Narrow Results

This study investigates the impact of a rotating tube on the flow characteristics within a tube bank consisting of nine cylinders arranged in an in-line configuration, with a pitch-to-diameter ratio of 1.44, using both experimental and numerical approaches. Experiments are performed in a subsonic wind tunnel to measure pressure distributions at various azimuthal angles along the tubes, using a multi-channel differential pressure system. Drag forces are determined via a wire-strain gauge balance. Numerical simulations are conducted using ANSYS Fluent, employing the URANS-based Shear-Stress Transport (SST) k–$\omega$ model, to replicate turbulent cross-flow at two values of Reynolds numbers $\mathrm{\text{Re}}=0.712\times 10^5$ and $\mathrm{\text{Re}}=1.42\times 10^5$. This study explores the effect of a tube rotation within the array on flow separation angles, pressure distributions and vortex shedding in the wake region, focusing on the effects of different positions of the rotating tube on the flow characteristics of the surrounding tubes. Numerical results closely match the experimental data, demonstrating that the rotational motion significantly mitigates flow separation and reduces drag forces on adjacent tubes. Additionally, the position of the rotating tube within the array plays a critical role in optimizing the fluid forces and pressure distribution, offering enhanced control over the flow within tube banks.

An aerofoil commonly used in aerospace engineering to produce lift is also employed in the motor sport industry to produce downforce for improving traction during cornering. This paper investigates aerofoil surface modification through ‘golf ball dimpling’, used to reduce flow separation behind a golf ball. The studies of other researchers have shown that this type of design can have a positive effect on improving aerofoil performance. However, no optimization information of dimple sizing is given in literature. Therefore, three types of dimpling sized at 5, 10 and 15 mm are applied to the surface of a NACA 6615 wing at 25% chord length from the leading edge in this study using Computational Fluid Dynamics (CFD) as an initial design process. Then a physical model, made through 3D printing additive manufacturing (AM), is tested at angles of attack (AoA) ranging from $0^{\circ }$ to $20^{\circ }$ and wind speed up to 30 m/s in a subsonic wind tunnel. Experimental and CFD results show that the smallest dimple size provides the most significant increase on lift to drag ratio at high AoA above $10^{\circ }$. This ratio increases further with the wind speed, indicating that a high AoA wing favors down force to improve drag reduction performance.

In order to investigate the drag reduction characteristics of a high-speed body with supercavitation shape, four types of typical disk cavitator models with different parameters were designed and tested. By measuring the velocity decrease histories during supercavitating flow experiments, the average drag coefficients were determined, which allows analysis and comparison of the influence of cavitator diameter, projectile aspect ratio, and cavitation number on the drag reduction. Based on the experimental results, numerical simulation of the drag reduction of supercavitation body was also carried out using a commercial software FLUENT6.2, and the results obtained agree well with the experimental data. Moreover, it is shown that the drag coefficients of the four bodies are in inverse ratio to the head area of cavitator when operating under natural supercavitating flow condition, and the smaller drag coefficient can be obtained by increasing the slender ratio of the bodies. Therefore, higher aspect ratio reduces drag coefficient, with the reduction of more than 95% under certain condition of cavitation number and supercaviation shape.

This paper describes a numerical study of vortex shedding from a quasi-streamlined 2D body with the trailing edge made in the form of a base cavity. Direct numerical simulations (DNS) are conducted to assess the effect of Reynolds number and streamwise cavity length on the flow. It was found that the flow is governed by the formation of a vortex within the base cavity, which either serves to retard or convolute the shedding process. For a shorter cavity length, this vortex is periodically replaced, and the result is a reduction in the shedding frequency and the time-averaged form drag. As the cavity length is increased, this periodic behavior ceases and the vortex which first forms within the cavity remains there permanently throughout the observation. The origins of the deflected wake pattern are traced to the presence of a vortex which resides within the base cavity. Reductions in drag are observed for all the investigated cavity configurations and additionally it is found that the magnitude of the reduction obeys a direct relationship with the length of the cavity up to a certain asymptotic value.

