Rms Velocity Research Articles

Flame characterisation of gas-assisted pulverised coal combustion using FPV-LES

A multiphase flamelet/progress variable (FPV) model for the large eddy simulation (LES) of gas-assisted pulverised coal combustion (PCC) is developed. The target of the simulation is the Darmstadt turbulent gas-assisted swirling solid fuel combustion chamber. The coal particles are treated as Lagrangian point particles, the position, momentum and energy of which are tracked. The gas phase is described by the low-Mach Navier-Stokes equations alongside the Eulerian transport equations of the governing variables for the FPV model. The set of chemical states of the PCC flame is pre-tabulated in a six-dimensional flamelet table and determined by the mixing of the primary fuel stream, volatiles and char off-gases with the oxidising air, the progress of chemical reactions, the interphase heat transfer, as well as sub-grid scale variations. A presumed β-PDF approach for the total mixture fraction is applied to capture sub-grid scale effects. The discrete ordinate method (DOM) with the weighted sum of grey gases model (WSGGM) is employed to model radiation. The FPV-LES results are validated against the experimental evidence and a good agreement of the predicted mean and RMS velocities, as well as the mean gas temperature between experiments and simulations is obtained. The contributions of the pilot, volatile and char off-gas fuel streams to the coal flame are analysed. It is found that most regions of the furnace are dominated by either pilot or volatile combustion, while char conversion only occurs in the far downstream and outer furnace regions. The pilot gas dominates the near-wall region inside the quarl, whereas the volatile gas mainly released from small particles dominates a first volatile combustion zone in the interior of the internal recirculation zone. Larger particles heat up more slowly and release their volatile content further downstream, leading to a secondary volatile combustion zone.

Analysis of the turbulent flow in a partially blocked wire-wrapped rod bundle using LES with wall functions

One of the most promising nuclear reactor designs is the Liquid-Metal Fast Reactor (LMFR). Its fuel assembly consists of rods containing the fuel material, arranged in a triangular lattice and covered by a hexagonal duct. The rods are separated with wire spacers that are helically wrapped around each fuel pin. Because this geometry is tight and compact, accumulation of debris, cladding deformation or detachment of the wires may generate a partial or total flow blockage of coolant subchannels at different locations. The understanding of the coolant behavior in LMFR fuels under the influence of a partially blocked flow area is required for a thorough safety assessment. A Large Eddy Simulation (LES) with wall functions was executed to simulate the flow on a 61-pin wire-wrapped rod bundle under the presence of a blockage located on the bundle periphery to simulate an abnormal operation condition. The Reynolds number was Re=17000, which is in the turbulent regime. Results of mean velocity and root mean square (rms) fluctuating velocity profiles were compared with PIV experimental data from the Texas A&M Thermal-Hydraulic Research Lab. Both mean and rms velocities were found in reasonable agreement with the experimental data among different locations because the simulation results were within 2σ from the PIV measurements. A pressure form loss coefficient due to the blockage was calculated and found to be Kloss=1.0455. The fluctuating velocities revealed a pair of shear layers that formed on each side of the blockage carrying the highest turbulent intensities. A spectral analysis was conducted to visualize the dominant frequencies on the turbulent flow in the recirculation region. The spatial–temporal two-point cross-correlations based on the fluctuating velocities were calculated at different points along the shear layer and were used to estimate integral lengths scales and convection velocities. The cross-correlations showed larger structures at higher axial elevations from the blockage rear wall. The length scales were found in the range of 3 to 21 mm while the magnitudes of the convection velocities presented values between 0.69U¯in to 1.39U¯in, where U¯in is the inlet velocity. Proper orthogonal decomposition (POD) analysis was utilized to extract the dominant modes in the fluctuating flow. Large flow structures and low frequency time signals were observed for the lowest order modes.

