Instantaneous Vortical Structures Research Articles

Effects of vortex generators on endwall film cooling in a turbine vane

Film cooling is widely applied on endwalls in high-performance gas turbines to guarantee the safety and endurance of the engines. Micro ramps are employed to optimize the flow and thermal performance in internal and external cooling designs. In the current investigation, flat-surface film cooling at the blowing ratio of 1.0 and the density ratio of 2.0 is investigated with and without the micro ramp by LES. The result shows that the micro ramp effectively enhances the cooling performance by creating the ACRVP, drawing the coolant toward the wall and enlarging the film coverage. Mean and instantaneous vortical structures are analyzed. Endwall film cooling with micro ramps is studied with high and low mass flow ratios (MFR). The cooling effectiveness of the micro ramp case is 89.9% higher than the baseline case when MFR is high, while 16.3% higher than the baseline when MFR is low. Micro ramps are more effective for endwall film cooling with high MFR. The cooling effectiveness of the region in the middle of the passage improves when applying the micro ramps. In the region upstream of the leading edge, cooling performance deteriorates due to the interaction of the horseshoe vortex and the micro ramps.

DNS investigation of flow and heat transfer characteristics of supercritical carbon dioxide over a backward-facing step

This study employed direct numerical simulation to investigate the turbulent flow and heat transfer characteristics of supercritical carbon dioxide (SCO2) in a vertical backward-facing step (BFS). Three cases were considered: constant property flow, SCO2 forced convection, and SCO2 mixed convection. All cases were conducted with an inlet Reynolds number of 4805, and the expansion rate of BFS is set at 1.5. Mean statistics like velocity, temperature, Reynolds stress, turbulent heat flux, and thermophysical properties are given and discussed. The results show that the turbulent and heat transfer characteristics of SCO2 over a backward-facing step are significantly different from those of conventional fluids due to the dramatic changes in its thermophysical properties near the pseudo-critical temperature. In the case of SCO2 forced convection, though the large thermophysical property variations do not have an obvious impact on the mean flow field, the near-wall mean density decreases largely and thus Reynolds stress and turbulent heat flux near the wall are reduced significantly, which in turn leads to a substantial reduction in the local skin friction coefficient and Nusselt number. In the SCO2 mixed convection, buoyancy substantially distorts the flow, resulting in a significant reduction in the size of the recirculation zone. Simultaneously, buoyancy acts as an additional driving force leading to the formation of a wall jet. These combined effects induce intensified turbulence near the heated wall, which enhances the turbulent heat flux and facilitates efficient heat transfer. The instantaneous vortical structures based on Q criterion also clearly show the intensified turbulence in the near-wall region.

Large eddy simulation of compound angle film cooling with vortex generators

Film cooling is a commonly used approach in high-performance gas turbine engines to insulate the components from the hot mainstream flow. Compound angle injection with orientation oblique to the freestream laterally are employed to achieve wide coolant coverage. The passive control devices of delta winglets are effective in improving thermal performance. In the current study of film cooling by LES, the compound angles are investigated with 35° vertical inclined angle to the cooling wall and 0 deg, 30 deg, and 60 deg lateral inclined angles to the streamwise crossflow. Vortex generators of delta winglets pair are installed upstream of the cooling hole. Adiabatic film cooling effectiveness is examined at the blowing ratio of 1.0 and density ratio of 2.0. The cooling performance improves significantly when the compound angled injection and delta winglets are applied. Mean and instantaneous vortical structures in the near-hole field are presented. The counter-rotating vortex pair (CRVP) associated with streamwise injection changes into a strong symmetric vortex in compound angled cases. Anticounter-rotating vortex pair (anti-CRVP) evolved from delta winglets cancels the strength of CRVP in the streamwise coolant injection case, and the downwash effect of anti-CRVP is weakened with lateral angle increasing.

