Energetic Modes Research Articles

Unsteady analysis of turbulent flow and heat transfer behind a wall-proximity square rib using dynamic delayed detached-eddy simulation

In the present study, turbulent wall heat transfer behind a wall-proximity square rib is numerically modeled using dynamic delayed detached-eddy simulations, with the objective of clarifying unsteady flow behaviors and their influence on wall heat transfer. Three configurations with gap-to-height ratios (G/d) of 0, 0.25, and 0.5 are comparatively evaluated at a Reynolds number (Red) of 7600. The wall heat transfer is overwhelmingly affected by the interaction between the upper separated shear layer and the lower wall jet flow, exhibiting distinctly different global characteristics with increases in the wall gap. A proper orthogonal decomposition analysis of the turbulent flow fields is conducted to effectively identify the energetic flow structures superimposed on the shear layers and demonstrates that transformative features are present, from energetic bubble-flapping modes (G/d= 0, 0.25) to Karman-like vortex street modes (G/d= 0.25, 0.5). Finally, the phase-dependent variation of the spatiotemporally varying flow structures is examined. In the G/d=0.25 configuration, the suppressed lower vortical structure oscillated irregularly, leading to a locally thin thermal boundary layer and strong wall heat-transfer augmentation in the 0<x/d<4 region. In the G/d=0.5 configuration, the wall jet flow constantly disrupted the thermal boundary layer, causing Nû to plateau in the 0<x/d<3 region. The periodic shedding of the vortical structures in the upper shear layer intermittently spread onto the wall surface in the 3<x/d<6 region, resulting in the gradual decline of Nû. Accordingly, the cause-and-effect mechanism linking the unsteady flow behaviors with wall heat removal is determined, and the coupling between the large-scale vortical structures and the corresponding thermal boundary distribution is established.

Analysis of the accuracy of residual heat removal and natural convection transients in reactor pools

The assessment of the inaccuracy and sensitivity of simulations of natural convection in pools is of the highest importance for the nuclear industry. Therefore, in this article we qualify the uncertainty on the calculation of three dimensional spatio-temporal evolution of Raileigh-Bènard instability.This assessment is carried out through the creation of a surrogate fast model. The model is build utilizing Proper Orthogonal Decomposition and Galerkin projection. The Proper Orthogonal Decomposition allows finding the most energetic modes. Those are derived post-processing a high fidelity CFD calculation. Those modes constitute the vectors of a new, reduced, basis. In the Galerkin Projection the governing equations are written in this reduced basis.Once the model is available, we discuss the uncertainties on the determination of the amplitudes of each mode. Concretely, we consider the hypothesis of deviations that follow the normal distribution for each mode. This normal distribution is centered in the nominal value of the amplitude and have a standard deviation which amounts 5% of the amplitude.The expected results and its uncertainty are henceforth computed by Monte-Carlo method, calculating hundreds of solutions of the Initial Value Problem with the surrogate model.

Investigation of the near wake of a horizontal-axis wind turbine model by dynamic mode decomposition

The wake flow of a horizontal-axis wind turbine has an important effect on the energy extraction and downstream turbulent flow. In this paper, large eddy simulation is employed to obtain the unsteady flow over a two-blade wind turbine model. The detailed geometry of the blade is resolved in the computational mesh. The wake flow is analyzed using dynamic mode decomposition (DMD). The DMD results for the wake flow in the absolute reference frame indicate that the tip vortices are the dominant unsteady flow features in the near wake. The leading energetic modes illustrate the downstream, swing and rotating motion of the tip vortices. The wake flow in a relative reference frame static to the rotor is also analyzed by DMD. The decomposition results illustrate the traveling wave structures and spatial-growth disturbances on the helical vortex filaments. The mode results suggest that the disturbances are gradually formed from the small-scale turbulences, grow rapidly on the helical vortex filament and eventually contribute to the mutual instability of the tip vortices. The breakdown of the tip vortices will lead to the wake recovery and affect the energy extraction of downstream turbines.

Turbulence properties in jets with fractal grid turbulence

Abstract

Analysis of the accuracy of residual heat removal in gen-IV reactors

The assessment of the inaccuracy and sensitivity of simulations of natural convection in pools is of the highest importance for the nuclear industry and concretely for the design of the 4th generation reactors.Therefore, in this article we qualify the uncertainty on the calculation of three dimensional spatio-temporal evolution of Rayleigh-Bénard instability.This assessment is carried out through the creation of a surrogate–fast–model. The model is build utilizing Proper Orthogonal Decomposition and Galerkin projection. The most energetic modes are derived from a high fidelity CFD calculation. The governing equations are henceforth projected into the space generated.Once the model is available, we consider the uncertainties on the dimensionless numbers, Re, Pr, Ra. The expected result and its uncertainty are computed by Monte-Carlo method, calculating hundreds of solutions of the Initial Value Problem with the surrogate model. By this we have verified the applicability of the methodology to this kind of problems and the viability of the approach for uncertainty qualification.

