Steady and oscillatory thermocapillary convection in liquid columns with free cylindrical surface

1983 ◽  
Vol 126 ◽  
pp. 545-567 ◽  
Author(s):  
F. Preisser ◽  
D. Schwabe ◽  
A. Scharmann
Keyword(s):  
Free Surface ◽  
Aspect Ratio ◽  
Cylindrical Surface ◽  
Thermocapillary Convection ◽  
Cylinder Surface ◽  
Axially Symmetric ◽  
Zone Length ◽  
Aspect Ratios ◽  
Temperature Oscillations ◽  
Free Cylindrical Surface

In liquid columns (Prandtl number 8·9) with free cylindrical surface heated from above, strong thermocapillary convection (TC) has been observed. Stationary thermocapillary convection exists in the form of a single axially symmetric roll bound to the free surface. For aspect ratios l/a < 1 the radial extension of the roll equals the zone length. The stream velocities and the temperature distribution were measured.The influence of buoyant forces due to horizontal temperature gradients in the experiments was also studied. Buoyant forces become obvious for a contaminated free surface and in bulk regions far from the cylinder surface.The thermocapillary convection shows a transition to time-dependent oscillatory motion when a critical Marangoni number Mac is exceeded. A unique Mac = 7 × 103 has been found for zones with lengths l < 3·5 mm. The oscillatory state of thermocapillary convection has experimentally been proved to be a distortion of the laminar state in form of a wave travelling in the azimuthal direction. A unique non-dimensional wavenumber ≈ 2·2 (near Mac) of the distortion has been found. The non-dimensional frequency of the temperature oscillations is independent of zone size if the aspect ratio is held constant. However, the non-dimensional frequency of temperature oscillations increases linearly with the aspect ratio of the zone. This result is interpreted as a dependence of the phase velocity of the running disturbance on the aspect ratio.

Three-Dimensional Thermocapillary Convection in Open Cylindrical Annuli

2000 ◽  
Author(s):  
Bok-Cheol Sim ◽  
Abdelfattah Zebib
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Critical Frequency ◽  
Thermocapillary Convection ◽  
Three Dimensional ◽  
Time Dependent ◽  
Infinite Layer ◽  
Aspect Ratios ◽  
Temperature Oscillations ◽  
Good Agreement

Abstract Three-dimensional, time-dependent thermocapillary convection in open cylindrical containers is investigated numerically. Results for aspect ratios (Ar) of 1, 2.5, 8, and 16 and a Prandtl number of 6.84 are obtained to compare the results of numerical simulations with ongoing experiments. Convection is steady and axisymmetric at sufficiently low values of the Reynolds number (Re). Transition to oscillatory states occurs at critical values of Re which depend on Ar. With Ar = 1.0 and 2.5, we observe, respectively, 5 and 9 azimuthal wavetrains travelling clockwise at the free surface near the critical Re. With Ar = 8.0 and 16.0, there are substantially more, but pulsating waves near the critical Re. In the case of Ar = 16.0, which approaches the conditions in an infinite layer, our results are in good agreement with linear theory. While the critical Reynolds number decreases with increasing aspect ratio in the case of azimuthal rotating waves, it increases with increasing aspect ratio in the case of azimuthal pulsating waves. The critical frequency of temperature oscillations is found to decrease linearly with increasing Ar. We have also computed supercritical time-dependent states and find that while the frequency increases with increasing Re near the critical region, the frequency of supercritical convection decreases with Re.