Large eddy simulations and modal decomposition analysis of flow past a cylinder subject to flow-induced vibration

Large eddy simulations (LES) are carried out to investigate the flow around a vibrating cylinder in the subcritical Reynolds number regime at Re = 3900. Three reduced velocities, Ur = 3, 5, and 7, are chosen to investigate the wake structures in different branches of a vortex-induced vibration (VIV) lock-in. The instantaneous vortical structures are identified to show different coherent flow structures in the wake behind the vibrating cylinder for various branches of VIV lock-in. The combined effects of the frequency and amplitude of the oscillation on the flow pattern in the wake region, the hydrodynamic quantities of the cylinder, and the spanwise length scale of the energetic wake flow structures are discussed in detail. It is found that the typical spanwise lengths of the flow structures are 0.22D at Ur = 5 and 0.3D at Ur=[3,7] in the near-wake region and level out at 0.5D further downstream. Furthermore, multiscale proper orthogonal decomposition (mPOD) is used to analyze the dominant flow features in the wake region. With the increasing Ur, the total kinetic energy contribution of superharmonic modes increases and the contribution of subharmonic modes decreases. The dominant flow characteristics associated with the vortex shedding and their super harmonics, and the low-frequency modulation of the wake flow can be captured by the mPOD modes.

On the three-dimensional wake flow behind a normal flat plate

Two-dimensional (2D) and three-dimensional (3D) numerical simulations are carried out for uniform flow past a normal flat plate with two semicircular ends to investigate the three-dimensional effects of the wake flow behind the plate. The Reynolds numbers under investigation based on the inflow velocity and the width of the plate (H) are Re= 125, 200, 250, 500, and 1000. The instantaneous vortical structures show that the three-dimensionality characterized by streamwise vortical ribs becomes obvious at Re=200. Their spatial densities increase with the increasing Re. The time-averaged drag coefficient remains almost the same from Re=500 to 1000. The relative difference of the predicted drag coefficients between 2D and 3D simulations can be up to 77%. The amplitude of the lift force obtained using 2D simulations can be up to 30 times that obtained using 3D simulations. A drag force element decomposition analysis is conducted for the results obtained using 2D and 3D simulations. It is found that although the time-averaged wake flows obtained using the 2D and 3D simulations are different, the relative difference of their contributions to the time-averaged drag coefficient is only 10% while the difference comes mainly from the contribution of the Reynolds shear stress. The three-dimensionality of the wake flow is further investigated using quadrant and the Lumley triangle analysis of the Reynolds stress. The spanwise spatial scales of the streamwise vortices are quantified using Hilbert transform along the spanwise direction. The width of the streamwise vortices is approximately 0.5–1H in the near wake region. The dependence of the spatial scales on Re is discussed.

COMPARISON OF SUB-GRID-SCALE MODELS BASED ON PREDICTION OF COHERENT STRUCTURES IN PLANE TURBULENT JETS

A compressible plane jet at Mach 0.9 is numerically studied by large-eddy simulation. The governing equations are discretized by fourth-order spatial and third-order temporal schemes. Five sub-grid-scale (SGS) models, namely, the standard Smagorinsky model (SM), the coherent structure kinetic model (CKM) and the selective mixed-scale model (SMSM), the localized dynamic Smagorinsky model (LDSM), and the coherent-structure Smagorinsky model (CSM), are employed for closure of the sub-grid scale (SGS) terms, respectively, and compared. Proper orthogonal decomposition (POD) method is applied to extract the leading modes of the fluctuating velocity components, i.e., \begin{document}$u'$\end{document} in the streamwise, \begin{document}$v'$\end{document} in the lateral and \begin{document}$w'$\end{document} in the spanwise. The averaged flow fields, dissipation, the instantaneous vortical structures and the coherent structures represented by the leading POD modes, are compared. The leading POD modes of \begin{document}$u'$\end{document} are two longitudinal stripes with fracted contours in the turbulent region, representing the multi-scaled flow and the decay of fluctuation strength. The leading modes of \begin{document}$v'$\end{document} are a row of ribs with the lateral size growing along the streamwise, while the modes of \begin{document}$\left( {u', v'} \right)$\end{document} are in a circular flow pattern around the ends of the ribs. The circular pattern penetrates both the jet flow and the peripheral flow, representing the flow entrainment. The leading modes of \begin{document}$w'$\end{document} are a train of ridges along streamwise. Their positive and negative values represent the spanwise stretching pattern of the coherent structures. In comparison of these five SGS models, the instantaneous multi-scale vortical flow is not effectively predicted by the SM and the CKM, as the small vortical scale is smeared. The POD modes are found to be sensitive to the sub-grid dissipation, since the valley regions of the mode of \begin{document}$u'$\end{document} coincide with the peak dissipation regions predicted by the CKM. The circular flow pattern is also not clearly predicted by the CKM. Neither the ridge pattern of the mode of \begin{document}$w'$\end{document} is predicted by the SM. On the other hand, the multi-scales turbulent flow and the flow patterns of POD modes are well predicted by the CSM, the SMSM, and the LDSM, in which the CSM is computationally more efficient.

