Instantaneous Vortex Structures Research Articles

Experimental study on hemodynamics of an end-to-side anastomosis

A three-dimensional and three-component velocity measurement on the flow field in a 45° end-to-side anastomosis model is conducted to investigate the hemodynamics, which is an important factor to the intimal hyperplasia formation and graft failure after surgery. Thanks to the advanced volumetric measurement technology of tomographic particle image velocimetry, the recirculation zone, low-speed region, and the spiral flow structures can be visualized. As a result, the flow field of three cases with the local maximum velocity of 0.15, 0.8, and 1.4 m/s are visible and the inlet velocity profile tends to be skewed as the flow rate increases. The mean vorticity contours indicate that the positive vortex center rotates 6.47°, 50.23°, and 90.4° and the negative vortex center rotates 20.44°, 15.73°, and 68.47°, respectively, in three cases. The instantaneous vortex structures identified by the λci criterion demonstrate two large-scale vortex structures in the distal section. The two vortices have the tendency to intertwine while one of them decays earlier. The wall shear stress (WSS) distributions on the entire model with the local maximum of 0.8, 5.8, and 13.8 Pa in three cases have been quantitatively achieved. The abnormal WSS and WSS gradient can help localize risk areas and understand the intimal hyperplasia formation. A detailed illustration of hemodynamics inside the 45° end-to-side anastomosis model has been provided, which demonstrates more comprehensive large-scale flow structures and abnormal WSS regions. Combined with the information of flow structures and WSS distribution, the understanding of the hemodynamics in the anastomosis can be strengthened.

Direct numerical simulation and mode analysis of turbulent transition flow in a compressor blade channel

The separation and turbulent transition of the flow in a compressor blade channel are investigated through direct numerical simulations (DNS) at a Reynolds number of 1.367 × 105. Based on the original DNS data, both time-averaged statistics and instantaneous vortex structures of the flow field are extensively analyzed. The vortices are visualized and studied by the Liutex method, and the streaming dynamic mode decomposition (SDMD), a low-storage variant of conventional DMD, is applied to the large datasets obtained on both pressure and suction sides. The physical quantity analyzed with SDMD is the Liutex magnitude R. The DNS results indicate that flow separation occurs on both sides of the blade. On the pressure surface, the separation is weak and the flow remains in a natural transition dominated by viscous Tollmien–Schlichting instabilities. In contrast, owing to the presence of a large laminar separation bubble, the flow experiences a separation transition governed by inviscid Kelvin–Helmholtz instabilities on the suction surface. The SDMD results suggest that a broad range of vortex frequencies exist in the transition flow, and the scale of the spatial structures is negatively correlated with the frequency of the mode. On the pressure surface, the extracted SDMD modes are primarily related to Kelvin–Helmholtz rolls, whereas on the suction side, influenced by the separated boundary layer, the modal structures exhibit greater diversity.

Effects of vortex generators and pulsation on compound angle film cooling

Compound angle film cooling is widely applied in modern turbine engines to provide reliable protection for the components. As passive and active methods, vortex generators enhance the thermal performance significantly, and pulsation coolant jet increases film cooling effectiveness when the compound angle is low. In the current study, the effects of vortex generators, pulsation, and their interactions have been investigated in film cooling with compound angles of 0 deg, 30 deg, and 60 deg. Cooling efficiency distribution is analyzed considering the coolant amount and separation condition. The downwash anticounter-rotating vortex pair (anti-CRVP) generated by vortex generators effectively increases film cooling effectiveness (FCE) in compound angle film cooling. Pulsation enhances the cooling performance by reducing the coolant separation when the lateral angle to the freestream is 0 deg; however, the performance is poor with 30 deg and 60 deg. Vortex generators are less efficient in enlarging the film coverage when pulsed jets are employed due to less coolant close to the surface. Moreover, mean and instantaneous vortex structures are illustrated to show the evolvements and interactions of the anti-CRVP, the counter-rotating vortex pair, the separated wakes, and the asymmetric vortex.