Turbulent Flow Around Rectangular Cylinders with Different Streamwise Aspect Ratios

2021 ◽  
Author(s):  
Sedem Kumahor ◽  
Mark F. Tachie
Keyword(s):  
Aspect Ratio ◽  
High Speed ◽  
Second Harmonic ◽  
Cylinder Surface ◽  
Large Aspect Ratio ◽  
Reynolds Stresses ◽  
Square Cylinder ◽  
Mean Flow ◽  
Aspect Ratios ◽  
The Mean

Abstract Turbulent flows around a square cylinder and a rectangular cylinder with a streamwise aspect ratio of 5 in a uniform flow were investigated using time-resolved particle image velocimetry. The Reynolds number based on the cylinder height and oncoming flow velocity was 16200. Similarities and differences in the flow dynamics over the cylinders and in the near wake region were examined in terms of the mean flow, Reynolds stresses and triple velocity correlations. The budget of turbulent kinetic energy as well as temporal and spectral analyses were also performed. The results show that the primary, secondary and wake vortexes are smaller for the square cylinder compared to the large aspect ratio cylinder. There are regions of elevated Reynolds stresses and triple velocity correlations along the mean separating streamlines, and the magnitudes of these statistics are an order of magnitude higher over the square cylinder compared to the large aspect ratio cylinder. The topology of the triple velocity correlations shows low-speed ejection and high-speed sweep events, respectively, transporting instantaneous Reynolds normal stresses away from the mean separating streamline into the free-stream and toward the cylinder surface, regardless of aspect ratio. Near the leading and trailing edges of both cylinders, regions of negative turbulence production are observed and the dominant components contributing to this occurrence are discussed. Temporal autocorrelation coefficients of the streamwise and vertical velocity fluctuations show a periodic trend, with a periodicity that is directly linked to the Kármán shedding frequency and its second harmonic.

scholarly journals WAVE FIELD GENERATED BY FINITE-SPAN HYDROFOILS OPERATING BENEATH A FREE SURFACE

Brodogradnja ◽  
2021 ◽  
Vol 72 (1) ◽  
pp. 145-167
Author(s):  
Yavuz Hakan Ozdemir ◽  
◽  
Taner Cosgun ◽  
Baris Barlas ◽  
◽  
...  
Keyword(s):  
Free Surface ◽  
Aspect Ratio ◽  
Wave Field ◽  
Surface Effect ◽  
Hydrodynamic Performance ◽  
Spanwise Direction ◽  
Two Dimensional ◽  
Aspect Ratios ◽  
Wide Range ◽  
2D And 3D

The present paper focuses on the numerical investigation of the flow around the fully submerged 2D and 3D hydrofoils operating close to a free surface. Iterative boundary element method is implemented to predict the flow field. This study aims to investigate the aspect ratio effect on the free surface interactions and hydrodynamic performance of the hydrofoils under a free surface by using potential flow theory. Three different submergence depths and aspect ratios are studied in the wide range of Froude Numbers. In 3D cases, spanwise width of the numerical wave tank model is selected both equal and wider to the foil span, to observe the sidewall effects. Wave field seems to be two dimensional at low Froude numbers. On the other hand, signs of three dimensionalities are observed on the free surface structure for higher Fn, even the predicted wave elevations are very close to 2D calculations in the midsection. Increment in the Fn give a rise to the amplitude of the generated waves first, however a further increase in Fn has a lowering effect with the beginning of waves spill in the spanwise direction in the form of Kelvin waves. Free surface proximity and resultant wave field are also seeming to be linked with the lift force on the hydrofoil. As aspect ratio of the foil increase, 3D lift values are getting closer to those of 2D calculations. However, it is seen that, 3D BEM predictions of a hydrofoil under free surface effect cannot be considered two-dimensional even the aspect ratio is equal to 8.

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.