Characterization of vortical structures in T-shaped branches depending on shear-thinning

The characteristics of vortical structures in T-shaped branches with respect to the shear-thinning effect are numerically investigated using a power-law fluid model. By varying the power-law index n, we observe three different flow structures, namely, steady-, harmonic-, and turbulent-like regimes. The time-averaged and instantaneous vortical structures are examined for different values of the local Reynolds number. In the steady regime, stationary vortical structures form near the corners of the T-shaped branch. As n decreases, the vortical structures oscillate back and forth, giving rise to the harmonic regime. Decreasing n further, we observe the turbulent-like regime. In this regime, the vortical structures are torn off near the tips of the vortices and small-scale structures are vigorously generated, constituting more violent behavior than in the harmonic regime. If the local Reynolds number near the wall and near the cores of the vortical structures reaches a critical value, the flow structure becomes turbulent-like after the bifurcation of the T-shaped branch. In addition, the modal characteristics of the vortical structures are analyzed using dynamic mode decomposition with respect to the degree of shear-thinning. As shear-thinning appears in the flow, various high-frequency modes with small-scale vortical structures are observed, and their energies are evenly distributed. This supports the present observation of the vortical structures depending on shear-thinning and -thickening.

Flow and thermal characteristics of three-dimensional turbulent wall jet

In the present work, a three-dimensional turbulent wall jet is simulated using large-eddy simulation to characterize its flow and thermal characteristics. The solver is first validated for streamwise velocity decay, wall-normal and spanwise spread rates, and mean and second-order flow statistics using reference experimental data from the literature. The mean vorticity transport equation for the streamwise component is analyzed to identify the dominant terms that contribute to the large spanwise spread of the jet. The terms that contain Reynolds normal stresses are identified as major contributors to a large mean streamwise component of vorticity. The mean streamwise and wall-normal components of vorticity are studied for their evolution and contribution to the spanwise spread. It was found that both these components together aid in the large spanwise spread of the jet. The heat transfer characteristics are studied for the jet flow on a heated isothermal wall. The profiles of mean and fluctuating temperatures, the evolution of the Nusselt number, and turbulent heat flux characteristics are studied. The streamwise evolution of Nusselt number behavior is explained using instantaneous vortical structures. A significant drop in heat transfer is observed in the potential core region. Further, the turbulent heat flux contours show that the transport of heat in the streamwise direction is different from that of the plane wall jet. A peculiar turbulent heat transport was found in the analysis of the spanwise heat flux. The heat transfer characteristics noted for the three-dimensional wall jet may help in the design and analysis of film-cooling applications.

