NDP Riser VIV Model Test With Staggered Buoyancy Elements

2016 ◽  
Author(s):  
Jie Wu ◽  
Halvor Lie ◽  
Yiannis Constantinides ◽  
Rolf J. Baarholm
Keyword(s):  
Aspect Ratio ◽  
Fatigue Damage ◽  
Vortex Shedding ◽  
Conceptual Development ◽  
Ocean Basin ◽  
Primary Objective ◽  
Model Tests ◽  
Aspect Ratios ◽  
Spacing Ratio ◽  
The Impact

The Norwegian Deepwater Program (NDP) has over several years working on conceptual development and design of new steel riser solutions for deep water and harsh environment. For the steel lazy wave riser (SLWR) design, the buoyancy section is a critical segment. When subjected to current, both the buoyancy elements and the riser may experience vortex induced vibrations (VIV), which can lead to fast accumulation of fatigue damage and amplified drag loads. As part of developing robust SLWR, it is crucial to understand the current induced loads and responses on various staggered buoyancy configurations. In order to investigate the interaction of bare pipe section and buoyancy elements and its effect on VIV, hydrodynamic model tests were carried out in MARINTEK’s ocean basin in 2014. The test pipe is 38 m in length and 3 cm in diameter. The length of each buoyancy element 0.15 m and its diameter is 0.15 m. The ratio between buoyancy element diameter and riser diameter is 5. VIV response may depend on the spacing ratio of the length of the buoyancy segments and the gaps between two adjacent buoyancy elements, aspect ratio of the buoyancy element, and coverage percentage of buoyancy elements. Hence these parameters were varied and 6 different staggered buoyancy configurations were tested in uniform flows. The use of strakes to suppress VIV was also evaluated. The primary objective is to determine a range for these parameters that leads to the smallest VIV curvature (and hence an optimal riser fatigue design). The shedding frequency of the bare riser section is significantly higher than the buoyancy element due to its smaller diameter. Therefore, the bare riser section will excite much higher modes. As observed from the present tests, the fatigue damage is dominated by the bare riser component for all of the test configurations. The CF displacement of cases with the highest buoyancy coverage (50%) is often dominated by the vortex shedding of the buoyancy element. The use of stakes can effectively suppress the vortex shedding and leads to lowest fatigue damage in the test. In addition, the non-dimensional frequency of the buoyancy element is low (about 0.087–0.122) due to its small aspect ratios (1/1 and 2/1) in present tests. The vortex shedding of the buoyancy element seems to be weakened when the spacing ratio is larger than 1/1, which is different than earlier tests using buoyancy element with larger aspect ratio (≥5/1). The impact on the SLWR design is evaluated based on both present and other relevant model tests results.

scholarly journals Numerical Computations for Flow Patterns and Force Statistics of Three Rectangular Cylinders

2021 ◽  
Vol 2021 ◽  
pp. 1-12
Author(s):  
Hamid Rahman ◽  
Shams-ul-Islam ◽  
Waqas Sarwar Abbasi ◽  
Raheela Manzoor ◽  
Fazle Amin ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Vortex Shedding ◽  
Lift Coefficient ◽  
Flow Characteristics ◽  
Drag And Lift Coefficients ◽  
Aspect Ratios ◽  
Spacing Ratio ◽  
Drag And Lift ◽  
Rectangular Cylinders

In this work, numerical simulations are performed in order to study the effects of aspect ratio (AR) and Reynolds number (Re) on flow characteristics of three side-by-side rectangular cylinders for fixed spacing ratio ( g ), using the lattice Boltzmann method (LBM). The Reynolds number varies within the range 60 ≤ Re ≤ 180, aspect ratio is between 0.25 and 4, and spacing ratio is fixed at g  = 1.5. The flow structure mechanism behind the cylinders is analyzed in terms of vorticity contour visualization, time-trace analysis of drag and lift coefficients, power spectrum analysis of lift coefficient and variations of mean drag coefficient, and Strouhal number. For different combinations of AR and Re, the flow is characterized into regular, irregular, and symmetric vortex shedding. In regular and symmetric vortex shedding the drag and lift coefficients vary smoothly while reverse trend occurs in irregular vortex shedding. At small AR, each cylinder experiences higher magnitude drag force as compared to intermediate and large aspect ratios. The vortex shedding frequency was found to be smaller at smaller AR and increased with increment in AR.

