Flow-Induced Vibrations of Riser Array System

2016 ◽  
Author(s):  
Vaibhav Joshi ◽  
Bin Liu ◽  
Rajeev K. Jaiman
Keyword(s):  
Dynamic Response ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Wake Flow ◽  
Stream Velocity ◽  
Elastic Instability ◽  
Short Term ◽  
Vortex Induced Vibration ◽  
Wake Interference ◽  
Flow Induced Vibrations

When a riser array system is subjected to a uniform flow, an unstable flow-induced vibration commonly occurs among cylinders, generally called fluid-elastic instability. It can cause long-term or short-term damage to the riser array system. A numerical investigation has been performed in the present study. Generally, flow-induced vibrations include vortex-induced vibration (VIV), wake-induced vibration (WIV), jet switching, turbulent buffeting and fluid-elastic instability. The dynamic interactions among the fluid-induced vibrations, wake interference and proximity interference pose difficulties in the design and operation of the riser array system. The dynamics of a riser array system is very different from that of basic canonical configurations such as side-by-side, tandem and staggered arrangements. In a riser array system, the interferences come from all possible nearby constituent risers. There is a synchronization phenomenon among the cylinders, which may lead to detrimental collisions and short-term failures. It is known that the vortex-induced vibration (VIV) of an isolated circular cylinder is self-limiting. An extensive vibration occurs in the lock-in region within which the frequency of the vortex shedding matches the structural frequency of the immersed structure. In a riser array system, there is a point at which the vibration of cylinder suddenly increases. The vibration of the constituent risers increases without bound with the increment of the free-stream velocity. This free-stream velocity is defined as the critical velocity. The interference not only comes from the inline and cross-flow directions, but also the wake interference from the diagonal upstream risers. In a riser array system, each riser vibrates independently. However, there is symmetry of frequency spectrum observed about the inline direction along the middle row of the risers. In this study, the dynamic response of the different risers in the array system is investigated with the help of the amplitude response results from the canonical arrangements (side-by-side and tandem) and wake flow structures. The long top-tensioned riser system can be idealized by two-dimensional elastically mounted cylinders to solve the complex fluid-structure interaction problem. The dynamic response of a typical riser array system has been analyzed at low and high Reynolds number. It is encouraging to see that the results reported in the present investigation can provide useful insight and suggestions in the design and optimization of riser systems to avoid collisions and various long-or short-term failures.

Control of global instability in a non-parallel near wake

2000 ◽  
Vol 404 ◽  
pp. 345-378 ◽  
Author(s):  
TZONG-SHYNG LEU ◽  
CHIH-MING HO
Keyword(s):  
Free Stream ◽  
Stream Water ◽  
Parallel Flow ◽  
Free Stream Velocity ◽  
Splitter Plate ◽  
Wake Flow ◽  
Stream Velocity ◽  
Global Instability ◽  
Unstable Flow ◽  
Trailing Edge

The effect of base suction on a plane wake was found to produce significant changes in wake dynamics. The wake is produced by merging two boundary layers from the trailing edge of a splitter plate in a two-stream water tunnel. A threshold suction speed exists which is approximately equal to half of the free-stream velocity. If the suction speed is below the threshold, the wake flow is unstable. If the suction speed is above the threshold, the wake becomes stable and no vortex shedding is observed. In the present experiment, the suction technique can stabilize a wake at a maximum tested Reynolds number of 2000.The suction significantly reduces the length of the absolutely unstable region in the immediate vicinity of the trailing edge of the splitter plate and produces a non-parallel flow pattern, resulting in the breakdown of global instability. The global growth rate changes from positive (unstable flow) to negative (stable flow) at the suction speed equalling 0.46 of the free-stream velocity. The threshold suction speed can be accurately predicted by the global linear theory of Monkewitz et al. (1993) with a non-parallel flow correction.