Flow developing properties of a compressible parallel jet

Two-point space–time correlation and Fourier spectrum analysis are applied to large-eddy simulation results of a compressible parallel jet flow at Reynolds number 2000 and Mach number 0.9. Flow properties including instantaneous vortical structures, flow intermittency, convection velocity of vortical structures, and pressure signals in the near flow-field are presented and discussed. The results show multi-scale features of vortical structures after flow transition. High intermittency is found to occur around the outer edges of the shear layer and the region surrounding the potential core, where the motion of vortices is vigorous. In order to analyze the motion of vortices, the convection velocity of vortical structures in the center plane of the jet is calculated by a designed solution procedure. The high convection velocity zone is shown to be a “V” shaped cap covering the potential core. The pressure fluctuation signals in the vortical flow are analyzed so as to study the effects of vortex motion. The signals show the feature of wave-packets and contain multiple dominant frequencies. The distribution of the dominant frequencies is characterized by a terrace shaped zone with stairs descending in both the streamwise and lateral directions. In addition, the phase velocity of the pressure signals shown by the frequency–wavenumber diagram unveils that the pressure fluctuations travel at a stable phase velocity along the lip-line of the jet, while the component with a supersonic phase velocity is salient in the downstream of the flow transition region outside the shear layer.

On the wake flow behind a sphere in a pipe flow at low Reynolds numbers

Numerical simulations are carried out to investigate the flow around a stationary sphere in a pipe. Seven sphere diameters d = 0.1D–0.9D (D is the diameter of the pipe) are chosen to investigate the effects of the blockage ratio on the flow characteristics. Three series of simulations are conducted. The first series of simulations is based on a fixed pipe flow Reynolds number Rep = 1250 (based on the inlet mean velocity and D) and the sphere Reynolds number Res (based on the sphere cross-sectional mean velocity and d) that is varying in the range of 249 ≤ Res ≤ 1360. The second series of simulations is based on a fixed Res = 500 and Rep varying between 460 ≤ Rep ≤ 678. The third series of simulations is based on a fixed Reg = 500 (based on the mean velocity of flow through the gap between the sphere and the pipe wall and d) and Rep varying between 113 ≤ Rep ≤ 773. The instantaneous vortical structures are presented to show different flow patterns behind the spheres with different d. For the sphere with the small diameter (d ≤ 0.5D), the vortex shedding in the wake flow behind the sphere is similar to that with the sphere subjected to a uniform flow. However, for the sphere with a larger diameter (d ≥ 0.7D), the flow behind the sphere is different from the sphere subjected to a uniform flow. At Rep = 1250, the large-scale vortex shedding behind the sphere is suppressed for d ≥ 0.8D and strong small-scale vortical structures are formed behind the sphere. At Res = 500, different behaviors of wake flow are observed with the increasing d. It is found that the vortex shedding is stabilized for 0.4D ≤ d ≤ 0.7D due to the confinement of the pipe wall, while the wake vortices become chaotic for d ≥ 0.8D due to the interaction between the wake flow and the pipe wall boundary layer. The vortex shedding is suppressed for d ≥ 0.9D. At Reg = 500, the wake flow behind the sphere is stabilized with the increasing d. The combined effects of the blockage ratio and Reynolds number on the flow pattern in the wake region, the hydrodynamic quantities of the sphere, and the power spectra of the velocities at different detection points are discussed in detail. Furthermore, in addition to power spectral analysis, sparsity-promoted Dynamic Mode Decomposition (SPDMD) is used to analyze the dominant flow modes in the wake region for different blockage ratios. The dominant flow characters associated with the hairpin vortex shedding, the Kelvin–Helmholtz instability, and the low-frequency modulation of the wake flow can be captured by the DMD modes, and their spatial structures are revealed by the mode shapes.

