Recent progresses in turbulent combustion researches are summarized in this paper with a focus on direct numerical simulation (DNS), large eddy simulation (LES) and laser diagnostics of turbulent combustion. In DNS studies of turbulent combustion, detailed kinetic mechanism tends to be included to investigate turbulence/flame interactions. LES of turbulent combustion can be classified into flame tracking method, PDF method and flame thickened method, and will play important roles in the developments of many combustors in engineering applications in near future. In the laser diagnostics of turbulent combustion, simultaneous measurements of several radical concentrations by planer laser induced fluorescence (PLIF) and multi-components velocity measurement by particle image velocimetry (PIV) are effective to investigate the local flame structure in turbulence. The simultaneous time-resolved PLIF and PIV will have great importance in future researches on turbulent combustion.
Many interesting and important basic problems still remain unsolved or ever untouched in the fields of experimental fluid dynamics. The present report explains a variety of newest results of wind tunnel experiments conducted chiefly in our laboratory with respect to the features of large scale quasi-isotropic turbulence, anisotropic turbulence, three-dimensional vortical structure in the wake of a sphere, the process of vortex pairing in a two-dimensional jet, the proposal for a new model on a boundary layer transition in a view point of hairpin or horse-shoe vortex and lastly the analysis on spontaneously generated internal gravity waves in a stably stratified mixing layer. These results, the author guesses, will promise the future possibility in the fields of experimental fluid dynamics.
The current understanding of supersonic boundary layer transition is reviewed in this paper. The material focuses on bypass transition in a plane supersonic boundary layer. Recent advances in DNS and engineering prediction of transition are discussed. It is shown that streaky structures appear in supersonic boundary layers and that the fluctuation energy growth is proportional to the downstream distance. Spatial DNS studies indicate a bypass transition scenario, breakdown mechanisms to turbulence, and turbulent boundary layer structures.
Direct numerical simulation of turbulent channel flow under spanwise system rotation is performed. When system rotation is imposed on turbulent Poiseuille flow, the Reynolds number decreases in one side of a channel, while it increases in the other side. Hence, the effects of the Reynolds number as well as spanwise system rotation cannot be negligible in turbulent Poiseuille flow. To exclude the Reynolds-number effects, we performed the numerical simulation of open channel flow where the free-slip boundary condition is imposed on one wall, and the Reynolds number defined by the friction velocity and channel width can be always constant. When system rotation is imposed to enhance the spanwise vorticity of the mean shear, large-scale turbulence is attenuated and the mean velocity gradient increases over an entire channel. In contrast, when system rotation is imposed in the opposite sense to the mean spanwise vorticity, the mean velocity is leveled and small-scale turbulence is markedly decreased. We also investigate the effects of system rotation on the longitudinal vortical structure typically observed in near-wall turbulence.
Large-eddy simulation of compressible transitional flows in a low-pressure turbine cascade is performed by a 6th-order compact finite-difference and 10th-order filtering method. Numerical results without free-stream turbulence and those with about 5% free-stream turbulence are compared. Based on the computed results, detailed investigations are presented on the effects of the free-stream turbulence on the boundary layer transition, and on the behavior of the pressure waves that originate near the trailing-edge along with the effects on the separation/transition of the boundary layer. Also, based on Snapshot Proper Orthogonal Decomposition (POD) Analysis, dominant behaviors of the transitional boundary layers are investigated.
Cooling performance is a significant issue for air-cooled motorcycle engines, because its performance depends on the vehicle motion. Commonly, a heat transfer coefficient is calculated by heat flux and temperature difference, which are solved by energy equation under conjugated condition between a solid and a fluid. However, this method is complicated. Therefore, Karman's analogy based on a relation between the fluid friction and the heat transportation was used to obtain a heat transfer coefficient without solving an energy equation. Partial Cells in Cartesian coordinate method was employed as a CFD (Computational Fluid Dynamics) method for an efficient calculation around complex obstacles. Characteristics of heat transfer in a straight pipe were confirmed by a comparison with the Colburn's empirical formula. The results showed good agreement within ±10% differences under Pr=0.7 and 104<Re<1.2×105. Furthermore, a calculation result of local heat transfer coefficients of an air-cooled motorcycle engine was shown. From the above, a new approach of heat analysis by a CFD was proposed.