Numerical study of the coherent structures in a transitional vertical channel natural convection flow

Numerical simulations of a spatially developing transitional flow in a vertical channel with one side uniformly heated and subjected to random velocity fluctuations at the inlet have been performed. Two characteristic frequency bands are observed in the flow, near the heated wall. The ability of the Proper Orthogonal Decomposition and the time-domain Spectral Proper Orthogonal Decomposition (SPOD) to decompose the flow is assessed, and SPOD is shown to be a powerful tool, as it is capable of separating the most energetic modes into two great families whose frequency content matches the frequency bands previously identified. The spatial structure of the modes is described, and their contribution to the turbulent heat transfer and velocity-temperature correlation is evaluated. Finally, the modes are linked to coherent structures that are observed in instantaneous visualizations of the flow, and a scenario of the development of the coherent structures in the laminar-turbulent transitional process is proposed.

Flow structures of a precessing jet in an axisymmetric chamber

This work investigates the flow structures of a precessing jet in an axisymmetric chamber with expansion ratio $$D/d=5$$ and length-to-diameter ratio $$L/D=2.75$$ at Reynolds number $${\mathrm{Re}}_{d}=2.4\times 10$$ 4 by use of planar particle image velocimetry. Time-averaged and statistical results indicate that the precessing jet flow is symmetric, on average, in the streamwise plane. Large velocity fluctuations and vorticity exist in the inner and outer shear layers. In addition, vorticity is found in the boundary layer of the chamber wall, due to backflow and confinement. Subsequent correlative analysis indicates that coherent structures are mainly distributed in the shear layer and decay with its development. The results also indicate that precession occurs in the region $$x/d>5$$ and may lead to changes in the structure and position of the shear layer. Further spectral analysis shows that a low-frequency structure with $$\mathrm{St}\approx 7.4\times 10$$ −3, which can be interpreted as precession, exists in the flow field and that the corresponding dominant frequency decreases as the fluid flows downstream. Finally, proper orthogonal decomposition (POD) analysis reveals the most energetic mode representing the flow structure of precession, which has the highest proportion of the total downstream energy. The precession induces an alternating flow, switching between outflow from one side of the chamber and inflow from another side. In addition, four typical instances of flow structures of precession with different intensities or phases are presented through low-order reconstruction of the specified POD modes. These cases show that due to the instability of the reattachment point, the mainstream oscillates up and down in the region near the nozzle exit and twists back and forth in the region near the chamber exit, with changes of scale and position of flow structures on the measurement plane, exposing complex behavior of the instantaneous flow field of precession.

Uncovering design principles for amorphous-like heat conduction using two-channel lattice dynamics

The physics of heat conduction puts practical limits on many technological fields such as energy production, storage, and conversion. It is now widely appreciated that the phonon-gas model does not describe the full vibrational spectrum in amorphous materials, since this picture likely breaks down at higher frequencies. A two-channel heat conduction model, which uses harmonic vibrational states and lattice dynamics as a basis, has recently been shown to capture both crystal-like (phonon-gas channel) and amorphous-like (diffuson channel) heat conduction. While materials design principles for the phonon-gas channel are well established, similar understanding and control of the diffuson channel is lacking. In this work, in order to uncover design principles for the diffuson channel, we study structurally-complex crystalline Yb14 (Mn,Mg)Sb11, a champion thermoelectric material above 800 K, experimentally using inelastic neutron scattering and computationally using the two-channel lattice dynamical approach. Our results show that the diffuson channel indeed dominates in Yb14MnSb11 above 300 K. More importantly, we demonstrate a method for the rational design of amorphous-like heat conduction by considering the energetic proximity phonon modes and modifying them through chemical means. We show that increasing (decreasing) the mass on the Sb-site decreases (increases) the energy of these modes such that there is greater (smaller) overlap with Yb-dominated modes resulting in a higher (lower) thermal conductivity. This design strategy is exactly opposite of what is expected when the phonon-gas channel and/or common analytical models for the diffuson channel are considered, since in both cases an increase in atomic mass commonly leads to a decrease in thermal conductivity. This work demonstrates how two-channel lattice dynamics can not only quantitatively predict the relative importance of the phonon-gas and diffuson channels, but also lead to rational design strategies in materials where the diffuson channel is important.

