Tangential Drag Force Research Articles

Superhydrophobic Strategy for Nature-Inspired Rotating Microfliers: Enhancing Spreading, Reducing Contact Time, and Weakening Impact Force of Raindrops

Wind-dispersal of seeds is a remarkable strategy in nature, enlightening the construction of microfliers for environmental monitoring. However, the flight of these microfliers is greatly affected by climatic conditions, especially in rainy days, they suffer serious raindrop impact. Here, a hierarchical superhydrophobic surface is fabricated and a novel strategy is demonstrated that the superhydrophobic coating can enhance spreading while reduce contact time and impact force of raindrops, all of which are beneficial for the rotating microfliers. When the surface rotating speed exceeds a critical value, the effect of centrifugal force becomes considerable so that the droplet spreading is enhanced. The rotating superhydrophobic surface can rotate an impacting droplet by the tangential drag force from the air boundary layer, and the rotation of the droplet generates a negative pressure zone inside it, reducing the contact time by more than 30%. The impact force by the droplet on the rotating superhydrophobic surface also has a remarkable reduction of 53% compared to that on unprocessed hydrophilic surfaces, which helps maintain the flight stability of the microfliers. This work pioneers in revealing the droplet impact effect on rotating microflier surfaces and demonstrates the effectiveness of protecting microfliers with superhydrophobic coatings, which shall guide the manufacture and flight of microfliers in rainy conditions.

Fluid force identification acting on snake-like robots swimming in viscous fluids

A snake-like robot can move in not only the dry ground but also various viscous environments of water, mud, and clay by exhibiting undulating motions. To construct the numerical model including the fluid–structure interaction for the snake-like robot under the above-mentioned environments, the added mass and drag coefficients for the snake-like robot must be identified via experimentation because they depend on the shape of the body, the roughness of the skin, and the viscosity of the fluid. In this work, firstly, we performed experiments in which a snake-like robot of eight links exhibits swimming motions in three fluids of different kinematic viscosities, and we measured its joint positions during swimming. Subsequently, we proposed a numerical model of the snake-like robot swimming in fluids with a wide range of viscosities, and the identification method of some unknown fluid force parameters using an unscented Kalman filter. After that, we identified the unknown added mass and drag coefficients for the fluid force acting on the snake-like robot by using the proposed method. Then, we clarified that the appropriate drag model is the inertia drag model for water with small viscosity and the viscous drag model for oil which is a highly viscous fluid. Moreover, we confirmed that the undulation increased the tangential drag force along the body by 1.4 times. The position and velocity of the center of gravity of the snake-like robot, calculated using the numerical models, also agreed with the experimental results.

Normal and Tangential Drag Forces of Nylon Nets, Clean and with Fouling, in Fish Farming. An Experimental Study

Experiments in a laboratory tank have provided measurements of the normal and tangential drag forces exerted on flat nets for different flow conditions. From those forces, normal and tangential drag coefficients of the nets have been obtained as functions of the Reynolds number and the solidity index. The experiments used two types of nets employed in the operation of a cultivation center: the fish net and the sea lion net, for the clean situation and for real operating conditions, with fouling adhered to the nets. Polyethylene ropes were used to characterize the presence of fouling in the nets. The experiments were carried out to determine equations for the normal and tangential drag coefficients. For the normal drag coefficient, the equations are linear with the Reynolds number, and the coefficients of the equations are linear with the solidity index. The equations are not so accurate for the tangential drag coefficient. The Reynolds number is not a relevant parameter for this coefficient and neither is the solidity index for the fish net, but the coefficient grows slightly with it for single and double sea lion nets with fouling. The literature review on the drag forces of nets reports that the tangential drag force is around 30% of the normal drag force. This value is approximately an average value of the ratio for the sea lion net and is higher for the clean fish net in this article.