Conditioned structure functions in turbulent hydrogen/air flames

Direct numerical simulation data obtained from two turbulent, lean hydrogen–air flames propagating in a box are analyzed to explore the influence of combustion-induced thermal expansion on turbulence in unburned gas. For this purpose, Helmholtz–Hodge decomposition is applied to the computed velocity fields. Subsequently, the second-order structure functions conditioned to unburned reactants are sampled from divergence-free solenoidal velocity field or irrotational potential velocity field, yielded by the decomposition. Results show that thermal expansion significantly affects the conditioned potential structure functions not only inside the mean flame brushes but also upstream of them. Upstream of the flames, first, transverse structure functions for transverse potential velocities grow with distance r between sampling points more slowly when compared to the counterpart structure functions sampled from the entire or solenoidal velocity field. Second, the former growth rate depends substantially on the distance from the flame-brush leading edge, even at small r. Third, potential root mean square (rms) velocities increase with the decrease in distance from the flame-brush leading edge and are comparable with solenoidal rms velocities near the leading edge. Fourth, although the conditioned axial and transverse potential rms velocities are always close to one another, thus implying isotropy of the potential velocity field in unburned reactants, the potential structure functions exhibit a high degree of anisotropy. Fifth, thermal expansion effects are substantial even for the solenoidal structure functions and even upstream of a highly turbulent flame. These findings call for development of advanced models of turbulence in flames, which allow for the discussed thermal expansion effects.

Effects of the synthetic jet on the flow field and heat transfer over a surface-mounted square rib

In this article, a synthetic jet manipulates the flow behavior behind the surface-mounted square rib for heat transfer enhancement. Effects of operating frequencies and amplitudes on the near wake flow field of the rib are studied. Experiments are performed at Re=32000 (based on the Hydraulic diameter of the rectangular test section) with the Synthetic jet flush mounted at the bottom of the test section. Particle Image Velocimeter (PIV) captures a plane's instantaneous and time-averaged spatial flow information. The hotwire anemometer in CTA mode is used for temporal information of the flow field. The effect of synthetic jet on heat transfer downstream of the square rib is measured using Liquid crystal thermography(LCT). The result includes time-averaged velocity magnitude, vorticity, streamlines, turbulence quantities such as turbulence intensity, Reynolds stress, and rms velocities. The concise study indicates that a drastic reduction of reattachment length occurs at frequencies and amplitude of the synthetic jet. The study's unique contribution is the complete removal of the recirculation bubble by using the synthetic jet at a particular upstream position(X/D=-1). At 50 Hz frequency, the maximum heat transfer rate is obtained.

Numerical prediction of loudness metrics for N-waves and shaped sonic booms in kinematic turbulence.

The effects of a kinematic field of velocity fluctuations on the loudness metrics of two waveforms are examined with a three-dimensional one-way propagation solver. The waveforms consist of an N-wave and a simulated low-boom from NASA's X-59 QueSST aircraft. The kinematic turbulence is generated using a von Kármán composite spectrum, which is dependent on a root mean square (rms) velocity and outer scale of the turbulence. A length scale is proposed to account for the effect of the rms velocity and integral scale on the focusing and defocusing of the sonic boom waveform. The probability density function of the location of the first caustic attains a maximum value when the propagation distance is equal to the proposed length scale. Simulation results indicate that for small values of the nondimensional propagation distance, the standard deviation of the loudness metrics increases linearly. The loudness metrics follow a normal distribution within a given range of the nondimensional propagation distance. Results indicate the potential to parameterize the loudness metric distributions by the rms velocity and integral length scale.

Comparison of a finite volume and two Lattice Boltzmann solvers for swirled confined flows

A finite volume and two Lattice–Boltzmann unsteady, flow solvers using LES (Large Eddy Simulation) were compared in a swirling flow configuration, typical of aeronautical combustion chambers. Numerical results were validated against experimental data collected at EM2C laboratory by comparing pressure losses, mean and RMS velocity profiles on multiple planes and axial velocity spectra. Meshes and the overall numerical setups were individually adjusted for each code to obtain the targeted accuracy before comparing CPU efficiencies.Results confirm that the three LES codes provide high fidelity results, much better than usual RANS especially in terms of RMS data. The analysis of CPU performances shows that LBM (Lattice–Boltzmann Method) solvers are faster than the finite volume solver, even if CPU efficiencies remains of the same order of magnitude. In addition, strong scaling tests from 36 to 900 cores show that the finite volume solver scales more efficiently than the LBM codes, specially when the number of grid points per core is not sufficient.

