Angle of attack dependence of flow past cactus-inspired cylinders with a low number of ribs

The aerodynamic coefficients and the Strouhal number of cylinders with three and four ribs, inspired by succulents Euphorbia trigona and Euphorbia Abyssinica are investigated using 2D Unsteady Reynolds-Averaged Navier-Stokes simulations at Reynolds number 20,000. Both configurations show a significant dependence of the studied characteristics on the angle of attack. The obtained results are compared to the smooth circular cylinder, previous results for cylinders with 24 ribs based on the Saguaro cactus, and cylinders with triangular and square cross-sections. Relative to the circular cylinder, the mean drag coefficient is lowered only for the four-rib case at high angles of attack. However, at some angular positions, the ability to reduce unsteady force fluctuations exceeds Saguaro-inspired cylinders. For both shapes studied, the Strouhal number at most angles of attack is lower compared to both the circular cylinder and cylinders with 24 ribs at the same Reynolds number. The minimum values of the aerodynamic coefficients for both configurations are related to the angular orientation. For the four-rib case a critical angle of αcr ≈ 40◦ is observed, at which the mean drag coefficient and the fluctuating lift coefficient attain their minima. The mean lift coefficient reaches at this angle its maximum value before a sudden drop for higher angles of attack. Therefore, for cactus-shaped cylinders with four ribs high angles of attack give the optimum orientation relative to prevailing winds

An Experimental and Numerical Investigation of Drag Reduction Through Biomimetic Modelling

This paper characterises flow around a circular cylinder and investigates the potential of a non-smooth surface to reduce air resistance in a Reynolds number range between Red = 8.09 104 and Red = 2.02 105. The two- and three-dimensional numerical simulations were performed using a steady-state solution and the Reynolds-averaged Navier-Stokes (RANS) approaches. Three different mesh designs and four turbulence models with various treatments were assessed and compared against experimental data. A total of 26 uneven preparations in the two-dimensional analysis and two riblet structures in the three-dimensional analysis were designed to investigate the effect of drag reduction. The results reported hold the potential of uneven structures to reduce the air resistance in the case of a circular cylinder. This research further demonstrates that engineering applications can benefit from mimicking nature's details and functions

A Review of Surface Texturing in Internal Combustion Engine Piston Assembly

This paper presents a brief review of surface texturing with a focus on piston assembly application. The paper begins with a general discussion on surface texturing and the manufacturing process of micro dimples.  Further, it discusses the theory of hydrodynamic lift generation and the effect of parameters of micro dimples texture on the surface-to-surface friction. Finally, the effect of surface texturing on heat transfer is briefly discussed. In pursuits to improve internal combustion engine (ICE) efficiency, tribological improvement of moving surfaces by means of micro surface texturing seems to be one of the way. However, texturing parameters have to be carefully designed as it can cause detrimental effect if the designs are wrong. Studies has shown micro surface texturing at piston ring could reduce friction around 20%-50% compare with un-textured piston ring and also reduce fuel consumption at 4%. Micro Surface texturing could also improve heat transfer between the surfaces to reduce piston slap and lubrication oil temperature. As reports on the surface texturing on friction reduction and heat transfer improvement in piston assembly are relatively scarce, it is suggested that optimization of micro dimple parameters for piston skirt application and its effect on engine tribology and heat transfer characteristics be further investigated

Numerical and Experimental Studies of Flow over Circular Cylinders Covered with Circumferential Riblets or Grooves

In this study, the flow past a circular cylinder partly or fully covered with circumferential riblets or grooves is investigated numerically and experimentally over the Reynolds number (Re D) range of 490 ~ 3900. The purpose is to explore the potential of a passive flow control strategy by placing circumferential riblets or grooves of various arrangements on the cylinder surface. Although many passive techniques have been developed for controlling the flow past circular cylinders, there is a lack of research of flow past cylinders covered with circumferential riblets or grooves. Given the extensive engineering applications of cylindrical structures, it is useful to understand the influence of circumferential riblets or grooves on the flow structures around a circular cylinder and the consequent hydrodynamic behaviour of the cylinder. Further, confined flows over circular cylinders with and without circumferential riblets at a fixed blockage ratio of 2/5 are investigated experimentally and numerically. Existing studies of confined flows (i.e., with a non-zero blockage ratio) exclusively focus on bare cylinders, but cylinders covered with riblets under confinement have not been considered. The outcome of this study may be applied for enhancing mixing and heat transfer. Unveiling the flow process and mechanism underlying the effects of circumferential riblets on the flow structures, wake dynamics and hydrodynamic characteristics of a circular cylinder in both confined and unconfined flows is another highlight of this thesis