Hydrodynamic characteristics of a full-scale kelp model for aquaculture applications

Hydrodynamic characteristics are needed to optimize the design of kelp aquaculture systems. To support this need, the objective of this study was to resolve both the normal and tangential drag forces acting on a dense aggregate of kelp blades using full-scale physical model tests. The physical model was designed to match the exposed length, individual blade flexural rigidity, the number of blades per unit width, the mass/length of biomass, and the aggregate mass density of kelp cultured at the University of New England experimental aquaculture lease site in Saco Bay, Maine USA. Tow tests were conducted at the United States Naval Academy in a tank with the dimensions of 116m × 7.9m × 4.9 m. The large tank size enabled the use of the full-scale physical model, minimizing dynamic similarity issues. In a series of tests, the model was towed in orientations both aligned and perpendicular to the tow direction. Horizontal and vertical reaction forces were measured for five tow speeds, along with the deflection of the dense kelp aggregate. With these datasets and the wet weight biomass per length of the model, normal and tangential drag forces were calculated. Drag components were processed into corresponding normal and tangential drag-area values. The drag-area representation was chosen since reference areas were ambiguous for both the actual kelp and model. At the higher speeds, the total horizontal drag in the aligned configuration were slightly lower than for the perpendicular orientation. Normal drag-areas as a function of tow speed ranged from 2.36 m2/m to 1.39 m2/m for the aligned case and from 2.49 to 1.88 m2/m for the perpendicular case. Tangential drag-areas as a function of tow speed ranged from 0.264 m2/m to 0.0325 m2/m for the aligned case and from 0.213 to 0.0415 m2/m for the perpendicular case. A transition from a bluff body to a streamlined body occurred as the tow speeds increased. To investigate this transition, horizontal components of the normal and tangential drag forces were reconstructed with the results of the tow tests. The reconstructed forces were obtained using a force balance system of equations with drag-area values for tow speeds less than 0.25 m/s extrapolated from the experimental datasets. For both aligned and perpendicular orientations, the model-aggregate reconfigured at a threshold of 0.25 m/s. We defined the threshold for reconfiguration as the tow speed at which the horizontal component of tangential drag equaled or exceeded the horizontal component of the normal drag. The drag-area results from this study can be incorporated into a dynamic fluid-structure interaction model representing kelp aggregates as a finite element beam prescribed with in-situ values of length, volume, mass density and flexural-rigidity of kelp material.

An improved quasi-static model for mooring-induced damping estimation using in the truncation design of mooring system

The horizontal motions of floating oil and gas structures by a mooring system can be modeled as a resonant oscillator whose natural frequencies may be much lower than wave frequencies. Thus, the low frequency (LF) excitation caused by the irregular wave results in resonant motion responses in the horizontal plane. For instance, mooring lines may provide 80% of the total LF surge motion damping of a moored tanker in water depth of 200m, thus reducing the floater resonant excursion significantly. Therefore, when designing truncated mooring system for model testing, mooring damping equivalence should be considered. Thus, simple but effective method for predicting mooring line damping is required for designing truncated mooring system. Different with other quasi-static methods for estimating mooing line damping, besides normal drag forces, tangential drag forces and seabed friction are also considered in the improved quasi-static model presented in this paper. The analytical results based on the improved method are compared to other quasi-static and time domain results for a chain mooring line and a wire line. In addition, model tests of a 1:70 scaled catenary mooring line are conducted. Then, the improved quasi-static results, the AQWA results and the experimental results are compared.

Axial Bladeless Turbine Suitable for High Supersonic Flows

The development of future propulsion systems may rely on the extraction of power from high supersonic flows. In conventional supersonic turbines operating under such extreme flow conditions, the presence of blades induces important flow perturbations that result in severe aerodynamic losses and restrictions in the operating range, seriously affected by starting issues. These limitations present design opportunities for a bladeless turbine configuration. The present paper documents, for the first time in literature, an axial bladeless turbine concept, which is able to extract mechanical power from high supersonic flows through an annular channel. The tangential drag force exerted on the rotating inner and outer end-walls produces power, together with enhanced simplicity, coolability, and maintainability. Reynolds-averaged Navier–Stokes simulations allowed the detailed evaluation of the aerodynamic performance of the bladeless configuration. This unconventional axial turbine concept becomes a design choice for extracting power from supersonic flows, with minimum pressure losses.

Rotating fluidized bed with a static geometry: Guidelines for design and operating conditions

Abstract Computational fluid dynamics (CFD) simulations of the hydrodynamic behavior of rotating fluidized beds in static geometry (RFB-SG) are carried out for gas–solid flows. The rotating motion of the reactor bed is induced by the tangential injection of the gas along the circumference of the fluidization chamber. Steep gradients in the gas velocity fields both in radial and tangential direction generate turbulence. The radial and tangential drag forces fluidize the particle bed in both radial and tangential direction. An Eulerian two-fluid model is used. Gas phase turbulence is accounted for by a k–e model adapted for rotational flows. The RFB-SG simulations provide guidelines for a design and operation with a high efficiency in gas–solid momentum transfer, excellent gas–solid separation and limited solids losses. Hydrodynamic variables like the centrifugal force, the injection pressure, the radial and tangential slip velocities, solids hold-up are calculated for both polymer particles (300 μm, 950 kg/m3, Geldart Group B) and glass beads (70 μm, 2500 kg/m3, Geldart Group A) to allow for a comparison among different fluidization chamber designs. Unstable bed behavior, like slugging and channeling, is also numerically predicted.

