共查询到20条相似文献,搜索用时 15 毫秒
1.
The interaction between the wake of a transverse circular cylinder and the underlying flat-plate boundary layer with a moderate gap ratio G/D=1.0 is investigated using both hydrogen-bubble-based and PIV-based visualization techniques. The spanwise rollers in the cylinder wake are found to be capable of inducing secondary vortices in the near-wall region. The mutual induction from the counter-clockwise rollers, which are closer to the wall, plays a primary role, so that these secondary vortices present linear lift-up motion at first. Their subsequent evolution dominantly determines the characteristics of the wake/boundary-layer interaction. Two different vortex interaction scenarios are observed: the secondary vortices can be either entrained into the rollers or pushed down towards the wall. This leads to a rapid three-dimensional destabilization process, through which streamwise vortices are generated. And it is suggested that these streamwise vortices are the dominant structures to promote the following boundary layer transition. 相似文献
2.
An efficient technique for drag reduction uses dilute solutions of a few p.p.m. of polymers. A possible reduction in drag of up to 80% is achieved. Several experimental observations have been made which tend to indicate that the polymers modify the turbulence structures within the buffer layer. Flow visualisations have shown that the changes consist of a weakening of the strength of the streamwise vortices. Existing literature reveals no attempts of numerical simulation of this phenomenon. In this paper an approach is pursued by using a constitutive equation which relates the elongation viscosity to the local properties of the flow. According to this model this viscosity is large in zones where the amount of strain rate is greater than the amount of vorticity, and is zero when the vorticity exceeds the strain rate. Simulations have been performed in a “minimal channel” to give good resolution with a limited number of grid points. The accuracy of the method is tested by comparison with the results of other techniques. For simulations with polymers, quantitative comparisons cannot be made, but the results reproduce the qualitative outputs of the experiments. The mean streamwise velocity is modified in the buffer layer; the peak of the streamwise turbulent intensity, in wall units, increases and its maximum moves far from the wall; and the vertical turbulent intensity is largely reduced in the wall region. An interesting outcome from both the simulation and the experiments is that the strength of the longitudinal vortices is reduced when the polymers are present. 相似文献
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The evolution of two spanwise-aligned low-speed streaks in a wall turbulent flow, triggered by the instability of the subharmonic varicose (SV) mode, is studied by a direct numerical simulation (DNS) method in a small spatial-periodic channel. The results show that the SV low-speed streaks are self-sustained at the early stage, and then transform into subharmonic sinuous (SS) low-speed streaks. Initially, the streamwise vortex sheets are formed by shearing, and then evolve into zigzag vortex sheets due to the mutual induction. As the intensification of the SV low-speed streaks becomes prominent, the tilted streamwise vortex tubes and the V-like streamwise vortex tubes can be formed simultaneously by increasing \( + \frac{{\partial u}}{{\partial x}}\). When the SV low-speed streaks break down, new zigzag streamwise vortices will be generated, thus giving birth to the next sustaining cycle of the SV low-speed streaks. When the second breakdown happens, new secondary V-like streamwise vortices instead of zigzag streamwise vortices will be generated. Because of the sweep motion of the fluid induced by the secondary V-like streamwise vortices, each decayed low-speed streak can be divided into two parts, and each part combines with the part of another streak, finally leading to the formation of SS low-speed streaks. 相似文献
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Using digital holographic microscopy for simultaneous measurements of 3D near wall velocity and wall shear stress in a turbulent boundary layer 总被引:2,自引:0,他引:2
A digital holographic microscope is used to simultaneously measure the instantaneous 3D flow structure in the inner part of
a turbulent boundary layer over a smooth wall, and the spatial distribution of wall shear stresses. The measurements are performed
in a fully developed turbulent channel flow within square duct, at a moderately high Reynolds number. The sample volume size
is 90 × 145 × 90 wall units, and the spatial resolution of the measurements is 3–8 wall units in streamwise and spanwise directions
and one wall unit in the wall-normal direction. The paper describes the data acquisition and analysis procedures, including
the particle tracking method and associated method for matching of particle pairs. The uncertainty in velocity is estimated
to be better than 1 mm/s, less than 0.05% of the free stream velocity, by comparing the statistics of the normalized velocity
divergence to divergence obtained by randomly adding an error of 1 mm/s to the data. Spatial distributions of wall shear stresses
are approximated with the least square fit of velocity measurements in the viscous sublayer. Mean flow profiles and statistics
of velocity fluctuations agree very well with expectations. Joint probability density distributions of instantaneous spanwise
and streamwise wall shear stresses demonstrate the significance of near-wall coherent structures. The near wall 3D flow structures
are classified into three groups, the first containing a pair of counter-rotating, quasi streamwise vortices and high streak-like
shear stresses; the second group is characterized by multiple streamwise vortices and little variations in wall stress; and
the third group has no buffer layer structures. 相似文献
7.