On the distributed blowing control of flow around a square cylinder at a low Reynolds number

This paper numerically investigates the distributed blowing control of the flow around a square cylinder at a low Reynolds number of Re=200. The air holes are placed at the windward or leeward stagnation points. The control effect of the unsteady aerodynamic forces is notably different with different spanwise distributions of air holes, which is represented by the dimensionless air hole area ratio G. The simulation results indicate that the windward blowing control (WBC) shows better control effect than the leeward blowing control (LBC) in reducing the time-averaged drag coefficient Cd¯ of the square cylinder while the fluctuating lift coefficient Cl′ can be significantly suppressed by the LBC. Besides, the surface pressure distribution, the turbulence kinetic energy (TKE) and Reynolds shear stress (RSS) distributions and the shedding vortices in the wake are analysed and compared in detail under different control conditions. Finally, through the instantaneous 3D vortex structures and the spanwise fluctuating velocity w′ distribution in the flow field, the suppression of the unsteady aerodynamic forces is mainly related to the 3D vortex pairs induced by the blowing jet.

Adjoint-based variational optimal mixed models for large-eddy simulation of turbulence

An adjoint-based variational optimal mixed model (VOMM) is proposed for subgrid-scale (SGS) closure in large-eddy simulation (LES) of turbulence. The stabilized adjoint LES equations are formulated by introducing a minimal regularization to address the numerical instabilities of the long-term gradient evaluations in chaotic turbulent flows. The VOMM model parameters are optimized by minimizing the discrepancy of energy dissipation spectra between LES calculations and a priori knowledge of direct numerical simulation using the gradient-based optimization. The a posteriori performance of the VOMM model is comprehensively examined in LES of three turbulent flows, including the forced homogeneous isotropic turbulence, decaying homogenous isotropic turbulence, and temporally evolving turbulent mixing layer. The VOMM model outperforms the dynamic Smagorinsky model, dynamic mixed model (DMM), and approximate deconvolution model in predictions of various turbulence statistics, including the velocity spectrum, structure functions, statistics of velocity increments and vorticity, temporal evolutions of the turbulent kinetic energy, dissipation rate, momentum thickness and Reynolds stress, as well as the instantaneous vortex structures at different grid resolutions and times. In addition, the VOMM model only takes up 30% time of the DMM model for all flow scenarios. These results demonstrate that the proposed VOMM model improves the numerical stability of LES and has high a posteriori accuracy and computational efficiency by incorporating the a priori information of turbulence statistics, highlighting that the VOMM model has a great potential to develop advanced SGS models in the LES of turbulence.

Effect of the wind speed on aerodynamic behaviours during the acceleration of a high-speed train under crosswinds

A high-speed train that accelerates under crosswinds can potentially encounter dramatic aerodynamic effects and safety risks. The goal of this study is to reveal the effect of wind speed on the instantaneous vortex structures, pressure distribution, aerodynamic loads and safety indicators of a high-speed train that is accelerating under crosswinds. The three-dimensional and incompressible IDDES method combined with moving overset grid technology is applied to obtain reliable results. The main results are summarized as follows: two typical inclined vortices on the leeward side are observed, and the amplitude and area of the negative pressure in the vortex core region increase with increasing wind speed. The leeward side and the roof exhibit severe pressure fluctuations, and the sensitivity of the pressure from wind speed on the head car is stronger than that on other carriages. For aerodynamic force/moment coefficients, local maximum/minimum values appear during the acceleration process; a reasonable inference for this phenomenon is that a certain range of yaw angles causes the force/moment coefficient to reach the local maximum/minimum values. Wind speed has a significant increasing effect on the maximum values of the lateral force coefficient and rolling moment coefficient, especially for the head car. Although the overturning coefficient and derailment coefficient are less than the prescribed safety thresholds, the overturning risk needs more attention than the derailment risk considering that the maximum value of the overturning coefficient is twice that of the derailment coefficient. The obtained numerical results can provide a preliminary reference for train accelerations under crosswind.

