Effect of the Free Surface on the Drag Forces on a Flat Plate Translating Normal to the Flow

2018 ◽  
Author(s):  
Sukruth Satheesh ◽  
Clément Haëck ◽  
Francisco J. Huera-Huarte
Keyword(s):  
Free Surface ◽  
Aspect Ratio ◽  
Flat Plate ◽  
Reynolds Numbers ◽  
Water Tank ◽  
Drag Forces ◽  
Aspect Ratios ◽  
Series Of Experiments ◽  
Abrupt Rise ◽  
Submergence Depth

A series of experiments were carried out with a flat plate towed normal to the flow in quiescent fluid. The focus was given to the analysis of the drag force seen by the plate as a function of its aspect ratio and hydraulic diameter. The effect of towing the plate near the water free surface was also investigated thoroughly. Plates of aspect ratio ranging from 0.25 to 4 were towed in a still water tank at different Reynolds numbers in the range from 15000 to 60000. Submergence depth was measured from the upper edge to the free surface and varied from zero to the centre of the tank. Forces on the plates were measured using a submersible bending beam load cell and the carriage motion was monitored by a rotary potentiometer. It was found that the drag increases abruptly prior subsiding with increasing submergence depth, with this effect being more dominant in lower aspect ratio plates. The abrupt rise in the drag is due to the interaction of the upper edge of the plate with the free surface resulting in a large shrinkage of the recirculation zone. The non-unit low aspect ratio plates also showed another drag peak around 50% depth, especially at lower speeds. Overall, the trends were Reynolds number independent, except when the aspect ratios was in the range from 0.75 to 1.33 and the plate was near the free surface.

Reynolds-number scaling of vortex pinch-off on low-aspect-ratio propulsors

2016 ◽  
Vol 799 ◽  
Author(s):  
John N. Fernando ◽  
David E. Rival
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Time Scales ◽  
Early Stage ◽  
Reynolds Numbers ◽  
Growth Time ◽  
Flat Plates ◽  
Low Aspect Ratio ◽  
Drag Forces ◽  
Aspect Ratios

Impulsively started, low-aspect-ratio elliptical flat plates have been investigated experimentally to understand the vortex pinch-off dynamics at transitional and fully turbulent Reynolds numbers. The range of Reynolds numbers investigated is representative of those observed in animals that employ rowing and paddling modes of drag-based propulsion and manoeuvring. Elliptical flat plates with five aspect ratios ranging from one to two have been considered, as abstractions of propulsor planforms found in nature. It has been shown that Reynolds-number scaling is primarily determined by plate aspect ratio in terms of both drag forces and vortex pinch-off. Due to vortex-ring growth time scales that are longer than those associated with the development of flow instabilities, the scaling of drag is Reynolds-number-dependent for the aspect-ratio-one flat plate. With increasing aspect ratio, the Reynolds-number dependency decreases as a result of the shorter growth time scales associated with high-aspect-ratio elliptical vortex rings. Large drag peaks are observed during early-stage vortex growth for the higher-aspect-ratio flat plates. The collapse of these peaks with Reynolds number provides insight into the evolutionary convergence process of propulsor planforms used in drag-based swimming modes over diverse scales towards aspect ratios greater than one.

Flow in an Open Tank With a Free Surface Driven by the Spinning Bottom

1985 ◽  
Vol 107 (4) ◽  
pp. 495-499 ◽  
Author(s):  
Jae Min Hyun
Keyword(s):  
Boundary Condition ◽  
Reynolds Number ◽  
Free Surface ◽  
Aspect Ratio ◽  
Numerical Solutions ◽  
Stokes Equations ◽  
Reynolds Numbers ◽  
Flow Patterns ◽  
Navier Stokes ◽  
Aspect Ratios

An investigation is made of flows of a viscous incompressible fluid inside a circular cylindrical tank. The flow is driven by the spinning bottom endwall disk of the tank. Numerical solutions of the Navier-Stokes equations were obtained over a range of rotational Reynolds number and of aspect ratio (cylinder height/radius) using two kinds of boundary condition at the top: a closed tank with a rigid lid and an open tank with a free surface. We provide descriptions of flow details for these two boundary conditions at the top. For small aspect ratios, the nature of the azimuthal flow is distinctly different depending on the type of the top boundary condition, i.e., a Couette flow under a rigid lid and a solid-body rotation under a free surface. These qualitative flow patterns are insensitive to the Reynolds number. For flows with a finite aspect ratio and at small Reynolds numbers, the change in the top boundary condition has little impact on the flow. For flows with a finite aspect ratio and at large Reynolds numbers, the prevailing flow patterns are of boundary layer-type. At a given vertical level, the angular velocity attains a larger value under a free surface than under a rigid lid.