Influence of nozzle exit geometrical parameters on supersonic jet decay

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Prasanta Kumar Mohanta ◽  
B. T. N. Sridhar ◽  
R. K. Mishra
Keyword(s):  
Aspect Ratio ◽  
Nozzle Exit ◽  
Ambient Air ◽  
Schlieren Image ◽  
Geometrical Parameters ◽  
Image Study ◽  
Aspect Ratios ◽  
Core Length ◽  
Supersonic Core Length ◽  
The Impact

Abstract Experiments and simulations were carried on C-D nozzles with four different exit geometry aspect ratios to investigate the impact of supersonic decay characteristics. Rectangular and elliptical exit geometries were considered for the study with various aspect ratios. Numerical simulations and Schlieren image study were studied and found the agreeable logical physics of decay and spread characteristics. The supersonic core decay was found to be of different length for different exit geometry aspect ratio, though the throat to exit area ratio was kept constant to maintain the same exit Mach number. The impact of nozzle exit aspect ratio geometry was responsible to enhance the mixing of primary flow with ambient air, without requiring a secondary method to increase the mixing characteristics. The higher aspect ratio resulted in better mixing when compared to lower aspect ratio exit geometry, which led to reduction in supersonic core length. The behavior of core length reduction gives the identical signature for both under-expanded and over-expanded cases. The results revealed that higher aspect ratio of the exit geometry produced smaller supersonic core length. The aspect ratio of cross section in divergent section of the nozzle was maintained constant from throat to exit to reduce flow losses.

scholarly journals Indentation of an S2 Floating Ice Sheet in the Brittle Range

1983 ◽  
Vol 4 ◽  
pp. 180-187 ◽  
Author(s):  
B. Michel ◽  
D. Blanchet
Keyword(s):  
Aspect Ratio ◽  
Research Work ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Theory Of Elasticity ◽  
Maximum Pressure ◽  
Shear Cracks ◽  
Aspect Ratios ◽  
Two Parameters ◽  
The Impact

The problem of a floating ice sheet hitting a structure with a vertical face appears to be a simple one but, in fact, has only been solved for a limited number of cases. Research work on this question usually reports on an indentation coefficient which relates the average pressure on the indenter to the uniaxial crushing strength of the ice. Very few tests have been made in the brittle range of ice failure. In this particular area of study, this paper reports on 27 tests that were conducted in a cold-room water basin where controlled S2floating ice sheets were produced with a surface area of 4 × 4 m, three sides being fully restrained and the other, freely float! no, being submitted to the impact of the moving indenter. All tests were carried out at computed indentation rates varying from 0.017 to 0.34 s-1. In this range this ice would normally be considered to act as a brittle material. The thickness of the ice sheets varied from 1.2 to 9.0 cm and the indenter width from 5 cm to 1 m. Overall, the aspect ratio relating these two parameters could be varied from 0.5 to 83.Results have shown that for aspect ratios <5, there was an important oscillatory effect which caused the formation of pi asti fi ed triangles in front of the indenter, increasing its resistance as it would under ductile conditions. Because of this plastification, an extrusion effect appeared in front of the indenter as the broken ice crystals were blown up and down in front of the fast-moving indenter. The theory of plasticity which gives an indentation coefficient of 2.97 seems to apply in this case. Another mode of failure which occurred with aspect ratios 5 was cleavage in the plane of the ice sheet which also gives a higher indentation coefficient for S2ice, but of the same order of magnitude as previously.For intermediate values of the aspect ratio, between 5 and 20, the theory of elasticity used by Michel (1978) seems to apply well. Shear cracks are first formed on both sides of the square indenter and control the maximum pressure when they propagate inside forming big triangles in front of it.Finally, for aspect ratios ~>20, buckling of the ice occurs, either after or at the same time as the formation of wedges, together with a reduction in the indentation coefficient to a value close to that given by the theory of buckling of a truncated 45° wedge with a hinged edge.

