Методы определения коэффициента сопротивления при вдуве газа с поверхности сферической частицы

Transitions in the unsteady wakes and aerodynamic characteristics of the flow past three square cylinders aligned inline

Numerical simulation of flow around a triangular cylinder placed both in left and right position at Reynolds number Re = 80–200 is presented in this study. The single relaxation time lattice Boltzmann method is used. The results are obtained in terms of vorticity contour visualization, drag and lift coefficients, and force statistics. Regular vortex shedding is observed for both cases, but the vortices generated for a triangular cylinder placed in the right position move toward the top downstream position as the range of Reynolds number increases due to effect of pressure (effect of thrust). The drag coefficient is constant, and lift coefficient is containing negative values from Re = 80–150 for that case, but negative Cdmean values are not observed for a triangular cylinder placed in the left position. In that case, it is due to the reason that the effect of thrust diminishes. The maximum value of Cdmean is obtained at Re = 200, that is, 1.5006 for the left triangular cylinder, but the value of the mean drag coefficient for the right triangular cylinder is zero throughout the cases for Re = 80–200. No fluid forces are produced for right triangular case. The Strouhal number (St) values increased by increasing the Reynolds number, i.e., Re = 80–200, for the right triangular cylinder, and it has a mixed trend for the left triangular cylinder. In literature, a lot of work has been found on rectangular and square cylinders, but triangular cylinder has not been studied much.

The results of the experimental and theoretical studies of the gas flow around a solid sphere under conditions of mass outflow from its surface are presented. A new experimental setup and a method for studying the flow around a solid sphere during the gas injection from its surface are proposed with the aim of improving the accuracy of determining the drag coefficient. In the range of the Reynolds number (Re = 133÷667), experimental results show that the drag coefficient decreases at the gas injection from the surface of the sphere. Moreover, the drag coefficient decreases with an increase in the density of the injected air flow at the fixed Reynolds number. Numerical simulation of a two-phase flow around a sphere with the gas injection from its surface is carried out for two calculation cases: with a uniform gas outflow from the surface of a porous sphere and with gas injection from perforated holes on the sphere surface. The numerical calculation results for the case of gas injection from the holes of the sphere are in quantitative and qualitative agreement with the experimental data.

Effect of cable surface characteristics and flow turbulence on the aerodynamic behaviour of stay cables in dry conditions

This study deals with the fluid dynamical drag coefficient of a plane netting concerning the Reynolds number based on the diameter of a netting twine. Six kinds of Weaver's-knot netting were used for this experiment; the range of the ratio of twine diameter and leg-length (d/l) were from 0.037 to 0.124. The nettings were fixed on a circular shaped frame hung 29.3% of hang-in ratio, and set into a flume tank normal to the flow direction.The drag was measured by a three component load-cell as a X-component and recorded on a chart-recorder. In this case the drag coefficients were calculated by Newton's law of resis-tance, based on projective area of netting twine.The current flow was set at 5, 10, 25, 50, 75, 100 and 125cm/s monitored by a super-sonic current meter, and the range of Reynolds number were from 1.7×10 to 1.8×103.The results obtained were as follows:1) A Reynolds number of 2×102 was determined to be the critical value for transition of drag coefficient.2) For Reynolds number greater than 2×102 the drag coefficient was determined to be 1.14.3) For Reynolds number less than 2×102 the drag coefficient followed the empirical equation: CD(90)=14.81(Re)-0.49.

It is well-known the dimples on the surface of a golf ball increase in flying distance and the drag force of the pipe jumper that used as the conductor of the high-voltage transmission is decreased by the grooves on the surface of the pipe jumper. But it has not been clarified completely how dimples and grooves manipulate the flow. The final goal of our research is to make clear the drag reduction mechanism of grooves and dimples and the detailes of the related flow structures. As the first stage of this research, this paper deals with the influence of the depth of grooves on the cross-flow around a circular cylinder. Pressure distribution and drag and lift coefficients were measured in a uniform flow in the Reynolds number range of 8.6×103≤Re≤5×105. The Reynolds number regime was classified by the variation in the drag coefficient with Reynolds number. Pressure distributions and drag and lift coefficients showed the characteristic changes for subcritical, critical, and supercritical Reynolds number ranges, respectively. As the depth of grooves decreased, drag coefficient became lower, and the critical points shifted into the high Reynolds number range. Lift was generated in the range of critical Reynolds number, even though a circular cylinder was not rotating.

Purpose This paper aims to introduce a numerical investigation of aquatic locomotion using the smoothed particle hydrodynamics (SPH) method. Design/methodology/approach To model this problem, a simple improved SPH algorithm is presented that can handle complex geometries using updatable dummy particles. The computational code is validated by solving the flow over a two-dimensional cylinder and comparing its drag coefficient for two different Reynolds numbers with those in the literature. Findings Additionally, the drag coefficient and vortices created behind the aquatic swimmer are quantitatively and qualitatively compared with available credential data. Afterward, the flow over an aquatic swimmer is simulated for a wide range of Reynolds and Strouhal numbers, as well as for the amplitude envelope. Moreover, comprehensive discussions on drag coefficient and vorticity patterns behind the aquatic are made. Originality/value It is found that by increasing both Reynolds and Strouhal numbers separately, the anguilliform motion approaches the self-propulsion condition; however, the vortices show different pattern with these increments.