Turbulent Parameters in the Middle Atmosphere: Theoretical Estimates Deduced from a Gravity Wave–Resolving General Circulation Model

Abstract We present a scaling analysis for the stratified turbulent and small-scale turbulent regimes of atmospheric flow with emphasis on the mesosphere. We distinguish rotating-stratified macroturbulence turbulence (SMT), stratified turbulence (ST), and small-scale isotropic Kolmogorov turbulence (KT), and we specify the length and time scales and the characteristic velocities for these regimes. It is shown that the buoyancy scale (Lb) and the Ozmidov scale (Lo) are the main parameters that describe the transition from SMT to KT. We employ the buoyancy Reynolds number and horizontal Froude number to characterize ST and KT in the mesosphere. This theory is applied to simulation results from a high-resolution general circulation model with a Smagorinsky-type turbulent diffusion scheme for the subgrid-scale parameterization. The model allows us to derive the turbulent root-mean-square (rms) velocity in the KT regime. It is found that the turbulent RMS velocity has a single maximum in summer and a double maximum in winter months. The secondary maximum in the winter MLT we associate with a secondary gravity wave–breaking phenomenon. The turbulent rms velocity results from the model agree well with full correlation analyses based on MF-radar measurements. A new scaling for the mesoscale horizontal velocity based on the idea of direct energy cascade in mesoscales is proposed. The latter findings for mesoscale and small-scale characteristic velocities support the idea proposed in this research that mesoscale and small-scale dynamics in the mesosphere are governed by SMT, ST, and KT in the statistical average. Significance Statement Mesoscale dynamics in the middle atmosphere, which consists of atmospheric turbulence and gravity waves, remains a complex problem for atmospheric physics and climate studies. Due to its high nonlinearity, the mesoscale dynamics together with the small-scale turbulence is the primary source of uncertainties and biases in high-altitude general circulation models (GCM) in the middle atmosphere. We use the stratified turbulence theory and the gravity wave–resolving GCM to characterize different scaling regimes and to define various length, time, and velocity scales, that are relevant for the mesoscale and small-scale dynamical regimes. Our results highlight the importance of stratified turbulence in the mesosphere and lower-thermosphere region.

Experimental assessment of the gap width effect on turbulent flow and forced convective heat transfer around a single rod suspended in a channel

The present work presents the experimental campaign carried out to assess the convective heat transfer around a single rod suspended in a rectangular channel under axial turbulent flow. The distance between the rod and the upper wall of the channel forms a rod-wall gap, whose width, d, can be varied. The experiments were performed for the dimensionless gap widths that yield W/D = 1.050, 1.100, 1.150 and 1.200. Hot-wire anemometry was used to measure mean velocities, the RMS velocity fluctuations and the velocity spectra. The rod is equipped with a heated cell that provides the local temperature and convective heat transfer coefficient distributions around the rod’s surface. The local convective heat transfer distribution is in good agreement with the literature. The lowest value for the heat transfer coefficient was not found right in the narrow gap for W/D = 1.050, but, slightly away from the gap. As W/D increases, the minimum local convective heat transfer coefficient occurred closer to the narrowest gap width. The Reynolds-Colburn analogy was investigated for all configurations through the measured convective heat transfer coefficient and the skin friction distributions. A linear relationship was found between the Stanton number and the skin friction coefficient. Moreover, data scattering was seen to decrease significantly when the local velocity was used instead of the global one. It suggests that the heat transfer process is governed, mainly, by the local parameters.

Nonlinear broadband time-domain admittance boundary condition for duct acoustics. Application to perforated plate liners

The behavior of perforated plates at high excitation level is generally modeled by a surface impedance that depends on the rms velocity in the perforations. A time-domain admittance boundary condition (TDABC) is developed to account for this variation using a multipole model. Two formulations are considered, based on the interpolation either of the admittance or of the multipole coefficients from a data set of reference values. These TDABC are implemented in a finite-difference time-domain solver of the linearized Euler equations and are validated by comparison with experimental results on an impedance tube. Application to a two-dimensional lined duct corresponding to the reference geometry of the NASA Grazing Incidence Tube is then performed. The spatial variation of the perforated plate liner impedance is highlighted and it is shown that assuming a uniform impedance can lead to an unacceptable prediction of the liner attenuation. These results are confirmed both for a harmonic or broadband excitation.

