Averaged Flow Field Research Articles

Synthetic turbulence using artificial boundary layers

Turbulent vortices in fluid flows are crucial for a visually interesting appearance. Although there has been a significant amount of work on turbulence in graphics recently, these algorithms rely on the underlying simulation to resolve the flow around objects. We build upon work from classical fluid mechanics to design an algorithm that allows us to accurately precompute the turbulence being generated around an object immersed in a flow. This is made possible by modeling turbulence formation based on an averaged flow field, and relying on universal laws describing the flow near a wall. We precompute the confined vorticity in the boundary layer around an object, and simulate the boundary layer separation during a fluid simulation. Then, a turbulence model is used to identify areas where this separated layer will transition into actual turbulence. We sample these regions with vortex particles, and simulate the further dynamics of the vortices based on these particles. We will show how our method complements previous work on synthetic turbulence, and yields physically plausible results. In addition, we demonstrate that our method can efficiently compute turbulent flows around a variety of objects including cars, whisks, as well as boulders in a river flow. We can even apply our model to precomputed static flow fields, yielding turbulent dynamics without a costly simulation.

직접수치모사를 이용한 난류경계층 내의 거대난류구조 연구

Direct numerical simulation (DNS) of a turbulent boundary layer with moderate Reynolds number was performed to scrutinize streamwise-coherence of hairpin packet motions. The Reynolds number based on the momentum thickness (θ<SUB>in</SUB>) and free-stream velocity (U<SUB>∞</SUB>) was varied in the range Re<SUB>θ</SUB>=1410~2540 which was higher than the previous numerical simulations in the turbulent boundary layer. In order to include the groups of hairpin packets existing in the outer layer, large computational domain was used (more than 50δo, where δ<SUB>o</SUB> is the boundary layer thickness at the inlet in the streamwise domain). Characteristics of packet motions were investigated by using instantaneous flow fields, two-point correlation and conditional average flow fields in xy-plane. The present results showed that a train of hairpin packet motions was propagating coherently along the downstream and these structures induced the very large-scale motions in the turbulent boundary layer.

Axisymmetric Swirling Flow Simulation of the Draft Tube Vortex in Francis Turbines at Partial Discharge

The flow in the draft tube cone of Francis turbines operated at partial discharge is a complex hydrodynamic phenomenon where an incoming steady axisymmetric swirling flow evolves into a three-dimensional unsteady flow field with precessing helical vortex (also called vortex rope) and associated pressure fluctuations. The paper addresses the following fundamental question: is it possible to compute the circumferentially averaged flow field induced by the precessing vortex rope by using an axisymmetric turbulent swirling flow model? In other words, instead of averaging the measured or computed 3D velocity and pressure fields we would like to solve directly the circumferentially averaged governing equations. As a result, one could use a 2D axi-symmetric model instead of the full 3D flow simulation, with huge savings in both computing time and resources. In order to answer this question we first compute the axisymmetric turbulent swirling flow using available solvers by introducing a stagnant region model (SRM), essentially enforcing a unidirectional circumferentially averaged meridian flow as suggested by the experimental data. Numerical results obtained with both models are compared against measured axial and circumferential velocity profiles, as well as for the vortex rope location. Although the circumferentially averaged flow field cannot capture the unsteadiness of the 3D flow, it can be reliably used for further stability analysis, as well as for assessing and optimizing various techniques to stabilize the swirling flow. In particular, the methodology presented and validated in this paper is particularly useful in optimizing the blade design in order to reduce the stagnant region extent, thus mitigating the vortex rope and expending the operating range for Francis turbines.

Characteristics of coherent vortical structures in turbulent flows over progressive surface waves

Vortical structures in turbulence over progressive surface waves are studied using the data from direct numerical simulation of a stress-driven turbulent Couette flow above a waving surface. Instantaneous flow field and its evolution, vorticity statistics, and conditionally averaged flow field with various sampling methods are examined. Unique vortical structures are identified, which are found to be strongly dependent on the wave motion. For a slow wave (with a small value of wave age c/u∗=2; here c is the phase speed of the wave and u∗ is the friction velocity), the vortical structures are characterized by reversed horseshoe vortices and quasistreamwise vortices. The former is concentrated above the wave trough and is associated with sweep events there; the latter has high intensity over the windward face of the wave and is associated with ejection events. Relative to the waveform, the coherent vortical structures propagate in the downstream direction. Vortex turning and vortex stretching play an important role in the vortex transformation and evolution processes. For an intermediate wave (c/u∗=14) and a fast wave (c/u∗=25), the dominant vortical structure is bent quasistreamwise vortices, which are predominantly horizontal but have a distinctive downward bending in their upstream ends near the wave trough. The vortices are found to propagate in the upstream direction with respect to the waveform. The above-wave coherent vortices identified in this study are found to play an important role in the turbulent transport process.

