共查询到20条相似文献,搜索用时 15 毫秒
1.
Istvan Bolgar Sven Scharnowski Christian J. Kähler 《Flow, Turbulence and Combustion》2018,101(3):653-680
The flow around a backward-facing step in the sub-, trans- and supersonic regimes was investigated at the Trisonic Wind Tunnel Munich with particle image velocimetry and dynamic pressure measurements. These two techniques were combined to simultaneously measure and correlate the velocity fluctuations in a streamwise vertical plane with the pressure fluctuations on the reattachment surface. The results show that the dynamic loads on the reattachment surface increase from subsonic up to the transonic regime while the mean reattachment location moves downstream. As soon as the flow becomes locally supersonic aft of the backward-facing step, the mean reattachment location suddenly moves upstream while the normalized dynamic loads drastically decrease. By correlating the velocity and the dynamic pressure data, it was shown that a clear separation between outer flow and the flow close to the surface aft of the step is responsible for the drastic load reduction. Due to the large difference in pressure/density, the disturbances from the locally supersonic flow do not have an effect on the flow close to the surface. This is also reflected in the power spectral densities of the pressure fluctuations on the surface, showing that at supersonic free-stream Mach numbers a low-frequency pumping motion of the locally subsonic flow is the dominant mode, while in sub-/transonic flow Kelvin-Helmholtz instabilities and a cross-pumping motion of the shear layer dominate the dynamic loads. 相似文献
2.
A passive control approach (no external energy input) for an unsteady separated flow case was investigated numerically. A surface-mounted control fence was positioned upstream of a backward-facing step, and as an oncoming flow a thin and fully developed turbulent boundary layer with a thickness of δ/h = 0.8 was used. The objective of the passive control was to enhance the entrainment rate of the shear layer bounding the separation zone behind the step, thereby reducing the mean reattachment length,〈 X r0 〉. Direct Numerical Simulations (DNS) and Large-Eddy Simulations (LES) at Re h = 3000 (based on the step height, h, and the free stream velocity, U ∞) were carried out for the uncontrolled and the controlled flow case. The LES results were in good agreement with the DNS reference solutions. Adaptively controlled feedback simulations showed that a certain minimum distance between the step edge and the upstream position of the control fence is required to achieve a maximum reduction of the reattachment length. 相似文献
3.
后台阶分离流动中大涡结构演变的数值模拟 总被引:4,自引:0,他引:4
本文对后台阶分离流动中涡结构的演变进行了大涡模拟,研究了流场结构的变化规律。详细讨论了随着雷诺数的增加流场结构的典型特征的变化规律,指出流场中的涡结构随着雷诺数的增大变得十分复杂和丰富,回流区的数目、大小及其出现的位置也显著地不同。这些结果与已有的一些实验值和流场显示结果是吻合的。在此基础上,进一步研究了高雷诺数时流场中大尺度涡结构的瞬时发展和演化过程,展示了其中大涡的产生、追随、吸引、合并和破碎等过程。对于高雷诺数情况,对大涡模拟得到的数值结果进行了统计,得到的时均速度分布以及台阶后方的回流区长度与现有的其他实验结果符合得很好。本研究是针对后台阶分离流动深入开展湍流控制以及两相流动研究的基础。 相似文献
4.
Subharmonic-perturbed shear flow downstream of a two-dimensional backward-facing step was experimentally investigated. The Reynolds number was Reh = 2.0 ×104, based on free-stream velocity and step height. Planar 2D-2C particle image velocimetry was employed to measure the separating and reattaching flow in the horizontal-vertical plane in the center position. The subharmonic perturbations were generated by an oscillating flap which was implemented over the step edge and driven by periodic Ampere force. The subharmonic frequency was 55 Hz as the half of the fundamental frequency of the turbulent shear layer. As a result of the subharmonic perturbations, the size of recirculation region behind the backward-facing step is reduced and the time-averaged reattachment length is 31.0% shorter than that of the natural flow. The evolution of vortices, including vortex roll-up, growth and breakdown process, is analyzed by using phase-averaging, cross-correlation function and proper orthogonal decomposition. It is found that Reynolds shear stress is considerably increased in which the vortices roll up and then break down further downstream. In particular, rapid growth of vortices based on the “step mode” occurs at approximate half of the recirculation region, caused by in interaction between the shear layer and the recirculation region. Furthermore, the coherent structures, which are represented by a phase-correlated POD mode pair, are reconstructed in phases in order to show regular patterns of the subharmonic-perturbed coherent structures. 相似文献
5.
