Instantaneous Flow Fields Research Articles

Large-Eddy Simulation of a wind turbine using a Filtered Actuator Line Model

When dealing with multirotor devices such as quadcopters or wind farms, the cost of blade-resolved large-eddy simulation (LES) becomes prohibitive. Combining LES with a family of lower-fidelity models, called actuator line models (ALMs), has grown in popularity in the past decade. ALM replaces full blade resolution with an array of actuator points or lines parameterized by aerodynamic lift/drag polar plots along the blades. Body forces computed based on these actuator points are then projected onto the LES flow mesh, mimicking the effect of rotating blades on the flow. However, the optimal projection radius and the associated LES grid size is often too restrictive for multirotor simulations. Recently, a new tip-correction-based filtered ALM (F-ALM) was proposed by Martínez-Tossas and Meneveau (2019), which allows coarser-than-optimal grids by avoiding the associated overprediction of thrust. In this work, F-ALM is implemented into a high-order, in-house LES code to simulate National Renewable Energy Laboratory Phase VI wind turbine. It is then followed by a comparison between the baseline ALM and the newly implemented F-ALM in terms of instantaneous and time-averaged flow fields and blade loads, revealing the advantages of F-ALM in preventing the overprediction of power on coarse grids. This encourages accurate and affordable simulations of multirotor devices in the future.

Investigation of unsteady secondary flows and large-scale turbulence in heterogeneous turbulent boundary layers

Following the findings by Wangsawijaya et al. (J. Fluid Mech., vol. 894, 2020, A7), we re-examine the turbulent boundary layers developing over surfaces with spanwise heterogeneous roughness of various roughness half-wavelengths $0.32 \leq S/\bar {\delta } \leq 3.63$, where $S$ is the width of the roughness strips and $\bar {\delta }$ is the spanwise-averaged boundary-layer thickness. The heterogeneous cases induce counter-rotating secondary flows, and these are compared with the large-scale turbulent structures that occur naturally over the smooth wall. Both appear as meandering elongated high- and low-momentum streaks in the instantaneous flow field. Results based on the triple decomposed velocity fluctuations suggest that the secondary flows are spanwise-locked turbulent structures, with $S/\bar {\delta }$ governing the strength of the turbulent structures and the efficacy of the surface in locking the structures in place (most effective when $S/\bar {\delta } \approx 1$). In terms of unsteadiness, we find additional evidence from conditional averages of the fluctuating velocity fields showing that the secondary flows exhibit maximum unsteadiness (or meandering) when $S/\bar {\delta } \approx 1$. The conditional averages of both spanwise heterogeneous and smooth-wall cases result in structures that are reminiscent of those proposed for the streak-vortex instability model for the inner cycle of wall-bounded turbulence. However, in this case these structures are larger and do not necessarily share the same formation mechanism with the inner cycle. Secondary flows and large-scale structures coexist in the limits where either $S/\bar {\delta } \gg 1$ or $S/\bar {\delta } \ll 1$, where the secondary flows scale on $\delta$ or $S$, respectively. When $S/\bar {\delta } \gg 1$, the secondary flows are locked about the roughness transition, while relatively unaltered large-scale structures occur further from the transition. In the case where $S/\bar {\delta } \ll 1$, $S$-scaled secondary flows are confined close to the surface, coexisting with unaltered larger-scale turbulent structures that penetrate much deeper into the layer.

Numerical investigation of supersonic flow over a parachute-like configuration including turbulent flow effects

Numerical simulations are conducted to investigate supersonic flow over a rigid parachute-like model with a small trailing distance at a freestream Mach number of 2.0 and freestream Reynolds number of 2.04×107 m−1. Turbulent effects on the flow evolution are highlighted by comparing the laminar and Reynolds-averaged Navier–Stokes (RANS) and detached eddy simulation (DES) predictions. The DES model can provide more accurate flow predictions both temporally and spatially than the laminar and RANS simulations. Comprehensive analyses are performed on the time sequence of instantaneous flow fields. The “pulsation” flow mode, as well as the lifecycle of vortex rings, are observed. Furthermore, although turbulent models have a minor effect on the dynamics of shock waves, they significantly alter the vortical structures. Analyses of the pressure fluctuation spectra also confirm that turbulent flows have a lower amplitude oscillation in high-frequency regions than laminar flows.

