Turbulence model choice for the calculation of drag forces when using the CFD method

Center-Line Velocity Change Regime in a Parallel-Flow Square Exhaust Hood

A parallel-flow exhaust hood is an effective ventilation device to control dust and toxic pollutants and protect the occupational health of workers, whether it is used alone or combined with a uniform air supply hood in a push-pull ventilation system. Some scholars have studied the outside air flow characteristics of the conventional exhaust hood with non-uniform air speed at the hood face, but the law of velocity variation outside the parallel-flow exhaust hood is not clear at present. Therefore, this paper uses the dimensionless method to study the center-line velocity change regime in a parallel-flow square exhaust hood based on simulation and experimental data. The results show that the dimensionless center-line velocity has a good change law with the characteristic length of exhaust hood in a parallel-flow square exhaust hood, which can eliminate the influence of hood face velocity and the hood size on the velocity change regime; and the experimental data is basically consistent with the calculated data, which shows that the regression equation method is reliable.

Aerodynamic investigation of the thermo-dependent flow structure in the wake of a cyclist

The main purpose of this study was to assess the influence of the environmental temperature on both the aerodynamic flow evolving around the bicycle and cycling power output. The CFD method was used to investigate the detailed flow field around the cyclist/bicycle system for a constant speed of 11.1 m/s (40 km/h) without wind. In complement, a mathematical model was used to determine the temperature-dependent power output in the range [-10; 40 °C]. The numerical investigation gives valuable information about the turbulent flow field in the cyclist's wake which evolves accordingly the surrounding temperature. A major result of this study is that the areas of overpressure upstream of the cyclist and of underpressure downstream of him are less extensive for a temperature of 40 °C compared to -10 °C. The results suggest that the aerodynamic braking effect of the bicycle is minimized when the air temperature is high, as a lower air density results in a reduction in drag on the cyclist. This study showed that the power required to maintain a constant speed is reduced when the temperature is high, the reason being a lower aerodynamic resistance.

Study on Flow Field Characteristics of the 90° Rectangular Elbow in the Exhaust Hood of a Uniform Push⁻Pull Ventilation Device

A uniform push–pull ventilation device can effectively improve indoor air quality (IAQ). The 90° rectangular elbow is an important part of the push–pull ventilation device. This paper analyzes the flow field characteristics of the 90° rectangular elbows under different working conditions. This was done by using computational fluid dynamics (CFD) simulation (Fluent). The flow lines, velocity and pressure distribution patterns of the elbow flow field are revealed in detail. The wind velocity non-uniformity and wind pressure non-uniformity of the 90° rectangular elbows with different coefficients of radius curvature R and rectangular section height h are also compared. The results show that when R ≥ 2.5 h, the wind flow traces inside the elbow are basically parallel lines. At the same time, the average wind velocity and the average wind pressure are stable. Also, the wind velocity non-uniformity and wind pressure non-uniformity decrease with the increase of R. Therefore, considering the space and material loss caused by an increase in radius of curvature, the elbow with R = 2.5 h can be used as the best design structure for the 90° rectangular elbow, which is of great significance for improving the control effect of dust and toxic pollutants in a uniform push–pull ventilation device.

Explosive lower limb extension mechanics: An on-land vs. in-water exploratory comparison

During a horizontal underwater push-off, performance is strongly limited by the presence of water, inducing resistances due to its dense and viscous nature. At the same time, aquatic environments offer a support to the swimmer with the hydrostatic buoyancy counteracting the effects of gravity. Squat jump is a vertical terrestrial push-off with a maximal lower limb extension limited by the gravity force, which attracts the body to the ground. Following this observation, we characterized the effects of environment (water vs. air) on the mechanical characteristics of the leg push-off. Underwater horizontal wall push-off and vertical on-land squat jumps of two local swimmers were evaluated with force plates, synchronized with a lateral camera. To better understand the resistances of the aquatic movement, a quasi-steady Computational Fluid Dynamics (CFD) analysis was performed. The force-, velocity- and power-time curves presented similarities in both environments corresponding to a proximo-distal joints organization. In water, swimmers developed a three-step explosive rise of force, which the first one mainly related to the initiation of body movement. Drag increase, which was observed from the beginning to the end of the push-off, related to the continuous increase of body velocity with high values of drag coefficient (C_D) and frontal areas before take-off. Specifically, with velocity, frontal area was the main drag component to explain inter-individual differences, suggesting that the streamlined position of the lower limbs is decisive to perform an efficient push-off. This study motivates future CFD simulations under more ecological, unsteady conditions.