Experimental Study on Flow-Induced Vibration of Floating Squared Section Cylinders With Low Aspect Ratio: Part I — Effects of Incidence Angle

2015 ◽  
Author(s):  
Rodolfo T. Gonçalves ◽  
Dênnis M. Gambarine ◽  
Felipe P. Figueiredo ◽  
Fábio V. Amorim ◽  
André L. C. Fujarra
Keyword(s):  
Aspect Ratio ◽  
Incidence Angle ◽  
Ocean Basin ◽  
Circular Cylinders ◽  
Structural Damping ◽  
Flow Induced Vibration ◽  
Double Frequency ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Elastically Supported

Experiments regarding flow-induced vibration on floating squared section cylinders with low aspect ratio were carried out in an ocean basin with rotating-arm apparatus. The floating squared section cylinders were elastically supported by a set of linear springs to provide low structural damping to the system. Three different aspect ratios were tested, namely L/D = 1.0, 2.0 and 3.0, and two different incidence angles, namely 0 and 45 degrees. The aims were to understanding the flow-induced vibration around single columns of multi-column platforms, such as semi-submersible and TLP. VIV on circular cylinders were also carried out to compare the results. The range of Reynolds number covered was 2,000 < Re < 27,000. The in-line and transverse amplitude results showed to be higher for 45-degree incidence compared with 0-degree, but the maximum amplitudes for squared section cylinders were lower compared with the circular ones. The double frequency in the in-line motion was not verified as in circular cylinders. The yaw amplitudes cannot be neglected for squared section cylinders, maximum yaw amplitudes around 10 degrees were observed for reduced velocities up to 15.

scholarly journals Characterization of Stereolithography Printed Soft Tooling for Micro Injection Molding

Micromachines ◽  
2020 ◽  
Vol 11 (9) ◽  
pp. 819 ◽  
Author(s):  
Daniel Dempsey ◽  
Sean McDonald ◽  
Davide Masato ◽  
Carol Barry
Keyword(s):  
Aspect Ratio ◽  
Injection Molding ◽  
Raw Materials ◽  
Tool Material ◽  
Mold Temperature ◽  
Micro Injection Molding ◽  
Digital Light Processing ◽  
Electron Microscopies ◽  
Aspect Ratios ◽  
The Impact

The use of microfeature-enabled devices, such as microfluidic platforms and anti-fouling surfaces, has grown in both potential and application in recent years. Injection molding is an attractive method of manufacturing these devices due to its excellent process throughput and commodity-priced raw materials. Still, the manufacture of micro-structured tooling remains a slow and expensive endeavor. This work investigated the feasibility of utilizing additive manufacturing, specifically a Digital Light Processing (DLP)-based inverted stereolithography process, to produce thermoset polymer-based tooling for micro injection molding. Inserts were created with an array of 100-μm wide micro-features, having different heights and thus aspect ratios. These inserts were molded with high flow polypropylene to investigate print process resolution capabilities, channel replication abilities, and insert wear and longevity. Samples were characterized using contact profilometry as well as optical and scanning electron microscopies. Overall, the inserts exhibited a maximum lifetime of 78 molding cycles and failed by cracking of the entire insert. Damage was observed for the higher aspect ratio features but not the lower aspect ratio features. The effect of the tool material on mold temperature distribution was modeled to analyze the impact of processing and mold design.

Experimental Study on Flow-Induced Vibration of Floating Squared Section Cylinders With Low Aspect Ratio: Part II — Effects of Rounded Edges

2016 ◽  
Author(s):  
Rodolfo T. Gonçalves ◽  
Dennis M. Gambarine ◽  
Aline M. Momenti ◽  
Felipe P. Figueiredo ◽  
André L. C. Fujarra
Keyword(s):  
Aspect Ratio ◽  
Reynolds Numbers ◽  
Ocean Basin ◽  
Separation Point ◽  
Flow Induced Vibration ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Significant Difference ◽  
Flow Induced Vibrations ◽  
Elastically Supported

