Bicycle aerodynamics:history, state-of-the-art and future perspectives

The importance of aerodynamics in cycling is not a recent discovery. Already in the late 1800s it was recognized as a main source of resistance in cycling. This knowledge was only rediscovered in the late 1970s and 1980s, when aerodynamic concepts were applied to bicycle equipment and cyclist positions, leading to new world hour records and Olympic medals. The renewed interest for cycling aerodynamics is significantly growing with the production of a vast literature, focused on increasing the comprehension of cycling aerodynamics and on improving the aerodynamics of bicycle equipment. Finding the connection between the different subfields of cycling aerodynamics and linking new research with past discoveries is crucial to efficiently drive future studies. Therefore, the present paper provides a comprehensive review of the history and the state-of-the-art in cycling aerodynamics, focusing on one of its main aspects: the bicycle. First, a short history of the bicycle is presented. Next, some cycling power models are outlined and assessment methods for aerodynamic drag are discussed, along with their main advantages and disadvantages. The core of this review paper addresses the components constituting the bicycle: frame and tubes, wheels, handlebar and other equipment. Finally, some future perspectives on bicycle aerodynamics are provided.</p

CFD simulations of a vertical axis wind turbine in dynamic stall: URANS vs. Scale-Adaptive Simulation (SAS)

Vertical axis wind turbines (VAWTs) are promising candidates for wind energy harvesting in the urban environment. However, their aerodynamic performance still falls behind of their horizontal axis counterparts. This could be associated to the comparatively small research they have received in the past decades as well as their complex unsteady aerodynamics. Computational Fluid Dynamics (CFD) has been widely used to evaluate and improve the aerodynamic performance of VAWTs. An extensive literature study reveals that the 2D unsteady Reynolds-Averaged Navier-Stokes (URANS) approach has been used in the majority of the CFD studies on VAWTs. The current study intends to evaluate the aerodynamic performance of a VAWT, calculated using 2D URANS, and compare it with that of 2.5D URANS and 2.5D scale-adaptive simulation (SAS). SAS is a hybrid RANS-LES model developed by Menter and Egorov [1]. The four-equation transition SST turbulence model is employed in the URANS simulations as well as in the RANS region of the hybrid RANS-LES simulation. The studied turbine is a one-bladed Darrieus H-type VAWT with a solidity of 0.125 operating at a low tip speed ratio of 2.0, which corresponds to the most complex case for VAWTs where dynamic stall is dominant. The reduced frequency is 0.125 representing the high unsteadiness in the flow. Significant benefits of the one-bladed turbine are: (i) less blade-wake interactions while the essential flow features, such as dynamic stall, are still present, (ii) reduced computational costs due to the smaller number of cells. The turbine characteristics is based on the experiment by Simão Fereira et al. [2]. Validation studies for the one-bladed turbine as well as the other turbines have been performed [3-5]. A comparative analysis of the instantaneous tangential and normal loads on the turbine (see Fig. 1), spatiotemporal distribution of pressure coefficient (see Figs. 2a-c) and skin friction coefficient (see Fig. d-f) on the blade suction side, the evolution of the shed vorticity by the blade, dynamic loads on the blade and the turbine wake are employed to evaluate the performance of URANS modeling in comparison to the SAS model. The instantaneous turbine loads calculated using the 2D and the 2.5D URANS, shown in Fig. 1, are in line with minor differences in the downwind side. Despite the 180 times higher number of cells and 10 times finer time step of the SAS modeling, an overall good agreement exists between the 2D URANS and the SAS results. The predicted thrust coefficients for 2D and 2.5D URANS and SAS are 0.422, 0.424 and 0.430, respectively. Nevertheless, there exist noticeable differences between the URANS and SAS results in the bursting location of the laminar separation bubble (LSB), the evolution of the dynamic stall vortex (DSV), the leading-edge secondary and tertiary vortices and the trailing-edge separation. The findings of the present study help to highlight the deficiencies of URANS modeling of VAWTs in dynamic stall

Air Curtain Optimization

The term “impinging jet” refers to a high-velocity fluid stream that is ejected from a nozzle, a narrow opening or an orifice, and which impinges on a surface. As applied to the built environment, impinging jets are used in air curtains to separate two environments subjected to different environmental conditions with the purpose of improving thermal comfort, air quality, energy efficiency and fire protection in buildings. The design and application of state-of-the-art air curtains requires detailed knowledge of the relationship between the separation efficiency of air curtains—their main performance criterion—and a wide range of jet and environmental parameters involving air curtain design. In order to address the current knowledge gaps in the field, this project encompasses an investigation into the impact of different jet and environmental parameters on the performance of air curtains while giving special attention to the study of innovative jet excitation techniques by means of optimizing the separation efficiency of air curtains.&nbsp; This project is being carried out in close collaboration with the air curtain manufacturer ‘Biddle B.V.’. &nbsp

Air Curtain Optimization

The term “impinging jet” refers to a high-velocity fluid stream that is ejected from a nozzle, a narrow opening or an orifice, and which impinges on a surface. As applied to the built environment, impinging jets are used in air curtains to separate two environments subjected to different environmental conditions with the purpose of improving thermal comfort, air quality, energy efficiency and fire protection in buildings. The design and application of state-of-the-art air curtains requires detailed knowledge of the relationship between the separation efficiency of air curtains—their main performance criterion—and a wide range of jet and environmental parameters involving air curtain design. In order to address the current knowledge gaps in the field, this project encompasses an investigation into the impact of different jet and environmental parameters on the performance of air curtains while giving special attention to the study of innovative jet excitation techniques by means of optimizing the separation efficiency of air curtains.&nbsp; This project is being carried out in close collaboration with the air curtain manufacturer ‘Biddle B.V.’. &nbsp