Two-dimensional unsteady laminar flow of a power law fluid across a square cylinder

Predicting aerodynamic forces on bluff bodies remains to be a challenging task due to the unpredictable flow behavior, specifically at higher Reynolds numbers. Experimental approaches to determine aerodynamic coefficients could be costly and time consuming. In the meantime, use of numerical techniques could also require a considerable computational cost and time depending on complexity of the flow behavior. The research focusses on developing an effective deep learning technique to predict aerodynamic force coefficients acting on elliptical bluff bodies for a given aspect ratio and given flow condition. Collecting data for drag and lift coefficients of several aspect ratios for flow conditions starting from onset of vortex shredding to verge of subcritical region is conducted by an accurate full order model. The specified region will provide a transient flow behavior and thus lift coefficient will be represented in terms of root mean square value and drag coefficient in terms of a mean value. With variations in flow behavior and vortex shredding frequencies, it requires to select an appropriate turbulence model, optimum discretization of fluid domain and time step to obtain an accurate result. Flow simulations are conducted primarily using Unsteady Reynolds Averaged Navier-Stokes Equations (URANS) model and Detached Eddy Simulations (DES) model. Effectiveness in using different turbulence models for specified flow regimes are also explored in comparison to available experimental results. At lower Reynolds numbers, aerodynamic force coefficients for a specified body will only depend on Reynolds number. But after a certain specific Reynolds number, aerodynamic forces are dependent on the Mach number in addition to Reynolds number. Therefore, for higher Reynolds numbers, aerodynamic force coefficients are recorded for multiple Mach numbers with same Reynolds number and will be fed to the neural network. With the development of the machine learning and neural network modelling, many of the fields have nourished and created effective and efficient technologies to ease complex functions and activities. Our goal is to ease the complexity in the computational fluid dynamic field with a deep neural network tool created to predict drag and lift coefficient of elliptical bluff bodies for a given aspect ratio with an acceptable accuracy level. Researchers have developed deep neural network tools to predict various flow conditions and have succeeded with sufficient accuracy and a satisfying reduction of computational cost. In our proposed deep learning neural network, we have chosen to model the network with inputs as the geometry setup and the flow conditions with validated drag and lift coefficients. The model will extract the necessary flow features into filters with the convolution operation performed on the inputs. Our main directive is to create a deep learned neural network tool to predict the target values within an acceptable range of accuracy while minimizing the computation cost.

We present a combination of laboratory experiments and computational fluid dynamics (CFD) simulations to understand the wind-induced drag force and drag coefficient for Saccharum contortum seeds. Seed drop experiments indicate that the settling fall velocities of hair-equipped seeds are within 1–2 m/s, compared to 2.34 times higher settling fall velocity of the seed without hairs. The experimental data illustrate a power-law relationship between drag coefficient (Cd) and Reynolds number (Re) under the free fall condition: Cd∼Re−1.1. CFD simulations show that both viscous and pressure drag force components are important in contributing to wind drag. The presence of hairs substantially increases pressure drag, and its relative importance depends on hair number and orientation. Seed morphology including hair number and orientation influences the drag coefficient under different flow directions relatively to the seed body. The lower drag coefficient observed with crossflow wind compared to free fall suggests that seeds encounter less air resistance while drifting horizontally in the wind, favoring extended flying time and distance. Based on the varying drag coefficients under different conditions, we propose the incorporation of varying drag coefficients in future wind-driven seed dispersal models.

The purpose of this experimental study is to measure the drag and heat transfer coefficients of the non-circular tubes including square, elliptic, and cam-shape tubes in a turbulent air cross flow. Studies have been made at 0° to 180° angles of attack () and Reynolds numbers (Re) based on the equivalent diameter of a circular tube within the range of 1.4 × 104 < Reeq < 7.5 × 104. The equivalent diameter Deq = 46 mm was chosen for the tubes. The results show that the minimum and maximum drag coefficients (Cd) belong to the cam-shape and square tubes at 0°, respectively. The (Cd) of the cam-shape and square tubes are 60% less and 25% more than the equivalent circular tube, respectively. Also, the ratio of the average Nusselt number () to the drag coefficient, /Cd, was used to compare non-circular tubes and with their equivalent circular ones. The results show that the maximum value belongs to the cam tube and the minimum occurs for the square tube at α = 0°. The /Cd ratio of the cam tube is 128% more than the equivalent circular tube, while the square tube yields a /Cd ratio that is13% less than its circular equivalent.