Numerical Study on the Flow Past Three Cylinders in Equilateral-Triangular Arrangement at Re = 3 × 106

One of the most common systems in engineering problems is the multi-column system in the form of an equilateral-triangular arrangement. This study used three-dimensional numerical simulations to investigate the flow around three cylinders in this arrangement at the super-critical Reynolds number Re=3×106, concentrating on the influence on the spacing ratio (L/D) among cylinders. The instantaneous vortex structures, Strouhal numbers, fluid force coefficients, and pressure distributions are analyzed thoroughly. The present study demonstrated that fluid dynamics is sensitive to L/D, by which five different flow patterns are classified, namely single bluff body flow (L/D≤1.1), deflected gap flow (1.2≤L/D≤1.4), anti-phase flow (1.5≤L/D≤2.3), in-phase flow (2.5≤L/D<3.5), and co-shedding flow (L/D≥3.5). Critical bounds are identified by significant transitions in the flow structure, discontinuous drop and jump of St, and force coefficients.

Dynamics of three-dimensional vortical structures behind a barchan dune based on tomographic particle image velocimetry

In this study, tomographic particle image velocimetry (PIV) was used to measure three-dimensional (3D) flow structures behind a fixed-bed barchan dune model at a Reynolds number of 2528 in a circulation water tunnel. The topological evolution of the 3D instantaneous vortex structures and their dynamic characteristics in the dune wake were analyzed. 3D instantaneous arch-shaped vortex “chains” shedding from the barchan and typical quasi-streamwise vortex structures induced by the development of an internal boundary layer were found to be located both before and after the reattachment region, which differs a little from the results of previous studies. Nevertheless, both conditionally averaged and typical instantaneous 3D flow fields revealed that the arch-shaped vortex system dominates the barchan dune wake, and the inclination angle of these spanwise-oriented structures was found to change as it propagates downstream. Additionally, the dynamic characteristics of the dune wake were found to be similar to a flapping phenomenon such as oscillation of the recirculation in a backward-facing step flow. To provide complementary insight into this phenomenon, the separated and reattached shear flow were also examined using two-dimensional time-resolved planar PIV. It was found that the basic unsteady flapping motion, vortex roll-up, and complex vortex interactions can all be analyzed with ease in a Lagrangian framework.

Large eddy simulation of structural characteristics in turbulent flow around a circular cylinder close to a wavy wall

The flow characteristics and vortex evolution behind a circular cylinder near a wavy wall are studied using a large-eddy simulation framework. The Reynolds number based on the cylinder diameter D is ReD = 1500, the gap ratio G/D is fixed to 1, and the slope S = λ/2k varies from 2 to 8 (where G is the distance between the lower surface of the cylinder and the crest, λ is the wavelength, and k is the wave amplitude). The flow field characteristics, hydrodynamic coefficients, pressure distributions, vortex evolution, and three-dimensional flow features are presented and discussed. The phase of the flow field is determined by proper orthogonal decomposition, and the vortex identification method is applied to capture vortex structures. Compared with a smooth wall, the wavy wall changes the pressure distribution of the flow field and decreases (increases) the mean drag (lift) coefficient. As the wall amplitude increases, the Strouhal number increases slightly, attaining values of 0.25 for S = 8, 0.25 for S = 4, and 0.26 for S = 2. The wall pressure coefficient has an approximately periodic distribution, while the wall friction coefficient varies according to the evolution of secondary vortices on the wall. Two small secondary vortices appear either side of the crest before merging into a new secondary vortex that moves downstream with the upper and lower wake vortices. The energy of the first two modes obtained by proper orthogonal decomposition decreases compared with the smooth-wall case, whereas that of the third and fourth modes increases, reflecting the greater large-scale vortex structures between the wavy walls. This is confirmed by examining the instantaneous three-dimensional vortex structures using the Q-criterion.

