1,720,969 research outputs found
Simulation of COVID-19 indoor emissions from coughing and breathing with air conditioning and mask protection effects
No full textThe COVID-19 infection has emerged as a disruptive pandemic at worldwide level. The study of the mechanism of contagion is one of the greatest challenges before a mass vaccination campaign that would protect populations. The study can support the development of knowledge and tools to develop possible strategies for containing its spread in future events. The saliva droplet aerosol expelled during breathing or coughing is the main cause for the propagation of the SARS-Cov-2. In this work, a URANS CFD approach was used to simulate the dispersion from the mouth of these particles in closed environments. The air conditioning system was considered. The conditions were varied to determine their impact on the diffusion of the aerosol. Lagrangian and Eulerian numerical approaches were used to model the coughing and the breathing events. These were validated with the puff theory, numerical and experimental results. A realistic case of a meeting room with two persons was simulated. Different characteristics of the expulsed aerosols and different ventilation system configurations were considered to demonstrate how these simulations can support management strategies for indoor occupation. Finally, the effect of the protective mask was introduced to quantify its beneficial effects to support safe indoor occupation
Flow and Thermal Analysis of a Racing Car Braking System
No full textThe braking system of a racing car is one of the main design challenges. The flow around and inside the wheel of an F1 car with all braking system components is analyzed in order to evaluate the heat transfer after a braking event. Very few studies have been published on this topic, mainly due to the high confidentiality level in the racing car sector. In the present work, using an actual geometry of an early 2000s F1 car, the braking system is simulated using a CFD approach. The boundary conditions for the wheel and brake system are taken from the simulation of a vehicle model with a front wing. Different heat transfer phenomena are progressively added to the model in order to understand their effects, including thermal convection only, radiation and conjugate heat transfer. Two different vehicle velocities are simulated to quantify and compare the heat removal after a braking event. The different heat transfer mechanisms have dramatic effects on the prediction of the brake cooling results, and these are quantified in order to understand the limitations of the simplified approaches. Finally, the influence of the ambient pressure at two different altitudes on the heat transfer from a braking event is studied
Criteria for the Stability Limit Prediction of High Speed Centrifugal Compressors With Vaneless Diffuser: Part II — The Development of Prediction Criteria
No full textThe challenge to be able to predict the stability limit in high speed centrifugal compressor is particularly strategic in an initial design phase. Furthermore, to be able to predict the limit massflow rate through the use of simplified numerical models (which does not require excessive computational resources) is very important. In the literature there are several methods to predict the chocking condition, while there is a lack as regards the surge condition. The authors have already presented a criterion to predict the surge line valid for centrifugal compressors with vaned diffuser. Instead those with vaneless diffuser have a very different behavior. For this reason, in the first paper an in-depth fluid dynamic analysis has been carried out, in order to identify the main phenomena linked to the trigger of instability in this type of compressors. This analysis has allowed understanding that the rotational speed is a discriminating factor in the phenomenology. In this second part, using the previous information, different criteria to predict the limit massflow rate for centrifugal compressors with vaneless diffuser are described. All the criteria are based on different simplified CFD approaches that can be routinely used during the design phase
Simulation and Modeling of Ported Shroud Effects on Radial Compressor Stage Stability Limits
No full textThe design features of a centrifugal compressor must guarantee high performance and a wide operating range. The ported shroud was developed specifically to extend the operating limit. It is a passive flow control device based on a cavity for flow recirculation to avoid blade passage blocking in near surge conditions. A CFD simulation campaign using a simplified model identified the differences in the performance of the centrifugal compressor with ported shroud, compared to the baseline case. The use of a stability criterion to determine the limit mass flow rate, developed in a previous study by the authors, highlighted and quantified the extension of the surge margin in the case with ported shroud for different rotational speeds. An increase in the surge margin of 11% was detected at design speed, but with a lower trend at higher speeds. An in-depth flow analysis showed the main physical mechanisms in the compressor that occur for different operating conditions: at near surge conditions the cavity recirculates the low momentum flow located in the inducer region; it re-energizes the mainstream decreasing the circumferential velocity component; an improvement of up to 7% of the pressure ratio was obtained. Instead, at best efficiency conditions the flow recirculation worsens the performance by reducing the flow incidence at the rotor leading edge. Finally, using unsteady simulations with a complete 3D model and with the application of the stability criterion it was possible to confirm that the ported shroud can effectively extend the operating range
Aerodynamic effect of the gurney flap on the front wing of a f1 car and flow interactions with car components
No full textThe design of a racing car needs several aerodynamic design steps in order to achieve high performance. Each component has an aerodynamic interaction with the others and high performance requires a good match between them. The front wing is undoubtedly one of the main components to determine car performance with a strong interaction with the downstream components. The Gurney Flap (GF) is a small appendix perpendicular to the pressure side of the front wing at the trailing edge that can dramatically improve the front wing performance. In the literature, the performance of a GF on a single profile is well documented, while in this paper the GF mounted on the front wing of a racing car has been investigated and the interactions through the 3D flow structures are discussed. The global drag and downforce performance on the main components of the vehicle have been examined by comparing the cases with and without a GF. The GF increases the downforce by about 24% compared to a limited increase in the drag force. A fluid dynamic analysis has been carried out to understand the physical mechanisms of the flow interaction induced to the other components. The GF, in fact, enhances the ground effect, by redistributing the flow that interacts differently with the other components i.e., the wheel zone