The structure of average turbulent flow field

Abstract A mathematical formalism to reflect the turbulent hierarchic eddy structure in an average turbulence description is discussed. The formalism is founded on simultaneous application of turbulent flow field splitting procedures by the motion variability and by the motion order explained in the paper. It is shown that this formalism permits the outlining of different types of cascading processes in turbulent media within the turbulence average description.

Structures in turbulent boundary layers subjected to adverse pressure gradients

The effects of adverse pressure gradients on turbulent structures were investigated by carrying out direct numerical simulations of turbulent boundary layers subjected to adverse and zero pressure gradients. The equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution U∞ ~ xm. Two-point correlations of velocity fluctuations were used to show that the spanwise spacing between near-wall streaks is affected significantly by a strong adverse pressure gradient. Low-momentum regions are dominant in the middle of the boundary layer as well as in the log layer. Linear stochastic estimation was used to provide evidence for the presence of low-momentum regions and to determine their statistical properties. The mean width of such large-scale structures is closely associated with the size of the hairpin-like vortices in the outer layer. The conditionally averaged flow fields around events producing Reynolds stress show that hairpin-like vortices are the structures associated with the production of outer turbulence. The shapes of the vortices beyond the log layer were found to be similar when their length scales were normalized according to the boundary layer thickness. Estimates of the conditionally averaged velocity fields associated with the spanwise vortical motion were obtained by using linear stochastic estimation. These results confirm that the outer region of the adverse pressure gradient boundary layer is populated with streamwise-aligned vortex organizations, which are similar to those of the vortex packet model proposed by Adrian, Meinhart &amp; Tomkins (J. Fluid Mech., vol. 422, 2000, pp. 1–54). The adverse pressure gradient augments the inclination angles of the packets and the mean streamwise spacing of the vortex heads in the packets.

Vortex Phase-Jitter in Acoustically Excited Bluff Body Flames

This paper describes an experimental study of the effect of acoustic excitation on bluff body stabilized flames, specifically on the flow field characteristics. The Kelvin-Helmholtz (KH) instability of the shear layer is excited due to the incident acoustics. In turn, the KH instability imposes a convecting, harmonic excitation on the flame, which leads to spatially periodic flame wrinkling and heat-release oscillations. Understanding the factors influencing these heat release oscillations requires an understanding of the generation, convection, and dissipation of these vortical disturbances. Phase locked particle image velocimetry was carried out over a range of conditions to characterize the vortical dynamics. It was found that the vortex core location exhibits “phase jitter”, manifested as cycle-to-cycle variation in flame and vorticity field at the same excitation phase. Phase jitter is shown to be a function of separation point dynamics, downstream convection time, and amplitude of acoustic excitation. It leads to fairly significant differences between instantaneous and ensemble averaged flow fields and, in particular, the decay rate of the vorticity in the axial direction.

The Entropy Wave Generator (EWG): A reference case on entropy noise

The entropy noise mechanism was investigated both experimentally and numerically on a generic test case. The model experiment features electrical heating to generate non-isentropic perturbations in a spatially varying average flow field. This flow field is set by the geometrical boundary conditions of an axisymmetric convergent–divergent nozzle in an otherwise straight tube. The considered flow conditions range from low subsonic to a transonic chocked base flow. The general response of the system to an abrupt heating pulse is studied experimentally and numerically. Furthermore, for two specific cases a detailed investigation of the pressure response in the outlet section is provided. Comprehensive experimental data are provided for the validation of numerical methods with respect to entropy noise. The numerical investigations use a commercial available computational fluid dynamics (CFD) method with partially and non-reflective boundary conditions for unsteady compressible simulations on one hand and a high-order CAA method with a time-domain impedance model on the other hand. It is found that the determination of reflections from the downstream and/or upstream open ends of the test configuration are necessary for the correct prediction of the experiment. The results of both methods are analyzed for the presence of acoustic sources considering the source term of an acoustic analogy and the acoustic intensity, respectively. Strong sources are found in the convergent/divergent nozzle by both methods. These sources show a much larger source strength than the direct sources due to the unsteady heat input. A saturation of the peak pressure response with a increasing Mach number in the nozzle throat above 0.8 is attributed to a phase shift of the source contributions between nozzle and diffuser. The presented results enable a deep understanding of the entropy noise phenomenon especially due to the combination of experiments and two fairly different numerical approaches. However, even in spite of the simplified model case the investigated entropy noise mechanism still appears in a comprehensive complexity. Therefore, and because of the increasing relevance as an aero-engine noise source, further research on entropy noise, also under application of the presented reference cases, should be performed.

