Flow over a step cylinder using partially-averaged Navier-Stokes equations with application towards dynamic subsea power cables

On vortex shedding from low aspect ratio dual step cylinders

Computational Fluid Dynamics (CFD) has gained recognition as a valuable simulation tool for many industrial applications, creating immense opportunities for introducing CFD into standard undergraduate curriculum. This paper is focused on an extensive aerodynamic CFD study related to the 2009 Supermileage Vehicle. Supermileage Competition is a trademark competition conducted by the Society of Automotive Engineers (SAE). This paper provides an aid for future undergraduate students and young researchers interested in designing low drag vehicles. The major part of this study includes airfoil selections and three dimensional CFD iterations to optimize the body design. The methodology focuses on an inside-out approach to optimization of body shape through computation of aerodynamic forces on a low mass vehicle. The 2008 Supermileage car, after four design iterations, had a drag coefficient of 0.16. The 2009 body, in its final full body design, has a drag coefficient equal to 0.12, which is the lowest drag coefficient ever calculated for a University of Windsor Supermileage car. Due to the current economic state and reduced funding, an additional low budget vehicle was also designed and extensive aerodynamic studies were conducted to validate the design. The new low budget design includes a scoop at the leading edge of the vehicle and the CFD model included a driver model for accuracy. After several design iterations the low budget design yielded a drag coefficient of 0.14. Due to the weight savings, conforming to SAE rules, and considering the current economic conditions, the low budget design was finalized for the 2009 Supermileage Vehicle.

In this paper, we describe the use of a new nonlinear partially-averaged Navier–Stokes (PANS) model with near-wall correction for simulating the cavitating flow around a Clark-Y hydrofoil. For comparison, the standard [Formula: see text]–[Formula: see text] PANS model is also used. The results demonstrate that compared to [Formula: see text]–[Formula: see text] PANS and experiment, the new PANS model shows better performance for cavitation flow, including time-averaged velocity, root mean square (rms) velocity and cavity shedding processing. Through the calculation of the lift and drag coefficient at [Formula: see text] and [Formula: see text], it can be concluded that the cavitation will decrease the lift and increase the drag of the hydrofoil, resulting in a decrease of the lift-to-drag ratio. From the analysis of different terms in both the turbulent kinetic energy (TKE) and dissipation rate transport equations of the cloud cavitation, it is found that the production term and the dissipation term are dominant in the turbulent transport, and they are mainly distributed in the vapor–liquid interface and the trailing edge of the hydrofoil.

The proximity interference and the vortex dislocation are fundamental fluid dynamic features that determine a wake pattern of dual-step circular cylinders in a side-by-side arrangement. Each dual-step cylinder consists of two coaxial cylinders with different diameters. A complex wake modulation is expected due to an intrinsic interaction between the vortex shedding of the larger-diameter cylinder (LC) and the smaller-diameter cylinder (SC), and to the cross-wake interaction of side-by-side cylinders in a uniform flow. To understand the physics of fluid wake modulations, three-dimensional direct numerical simulations of the flow around a pair of dual-step circular cylinders in side-by-side arrangements are performed and investigated. Six simulation cases are presented with different combinations of diameter ratio (1.33–2.00), gap ratio (1.00–3.17), and the Reynolds number (60–200) in a laminar flow regime, providing key insights into the two distinct LC-dominated and SC-governing wake modulation characteristics. For the LC-dominated wake modulation, the wake pattern of side-by-side cylinders is determined by the predominant vortex dynamics in the LC wake. For the SC-governing wake modulation, the LC wake pattern becomes modulated and controlled by the SC wake through the connections of vortex tubes shed downstream of the two nonuniform cylinders. Owing to the proximity interference and vortex dislocation, these new evolutionary wake modulations result in distinct wake patterns, vortex structures, and associated hydrodynamic force features for the pair of side-by-side dual-step circular cylinders.

In human swimming, the total drag comprises the skin friction drag, form drag and wave drag. The relative contribution of each component to the overall hydrodynamic drag is controversy issue. PURPOSE: To analyse the relative contribution of the skin friction drag and the form drag for the total drag during the gliding, using computational fluid dynamics. METHODS: A 3-D domain was created to simulate the fluid flow around a swimmer model. The numerical simulation analysis consisted of the use of a three-dimensional mesh of cells that simulates the flow around the human body. Computational fluid dynamics methodology uses the finite volume approach, where the equations are integrated over each control volume. The k-epsilon turbulent model was applied to the flow around a three-dimensional model of a male adult swimmer in two gliding positions: in ventral position with the arms extended at the front and in ventral position with the arms aside the trunk. The swimmer model middle line was placed at a water depth of 0.90 m, equidistant from the top and bottom surfaces of the 3-D domain. The coefficient of drag (CD) was computed using a steady flow velocity of 2 m/s for both gliding situations. The CD was decomposed into form and skin friction drag. RESULTS: The position with the arms extended at the front presented a CD value of 0.43 whereas the position with the arms aside the trunk presented a CD value of 0.74. In the position with the arms extended at the front, form drag and skin friction drag represented, approximately, 87% (CD = 0.37) and 13% (CD = 0.06) of the total drag, respectively. In the position with the arms aside the trunk, form drag and skin friction drag represented, approximately, 92% (CD = 0.68) and 8% (CD = 0.06) of the total drag, respectively. CONCLUSIONS: The gliding position with the arms extended at the front produced lower drag coefficients than with the arms placed aside the trunk. Although form drag was dominant, skin friction drag was by no means negligible during the swimming gliding. Supported by FCT (SFRH/BD/25241/2005; POCTI/DES/58872/2004).