J.S. Marshall 《Theoretical and Computational Fluid Dynamics》2003,16(3):231-247
This computational study examines the unsteady cross-stream vorticity structures that form when one or more streamwise vortices
are immersed in homogeneous and boundary-layer shear flows. A quasi-two-dimensional limit is considered in which the velocity
and vorticity fields, while still possessing three nonzero components, have vanishing gradient in the streamwise direction.
This idealization is suitable to applications such as streamwise vortices that occur along a ship hull or airplane fuselage
and it can be used as an idealized representation of the quasi-streamwise vortices in the near-wall region of a turbulent
boundary layer. In this quasi-two-dimensional idealization, the streamwise velocity has no effect on the cross-stream velocity
associated with the vortex. However, the vortex acts to modify the cross-stream vorticity component, resulting in regions
of the flow with strong deviations in streamwise velocity. This paper examines the complex structures that form as the cross-stream
vorticity field is wrapped up by the vortex and the effect of these structures on the streamwise velocity field, first for
vortices immersed in homogeneous shear flow and then for vortices immersed in a boundary layer along a flat wall.
Received 2 January 2002 and accepted 13 August 2002 Published online 3 December 2002
RID="*"
ID="*" This project was supported by the Office of Naval Research under Grant Number N00014-01-1-0015. Dr. Thomas Swain is
the program manager.
Communicated by T.B. Gatski 相似文献
8.
On swirl development in a square cross-sectioned, S-shaped duct 总被引:1,自引:0,他引:1
The flow in a uniform square cross-sectioned, S-shaped duct was investigated experimentally, at Reynolds number (Re) = 4.73 × 104 and 1.47 × 105, using three S-ducts of different curvature and turning angle. The hydraulic diameter (D) for each S-duct is 150 mm. Besides studying the square cross-sectioned S-duct flow at moderately higher Re than current literature, the S-ducts’ geometry used in this study also have larger curvatures and higher turning angles than those reported in the literature. With surface pressure measurement and smoke wire flow visualization, flow separation at the inside wall of the first bend was detected. Using surface oil flow visualization on the bottom wall of the S-duct and cross-wires measurement at the duct exit, it is shown here that the swirl developed in the first bend was partly attenuated in the second bend due to the formation of swirl of opposite direction. The swirl of an opposite sign results in the formation of a clear dividing or separation line on the bottom wall (and top wall) of the duct. Additional flow features include the formation of streamwise vortices on the outer-wall of the second bend. These streamwise vortices can either be a pair of counter-rotating vortices or a single vortex. The formation mechanism of these streamwise vortices is explained using the Squire and Winter [J Aeronaut Sci 18(4):271–277, 1951] formula and it is shown that the said mechanism is applicable to both Re in the present study. 相似文献
9.
Sedat F. Tardu 《Applied Scientific Research》1995,54(4):349-385
This work deals with the effect of the riblets on the coherent structures near the wall. The emphasis is put on the genesis of the quasi-streamwise vortices in the presence of the riblets. The quasi-streamwise vortices regenerate by the tilting of wall normal vorticity induced by prevailing structures. This requires a mechanism which leads to a temporal streamwise dependence near the elongated flow structures and to a subsequent formation of new wall normal vorticity. It is suggested here that the action of existing quasi-streamwise vortices on the sidewalls of wall normal vorticity may create a local, streamwise dependent spanwise velocity and therefore, a secondary wall normal vorticity field. A preliminary analysis of the set-up and the time and space development of this secondary three-dimensional flow associated with the regeneration mechanism, is given. An attempt is made, in order to explain the drag reduction performed by the riblets through an intermittent model, based on the protrusion height. Logical estimates of the amount of drag reduction are obtained. The differences between the mechanism suggested here and those based on forced control experiments are also discussed. 相似文献
10.