Wang Chaozheng Zhao Pinghui Lei Mingzhun Lu Kun Ge Zhihao Liu Jiaming Li Yuanjie Pei Gang 《Flow, Turbulence and Combustion》2021,107(1):125-147
Flow, Turbulence and Combustion - A direct numerical simulation has been performed to study instantaneous behavior in lead-bismuth eutectic flows past a vertical, backward-facing step. A turbulent... 相似文献
6.
Experimental Investigation of Three-Dimensional Vortex Structures Downstream of Vortex Generators Over a Backward-Facing Step 总被引:1,自引:0,他引:1
An experimental investigation of vortex generators has been carried out in turbulent backward-facing step (BFS) flow. The Reynolds number, based on a freestream velocity U0 = 10 m/s and a step height h = 30 mm, was Reh = 2.0 × 104. Low-profile wedge-type vortex generators (VGs) were implemented on the horizontal surface upstream of the step. High-resolution planar particle image velocimetry (2D-2C PIV) was used to measure the separated shear layer, recirculation region and reattachment area downstream of the BFS in a single field of view. Besides, time-resolved tomographic particle image velocimetry (TR-Tomo-PIV) was also employed to measure the flow flied of the turbulent shear layer downstream of the BFS within a three-dimensional volume of 50 × 50 × 10 mm3 at a sampling frequency of 1 kHz. The flow control result shows that time-averaged reattachment length downstream of the BFS is reduced by 29.1 % due to the application of the VGs. Meanwhile, the Reynolds shear stress downstream of the VGs is considerably increased. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) have been applied to the 3D velocity vector fields to analyze the complex vortex structures in the spatial and temporal approaches, respectively. A coherent bandwidth of Strouhal number 0.3 < Sth < 0.6 is found in the VG-induced vortices, and moreover, Λ-shaped three-dimensional vortex structures at Sth = 0.37 are revealed in the energy and dynamic approaches complementarily. 相似文献
7.
8.
9.
Several studies of enclosed turbulent flows within rotating discs or cylinders (e.g. [6, 7]) have revealed that, while the
geometry may be strictly axisymmetric, it is possible for non-axisymmetric flow patterns to be created within the space. Here
we report a visualization study off low induced in the cavity formed between two discs, one rotating, the other stationary.
This is an idealization of the flow configuration that occurs between successive stages in the `hot section' of a gas turbine.
Such rotor-stator cavities have hitherto been regarded as creating asymmetric flow pattern but Owen [8] has conjectured that
the failure to predict heat transfer coefficients accurately for certain radius-to-height ratios may indicate that here, too,
organized rotating vortex structures were playing a crucial role. The present study has made an experimental visualization
of this flow over a range of conditions in order to test this conjecture and to help guide future numerical explorations.
The apparatus comprised a rotating disc over which is fitted a Perspex stationary disc and shroud. The lower disc was rotated
for a number of distinct speeds between 30 and 120 rpm and for two ratios of gap-height to radius (H/R). The spin Reynolds number based on gap height and maximum rotational speed, ρΩRh/μ, ranged from 3.7 × 10E4 to 2.24 × 10E5. The flow structures were visualized by injecting ink through a small hypodermic
tube at various radii and depths within the cavity and recording the ensuing dye streaks with a video camera mounted above
the discs. The results show that, for a wide range of conditions,structured flow with large-scale vortices does indeed arise,
the number of vortices diminishing as the spin Reynolds number is increased. The two-vortex S-shaped pattern is stable over
a wide range of conditions but three, five and seven vortices have also been observed.
These results suggest that an accurate numerical simulation of the flow within rotor-stator disc cavities may require unsteady,three-dimensional
CFD modelling over at least certain ranges of flow parameters.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
Investigation on Heat Transfer Characteristics over Hypersonic Backward-Facing Step of a Blunt Plate
Fluid Dynamics - The combined effect of leading-edge bluntness and backward-facing step (BFS) is critical because they affect the surface thermal loads and boundary layer transition of hypersonic... 相似文献
11.
V. I. Terekhov N. I. Yarygina R. F. Zhdanov 《Journal of Applied Mechanics and Technical Physics》2002,43(6):888-894
The influence of the shape and size of the obstacle on separated flow and heat transfer is studied experimentally. Results of investigation and comparative analysis of the hydrodynamic structure of a separated flow behind a step and a rib are presented. A principally different character of transfer processes in the separated flow behind obstacles of these types is demonstrated. The flow structure in the secondary vortex region is considered. 相似文献
12.