Assessment on heat transfer deterioration to supercritical carbon dioxide in upward flows via large eddy simulation

The direct numerical simulation (DNS) of supercritical carbon dioxide flowing in heated circular tubes by Bae et al. [Phys. Fluids, 2005, 17: 105104] is reproduced by the present large eddy simulation (LES). Cases of forced convection and mixed convection of upward flows in different-diameter tubes are considered, which were also re-run by Nemati et al. [Int. J. Heat Mass Transfer, 2015, 83: 741–752] and Chu et al. [J. Nucl. Eng. Raidat. Sci., 2016, 2: 031019] using DNS technology. Heat transfer deterioration and recovery are well captured in the simulation. The wall temperature predicted by current work shows excellent consistency with those DNS data, even lower than deviations among them. Buoyancy effect, mean flow characteristics and turbulence statistics are comparatively analyzed for the mixed-convection cases. For upward flow in the 1-mm tube, the turbulence kinetic energy and Reynolds stresses are continuously reduced along the tube, where heat transfer is inhibited by buoyancy. For upward flow in the 2-mm tube, the mean velocity profile distorts to an M−shaped distribution in the downstream of the tube, corresponding to the regeneration of turbulence kinetic energy and Reynolds stresses. The instantaneous turbulent streaks and vortex structures are displayed to intuitively understand the development of turbulence along the tube. Considering the huge cost of DNS on computing resources and time consumption, LES technology is a reliable and feasible means to explore supercritical heat transfer.

Effect of MDI Actuation Timing on Inhalation Dosimetry in a Human Respiratory Tract Model.

Accurate knowledge of the delivery of locally acting drug products, such as metered-dose inhaler (MDI) formulations, to large and small airways is essential to develop reliable in vitro/in vivo correlations (IVIVCs). However, challenges exist in modeling MDI delivery, due to the highly transient multiscale spray formation, the large variability in actuation–inhalation coordination, and the complex lung networks. The objective of this study was to develop/validate a computational MDI-releasing-delivery model and to evaluate the device actuation effects on the dose distribution with the newly developed model. An integrated MDI–mouth–lung (G9) geometry was developed. An albuterol MDI with the chlorofluorocarbon propellant was simulated with polydisperse aerosol size distribution measured by laser light scatter and aerosol discharge velocity derived from measurements taken while using a phase Doppler anemometry. The highly transient, multiscale airflow and droplet dynamics were simulated by using large eddy simulation (LES) and Lagrangian tracking with sufficiently fine computation mesh. A high-speed camera imaging of the MDI plume formation was conducted and compared with LES predictions. The aerosol discharge velocity at the MDI orifice was reversely determined to be 40 m/s based on the phase Doppler anemometry (PDA) measurements at two different locations from the mouthpiece. The LES-predicted instantaneous vortex structures and corresponding spray clouds resembled each other. There are three phases of the MDI plume evolution (discharging, dispersion, and dispensing), each with distinct features regardless of the actuation time. Good agreement was achieved between the predicted and measured doses in both the device, mouth–throat, and lung. Concerning the device–patient coordination, delayed MDI actuation increased drug deposition in the mouth and reduced drug delivery to the lung. Firing MDI before inhalation was found to increase drug loss in the device; however, it also reduced mouth–throat loss and increased lung doses in both the central and peripheral regions.