《应用数学和力学(英文版)》2019,(3)
The relationship between wall shear stresses and near-wall streamwise vortices is investigated via a direct numerical simulation(DNS) of turbulent flows over a wavy boundary with traveling-wave motion. The results indicate that the wall shear stresses are still closely related to the near-wall streamwise vortices in the presence of a wave. The wave age and wave phase significantly affect the distribution of a two-point correlation coefficient between the wall shear stresses and streamwise vorticity. For the slow wave case of c/Um = 0.14, the correlation is attenuated above the leeward side while the distribution of correlation function is more elongated and also exhibits a larger vertical extent above the crest. With respect to the fast wave case of c/U_m=1.4, the distribution of the correlation function is recovered in a manner similar to that in the flat-wall case. In this case, the maximum correlation coefficient exhibits only slight differences at different wave phases while the vertical distribution of the correlation function depends on the wave phase. 相似文献
11.
Experiments were conducted for the flow in a straight-walled 3D diffuser fed by a fully developed turbulent duct flow. Previous
work found that this diffuser has a stable 3D separation bubble whose configuration is affected by the secondary flows in
the upstream duct. Dielectric barrier discharge plasma actuators were used to produce low-momentum wall jets to determine
if the separation behavior could be modified by weak forcing. Actuators producing a streamwise force along the wall where
separation occurred in the baseline flow had a relatively small effect. However, spanwise acting plasma actuators that produced
a pair of streamwise vortices in the inlet section of the diffuser had a strong effect on the diffuser pressure recovery.
The diffuser performance could be either improved or degraded depending on the actuation parameters, including the actuator
modulation frequency, duty cycle, and drive voltage. Velocity profile measurements in the diffuser inlet showed that the streamwise
vortices affect the uniformity of the streamwise mean velocity accounting for some of the performance changes. However, phase-locked
hotwire measurements at the diffuser exit indicate that the periodic nature of the forcing also plays an important role for
cases with enhanced pressure recovery. 相似文献
12.
Adrian Sescu Lamiae Taoudi Mohammed Afsar 《Theoretical and Computational Fluid Dynamics》2018,32(1):63-72
Görtler vortices develop along concave walls as a result of the imbalance between the centrifugal force and radial pressure gradient. In this study, we introduce a simple control strategy aimed at reducing the growth rate of Görtler vortices by locally modifying the surface geometry in spanwise and streamwise directions. Such wall deformations are accounted in the boundary region equations by using a Prandtl transform of dependent and independent variables. The vortex energy is then controlled via a classical proportional control algorithm for which either the wall-normal velocity or the wall shear stress serves as the control variable. Our numerical results indicate that the control algorithm is quite effective in minimizing the wall shear stress. 相似文献
13.
A physical mechanism of onset of large-scale organized structures in turbulent flows along a plane wall which are the cause
of intensification of turbulent fluctuations is formulated. The structures take the form of high-speed and low-speed streaks
caused by streamwise vortices, i.e., motions in the plane of the transverse cross-section. The streamwise vortices are excited
as a result of instability under the action of the anisotropy of the normal components of the Reynolds stress tensor. A model
for describing these vortices that gives characteristics in qualitative and quantitative agreement with the experimental data
is proposed. In particular, the most probable and mean distances between neighboring vortices are correctly reproduced. The
theory makes it possible to explain certain methods of turbulent flow control for the purpose of drag reduction.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 24–30, January–February,
1997.
The work was carried out with financial support from the Russian Foundation for Fundamental Research (project No. 96-01-00602). 相似文献
14.
A direct numerical simulation dataset of a fully developed turbulent Couette-Poiseuille flow is analyzed to investigate the spatial organization of streamwise velocity-fluctuating u-structures on large and very large scales. Instantaneous and statistical flow fields show that negative-u structures with a small scale on a stationary bottom wall grow throughout the centerline due to the continuous positive mean shear, and they penetrate to the opposite moving wall. The development of an initial vortical structure related to negative-u structures on the bottom wall into a large-scale hairpin vortex packet with new hairpin vortices, which are created upstream and close to the wall, is consistent with the auto-generation process in a Poiseuille flow (Zhou et al., J. Fluid Mech., vol. 387, 1999, pp. 353–396). Although the initial vortical structure associated with positive-u structures on the top wall also grows toward the bottom wall, the spatial development of the structure is less coherent with weak strength due to the reduced mean shear near the top wall, resulting in less turbulent energy on the top wall. The continuous growth of the structures from a wall to the opposite wall explains the enhanced wall-normal transport of the streamwise turbulent kinetic energy near the centerline. Finally, an inspection of the time-evolving instantaneous fields and conditional averaged flow fields for the streamwise growth of a very long structure near the centerline exhibits that a streamwise concatenation of adjacent large-scale u-structures creates a very-large-scale structure near the channel centerline. 相似文献
15.