Numerical investigations of the wake behind a confined flat plate

The two-dimensional flow around a confined flat plate is simulated with the spectral element code, Nek5000. Direct numerical simulations were carried out at low Reynolds numbers Re (between 10⩽Re⩽200) and various blockage ratios β (between 0.1⩽β⩽0.9) and compared with the confined two-dimensional circular cylinder flow previously reported by Sahin and Owens (2004). Four distinct flow regimes are observed for the flow over a confined flat plate. The first is a steady symmetric wake where the vortices are evenly distributed along the centerline of the channel. The second is an unsteady symmetric regime which upon time-averaging the instantaneous flow field, the wake profile is symmetric. The third is an asymmetric steady wake where the flow is steady but the vortices in the wake are skewed towards one of the channel walls. The first three have been reported for the confined flow over a circular cylinder. However, a fourth flow regime defined as unsteady asymmetric wake is found for a flat plate at β = 0.8. This flow regime was not reported in prior studies on the confined flow over a confined cylinder. Analysis using linear stability analysis is also reported in this paper which confirms the trends reported in this paper. Effects of the confinement on the wake of a flat plate on the hydrodynamic forces such as the mean drag coefficient and Strouhal number have also been documented for the same range of Re and β and compared with the flow over a confined cylinder.

Quantifying the spatiotemporal evolution of the turbulent horseshoe vortex in front of a vertical cylinder

The spatiotemporal evolution of the turbulent horseshoe vortex (THV) in front of a cylinder vertically mounted on a hydraulically smooth flat-bed was physically modeled in a large water flume. A particle image velocimetry (PIV) system with upward-illumination was, in particular, employed for the junction flow visualization. The examined Reynolds number was varied from 1.28 × 104 to 1.08 × 105, which is above the threshold of turbulent transition for a junction flow. Based on the PIV measurements, the characteristic features were presented for both the time-averaged and the instantaneous flow fields in a sheet flow at the upstream wall-cylinder junction. Statistical analyses on the experimental data are performed to characterize the spatial-temporal evolution of THV strength. The normalized THV strength in the time-averaged flow field increases first and then approaches a constant value with increasing the ratio of cylinder diameter to water depth. Two alternating patterns, i.e., the regular oscillation and the random wandering, are identified for the quasi-periodic oscillating behaviors of the instantaneous THV. It is found that the cumulative distribution curves for the normalized instantaneous THV strength can be described by the Weibul distribution. The present results provide a physical insight and quantitative characterization for the spatiotemporal evolution of THV, which is critical for predicting the associated wall shear stresses.

On 3D structure of a circular cylinder wake

The 3D structure of the wake behind a circular cylinder in a cross-flow is studied experimentally using stereo PIV technique, Reynolds number was around 5 thousands. The statistical characteristics along the cylinder axis are effectively constant and thus 2D, however instantaneous flow-field in the wake is fully 3D. Within the wake dynamical flow, the streamwise vortices and spots of streamwise velocity component pulsations are detected.

Numerical Study on the Periodicity of Urban Heat Dome Flow

Under calm conditions where the background synoptic wind is weak or absent, the difference between city and rural temperature allows natural convection to develop. The turbulent circulation is also referred as urban heat island circulation or urban heat dome flow. The coupling of buoyancy, inertia, and vertical stability induces a natural periodicity in the urban heat dome flow. The periodicity in urban heat dome flow is often neglected in previous urban heat dome flow research. In this article, a high-precision CFD numerical method is applied, and the city is treated as a homogeneous flat plate with a specific shape. First of all, the real atmosphere model (around 20 km in length scale) are established and validated with water tank experiments. Convergent inflows at lower level and divergent outflows at upper levels can be observed. Then, the instantaneous 3D flow field distribution is analyzed, and the periodicity of the upper outflow region (fout) after reaching the quasi-steady state is determined. Finally, cases with different city diameters (D), regional heat flux (H0), and ambient potential temperature gradients (∂θ/∂z) are designed and solved. A quantitative description of periodicity (fout) and its relationship to buoyancy (B0), inertia (Ud) and vertical stability (N) is presented.