Very-Large-Eddy Simulation of Nonreactive Turbulent Flow for Annular Trapped Vortex Combustor

A hybrid Reynolds-averaged Navier-Stokes (RANS) large eddy simulation (LES) method is applied in this work. It called very-large-eddy simulation (VLES) turbulence closure model. The aim of this present study is firstly to validate the accuracy of this method for a specific engineering application (a trapped vortex combustor) and secondly to describe its flow characteristics. The trapped vortex combustor is a new concept that utilizes a large recirculation vortex to stabilize the flame. An accurate prediction of the turbulent flow is meaningful for the trapped vortex combustor. The time-averaged velocity, root-mean-square (rms) velocity, and flow pattern are compared with the experimental data. And the LES model, RANS BSL k-ω model, and RANS k-ɛ model are also applied for the simulation with different mesh resolutions. The results show that the VLES BSL k-ω model provides improved accuracy for velocity prediction. The classical large vortex structure for the trapped vortex combustor is captured qualitatively by the VLES BSL k-ω model also. In addition, the vortex breakdown and processing vortex cone are visualized using the Q-criterion. Furthermore, the VLES BSL k-ω model is not sensitive to the gird resolution. The VLES method is able to predict the turbulent flow of trapped vortex combustor relatively well‎.

Art Exists Because the Viewer Exists: How François Morellet’s Triple X Neonly Influences Postural Control and Subjective Aesthetic Appreciation

Abstract The authors study the effects of viewing François Morellet’s Triple X Neonly (2012) on postural control and subjective appreciation. Participants were asked to view monocularly and binocularly. Their average forward-backward (anteroposterior) body sway root mean square (rms) velocity was significantly greater viewing it monocularly. Participants showed diverse reactions to the artwork, yet comments mainly concerned apparent movement and depth. The authors conclude that binocular viewing facilitates viewer appreciation of the proximity of the central X, thus requiring less energy to keep the body stable. This study confirms the artist’s success in involving viewers actively in their aesthetic experience through their eye and body movements.

Experimental parametric study on dynamic divergence instability and chaos of circular cylindrical shells conveying airflow

Experiments have shown that soft circular cylindrical shells supported at both ends and conveying airflow lose stability by so-called dynamic divergence. The present study investigates experimentally the effect of geometric parameters of a silicone rubber shell, namely length-to-radius (L/R) and thickness-to-radius (h/R) ratios, on the dynamic divergence instability. Bifurcation diagrams of the rms velocity of the shell vibration versus flow velocity are obtained for different shells, displaying a strongly subcritical nonlinear behaviour. Then, the onset of instability and post-critical behaviour of the shells are compared: (i) thinner and longer shells lose stability at lower flow velocities, (ii) thinner shells have higher rms vibration velocity, and (iii) by decreasing L/R, the subcritical behaviour is weakened for thin shells, while it is strengthened for thick shells. The existence of chaos and the influence of geometric parameters on the chaotic behaviour of the system are deeply examined by means of several measures.

Predictions of Vortex Flow in a Diesel Multi-Hole Injector Using the RANS Modelling Approach

The occurrence of vortices in the sac volume of automotive multi-hole fuel injectors plays an important role in the development of vortex cavitation, which directly influences the flow structure and emerging sprays that, in turn, influence the engine performance and emissions. In this study, the RANS-based turbulence modelling approach was used to predict the internal flow in a vertical axis-symmetrical multi-hole (6) diesel fuel injector under non-cavitating conditions. The project aimed to predict the aforementioned vortical structures accurately at two different needle lifts in order to form a correct opinion about their occurrence. The accuracy of the simulations was assessed by comparing the predicted mean axial velocity and RMS velocity of LDV measurements, which showed good agreement. The flow field analysis predicted a complex, 3D, vortical flow structure with the presence of different types of vortices in the sac volume and the nozzle hole. Two main types of vortex were detected: the “hole-to-hole” connecting vortex, and double “counter-rotating” vortices emerging from the needle wall and entering the injector hole facing it. Different flow patterns in the rotational direction of the “hole-to-hole” vortices have been observed at the low needle lift (anticlockwise) and full needle lift (clockwise), due to their different flow passages in the sac, causing a much higher momentum inflow at the lower lift with its much narrower flow passage.