Transitional Flow Dynamics Behind a Micro-Ramp

Micro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number. Results of our Direct Numerical Simulations (DNS) are compared with results of BiLocal stability analysis on the DNS base flow and independent tomographic Particle Image Velocimetry (tomo-PIV) experiments. We analyse relevant flow structures developing in the micro-ramp wake and assess their role in the micro-ramp functionality, i.e., in increasing the near-wall momentum. The main flow feature of the base flow is a pair of streamwise counter-rotating vortices induced by the micro-ramp, the so-called primary vortex pair. In the instantaneous transitional flow, the primary vortex pair breaks up into large-scale hairpin vortices, which arise due to linear varicose instability of the base flow, and unsteady secondary vortices develop. Instantaneous vortical structures obtained by DNS and experiments are in good agreement. Matching linear disturbance growth rates from DNS and linear stability analysis are obtained until eight micro-ramp heights downstream of the micro-ramp. For the setup considered in this article, we show that the working principle of the micro-ramp is different from that of classical vortex generators; we find that transitional perturbations are more efficient in increasing the near-wall momentum in the mean flow than the laminar primary vortices in the base flow.

Analysis of heat transfer of supercritical water by direct numerical simulation of heated upward pipe flows

Direct numerical simulation (DNS) was performed to investigate the turbulent heat transfer of supercritical water in an upward pipe with and without buoyancy. The heat transfer deterioration mechanisms were comprehensively studied by analyzing the effects of large thermophysical property variations, flow acceleration and buoyancy. It is shown that the large property variations near the wall severely impair the heat transfer in supercritical water especially for forced convection. The flow acceleration induced by thermal expansion shows an obvious suppression on turbulence but does not affect the heat transfer significantly due to the quick developing of thermal boundary layer. A small buoyancy first impairs the heat transfer and then improves the heat transfer due to the turbulence attenuation and recovery. With buoyancy increasing, the turbulence recovery gets earlier and stronger, and the wall temperature is significantly reduced due to the large buoyancy production of turbulent kinetic energy (TKE) in supercritical water flows. The instantaneous vortical structures based on Q criterion also clearly show the process of turbulence attenuation and recovery. In addition, the simulation using two eddy-viscosity turbulent models is compared to DNS data. The results show that the failed prediction of wall temperature and Nusselt number by those turbulent models is possibly due to the bad results of TKE, production of TKE and turbulent Prandtl number (Prt).

Numerical investigation of nonlinear wave passing through finite circular array of slender cylinders

ABSTRACTCylinders extend from the plat bottom of an open channel piercing the water free surface to mimic a patch of emergent vegetation and are often used as models in numerical simulations or experiments related to flow in the presence of vegetation. This work focuses on investigating the flow field structures induced by nonlinear waves passing through a cylinder patch with a diameter of DG and different solid volume fractions . A 3D non-hydrostatic numerical model is used to simulate the nonlinear wave passing through the emergent circular array of slender cylinders. The instantaneous vortical structures within the cylinder patch are clearly predicted by the numerical model. The residual flow reveals abundant flow characteristics of a long term-averaging wave motion. The 3D numerical model effectively presents the spatial and temporal wave process evolution, and contributes to highlighting vegetation flows under nonlinear waves.

Stereoscopic PIV measurements of near-wall flow in a tightly packed rod bundle with wire spacers

In this study, we experimentally investigated the flow field characteristics in the complex geometry of a tightly packed 61-rod bundle with wire spacers by employing matched-index-of-refraction (MIR) and stereoscopic particle image velocimetry (SPIV) techniques. SPIV measurements were performed on a cross-flow plane at Reynolds numbers of 500, 2500, and 6300, and the measurement area covered the interior and edge sub-channels that were located near the enclosure wall. The SPIV results showed that peaks of the velocity magnitude occurred in the edge sub-channel, indicating the wall effects on the flow symmetry in the cross-flow plane. Mean velocities at the central areas of the edge and interior sub-channels showed flat profiles for Re=500 and local peaks around 0.15<y/Drod<0.3, due to the wire’s presence and near-wall location of the edge sub-channel. The rms fluctuating profiles velocities showed that local maximal peaks linearly shifted from y/Drod=-0.44 to −0.36 and −0.28 as Re increased. Taylor’s hypothesis was applied to the SPIV velocity vector fields for reconstructing quasi-instantaneous three-dimensional (3D) vortical structures in the wire-wrapped rod bundle. The visualization of 3D vorticity iso-surfaces showed that vortical structures were generated from shear layers of neighboring rods and traveled in the streamwise direction. Finally, proper orthogonal decomposition (POD) analysis was applied to SPIV velocity snapshots taken at the edge and interior sub-channels to extract the most statistically dominant flow structures. The POD velocity decomposition revealed coherent structures with sizes and shapes comparable to instantaneous vortical structures identified by iso-surfaces. Energy fractions of the first POD modes were found to reduce from 46% and 31% to 26% and 18% when Reynolds numbers increased from Re=500 to 6300, respectively. On the other hand, kinetic energy levels of low-order POD modes (modes 2, 3, 4, etc.) increased when the Reynolds number increased.