CFD analysis of an exceptional cyclist sprint position

A few riders have adopted a rather exceptional and more aerodynamic sprint position where the torso is held low and nearly horizontal and close to the handle bar to reduce the frontal area. The question arises how much aerodynamic benefit can be gained by such a position. This paper presents an aerodynamic analysis of both the regular and the low sprint position in comparison to three more common cycling positions. Computational fluid dynamics simulations are performed with the 3D RANS simulations and the transition SST k–ω model, validated with wind-tunnel measurements. The results are analyzed in terms of frontal area, drag coefficient, drag area, air speed and static pressure distribution, and static pressure coefficient and skin friction coefficient on the cyclist surfaces. It is shown that the drag area for the low sprint position is 24% lower than for the regular position, which renders the former 15% faster than the latter. This 24% improvement is not only the result of the 19% reduction in frontal area, but also caused by a reduction of 7% in drag coefficient due to the changed body position and the related changes in pressure distribution. Evidently, specific training is required to exert large power in the low sprint position.</p

A computational framework for the lifetime prediction of vertical-axis wind turbines:CFD simulations and high-cycle fatigue modeling

A novel computational framework is presented for the lifetime prediction of vertical-axis wind turbines (VAWTs). The framework uses high-fidelity computational fluid dynamics (CFD) simulations for the accurate determination of the aerodynamic loading on the wind turbine, and includes these loading characteristics in a detailed 3D finite element method (FEM) model to predict fatigue cracking in the structure with a fatigue interface damage model. The fatigue interface damage model allows to simulate high-cycle fatigue cracking processes in the wind turbine in an accurate and robust fashion at manageable computational cost. The FEM analyses show that the blade-strut connection is the most critical structural part for the fatigue life of the VAWT, particularly when it is carried out as an adhesive connection (instead of a welded connection). The sensitivity of the fatigue response of the VAWT to specific static and fatigue modeling parameters and to the presence of a structural flaw is analyzed. Depending on the flaw size and flaw location, the fatigue life of the VAWT can decrease by 25%. Additionally, the decrease of the fatigue resistance of the VAWT appears to be mainly characterized by the monotonic reduction of the tensile strength of the adhesive blade-strut connection, rather than by the reduction of its mode I toughness, such that fatigue cracking develops in a brittle fashion under a relatively small crack opening. It is emphasized that the present computational framework is generic; it can also be applied for analyzing the fatigue performance of other rotating machinery subjected to fluid–structure interaction, such as horizontal-axis wind turbines, steam turbine generators and multistage pumps and compressors

Indicators for the evaluation of test section flow quality

ABSTRACT: The flow in a wind tunnel test section must meet high standards to obtain accurate and reliable measurement data. Good flow quality demands a certain degree of spatial uniformity and temporal steadiness of velocity and pressure. In this paper, a set of six new indices is developed and presented that relate spatial aspects of the mean velocity field to flow quality. One index quantifies the degree of uniformity of the velocity field and can be used directly as a flow quality indicator. The five other indices are related to different types of deviations from spatially uniform flow; skewed flow and angularity (up-flow and down-flow, swirl, cross-flow, diverging and converging flow). The indices can be used to evaluate the flow quality in existing tunnels and to assess the impact of design modifications. They can also be used in the CFD-based design of new wind tunnels. KEYWORDS: Wind tunnel testing; Flow quality; Skewness; Angularity; Uniformity. INTRODUCTION Wind tunnel test section flow quality relates to temporal and spatial aspects of the flow. In this paper, only spatial aspects of the flow will be addressed. Strictly speaking, spatial uniformity is required in the entire empty test section of the wind tunnel. Deviations from spatial uniformity can have negative repercussions on the test results (Rae and Pope, 1984; Barlow and Rae, 1999). A skewed flow for example (i.e. with a streamwise velocity that is not symmetrically distributed over the width of the test section) will cause the static pressure over the front face of an object placed in the test section and the position of the stagnation point to be shifted. This can have a significant influence on all measured quantities around the object. Spatial flow uniformity is often documented by contour plots of velocity magnitude or static pressure that are shown in one or more cross-sectional planes of the wind tunnel (e.g. Selig and McGranahan, 2004). Other authors provide only numerical information in the form of a single mean value and spatial standard deviation for the quantity for the entire cross-sectional plane. The first method allows determining the presence or absence of skewness and angularity. However, multiple sections are required to obtain a complete view of the flow quality in the entire test section. Mean values and spatial standard deviations have the advantage that the flow in a specific (part of the) cross-section can be characterised numerically, although the interpretation of these characteristic values is not always clear. The existing techniques do not allow for a complete and straightforward evaluation of test section flow quality. However, it is important to be able to quantify wind tunnel test section flow quality and to assess and compare the impact of features such as honeycombs, corner or guide vanes, screens, etc. for wind tunnel and flow quality optimization. To this extent, a set of six new complementary indices describing spatial uniformity and the different types of spatial non-uniformity is developed in this paper