Effect of Free-Stream Velocity Definition on Boundary Layer Thickness and Losses in Centrifugal Compressors

2017 ◽  
Author(s):  
Jonna Tiainen ◽  
Ahti Jaatinen-Värri ◽  
Aki Grönman ◽  
Teemu Turunen-Saaresti ◽  
Jari Backman
Keyword(s):  
Boundary Layer ◽  
Layer Thickness ◽  
Centrifugal Compressor ◽  
Boundary Layer Thickness ◽  
Free Stream ◽  
Compressor Blade ◽  
Free Stream Velocity ◽  
Wake Flow ◽  
Stream Velocity ◽  
Centrifugal Compressors

The estimation of boundary layer losses requires the accurate specification of the free-stream velocity, which is not straightforward in centrifugal compressor blade passages. This challenge stems from the jet-wake flow structure, where the free-stream velocity between the blades cannot be clearly specified. In addition, the relative velocity decreases due to adverse pressure gradient. Therefore, the common assumption of a single free-stream velocity over the blade surface might not be valid in centrifugal compressors. Generally in turbomachinery, the losses in the blade cascade boundary layers are estimated e.g. with different loss co-efficients, but they often rely on the assumption of a uniform flow field between the blades. To give guidelines for the estimation of the mentioned losses in highly distorted centrifugal compressor flow fields, this paper discusses the difficulties in the calculation of the boundary layer thickness in the compressor blade passages, compares different free-stream velocity definitions, and demonstrates their effect on estimated boundary layer losses. Additionally, a hybrid method is proposed to overcome the challenges of defining a boundary layer in centrifugal compressors.

The Wake Dynamics Behind a Near-Wall Square Cylinder

2021 ◽  
Author(s):  
Samuel Addai ◽  
Xingjun Fang ◽  
Afua A Mante ◽  
Mark F. Tachie
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Wake Flow ◽  
Stream Velocity ◽  
Reynolds Stresses ◽  
Square Cylinder ◽  
Gap Ratio ◽  
The Mean

Abstract Particle image velocimetry is used to experimentally study the wake dynamics behind a near-wall square cylinder subjected to a thick oncoming turbulent boundary layer. The turbulent boundary layer thickness was 3.6 times the cylinder height (h) while the Reynolds number based on the free-stream velocity and the cylinder height was 12750. The gap distance (G) between the bottom face of the cylinder and the wall was varied, resulting in gap ratios (G/h) of 0, 0.3, 0.5, 1.0, 2.0, 4.0 and 8.0. The effects of varying the gap ratio on the mean flow, Reynolds stresses, triple velocity correlation, two-point autocorrelation and the unsteady wake characteristics were examined. The results indicate that as gap ratio decreases, asymmetry in the wake flow becomes more pronounced and the size of the mean separation bubbles increases. The magnitudes of the Reynolds stresses and triple velocity correlations generally decrease with decreasing gap ratio. Moreover, the size of the large-scale structures increases with decreasing gap ratio and the critical gap ratio, below which Kármán vortex shedding suppression occurs, is found to be 0.3. The dominant Strouhal number in the wake flow expressed in terms of the streamwise mean velocity at the cylinder vertical midpoint increases as gap ratio decreases while that based on the free-stream velocity is less sensitive to gap ratio for the offset cases (G/h > 0).

scholarly journals Boundary-Layer Type Solutions for Initial Platelet Activation and Deposition

2002 ◽  
Vol 4 (2) ◽  
pp. 95-108 ◽  
Author(s):  
T. David ◽  
P. G. de Groot ◽  
P. G. Walker
Keyword(s):  
Platelet Activation ◽  
Peclet Number ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Bulk Fluid ◽  
Péclet Number ◽  
Activated Platelets ◽  
Activation Rate ◽  
Initial Adhesion

This paper presents, on the basis of high Peclet number, a mathematical model for the activation and initial adhesion of flowing platelets onto a surface. In contrast to past work, the model is applicable to general 2D and axi-symmetric flows where the wall shear stress is knowna priori. Results indicate that for high activation reaction rates there exist two layers, one containing only activated platelets and the other both activated and non-activated platelets. Fundamental relationships are proposed between the adhesion rate of platelets to the surface and the characteristic parameters of Peclet number and Reynolds number. Activation in the bulk fluid (blood) is characterised by the Damkohler number, which is a function of activation rate and the free-stream velocity. It is shown that, as the free-stream velocity varies, there exists a maximum of activated platelet flux to the wall for particular values of the velocity. These values, at which the maximum occur, are themselves functions of the platelet activation rate. As the free-stream velocity increases the activation of platelets ceases altogether and adhesion is reduced to a very small value strengthening the hypothesis of the correlation between atherogenesis/thrombogenesis and areas of low shear.

