Browsing by Author "Gori, Fabio"

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    A 2d-Numerical Study on Slot Jet Applied to a Wind Turbine as a Circulation Control Technique
    (XII International Conference on Computational Heat, Mass and Momentum Transfer, 2019-09) Petracci, Ivano; Manni, Luca; Angelino, Matteo; Corasaniti, Sandra; Gori, Fabio
    A study on the feasibility of the Circulation Control (CC) technique for wind turbines is proposed. The CC was born in aeronautic field to improve the lift force on the wings, allowing the short take-off and landing of aircraft. It consists in blowing air at a relatively high speed over a rounded trailing edge. The thin jet of air remains attached to the convex curved surface, imposing a certain curvature to the outer streamlines, and, hence, increasing the lift force of the airfoil. Aim of this study is to numerically investigate the advantages on a wind turbine, based on the S809 airfoil, taking into account the energy related considerations, as the cost of the jet production. The paper, after a thorough evaluation of the increase of the generated power, finds that this technique could be promising in the energy harvesting aim.
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    Experiments and numerical simulations of mass transfer and flow evolution in transient rectangular free jet of air
    (Elsevier, 2019-09-12) Petracci, Ivano; Angelino, Matteo; Di Venuta, Ivan; Boghi, Andrea; Gori, Fabio
    The paper investigates the transient flow evolution, up to the steady state, of a rectangular free jet of air with aspect ratio AR=6.3. The Reynolds number, Re, defined according to the hydraulic diameter, D, of the rectangular slot of height H, equal to about D=2H, spans in the range from Re=48,000 to Re=3400. A centrifugal fan generates the jet flow with a velocity increasing, as a ramp, from zero up to a constant steady state value. In the transient state of the flow the jet is composed of a primary vortex (PV) and a quasi-stable stem. In the transient 2D experimental instant images the stem of the jet has a height equal to that of the slot and a length increasing with the time up to a maximum value, which is greater than the lengths observed in the steady state, i.e. Negligible Disturbance Flow (NDF) and Small Disturbance Flow (SDF). The experimental jet evolution, observed with measurements and image processing, show that the primary vortex pinch-off does not occur for all the Reynolds numbers investigated, while the leading vortex structure becomes progressively incoherent and breaks down into turbulence. Two-dimensional (2D) Large Eddy Simulations (LES) of mass transfer and fluid flow evolution confirm the experimental images. The transient LES simulations are in good agreement with the experimental results.
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    Influence of the Reynolds number on the instant flow evolution of a turbulent rectangular free jet of air. Average Particle Image Velocimetry (PIV) visualizations and measurements
    (Elsevier, 2014-10-29) Gori, Fabio; Petracci, Ivano; Angelino, Matteo
    The paper is aimed at investigating the influence of the Reynolds number on the instant flow evolution of a rectangular free jet of air in the range of Reynolds numbers from Re = 35,300 to Re = 2,200, where the Reynolds number, Re, is defined according to the hydraulic diameter, D, of a rectangular slot of height H, equal to about D = 2H. The Particle Image Velocimetry (PIV) technique allows obtaining the instant PIV visualizations on the central symmetry section of the rectangular jet. The visual inspection of the instant frames with one and two vortices, except for Re = 35,300 where only one vortex images are detected, shows that after the jet exit is present the Flow with Constant Instant Height, with a length LCIH which increases with the decrease of the Reynolds number, from a ratio LCIH/H equal to LCIH/H = 0.9 at Re = 35,300 to LCIH/H = 4.0 at Re = 2,200. The instant PIV measurements, carried out at several distances from the jet exit, show that the variations of the ratio U=U0 of the centerline instant velocity, U, to the exit average velocity, U0, remain below ±4% for a length LCIV, defining the Flow with Constant Instant Velocity on the centerline. The ratio LCIV/H increases from LCIV/H = 1.1 at Re = 35,300 to LCIV/H = 4.1 at Re = 2,200 and is quite similar to LCIH/H. The instant PIV measurements of the centerline turbulence intensity, Tu, show that its variations remain below ±4% for a length LCIT, defining the Flow with Constant Instant Turbulence on the centerline. The ratio LCIT/H is equal to LCIV/H. The instant PIV velocity profiles in the transverse direction, are practically unchanged for a length LUVP which defines the flow with Unchanged Velocity Profile. The four lengths