The Experts below are selected from a list of 4293 Experts worldwide ranked by ideXlab platform
Jørgen Fredsøe - One of the best experts on this subject based on the ideXlab platform.
-
Time scale of scour around a large vertical cylinder in waves
2002Co-Authors: B. Mutlu Sumer, Jørgen FredsøeAbstract:This paper presents the results of an experimental work on the time scale of scour process around a large vertical circular cylinder exposed to a progressive wave. The flow is in the unseparated flow regime; the Keulegan-Carpenter Number is KC < O(1), and the diffraction effect exists. The time scale is mainly a function of the Keulegan-Carpenter Number, the Shields parameter and the diffraction parameter, D/L (D being the cylinder diameter, and L the wave length). Although the size of the data is somewhat limited, the results seem to give a consistent picture. It is found that the time scale decreases with the Shields parameter, and increases with KC. The range of the diffraction parameter, D/L, in the present experiments is not very broad, however, to resolve the D/L dependence.
-
Onset of scour below pipelines and self-burial
Coastal Engineering, 2001Co-Authors: B. M. Sumer, Christoffer Truelsen, T. Sichmann, Jørgen FredsøeAbstract:Abstract This paper summarizes the results of an experimental study on the onset of scour below and self-burial of pipelines in currents/waves. Pressure was measured on the surface of a slightly buried pipe at two points, one at the upstream side and the other at the downstream side of the pipe, both in the sand bed. The latter enabled the pressure gradient (which drives a seepage flow underneath the pipe) to be calculated. The results indicated that the excessive seepage flow and the resulting piping are the major factor to cause the onset of scour below the pipeline. The onset of scour occurred always locally (but not along the length of the pipeline as a two-dimensional process). The critical condition corresponding to the onset of scour was determined both in the case of currents and in the case of waves. Once the scour breaks out, it will propagate along the length of the pipeline, scour holes being interrupted with stretches of soil (span shoulders) supporting the pipeline. As the span shoulder gets shorter and shorter, more and more weight of the pipeline is exerted on the soil. In this process, a critical point is reached where the bearing capacity of the soil is exceeded (general shear failure). At this point, the pipe begins to sink at the span shoulder (self-burial). It was found that the self-burial depth is governed mainly by the Keulegan–Carpenter Number. The time scale of the self-burial process, on the other hand, is governed by the Keulegan–Carpenter Number and the Shields parameter. Diagrams are given for the self-burial depth and the time scale of the self-burial process.
-
Scour at the head of a vertical-wall breakwater
Coastal Engineering, 1997Co-Authors: B. M. Sumer, Jørgen FredsøeAbstract:Abstract This paper presents the results of an experimental investigation on the near-bed flow patterns, the bed shear stress amplification and scour around the head of a vertical-wall breakwater, using regular waves. The Keulegan-Carpenter Number (KC), based on the diameter of the breakwater head, is found to be the major parameter that governs the flow and the equilibrium scour depth. Basic flow structures are identified as function of KC. The scour depth is found to increase with increasing the Keulegan-Carpenter Number. The necessary extent of the conventional stone protection is studied. An empirical formula is worked out for the width of the protection layer as function of KC. Also, the effects of head shape, the angle of attack and the presence of a co-directional current are investigated. The results indicate that the scour depth is increased considerably in the presence of a current. Likewise, the scour depth is increased when the head shape is changed from a round shape to a sharp-edged one. It is found that the angle of attack is also an influencing factor as regards the scour depth.
-
Scour Around Vertical Pile in Waves
Journal of Waterway Port Coastal and Ocean Engineering, 1992Co-Authors: B. Mutlu Sumer, Jørgen Fredsøe, Niels ChristiansenAbstract:This paper presents the results of an experimental investigation on scour around piles exposed to waves. In addition to the actual scour tests, bed shear‐stress measurements and a flow visualization study are carried out. The effects of lee wake and horseshoe vortex are demonstrated to be the two key elements in the scour process. The development of these flow structures mainly depends on the Keulegan‐Carpenter (KC) Number that hereby becomes the main parameter that governs the equilibrium scour depth on a live bed. Based on the present data, a design equation is established, relating the scour depth to the Keulegan‐Carpenter Number. For the values of the Keulegan‐Carpenter Number below six, the scour around the pile practically ceases to exist. The scour depth normalized by the pile diameter is found to increase with increasing Keulegan‐Carpenter Number and approaches its steady‐current value for Keulegan‐Carpenter Numbers above approximately 100.