Numerical investigation of the influence of wave parameters on maneuvering hydrodynamic derivatives in regular head waves

To evaluate the wave effect on the maneuvering hydrodynamic derivatives in the research of maneuverability in waves, the experimental and numerical studies on the Planar Motion Mechanism (PMM) tests in regular head waves for the standard ship model KVLCC2 are carried out. The simulations are performed by an up-to-date Unsteady Reynolds Averaged Navier-Stokes (URANS) solver. Overset mesh technique and Euler Overlay method are adopted. The simulation results are validated using the experimental data obtained from the PMM tests in towing tank of Shanghai Jiao Tong University. The comparison of the hydrodynamic derivatives in waves and calm water conditions reveals that the wave effect on Yr′ can be ignored except for the extremely steep wave condition. However, Nr′ is significantly changed by the wavelength and wave steepness. This finding shows that there is an error in the previous assumption that the hydrodynamic derivatives in waves are the same as those in calm water. In addition, a simple modification for the wave effect on high-order hydrodynamic derivatives is hard to be proposed due to the random changing tendency of these parameters. The instantaneous flow field information is also obtained and indicates that the influence of waves on the free surface contour and dynamic pressure is more significant than that of low-frequency maneuvering motions.

CFD simulation of hydrodynamics characteristics in a tank with forward-reverse rotating impeller

BackgroundA forward-reverse rotating impeller was used in an unbaffled tank to improve the mixing efficiency. Flow field and power characteristics in the tank were numerically studied. MethodsComputational fluid dynamics(CFD) simulations were performed to investigate hydrodynamics in the unbaffled tank. The Reynolds stress turbulence (RST) model along with a sliding mesh approach was used to predict instantaneous flow fields in the stirred tank. Significant findingsFluid in the unbaffled tank flows periodically with the forward-reverse impeller agitation. Zonal flow phenomenon was found for the fluid affected by agitation in different directions. The effects of operating conditions on instantaneous fluid velocity vary with positions in the tank. Flow field characteristics in the forward-reverse agitated unbaffled tank are very different from those in constant agitated unbaffled and baffled tanks. Generally, transient power consumption is only the function of its corresponding transient rotational speed. The averaged impeller power number is a constant and is not affected by maximum rotational speed and cycle time. The averaged power number is the largest in the unbaffled tank with forward-reverse agitation, followed by the baffled tank at a constant agitation, and the smallest in the unbaffled tank at a constant impeller speed.

Design and optimization of aerospike nozzle for rotating detonation engine

Rotating detonation engine (RDE) is increasingly viewed as a viable technology to improve the performance of propulsion devices. At now, the design of RDE faces challenges when integrated with aerospike nozzle due to the large pressure fluctuations of rotating detonation waves. To optimize the nozzle configuration for thrust generation, the current work investigates the performance of RDE combined with various types of aerospike nozzles. The method of designing optimal aerospike nozzle for RDE with annulus combustion chamber is proposed. A series of three-dimensional numerical simulations are conducted to obtain the thrust performance of nozzles based on open-source software OpenFOAM. The flow-field structure, pressure gain in combustor and thrust performance of RDE are investigated. The results suggest that although the instantaneous flow-field in RDE is time-varying, the averaged plume field can be viewed as axisymmetric under-expanded supersonic jets. It's reasonable to use the time-averaged value of physical quantities for nozzle design. For nozzle with inward-bend cowl lip and isentropic ramp surface, the supersonic flow in nozzle undergoes approximate isentropic expansion with minimum loss. The flow at nozzle exit is basically parallel to chamber axis, generating the optimal thrust. This indicates that the aerospike nozzle is a good match for efficient expansion of rotating detonation products for propulsion. Moreover, the stagnation pressure gain in combustor has been greatly improved with the constriction nozzle throat. This suggests that in order to elevate the chamber pressure, the convergence section with nozzle throat should be adopted in RDE just as in traditional rocket engines.

