Time-averaged Velocity Fields Research Articles

Flow around triangular prisms with varying vertex angle at low Reynolds numbers

The present work investigates the unsteady flow around triangular prisms with vertex angles of 30°,45°,60°, and 90° for shedding Reynolds number between 50 and 150. The numerical simulations of flow around triangular prisms at different vertex angles and Reynolds number has been carried out using the open-source code OpenFOAM. The wake of the prisms in different cases has been examined using instantaneous and time-averaged velocity and vorticity fields. The energy dynamics in the wake are demonstrated using enstrophy. The paper explains the shedding around a prism and reports the differences in the wake due to the different vertex angles of the prisms employed in the present work. Strouhal number and force coefficients have been obtained and compared for different prisms. The coefficient of lift and drag phase plot indicates a higher spread for prisms with larger vertex angles at higher Reynolds number. The shedding frequency has a linear variation with Reynolds numbers for the prisms. The obtained results were compared with earlier works on square cylinders and 45° oriented square cylinder.

A multi-directional redundant 3D-LPT system for ship–flight–deck wind interactions

In the past years, volumetric velocimetry measurements with helium-filled soap bubbles as tracer particles have been introduced in wind tunnel experiments and performed at large-scale, enabling the study of complex body aerodynamics. A limiting factor is identified in the field of wind engineering, where the flow around ships is frequently investigated. Considering multiple wind directions, the optical access for illumination and 3D imaging rapidly erodes the measurement regions due to shadows and incomplete triangulation. This work formalizes the concepts of volumetric losses and camera redundancy, and examines the performance of multi-directional illumination and imaging for monolithic and partitioned modes. The work is corroborated by experiments around a representative ship model. The study shows that a redundant system of cameras yields the largest measurement volume when partitioned into subsystems. The 3D measurements employing two illumination directions and seven cameras, yield the time-averaged velocity field around the ship. Regions of flow separation and recirculation are revealed, as well as sets of counter-rotating vortices in several stations from the ship bow to the flight–deck. The unsteady regime at the flight–deck is examined by proper orthogonal decomposition, indicating that the technique is suited for the analysis of large-scale unsteady flow features.

An Experimental Investigation Of Sweeping Jets Impinging Flow Field: Effects Of Nozzle-To-Plate Distance And Feedback Channel Minimum Cross-Sectional Area

Without requiring any moving part or piezoelectrical elements, fluidic oscillators are actuators able to convert a steady jet into an oscillating one, only due to internal fluid dynamic mechanisms. Since the oscillations of such jets are self-induced and self-sustained, the interest in fluidic oscillators has grown over the last years, especially for film cooling, flow control and heat transfer applications. The oscillation frequency, the sweeping angle and the flow field characteristics of sweeping jets greatly depend on various geometric and fluid dynamic parameters. Therefore, in the current work, through the application of the planar PIV technique, the effects of nozzle-to-plate distance and feedback channel minimum cross-sectional area on the external flow field of impinging sweeping jets are investigated. Non-dimensional nozzle-to-plate distances H/w equal to 2, 4, 6, 8 and 10 are taken into account, being w the exit nozzle throat section width. To analyze the effects of the feedback channel minimum cross-sectional area, a fluidic oscillator having a mixing chamber length Lf/w = 4.5 is provided with threaded holes in correspondence of the feedback channels, in which a micrometric screw is fastened to vary the parameter g/w, being g the feedback channel minimum passage width; values of g/w equal to 0.17, 0.33, 0.50, 0.67 and 1 are considered. Time-averaged and phase-averaged analyses are performed; indeed, the pressure signal measured between the inlet and outlet sections of one feedback channel is used to identify the phase of each velocity snapshot, despite the velocity measurements are not time-resolved. Firstly, the configuration with g/w=1 is taken as reference case, thus analyzing the nozzle-to-plate distance effect. The phase-averaged velocity and fluctuating kinetic energy fields show that the jet sweeps in the external flow field, with a sweeping angle approximately equal to 25° and a frequency of 50 Hz. Due to this behavior, the time-averaged velocity field exhibits a twin-jet pattern, which can also be identified from the time-averaged Phase-correlated Kinetic Energy (PKE) distributions (see Figure 1, left). Indeed, the extreme positions of the oscillating motion are the zones in which the coherent oscillation of the velocity values is greater and, therefore, the PKE values are the greatest. Furthermore, while at H/w=2 the jet sweeps in an almost rigid-like way between the two extreme positions reached during its oscillating motion, at greater nozzle-to-plate distances a phase delay between the motion of the jet near the impinging plate and near the exit nozzle section can be observed. Considering the effects of the feedback channel minimum cross-sectional area, by reducing g/w the sweeping angle of the jet decreases, while the oscillation frequency increases. Still, a twin-jet pattern can be identified for g/w≥0.33, as it can be seen from the crosswise profiles of the dimensionless time-averaged axial velocity (see Figure 1, right), while for g/w=0.17 a single-jet velocity structure can be identified. Nevertheless, as g/w decreases the maximum values of the velocity increase while the oscillation intensity decreases, resulting in a decrease of the PKE values.