Aerodynamic study of time-trial helmets in cycling racing using CFD analysis

The aerodynamic drag of three different time-trial cycling helmets was analyzed numerically for two different cyclist head positions. Computational Fluid Dynamics (CFD) methods were used to investigate the detailed airflow patterns around the cyclist for a constant velocity of 15 m/s without wind. The CFD simulations have focused on the aerodynamic drag effects in terms of wall shear stress maps and pressure coefficient distributions on the cyclist/helmet system. For a given head position, the helmet shape, by itself, obtained a weak effect on a cyclist's aerodynamic performance (<1.5%). However, by varying head position, a cyclist significantly influences aerodynamic performance; the maximum difference between both positions being about 6.4%. CFD results have also shown that both helmet shape and head position significantly influence drag forces, pressure and wall shear stress distributions on the whole cyclist's body due to the change in the near-wake behavior and in location of corresponding separation and attachment areas around the cyclist.

Individual-Environment Interactions in Swimming: The Smallest Unit for Analysing the Emergence of Coordination Dynamics in Performance?

Displacement in competitive swimming is highly dependent on fluid characteristics, since athletes use these properties to propel themselves. It is essential for sport scientists and practitioners to clearly identify the interactions that emerge between each individual swimmer and properties of an aquatic environment. Traditionally, the two protagonists in these interactions have been studied separately. Determining the impact of each swimmer's movements on fluid flow, and vice versa, is a major challenge. Classic biomechanical research approaches have focused on swimmers' actions, decomposing stroke characteristics for analysis, without exploring perturbations to fluid flows. Conversely, fluid mechanics research has sought to record fluid behaviours, isolated from the constraints of competitive swimming environments (e.g. analyses in two-dimensions, fluid flows passively studied on mannequins or robot effectors). With improvements in technology, however, recent investigations have focused on the emergent circular couplings between swimmers' movements and fluid dynamics. Here, we provide insights into concepts and tools that can explain these on-going dynamic interactions in competitive swimming within the theoretical framework of ecological dynamics.

Hydrodynamic analysis of human swimming based on VOF method

A 3-D numerical model, based on the Navier-Strokes equations and the RNG k-ε turbulence closure, for studying hydrodynamic drag on a swimmer with wave-making resistance taken into account is established. The volume of fluid method is employed to capture the undulation of the free surface. The simulation strategy is evaluated by comparison of the computed results with experimental data. The computed results are in good agreement with data from mannequin towing experiments. The effects of the swimmer's head position and gliding depth on the drag force at different velocities are then investigated. It is found that keeping the head aligned with the body is the optimal posture in streamlined gliding. Also wave-making resistance is significant within 0.3 m depth from the free surface.

Numerical Investigation of Swimmer's Gliding Stage with 6-DOF Movement

The purpose of this study is to analyze the motion status of swimmers during their gliding stage using a numerical simulation method. This simulation strategy is conducted by solving the 3D incompressible Navier-Stokes equations using the Realizable k-ε turbulence closure equations in combination with the Six Degrees of Freedom (6-DOF) method. The uneven mass distribution of a swimmer and the roughness of the surface of the body are taken into consideration. The hydrodynamic characteristics and movement characteristics of the swimmers at different launch speeds were analyzed. The calculated results suggest that an optimal instant for starting propulsive movement is when the velocity of the swimmer decreases by 1.75 m/s to 2.0 m/s from an initial horizontal velocity of 3.1 m/s to 3.5 m/s.