-
Onset Of Scour Below A Pipeline Exposed To Waves
International Journal of Offshore and Polar Engineering, 1991Co-Authors: B. M. Sumer, Jørgen FredsøeAbstract:Experiments are carried out in a wave flume to investigate whether tunnel erosion will take place or otherwise. The main parameters studied are the initial burial depth and the Keulegan-Carpenter Number, KC. The results indicate that the critical burial depth beyond which no scour occurs is a function of KC Number. The larger the KC Number, the larger the critical burial depth. Also presented in the paper are the results of bed shear-stress measurements in the vicinity of a pipeline exposed to an oscillatory flow as well as the pressure distributions around a pipeline mounted on the bed. The latter information shed light onto the mechanism responsible for the onset of scour in waves.
Ladislav Skrbek - One of the best experts on this subject based on the ideXlab platform.
-
Experiments relating to the flow induced by a vibrating quartz tuning fork and similar structures in a classical fluid.
Physical Review E, 2010Co-Authors: D. Schmoranzer, M. Král’ová, V. Pilcová, W. F. Vinen, Ladislav SkrbekAbstract:We report on an experimental study of the behavior of a Number of commercially available quartz tuning forks oscillating in a classical cryogenic fluid, in the form of either liquid helium I or gaseous helium, extending our previous studies [M. Blazkova et al. Phys. Rev. E 75, 025302 (2007)]. Measurements of the damping of the oscillations allowed us to deduce the drag on the prong of a fork, as a function of the velocity with which the prong moves, for various sizes of fork and various oscillation frequencies. Transitions to turbulent flow have been identified, and the dependence of the critical velocity, expressed as a dimensionless critical Keulegan-Carpenter Number, on the dimensionless Stokes Number has been established. These measurements have not allowed us to visualize the flow, so we have carried out visualization experiments with oscillating rods in water, the rod dimensions, and the frequencies of oscillation, being chosen so that the relevant dimensionless parameters are similar to those for the prongs of the forks. Some information about the nature of the instability that leads to turbulence has been obtained in this way, and the results for the critical Keulegan-Carpenter Number for the rods in water have been compared with values for the tuning forks in a cryogenic fluid.
-
Experiments relating to the flow induced by a vibrating quartz tuning fork and similar structures in a classical fluid.
Physical review. E Statistical nonlinear and soft matter physics, 2010Co-Authors: D. Schmoranzer, V. Pilcová, W. F. Vinen, M Král'ová, Ladislav SkrbekAbstract:We report on an experimental study of the behavior of a Number of commercially available quartz tuning forks oscillating in a classical cryogenic fluid, in the form of either liquid helium I or gaseous helium, extending our previous studies [M. Blazkova Phys. Rev. E 75, 025302 (2007)]. Measurements of the damping of the oscillations allowed us to deduce the drag on the prong of a fork, as a function of the velocity with which the prong moves, for various sizes of fork and various oscillation frequencies. Transitions to turbulent flow have been identified, and the dependence of the critical velocity, expressed as a dimensionless critical Keulegan-Carpenter Number, on the dimensionless Stokes Number has been established. These measurements have not allowed us to visualize the flow, so we have carried out visualization experiments with oscillating rods in water, the rod dimensions, and the frequencies of oscillation, being chosen so that the relevant dimensionless parameters are similar to those for the prongs of the forks. Some information about the nature of the instability that leads to turbulence has been obtained in this way, and the results for the critical Keulegan-Carpenter Number for the rods in water have been compared with values for the tuning forks in a cryogenic fluid.