Characterization of near-wall flow and in-cylinder free-stream turbulence during the intake phase of a direct-injection engine: A flow bench study

This study investigates the characteristics of the near-wall flow and pressure-induced wall-influenced turbulence inside a direct-injection engine during the intake phase by utilizing a wall-resolved Large Eddy Simulation. An engine flow bench operating under stationary conditions is used to reduce the complexity of engine flows and to shed light on the in-cylinder flow during the intake phase. The investigation focuses on in-cylinder turbulence on symmetry (SP) and valve planes (VP), particularly emphasizing the flow close to the intake valves and the impingement region on the liner wall. An experimental dataset acquired through 2D high-speed particle image velocimetry (PIV) facilitates validation of the simulation of both time-averaged velocity field and associated turbulence structure. Turbulence anisotropy analysis shows that, in complex wall-bounded flows, the IC engine-related in-cylinder turbulence structure is characterized by the anisotropic states that cope with both axisymmetric expansion and contraction straining events. Due to the strong influence of the intake flow, the turbulence anisotropy level, expressed in terms of the two-componentality parameter F, retains its value of less than 0.85, and no turbulence occurs in accordance with the fully three-component isotropic state, characterized by the F-parameter value of 1. In the vicinity of the intake valve near wall area, the turbulence anisotropy state transitions from that resembling a canonical channel flow characteristic to curvature-induced acceleration after impingement, which aligns with axisymmetric expansion in the anisotropy invariant map. The presence of intake flow sustains turbulence anisotropy. Limitations in applying wall-function treatments for near-wall flow in engine contexts due to simplifications, such as the zero-pressure gradient assumption, are highlighted. In this respect, the interplay of non-equilibrium effects in the near-wall region needed to be further explored. Accordingly, analysis of the viscosity-affected layer near the intake valve and the flow impingement area of the liner wall underscores the importance of the wall-tangential pressure gradient in shaping the near-wall behavior. A budget analysis of the turbulent kinetic energy equation at the intake valve emphasizes the dominance of fluctuating pressure-induced diffusive transport in turbulence generation.

Data Assimilation of Ideally Expanded Supersonic Jet Using RANS Simulation for High-Resolution PIV Data

Data assimilation using particle image velocimetry (PIV) and Reynolds-averaged Navier–Stokes (RANS) simulation was performed for an ideally expanded supersonic jet flying at a Mach number of 2.0. The present study aims to efficiently reconstruct all the physical quantities in the aeroacoustic fields that match well with a realistic, experimentally obtained flow field. The two-dimensional, two-component PIV measurement was applied to the jet axis plane, and the time-averaged velocity field was obtained using single-pixel ensemble correlation. Two-dimensional axisymmetric RANS simulation using the Menter shear stress transport (SST) model was also performed, and the parameters of the SST model were optimized via data assimilation using the ensemble Kalman filter. The standard deviation of the observation noise σ, which is a parameter of the ensemble Kalman filter, is estimated by the previously proposed method (Nakamura et al., Low-Grid-Resolution-RANS-Based Data Assimilation of Time-Averaged Separated Flow Obtained by LES. Int. J. Comp. Fluid. Dyn., 2022), and its effectiveness was investigated for the first time. This method effectively estimated the magnitude of σ at each generation without tuning the hyperparameters. The assimilated flow fields exhibited similar flow structures observed in PIV such as the potential core length or shear layer. Therefore, the present framework can be used to estimate time-averaged full flow fields that match well with experimentally observed flow fields, and has the potential to construct a database for the Navier-Stokes-based stability analysis that requires a full flow field.