Dynamic modes of inflow jet in brain aneurysms

The transition of the inflow jet to turbulence is crucial in understanding the pathology of brain aneurysms. Previous works Le et al. (2010, 2013) have shown evidence for a highly dynamic inflow jet in the ostium of brain aneurysms. While it is highly desired to investigate this inflow jet dynamics in clinical practice, the constraints on spatial and temporal resolutions of in vivo data do not allow a detailed analysis of this transition. In this work, Dynamic Mode Decomposition (DMD) is used to identify the most energetic modes of the inflow jet in patient-specific models of internal carotid aneurysms via the utilization of high-resolution simulation data. It is hypothesized that dynamic modes are not solely controlled by the blood flow waveform at the parent artery. They are also dependent on jet-wall interaction phenomena. DMD analysis shows that the spatial extent of low- frequency modes corresponds well to the most energetic areas of the inflow jet. The high-frequency modes are short-lived and correspond to the flow separation at the proximal neck and the jet’s impingement onto the aneurysmal wall. Low-frequency modes can be reconstructed at relatively low spatial and temporal resolutions comparable to ones of in vivo data. The current results suggest that DMD can be practically useful in analyzing blood flow patterns of brain aneurysms with in vivo data.

Validation of large eddy simulation of flow behind a circular cylinder

The flow in the wake behind a circular cylinder in a cross-flow at Reynolds number of 4815 was studied both experimentally and via mathematical modeling. The mathematical model was performed as a Large Eddy Simulation (LES), while the experiments were carried out using the time-resolved variant of the Particle Image Velocimetry (PIV) method. Both the simulation and experiment took into account the dynamical aspects of the studied phenomenon, which enabled a detailed validation of the mathematical model. The overall statistical properties of the simulated flow were validated via comparing the time-averaged measured and computed velocity and vorticity fields. To validate the dynamical behavior, the velocity spectra were examined first. Next, the Proper Orthogonal Decomposition (POD) of the spatio-temporal velocity data was performed on both the experimental and numerical data and a comparison of the obtained energetic modes was carried out. All the performed validations have shown a satisfactory agreement between the simulation and the experiment.

Experimental Study on Physical Behavior of Fluidic Oscillator in a Confined Cavity with Sudden Expansion

In this study, two-dimensional time-resolved particle image velocimetry (2D-TR-PIV) was used to investigate the effect of the external domain on oscillating jets from double-feedback fluidic oscillators. Two different cases with different Re numbers (2680–10,730), as free external domain and fully confined were studied. Time-averaged results showed although a self-oscillating jet was attained for the free external domain, it could not be achieved for a fully confined geometry. For a fully confined geometry at Re = 2680, two symmetric vortices did not allow the jet to oscillate and at Re = 6440, the flow pattern in the external region became non-symmetric due to the Coanda vortex, subsequently, the self-oscillating jet was not observed. At Re = 10,730, the strength of the jet was inclined to cope with such vortices and tended to oscillate. However, strong vortices were created near the exit region of the fluidic oscillator, which led to an almost non-symmetric pattern. In addition, the proper orthogonal decomposition (POD) method and phase-averaged analysis were applied to obtain the unsteady behavior of flow and the most energetic dynamic structure. Interestingly, at Re = 6440, the third mode was still energetic for fully confined, but for other cases, the first two modes were the most energetic modes, which showed vigorous coherent structures.

POD‐based analysis of a wind turbine wake under the influence of tower and nacelle

AbstractThe wake produced by a model wind turbine is investigated using proper orthogonal decomposition (POD) of numerical data obtained by large eddy simulations at a diameter‐based Reynolds number . The blades are modeled employing the actuator line method and an immersed boundary method is used to simulate tower and nacelle. Two simulations are performed: one accounts only for the blades effect; the other includes also tower and nacelle. The two simulations are analyzed and compared in terms of mean flow fields and POD modes that mainly characterize the wake dynamics. In the rotor‐only case, the most energetic modes in the near wake are composed of high‐frequency tip and root vortices, whereas in the far wake, low‐frequency modes accounting for mutual inductance instability of tip vortices are found. When tower and nacelle are included, low‐frequency POD modes are present already in the near wake, linked to the von Karman vortices shed by the tower. These modes interact nonlinearly with the tip vortices in the far wake, generating new low‐frequency POD modes, some of them lying in the frequency range of wake meandering. An analysis of the mean kinetic energy (MKE) entrainment of each POD mode shows that tip vortices sustain the wake mean shear, whereas low‐frequency modes contribute to wake recovery. This explains why tower and nacelle induce a faster wake recovery.