LUVP = LCIH = LCIV = LCIT are considered equal, defining the Flow with Negligible Disturbances, with length LND, or first type of flow, L1, which is in agreement with the average length of the flow with Constant Height, LCH, in the turbulent and laminar regime, and with the average length of the Undisturbed flow, LU, in the turbulent regime. The instant PIV frames show the presence, after the flow with negligible disturbances, of the Flow with Small Variations of the Height, where the jet height varies slightly without forming the vortex. The length of the flow with Small Variation of the Height is LSVH, and the first vortex appears after the total length (LND + LSVH). The variations of the instant PIV measurements of the centerline turbulence intensity, Tu, increase after the length LCIT, but remaining below ±5–9% for a length LSVT, which defines the Flow with Small Variation of the Turbulence. The total length (LCIT + LSVT) is comparable to (LCIH + LSVH) in the turbulent and laminar regimes, and can be assumed equal to the sum of the lengths of the flows with negligible and small disturbances, LND + LSD = L2, which is in agreement with the average length of the undisturbed flow, LU, in the laminar regime. The instant PIV frames show the flow with Coherent Vortices after L2, with a length LCV, followed by the vortices breakdown. The instant PIV frames allow visualizing the end of the flow with Coherent Vortices, defining the total length of the flows with negligible and small disturbances, and coherent vortices, as LND + LSD + LCV = L3, which is in good agreement with the average length of the potential core, LP.
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    Large eddy simulation and self-similarity analysis of the momentum spreading in the near field region of turbulent submerged round jets
    (Elsevier, 2019-09-24) Boghi, Andrea; Di Venuta, Ivan; Angelino, Matteo; Gori, Fabio
    Self-Similarity in turbulent round jets has been the object of investigation from several decades. The evolution of turbulent submerged jets is characterized by the presence of two regions: the region of flow establishment, or near field region (NFR) and the fully developed region (FDR), or far-field region (FFR). The momentum spreading in the FDR is known to be self-similar and few mathematical models have been presented in the past to describe it. The flow evolution in the NFR has been rarely studied since there is a certain consensus on the idea that the flow in the NFR is not self-similar. In this work, we study the flow evolution of a turbulent submerged round jet by means of large eddy simulation (LES) at several Reynolds numbers ranging from 2492 to 19,988. Three new self-similar laws are proposed to describe the flow evolution in the NFR, one for the initial region, called Undisturbed Region of Flow, (URF), and two for the final region, the potential core region (PCR). The numerical results presented in this work are also validated with the self-similar laws for the FDR proposed by Tollmien (1926) and Görtler (1942), and the experimental data of Hussein et al. (1994), and Panchapakesan and Lumley (1993), in the FDR; those of Davies et al. (1963), in the PCR; and van Hout et al. (2018), in the URF. The conclusion is that previous inability to find the self-similarity law in the NFR is due to the attempt to find a unique self-similar variable to describe the momentum spreading in both the URF and the PCR.
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    Lateral edge effects on heat/mass transfer on a finite width surface within a turbulent boundary layer
    (Elsevier, 2019-04-12) Angelino, Matteo; Goldstein, Richard J.; Gori, Fabio
    Numerical simulations of the local heat/mass transfer on a finite width surface within a turbulent boundary layer are presented. Different approaches to the RANS modelling of the turbulent heat/mass flux are compared to Large Eddy Simulations (LES). Mass transfer experiments conducted with the naphthalene sublimation technique are used as validation. The isotropic eddy viscosity model, Simple Gradient Diffusion Hypothesis (SGDH), is shown to underestimate the span-wise effects. Its anisotropic extension, Generalized Gradient Diffusion Hypothesis (GGDH), improves the prediction, but still does not account for near-wall contribution in strongly dissimilar velocity and temperature/concentration fields, even in combination with a wall-sensitive second-moment closure model such as the Elliptic Blending Reynolds Stress Model (EB-RSM). A more complete turbulent heat flux model based on the elliptic blending approach, the Elliptic Blending GGDH (EB-GGDH) presents very good agreement with LES and with the experiments, confirming the need for more advanced turbulent heat flux modelling in applications with strong three-dimensional effects.