Combined buoyant-thermocapillary flow in a cavity

1989 ◽  
Vol 207 ◽  
pp. 121-132 ◽  
Author(s):  
Bradley M. Carpenter ◽  
G. M. Homsy
Keyword(s):  
Boundary Layer ◽  
Free Surface ◽  
Aspect Ratio ◽  
Prandtl Number ◽  
Large Scale ◽  
Thermocapillary Convection ◽  
Numerical Solutions ◽  
Thermocapillary Flow ◽  
Large Scale Flow ◽  
Rayleigh Numbers

We treat the problem of combined buoyancy-thermocapillary convection in a cavity with a free surface heated differentially in the horizontal. Attention is focused on the structure and strength of the flow for large ΔT, i.e. large Marangoni and Rayleigh numbers. In the combined problem, the boundary-layer scalings for buoyant and thermocapillary convection suggest that in the limit of large ΔT, thermocapillarity will dominate the large-scale flow. Accurate numerical solutions are used to study this question at fixed cavity aspect ratio and Prandtl number, with G = Ra/Ma as a parameter. For G = 1, the flow evolves toward its boundary-layer limit in a fashion identical to that for G = 0, i.e. pure thermocapillary flow. For G = 10, the evolution is from a buoyancy-dominated structure, through a transition, to a thermocapillary-dominated structure. We infer that thermocapillarity will ultimately dominate all such flows at sufficiently large ΔT, for any fixed values of G, the aspect ratio, and the Prandtl number.

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.

scholarly journals Three-dimensional theory on supercavitating hydrofoils near a free surface

1975 ◽  
Vol 71 (2) ◽  
pp. 339-359 ◽  
Author(s):  
Okitsugu Furuya
Keyword(s):  
Free Surface ◽  
Aspect Ratio ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reference Level ◽  
Two Dimensional ◽  
Large Aspect Ratio ◽  
Dimensional Solution ◽  
Aspect Ratios ◽  
Lifting Line

Supercavitating hydrofoils of large aspect ratio operating near a free surface are investigated, assuming an inviscid and irrotational flow with the effects of gravity and surface tension neglected. The flow near the foil, treated as two-dimensional, is solved by a nonlinear free-streamline theory, then a three-dimensional ‘downwash’ correction is made using Prandtl's lifting-line theory. The strength of the lifting-line vortex is determined by information from the two-dimensional solution through a matching procedure, in which the inverse of aspect ratio is used as a small parameter for asymptotic expansions. The analysis incorporates a free-surface reference level to determine the submergence depth of the foil. The present method can be applied to any type of foil having an arbitrary planform or profile shape, including a rounded leading edge, a twist and even a small dihedral angle, within the assumption of large aspect ratio. Numerical computations made on rectangular flat-plate hydrofoils show excellent agreement of results with existing experimental data, even for large angles of attack and relatively low aspect ratios. The pressure distributions, shapes of the cavity and free surface are also calculated as a function of spanwise position.

scholarly journals Effects of Steel Fiber Percentage and Aspect Ratios on Fresh and Harden Properties of Ultra-High Performance Fiber

2021 ◽  
Vol 2 (3) ◽  
pp. 501-515
Author(s):  
Rajib Kumar Biswas ◽  
Farabi Bin Ahmed ◽  
Md. Ehsanul Haque ◽  
Afra Anam Provasha ◽  
Zahid Hasan ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Aspect Ratio ◽  
High Performance ◽  
Steel Fiber ◽  
Setting Time ◽  
Fiber Reinforced Concrete ◽  
Future Research ◽  
Manufacturing Cost ◽  
Aspect Ratios ◽  
High Performance Fiber

Steel fibers and their aspect ratios are important parameters that have significant influence on the mechanical properties of ultrahigh-performance fiber-reinforced concrete (UHPFRC). Steel fiber dosage also significantly contributes to the initial manufacturing cost of UHPFRC. This study presents a comprehensive literature review of the effects of steel fiber percentages and aspect ratios on the setting time, workability, and mechanical properties of UHPFRC. It was evident that (1) an increase in steel fiber dosage and aspect ratio negatively impacted workability, owing to the interlocking between fibers; (2) compressive strength was positively influenced by the steel fiber dosage and aspect ratio; and (3) a faster loading rate significantly improved the mechanical properties. There were also some shortcomings in the measurement method for setting time. Lastly, this research highlights current issues for future research. The findings of the study are useful for practicing engineers to understand the distinctive characteristics of UHPFRC.

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