Microbubble drag reduction on the axisymmetric body is experimentally investigated in the turbulent water tunnel. Microbubbles are created by injecting compressed air through the porous medium with various average pore sizes. The morphology of microbubble flow and the size distribution of microbubble are observed by the high-speed visualization system. Drag measurements are obtained by the balance which is presented as the function of void ratio. The results show that when the air injection flow rate is high, uniformly dispersed microbubble flow is coalesced into an air layer with the larger increment rate of drag reduction ratio. The diameter distributions of microbubble under various conditions are submitted to normal distribution. Microbubble drag reduction can be divided into three distinguishable regions in which the drag reduction ratio experiences increase stage, rapid increase stage and stability stage, respectively, corresponding to the various morphologies of microbubble flow. Moreover, drag reduction ratio increases with the decreasing pore sizes of porous medium at the identical void ratio in the area of low speeds, while the effect of pore sizes on drag reduction is reduced gradually until it disappears with the increasing free stream speeds, which indicates that smaller microbubbles have better efficiency in drag reduction. This research results help to improve the understanding of microbubble drag reduction and provides helpful references for practical applications.

In this paper, the hydrodynamics of streamwise and normal vibration wall are studied using the Lattice Boltzmann method. Firstly, based on the two-dimensional flow geometry model, which is made up of flat wall and water fluid, the characters of the fluid near the streamwise and normal vibration wall are simulated under the condition of mutative vibration parameters. By rigorous data treating, some notable results such as the velocity distribution, density distribution curves of the flow field, and the frictional force of the solid-liquid interface are gained. Secondly, the reason of the change of frictional resistance at the solid-liquid interface by wall vibration are studied. And the results are evidence that well drag reduction effect can be obtained by applying appropriate flow vibration parameters to the solid wall. In addition, the reduction in fluid density near the solid-liquid wall is another significant cause behind the frictional drag decrease.

In this study, the drag force and heat flux reduction mechanism induced by the aerodisk (with disks on its nose) with the freestream Mach number being 4.937 has been numerically investigated, and the simulations have been carried out by the three-dimensional Reynolds-averaged Navier–Stokes equations coupled with the SST $k-\omega$ turbulence model. The influence of the angle of attack on the drag and heat flux reduction has been analyzed comprehensively. The obtained results show that the drag force of the spiked blunt body can be reduced by the aerodisk, and the drag force decreases by 24.63%. The flow mechanism of the complex flow is drastically modified by the angle of attack, and this results in a strong flow asymmetry. This asymmetry becomes more and more obvious as the angle of attack increases. Both the pressure force and viscous force increase with the increase of the angle of attack. Moreover, both the lift and drag coefficients increase as the angle of attack increases, and the lift-to-drag ratio increases first and then decreases with the increase of the angle of attack. When the angle of attack is $6^{\circ }$, the maximum lift-to-drag ratio is close to 0.36.

The drag reduction effect of interceptors on planing boats has been widely proven, but the mechanism of the effect has been rarely studied in terms of drag components, especially for spray resistance. The resistance was caused by the high gauge pressure under the boats transformed from the dynamic pressure, and it is the largest drag component in the high-speed planing mode. In this study, numerical simulations of viscous flow fields around a planing boat with and without interceptors were conducted. A two degrees of freedom motion model was employed to simulate the trim and sinkage. The numerical results were validated against the experimental data. The flow details with and without the interceptor were visualized and compared to reveal the underlying physics. A thinner and longer waterline could be achieved by the interceptor, which made the boat push the water away more gradually, and hence, the wave-making resistance could be decreased. The improved waterline also reduced the component of the freestream normal to the hull surface and led to the less transformed dynamic pressure, resulting in the lowAer spray resistance. Furthermore, the suppression of the flow separation could also be benefited from the interceptor; the viscous pressure resistance was therefore decreased.