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    Local heat transfer on a finite width surface with laminar boundary layer flow
    (ASME-ATI-UIT, 2015-05-17) Taliaferro, Matthew E.; Angelino, Matteo; Gori, Fabio; Goldstein, Richard J.
    The effect of a lateral discontinuity in the thermal boundary conditions in two dimensional laminar flow on a flat plate, is investigated by numerical and analytical modeling. When the thermal and momentum boundary layers start at the same location, the resulting self-similar two dimensional boundary layer equations were solved numerically. For an unheated starting length, three dimensional numerical simulations were required. For both the three and two dimensional thermal simulations, a Blasius velocity field was assumed. It is found that all the Nusselt numbers collapse to a single curve when graphed as a function of a spanwise similarity variable. Simple correlations for the local Nusselt number on a rectangular flat plate are presented for a variety of boundary conditions.
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    Local heat transfer on a finite width surface with laminar boundary layer flow
    (Elsevier, 2016-02-16) Taliaferro, Matthew E.; Angelino, Matteo; Gori, Fabio; Goldstein, Richard J.
    The effect of a lateral discontinuity in the thermal boundary conditions in two dimensional laminar flow on a flat plate is investigated with numerical and analytical modeling. When the thermal and momentum boundary layers start at the same location, the resulting self-similar two dimensional boundary layer equations were solved numerically. For flow with an unheated starting length, three dimensional numerical simulations were required. For both the two and three dimensional thermal simulations, the Blasius solution for a two dimensional momentum boundary layer was assumed. It is found that all the Nusselt numbers collapse to a single curve when graphed as a function of a spanwise similarity variable. Simple correlations for the local Nusselt number on a rectangular flat plate are presented for a variety of boundary conditions.
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    Numerical evidence of an undisturbed region of flow in a turbulent rectangular submerged free jet
    (Taylor and Francis, 2016-07-02) Boghi, Andrea; Angelino, Matteo; Gori, Fabio
    The evolution of turbulent rectangular submerged free jets is described in the literature by the presence of two regions of flow: the potential core region (PCR) and the fully developed region (FDR). However, experiments carried out in the last decade showed that a third region of flow is present, the undisturbed region of flow (URF), so-called in the average visualization, or the negligible disturbances flow (NDF) plus the small disturbances flow (SDF), so-called in the instant visualization. The URF is located between the slot exit and the beginning of the PCR. The main characteristics of URF, and NDF, are that velocity and turbulence profiles remain almost equal to those measured on the slot exit, and the height of the jet remains equal to the slot one. In the SDF the jet height undergoes small variations, i.e., contractions or expansions, but without formation of the vortex. To date, no numerical evidence of the presence of URF has been given by the literature. The present study, which concerns a two-dimensional jet, presents Large Eddy Simulations (LES), carried out at four Reynolds numbers, which are able to predict and characterize URF. The present numerical results are compared to previous theoretical approaches and confirm the presence of URF, between the slot exit and the PCR. Moreover, URF has a self-similar behavior and a new law for the evolution of the momentum is proposed.