Hydrodynamics of linked sphere model swimmers

We describe in detail the hydrodynamics of a simple model of linked sphere swimmers. Wecalculate the asymptotic form of both the time averaged flow field generated by asingle swimmer and the interactions between swimmers in a dilute suspension,showing how each depends on the parameters describing the swimmer and itsswimming stroke. We emphasize the importance of time reversal symmetry indetermining the far field flow around a swimmer and show that the interactionsbetween swimmers are highly dependent on the relative phase of their swimmingstrokes.

Surface current patterns in front of the Venice Lagoon

Surface patterns of the low-frequency current in a 20 × 30 km region in front of the Venice Lagoon were analysed from a 13-month-long HF radar data set. Surface circulation was related to prevalent wind regimes in the area and to the tidal flow through the lagoon inlet. Three different categories of wind-forcing were defined: bora (NE wind), sirocco (SE wind), and finally the category containing all other wind directions and calms (winds lower than 3 m/sec). Mean flow and vorticity spatial distributions were discussed for different wind conditions. The coastal area about 5 km wide is characterized by a flow field with maximum vorticity. Outside the coastal boundary layer the interior flow is part of the Adriatic basin-wide cyclonic circulation. Two counter-rotating vortices of the dimension of about 4–5 km were evidenced in the average flow field in front of the lagoon inlet (Malamocco inlet) in all situations except for the bora. The vortex-pair is probably associated with the tidal flow through the inlet. The bora wind induces a strong southward coastal jet detached from the coast by about 5 km homogenizing the flow and eliminating the dipole. The average coastal flow pattern in calm wind conditions was also analysed as a function of the strong inflow/outflow (currents in the inlet > 0.7 m s −1) from the lagoon inlet. In both cases the vorticity pattern is similar, with the negative vorticity to the left of the inlet and positive to the right looking seaward.

S0504-4-6 突起物によるエッジ音の制御に関する研究 : 噴出口長辺方向の渦度分布について(噴流,後流および剥離流れの流動解析と応用(その4))

The purpose of this study is to suppress edge tone generated by jet-wedge interaction. A passive control was achieved by means of a pair of tabs attached to the center of the nozzle exit, and revealed that the edge tone is reduced by 5dB. Velocity and vorticity measurement was also conducted to clarify the suppression mechanism by use of sound wave excitation. The excitation was synchronized with the self-oscillating flow for selective sampling, so that hot-wire measurement directly provide the phase averaged flow fields. We investigated oscillating phase and vorticity fields. As a result, the difference of flow field between with tabs and without tabs becomes clear.

Large eddy simulation of a swirling transverse jet into a crossflow with investigation of scalar transport

The flow field of a turbulent jet emerging from a straight round pipe into a laminar crossflow is investigated by means of large eddy simulations. The concentration of a passive scalar, introduced with the jet, is calculated in order to quantify the mixing of the jet and the crossflow. In the jet, swirl is introduced by means of body forces and a range of jet swirl numbers from S=0 up to S=0.6 is studied. The impact of the jet swirl on the flow field, on the coherent structures, and on the mixing efficiency is investigated and quantified by means of various analyses. It is found that for all swirl numbers larger than zero a clear asymmetry appears in all quantities studied. Additional to the two hanging vortices at both sides of the jet a third vortex is introduced by the swirling pipe flow which interacts with the former. This feature is described in detail as it is not mentioned in the literature. For the strongest swirl investigated a recirculation zone near the jet exit is observed. Despite the asymmetry and even with a recirculation zone at the outlet, the counter-rotating vortex pair still exists in all cases in the downstream flow, where it entrains a large amount of crossflow fluid into the jet. The near field, however, is altered by the jet swirl in several respects. The jet more and more approaches the bottom wall with increasing swirl. As a result, the entrainment is gradually attenuated due to the larger blocking of the secondary flow by the wall. Increased swirl increases both the turbulent kinetic energy in the pipe and the vorticity of the average flow field near the jet exit, and thus stimulates the mixing in these regions. However, this stimulating effect is overwhelmed by the closer position of the jet trajectory to the wall of the channel with increasing swirl, which in turn reduces entrainment of fresh crossflow fluid into the jet. As a final result of these two competing effects, the overall mixing efficiency of a jet into a crossflow is merely unchanged with the addition of swirl. Various mixing indices, both spatial and temporal, are used for this analysis. Their respective advantages and disadvantages are discussed and detailed illustrations provide a sound understanding of their behavior.