B. M. Sumer - One of the best experts on this subject based on the ideXlab platform.
-
Onset of scour below pipelines and self-burial
Coastal Engineering, 2001Co-Authors: B. M. Sumer, Christoffer Truelsen, T. Sichmann, Jørgen FredsøeAbstract:Abstract This paper summarizes the results of an experimental study on the onset of scour below and self-burial of pipelines in currents/waves. Pressure was measured on the surface of a slightly buried pipe at two points, one at the upstream side and the other at the downstream side of the pipe, both in the sand bed. The latter enabled the pressure gradient (which drives a seepage flow underneath the pipe) to be calculated. The results indicated that the excessive seepage flow and the resulting piping are the major factor to cause the onset of scour below the pipeline. The onset of scour occurred always locally (but not along the length of the pipeline as a two-dimensional process). The critical condition corresponding to the onset of scour was determined both in the case of currents and in the case of waves. Once the scour breaks out, it will propagate along the length of the pipeline, scour holes being interrupted with stretches of soil (span shoulders) supporting the pipeline. As the span shoulder gets shorter and shorter, more and more weight of the pipeline is exerted on the soil. In this process, a critical point is reached where the bearing capacity of the soil is exceeded (general shear failure). At this point, the pipe begins to sink at the span shoulder (self-burial). It was found that the self-burial depth is governed mainly by the Keulegan–Carpenter Number. The time scale of the self-burial process, on the other hand, is governed by the Keulegan–Carpenter Number and the Shields parameter. Diagrams are given for the self-burial depth and the time scale of the self-burial process.
-
Scour at the head of a vertical-wall breakwater
Coastal Engineering, 1997Co-Authors: B. M. Sumer, Jørgen FredsøeAbstract:Abstract This paper presents the results of an experimental investigation on the near-bed flow patterns, the bed shear stress amplification and scour around the head of a vertical-wall breakwater, using regular waves. The Keulegan-Carpenter Number (KC), based on the diameter of the breakwater head, is found to be the major parameter that governs the flow and the equilibrium scour depth. Basic flow structures are identified as function of KC. The scour depth is found to increase with increasing the Keulegan-Carpenter Number. The necessary extent of the conventional stone protection is studied. An empirical formula is worked out for the width of the protection layer as function of KC. Also, the effects of head shape, the angle of attack and the presence of a co-directional current are investigated. The results indicate that the scour depth is increased considerably in the presence of a current. Likewise, the scour depth is increased when the head shape is changed from a round shape to a sharp-edged one. It is found that the angle of attack is also an influencing factor as regards the scour depth.
-
Onset Of Scour Below A Pipeline Exposed To Waves
International Journal of Offshore and Polar Engineering, 1991Co-Authors: B. M. Sumer, Jørgen FredsøeAbstract:Experiments are carried out in a wave flume to investigate whether tunnel erosion will take place or otherwise. The main parameters studied are the initial burial depth and the Keulegan-Carpenter Number, KC. The results indicate that the critical burial depth beyond which no scour occurs is a function of KC Number. The larger the KC Number, the larger the critical burial depth. Also presented in the paper are the results of bed shear-stress measurements in the vicinity of a pipeline exposed to an oscillatory flow as well as the pressure distributions around a pipeline mounted on the bed. The latter information shed light onto the mechanism responsible for the onset of scour in waves.
J.c. Wilson - One of the best experts on this subject based on the ideXlab platform.
-
tuned liquid dampers with a keulegan Carpenter Number dependent screen drag coefficient
Journal of Fluids and Structures, 2013Co-Authors: J.a. Hamelin, J.s. Love, Michael Tait, J.c. WilsonAbstract:Abstract The amplitude-dependent damping associated with a tuned liquid damper (TLD) equipped with slat-type screens produces a device that performs optimally at a targeted response amplitude. Increasing the slat height produces a screen whose drag coefficient is dependent on the Keulegan–Carpenter Number (KC), which may improve the TLD performance. This new type of TLD is modeled as an equivalent mechanical model with damping that is dependent on both KC and the response amplitude. An experimental shake table testing program is undertaken to study the influence of KC on the TLD response and to validate the model. A power fit is performed on the experimentally determined screen drag coefficient and KC values to express the drag coefficient as a function of KC and the steady flow drag coefficient. Predicted frequency response plots of sloshing forces and energy dissipation per cycle are in agreement with experimental results. A structure–TLD system model is developed to theoretically study the performance of this new TLD. Nonlinear shallow water wave theory is used to validate the output of the mechanical model. Results indicate that a KC-dependent screen drag coefficient produces a more robust TLD whose performance is maintained over a broader range of structural response amplitudes.