Numerical and experimental investigations of human swimming motions

This paper reviews unsteady flow conditions in human swimming and identifies the limitations and future potential of the current methods of analysing unsteady flow. The capability of computational fluid dynamics (CFD) has been extended from approaches assuming steady-state conditions to consideration of unsteady/transient conditions associated with the body motion of a swimmer. However, to predict hydrodynamic forces and the swimmer's potential speeds accurately, more robust and efficient numerical methods are necessary, coupled with validation procedures, requiring detailed experimental data reflecting local flow. Experimental data obtained by particle image velocimetry (PIV) in this area are limited, because at present observations are restricted to a two-dimensional 1.0 m(2) area, though this could be improved if the output range of the associated laser sheet increased. Simulations of human swimming are expected to improve competitive swimming, and our review has identified two important advances relating to understanding the flow conditions affecting performance in front crawl swimming: one is a mechanism for generating unsteady fluid forces, and the other is a theory relating to increased speed and efficiency.

The Role of the Hand During Freestyle Swimming

The connections between swimming technique and the fluid dynamical interactions they generate are important for assisting performance improvement. Computational fluid dynamics (CFD) modeling provides a controlled and unobtrusive way for understanding the fundamentals of swimming. A coupled biomechanical–smoothed particle hydrodynamics (SPH) fluid model is used to analyze the thrust and drag generation of a freestyle swimmer. The swimmer model was generated using a three-dimensional laser body scan of the athlete and digitization of multi-angle video footage. Two large distinct peaks in net streamwise thrust are found during the stroke, which coincide with the underwater arm strokes. The hand motions generate vortical structures that travel along the body toward the kicking legs and the hands are shown to produce thrust using both lift and drag. These findings advance understanding of the freestyle stroke and may be used to improve athlete technique.

Cyclist Drag in Team Pursuit: Influence of Cyclist Sequence, Stature, and Arm Spacing

In team pursuit, the drag of a group of cyclists riding in a pace line is dependent on several factors, such as anthropometric characteristics (stature) and position of each cyclist as well as the sequence in which they ride. To increase insight in drag reduction mechanisms, the aerodynamic drag of four cyclists riding in a pace line was investigated, using four different cyclists, and for four different sequences. In addition, each sequence was evaluated for two arm spacings. Instead of conventional field or wind tunnel experiments, a validated numerical approach (computational fluid dynamics) was used to evaluate cyclist drag, where the bicycles were not included in the model. The cyclist drag was clearly dependent on his position in the pace line, where second and subsequent positions experienced a drag reduction up to 40%, compared to an individual cyclist. Individual differences in stature and position on the bicycle led to an intercyclist variation of this drag reduction at a specific position in the sequence, but also to a variation of the total drag of the group for different sequences. A larger drag area for the group was found when riding with wider arm spacing. Such numerical studies on cyclists in a pace line are useful for determining the optimal cyclist sequence for team pursuit.

Numerical Streamline Patterns at Swimmer’s Surface Using RANS Equations

The objective of this article is to perform a numerical modeling on the flow dynamics around a competitive female swimmer during the underwater swimming phase for a velocity of 2.2 m/s corresponding to national swimming levels. Flow around the swimmer is assumed turbulent and simulated with a computational fluid dynamics method based on a volume control approach. The 3D numerical simulations have been carried out with the code ANSYS FLUENT and are presented using the standard k-ω turbulence model for a Reynolds number of 6.4 × 106. To validate the streamline patterns produced by the simulation, experiments were performed in the swimming pools of the National Institute of Sports and Physical Education in Paris (INSEP) by using the tufts method.

Three-Dimensional CFD Analysis of the Hand and Forearm in Swimming

The purpose of this study was to analyze the hydrodynamic characteristics of a realistic model of an elite swimmer hand/forearm using three-dimensional computational fluid dynamics techniques. A three-dimensional domain was designed to simulate the fluid flow around a swimmer hand and forearm model in different orientations (0°, 45°, and 90° for the three axes Ox, Oy and Oz). The hand/forearm model was obtained through computerized tomography scans. Steady-state analyses were performed using the commercial code Fluent. The drag coefficient presented higher values than the lift coefficient for all model orientations. The drag coefficient of the hand/forearm model increased with the angle of attack, with the maximum value of the force coefficient corresponding to an angle of attack of 90°. The drag coefficient obtained the highest value at an orientation of the hand plane in which the model was directly perpendicular to the direction of the flow. An important contribution of the lift coefficient was observed at an angle of attack of 45°, which could have an important role in the overall propulsive force production of the hand and forearm in swimming phases, when the angle of attack is near 45°.