《European Journal of Mechanics - B/Fluids》2005,24(5):539-554
A high Reynolds number flat plate turbulent boundary layer is investigated in a wind-tunnel experiment. The flow is subjected to an adverse pressure gradient which is strong enough to generate a weak separation bubble. This experimental study attempts to shed some new light on separation control by means of streamwise vortices with emphasize on the change in the boundary layer turbulence structure. In the present case, counter-rotating and initially non-equidistant streamwise vortices become and remain equidistant and confined within the boundary layer, contradictory to the prediction by inviscid theory. The viscous diffusion cause the vortices to grow, the swirling velocity component to decrease and the boundary layer to develop towards a two-dimensional state. At the position of the eliminated separation bubble the following changes in the turbulence structure were observed. The anisotropy state in the near-wall region is unchanged, which indicates that it is determined by the presence of the wall rather than the large scale vortices. However, the turbulence in the outer part of the boundary layer becomes overall more isotropic due to an increased wall-normal mixing and a significantly decreased production of streamwise fluctuations. The turbulent kinetic energy is decreased as a consequence of the latter. Despite the complete change in mean flow, the spatial turbulence structure and the anisotropy state, the process of transfer of turbulent kinetic energy to the spanwise fluctuating component seems to be unchanged. Local regions of anisotropy are strongly connected to maxima in the turbulent production. For example, at spanwise positions in between those of symmetry, the spanwise gradient of the streamwise velocity cause significant production of turbulent fluctuations. Transport of turbulence in the spanwise direction occurs in the same direction as the rotation of the vortices. 相似文献
16.
Results of an experimental investigation of the heat elimination from perforated plates to cooling air during its motion along walls and partial drainage through the perforations are elucidated. Boundary-layer characteristics on perforated plates, their thermal state, and their efflux coefficients are presented for a set of orifices. An approximate method to compute the temperature of the permeable wall under combined cooling is given and its domain of application is estimated.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 182–187, March–April, 1977. 相似文献
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The mechanisms of laminarization in wall-bounded flows have been investigated by performing direct numerical simulations (DNS) of turbulent channel flows. By decreasing Reynolds numbers systematically, the effects of the low Reynolds number are studied in connection with the near-wall turbulent structure and turbulent statistics. At approximately the critical Reynolds number, the turbulent skin friction is reduced, and the turbulent structure changes qualitatively in the very near-wall region. Instantaneous turbulent structures reveal that streamwise vortices, the cores of which are at y+ 10, disappear, although low speed streaks and Reynolds shear stress are still produced by larger streamwise vortices located in the buffer region y+ > 10. Sweep motions induced by these vortical structures are shifted toward the center of a channel and also significantly deterred, which may heighten the effects of the viscous sublayer over most of the channel section and suppress the regeneration mechanisms of new streamwise vortices in the very near-wall region. To investigate the details of how large-scale coherent vortices affect the viscous sublayer and the relevant small-scale streamwise vortices, a body force is virtually imposed in the wall-normal direction to enhance the large streamwise vortices. As a result, it is found that when they are sufficiently enhanced, the small-scale vortices reappear, and the sweep events are again dominant in the viscous sublayer. 相似文献
20.
《European Journal of Mechanics - B/Fluids》2000,19(4):469-490
The upstream perturbations that maximise the spatial energy growth in a boundary layer are called optimal perturbations. The optimal perturbations correspond to streamwise vortices and the downstream response corresponds to streamwise streaks.The aim of the present paper is to find a control by blowing and suction at the wall that zeros the energy of perturbation, when the initial disturbance is itself optimal. We shall also address the question: which kind of blowing and suction at the wall is most effective in controlling optimal disturbances?The problem is examined by a method of receptivity analysis based on a numerical solution of a system of equations adjoint to the linearised boundary layer equations. We shall investigate both cases of a flat and a concave wall. 相似文献