Wind tunnel measurement of pedestrian-level gust wind flow and comfort around irregular lift-up buildings within simplified urban arrays

The implementation of lift-up buildings has demonstrated their practicality in enhancing urban wind comfort in high-density cities. The pedestrian-level wind environment around a central lift-up building surrounded by low-rise buildings was measured using wind tunnel experiments. The time-averaged velocity field and turbulence statistics, including the turbulent kinetic energy, normal component of the Reynolds stresses, gust factor, and gust wind velocity field, were discussed. Based on the results, the pedestrian-level gust wind flow and comfort for two typical building shapes (Arc, V-shaped) were compared with Rectangular lift-up buildings under different wind directions (0∘, 90∘ and 180∘), the pedestrian-level wind environments (PLWEs) were assessed following two wind criteria for weak and windy wind conditions by wind speed thresholds and maximum allowed exceedance probabilities. The results showed that the erection of the high-rise lift-up building modified the pedestrian-level wind environment around the building groups, owing to the interaction between the downwash flow around the building and the surrounding building blocks. A region of high mean and gust wind velocity was observed at the lateral and rear side of the central lift-up building due to the lateral entrained wind and Venturi effect between elevated columns, indicating that lift-up buildings can enhance PLWEs even in the presence of surrounding buildings. The Arc-shaped lift-up building tends to generate a larger “Intolerable” area compared with V- and Rectangular-shaped buildings due to its curved surface. The wind impacts the convex portion of the building, leading to airflow divergence and facilitating downstream ventilation.

Global dynamics and topology of two-phase mixing layer flow through simultaneous gas and liquid velocity measurements

This study reports the first time-resolved particle image velocimetry characterization of a planar two-phase mixing layer flow, whose velocity field is measured simultaneously in gas and liquid streams. Two parallel air and water flows meet downstream of a splitter plate, giving rise to an initially spanwise invariant configuration. The aim is to elucidate further the mechanisms leading to the flow breakup in gas-assisted atomization. The complete experimental characterization of the velocity field represents a database that could be used in data-driven reduced-order models to investigate the global behaviour of the flow system. After the analysis of a selected reference case, a parametric study of the flow behaviour is performed by varying the liquid ( $Re_l$ ) and gas ( $Re_g$ ) Reynolds numbers, and as a consequence also the gas-to-liquid dynamic pressure ratio ( $M$ ), shedding light on both time-averaged (mean) and unsteady velocity fields. In the reference case, it is shown that the mean flow exhibits a wake region just downstream of the splitter plate, followed by the development of a mixing layer. By increasing both $Re_l$ and $Re_g$ , the streamwise extent of the wake decreases and eventually vanishes, the flow resulting in a pure mixing layer regime. The spectral analysis of the normal-to-flow velocity fluctuations outlines different flow regimes by variation of the governing parameters, giving more insights into the global characteristics of the flow field. As a major result, it is found that at high $Re_g$ and $M$ values, the velocity fluctuations are characterized by low-frequency temporal oscillations synchronized in several locations within the flow field, which suggest the presence of a global mode of instability. The proper orthogonal decomposition of velocity fluctuations, performed in both gas and liquid phases, reveals finally that the synchronized oscillations are associated with a low-frequency dominant flapping mode of the gas–liquid interface. Higher-order modes correspond to interfacial wave structures travelling with the so-called Dimotakis velocity. For lower gas Reynolds numbers, the leading modes describe higher frequency fingers shedding at the interface.