Guang Yang Bernhard Weigand Alexandros Terzis Kilian Weishaupt Rainer Helmig 《Transport in Porous Media》2018,122(1):145-167
This study investigates numerically the turbulent flow and heat transfer characteristics of a T-junction mixing, where a porous media flow is vertically discharged in a 3D fully developed channel flow. The fluid equations for the porous medium are solved in a pore structure level using an Speziale, Sarkar and Gatski turbulence model and validated with open literature data. Overall, two types of porous structures, consisted of square pores, are investigated over a wide range of Reynolds numbers: an in-line and a staggered pore structure arrangement. The flow patterns, including the reattachment length in the channel, the velocity field inside the porous medium as well as the fluctuation velocity at the interface, are found to be strongly affected by the velocity ratio between the transversely interacting flow streams. In addition, the heat transfer examination of the flow domain reveals that the temperature distribution in the porous structure is more uniform for the staggered array. The local heat transfer distributions inside the porous structure are also studied, and the general heat transfer rates are correlated in terms of area-averaged Nusselt number accounting for the effects of Reynolds number, velocity ratio as well as the geometrical arrangement of the porous structures. 相似文献
13.
V. I. Terekhov N. I. Yarygina R. F. Zhdanov 《Journal of Applied Mechanics and Technical Physics》2003,44(4):522-531
Results of an experimental study of heat transfer in a separated flow behind a step and a rib are presented. The influence of the obstacle height (H = 6–30 mm) on heat and mass transfer and the structure of the thermal boundary layer is studied. The features of heat transfer in recirculation and relaxation zones of the separated flow are analyzed, and the effect of separation on intensification and suppression of turbulent heat transfer is determined. 相似文献
14.
Jacek Pozorski Sergei Sazhin Marta Wacławczyk Cyril Crua David Kennaird Morgan Heikal 《Flow, Turbulence and Combustion》2002,68(2):153-165
Analytical expressions for mass concentration of liquid fuel in a spray are derived taking into account the effects of gas
turbulence, and assuming that the influence of droplets on gas is small (intitial stage of spray development). Beyond a certain
distance the spray is expected to be fully dispersed. This distance is identified with the maximum spray penetration. Then
the influence of turbulence on the spray stopping distance is discussed and the rms spray penetration is computed from a trajectory
(Lagrangian) approach. Finally, the problem of spray penetration is investigated in a homogeneous two-phase flow regime taking
into account the dispersion of spray away from its axis. It is predicted that for realistic values of spray parameters the
spray penetration at large distances from the nozzle is expected to be proportional to t
2/3 (in the case when this dispersion is not taken into account this distance is proportional to t
1/2). The t
2/3 law is supported by experimental observations for a high pressure injector.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
It is known that turbulence is characterized by intermittence which is closely related to the development of unsteady nonisotropic intense small-scale vortex structures. In this study, small fluid particles from the inertial range of isotropic turbulence are considered. It is shown that the phenomenon of rotation intensification and stretching of the particles can be analyzed theoretically. In recent experimental and numerical studies, where this phenomenon was called “the pirouette effect”, its significance in the mechanism of the intense small-scale structures generation was discussed. In this study, a linear stochastic Lagrangian model for the effect is developed. In this model, the kinetic equation for the distribution function of the squared cosine of the angle between the vorticity and the eigenvector of the strain rate tensor of a fluid particle is derived and time history asymptotics of this quantity are analytically calculated at large and small times. The results are in good agreement with the recent experiments and numerical calculations. An analysis made in this study shows that the linear processes probably play the crucial role in certain processes in the isotropic turbulence, which is known to be a principally nonlinear phenomenon. The model developed makes it possible to analyze the statistics of the Lagrangian dynamics of small fluid particles in the inertial range which can be useful in some computational approaches to turbulence. 相似文献
16.
Hairpin-like vortical structures that form in the wall region of turbulent channel flow are investigated. The analysis is
performed by following a procedure in which the Navier-Stokes equations are first integrated by means of a computational code
based on a mixed spectral-finite difference technique in the case of the flow in a plane channel. A DNS turbulent-flow database,
representing the turbulent statistically steady state of the velocity field through 10 viscous time units, is computed and
the vortex-detection method of the imaginary part of the complex eigenvalue pair of the velocity-gradient tensor is applied to the velocity field. As a result, hairpin-like vortical structures are educed. Flow visualizations are provided
of the processes of evolution that characterize hairpin vortices in the wall region of turbulent channel flow. The relationship
is investigated between vortex dynamics and 2nd- and 4th- quadrant events, showing that ejections and sweeps play a fundamental
role in the way the morphological evolution of a hairpin vortex develops with time. 相似文献
17.