Characterization of the thermal performance of multi piezoelectric fans for cooling a semi-cylindrical concave surface

Piezoelectric fan is widely used in the heat removal of high-temperature devices for its advantages of simple structure, low energy consumption, low noise, and good wind orientation. A theoretical and numerical study of characterizing the flow and heat transfer performances induced by single and multi-vibrating piezoelectric fans for cooling heated semi-cylindrical concave surface is conducted in this work. Two kinds of numerical models based on a dynamic meshing scheme and a user defined function describing the time-varying displacement of vibrating fans are employed to capture the instantaneous vortex structures and temperature contours on the semi-cylindrical concave surface. The influence mechanisms on heat removal performance of some vital factors, such as the dimensionless fan-to-fan pitch (P/W), vibrating phase difference and the dimensionless fan tip-to-concave surface distance (G/A) are analysed. The results prove that the evolution of the transient vortical structures around a single vibrating piezoelectric fan for cooling concave surface is almost the same as the case for cooling plane surface. The interaction of the streaming flow induced by multi fans is mainly concentrated in the clearance area between adjacent fans rather than the vibration envelopes. Moreover, the in-phase vibrating condition is more sensitive to the change of P/W and G/A than that of the out-of-phase. The simulations furnish a good reference for the design and optimization of multi piezoelectric fans.

Experimental investigation of upstream cube effects on the wake of a wall-mounted cylinder: Wake rising reduction, TKE budget and flow organization

The development of Marine Renewable Energy based on tidal currents at a selected area needs to first analyze the flow characteristics and thus the bathymetry induced turbulence in this area. The present study aims to investigate turbulence characteristics around two wall mounted obstacles in tandem, which represent seabed obstacles in a high Reynolds number flow. 2D planar PIV measurements are performed in multiple planes to access the main flow organization. Based on successive analyses of mean flow characteristics as well as of the flow organization, it is demonstrated that the presence of an upstream cube clearly impacts the cylinder wake development by preventing the rising of the large scale flow structures observed for an isolated wall-mounted cylinder in the same flow conditions. The TKE budget analysis shows that the energy is mainly produced in the upstream cube shear layer, and TKE exchanges are very small in the far wake field, in comparison with a single cylinder flow configuration. A Fourier analysis shows that the cube–cylinder interaction generates a low frequency peak in the far wake field which persists for long downstream distances. This particular frequency is the signature of the cylinder wake coherent structures. A statistical analysis based on the two-point correlation velocity tensor confirms that energetic flow structures present in the far wake are mainly of 2D nature and develop horizontally. Elongated structures are identified in the wake and seem to correspond to hairpin-like structures. Instantaneous vortex structures are also characterized with local vortex identification swirling strength criterion, confirming previous FFT and statistical results. The impact of the upstream cube is strongest in the near wake in the symmetrical transverse plane and prevents the large scale flow structures from rising in the water column.

Large eddy simulation of non-reacting flow and mixing fields in a rotating detonation engine

Large Eddy Simulations (LES) of the non-reacting flow and mixing fields in a Rotating Detonation Engine (RDE) from Air Force Research Laboratory (AFRL) are performed. Effects of the total number of fuel injection orifices and air flow rates on the mixing in the AFRL RDE are studied using a periodic burner sector with five injection orifices. The instantaneous vortex structures and shock wave in the non-reactive AFRL RDE are identified, and the structures are found to be similar to those in jet in crossflow. Also, the compositional non-uniformity in the height and azimuthal directions of the burner is observed. The mixing efficiency, root-mean-square value of the hydrogen mass fraction and mixing area ratios are introduced to quantify the mixing process. The observations from LES qualitatively justify the behaviors of detonation fronts observed in the experiments. Finally, results based on full scale AFRL RDE are briefly discussed.