DNS analysis of the effect of control parameters on the heat transfer performance of intermittently controlled impinging jets

Impinging jets are used industrially in various applications such as heating and cooling. However, multiple impinging jets (MIJ) create a complex flow field due to interference between the individual jets, which hinders heat transfer. In order to solve the problem of heat transfer performance in MIJ, a new control method must be developed. This study introduces intermittent control to 13 hexagonally arranged circular jets to investigate the possibility of control. In the intermittent control, the 13 jets are divided into three groups, and each group is intermittently issued. The distance between jets and the issuing period are changed as control parameters, and the heat transfer characteristics are evaluated using direct numerical simulation (DNS). The time-averaged velocity field revealed the effects of the intermittent period and the distance between jets on the flow characteristics, and the Nusselt number distribution on the wall surface indicated that the proposed method improves the heat transfer characteristics.

Flow characteristics in a semi-confined circular-pipe impinging jet

The impinging jet is a complex heat and mass transfer technique that involves several process variables, such as the jet Reynolds number, impingement distance, and jet configuration. In this study, the flow characteristics of a semi-confined circular-pipe impinging jet over different Reynolds numbers and impingement distances were experimentally investigated using a two-dimensional particle image velocimetry technique. The confinement was achieved by positioning a plate parallel to the impinging plate at the nozzle exit. The time-averaged velocity field exhibited a recirculation structure that gradually shifted downstream with increasing Reynolds numbers or impingement distances. Notably, at H/d = 2, this downstream shift of the structure was accompanied by an increase in the vortex intensity. Moreover, the confined plate induced alterations in the overall flow pattern within the confined region, significantly reducing the wall jet decay rate compared with both unconfined and confined radial wall jets for H/d ≥ 3. Conversely, the confinement did not affect the expansion of the wall jet. Unlike the free (unconfined) impinging jets, the semi-confined circular-pipe impinging jet did not exhibit self-similar behavior in the conventional outer-scaled coordinates, particularly concerning the turbulence intensity and Reynolds shear stress. Finally, self-similarity in the time-averaged velocity and various turbulence parameters was achieved using the parameter scale proposed in this study, thereby obtaining the corresponding scaling laws in the wall jet region. Our study results can deepen the current understanding of the flow characteristics of semi-confined circular-pipe impinging jets and are significant for optimizing the performance and efficiency of compact electronic packaging equipment.

Direct Numerical Simulation of liquid metal forced and mixed convection in a square rod bundle

This paper reports results of Direct Numerical Simulations of fully developed, forced and mixed convection of Liquid Lead Bismuth Eutectic (LBE) around four vertical cylindrical rods arranged in a square lattice at Pr = 0.031. The solutions provided here are placed in a context where very few data are available for low-Prandtl flows, especially in mixed convection conditions, and can hence be useful for the development and validation of advanced turbulent heat transfer models. The equations are discretized using a Finite Volume implementation of a second order projection method. The irregular cylindrical boundaries are handled with an original Immersed Boundary technique. A single friction Reynolds number value is set (Reτ = 180) and buoyancy effects are accounted for by imposing the Rayleigh number, under the Boussinesq approximation. A Ra-value Ra = 2.5 ×104 is selected in order to investigate the main differences between forced and mixed convection at low Prandtl number values. Time-averaged velocity and temperature fields are shown, in order to discuss the main features of the flow and thermal fields. First order statistics are also presented, highlighting the effect of aiding buoyancy on turbulent heat and momentum transport.

LES study of the precessing vortex core mitigation by radial injection in a Francis turbine air model

This work is devoted to a numerical study of the precessing vortex core (PVC) mitigation in a Francis turbine air model. The approach utilized to suppress the PVC, injection of radial jets, was based on influencing flow zones that are most susceptible to control. The analysis of the time-averaged velocity fields revealed extending of a recirculation zone. The time-averaged pressure pulsations in the flow decreased significantly, that indicates a mitigation of the PVC dynamics. Spectra of the azimuthally decomposed pressure pulsations signal demonstrated a suppression of the m = 1 mode by 25% and m = 2 by 40%, however, it does not coincide with the control efficiency in experimental work of I. Litvinov et al. In further investigations it is planned to vary the spatial orientation of the injection jets in order to achieve more effective PVC suppression.