Three-dimensional homogeneous isotropic turbulence is simulated using the finite difference lattice Boltzmann method (FDLBM). First, we analyze the characteristics of numerical viscosity of FDLBM, and present a method in which the numerical viscosity coincides with that of the Navier-Stokes based FDM. Second, we conducted simulations with the Smagorinsky model. In the energy spectrum E(k) one detects the inertial range E(k) ∼k-5/3; however, the large k range presents a persistent energy “tail,” which has also been found in direct simulations using conventional lattice Boltzmann scheme. This energy “tail” is is due to generation of non-physical longitudinal noise. However, for high Reynolds number flows this noise is shown to be negligible.
An experimental study was conducted to examine the management of a two-dimensional turbulent channel flow with a pair of streamwise vortices. A common-flow down type streamwise vortex pair, generated by a pair of half-delta wings mounted on the wall, was introduced into a fully developed turbulent channel flow. The half-delta wings were as high as the inner layer thickness of the channel flow. The mean velocity and Reynolds shear stress distributions were measured and various properties were obtained in order to find meanings of the vortex generator for management of the turbulent channel flow. The convective motion of the secondary current is responsible for most of the streamwise momentum transfer toward the wall in the interaction between the vortices and the shear layer. In the velocity profile averaged over the spanwise extent, the velocity is accelerated below the vortex center and decelerated above the vortex center. Deformation of the mean velocity profile remained at the remarkable downstream distance of 250 times the wing height, which corresponds to 50 times channel the half-width H.
Slit nozzles are used in some gasoline direct injection engines and makes fan shaped spray. Spray injecting flows to turbulent flows from a slit nozzle have been analyzed numerically using combination of Large Eddy Simulation (LES) and lagrangian Discrete Droplet Method (DDM). As a result, LES can resolve the internal structure of the spray and irregular droplet distribution made by small eddies that momentum of spray itself induced. In conventional Reynolds Averaged Navier-Stokes (RANS) combined DDM calculation such eddies cannot be resolved, and also the internal spray structure or irregularity doesn't appear since all effects of turbulence are averaged. But such structure or irregularity is important for stable combustion in gasoline direct injection engines. Therefore the combination of LES and DDM method will play essential role for developing more robust and high efficient engines under wide operating conditions. We also proposed the way of constructing pseudo particle image in order to compare calculation results with sliced spray pictures obtained by experiments. We show time changes of the shape of brightness Probability Density Function (PDF) can be used to evaluate variance of spray droplets.
Fluctuating static pressure is closely related with fluctuating velocity in a turbulent flow, and it plays an important role in the energy balance and anisotropy of turbulence. Thus, the measurement of the fluctuating static pressure is significantly effective for the clarification of the organized structure of the turbulent flow. In this study, a static probe that is less sensitive to the yaw angle and having a good frequency response was developed, and the simultaneous measurement of velocity and pressure field was performed in a two-dimensional jet flow by a combination of this static pressure probe and an I-type hot-wire probe. In the pressure spectrum, the κ-7/3 power law range could be clearly observed, and the distribution of the cross-correlation between streamwise velocity and static pressure is demonstrated to be consistent with the previous model of coherent vortex structure.
With improvements in a computer hardware and CFD software, the problems treated in industry are becoming more complex, both physically and geometrically. Turbulent flow with strong unsteadiness is one such physically complex example. Since Large Eddy Simulation (LES) is still too time-consuming, a great number of unsteady Reynolds-Averaged Navier-Stokes (RANS) computations have been employed in such engineering applications. However, the applicability of RANS to unsteady flows remains unclear. In the present study, RANS computations for two-dimensional turbulent flow with periodic perturbation over a backward-facing step are performed in order to verify the performance of a low-Reynolds-number type κ-ε turbulence model. Visualization and investigation of the temporal change of the flow pattern and the instantaneous term-by-term budget of the governing equations reveals that the RANS computation can reproduce the unsteady nature satisfactorily, and clarifies why the RANS model captures the unsteady turbulent flow reasonably.