To investigate the hydrodynamics of undulatory swimming, a key issue in numerical analysis is to determine the correlation between undulatory locomotion and the flow characteristics. In this study, a novel dynamic-grid generation method, the adaptive control method, is implemented to deal with the moving and morphing boundaries in an unsteady flow field at all Reynolds numbers. This method, based on structured grids, can ensure the orthogonality and absolute controllability of the grids and is performed to precisely simulate the wake and the boundary layer. The NACA0010 wing is employed as a two-dimensional (2D) body model of a fish in the simulations. To maintain the calculation stability, the increase stage of the amplitude is defined as a smooth transitional stage. Analysis of hydrodynamic coefficients reveals that undulation results in a significant increase of frictional force in laminar flow $(\mathrm{\text{Re}}\le 10^4)$. However, the undulation also results in a reduction of the frictional force when the fish swims in turbulent flow $(\mathrm{\text{Re}}\ge 10^6)$. The vorticity distribution and the $Q$-criterion are both used to accurately capture the shedding vortexes in the wake. Furthermore, these vortex pairs have a substantial impact on the turbulence and the wake, in which the turbulent kinetic energy and the turbulent viscosity ratio both decrease at $\mathrm{\text{Re}}\ge 10^6$. The wake of an undulatory fish presents different vortex patterns with various kinematic parameters. When the phase velocity is greater than the incoming velocity and the wave number is sufficiently large, thrust is yielded, accompanying the distinct reverse Karman Street in the wake.

To improve the performance of a low-specific-speed centrifugal pump, the bionic structure extracted from the dung beetle was distributed on the blade surface to reduce the drag. The flow fields in the centrifugal pumps with and without the bionic structure blade were calculated based on the RNG $k$–$𝜀$ turbulence model. The results showed that the bionic structure can reduce the energy loss and improve the hydraulic efficiency of the centrifugal pump, with a maximum increase of 5%. The bionic structure can affect the turbulence structure of the boundary layer near the wall and reduce the blade outlet velocity gradient and wall shear, thereby improving the pressure pulsation characteristics of the centrifugal pump. With the increase in flow, the drag reduction gradually becomes evident for the impeller with a bionic structure.

Numerous facts have validated that sharkskin possesses the obvious drag reduction effect in certain turbulent flowing stations, and it has huge potential and important applications in the fields of agriculture, aerospace, industry, transportation, daily life and so on, which have attracted increased attention throughout the world. To meet the increasing requirements of practical applications, it has been progressively developing into an urgent problem to manufacture sharkskin surfaces with perfect forming quality and high drag-reducing effect. In this paper, the vacuum casting method is put forward to fabricate the drag-reducing surface with the real sharkskin morphology by eliminating the air bubbles from the bottom of sophisticated morphology in the pouring process. Meanwhile, a novel and facile “marking key point” method is explored and adopted to search for the corresponding biological sharkskin and negative template, a more convincing way to evaluate the replicating precision is systematically illustrated and the hydrodynamic experiment is carried out in the water tunnel. The results indicate that wall resistance over sharkskin surface replicated by the vacuum casting method can be decreased by about 12.5% compared with the smooth skin. In addition, the drag reduction mechanism hypotheses of sharkskin are generalized from different respects. This paper will improve the comprehension of the sharkskin fabrication method and expand biomimetic sharkskin technology into more applications in the fluid engineering.

It has gradually developed into an undisputable fact that sharkskin surface has the obvious drag reduction effect compared with the absolutely smooth skins, and it has been put into application widely, which has brought great advantages and profits in daily life, industry and agriculture. Because some problems in turbulence are not resolved completely and perfectly, the drag reduction mechanism of real sharkskin has also not been understood absolutely and thoroughly so far. However, many researchers have carried out lots of the relevant experiments and analyses, very plentiful and important conclusions are obtained, which can explain some certain phenomena of sharkskin drag reduction effect. An overview of exploring drag reduction mechanism of real sharkskin surface is systemically presented in detail. These mechanisms include inhibition of turbulence using micro/nano structured morphology, influence of scale's attack angles, nano-long chains and boundary layer slipping based on superhydrophobicity. This paper will improve the comprehension of the drag reduction mechanism and expand biomimetic sharkskin technology into more applications.

Reducing energy consumption and protecting the environment has always been becoming the pursing goal and object for mankind, especially in the past several decades. Our living environment is all surrounded by the fluids, and the friction in turbulence has progressively developed into the important proportion of energy consumption. Therefore, how to realize the drag reduction in turbulence has turned into the imperative issue to be resolved, and many scholars have made great contributions in the field with achieving so many profits. In this paper, three different typical biological functional surfaces including sharkskin, lotus leaf and dolphin skin are reviewed and generalized systematically and comprehensively, the drag reduction mechanisms based on boundary layer control are generalized. The paper will enable the potential readers to better understand the recent progresses in exploring drag reduction technologies in turbulence, with improving the comprehension of the drag reduction mechanism and extending the relevant technologies into more applications.