Extreme events in wall turbulence

Abstract

Nonlinear simulations of energetic particle-driven instabilities interacting with Alfvén continuum during frequency chirping

Toroidal Alfvén eigenmodes excited by energetic particles (EPs) and their transition to energetic particle modes are investigated using a three-dimensional nonlinear kinetic-magnetohydrodynamic hybrid simulation code. Both the symmetric chirping inside the Alfvén continuum gap for upward and downward directions and the asymmetric chirping affected by the Alfvén continuum are found in the simulations depending on the EP drive and the mode damping. For weak drive and damping, symmetric frequency chirping is observed with a chirping rate consistent with the Berk–Breizman theory (Berk et al 1997 Phys. Lett. A 234 213). For moderate drive and damping, upward chirping is dominant because downward chirping is interrupted by interaction with the Alfvén continuum. For strong drive and damping, two branches of frequency chirping are found for different poloidal harmonics. The dominant chirping is downward into the Alfvén continuum and the other is an upward chirping inside the Alfvén continuum gap. The creation and the propagation of holes and clumps are found in EP phase space, which is qualitatively consistent with the Berk–Breizman theory.

Experimental Analysis of Multiscale Vortex Shedding in Turbulent Turbomachine Blade Wakes

Wakes behind turbomachine blades are generally highly turbulent and seemingly chaotic. Within the apparent chaos, coherent structures of various strength and length scales are present that are challenging to identify within individual snapshots of the wake flowfield. We propose a strategy to identify and separate multiscale vortices in the turbulent wake of a turbomachine blade from experimental data. Uncorrelated velocity field snapshots were obtained in the wake of a compressor blade in a linear cascade wind tunnel at Reynolds using particle image velocimetry. The snapshots were analyzed using proper orthogonal decomposition to identify the most energetic mode pairs representing vortex shedding patterns of different length scales with their own shedding frequency. The phase angles associated with the individual shedding patterns are extracted for each snapshot based on the time coefficients of the mode pairs. By averaging snapshots with the same phase angle for selected mode pairs, we were able to calculate mode- or scale-dependent phase average flowfields. These scale-dependent phase averages visually highlight the multiscale vortex character of the turbulent wake and allow for a quantitative analysis of the size, emerging location, and shedding frequency of the different structures.

Identification of coherent structures in horizontal slug flow

Multiphase flow measurement devices are significantly affected by the flow pattern, such as, e.g., slug flow, leading to large uncertainties. In this context, the slug flow pattern in horizontal pipes is investigated with the aim of finding a statistical characterization of the structures in space and time. For this, two different instances of slug flow are analyzed with a snapshot proper orthogonal decomposition and an additional mode coupling algorithm, which provides an energy-ranked mode basis of the underlying coherent structures. For the considered flows, the most energetic mode pair has been identified with the corresponding slugging structures. Thereby, the temporal and spatial information of these mode pairs enables a statistical characterization of the slugs. In this context, a length scale, a dominant frequency, and an energy representation of the slugging structures are obtained from this method.

Wake dynamics behind a rotary oscillating cylinder analyzed with proper orthogonal decomposition

Two-dimensional high-order spectral/hp computations are carried out for a cylinder undergoing a sinusoidal rotary oscillation about its own axis. Results are examined for Re=200 and a fixed oscillation amplitude of θ0=π. The study concentrates on a domain of forcing frequencies ranging from 0 to 5f0, with f0 being the natural shedding frequency of the fixed cylinder. The drag of the cylinder is measured to reduce by up to 22% at the optimal frequency. This drag reduction is expected to result from the appearance of negative pressure in the windward regions of cylinder surface. Proper orthogonal decomposition (POD) is then utilized to extract the energetic modes that govern the dynamics of the flow. A novel force decomposition technique proposed by Miyanawala and Jaiman (2019) is reformulated, to allow quantification of the time-dependent contribution from each mode to the pressure drag. Such contribution is found to be affected, in a manner, by the forcing frequency. POD is further used to characterize the spatially evolving nature of the forced wake as it undergoes a transition from the near-wake two-layer shedding pattern to the far-wake Kármán-like shedding pattern. It is also found that a few modes suffice to reconstruct the near-wake accurately, while more modes must be retained to ensure an accurate approximation of the far-wake.