Fluid Dynamics - The laminar-turbulent boundary (edge) separates trajectories approaching a turbulent attractor from those approaching a laminar one, at least for a finite time. To investigate the... 相似文献
18.
Andrew S. Magstadt Matthew G. Berry Zachary P. Berger Patrick R. Shea Christopher J. Ruscher Sivaram P. Gogineni Mark N. Glauser 《Flow, Turbulence and Combustion》2017,98(3):725-750
A jet from an axisymmetric convergent nozzle is studied at ideal and underexpanded conditions using velocity and acoustic data. Two particle imaging velocimetry setups, a 10 kHz system and a multi-camera configuration, were used to capture near-field velocities while simultaneously sampled with far-field microphones. Proper orthogonal decomposition is performed on the velocity data to extract modes representative of physical processes in the flow. The decomposed velocity fields are then correlated with acoustic data to identify modes related to specific noise spectra. Specifically, four modes are associated with noise production in the sonic plume. Selective flow-field reconstruction is carried out, revealing interesting dynamics associated with loud flow states. In the supersonic case, screech-containing and turbulent mixing modes are isolated. The spatial modes of each data set are then compared for similarities in structures. 相似文献
19.
The transient dynamics of a high Reynolds number separated flow over a two-dimensional ramp submitted to pulsed fluidic control is investigated. A spanwise array of 22 round jets, located upstream of the flap leading edge, is used as actuator to generate co-rotating vortical structures. Simultaneous measurements of wall friction using hot-film anemometry and phase-averaged velocity using 2D2C PIV are conducted. The PIV plane encompasses the incoming boundary layer upstream the flap leading edge, the separation bubble and the natural reattachment region. The dynamics of the separated flow is studied for successive sequences of pulsed actuation. Pockets of turbulence are periodically generated by the separation process and pushed downstream. After the transition period, the controlled flow shows large amplitude oscillations around a steady mean, particularly for the separation area. The transient dynamics of the flow at the actuation activation is also studied. The separated flow is strongly modified by the actuation from the first pulse. Characteristic times of the transient dynamics can be determined by fitting a first-order model with delay on the data. For the reattachment, the dimensionless characteristic rising times defined as \(\tau _{r}^{+} = \tau _{r} ~ U_{0} ~/~ L_{sep}\) of 11.7 for the friction gain, 4.8 for the separation length and 4.1 for the first mode of a Conditional Proper Orthogonal Decomposition analysis of the phase-averaged velocity fields were found. These values are in good agreement with previous studies and are of particular interest for modeling the transients and for further closed-loop control applications. 相似文献
20.
Large-eddy simulations (LES) of a planar, asymmetric diffuser flow have been performed. The diverging angle of the inclined
wall of the diffuser is chosen as 8.5°, a case for which recent experimental data are available. Reasonable agreement between
the LES and the experiments is obtained. The numerical method is further validated for diffuser flow with the diffuser wall
inclined at a diverging angle of 10°, which has served as a test case for a number of experimental as well as numerical studies
in the literature (LES, RANS). For the present results, the subgrid-scale stresses have been closed using the dynamic Smagorinsky
model. A resolution study has been performed, highlighting the disparity of the relevant temporal and spatial scales and thus
the sensitivity of the simulation results to the specific numerical grids used. The effect of different Reynolds numbers of
the inflowing, fully turbulent channel flow has been studied, in particular, Re
b
= 4,500, Re
b
= 9,000 and Re
b
= 20,000 with Re
b
being the Reynolds number based on the bulk velocity and channel half width. The results consistently show that by increasing
the Reynolds number a clear trend towards a larger separated region is evident; at least for the studied, comparably low Reynolds-number
regime. It is further shown that the small separated region occurring at the diffuser throat shows the opposite behaviour
as the main separation region, i.e. the flow is separating less with higher Re
b
. Moreover, the influence of the Reynolds number on the internal layer occurring at the non-inclined wall described in a recent
study has also been assessed. It can be concluded that this region close to the upper, straight wall, is more distinct for
larger Re
b
. Additionally, the influence of temporal correlations arising from the commonly used periodic turbulent channel flow as inflow
condition (similar to a precursor simulation) for the diffuser is assessed. 相似文献