The spectral element method, a direct numerical technique, is used to study the behavior of flow past two cylinders in tandem array. A control cylinder is employed in front of the main cylinder to study the drag reduction performance and flow patterns. Three major flow patterns are found, including single cylinder vortex shedding, two cylinders vortex shedding and suppression. The flow patterns are affected by the distance between two cylinders, Reynolds number and the diameter ratios of cylinders. In a bistable regime, when there is a critical distance between cylinders, drag is reduced dramatically.

Swirling flows in conical pipe can be found in a number of industrial processes, such as hydrocyclone, separator and rotating machinery. It has been found that wall oscillations can reduce the drag in water channel and pipe flows, but there is no study of a swirling flow combined with a vibrating wall in conical pipes, though there are many applications of such combination in engineering processes. A two-dimensional particle image velocimetry (PIV) is used to measure the swirling flow field in a water conical pipe subjected to a periodic vibrating wall for a Reynolds number 3800. The flow velocity statistics are studied under different vibration frequencies corresponding to Strouhal numbers from 60 to 242. The instantaneous axial and vertical velocity in one vibrating period, the mean velocities, and Reynolds stresses were obtained. The results show the existence of an intermediary recirculation cell in the middle of the pipe. They also show that the vibration improves the symmetry for the swirling flow while decreasing dramatically the velocity fluctuation.

Under the aerated conditions of wall and top in tube, the turbulent flow field in the tube was measured by using LDA. The turbulent structure of the flow field and the mechanism of aerating drag reduction in the tube were discussed. It is shown that the energy dissipations of turbulence flow and mean flow will reduce and the flow velocity (or flow rate) will increase by injecting mini-bubbles to the wall or top of tube, namely the effect of aerating drag reduction is attained.

Drag reduction by grooves, as an easily realizable turbulent passive drag reduction technique and with the bionic structure and drag reduction mechanism similar to shark skin, becomes an important direction for research. In recent years, the research on drag reduction by grooves gradually shifts from two dimensional to three dimensional and begins to examine the impact of more complicated groove shapes. This paper conducts the numerical simulation of drag reduction by three dimensional groove surfaces, namely shark skin, the typical three dimensional groove surfaces. The RANS equations and RNG k-ε turbulence model are utilized for computation and analysis of the two three-dimensional features, U-grooves with attack angle and alternating configuration. The results show that the drag reduction by U-grooves with attack angle and alternating configuration performs worse than two dimensional U-grooves. For U-grooves with attack angle, the larger the attack angle, the less its drag reduction; for alternating U-grooves, the smaller the length of groove section, the worse its drag reduction. And different mechanisms exist for the impact of grooves with attack angle and alternating configuration.

The features of flow past a cylinder with a vertical slot at Reynolds number of 3500 based on high-speed visualizations of smoke-flow and computations are presented. The pressure differential across the slot causes natural periodic suction and blowing across the cylinder. The ‘dead-water’ region that forms behind the basic circular cylinder is replaced by a highly dynamic and complex vortex dominated periodic base flow. The separated shear layer on one side rolls up into a large vortex and reattaches at the base. The reattaching vortex entrains the opposite shear layer and promotes extended regions of attached flow featuring multiple separations and reattachments. Computations show qualitative agreement with experiments and clearly bring out the occurrence of periodic wall jets transverse to the cylinder, caused by oscillatory flow across the slot excited by the external shedding. These wall jets lead to the formation of multi-pole vortices in the shear layers and periodic ejection of vorticity leading to ‘aerodynamic tripping’ of the shear layers. It seems likely that the separated shear layers undergo bypass transition to turbulence and reattach onto the base, resulting in formation of periodic separation bubbles around the base of the slotted cylinder. Variation of instantaneous static pressures around the slotted cylinder shows oscillations with increasing amplitude from the stagnation point, which tend to coalesce towards the beginning of the slot. Soon after the slot, large amplification of pressure occurs due to the wall-jet. Time-averaged pressure distributions indicate increased pressures in the entire separated flow region of the slotted cylinder as compared to that of the basic cylinder. The slotted cylinder shows dramatic reductions of more than 80% in the mean and fluctuating drag and lift forces. The present studies show that the vortex-induced vibrations of a circular cylinder could be passively controlled using natural ventilation through a slot across the cylinder.