Effects of an incoming vortex on the film cooling jet

In a high pressure turbine, film cooling is applied to protect the blades and endwalls from the high temperature main flow. The horseshoe and passage vortex near the endwall can significantly affect the film cooling performance and lead to a detrimental consequence for certain holes. In this study, large eddy simulations (LES) and laser-induced fluorescence (LIF) experiments were conducted to investigate the effects of the incoming vortex on the flow physics and cooling performance of a film cooling jet. Two cases with the incoming vortex passing from different sides of the cooling hole were compared with the baseline case with the uniform incoming flow. The time-averaged results showed that two cases with incoming vortex have more than three times variation in the film cooling effectiveness due to the downwash and upwash effects of the incoming vortex. The analysis based on the instantaneous flow field provides more insight into the flow physics and cooling mechanism. The instantaneous flow field showed that the incoming vortex changes the shape and the distributions of the coherent structures, which affects the mixing and surface area covered by the coolant and hence the cooling effectiveness. The norm of turbulent heat flux was found to better illustrate the mixing process between the main flow and the coolant due to the transportation and breakdown of the coherent structures. In addition, the cases with different vortex strengths were also studied to find the degree of improvement of film cooling effectiveness.

Numerical research on the instabilities of CLT propeller wake

Optimising contracted and load tip (CLT) propellers has proven difficult due to the lack of research on flow fields. In this study, the hydrodynamics and wake of the P1727 propeller, a CLT propeller, are investigated using large eddy simulation for three advance coefficients. Fluctuations in the thrust and torque are minimal, with a frequency of up to 1000 Hz, as indicated by the simulations, which are verified based on experimental data and study on spatial and temporal convergences. Instantaneous, phase-averaged, and azimuthal-averaged flow fields are used to analyse propeller wake, including the tip vortices of endplate, secondary tip vortices, merged and trailing sheet vortices and the hub vortex. Based on previous works, we present a novel mutual inductance mechanism for the instabilities in CLT propeller wake. The merged vortices formed by merging between the tip vortices of endplate and the secondary tip vortices are strengthened, stretched by the outer trailing vortices that are caused by the crack of trailing sheet vortices, then the reinforced merged vortices trigger the leapfrogging phenomenon and induce the outer instabilities. While the inner trailing vortices, or root vortices, remain coherent, they undergo contraction and approach the hub vortex. Thereafter, they intertwine with the spiraling hub vortex, acting as the dominant mutual inductance mode of the inner CLT propeller wake.

Evolution characteristics of 3D vortex structures in stratified swirling flames studied by dual-plane stereoscopic PIV

Stratified swirling flames are typically formed in centrally-staged combustors of modern aero engines. The pilot flame plays an indispensable role in stabilizing the stratified swirling flame. Evolutions of the primary recirculation zone (PRZ) and vortices in the central shear layer (CSL) notably affect dynamics of the pilot flame. This work employed a dual-plane stereoscopic particle image velocimetry (DP-s-PIV) to investigate the evolution characteristics of three-dimensional vortex structures in stratified swirling flows/flames at different Re numbers and working conditions. Time-resolved measurements obtained temporal evolutions of vortex structures and the scanning process resolved time-averaged volumetric flow fields under different working conditions. Results show that the flame-induced flow acceleration near the burner outlet enhances the vorticity and the strain rate of the central shear layer (CSL), where a precessing vortex core (PVC) resides and its dynamics is strongly coupled with the primary recirculation zone (PRZ). The lengths of the PRZ in swirling flames were found to be smaller than that of the swirling flow, owing to the reduced swirl intensity and the enhanced turbulent-induced momentum transport. Furthermore, statistical analyses based on the instantaneous flow fields of the reacting cases demonstrated that the averaged vorticity magnitude and averaged turbulent kinetic energy (TKE) for vortices in the CSL essentially obey the linear dependency and square dependency with the Re number, respectively. And, there exists a linear correlation between the average turbulent kinetic energy and averaged vorticity magnitude for vortices in the CSL. This implies that the enhanced vortices due to the flame-induced effect downstream of the pilot stage is the main source for local turbulence generation in stratified swirling flames.