A numerical study of correlation between recirculation length and shedding frequency in vortex shedding Phenomena
No full textThe purpose of this paper is to characterize and to estimate the recirculating length behind an aerodynamic profile in ground effect with Gurney Flap. The flow characterization at high Reynolds numbers was performed by means of numerical analysis. A correlation between the size of the recirculation length and the frequency of vortex shedding was studied. The vortex shedding has a characteristic frequency, which, in this work, is correlated to the size of a recirculation length defined by the authors. The numerical investigation methodology applied to the profile with Gurney Flap, was previously developed on the well-documented test case of the flow around a cylinder at high Reynolds. The case was chosen to investigate and to validate the numerical approach with experimental data
Numerical Prediction of Tonal Noise in Centrifugal Blowers
No full textCentrifugal blowers are widely used in automotive, heating, ventilating, air-conditioning and other industrial purposes. In fact, they allow high-pressure increase to a moderately high mass flow rate, despite the reduced overall dimensions of the system. Numerous authors have worked to increase the fluid dynamic efficiency of the impeller and the volute. However, the recent sound emission standards have imposed tighter constraints, making the noise reduction one of the most challenging target for the design. The reduction of the tonal noise is the main concern for these blowers because the noise at the first harmonic is clearly distinguishable in the noise spectrum. Techniques to reduce this peak generally rely on experimental measurements in aeroacoustics laboratory. In this work, a CFD procedure has been developed to accurately predict the tonal noise in radial bowers. The approach has been validated using real geometries and experimental data. Various turbulence models have been tested to find the best results without the use of excessive computational resources. Moreover, unsteady simulations of the 3D blower have been carried out to analyze the influence of the main geometric parameters on the tonal noise reduction. A parametric design code developed by the authors have been used to change the geometry in order to identify the effect of the main geometrical design parameters on both fluid dynamic and aeroacoustics performance
Criteria for the Stability Limit Prediction of High Speed Centrifugal Compressors With Vaneless Diffuser: Part I — Flow Structure Analysis
No full textHigh-speed centrifugal compressor requirements include a wide operating range between choking and stall especially for turbocharging applications. The prediction of the stability limit at different speeds is still challenging. In literature, several studies have been published on the phenomena that trigger the compressor instability. However, a comprehensive analysis of criteria that can be used in the first steps of centrifugal compressors design to predict the stability limit is still missing. In previous work the authors have already presented a criterion, so called “Stability Parameter”, to predict the surge line of centrifugal compressors based on a simplified CFD approach that does not require excessive computational resources and that can be efficiently used in the preliminary design phases. The above methodology has demonstrated its accuracy for centrifugal compressors with vaned diffuser, but a lower accuracy has been detected for vaneless diffusers. Before proceeding to identify additional criteria focused on compressors with vaneless diffuser, an in-depth fluid dynamics analysis has been necessary. This analysis has been also carried out through fully 3D unsteady simulations to allow identifying the real phenomena linked to the trigger of the instability of centrifugal compressors. It has been found how these phenomena are strongly related to the rotational speed, in particular have been shown the key role of the volute at high rotational speed
CFD simulation of the slot jet impingement heat transfer process and application to a temperature control system for galvanizing line of metal band
No full textA CFD model to simulate the cooling technique trough slot jet impingement has been developed. Such a technique has been tested on an existing vertical galvanizing industrial line, which initially envisaged a round jet configuration, the subject of a previous work. Two different slot jet configurations have been simulated and compared to the pre-existing one, in order to provide design information for a possible new jet cooler after exploring different solutions. The numerical model has been appropriately calibrated and validated by comparing it with experimental measurements from a literature case. At first, a single slot jet configuration was simulated through a 2D model, then multi slot configurations were calculated using 3D models. Different turbulence models were compared to select the best candidate for the CFD approach. Finally, several configurations with different slots numbers and jet-wall distances were considered. It was possible to understand the physical mechanisms underlying this cooling technique and to be able to select the most promising configuration for the reference industrial cooling process
Modelling of hydrogen diffusion leading to embrittlement in austenitic stainless steels
Full text linkThe paper presents the 3-D model of the hydrogen diffusion in austenitic stainless steel. In order to model the material behaviour, a real microstructure taking into account the grain boundaries, the dislocations density, the vacancies number and the precipitates state was analysed in order to the implemented in the employed software (ANSYS). The effect of each single hydrogen trap was physically determined. The simulations were carried out by modelling the material microstructure with a high number of elements and nodes in order to improve the affordability of the obtained results. The model allowed to identify the hydrogen diffusion mode in different conditions by evaluating the weight that each single trap has on the overall process. The developed model allowed also to define the hydrogen saturation of the microstructure in different conditions of temperature
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