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    Numerical simulation of mass transfer and fluid flow evolution of a rectangular free jet of air
    (Elsevier, 2017-10-12) Di Venuta, Ivan; Petracci, Ivano; Angelino, Matteo; Boghi, Andrea; Gori, Fabio
    The paper presents Large Eddy Simulations (LES) of mass transfer and fluid flow evolutions of a submerged rectangular free jet of air in the range of Reynolds numbers from Re = 3400 to Re = 22,000, with the Reynolds number, Re, defined with the hydraulic diameter of the rectangular slot, of height H. The numerical simulations are 3D for Re = 3400 and 6800, while 2D for Re = 10,400 and 22,000 to reduce computational time costs. The average and instant LES numerical simulations are compared with the concentration visualizations, obtained with the Particle Image Velocimetry (PIV) technique, and the fluid dynamics variables, velocity and turbulence, measured with the PIV technique and the Hot Film Anemometry (HFA). In the numerical simulations, the Schmidt number is equal to 100 to compare the air concentration in the PIV experiments, while the turbulence on the exit of the slot is equal to the value measured experimentally, and ranging between 1% and 2%. The average 2-3D LES simulations are in agreement with the concentration and the fluid dynamics experimental results in the Undisturbed Region of Flow (URF) and in the Potential Core Region (PCR), while the vortex breakdown is captured only by the 3D LES approach. As far as the instant flow evolution is concerned, the 2-3D LES simulations reproduce the Negligible Disturbances Flow (NDF), where the jet height maintains constant, and the Small Disturbances Flow (SDF), where the jet height oscillates, with contractions and enlargements, but without the vortex formation. Average and instant velocity and turbulence numerical simulations on the centreline are in good agreement to the experimental PIV measurements.
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    Numerical solution of three-dimensional rectangular submerged jets with the evidence of the undisturbed region of flow
    (Taylor & Francis, 2016-09-20) Angelino, Matteo; Boghi, Andrea; Gori, Fabio
    The evolution of turbulent rectangular submerged free jets has been investigated numerically with a two-dimensional (2D) approach by the present authors and, by using the large eddy simulations (LES) at several Reynolds numbers. The average numerical results confirmed the presence of the undisturbed region of flow (URF) located between the slot exit and the beginning of the potential core region (PCR) previously observed experimentally at the University of Rome “Tor Vergata” by Gori and coworkers. The 2D study of the present authors carried out under the conditions previously investigated in the literature, showed that the URF has a self-similar behavior, and proposed a new law for the evolution of the momentum. The present paper extends the LES to three-dimensional (3D) rectangular submerged free jets, in the range from Re =5,000 to Re =40,000, showing that the self-similar behavior of URF is also present in the 3D numerical simulations, as well as in the PCR and in the fully developed region (FDR).
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    Passive scalar diffusion in three-dimensional turbulent rectangular free jets with numerical evaluation of turbulent Prandtl/Schmidt number
    (Elsevier, 2018-05-08) Di Venuta, Ivan; Boghi, Andrea; Angelino, Matteo; Gori, Fabio
    The passive scalar spreading of fluids with laminar Prandtl or Schmidt number, Pr, Sc, equal to 1 in turbulent rectangular submerged free jets is analyzed by means of numerical simulation and theoretical analysis in the Reynolds number range 5000–40,000. The numerical investigation is carried out by means of a three-dimensional (3D) Large Eddy Simulation (LES) approach with the dynamic Smagorinsky model. A new mathematical model allows to obtain a simplified description of the passive scalar spreading in the largest area of the flow field, the Fully Developed Region (FDR). The present three-dimensional (3D) investigation shows that the passive scalar spreading follows a self-similarity law in the Fully Developed Region (FDR), as well as in the mean Undisturbed Region of Flow (URF) and in the Potential Core Region (PCR), similarly to what found in the Near Field Region (NFR) of rectangular submerged free jets, investigated with a two-dimensional (2D) approach. The turbulent Prandtl or Schmidt number is evaluated numerically and is found to be inversely proportional to the mean velocity gradient in the PCR. The present 3D numerical results show that the turbulent Prandtl or Schmidt number is zero in most part of the mean URF, and PCR, while it assumes different values outside. In the FDR the turbulent Prandtl or Schmidt number is constant and approximately equal to 0.7, in agreement with the literature, showing that turbulence affects momentum and passive scalar in a different way.