T-Shaped Control Plate Effect on Flow past a Square Cylinder at Low Reynolds Numbers

In this study, the influence of the T-shaped control plate on the fluid flow characteristics around a square cylinder for a low Reynolds numbers flow is systematically presented. The introduction of upstream attached T-shaped control plate is novel of its kind as T-shaped control plate used for the first time rather than the other passive control methods available in the literature. The Reynolds numbers (Re) are chosen to be Re = 100, 150, 200, and 250, and the T-shaped control plate of the same width with varying length is considered. A numerical investigation is performed using the single-relaxation-time lattice Boltzmann method. The numerical results reveal that there exists an optimum length of T-shaped control plate for reducing fluid forces. This optimum length was found to be 0.5 for Re = 100, 150, and 200 and 2 for Re = 250. At this optimum length, the fluctuating drag forces acting on the cylinder are reduced by 134%, 1375, 133%, and 136% for Re = 100, 150, 200, and 250, respectively. Instantaneous and time-averaged flow fields were also presented for some selected cases in order to identify the three different flow regimes around T-shaped control plate and square cylinder system.

Performance and wake analysis of horizontal axis tidal current turbine using Improved Delayed Detached Eddy Simulation

This paper presents the results of hydrodynamic performance and wake assessment for a horizontal axis tidal current turbine. The three-dimensional instantaneous flow field is resolved utilizing the Improved Delayed Detached Eddy Simulation (IDDES) turbulence model and polyhedral mesh. The rotor rotation is simulated by employing the sliding mesh approach. The simulation results using turbulence model of k-ω Shear Stress Transport are also presented for comparison. The simulated thrust and power coefficients and wake characteristics are validated against published experimental results. It is shown that the performance and wake velocity predicted by IDDES are closer to the experimental values than those predicted by k-ω Shear Stress Transport model. In determining the pressure coefficient, only IDDES captures the time-varying fluctuations pertaining to blade generated turbulence. The oscillations in phase-averaged performance coefficients are substantially larger for IDDES because the inflow turbulence in this model is more realistically resolved using Synthetic Eddy Method. The IDDES predicts more detailed vortex structures which has impact on accurate determination of blade pressure distribution, energy dissipation rate and downstream flow field. It is demonstrated that the IDDES model is capable to satisfactorily simulate the hydrodynamics of a horizontal axis tidal current turbine.

Numerical simulation of wind veering effects on aeroelastic responses of thousand-meter-scale super high-rise buildings

The atmospheric boundary layer (ABL) consists of the atmospheric surface layer (ASL) and the Ekman layer. In the Ekman layer, which typically accounts for the upper 90% of the ABL, the wind veering is generally remarkable along the height. In the wind-resistant design of thousand-meter-scale super high-rise buildings, it is of vital importance to consider the wind veering effects. However, the relevant research on this issue has been rare so far. In this study, based on the authors’ previous study on wind loads acting on thousand-meter-scale super high-rise buildings immersed in the veering wind field, the structural aeroelastic responses of a thousand-meter-scale square-section building model with an aspect ratio of 9:1 under various reduced wind velocities without considering wind veering at 0° wind direction and considering wind veering at 75° wind direction are numerically simulated by utilizing the commercial software ANSYS Workbench 19.1. The wind veering effects on structural aeroelastic responses are comprehensively analyzed. Additionally, the structural dynamic responses with and without considering aeroelasticity are compared. The effects of aerodynamic damping on wind-induced vibration responses are further discussed, and the underlying mechanisms are illustrated in the light of instantaneous flow fields.

Large eddy simulation on the flow characteristics of an argon thermal plasma jet

Large eddy simulations based on the CFD software OpenFOAM have been used to study the effect of Reynolds number and turbulence intensity on the flow and mixing characteristics of an argon thermal plasma jet. Detailed analysis was carried out with respect to four aspects: the average flow field, the instantaneous flow field, turbulence statistical characteristics and the self-similarity. It was shown that for the argon thermal plasma jet with low Reynolds number, increasing the turbulence intensity will increase the turbulent transport mechanism in the mixing layer rather than in the jet axis, leading to the faster development of turbulence. The effect of the turbulent transport mechanism increases with increasing Reynolds number. However, the characteristics of flow and mixing are not affected by turbulence intensity for high Reynolds number situations. It was also found that the mean axial velocity and mean temperature in the axis of the turbulent thermal plasma jet satisfy the self-similarity aspects downstream. In addition, decay constant K is 1.25, which is much smaller than that (5.7–6.1) of the turbulent cold gas jet and has nothing to do with the Reynolds number or turbulence intensity in the jet inlet.