Heat Transfer for High Aspect Ratio Rectangular Channels in a Stationary Serpentine Passage With Turbulated and Smooth Surfaces

2013 ◽  
Author(s):  
Matthew A. Smith ◽  
Randall M. Mathison ◽  
Michael G. Dunn
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Flow Behavior ◽  
Reynolds Numbers ◽  
Hydraulic Diameter ◽  
Internal Cooling ◽  
Number Range ◽  
Aspect Ratios ◽  
Parallel Configuration

Heat transfer distributions are presented for a stationary three passage serpentine internal cooling channel for a range of engine representative Reynolds numbers. The spacing between the sidewalls of the serpentine passage is fixed and the aspect ratio (AR) is adjusted to 1:1, 1:2, and 1:6 by changing the distance between the top and bottom walls. Data are presented for aspect ratios of 1:1 and 1:6 for smooth passage walls and for aspect ratios of 1:1, 1:2, and 1:6 for passages with two surfaces turbulated. For the turbulated cases, turbulators skewed 45° to the flow are installed on the top and bottom walls. The square turbulators are arranged in an offset parallel configuration with a fixed rib pitch-to-height ratio (P/e) of 10 and a rib height-to-hydraulic diameter ratio (e/Dh) range of 0.100 to 0.058 for AR 1:1 to 1:6, respectively. The experiments span a Reynolds number range of 4,000 to 130,000 based on the passage hydraulic diameter. While this experiment utilizes a basic layout similar to previous research, it is the first to run an aspect ratio as large as 1:6, and it also pushes the Reynolds number to higher values than were previously available for the 1:2 aspect ratio. The results demonstrate that while the normalized Nusselt number for the AR 1:2 configuration changes linearly with Reynolds number up to 130,000, there is a significant change in flow behavior between Re = 25,000 and Re = 50,000 for the aspect ratio 1:6 case. This suggests that while it may be possible to interpolate between points for different flow conditions, each geometric configuration must be investigated independently. The results show the highest heat transfer and the greatest heat transfer enhancement are obtained with the AR 1:6 configuration due to greater secondary flow development for both the smooth and turbulated cases. This enhancement was particularly notable for the AR 1:6 case for Reynolds numbers at or above 50,000.

Power Augmentation of a HAWT by Mie-type Tip Vanes, considering Wind Tunnel Flow Visualisation, Blade-Aspect Ratios and Reynolds Number

2003 ◽  
Vol 27 (3) ◽  
pp. 183-194 ◽  
Author(s):  
Yukimaru Shimizu ◽  
Edmond Ismaili ◽  
Yasunari Kamada ◽  
Takao Maeda
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Aspect Ratio ◽  
Reynolds Numbers ◽  
Flow Visualisation ◽  
Velocity Measurements ◽  
Horizontal Axis Wind Turbine ◽  
Aspect Ratios ◽  
Detailed Surface ◽  
Blade Tips

Wind tunnel results are reported concerning the effects of blade aspect ratio and Reynolds number on the performance of a horizontal axis wind turbine (HAWT) with Mie-type1 tip attachments. The flow behaviour around the blade tips and the Mie-type tip vanes is presented. Detailed surface oil film visualization and velocity measurements around the blade tips, with and without Mie vanes, were obtained with the two-dimensional, Laser-Doppler Velocimetry method. Experiments were performed with rotors having blades with different aspect ratio and operating at different Reynolds numbers. The properties of the vortices generated by the Mie vanes and the blade tips were carefully studied. It was found that increased power augmentation by Mie vanes is achieved with blades having smaller aspect ratio and smaller Reynolds number.

Topology of vortex breakdown in closed polygonal containers

2017 ◽  
Vol 820 ◽  
pp. 263-283 ◽  
Author(s):  
Igor V. Naumov ◽  
Irina Yu. Podolskaya
Keyword(s):  
Aspect Ratio ◽  
Cross Section ◽  
Vortex Breakdown ◽  
Laser Doppler Anemometry ◽  
Reynolds Numbers ◽  
Cylindrical Container ◽  
Stagnation Points ◽  
Aspect Ratios ◽  
Particle Visualization ◽  
Cross Section Geometry

The topology of vortex breakdown in the confined flow generated by a rotating lid in a closed container with a polygonal cross-section geometry has been analysed experimentally and numerically for different height/radius aspect ratios $h$ from 0.5 to 3.0. The locations of stagnation points of the breakdown bubble emergence and corresponding Reynolds numbers were determined experimentally and numerically by STAR-CCM+ computational fluid dynamics software for square, pentagonal, hexagonal and octagonal cross-section configurations. The flow pattern and velocity were observed and measured by combining seeding particle visualization and laser Doppler anemometry. The vortex breakdown size and position on the container axis were identified for Reynolds numbers ranging from 500 to 2800 in steady flow conditions. The obtained results were compared with the flow structure in the closed cylindrical container. The results allowed revealing regularities of formation of the vortex breakdown bubble depending on $Re$ and $h$ and the cross-section geometry of the confined container. It was found in a diagram of $Re$ versus $h$ that reducing the number of cross-section angles from eight to four shifts the breakdown bubble location to higher Reynolds numbers and a smaller aspect ratio. The vortex breakdown bubble area for octagonal cross-section was detected to correspond to the one for the cylindrical container but these areas for square and cylindrical containers do not overlap in the entire range of aspect ratio.

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.