Simultaneous measurements of lift and drag forces have been performed in order to study passive control of unsteady loads induced on a circular cylinder. For this purpose, an aerodynamic balance has been developed. The balance, developed for a cylinder of 25.4 mm in diameter, was designed to operate in the subcritical regime (Re = 32000). This instrument is characterized by its sensitive element that forms a small central part of the cylinder. The static and dynamic calibrations of the balance show the appropriateness of the present design. Moreover, qualification experiments carried out with a single cylinder gave results (mean and rms values of the lift and drag coefficients) that are in good agreement with those found in the literature. The purpose of this paper is to present a passive control experiment performed by means of the wake of a smaller cylinder interacting with a larger one. Firstly, a parametric study was performed by varying the following: i) the diameter d_s of the small cylinder for one large cylinder diameter d (7 values in the range 0.047 ≤ d_s/d ≤ 0.125); ii) the center-to-center spacing S/d (11 values in the range 1.375 ≤ S/d ≤ 2.5); and iii) the stagger angle a (0 ≤ a ≤ 90 with a fine angular step Da for a ≤ 15 .). A maximum mean drag reduction of about 48% is achieved. At a = 4· to 8·, one can observe a peak of mean lift coefficient. Then unsteady fluid forces, vortex shedding frequency and flow pattern were systematically investigated for the small cylinder having a diameter d_s = 2.4 mm (0.094 d). Reductions of 78% and 56% of the rms lift and drag fluctuations respectively were obtained with the small cylinder placed at a slight stagger angle in the range 6· < a < 9·. This leads to an instantaneous force vector that exhibits more steadiness both in angle and amplitude. Moreover, at these stagger angles, the energy of the lift fluctuations at the shedding frequency is significantly reduced compared to the single cylinder case.

CFD simulations and experimental measurements of flow past free-surface piercing, finite length cylinders with varying aspect ratios

This study uses numerical simulations to investigate the complex fluid dynamics around two adjacent rotating cylinders arranged side by side. The primary focus is on understanding the influence of Reynolds number (Re), rotational speed ([Formula: see text]), gap spacing ([Formula: see text]), and compressibility effects at different Mach numbers ([Formula: see text]). Simulations were performed using ANSYS Fluent 19.0, employing a finite volume-based solver and the shear stress transport (SST) [Formula: see text]–[Formula: see text] turbulence model. The numerical methodology is validated against experimental results for flow over a single rotating cylinder and numerical studies for the two-cylinder configuration. The analysis encompasses velocity magnitude contours, pressure distributions, vorticity patterns, and drag coefficients to elucidate the intricate flow interactions over two vertically placed side-by-side cylinders. Across Reynolds numbers ranging from 50 to 200, the flow exhibits distinct transitions from laminar to turbulent regimes, characterized by forming vortices and emerging flow separation zones. Higher Reynolds numbers generally lead to more turbulent and asymmetric flow patterns, increasing drag coefficients due to intensified wake formation and vortex shedding. The cylinders’ rotational speed significantly influences flow dynamics: lower speeds favor symmetric vortex formations, while higher speeds induce complex turbulent flows with enhanced vortex shedding. Outward rotation of the cylinders accelerates flow between them, increasing velocities and drag coefficients, whereas inward rotation reduces velocities and drag coefficients due to reduced flow interactions. Alterations in the gap spacing ([Formula: see text]) between cylinders reveal critical insights: smaller gaps intensify rotational effects, enhancing vortex shedding and increasing drag coefficients, whereas larger gaps promote stable flows with reduced vortex-shedding tendencies and lower drag coefficients. Compressibility effects at Mach numbers ranging from 0.2 to 0.8 amplify wake elongation, flow compression, and asymmetric vortex shedding, contributing to higher drag coefficients in transonic and supersonic regimes. This study advances understanding of fluid dynamics around cylindrical configurations, providing insights essential for optimizing engineering designs and performance across various applications. Future research directions may involve refining models, exploring additional parameters, and advancing predictive capabilities further to enhance practical insights into similar fluid flow phenomena.