Large-eddy simulations of flow normal to a circular disk at Re=1.5×105

Abstract Numerical simulations of the flow normal to a circular disk have been carried out using the large-eddy simulation (LES) method with Smagorinsky subgrid scale model. The Reynolds number ( Re ) based on the free stream velocity and the diameter of the disk is 1.5 × 10 5 . The thickness to diameter ratio of the disk is 0.02. The instantaneous vortical structures in wake of the disk are revealed. Toroidal vortices are formed along the disk edges due to Kelvin–Helmholtz instability. The toroidal vortices break up as the flow goes downstream and then hairpin-like vortices are formed. Worm-like vortices parallel to the axis of the disk are observed in the center region of the medium wake. Two distinct vortex spreading modes in the far wake are observed: one has a main shedding plane; the other has not. Three dominant frequencies at the Strouhal number S t 1 = 0.01 , S t n = 0.148 (corresponding to the natural frequency) and St 3 ≈ 0.8–1.35, characterizing that the wake instability mechanisms are found through frequency analysis. The time and azimuth averaged statistics, e.g., streamlines, pressure, vorticity and profiles of velocity and kinetic energy, are also presented and discussed.

Numerical and experimental investigation on droplet dynamics and dispersion of a jet engine injector

In the case of turbine combustors operating with liquid fuel the combustion process is governed by the liquid fuel atomization and its dispersion in the combustion chamber. By highly unsteady flow field conditions the transient interaction between the liquid and the gaseous phase is of interest, because it results in a temporal variation of air–fuel ratio which leads to a fluctuating temperature distribution. The objective of this research was the investigation of transient flow field phenomena (e.g. large coherent structures) on droplet dynamics and dispersion of an isothermal flow (of inert water droplets) as a necessary first step towards a full analysis of spray combustion in real-life devices. The advanced injector system for lean jet engine combustors PERM (Partial Evaporated Rapid Mixing) was applied, generating a dilute polydispersed spray in a swirled flow field. Experiments were performed using Phase Doppler Anemometry (PDA) and a patternator to determine the droplet polydispersity, concentration maps, and velocity profiles in the flow. An important finding is the effect of large-scale coherent structures due mainly to the precessing of the vortex core (PVC) of the swirling air jet on the particle dispersion patterns. The experimental results then serve as reference data to assess the accuracy of the Eulerian–Lagrangian computations using a Large Eddy Simulation (LES), a Unsteady Reynolds-Average Navier–Stokes Simulation (URANS) and two simplified (steady-state) simulations. There, a simplified droplet injection model was used and the required boundary conditions of injected droplet sizes were obtained from measurements. Important transient effects of deterministic droplet separation observed during experiments, could be perfectly replicated with this injection model. It is convincingly shown, through extensive computations, that the resolution of instantaneous vortical structures is indeed crucial; hence the LES, or a reasonably-well resolved URANS are preferred over the steady-state solutions with additional, stochastic-type, turbulent dispersion models.