Large-Eddy Simulation of the airflow around a truck

Understanding the complex and unsteady flow around commercial vehicles is crucial for improving the aerodynamic performance and safety. Large-Eddy Simulation (LES) is an excellent tool for understanding and visualising the flow structures, which is normally difficult to achieve in experiments. This paper examines a 1:25 scale model truck subjected to a headwind. Both LES and Reynolds-Averaged Navier-Stokes (RANS) techniques were used to acquire the aerodynamic coefficients and surface pressure. Pressure coefficients from LES show good agreement with the full-scale data while discrepancies were found for RANS. The complex flow structures around the truck are presented in detail through streamlines, isosurface contours and surface trace lines, based on the LES results. Both time-averaged and instantaneous vortex structures are identified, highlighting the highly turbulent regions with high energy dissipations, as well as the propagation of small vortices along the truck surface. Spectral analysis is carried out on the time-varying aerodynamic coefficients, showing the dominant frequencies in vortex shedding. Two potential instability modes were identified corresponding to large-scale vortex shedding at low Strouhal number and small-scale vortex shedding due to Kelvin Helmholtz instability. The outcome of the work will help the designers and manufacturers improve the aerodynamic performance and safety of commercial trucks.

Direct numerical simulation and in-depth analysis of thermal turbulence in square annular duct

The thermal turbulence in an infinitely-long square annular duct (SAD) was studied by the direct numerical simulation with the Reynolds (Re) number Reb≈6400 based on the bulk velocity. The isothermal boundary condition was applied to the side walls in SAD with the higher and lower temperatures on the outer and inner walls, respectively. As one of the most fascinating features for the corner turbulence, the turbulent-driven secondary flow (TDSF) presented the twin counter-rotating vortex pairs symmetrically located at the outer concave and inner convex corners. The TDSF was found significantly increase the spatially-averaged Nusselt (Nu) number comparing to the laminar flow. The instantaneous vortex structures were visualized by the third-generation vortex-identification method based on Liutex. The budget analyses of the governing-equation for the turbulent boundary layer (TBL) in SAD distinguished the balancing mechanisms from the one in the traditional flat-plate TBL, which justified the necessity to classify the generic TBLs into their sub-categories of Type-A, B and C TBLs. Moreover, the current study focused on a number of issues in the thermal TBL, such as the TDSF structures with their thermal effects and the TBL law formulations. For the present TBL in SAD, it was found that the buffer layer located in between the inner and the semi-log layers tended to be wider than that in a flat-plate TBL, and an outer linear conduction layer was, for the first time, found for the Type-B temperature TBL. Based on the indicator function, the quantitative sublayer partitions were first conducted for both the velocity and temperature TBLs and then the applicable ranges for the linear and semi-log sublayers were rigorously determined. Further, the profound damping mechanism was first introduced and applied to the traditional linear and semi-log laws to derive more accurate analytical law formulations for the entire velocity and temperature TBL sublayers.

Experimental study of human thermal plumes in a small space via large-scale TR PIV system

In small occupied spaces such as vehicle cabins, in-depth information about human thermal plumes can be important for designing ventilation systems, especially in the case of displacement ventilation. In this study, large-scale time-resolved particle image velocimetry measurements were performed to reveal airflow characteristics of thermal plumes inside a small space with high temporal and spatial resolutions. The measured time-averaged velocity showed that the development of thermal plumes was limited by the small space, with maximum vertical velocity of 0.184 m/s above the head. The standard deviation of velocity and the turbulence intensity (TI) indicated high fluctuation characteristics, with TI of approximately 0.4 in the mainstream area. With these time-resolved data, the integral, Taylor and Kolmogorov scales were calculated, which provided recommended grid sizes and time steps for different numerical simulations. For investigation of instantaneous characteristics and vortex structures, three vortex identification parameters were compared. The vorticity index identified bulky attached and detached vortexes around the head; the Q-criterion revealed that the mainstream area was controlled mainly by deformation structures; and the λci criterion, which was the most effective means of identification, avoided the influences of deformation structures and focused only on the rotation structures with directions of rotation. Furthermore, multi-scaled characteristics of thermal plumes were revealed by proper orthogonal decomposition, and the period of ascending plumes was estimated as 5 s.