The direct numerical simulations of turbulent flow in a rotating channel are executed, with the attention on a transient state after an onset of the rotation. Some different rotation numbers which contain 0, 0.5, 1, 2.5 and 5 are tested. To clarify a relationship between rotation numbers and the alteration of turbulence structures, we paid notice to temporal evolutions of wall friction velocities on both sides, volume-averaged velocity, kinetic energy and its production and dissipation. We found that the time evolutions of flow rates indicate non-monotonic behaviors due to the system rotation. The major reason of it is the difference in response of wall friction velocities on both walls. Moreover, we applied an analysis based on the theory of the invariants of Reynolds stress tensor to associate the status of turbulence with turbulence structures. From the result of the analysis, the three-dimensionality of turbulence is emphasized by the system rotation except for the vicinity of the wall on the pressure side. On the suction side, however, flow field indicates the feature of two-dimensionality.
The wind tunnel used is equipped with an array of small fans (9 columns×11 rows), each of which is independently controlled via a personal computer. To establish an efficient driving mode for generating high-Reynolds number isotropic turbulence, a new driving mode (“active grid mode”) is attempted where the active and inactive fans are grid-like arranged. The characteristics of turbulence are investigated for two kinds of external fluctuated signals. The flow structure of the turbulence generated by the active grid mode is proposed.
This study is to clarify the flow structure of a three-dimensional wall jet discharged tangentially on a flat surface from a circular nozzle. Distributions of the mean velocities and fluctuating velocities in a cross section of a fully-developed flow region were measured by using a single hot-wire probe and cross-wire probes. It is confirmed in any position of the lateral direction that the distributions of the mean velocities U and W in the direction normal to the wall are made similar to each other when they are made dimensionless by the maximum local velocity while the jet height remains almost invariable in the spanwise direction. The direction of the mean flow vector in the inner layer is remained constant while the direction of the mean flow vector outside the half-value height is coincided with the direction of the jet axis. The distributions of the turbulent statistics in the direction normal to the wall are almost similar to each other outside the half-value height due to the scaling by the maximum local velocity, and on the other hand, the magnitude of the turbulent energy and Reynolds stress in the wall-side region becomes larger at the position farther from the jet axis.
Coherent vortical structures in a high-Reynolds number turbulent jet indicating edgetone oscillations are extracted from velocity and pressure data. Three statistical quantities are adopted as the candidate indicators of coherent structures; (i) phase-averaged vorticity, (ii) phase-averaged pressure, and (iii) phase-averaged velocity gradient tensor. All these quantities work well for extracting coherent structures in a test case of low-Reynolds number cylinder wake. However, when applied to the high-Reynolds number edgetone flow, (ii) and (iii) indicate much better performance than (i), because (i) is affected by the background time-averaged vorticity field. The advection speed of coherent structures is evaluated and compared with Powell's feedback mechanism.
This paper describes an experimental investigation of the transitional mechanism of a wake generated behind a thin airfoil with a small angle of attack in a towing wind tunnel. A linear stability analysis shows that the wake is characterized by a region of absolute instability in the near wake (x=30mm) and one of convective instability further downstream. When the airfoil starts to run in the tunnel, boundary layers develop on the upper/lower airfoil surfaces with different thickness. Since the asymmetric wake is generated, starting vortices of a single row are observed first in the wake, which is different from the Karman vortex street. The experimental results show that time-harmonic fluctuations of the starting vortex sustain in the natural transition process due to a self sustained resonance in the absolutely unstable region behind the trailing edge. The wake profile in the saturation steady state yields the vortex street structure, where the fluctuation frequency defined as the fundamental unstable mode is found in the final saturation steady state. The growth of the fundamental unstable mode in the convectively unstable region suppresses the high frequency fluctuations associated with the starting vortex generation. On the other hand, low-frequency fluctuations in the quasi-steady state sustaining in the saturation state grow gradually during the vortex street formation, which lead to the vortex deformation downstream.