突起物による発振噴流の制御と流れ特性

The purpose of this study is to suppress edge tone generated by jet-wedge interaction. A passive control was achieved by means of a pair of tabs attached to the center of the nozzle exit, and revealed that the edge tone is reduced by 5dB. Velocity measurement was also conducted to clarify the suppression mechanism by use of sound wave excitation. The excitation was synchronized with the self-oscillating flow for selective sampling, so that hot-wire measurement directly provide the phase averaged flow fields. It was revealed that the velocity is weakened at the position corresponding to tabs, and the spanwise uniformity of the jet is broken. In addition periodic fluctuation of vorticity is reduced by tabs. These phenomena weaken feedback loop, as a result the edge tone was suppressed.

PIV measurements of flows around the wind turbines with a flanged-diffuser shroud

The wind turbines with a flanged-diffuser shroud-so called “wind lens turbine”-are developed as one of high performance wind turbines by Ohya et al. In order to investigate the flow characteristics and flow acceleration, the paper presents the flow velocity measurements of a long-type and a compact-type wind turbines with a flanged-diffuser shroud by particle image velocimetry. In the case of the long type wind turbine, the velocity vectors of the inner flow field of the diffuser for turbine blades rotating and no blades rotating are presented at Reynolds number, 0.9×105. Furthermore the flow fields between with and without rotating are compared. Through the PIV measurement results, one can realize that the turbine blades rotating affects as suppress the disturbance and the flow separation near the inner wall of the diffuser. The time average velocity vectors are made on the average of the instantaneous velocity data. There are two large vortices in downstream region of the diffuser. One vortex behind the flange acts as suck in wind to the diffuser and raise the inlet flow velocity. Another large vortex appears in downstream. It might be act as blockage vortex of main flow. The large blockage vortex is not clear in the instantaneous velocity vectors, however it exists clearly in the time average flow field. The flow field around the wind turbine with a compact-type flanged-diffuser shroud is also investigated. The flow pattern behind the flange of the compact-type turbine is the same as the long-type one. It means that the effect of flow acceleration is caused by the unsteady vortices behind the flange. The comparison with CFD and PIV results of meridional time-average streamlines after the compact-type diffuser is also presented.

Coherent structure eduction from PIV data of an electromagnetically forced separated flow

Abstract Periodic addition of momentum by wall parallel electromagnetic forces has a strong influence on the separated region of a stalled airfoil. As in the case of periodic blowing and suction, actuation frequency and momentum input are the main factors of influence. However, the control authority depends as well on the actuation wave form. This latter aspect is investigated in the present paper by means of time resolved particle image velocimetry data from the suction side of an inclined flat plate. The control effect is rated by the size of the remaining recirculation region in the time averaged flow fields. Typically, the controlled flow possesses a small number of relatively large vortices, which are assumed to be related to the control mechanism. Consequently, the time resolved flow fields have been analysed by proper orthogonal decomposition and continuous wavelet transform to extract dominant features of the flow.

Experimental study of unsteady flame structures of an oscillating swirl flame in a gas turbine model combustor

Velocity fields and flame structures of a partially premixed swirl flame in a gas turbine model combustor are measured in axial and transverse sections using simultaneous (Stereo-)PIV and OH-PLIF. The flame, operated under atmospheric pressure with air and methane at a thermal power of 10.3 kW and a global equivalence ratio of ϕ = 0.75, features thermoacoustic oscillations at a frequency f ≈ 295 Hz. The averaged flow field with inner and outer recirculation zones is typical of swirl-stabilized flames, and the instantaneous measurements show the presence of a helical vortex (PVC) located in the inner shear layer. The PVC, which rotates with a different frequency than the thermoacoustic oscillation, leads to an enhanced mixing of burned and unburned gas and thus to stabilization of the flame. Two distinct large-scale structures of velocity and OH are found in the transverse cross-sections. The first type is characterized by a roughly annular region of inflowing unburned gas and distinct inner and outer recirculation zones. In the second type, the region of positive axial velocity forms a spiral, and the recirculation zone consists of an inner region that is connected to the outer parts by a narrow curved zone along the spiral arm. Whereas the first type is a typical scenario of vortex breakdown with a PVC, the transient spiral recirculation zone observed here has, to our knowledge, not yet been reported. A phase-resolved analysis shows that the annular form correlates with low, and the spiral form with high rates of global heat release.

