The Experts below are selected from a list of 4848 Experts worldwide ranked by ideXlab platform
Liang Cheng - One of the best experts on this subject based on the ideXlab platform.
-
On the study of vortex-induced vibration of a cylinder with helical strakes
Journal of Fluids and Structures, 2011Co-Authors: Tongming Zhou, Siti Fatin Mohd Razali, Zhiyong Hao, Liang ChengAbstract:Abstract While the effect of helical strakes on suppression of Vortex-Induced Vibrations (VIV) has been studied extensively, the mechanism of VIV mitigation using helical strakes is much less well documented in the literature. In the present study, a rigid circular cylinder of diameter d =80 mm attached with three-strand helical strakes of dimensions of 10 d in pitch and 0.12 d in height was tested in a wind tunnel. It was found that the helical strakes can reduce VIV by about 98%. Unlike the bare cylinder, which experiences lock-in over the reduced velocity in the range of 5–8.5, the straked cylinder does not show any lock-in region. In exploring the mechanism of VIV reduction by helical strakes, measurements in stationary bare and straked cylinder wakes using both a single X-probe at four different Reynolds numbers, i.e. Re=10 240, 20 430, 30 610 and 40 800, and two X-probes with variable separations in the Spanwise Direction at R e=20 430 were conducted. It was found that vortices shed from the straked cylinder are weakened significantly. The dominate frequency varies by about 30% over the range of x / d =10–40 in the streamwise Direction while that differs by about 37.2% of the averaged peak frequency over a length of 3.125 d in the Spanwise Direction. The latter is supported by the phase difference between the velocity signals measured at two locations separated in the Spanwise Direction. The correlation length of the vortex structures in the bare cylinder wake is much larger than that obtained in the straked cylinder wake. As a result, the straked cylinder wake agrees more closely with isotropy than the bare cylinder wake. Flow visualization on the plane perpendicular to the cylinder axis at Reynolds number of about 300 reveals small-scale vortices in the shear layers of the straked cylinder wake. However, these vortices do not roll up and interact with each other to form the well-organized Karman-type vortices. Flow visualization on the plane parallel to the cylinder axis shows vortex dislocation and swirling flow, which should be responsible for the variations of the peak frequency in the streamwise as well as Spanwise Directions.
Rajeev K. Jaiman - One of the best experts on this subject based on the ideXlab platform.
-
Passive control of vortex-induced vibration by Spanwise grooves
Journal of Fluids and Structures, 2018Co-Authors: Yun Zhi Law, Rajeev K. JaimanAbstract:Abstract The objective of this numerical study is to investigate the effect of Spanwise grooves on the suppression of vortex-induced vibration (VIV) and the reduction of drag force. For this purpose, we consider a standard configuration of the elastically mounted circular cylinder, which is free to vibrate in both streamwise and transverse Directions with identical natural frequency. We introduce a novel staggered groove configuration whose geometry is especially designed by offsetting the cross-sectional portion of the cylinder continuously along the Spanwise Direction. We assess the characteristic VIV responses of the proposed staggered groove configuration against the helical surface grooves for the identical dimensions and physical conditions. The staggered and helical groove configurations differ only in their arrangement of cross-section geometry along the Spanwise Direction. Three-dimensional coupled fluid–structure simulations are conducted at low mass and damping values with moderate Reynolds number of R e = 4800 . The effective width and the depth of surface grooves are determined to characterize the size effects for the assessment of staggered and helical groove configurations. Results show that the staggered groove configuration is effective in suppressing VIV, wherein the net reductions of 37% in the peak transverse amplitude and about 25% in the mean drag coefficient are observed in comparison to the plain cylinder counterpart. Staggered groove configuration produces three dominant effects by introducing a continuous jump in the cross-sectional geometry along the Spanwise Direction: (i) reduction of the Spanwise correlation, (ii) enhancement of the three-dimensional effects in the near-wake flow, and (iii) broadening of the frequency spectra of fluid forces. As a result of these physical effects, the transfer of energy from the surrounding fluid flow to the vibrating grooved-cylinder system is reduced as compared to its plain cylinder counterpart. Owing to the simplicity of mechanical design and the ease of installation, the proposed passive control concept has a potential application to deepwater marine risers and tall structures in a wind environment.