An empirical velocity scale relation for modelling a design of large mesh pelagic trawl

The purpose of this thesis is to investigate the drag coefficient of spheres in free motion, falling and rising, in water. Following the introduction the test set-up, sphere release mechanism, spheres and timing device and recorder are described. Section 3 gives the results of the drag coefficient measurements for free falling spheres which show good agreement with the known measurements, quoted, for instance, in the Handbuch der Experimentalphysik. Section 4 deals with the measurement of drag coefficients of free-rising spheres. It was found that the freely rising spheres move in an oscillatory path of which wavelength and amplitude depend on the ratio of sphere density and water density. The local drag coefficient of the spheres was measured to be the same as for falling spheres. It was found to be independent of the sphere motion. No critical Reynolds number was found for the onset of the oscillatory motion of the sphere. The oscillatory motion appeared to be independent of initial disturbances of the sphere motion. From photographs of the sphere paths Strouhal numbers were formed which are about one third the value given for circular cylinders in the same Reynolds number range. A theoretical oscillatory force coefficient, based on a force balance on the sphere, was obtained. It was found to be of the same order of magnitude as the correspondent, actually measured oscillatory force coefficient on a circular cylinder. The Strouhal number based on the main theoretical frequency was found to be four times as high as the one calculated from the frequency of the sphere path. It could be, dependent on the sphere density, as high as twice the Strouhal number found for circular cylinders in the same Reynolds number range.

A constant drag coefficient of 0.61 was calculated over a Reynolds number range of 2×105 to 8×105 from simultaneous measurements of-wave-induced velocities and forces. The average inertia coefficient was 1. 20. The constant drag coefficient found for a Reynolds number as low as 2×105 allows a simulation of design wave force with a smaller scale space frame structure. The drag (inertia) coefficients were determined from drag (inertia11y) dominant portions of individual wave cycles. The measured wave force power spectrum agrees well with that calculated from measured velocities with average drag and inertia coefficients. INTRODUCTION The objective of this study was to establish the feasibility of measuring wave forces on a small-scale structure that could be scaled up to typical design loads on a conventional jacket-type offshore platform. It is essential to show that a supercritical flow condition (typified by constant drag coefficient) occurs over the entire structure to make such scaling possible. It is not possible, however, with existing wave-force data to show at what Reynolds number this supercritica1 condition begins, because the water-particle velocity must be inferred from the surface wave elevation. By measuring both the wave force and the particle velocity, one can remove the uncertainty due to wave theory and possible steady ocean currents affecting the calculated drag coefficient. When unambiguous data that can be scaled up to typical design loads are obtained from a smaller scale test structure, they can be used to calibrate present-day design wave-force calculations [4]. Currently used design force calculations for a space-frame structure are based on wave-force measurements on a single pile, typically the wave-force data collected by Chevron during Hurricane Carla [2,3,9]. Design waves are assumed to be smooth and long crested (two-dimensional). Actual waves, however, are neither. The spatial distribution of particle velocities calculated for idealized design waves can be different from those for real, random waves. For the same average velocities, the drag forces on structural members are subject to random variation due to turbulence. The net result of these effects can be randomly varying force vectors over the space of a structure. Because of the changing directions of force vectors, not all of the wave forces are additive. The actual structural load in real waves may be different from when the forces are assumed in the same direction over the entire structural space, as in present design wave-force calculations. Scaling Wave Forces In order to simulate one physical system with another of different size, the two systems are required to be geometrically similar. In addition to geometrical similarity, certain dimensionless physical parameters must be preserved so that the experimental results of one system can be applied to another. Such parameters can be found by dimensional analysis of the governing equations for the physical system under consideration.

The square lattice tower with tubular members (SLTTM) is widely used in civil engineering. Wind load is the primary factor affecting the stability of these structures. This paper presents the wind loads of the SLTTM at different Reynolds numbers (Re) through wind tunnel testing and large eddy simulation (LES). A series of wind tunnel tests with four segment models were first conducted to discuss the effects of wind direction, turbulence intensity, and solidity ratio on the aerodynamic force under low and subcritical Reynolds numbers. The LES method was subsequently employed to determine the aerodynamic force coefficient at Re = 191–1.34 × 106 in both the smooth and uniform turbulent wind fields. The results from wind tunnel tests indicate that the mean and fluctuating drag coefficients at low Reynolds numbers are somewhat different from those at subcritical Reynolds numbers. The solidity ratio and wind direction exert a notable influence on the mean drag coefficient, whereas their impact on the fluctuating drag coefficient is relatively minimal. As the turbulence intensity increases, both the mean and fluctuating drag coefficients exhibit a notable rise. The results of the LES at Re = 191–1.34 × 106 show that an increase in turbulence intensity can markedly elevate the fluctuating drag coefficient. However, the effect on the mean drag coefficient is found to depend on the Reynolds number range. The Reynolds number effect on the mean and fluctuating drag coefficients of the SLTTM may be primarily attributed to the members without and with a shielding effect, respectively.