Coherence and Reynolds stresses in the turbulent wake behind a curved circular cylinder

The turbulent wake behind a curved circular cylinder is investigated based on data obtained from a direct numerical simulation. Here, emphasis is placed in the assessment of two approaches for simplified modelling: reduced-order modelling (ROM) and Reynolds-averaged Navier–Stokes equations. To this end, the instantaneous vortical structures, the proper orthogonal decomposition (POD) of the flow, and relevant Reynolds stress components have been analysed. The results show that despite the complexity of the instantaneous vortical structures, the wake dynamics are governed by the quasi-periodic shedding of primary vortices. Between 24% and 50% of the kinetic energy in the POD is captured by the two most energetic modes, and about 200 modes are needed to capture 90% of the kinetic energy. These findings suggest that, as long as the large-scale structures of the von Kármán vortex shedding are concerned, the present case can be approached by ROM; but a detailed representation of the flow dynamics without an eddy viscosity model that accounts for the unresolved turbulent fluctuations would require a large amount of degrees of freedom. Concerning the Reynolds stresses, their magnitude varies considerably depending on the depth at which they have been sampled. This dependence is related to the strength of the vortex shedding, and the intensity of the secondary flows induced by the curvature of the cylinder. As a consequence of the combination of these two effects, the correlation between streamwise and vertical velocity fluctuations is highest in the wake behind the midspan of the curved cylinder, and the correlation between cross-flow and vertical velocity fluctuations reaches large values in the lower wake.

Experimental evidence of deterministic turbulence

This paper concentrates on a brief illustration of some recent experimental results devoted to substantiate the hypothesis on deterministic turbulence described by Kachanov (2013) [1] and to apply the deterministic turbulence method to turbulence control experiments. The idea of the deterministic turbulence has been suggested about a decade ago. In contrast to the usual (random) turbulence, the deterministic turbulent flows have reproducible instantaneous structure, representing one particular realization from infinite number of possible ones. It is shown experimentally that the deterministic turbulence does exist in nature. The behavior of such flows only appears chaotic, due to its great complexity, but it can be deterministic, i.e. reproducible, in the main, from one realization to another. The application of the deterministic turbulence method to investigate one of previously known methods of turbulent-flow control (the so-called LEBU-device method) shows the great potential of the former. This approach allows the observation of distortions of instantaneous vortical structures of the turbulent boundary layer produced by the presence of the LEBU-devices. No other experimental approach is able to give this information.

S054023 DNSによる多重衝突噴流の熱流動特性解析([S054-02]噴流、後流およびはく離流れ現象の探求と技術革新(2))

In order to improve the performance of heat transfer with multiple impinging jet(MIJ), we investigate the DNS (direct numerical simulation) of four round impinging jet arranged at an inflow of flow field. As a control parameters, a separation between each jet is varied. From view of instantaneous vortical structures and time-averaged velocity distribution, it reveals that the generation of vortical structures are enhanced due to an interaction between each jet, compared to that of a single impinging jet, and that various type of upward flow which does not exist in the single one, appears according to the control parameter. In addition the heat transfer performance of MI J is numerically turn out.

Dynamics in the turbulent wake of a curved circular cylinder

Understanding the physics of turbulent wakes is an essential, yet complex task in the study of turbulent flows. In the present paper we investigate the flow past a curved body of circular cross-section. The inflow velocity is aligned with the curvature of the cylinder and directed towards its convex face. We conduct direct numerical simulations at a Reynolds number of 3900 in order to obtain a fully turbulent wake. The instantaneous vortical structures reveal that the primary vortices are roughly aligned with the curved axis. Despite the presence of isolated splitting events in the frequency of the vortex shedding, there is one single shedding frequency that dominates this process. Velocity time-traces confirm that the shear layers exhibit intermittency, which is manifested as large amplitude fluctuations. The intensity of these instabilities is increased by the secondary flow along the recirculation region, thereby influencing the dynamics of the near wake. Several spots of zero mean velocity reside next to the baseline within the recirculation region, displacing the back-flow region further downstream. It is suggested that this displacement is induced by the secondary flow, in combination with the symmetry boundary condition imposed at the top plane.