The Shell Eco-Marathon challenge is an annual competition held to challenge students in innovating the most fuel efficiency vehicle for either a prototype or an urban concept vehicle. An urban concept vehicle is designed for fuel efficiency using electricity as source of power. Apart from the use of electricity as an alternative to internal combustion engines, the design of the vehicle is also crucial for efficiency. The car bodywork design needs to be aerodynamically designed to minimise drag and subsequently use less energy to move. The design must also incorporate structural integrity to protect the driver as well as providing airflow for sufficient ventilation both inside the passenger and the engine compartment. Five models for the rear and front designs were produced using CATIA and analysed using Computational Fluid Dynamics in ANSYS Fluent. The models underwent a virtual wind tunnel on three different air velocity speeds, 20 km/h, 30 km/h and 40 km/h to generate a force report of drag force and coefficient on each design. The front design is chosen based on the lowest drag coefficient and force while the rear design is selected based on a balanced downforce while achieving the lowest practical drag force. The results demonstrated that the air resistance faced by a car was highly influenced by both front and rear design of the body.

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.

Study of active flow control for a simplified vehicle model using the PANS method

Unmanned aerial vehicles (UAV) are used in the military field for missions such as observation, surveillance and seeking object. UAV observation can be used to monitor humans, weather, forests, agriculture, and fisheries and so on. UAV can be operated for very broad missions with low development costs compared to manned aerial vehicles with a low risk of death. The design of an aircraft affects the performances of the aircraft itself. Aircraft flying characteristics are influenced by the value of coefficient of drag (CD) and coefficient of lift (CL). The value of CD affects how much aircraft friction with the air, while the value of CL affects how much lifting force the aircraft is produced so it can fly. The design output needs the most optimal CD and CL values to get the optimal aircraft performance. In this study, we simulated the effect of 4 digit NACA changes on the characteristics of the plane’s CD and CL. This study uses variations modification of NACA digits with different variables. NACA airfoil 4 digit series (MPXX) designates camber lines, maximum camber position and thickness. Based on the results of this study, the optimal CD and CL values for the Airfoil NACA with the Aerosonde UAV model are designs that have a large ratio of lift and drag (CL / CD). The lower the CD value and the higher the CL value, the better the performance of the UAV. The design that has the largest CL / CD is the NACA 6412 Airfoil.

In this work, nonequilibrium molecular dynamics (MD) simulations were performed to investigate uniform liquid argon flow past a carbon nanotube. In the simulation, nanotubes were modeled as rigid cylinders of carbon atoms. Both argon-argon and argon-carbon interactions were calculated based on Lennard-Jones potential. Simulated drag coefficients were compared with (i) published empirical equation which was based on experiments conducted with macroscale cylinders and (ii) finite element (FE) analyses based on Navier-Stokes equation for flow past a circular cylinder using the same dimensionless parameters used in MD simulations. Results show that classical continuum mechanics cannot be used to calculate drag on a nanotube. In slow flows, the drag coefficients on a single-walled nanotube calculated from MD simulations were larger than those from the empirical equation or FE analysis. The difference increased as the flow velocity decreased. For higher velocity flows, slippage on the surface of the nanotube was identified which resulted in lower drag coefficient from MD simulation. This explains why the drag coefficient from MD dropped faster than those from the empirical equation or FE simulation as the flow velocity increased. It was also found that the drag forces are almost equal for single- and double-walled nanotubes with the same outer diameter, implying that inner tubes do not interact with fluid molecules.

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

This paper presents a comparative aerodynamic analysis of two vehicle models—a hatchback and a sedan—using computational fluid dynamics (CFD) simulations conducted with SimScale software. The primary objective is to evaluate the behavior of the external flow around the body, focusing on determining the drag (Cd) and lift (Cl) coefficients, as well as analyzing the flow patterns and recirculation zones. The vehicle geometries were generated using the artificial intelligence tool Tripo3D.AI, to investigate the feasibility of automated models for aerodynamic analysis. The study was conducted with the k – ω SST turbulence model, suitable for simulations in regions with flow separation and boundary layers. The mesh was composed of polyhedral elements with local refinements in regions of interest, automatically generated by SimScale. The boundary conditions included velocity inputs of 25 m/s, walls with no-slip and slip conditions, and a lower wall with movement equal to the flow velocity. The results indicated a drag coefficient (Cd) of 0.308 for the sedan model and 0.601 for the hatchback, representing an increase of approximately 95% in Cd for the hatchback compared to the sedan. Regarding the lift force, a value of -0.00014 was observed for the sedan and -1.97 for the hatchback, indicating a greater vertical aerodynamic effect in the hatchback. The simulations showed good numerical convergence, validating the adopted approach.