Heat transfer in shallow caves: influence of turbulent convection

The enhancement of the conservation conditions of cave paintings requires a detailed understanding of heat transfer in such cavities. This article presents a numerical investigation of turbulent free convection in a parallelepipedic cavity. Non-homogeneous wall temperatures are prescribed from a large-scale model taking into account external temperature fluctuations damped by heat diffusion in the rock massif above the cavity. Large Eddy Simulation is performed to solve the turbulent flow fields for a given wall temperature field corresponding to a Rayleigh number of 8.5 x 109. The outcomes of the model are analysed in terms of statistical mean. Results show complex large scale flow patterns with regions of high turbulent intensity. The Q-criterion is used to identify turbulent structures for an instantaneous flow field. Then we analyse the spatial distribution of the conductive heat flux at the walls to locate the regions with intense convection. We show that the conductive flux smaller than the wall-to-wall radiative flux in the major part of the cavity, and close to its value at some spots.

Large‐scale turbulent mixing at a mesoscale confluence assessed through drone imagery and eddy‐resolved modelling

AbstractConfluences are sites of intense turbulent mixing in fluvial systems. The large‐scale turbulent structures largely responsible for this mixing have been proposed to fall into three main classes: vertically orientated (Kelvin–Helmholtz) vortices, secondary flow helical cells and smaller, strongly coherent streamwise‐orientated vortices. Little is known concerning the prevalence and causal mechanisms of each class, their interactions with one another and their respective contributions to mixing. Historically, mixing processes have largely been interpreted through statistical moments derived from sparse pointwise flow field and passive scalar transport measurements, causing the contribution of the instantaneous flow field to be largely overlooked. To overcome the limited spatiotemporal resolution of traditional methods, herein we analyse aerial video of large‐scale turbulent structures made visible by turbidity gradients present along the mixing interface of a mesoscale confluence and complement our findings with eddy‐resolved numerical modelling. The fast, shallow main channel (Mitis) separates over the crest of the scour hole's avalanche face prior to colliding with the slow, deep tributary (Neigette), resulting in a streamwise‐orientated separation cell in the lee of the avalanche face. Nascent large‐scale Kelvin–Helmholtz instabilities form along the collision zone and expand as the high‐momentum, separated near‐surface flow of the Mitis pushes into them. Simultaneously, the strong downwelling of the Mitis is accompanied by strong upwelling of the Neigette. The upwelling Neigette results in ∼50% of the Neigette's discharge crossing the mixing interface over the short collision zone. Helical cells were not observed at the confluence. However, the downwelling Mitis, upwelling Neigette and separation cell interact to generate considerable streamwise vorticity on the Mitis side of the mixing interface. This streamwise vorticity is strongly coupled to the large‐scale Kelvin–Helmholtz instabilities, which greatly enhances mixing. Comparably complex interactions between large‐scale Kelvin–Helmholtz instabilities and coherent streamwise vortices are expected at other typical asymmetric confluences exhibiting a pronounced scour hole.

Average-based mesh adaptation for hybrid RANS/LES simulation of complex flows

Generating meshes with the right resolution is crucial for hybrid RANS/LES simulations of high Reynolds number flow with complex physical phenomena and geometries. This makes automatic mesh generation through adaptive refinement an interesting option. However, since the behavior of these turbulence models depends on the local grid size, mesh changes in time as a result of adaptive refinement may affect the production and destruction of turbulence. Therefore, grid adaptation with refinement criteria based on time-averaged quantities is proposed here to produce meshes that do not evolve rapidly in time and that are suitable for capturing the unsteady flow at each instant. Two averaging approaches, averaging over the instantaneous flow field, and averaging over the refinement criterion, are tested to simulate a turbulent flow behind a backward-facing step with a Detached Eddy Simulation type turbulence model. Compared to grid adaption based on instantaneous solutions, the computational cost is reduced, the accuracy of the solutions improves and an adapted mesh which has a generally static topology based on the main flow features is obtained. The investigation of this refinement process for two realistic test cases, a ship and a hybrid delta wing, confirms the reliability of the average-based adaptation and shows that automatic meshing for hybrid RANS/LES simulations of complex flows is possible.