Experiments regarding flow-induced vibration on floating rounded squared section cylinders with low aspect ratio were carried out in an ocean basin equipped with a rotating-arm apparatus. Floating squared section cylinders with rounded edges and aspect ratios of L/D = 2.0 were elastically supported by a set of linear springs in order to provide low structural damping to the system. Two different incidence angles were tested, namely 0 and 45 degrees. The Reynolds numbers covered the range from 2,000 to 30,000. The aim was to understand the flow-induced vibrations around single columns, gathering information for further understanding the causes for the Vortex-Induced Motions in semi-submersible and TLP platforms. Experiments on circular and squared sections cylinders (without rounded edges) were also carried out to compare the results with the rounded square section cylinders (with rounded edges). The amplitude results for in-line, transverse and yaw amplitude for 0-degree models showed to be higher for squared section cylinders compared to those for the rounded square section cylinders. No significant difference between the 45-degree models was observed. The results of ratio between frequency of motion in the transverse direction and natural frequency in still water confirmed the vortex-induced vibration behavior for the squared and rounded square section cylinders for 45-degree incidence; and also the galloping characteristics for 0-degree incidence cases. The rounded effect on the square section cylinders showed to be important only for reduced velocity larger than 8, which is probably related to the position of the separation point that changes around the rounded edge, behavior that did not occurr for the squared edge that fixed the separation point for any reduced velocity.

scholarly journals Study on Strain Rate Effect and Size Effect of Dynamic Response Characteristics of Granite

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Zhihang Hu ◽  
Yuying Ning ◽  
Jiuyang Zhang ◽  
Jianyu Zhao
Keyword(s):  
Aspect Ratio ◽  
Strain Rate ◽  
Split Hopkinson Pressure Bar ◽  
Impact Load ◽  
Hopkinson Pressure Bar ◽  
Aspect Ratios ◽  
Rate Of Increase ◽  
Density Index ◽  
The Impact ◽  
Time Density

Under impact load, the dynamic mechanical properties of rock are complex and changeable. The Split Hopkinson Pressure Bar (SHPB) system was used to change the impact load to carry out different strain rate loading tests on granite with different aspect ratios, and to analyze the influence of strain rate and aspect ratio on the dynamic energy consumption of granite crushing. The results show that at an impact velocity of 14 m/s, the granite with an aspect ratio of 1.4 appears to be strip-shaped fragments after being broken; the granite with an aspect ratio of 1.0 uniform square fragments after being broken; the granite with an aspect ratio of 0.6 appears to be a large number of flat fragments after being broken. When the load strain rate of the granite with an aspect ratio of 0.6 increases from 50 s-1 to 150 s-1, the energy-time density index increases significantly; when the load strain rate exceeds 150 s-1, the energy-time density index decreases. When the strain rate of granite with an aspect ratio of 1.0 exceeds 80 s-1, the energy-time density increases significantly. When the strain rate of the granite with an aspect ratio of 1.4 exceeds 60 s-1, the rate of increase of the energy-time density of the rock increases significantly.

Wake Flow and Vortex Shedding Patterns Behind Rotating Finite Length Cylinders

2019 ◽  
Author(s):  
Amber Donaldson ◽  
John C. Vaccaro ◽  
David M. Rooney
Keyword(s):  
Aspect Ratio ◽  
Vortex Shedding ◽  
Rotational Speed ◽  
Maximum Velocity ◽  
Velocity Profiles ◽  
Wake Flow ◽  
Circular Cylinders ◽  
Mean Velocity ◽  
Constant Length ◽  
Aspect Ratios

Abstract An experimental wind tunnel study was performed to assess the effect of aspect ratio and rotational speed of circular cylinders of varying diameter on the flow patterns behind the cylinders in the presence of a uniform upstream crossflow. Six circular cylinders of constant length but different diameters, producing aspect ratios 6 ≤ AR ≤ 32 were examined at a single upstream velocity such that the Reynolds number varied between 1920 ≤ Re ≤ 10240. Rotational speeds from stationary up to 3600 rpm were applied to the cylinders, so that the maximum relative velocity α = πfD/U∞ = 0.80. Mean velocity profiles were measured three diameters downstream of the cylinder axis at 6 equidistant locations, and PSD power spectral density were generated for 26 equidistant locations along the cylinder, to create a comprehensive record of spanwise variations under all rotational conditions. For the highest aspect ratio tested, the wake velocity profiles were independent of rotational speed at all spanwise locations, whereas at lower aspect ratios, the maximum velocity defect diminished with increasing rotational speed along most of the span and became asymmetric near the free end. Two distinct shedding cells were found only for a cylinder with an aspect ratio of twelve at three relative spin rates of 0.067, 0.27, and 0.4. In cases where only a single cell existed, increased rotational speed produced a higher vortex shedding frequency on a given aspect ratio cylinder.