Effects of background noise on generating coherent packets of hairpin vortices

We examine the autogeneration process by which new hairpin vortices are created from a sufficiently strong hairpin vortex, leading to the formation of a hairpin packet. Emphasis is placed on the effects of background noise on packet formation. The initial conditions are given by conditionally averaged flow fields associated with the second quadrant (Q2) event in the fully turbulent channel flow direct numerical simulation (DNS) database at Reτ=395. The nonlinear evolution of the initial vortical structure is tracked by performing a spectral simulation. Background noise is introduced by adding small amplitude perturbations to the initial field or by imposing momentum forcing. The background noise gives rise to chaotic development of a hairpin packet. The hairpins become asymmetric, leading to much more complicated packet structures than are observed in the symmetric hairpin vortex train of the flow with a clean background. However, the chaotic packets show the same properties as the clean packet in terms of the rate of growth of vertical and spanwise dimensions and the distance between successive vortices, suggesting that the autogeneration mechanism is robust. The background noise leads to a decrease in the minimum value of the Q2 strength required to trigger autogeneration, indicating that background noise enhances autogeneration, especially in the buffer layer. The autogeneration process is more enhanced by the background noise with wavenumbers kx&amp;lt;kz. Conditionally averaged flow fields around the tall attached vortices in the hairpin packet show that they are associated with elongated low-momentum structures in the streamwise direction. Finally, the autogeneration process was tested in a real turbulent environment taken from an instantaneous field of a turbulent channel flow DNS. The generation of secondary hairpin vortices is clearly observed upstream of the primary hairpin.

Analysis of liquid circulation in a rectangular tank with a gas source at a corner

Abstract In this work, we investigate experimentally and numerically the hydrodynamics induced by a bubble plume introduced at a corner of a rectangular tank. Such gas–liquid flows are inherently unsteady. Particle Image Velocimetry (PIV) was used to experimentally determine transient velocity fields in the system. For this gas–liquid flow system, both the fluctuating and mean liquid velocities were determined experimentally by PIV. This technique enables us to determine velocity fields in a 2D plane. The behavior of the system was simulated in FLUENT 6.2 using a two fluid Euler–Euler model with a constant bubble size of 3 mm. Water is treated as the continuous phase and the gas bubbles are treated as the dispersed phase. The motion of the bubbles renders the flow turbulence and this effect is captured by the mixture k–ɛ turbulence model. Two and three-dimensional simulations were carried out to predict the flow behavior. The predictions of the time averaged flow field, turbulent intensity etc. are compared with experimental observations. We also calculate the magnitude of the turbulent viscosity from our model. For the case of corner injection of bubbles, we conclude that the velocity at a point does not show sustained periodic oscillations in time.

Development of a high-speed UV particle image velocimetry technique and application for measurements in internal combustion engines

A flexible, high-frame rate particle image velocimetry technique that can be applied to operating internal combustion engines in highly luminous combustion situations was developed. Two high-repetition rate diode-pumped Nd:YAG lasers operated at 355 nm and a CMOS camera were used to devise a system that allowed measurements of velocity fields near the spark plug in a firing engine at a rate of 6 kHz for 500 consecutive cycles. The 6 kHz acquisition rate enables recording one velocity field every other crank angle at 2,000 RPM engine speed. Sample results such as individual and average flow fields and kinetic energy evolutions are presented.

Experimental investigation on mixing enhancement mechanism of turbulent jet flow with tabbed nozzle

Flow visualization and hot-wire measurement techniques were combined to investigate the influence of the size and number of tabs on jet flow field and vortex structure generation mechanism. Streamwise vortices generated by the tabs of different sizes and numbers were observed from the flow visualization images. Combined with flow visualization, hot-wire measurement gave a quantitative insight of the effect of various tabbed jet flows. Instantaneous two-component velocity signals (longitudinal and transverse velocity components) at different cross sections along radius direction and streamwise direction with different tabbed jet nozzles were measured using hot-wire anemometer. Average flow field parameters of tabbed jet flow such as mean velocity, turbulence intensity, vorticity were analyzed and the effects of tabs with different sizes and numbers were compared with that of circular no-tab jet flow. It is revealed that the generation of a series of counter-rotating quasi-streamwise vortices, azimuthal vortices and double-row azi-muthal vortex are the reasons for mixing enhancement of tabbed turbulent jet flow.