Plasma texturing for enhanced tribological performance of cast iron

Cathodic plasma electrolysis (CPE) was used to create surface texturing on gray iron samples, which could reduce the friction and increase the wear resistance. During the treating process, cast iron sample served as a cathode where the plasma discharging occurred, increasing the surface hardness and leaving an irregular array of micro craters on the surface. Modified surface morphology was determined from scanning electron microscope (SEM) and surface profiler. Recessed and protruded surface textures were observed when the CPE was applied at low and high voltages, respectively. Pin-on-disk tribotests were conducted on CPE-treated samples and untextured sample. The friction of as-treated samples could be reduced in boundary lubrication regime at low sliding speed due to the ability to store lubricant. Besides that, the surface texture generated extra hydrodynamic pressure that separated two sliding surfaces, increased the oil film thickness and accelerated the transition from boundary to mixed lubrication at high sliding speeds

A selection of biomechanical research problems : from modeling to experimentation

The research undertakings within this manuscript illustrate the importance of biomechanics in todays science. Without doubt, biomechanics can be utilized to obtain a better understanding of many unsolved mysteries involved in the field of medicine. Moreover, biomechanics can be used to develop better prosthetic or surgical devices as well. Chapter 2 represents a medical problem, which has not been solved for more than a century. With the use of fundamental principles of biomechanics, a better insight of this problem and its possible causes were obtained. Chapter 3 investigates the mechanical interaction between the human teeth and some processed food products during mastication, which is a routine but crucial daily activity of a human being. Chapter 4 looks at a problem within the field of surgery. In this chapter the stability and reliability of two different suturing techniques are explored. Chapters 5 and 6 represent new patent designs as a result of the investigations made in Chapter 4. Chapter 7 studies the impact and load transfer patterns during the collision between a child\u27s head and the ground. All of the above mentioned chapters show the significance of biomechanics in solving a range of different medical problems that involve physical and or mechanical characters.\u2

Effects of Surface Topographies on Heat and Fluid Flows

Responses of annular and planar flows to the introduction of grooves on the bounding surfaces have been analyzed. The required spectral algorithms based on the Fourier and Chebyshev expansions have been developed. The difficulties associated with the irregularities of the physical domain have been overcome using either the immersed boundary conditions (IBC) concept or the domain transformation method (DT). Steady flows in annuli bounded by walls with longitudinal grooves have been studied. Analysis of pressure losses showed that the groove-induced changes can be represented as a superposition of a pressure drop due to a change in the average position of the bounding cylinders and a pressure drop due to the flow modulations induced by the shape of the grooves. The former effect can be evaluated analytically while the latter requires explicit computations. It has been shown that the reduced-order model is an effective tool for extraction of features of the groove geometry that lead to flow modulations relevant to drag generation. It has been shown that the presence of the longitudinal grooves may lead to a reduction of the pressure loss in spite of an increase of the wetted surface area. The form of the optimal grooves from the point of view of the maximization of the drag reduction has been determined. When mixing augmentation is not available, heat can be transported across micro-channels by conduction only. A method to increase this heat flow has been proposed. The method relies on the use of grooves parallel to the flow direction. It has been shown that it is possible to find grooves that can increase the heat flow and, at the same time, can decrease the pressure losses. The optimal groove shape that maximizes the overall system performance has been determined. Since it has been assumed that the flow must be laminar, it is of interest to determine the maximum Reynolds number for which this assumption remains valid. The stability characteristics of flow in a grooved channel have been studied. Only disturbances corresponding to the travelling waves in the limit of zero groove amplitude have been found. It has been shown that disturbances corresponding to the two-dimensional waves in a smooth channel play the critical role in the grooved channel. The highly three-dimensional disturbance flow topology at the onset of the instability has been described. It has been demonstrated that the presence of the grooves leads to flow stabilization for the groove wave numbers 4.22 and flow destabilization for larger . The stabilizing/destabilizing effects increase with the groove amplitude. Variations of the critical Reynolds number over the whole range of the groove wave numbers and the groove amplitudes of interest have been determined. Special attention has been paid to the effects of the long wavelength, drag reducing grooves. It has been shown that such grooves lead to a small increase of the critical Reynolds number compared with the smooth channel

Case of a hydraulic splashing nozzle: Effect of the disk geometry on the sheet breakup