Study of unsteady cavitating flow around Clark-Y hydrofoil using nonlinear PANS model with near-wall correction

In this paper, we describe the use of a new nonlinear partially-averaged Navier–Stokes (PANS) model with near-wall correction for simulating the cavitating flow around a Clark-Y hydrofoil. For comparison, the standard [Formula: see text]–[Formula: see text] PANS model is also used. The results demonstrate that compared to [Formula: see text]–[Formula: see text] PANS and experiment, the new PANS model shows better performance for cavitation flow, including time-averaged velocity, root mean square (rms) velocity and cavity shedding processing. Through the calculation of the lift and drag coefficient at [Formula: see text] and [Formula: see text], it can be concluded that the cavitation will decrease the lift and increase the drag of the hydrofoil, resulting in a decrease of the lift-to-drag ratio. From the analysis of different terms in both the turbulent kinetic energy (TKE) and dissipation rate transport equations of the cloud cavitation, it is found that the production term and the dissipation term are dominant in the turbulent transport, and they are mainly distributed in the vapor–liquid interface and the trailing edge of the hydrofoil.

Turbulent Flow over Confined Backward-Facing Step: PIV vs. DNS

Particle Image Velocimetry measurements of the liquid velocity fields in the flow over the backward-facing step were performed in the same flow configuration as in the existing Direct Numerical Simulation (DNS). The experiment and the simulation were performed in an identical cross-section geometry with step expansion rate 2.25 and the square shape of the outlet duct at the Reynolds number in an inlet part of the section 7100. The experiment was performed in transparent test section, 1.2 m long, with 20 × 45 mm2 cross-section upstream and 45 × 45 mm2 downstream, while a domain that was three times shorter was used in the DNS. A 2D-2C PIV system with a single high-speed camera and a pulse laser was used for a series of two-dimensional measurements of the velocity field at several cross-sections from two different perspectives. Variables analyzed in the experiment are time-averaged fluid velocities, velocity RMS fluctuations and two components of the Reynolds stress tensor. The key novelty is the comparison of two very accurate approaches, PIV and DNS, in the same cross-section geometry. Comparison of the similarities, and especially the differences between the two approaches, elucidates uncertainties of both studies and answers the question on what kind of agreement is expected when two very accurate approaches are compared.

Experimental study of a natural convection flow in a cubic enclosure with a partially heated inner block

In many industrial contexts, buoyancy driven flows are the only cooling strategy in case of breakdown of the forced convection cooling system. In order to study those flows in a simplified configuration, a buoyancy-driven flow is generated inside a cubic enclosure by a partially heated block (Ra = 1.4 × 109). The flow is studied experimentally in the vertical median plane, in the part of the enclosure where the flow is generated i.e. close to the heated side of the block. Velocity fields, mean profiles and RMS statistics are analyzed. The results show the presence of boundary layer flows with a central zone nearly at rest and stratified. RMS velocities are intensified with elevation.

Prandtl number dependence of stellar convection: Flow statistics and convective energy transport

