values is investigated. It is found that the contoured cavities are effective as long as the flow is able to reattach immediately downstream of the cavities

Effects of flow accelerations typical of thunderstorm outflows on the vortex-shedding from a square cylinder

Accelerating flows acting on a sectional model of a sharp-edged square cylinder are reproduced through the action of a multiple-fan wind tunnel. The model is equipped with 94 pressure sensors, while the wind flow around the body is characterized through three Pitot-static tubes. The accelerations generated by the 72 individually controlled fans of the facility are compatible with those typical of thunderstorm outflows. 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 proposed. Time intervals in which the shedding frequency is constant, separated by discontinuities, are found during the transients. The number and magnitude of such discontinuities seem to be connected with the flow acceleration. The appearance of constant-frequency time cells is not strictly repetitive; moreover, the Strouhal number is seen to decrease for higher levels of acceleration

Confronto tra metodi di diradamento in un impianto puro d farnia (Quercus robur L.)

Comparison between different thinning systems in a pure common oak plantation.- First results of thinning trials, in a 19 years old pure common oak plantation, carried out during winter 2001/2 are reported. A previous thinning had been applied in 1993. Two different thinning methods were tested: systematic-selective method (50% tree felling) and selective thinning method (felling around 80 target tree per hectare). In spite of different thinning intensity (43% and 28% of basal area) in order to keep constant the diameter increments of target trees, the effects of both methods were similar. During the first five years after thinning, the target trees in both thinned plots showed constant diameter increments around 1,1 cm , while in the control plot the diameter increment decreased progressively to 0,7 cm. After thinning and pruning, epicormic branches developed on target trees. In the specific, about 68% of target trees in the thinned plots are characterized by epicormic branches, although only the 17% are vigorous. In the control plot, 62% of target trees developed epicormic branches, none of them vigorous

Professional activity, information demands, training and updating needs of occupational medicine physicians in Italy: National survey.

Objectives: Occupational medicine is a discipline continually evolving in response to technological advances, changes in workplaces and production processes, emergence of new occupational risks and diseases and modifications in regulatory framework for occupational health and safety. Therefore, the recurrent revaluation of professional activity, information demands and education and training needs of occupational physicians is essential in order to identify methodologies and tools that may contribute to improvement of their professional knowledge and competency. In this regard, we conducted the first large-scale national survey of Italian occupational medicine physicians to define their demographic and professional activity and to assess their information demands, training and updating needs. Material and Methods: A random sample of occupational physicians, listed in the national register of the Italian Ministry of Health, was selected to complete a voluntary survey. Subjects recruited in this study were asked to complete 3 different sections (personal and professional information, training and updating needs, professional activity and practice characteristics) of a questionnaire for a total of 35 questions. Results: Most of participants were specialized in occupational medicine, worked for a large number of companies and carried out health surveillance on a total number of workers that exceeds 1500. Occupational physicians would like to have a higher training offer towards practical aspects of health surveillance, risks assessment, manual handling of loads, chemical substances and upper limb biomechanical overload. Interestingly, statistically significant differences were observed subdividing the sample into different groups according to the legal requirements to perform the professional activity of occupational physicians in Italy or according to particular aspects of their professional activity. Conclusions: This study has provided interesting findings that may help to guide future discussion on alternative and additional instruments and/or methodologies that may be adopted to implement the quality and effectiveness of occupational medicine practice. Int J Occup Med Environ Health 2016;29(5):837–85

Experimental investigation on the aerodynamic loads and wake flow features of a low aspect-ratio circular cylinder

An experimental investigation on the flow features of the wake generated from a circular cylinder with finite height and placed vertically on a plane is presented. Through force measurements the mean drag coefficient is found to be roughly invariant by varying Reynolds number in a range between 6×104 and 11×104. As for the fluctuating forces, a dominant spectral component is clearly detected for the signals of the cross-flow force. A spectral contribution with roughly the same Strouhal number is detected from velocity signals acquired, through hot-wire anemometry, in proximity to the lateral wake boundary; its energy is found to decrease by moving the probe away from the wake and upwards. Simultaneous velocity measurements showed that these fluctuations can confidently be ascribed to an alternate vortex shedding. Subsequently, dynamic measurements of the pressure field over the lateral surface and the free-end of the model were carried out, which highlight that the spectral component connected to vortex shedding is found over the lateral surface, with maximum energy at an azimuthal position just before the separation of the shear layers. The fluctuating energy connected to vortex shedding decreases by moving towards regions immersed in the separated wake, and with increasing vertical coordinate; as a matter of fact, above about half model height an evident energy peak cannot be detected anymore. This feature highlights that a regular alternate vortex shedding occurs only for the lower half-span of the model and that the remaining part is dominated by the upwash generated by the flow passing over the free-end. From the spectral analysis of the pressure measurements carried out over the model free-end no evidence of the presence of the spectral component connected to the alternate vortex shedding is found, as expected. However, a significant fluctuating energy is observed at lower dominating frequencies