Towards time-resolved multi-property measurements by filtered Rayleigh scattering: diagnostic approach and verification

The use of multiple perspective views is a possible pathway towards the combined measurement of multiple time-resolved flow properties by filtered Rayleigh scattering (FRS). In this study, a six view observation concept is experimentally verified on a aspirated pipe flow. The concept was introduced in our previous work, and it has the ability to simultaneously measure high-accuracy time-averaged and time-resolved three-component velocity, pressure and temperature fields. To simulate time-resolution, multi-view FRS data at a single optimised excitation frequency are selected and processed for multiple flow properties. Time-averaged and quasi-time-resolved FRS results show very good agreement with differential pressure probe measurements and analytical temperature calculations and lie within ±2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm 2$$\\end{document} m/s of complementary laser Doppler anemometry (LDA) velocity measurements for all operating points. The introduction of a multistage fitting procedure for the time-resolved analysis leads to a significant improvement of the precision by factors of 4 and 3 for temperature and axial velocity and 18 for pressure. Moreover, both processing methods show their capacity to resolve flow structures in a swirling flow configuration. It is demonstrated that the developed multi-view concept can be used to determine multiple flow variables from a single-frequency measurement, opening the path towards time-resolved multi-parameter measurements by FRS.

Parallel ribbon vortex: A phenomenological flow feature in an atmospheric boundary layer near sloped terrain

In this study, flow behaviors and vortex morphology around the sloping terrain that is covered by an atmospheric boundary layer are investigated using a combination of numerical and experimental methods. An array of wind tunnel experiments and high-fidelity large eddies simulations have been conducted, considering different slope angles θ = 90°, 45°, 26.5°, and 18.2°. Systematic analysis of turbulence phenomena pertaining to the flow field's topography-driven distortions and accelerations was performed quantitatively via time-averaged velocity field, terrain-induced turbulence intensity, velocity spectrum, and acceleration ratio. Two recirculation zones, one originating from the upstream boundary layer's gradual separation bubble (GSB) and the other from the forced separation bubble (FSB) behind the downstream sharp leading-edge, were confirmed by detailed visualization of the flow field. An acceleration zone of considerable magnitude is also observed, connecting the GSB and FSB. Subsequently, a new vortex structure—the parallel ribbon vortex (PRV)—was disclosed. The PRVs originate from the disordered turbulence structures inside the GSB, spiraling into coherent structures. The accelerated mainstream up the step then stretches and elongates the coherence structures into ribbon-like vortices parallel to the mainstream, completing the formation process. Finally, the PRVs transport fluid up the step before encountering the strong spanwise vortices and dissipating thereafter. The investigation into the slope flow characteristics and turbulence structure conducted in this study has the potential to enhance the prediction of wind characteristics and wind load specifications in mountainous regions.

Air Interactions of Magnetically Driven Plasma Discharges in Crossflow

Motivated by the use of magnetohydrodynamic devices in fluid dynamic flow control research, the current work experimentally studied the actuation mechanism and the resulting aerodynamic interactions of magnetically driven low-current arc-plasma discharges. The investigation was conducted in the context of boundary-layer interactions in low-speed crossflow conditions through the use of coaxial and v-shaped geometries. Time-averaged velocity field measurements showed that the toroidal region of vorticity induced by the coaxial geometry in quiescent air resulted in the formation of a horseshoe vortex in crossflow. Similarly, the v-shaped actuator resulted in the formation of streamwise vortices. The resulting boundary layers had s-shaped streamwise velocity profiles as measured in the wall-normal direction characteristic for the flow downstream of a conventional vortex generator pair, due to the three-dimensional mixing induced by coherent vortex structures. A simplified model based on intermolecular momentum transfer through collisions is proposed to explain the fundamental discharge–air interactions and the induced flowfields. These results inform the applications of various magnetically driven discharges in the field of aerodynamic flow control.