Effects of inert gas jet on the transition from deflagration to detonation in a stoichiometric methane-oxygen mixture

Detonation is an energetic combustion mode augmenting high flow momentum and thermodynamic efficiency, it has been applied in detonation engines, such as pulse detonation engines (PDEs) and rotating detonation engines (RDEs), they have become potential aerospace propulsion equipment. Recently, fluidic jet-in-cross flow (JICF) has been demonstrated experimentally and numerically that can accelerate the deflagration-to-detonation transition (DDT) process. Nonetheless, most of previous studies focused on the jets using combustible mixture or oxygen, which may bring additional risk for turbulence-generated system in detonation engines. In this study, a more safe and controllable inert gas (i.e., Ar) is applied for JICF, experiments are carried out to investigate effects of argon jet as an enhancement method on promoting the DDT in a stoichiometric methane-oxygen mixture. The effects of local argon concentration, turbulence intensity and injection position on the DDT process are systematically examined. Two-dimensional numerical simulations are also performed to elucidate the details of the injection evolution. The experimental results show that turbulence generated by the argon injection can promote flame acceleration and the onset of detonation only in the fast deflagration regime. The enhancing effect is more prominent at higher turbulence intensity by increasing jet injection pressure and shorter injection time. Too long injection duration increases argon local concentration that leads to an adverse effect prohibiting the DDT occurrence. During the initial laminar flame acceleration, referred to as the slow deflagration regime, no enhancement by the argon jet on DDT can be observed. By looking numerically at the flow structure of the argon jet, the vortical features enhance the transport and mixing between reactants and products. The interaction between the reactive travelling wave and the jet structure further induces turbulence and thus accelerates the chemical reaction rate. With the time elapsed, the injected argon entrains largely and dilutes the ambient combustible mixture, and restrains the DDT. Furthermore, a novel dimensionless criterion and a characteristic parameter Turc are proposed, quantitatively analyzing the dominate mechanism in flame propagation and the initial stage of DDT as inert jet is introduced.

Modeling of the ECCD injection effect on the Heliotron J and LHD plasma stability

The aim of the study is to analyze the stability of the energetic particle modes (EPM) and Alfven Eigenmodes (AE) in Helitron J and LHD plasma if the electron cyclotron current drive (ECCD) is applied. The analysis is performed using the code FAR3d that solves the reduced MHD equations describing the linear evolution of the poloidal flux and the toroidal component of the vorticity in a full 3D system, coupled with equations of density and parallel velocity moments for the energetic particle (EP) species, including the effect of the acoustic modes. The Landau damping and resonant destabilization effects are added via the closure relation. The simulation results show that the n = 1 EPM and n = 2 global AE (GAE) in Heliotron J plasma can be stabilized if the magnetic shear is enhanced at the plasma periphery by an increase (co-ECCD injection) or decrease (ctr-ECCD injection) of the rotational transform at the magnetic axis (). In the ctr-ECCD simulations, the EPM/AE growth rate decreases only below a given , similar to the ECCD intensity threshold observed in the experiments. In addition, ctr-ECCD simulations show an enhancement of the continuum damping. The simulations of the LHD discharges with ctr-ECCD injection indicate the stabilization of the n = 1 EPM, n = 2 toroidal AE (TAE) and n = 3 TAE, caused by an enhancement of the continuum damping in the inner plasma leading to a higher EP β threshold with respect to the co- and no-ECCD simulations.

POD analysis of the recovery process in wind turbine wakes

The wake produced by a single three-bladed wind turbine is investigated using the proper orthogonal decomposition (POD) of numerical data obtained by a large eddy simulation. The rotor blades are modeled using the actuator line method, whereas tower and nacelle are simulated through an immersed boundary method. The POD is performed in a three-dimensional subdomain enclosing the wake after conducting a convergence test, which demonstrates that the first ten modes are well converged. Most energetic POD modes identify and isolate different flow features characterising the wake dynamics, such as the tip-vortices spirals, the von Kárman vortices shed by the tower, the Kelvin-Helmholtz instability linked to the wake shear layer. Very low frequency modes are also found, which could be related to the wake meandering phenomenon. Moreover, the wake recovery process is studied by computing the contribution of each POD mode to the mean-kinetic-energy entrainment. This analysis indicates that tip vortices negatively affect the wake recovery, since they provide a negative entrainment. On the contrary, flow structures related to the tower wake are found to be beneficial to wake recovery, demonstrating the importance of including tower and nacelle in numerical simulations.