-
Tuned liquid dampers with a Keulegan–Carpenter Number-dependent screen drag coefficient
Journal of Fluids and Structures, 2013Co-Authors: J.a. Hamelin, J.s. Love, Michael Tait, J.c. WilsonAbstract:Abstract The amplitude-dependent damping associated with a tuned liquid damper (TLD) equipped with slat-type screens produces a device that performs optimally at a targeted response amplitude. Increasing the slat height produces a screen whose drag coefficient is dependent on the Keulegan–Carpenter Number (KC), which may improve the TLD performance. This new type of TLD is modeled as an equivalent mechanical model with damping that is dependent on both KC and the response amplitude. An experimental shake table testing program is undertaken to study the influence of KC on the TLD response and to validate the model. A power fit is performed on the experimentally determined screen drag coefficient and KC values to express the drag coefficient as a function of KC and the steady flow drag coefficient. Predicted frequency response plots of sloshing forces and energy dissipation per cycle are in agreement with experimental results. A structure–TLD system model is developed to theoretically study the performance of this new TLD. Nonlinear shallow water wave theory is used to validate the output of the mechanical model. Results indicate that a KC-dependent screen drag coefficient produces a more robust TLD whose performance is maintained over a broader range of structural response amplitudes.
John R. Chaplin - One of the best experts on this subject based on the ideXlab platform.
-
Hydrodynamic damping of the vertical motion of a horizontal cylinder beneath waves at large scale
Journal of Fluids and Structures, 2001Co-Authors: John R. Chaplin, C.h. RetzlerAbstract:Large-scale laboratory measurements are presented of the hydrodynamic damping of the vertical oscillations of a circular cylinder beneath waves, where the wave crests are parallel to the cylinder axis. In two series of tests, the Stokes parameter ? was 647 000 and 997 000, and the Keulegan Carpenter Number of the cylinder motion was in the range 0·01–0·1. Observations of the decaying motion of the cylinder, and of steady-state oscillations generated by continuous force excitation, were in reasonable agreement with the vector form of the relative velocity Morison equation with constant coefficients. In some conditions, the motion of the cylinder became phase locked to the wave-induced flow, at an integer frequency ratio.
-
Hydrodynamic damping of a cylinder at β 106
Journal of Fluids and Structures, 2000Co-Authors: John R. ChaplinAbstract:This paper describes delicate, but large-scale, experiments aimed at measuring the hydrodynamic damping of a circular cylinder oscillating in still water and transversely in a current. Attention is concentrated on the regime of very small Keulegan–Carpenter Numbers, in which the drag coefficient is inversely proportional to the Keulegan–Carpenter Number. Measurements in still water at ?=650 000 and 1250 000 point to drag coefficients about twice those appropriate to two-dimensional laminar flow, in common with earlier measurements at ??105. In the presence of a slowly varying transverse current (generated by placing the cylinder at the node of standing waves of long period), the damping increased with the reduced velocity of the ambient flow at a rate that increased with the Reynolds Number.
-
Damping Of A Vertical Cylinder Oscillating In Still Water
1991Co-Authors: K Hayashi, John R. ChaplinAbstract:To estimate the unknown damping which is important in the study of dynamic response of the offshore structures to the wave forces, the damping of a circular cylinder oscillating in various depth of still water has been investigated using a rigid cylindrical pendulum. The damping factor is constant and independents range of low amplitude, and its value is explained theoretically by using the Stokes' solution introduced to explain the viscous effect of cylinder at low Keulegan-Carpenter Number. At large amplitude, the damping factor becomes amplitude dependent and the drag coefficient associated with damping factor corresponds well the drag coefficients obtained for the circular cylinder in planar oscillatory flow at Keulegan-Carpenter Number above 2.