Triple Decomposition of Unsteady Flow in a Low-Speed Mixed-Flow Fan

Numerous studies (including theoretical modeling, numerical simulation, and experimental measurement) have confirmed that wake interaction is a primary factor affecting tonal and broadband noise emitted by aeroengine fans. The knowledge and techniques derived from these studies have often been applied to analyze and control the aerodynamic noise of low-speed axial/mixed-flow fans used in ventilation and cooling applications. Despite the structural similarity between aeroengine fans and axial-flow fans for ventilation and cooling, the differences in their rotor–stator spacing and operating parameters raise the question of whether or not they share completely identical or approximately similar unsteady flow and noise generation mechanisms. In this paper, we employ a large-eddy simulation to investigate the low-speed unsteady flow inside a mixed-flow fan. The triple decomposition method is used to analyze the spatiotemporal characteristics of the time-averaged, ensemble-averaged, and stochastic fluctuation velocities and pressures in both stationary and rotating coordinate systems. The results show that the unsteady flow and noise generation mechanisms inside the fan investigated in this paper are significantly different from those of aeroengine fans, mainly due to two aspects. First, the wake effect on the velocity field of the fan studied in this paper is almost negligible, and the potential interaction is the primary source of the nonuniform spatial distribution of the time-averaged velocity field and the periodic velocity disturbance. Second, the turbulent energy spectrum of the stochastic fluctuation velocity is greatly distinct from the classic turbulence spectra in the low-frequency bands.

On the delineation of the flow separation zone in open-channel confluences

In the context of a T-shaped open-channel confluence, where the tributary flow separates at the downstream junction edge and creates a separation zone in the post-confluence channel, this paper provides a critical reflection upon the terminology and the delineation methods for the separation zone and its proxies: the recirculation zone and the reverse flow zone. Following the fluid-mechanical literature on flow separation, the separation zone is delineated by a dividing surface formed by the three-dimensional (3D) streamlines separating from the downstream junction edge. Based on a dense 3D time-averaged velocity field obtained from a Large-Eddy Simulation of a lab-style confluence flow, the separation zone turns out to be open. Near the bed, the dividing surface exhibits a gap, through which mainly tributary fluid enters the separation zone, leading to a limited discharge flowing through that zone. Downstream of the section where the merging flows are the most contracted, the dividing surface does not reattach to the inner bank, prohibiting the determination of a maximum length of the separation zone. Downstream of that section, the dividing surface gradually loses its role as a skeleton for the separation zone shear layer. Due to mixing, the separation zone and the adjacent maximum velocity zone gradually dissolve and transform into the flow recovery zone, as in a wake. By means of flow visualization with 3D streamlines, revealing different types of recirculating motions, the maximum recirculation zone extent is determined. Then three simplified methods commonly used in the literature to quantify the dimensions of the separation zone or its proxies are evaluated by comparison with the 3D streamline method. By adopting various simplifying assumptions about the flow, the pseudo-streamline method, the zero-unit-discharge method and the zero-velocity method yield definite reasonable maximum lengths for the recirculation zone and the reverse flow zone. With the exception of the zero-velocity method, all simplified methods yield reasonable estimates for the maximum widths and the corresponding contraction coefficients of the separation zone proxies. Still, contrary to the simplified methods, only the 3D streamline method can provide physical insight into the flow features occurring in the separation zone or its proxies.

Numerical evaluation of wind direction effects on the turbulence aerodynamics of a ship airwake

The airwake characteristics around a ship body are an important consideration for helicopter pilots during shipboard operations, as airwake can lead to the formation of massively turbulent flow over the ship deck. Therefore, this study conducted a delayed detached eddy simulation (DDES) to investigate the impact of the bow part of the ship and the wind direction on the turbulent flow characteristics of the ship deck airwake. Numerical simulations were performed for two different ship models (1:12.5) of the Simple Frigate Shape 1 and 2 (SPF1 and SFS2) to analyze the influence of the bow part on the deck flow-field. Seven different orientations of wind direction angle (HW, RW15, RW30, RW45, RW60, RW75, and RW90) were numerically simulated for SFS1. The time-averaged velocity and pressure fields computed by the DDES approach were validated in comparison to those of the experimental data measured by the PIV data and Kulite pressure sensors, respectively, at various locations over the ship deck. The numerical data revealed that, while the deck flow-field is not influenced by the bow of the ship, the airwake flow is significantly affected by the wind direction angle. Specifically, as the wind direction angle is increased, the turbulent kinetic energy is also increased and the flow-field becomes more asymmetric. Moreover, the spectral analysis and time/space assessment of the flow field showed more pressure fluctuations and a higher level of unsteadiness over the ship deck with a larger wind direction angle, which are important parameters in shipboard operations.