Fuel Cell Bionic Flow Slab Design

2009 ◽  
Vol 7 (1) ◽  
Author(s):  
C. T. Wang ◽  
C. P. Chang ◽  
C. K. Shaw ◽  
J. Y. Cheng
Keyword(s):  
Fuel Cells ◽  
Fuel Cell ◽  
Pressure Drop ◽  
Aspect Ratio ◽  
Reynolds Numbers ◽  
Cell Performance ◽  
High Potency ◽  
Fuel Cell Performance ◽  
Aspect Ratios ◽  
New Generation

Fuel cells possessing high potency and low pollution are well known and are considered the new generation of power technology. This study presents a novel bionic concept flow slab design to improve fuel cell performance. A series of 2D simulations was executed at Re=10 and 100 for the bionic flow and traditional flow slabs. In addition, the effect of aspect ratio was studied using 3D simulation. Numerical results obtained show that this novel bionic flow slab design will exhibit better performance than traditional flow slabs regardless of Reynolds numbers and aspect ratios because it possesses a more uniform velocity and a lower pressure drop. Finally, the performance in the bionic flow slab’s reaction area was determined to be superior. These findings show that the bionic concept and flow slab design addressed in this paper will be useful in enhancing fuel cell performance.

Vortex-Excited Response of Large-Scale Cylinders in Sheared Flow

1988 ◽  
Vol 110 (3) ◽  
pp. 272-277 ◽  
Author(s):  
J. A. Humphries ◽  
D. H. Walker
Keyword(s):  
Large Scale ◽  
Shear Velocity ◽  
Reynolds Numbers ◽  
Hydrodynamic Drag ◽  
Flow Induced Vibration ◽  
Sheared Flow ◽  
Drag Forces ◽  
Linear Shear ◽  
Series Of Experiments ◽  
The Mean

A series of experiments were performed to measure the vortex-excited response of a 0.168-m-dia slender circular cylinder in a range of linear shear velocity profiles. Reynolds numbers of up to 2.5 × 105 were achieved. The results clearly showed that regular large-amplitude cylinder vibrations occurred well within the critical drag transition region. It was found that increasing the linear shear profile decreased the peak amplitude response but broadened the range of lock-on over which large oscillations occurred. The flow-induced vibration of the cylinder caused amplification of the mean hydrodynamic drag forces acting on the cylinder when compared with those expected for a similar rigid cylinder.

An experimental investigation of the flow around a circular cylinder: influence of aspect ratio

1994 ◽  
Vol 258 ◽  
pp. 287-316 ◽  
Author(s):  
C. Norberg
Keyword(s):  
Aspect Ratio ◽  
Free Stream ◽  
Reynolds Numbers ◽  
Infinite Cylinder ◽  
Stream Velocity ◽  
Subcritical Regime ◽  
End Plate ◽  
Aspect Ratios ◽  
The Mean ◽  
Stable Flow

The investigation is concentrated on two important quantities – the Strouhal number and the mean base suction coefficient, both measured at the mid-span position. Reynolds numbers from about 50 to 4 × 104 were investigated. Different aspect ratios, at low blockage ratios, were achieved by varying the distance between circular end plates (end plate diameter ratios between 10 and 30). It was not possible, by using these end plates in uniform flow and at very large aspect ratios, to produce parallel shedding all over the laminar shedding regime. However, parallel shedding at around mid-span was observed throughout this regime in cases when there was a slight but symmetrical increase in the free-stream velocity towards both ends of the cylinder. At higher Re, the results at different aspect ratios were compared with those of a ‘quasi-infinite cylinder’ and the required aspect ratio to reach conditions independent of this parameter, within the experimental uncertainties, are given. For instance, aspect ratios as large as L/D = 60–70 were needed in the range Re ≈ 4 × 103–104. With the smallest relative end plate diameter and for aspect ratios smaller than 7, a bi-stable flow switching between regular vortex shedding and ‘irregular flow’ was found at intermediate Reynolds number ranges in the subcritical regime (Re ≈ 2 × 103).

Effects of Aspect Ratios on Average Daily Solar Energy Collection for Different Flat-Plate Collector Arrays in Winter Months

2013 ◽  
Vol 368-370 ◽  
pp. 1228-1231
Author(s):  
Fen E Hu ◽  
Sheng Xian Wei ◽  
Neng Bang Hou
Keyword(s):  
Solar Radiation ◽  
Solar Energy ◽  
Aspect Ratio ◽  
Flat Plate ◽  
Solar Collector ◽  
Radiation Model ◽  
Aspect Ratios ◽  
Solar Radiation Model ◽  
Flat Plate Collector

A solar radiation model to determine solar energy collection on solar collector array with different aspect ratios has been developed. The relations between the aspect ratio and the average daily solar radiation collection on the collector array have been deeply studied. The results show that there is an optimum aspect ratio to maximize the solar energy collection on the collector arrays. The optimum aspect ratios of the 1000 m2 collector array for Haikou, Kunming, Lhasa and Beijing are 10/1, 1/3, 5/1 and 10/1.The optimum aspect ratios of 1000 m2, 500 m2, 200 m2 and 100 m2 collector arrays for Kunming are 1/3, 3/1, 7/1 and 1/5, respectively.

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