F. Daviaud - One of the best experts on this subject based on the ideXlab platform.
-
Streamwise vortices in plane Couette flow
Physics of Fluids, 1995Co-Authors: Olivier Dauchot, F. DaviaudAbstract:Experimental observations of various flows have led to the conclusion of the existence of streamwise vortices involved in the destabilization process of these flows. In the plane Couette flow, a linear shear flow, such structures have never been observed, because of the linear stability of the flow. The flow was slightly modified by introducing a wire in its central plane, parallel to the Spanwise Direction. A destabilization then occurs. It generates streamwise structures periodically spaced in the Spanwise Direction. These structures have been identified as pairs of counter‐rotating streamwise vortices. This Letter characterizes the dependence on the Reynolds number of the behavior of these vortices and how their destabilization leads to turbulence.
Denis Sipp - One of the best experts on this subject based on the ideXlab platform.
-
Link between subsonic stall and transonic buffet on swept and unswept wings: from global stability analysis to nonlinear dynamics
Journal of Fluid Mechanics, 2021Co-Authors: Frédéric Plante, Julien Dandois, Samir Beneddine, Éric Laurendeau, Denis SippAbstract:This paper examines the three-dimensional cellular patterns appearing on wings in subsonic stall and transonic buffet conditions. Unsteady Reynolds-averaged Navier–Stokes simulations are carried out for three-dimensional infinite swept configurations closed by periodic boundary conditions in the Spanwise Direction. In both flow conditions the occurrence of stall/buffet cells is observed, as well as their convection at a speed proportional to the sweep angle. In transonic buffet conditions, this phenomenon is superimposed to the well-documented two-dimensional buffet instability. These results indicate that the discrepancies between two-dimensional and three-dimensional buffet are caused by the occurrence of buffet cells and that this phenomenon is similar to the one observed at low speed. These phenomena are then studied using global linear stability analysis with the assumption of a periodic flow in the Spanwise Direction. From these analyses a mode coherent with the two-dimensional buffet is obtained, as well as a mode coherent with two-dimensional vortex shedding in stall conditions. In addition, in both flow conditions an unstable mode reminiscent of stall/buffet cells is observed.
Tongming Zhou - One of the best experts on this subject based on the ideXlab platform.
-
On the study of vortex-induced vibration of a cylinder with helical strakes
Journal of Fluids and Structures, 2011Co-Authors: Tongming Zhou, Siti Fatin Mohd Razali, Zhiyong Hao, Liang ChengAbstract:Abstract While the effect of helical strakes on suppression of Vortex-Induced Vibrations (VIV) has been studied extensively, the mechanism of VIV mitigation using helical strakes is much less well documented in the literature. In the present study, a rigid circular cylinder of diameter d =80 mm attached with three-strand helical strakes of dimensions of 10 d in pitch and 0.12 d in height was tested in a wind tunnel. It was found that the helical strakes can reduce VIV by about 98%. Unlike the bare cylinder, which experiences lock-in over the reduced velocity in the range of 5–8.5, the straked cylinder does not show any lock-in region. In exploring the mechanism of VIV reduction by helical strakes, measurements in stationary bare and straked cylinder wakes using both a single X-probe at four different Reynolds numbers, i.e. Re=10 240, 20 430, 30 610 and 40 800, and two X-probes with variable separations in the Spanwise Direction at R e=20 430 were conducted. It was found that vortices shed from the straked cylinder are weakened significantly. The dominate frequency varies by about 30% over the range of x / d =10–40 in the streamwise Direction while that differs by about 37.2% of the averaged peak frequency over a length of 3.125 d in the Spanwise Direction. The latter is supported by the phase difference between the velocity signals measured at two locations separated in the Spanwise Direction. The correlation length of the vortex structures in the bare cylinder wake is much larger than that obtained in the straked cylinder wake. As a result, the straked cylinder wake agrees more closely with isotropy than the bare cylinder wake. Flow visualization on the plane perpendicular to the cylinder axis at Reynolds number of about 300 reveals small-scale vortices in the shear layers of the straked cylinder wake. However, these vortices do not roll up and interact with each other to form the well-organized Karman-type vortices. Flow visualization on the plane parallel to the cylinder axis shows vortex dislocation and swirling flow, which should be responsible for the variations of the peak frequency in the streamwise as well as Spanwise Directions.