In the agricultural field, conventional farmers use Plant Protection Products (PPP) to control crop enemies as for instance weeds, diseases and pests. In practice, PPP application techniques are based on droplet clouds to carry the spray mixture containing the active ingredient and the adjuvant to the surface target. The PPP application efficiency consists in maximizing the deposition of the mixture on the target while reducing the environmental losses. However, the droplet characteristics within the spray affect drastically the treatment efficiency. For instance, small droplets ( 300 µm) have the tendency to splash on a specific target surface. The widely used agricultural nozzles produce a liquid sheet that disintegrates into ligaments leading to droplets with various diameters and velocities. Hence, the generated spray is characterized by a wide droplet size distribution (RSF = 1) resulting in potential drifting or efficiency losses due to splashing phenomena. The spray must deliver an optimal droplet distribution in term of diameters and velocities by reducing the extent of the droplet size distribution. The design of new agricultural nozzles is a challenge for the practitioners in the field of agricultural nozzles. As the simplicity, the robustness, the low cost and the high flow rate ranges are required for the agronomic application, the Savart configuration namely a round jet impacting vertically a motionless disk is the ideal candidate for the massive production of droplets. The Savart sheet develops in the air (outside the disk) and it results in random breakup leading to wide droplet size distribution as in the case in hydraulic nozzles. As the used flow ranges are high in the PPP application, the obtained sheet on the disk is turbulent. The challenge is to tame the turbulent sheet. I propose to split the sheet into individual jets using textured disks by acting on the semi-free film or by inserting the right structures directly in the free sheet. Then, the jets break up according the Plateau-Rayleigh mechanism and lead to a narrower droplet size distribution. Therefore, this thesis aims to study experimentally the effect of the disk geometry on the sheet break up. This study is seen as a practical guide for specialists in fluid mechanics who desire developing the generation of drops with controlled sizes. One firstly detailed the experimental setup based on the impact of a turbulent jet on a non-textured disk. The disk configuration constitutes our reference case to which results on textured disks are to be confronted. The sheet was characterized through several parameters: The mean velocity U, the mean thickness h and the ejection angle φ that depends on the disk geometry and on the flow rate Q. As the jet flow is turbulent at the impact, local disturbances in the film triggered downstream the disk edge the appearance of random holes at a distance R′ (from the jet axis) in the liquid sheet. These holes lead to the disintegration of the sheet into droplets. Furthermore, the produced droplets are characterized in term of diameters. The droplet size distribution is clearly wide (RSF values are close to 1) that is similar to the case of hydraulic nozzles. Moreover, one perturbed the semi-free film evolving on the disk surface. The case of a turbulent round water jet impacting a disk engraved along its circumference by a number N of radial grooves is addressed. By the insertion of grooves, one controls the turbulent flow and the film splits into a number n of liquid jets before reaching the disk edge. The phase diagrams presented as a function of the inside gap between grooves d1 and the flow rate Q illustrate the transition between jet regimes. Droplets were characterized in term of their diameters and velocities. For all configurations including an engraved disk, the obtained droplet size distribution is narrower compared to the ungraved disk and to the standard flat fan nozzle Teejet TP 65 15. The V50 is reduced towards smaller droplets in the case of the engraved disk configurations but it is still coarse compared to that of the Teejet nozzle. Furthermore, one perturbed directly the Savart sheet, i.e. the free sheet evolves in the air. A number N of triangular prisms are set in the sheet at a radial distance r from the jet axis. This radial distance is strictly greater than D/2 (D = disk diameter) and less than or equal to R’ (for which the holes appear in the sheet). Once the number of prisms and the distance from the injector is fixed, the geometry is determined by the size of the prism and the distance b between two successive prisms. When increasing the flow rate Q, the jet numbers are n = 0, n = N and n = N* (elastic coalescence of jets) for large Q. One used a geometrical model that explains the generation of individual jets through these structures. Then, the emitted droplets are characterized in term of diameters and velocities. The droplet size distribution is narrower compared to the non-textured disk and the hydraulic nozzle TP 65 15 with the same spray class. The V50 decreased by decreasing the external gap b between two neighboring textures at the same radial distance r from the jet axis. Also, the V50 decreased by increasing the radial distance from the jet axis as the sheet thickness decreases. However, we are limited by R’ due to holes appearances. Finally, one concludes on the fundamental findings and on the role that the developed nozzle could play in applications. Also, one proposed some original perspectives to the present thesis such as testing the triangular prism textures in the case of the liquid sheet produced by a standard hydraulic nozzle