Couette Flows over a Rough Boundary and Drag Reduction

We consider the Couette flow between two plates. The lower plate is fixed and has periodically placed riblets of the characteristic size ɛ on it. In the limit ɛ → 0 we find the effective Couette-Navier flow as an O(ɛ2) approximation for the effective mass flow and an O(ɛ2)L1-approximation for the velocity. In the effective solution the effect of roughness enters through the Navier slip condition with the matrix coefficient in front of the effective shear stress, calculated using a boundary layer problem. Furthermore, an O(ɛ2) approximation for the tangential drag force is found. In all estimates explicit dependence on the kinematic viscosity ν, the velocity \(\) of the upper plate and the distance between the plates L3 is kept. Also the uniqueness of the solution is expressed through a non-linear algebraic condition linking \(\) and L3. Then the result is applied to the viscous sub-layers around immersed bodies, strictly containing the surface riblets. It is found that for the riblets of the characteristic size ɛ, being of the order smaller or equal to \(\), the approximation obtained for the tangential drag could be applied. We compare ɛ and \(\) for realistic data and our results lead to the conclusion that the riblets reduce significantly tangential drag, which may explain their presence on the skin of Nektons.

On the Roughness-Induced Effective Boundary Conditions for an Incompressible Viscous Flow

We consider the laminar viscous channel flow with the lateral surface of the channel containing surface irregularities. It is supposed that a uniform pressure gradient is maintained in the longitudinal direction of the channel. After studying the corresponding boundary layers, we obtain rigorously the Navier friction condition. It is valid when the size and amplitude of the imperfections tend to zero. Furthermore, the coefficient in the law is determined through an auxiliary boundary-layer type problem, and the tangential drag force and the effective mass flow are determined up to order O(ε3/2). The value of the effective coefficient is shown to be independent with respect to the position of the mean surface in the range of O(ε).

Flexible riser analysis — comparison of results from computer programs

The paper presents results from static and dynamic analyses of standard flexible riser systems performed by 11 different institutions using their in-house computer programs. The study was initiated by the International Ship and Offshore Structures Congress (ISSC) Committee V.7 ‘Slender Marine Structures’, and partly included in the 1991 Congress report presented in Wuxi, China. This study presents only the results from the analyses and does not comment on the programs' efficiency or ability to model the various physical effects. The main conclusions from the study are: - static analysis of flexible risers is today a straightforward task although it does not represent important uncertainties when dealing with simple riser systems. In some cases it is possible that significant discrepancies may be obtained if different modelling of tangential drag forces and riser/seafloor interaction are used. - Results from dynamic analyses show a larger scatter, and a coefficient of variation of 10–15% in dynamic tension must be expected. Hydrodynamic loading and structural damping models are seen to be the main sources for this uncertainty. The paper contains a complete description of the test cases and reports results in such a way that it can provide a ‘benchmark’ test for users and programmers of computer codes for flexible riser analysis.

3‐D Static analysis of cable in nonuniform current

Abstract The analysis of a cable in a current involves geometric, material, and loading nonlinearity. This paper derives the 3‐D static equations for cable analysis and discusses their boundary conditions and the numerical technique. After the simplification to a 2‐D case, the equations are identical to those of Goodman and Breslin. Traditionally, the buoyancy force on a cable element has been taken in the gravitational direction; however, this paper cautiously avoids this conceptual error and verifies that the buoyancy force is in the normal direction of the cable. The result shows that, in the three‐dimensional analysis, the variation of current velocity has a significant effect on the cable geometry, and the neglect of tangential drag force of the assumption of v=0.5 for the material of low modulus of elasticity may cause significant error.

Equilibrium configuration and tensions of a flexible cable in a uniform flowfield.

Equilibrium configuration and tension of flexible cable in uniform flow field, considering tangential drag force and weight effects