Context. The ratio of kinematic viscosity to thermal diffusivity, the Prandtl number, is much smaller than unity in stellar convection zones. Aims. The main goal of this work is to study the statistics of convective flows and energy transport as functions of the Prandtl number. Methods. Three-dimensional numerical simulations of compressible non-rotating hydrodynamic convection in Cartesian geometry are used. The convection zone (CZ) is embedded between two stably stratified layers. The dominant contribution to the diffusion of entropy fluctuations comes in most cases from a subgrid-scale diffusivity whereas the mean radiative energy flux is mediated by a diffusive flux employing Kramers opacity law. Here, we study the statistics and transport properties of up- and downflows separately. Results. The volume-averaged rms velocity increases with decreasing Prandtl number. At the same time, the filling factor of downflows decreases and leads to, on average, stronger downflows at lower Prandtl numbers. This results in a strong dependence of convective overshooting on the Prandtl number. Velocity power spectra do not show marked changes as a function of Prandtl number except near the base of the convective layer where the dominance of vertical flows is more pronounced. At the highest Reynolds numbers, the velocity power spectra are more compatible with the Bolgiano-Obukhov k−11/5 than the Kolmogorov-Obukhov k−5/3 scaling. The horizontally averaged convected energy flux (F̅conv), which is the sum of the enthalpy (F̅enth) and kinetic energy fluxes (F̅kin), is independent of the Prandtl number within the CZ. However, the absolute values of F̅enth and F̅kin increase monotonically with decreasing Prandtl number. Furthermore, F̅enth and F̅kin have opposite signs for downflows and their sum F̅↓conv diminishes with Prandtl number. Thus, the upflows (downflows) are the dominant contribution to the convected flux at low (high) Prandtl numbers. These results are similar to those from Rayleigh-Benárd convection in the low Prandtl number regime where convection is vigorously turbulent but inefficient at transporting energy. Conclusions. The current results indicate a strong dependence of convective overshooting and energy flux on the Prandtl number. Numerical simulations of astrophysical convection often use a Prandtl number of unity because it is numerically convenient. The current results suggest that this can lead to misleading results and that the astrophysically relevant low Prandtl number regime is qualitatively different from the parameter regimes explored in typical contemporary simulations.

Combustion and vibration study of a direct injection compression ignition engine fuelled with Moringa oleifera biodiesel–diesel blends

ABSTRACT The intention of the current experimental analysis was for the assessment of combustion and vibration using a direct injection single cylinder compression ignition engine. The tests were performed at different loads of 25%, 50%, 75%, and 100% separately with the Moringa oleifera-diesel mixed test fuels samples such as; B0D100, B10D90, B20D80, and B30D70. The parameters concerning combustion i.e. cylinder pressure (CP) and net heat release rate (NHRR) of the diesel engine was improved somewhat while fuelling with M. oleifera biodiesel blends compared to conventional diesel. Besides, the engine vibrations were brought down for biodiesel usage rather than regular diesel. The improvement in combustion and vibration was observed at higher loads. B20D80 was shown best the outcome than the remaining fuel samples. At 100% load, the peak values of CP and NHRR intended for B20D80 were enhanced by 2.91% and 9.5%, while the RMS velocity (vibration) was lowered by 14.27%.

Detecting and locating microseismic events with stacking velocity analysis for surface monitoring

Microseismic surface monitoring may suffer from the signal-to-noise ratio (SNR) in the data and lack of datasets to constrain a realistic velocity model. Therefore, an 1D depth velocity, which is derived from the well log and calibrated with perforation shot, is commonly utilized in microseismic monitoring. In this study, we simultaneously detect and locate microseismic events by applying a stacking method without using a depth velocity model. Since the 1D layered velocity model could be approximately equivalent to a RMS velocity, we borrow the velocity analysis concept in traditional exploration seismic data processing and apply it on mocroseismic surface monitoring. We stack the perforation shot waveform along a hyperbolic curve and pick the maximum stacking energy to obtain a stacking velocity above the target zone. The microseismic events are automatically detected and located by a stacking procedure with the known stacking velocity in time domain. The events are then converted from time to depth using the average velocity calculated from the perforation shot. Synthetic examples show that our approach can successfully recover the true locations of events with a large amount of noises added to the data. The real data results also suggest that this method is applicable for effectively detecting and locating events with low SNR. Although the approach with stacking velocity is limited to the layered media, this restriction is acceptable for many unconventional reservoirs.

Predicting the Maximum Seismic Response of the Soil-Pile-Superstructure System Using Random Forests

ABSTRACT Seismic fragility analysis has been considered an efficient method for seismic performance assessment of soil-pile-superstructure systems (SPSSs). However, seismic fragility analysis based on widely used numerical methods often requires a significant investment of time. In this study, a new approach based on the random forest technique, which can be applied to fragility analysis, is proposed to accurately and efficiently predict the maximum seismic response of SPSSs. Moreover, based on the random forest technique, the velocity RMS is found to exhibit a stronger correlation with the maximum seismic response of SPSSs in liquefiable soils.