Tomographic PIV investigation on 3D wake structures for flow over a wall-mounted short cylinder

Tomographic particle image velocimetry (TPIV) measurement with six high-resolution charge-coupled device (CCD) cameras is conducted to investigate flow structures over a finite circular cylinder with an aspect ratio of 2 ($h/d=2$). This short wall-mounted cylinder is fully immersed in a thick turbulent boundary layer ($\unicode[STIX]{x1D6FF}/h=1.025$). Focus is placed on the three-dimensional instantaneous vortex structures and their dynamic characteristics in the wake flow fields. Based on the present results, a refined topological model of the mean wake field behind the finite circular cylinder is proposed, where the spatial locations of the typical vortex structures and their interactions are described in more detail. Among the reported typical vortex structures (i.e. the horseshoe, tip, base, trailing and arch vortex), emphasis is laid on discussion of the tip and arch vortex. The instantaneous 3D M-shape arch vortex and an alternating large-scale streamwise vortex are first found in the present experiment, and their developments are also discussed. Therefore, it is suggested that the instantaneous finite-cylinder wake is dominated by the arch vortex system and the large-scale streamwise vortices. Moreover, in the instantaneous volumetric flow fields, both the antisymmetric and the symmetric wake behaviours are observed. With proper orthogonal decomposition (POD) analysis, the dynamic characteristics of the wake field are clarified. Different from the flow around an infinite cylinder without control, the third and fourth POD modes are characterized by low-frequency symmetric shedding. The low-frequency feature shown in the second mode pair is observed and associated with the occurrence of instantaneous symmetric 3D wake behaviour triggered by the low-aspect-ratio effect and the extension of the separated shear layer. The low frequency seems be attributed to the flapping phenomenon, i.e. oscillation of the recirculation in the backward-facing step flow. It is found that the flapping motion has a modulating effect on the occurrence of the antisymmetric shedding vortex and thus the large-scale streamwise vortex.

S0510304 DNSを用いたノズル回転制御下における衝突噴流の熱流動解析

In order to develop a new heat transfer procedure of impinging jet, we conduct a DNS (direct numerical simulation) of controlled impinging jet. As a proposed procedure, a nozzle is rotated around a given axis normal to the impinging wall. To realize the high accurate computation, the discretization in space is performed with hybrid scheme in which sine or cosine series and 6th order compact scheme are used. For the surrounding boundary condition, in order to reduce the artificial noise due to the occurrence of vortex at the outflow boundary, a fringe technique is newly developed and introduced at the surroundings of computational volume. As a control parameters, a rotating frequency is varied. From view of instantaneous velocity magnitude and instantaneous vortex structures, it is found that jet circumferentially rotates depend on the rotating frequency in the free jet region, and that the jet diffuses largely compared to the case of the uncontrolled jet. Furthermore, in order to investigate dominant flow structures, DMD (dynamic mode decomposition) method is applied. Using DMD, for controlled case, the large-scale structures spiral flow are extracted. The local Nusselt number distribution is calculated too to estimate the heat transfer performance. For the controlled case, the heat transfer performance around the impinging position markedly decreases compared to the uncontrolled one, however it is found that a homogeneous heat transfer appears around the impinging position under a specific frequency of rotation.

0504 DNSによるノズル回転制御下における衝突噴流の流動・伝熱特性

In order to develop a new heat transfer procedure of impinging jet, we conduct a DNS (direct numerical simulation) of controlled impinging jet. As a proposed procedure, a nozzle is rotated around a given axis normal to the impinging wall. Prom view of instantaneous velocity magnitude and instantaneous vortex structures, it is found that jet circumferentially rotates depend on the rotating frequency in the free jet region, and that the jet diffuses largely compared to the case of the uncontrolled jet. In order to estimate the heat transfer performance, the local Nusselt number distribution is calculated. For the controlled case, the heat transfer performance around the impinging position markedly decreases compared to the uncontrolled case, however it is found that a homogeneous heat transfer appears around the impinging position under a specific frequency of rotation and a rotating radius.