Free Convective Couette flow1of1a1Dusty1Fluid1through1a Porous Medium1with1Periodic1Permeability1in1the1Absence1of Electrically Magneto Hydro Dynamic Model

2021 ◽  
Vol 58 (2) ◽  
pp. 6072-6083
Author(s):  
K. Rajesh, A. Govindarajan, M. Vidhya
Keyword(s):  
Flow Field ◽  
Dynamic Model ◽  
Analytical Solutions ◽  
Free Stream ◽  
Porous Plate ◽  
Free Stream Velocity ◽  
Dust Particles ◽  
Stream Velocity

“The purpose of this investigation stands to discuss the effects of periodic permeability on1the; free1convective flow of a dusty viscous; incompressible1fluid through a1highly1porous1channel. The porous1medium is confined by an infinite perpendicular porous plate supercilious the free stream velocity to be uniform. Analytical solutions are gained for the dusty flow field, the1temperature field, the1skin1friction and the rate1of heat1transfer. when there is an increase in mass concentration1of dust1particles, it is found that the1velocity profile of fluid and dust particles reduces.”

scholarly journals Prediction of Heat Transfer From Laminar Boundary Layers, With Emphasis on Large Free-Stream Velocity Gradients and Highly Cooled Walls

1966 ◽  
Vol 88 (3) ◽  
pp. 249-256 ◽  
Author(s):  
L. H. Back ◽  
A. B. Witte
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Shear Stress ◽  
Velocity Gradient ◽  
Free Stream ◽  
Variable Density ◽  
Similarity Solutions ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Gradient Parameter

Laminar boundary-layer heat transfer and shear-stress predictions from existing similarity solutions are extended in an approximate way to perfect gas flows with a large free-stream velocity gradient parameter β and variable density-viscosity product ρμ across the boundary layer resulting from a highly cooled wall. The dimensionless enthalpy gradient at the wall gw′, to which the heat flux is related, is found not to vary appreciably with β. Thus the application of similarity solutions on a local basis to predict heat transfer from accelerated flows to an arbitrary surface may be a reasonable approximation involving a minimum amount of calculation time. Unlike gw′, the dimensionless velocity gradient at the wall fw″, to which the shear stress is related, is strongly dependent on β.

Effects of Partial Slip on Chemically Reactive Solute Distribution in MHD Boundary Layer Stagnation Point Flow Past a Stretching Permeable Sheet

2015 ◽  
Vol 13 (1) ◽  
pp. 29-36 ◽  
Author(s):  
Swati Mukhopadhyay
Keyword(s):  
Boundary Layer ◽  
Differential Equations ◽  
Stagnation Point ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Stagnation Point Flow ◽  
Partial Slip ◽  
Stream Velocity ◽  
Slip Parameter ◽  
Mhd Boundary Layer

Abstract This paper presents the magnetohydrodynamic (MHD) boundary layer stagnation point flow with diffusion of chemically reactive species undergoing first-order chemical reaction over a permeable stretching sheet in presence of partial slip. With the help of similarity transformations, the partial differential equations corresponding to momentum and the concentration equations are transformed into non-linear ordinary differential equations. Numerical solutions of these equations are obtained by shooting method. It is found that the horizontal velocity increases with the increasing value of the ratio of the free stream velocity and the stretching velocity. Velocity decreases with the increasing magnetic parameter when the free-stream velocity is less than the stretching velocity but the opposite behavior is noted when the free-stream velocity is greater than the stretching velocity. Due to suction, fluid velocity decreases at a particular point of the surface. With increasing velocity slip parameter, velocity increases when the free-stream velocity is greater than the stretching velocity. But the concentration decreases in this case. Concentration decreases with increasing mass slip parameter.

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