Evaluation of the PANS method for the prediction of the fluid flow in a cyclone separator

The simulation of the cyclone separation efficiency can rapidly become very expensive in time of calculation, which is not suitable for industrial application. The prediction of the complex turbulent gas flow inside a cyclone is necessary to estimate its pressure drop, its characteristics and finally its separation efficiency. Even though classic models such as RSM and LES can effectively predict such type of flow, the quality of the results and the computation cost can be discussed. In this work, Partially Averaged Navier–Stokes (PANS) method is proposed to simulate the fluid flow inside a gas cyclone. Compared to experimental and numerical results of a literature review, this model predicts reasonably the time-averaged tangential and axial velocity fields, and more accurately the pressure drop across the cyclone in shorter CPU times. The sensitivity of the PANS approach to the unresolved to total kinetic energy ratio f k was also evaluated with two different definitions. The first one was the standard definition developed by Girimaji et al. [1] and the second one was based on a statistical equivalence between the DES and PITM. The proposed model offers a better cost to quality ratio compared to the classic RANS approach.

Influence of the adjustable EMBr on the asymmetric flow in a thin slab caster with a misaligned SEN

The thin slab casting (TSC) of steel is a type of the continuous casting (CC) with a narrow funnel-shaped mold, characterized by the rapid solidification and fast production rates. A highly turbulent flow impacts on a growing solid shell due to the constant feeding of the fresh hot melt. That strongly affects the solidification profiles and final quality of the TSC slabs. The presented work numerically investigates the solidification inside the TSC mold with the asymmetric flow pattern caused by the misalignment (tilting) of the submerged entry nozzle (SEN). These effects were considered with and without the applied electromagnetic brake (EMBr). The influence of the adjustable EMBr on the asymmetric flow and solidification profiles including turbulent and magnetohydrodynamic (MHD) effects were studied. During consistent series of simulations, the EMBr was varied between the magnetic poles and the time-averaged velocity and temperature fields were collected. The results showed that symmetric EMBr of a local type could partially improve the asymmetry. An optimal braking scenario was found for the casing speed of 5.5 m/min and maximum EMBr value of 180 mT. The solidification and MHD models including turbulence were developed using OpenFOAM®.

The near wake of discrete roughness elements on swept wings

This work presents the first experimental characterization of the flow field in the vicinity of periodically spaced discrete roughness elements (DRE) in a swept wing boundary layer. The time-averaged velocity fields are acquired in a volumetric domain by high-resolution dual-pulse tomographic particle tracking velocimetry. Investigation of the stationary flow topology indicates that the near-element flow region is dominated by high- and low-speed streaks. The boundary layer spectral content is inferred by spatial fast Fourier transform (FFT) analysis of the spanwise velocity signal, characterizing the chordwise behaviour of individual disturbance modes. The two signature features of transient growth, namely algebraic growth and exponential decay, are identified in the chordwise evolution of the disturbance energy associated with higher harmonics of the primary stationary mode. A transient decay process is instead identified in the near-wake region just aft of each DRE, similar to the wake relaxation effect previously observed in two-dimensional boundary layer flows. The transient decay regime is found to condition the onset and initial amplitude of modal crossflow instabilities. Within the critical DRE amplitude range (i.e. affecting boundary layer transition without causing flow tripping) the transient disturbances are strongly receptive to the spanwise spacing and diameter of the elements, which drive the modal energy distribution within the spatial spectra. In the super-critical amplitude forcing (i.e. causing flow tripping) the near-element stationary flow topology is dominated by the development of a high-speed and strongly fluctuating region closely aligned with the DRE wake. Therefore, elevated shears and unsteady disturbances affect the near-element flow development. Combined with the harmonic modes transient growth these instabilities initiate a laminar streak structure breakdown and a bypass transition process.