scholarly journals Indentation of an S2 Floating Ice Sheet in the Brittle Range

1983 ◽  
Vol 4 ◽  
pp. 180-187 ◽  
Author(s):  
B. Michel ◽  
D. Blanchet
Keyword(s):  
Aspect Ratio ◽  
Research Work ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Theory Of Elasticity ◽  
Maximum Pressure ◽  
Shear Cracks ◽  
Aspect Ratios ◽  
Two Parameters ◽  
The Impact

The problem of a floating ice sheet hitting a structure with a vertical face appears to be a simple one but, in fact, has only been solved for a limited number of cases. Research work on this question usually reports on an indentation coefficient which relates the average pressure on the indenter to the uniaxial crushing strength of the ice. Very few tests have been made in the brittle range of ice failure. In this particular area of study, this paper reports on 27 tests that were conducted in a cold-room water basin where controlled S2 floating ice sheets were produced with a surface area of 4 × 4 m, three sides being fully restrained and the other, freely float! no, being submitted to the impact of the moving indenter. All tests were carried out at computed indentation rates varying from 0.017 to 0.34 s-1. In this range this ice would normally be considered to act as a brittle material. The thickness of the ice sheets varied from 1.2 to 9.0 cm and the indenter width from 5 cm to 1 m. Overall, the aspect ratio relating these two parameters could be varied from 0.5 to 83.Results have shown that for aspect ratios <5, there was an important oscillatory effect which caused the formation of pi asti fi ed triangles in front of the indenter, increasing its resistance as it would under ductile conditions. Because of this plastification, an extrusion effect appeared in front of the indenter as the broken ice crystals were blown up and down in front of the fast-moving indenter. The theory of plasticity which gives an indentation coefficient of 2.97 seems to apply in this case. Another mode of failure which occurred with aspect ratios 5 was cleavage in the plane of the ice sheet which also gives a higher indentation coefficient for S2 ice, but of the same order of magnitude as previously.For intermediate values of the aspect ratio, between 5 and 20, the theory of elasticity used by Michel (1978) seems to apply well. Shear cracks are first formed on both sides of the square indenter and control the maximum pressure when they propagate inside forming big triangles in front of it.Finally, for aspect ratios ~>20, buckling of the ice occurs, either after or at the same time as the formation of wedges, together with a reduction in the indentation coefficient to a value close to that given by the theory of buckling of a truncated 45° wedge with a hinged edge.

Vortex Shedding From Low Aspect Ratio Dual Step Cylinders

2012 ◽  
Author(s):  
Chris Morton ◽  
Serhiy Yarusevych
Keyword(s):  
Aspect Ratio ◽  
Vortex Shedding ◽  
Small Diameter ◽  
Dominant Frequency ◽  
Cylinder Wake ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Half Loop ◽  
Large Cylinder ◽  
Small Cylinder

A low aspect ratio dual-step cylinder is comprised of two cylinders of different diameters (D and d). The large diameter cylinder (D) with low aspect ratio (L/D) is attached to the mid-span of a small diameter cylinder (d). This geometry is relevant to many engineering applications, e.g., finned-tube heat exchangers, underwater cables, and cylindrical support structures. The present study investigates the effect of Reynolds number (ReD) and L/D on dual step cylinder wake development for 1050 ≤ ReD ≤ 2100, D/d = 2, and 0.2 < L/D ≤ 3. Experiments have been performed in a water flume facility utilizing flow visualization, Laser Doppler Velocimetry (LDV), and Particle Image Velocimetry (PIV). The results show that vortex shedding occurs from the large and small diameter cylinders at distinct frequencies for L/D ≥ 1 & ReD = 2100 and L/D ≥ 2& ReD = 1050. At these higher aspect ratios investigated, large cylinder vortices predominantly form closed vortex loops in the wake and small cylinder vortices form half-loop vortex connections. In contrast, at lower aspect ratios, vortex shedding from the large cylinder ceases, with the dominant frequency centred-activity in the large cylinder wake attributed to the passage of vortex filaments connecting small cylinder vortices. The presence of the large cylinder distorts the vortex filaments causing cyclic vortex dislocations accompanied by the formation of half-loop vortex connections. Increasing L/D decreases the frequency of occurrence of vortex dislocations and increases the dominant frequency in the large cylinder wake. The results also show that the Reynolds number has a substantial effect on wake vortex shedding frequency, which is more profound than that expected for a uniform cylinder.

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