David Burton

Computational Fluid Dynamics Study of the Effect of Leg Position on Cyclist Aerodynamic Drag

An experimental and numerical analysis of cycling aerodynamics is presented. The cyclist is modeled experimentally by a mannequin at static crank angle; numerically, the cyclist is modeled using a computer aided design (CAD) reproduction of the geometry. Wind tunnel observation of the flow reveals a large variation of drag force and associated downstream flow structure with crank angle; at a crank angle of 15 deg, where the two thighs of the rider are aligned, a minimum in drag is observed. At a crank angle of 75 deg, where one leg is at full extension and the other is raised close to the torso, a maximum in drag is observed. Simulation of the flow using computational fluid dynamics (CFD) reproduces the observed variation of drag with crank angle, but underpredicts the experimental drag measurements by approximately 15%, probably at least partially due to simplification of the geometry of the cyclist and bicycle. Inspection of the wake flow for the two sets of results reveals a good match in the downstream flow structure. Numerical simulation also reveals the transient nature of the entire flow field in greater detail. In particular, it shows how the flow separates from the body of the cyclist, which can be related to changes in the overall drag.

Aerodynamic performance and riding posture in road cycling and triathlon

Cycling performance is strongly dependent on aerodynamic drag, of which the majority is attributable to the rider. Previous studies have shown the importance of optimising athlete posture on the bicycle for individual time-trial events. This article identifies that performance in road cycling and draft-legal triathlon can be improved through aerodynamic optimisation of the athlete’s posture. Nine relevant cycling postures have been studied, and it was found that for road cycling, gripping the hoods with horizontal forearms can reduce the required cyclist power by 13.4%, and for draft-legal triathlon applications, the use of short bar extensions reduced the required power by up to 16.7%. It was also found that lowering the eyes and head increased drag in both drops and triathlon postures. Measurements of the velocity profiles of the wake of a cyclist in four different postures are presented, and it is shown that differences in drag coefficients between postures can be correlated with changes in the wake velocity defect and turbulence intensity distribution.

The nature of the vortical structures in the near wake of the Ahmed body

This study presents the results from high-spatial-resolution water-channel velocity-field measurements behind an Ahmed body with 25° rear slant angle. The Ahmed body represents a simplified generic model of a hatchback automobile that has been widely used to study near-wake flow dynamics. The results help clarify the unresolved question of whether the time-mean near-wake flow structure is topologically equivalent to a toroidal vortex or better described by a pair of horizontally aligned horseshoe vortices, with their legs pointing downstream. The velocimetry data presented allows the tracking of the vortical structures throughout the near wake through a set of orthogonal planes, as well as the measurement of their circulation. The spanwise vortices that form as the flow separates from the top and bottom rear edges are shown to tilt downstream at the sides of the body, while no evidence is found of a time-mean attached toroidal vortex, at least for the Reynolds number (based on the square root of the frontal area) of R e FA ~ 30 , 000 under consideration.

Surface flow visualisation over forward facing steps with varying yaw angle

Many Australian wind farms are located near escarpments and cliffs where flow separation occurs. An absence of literature addressing the effect of wind direction over cliffs have motivated surface shear stress visualisations on forward facing steps at yaw angles between 0° and 50°. These visualisations have been conducted in the Monash University 450 kW wind tunnel. Mean reattachment lengths were measured and shown to vary as a function of the boundary layer thickness to step height ratio and the yaw angle. Vortices shed off the crest of the step induced surface shear stresses on the top surface of the step. The orientation of these shear stresses varied linearly with the yaw angle. Three-dimensional structures of different forms were also observed. At zero yaw angle the flow converged at points along the crest. At high yaw angles distinct sections of misaligned flow were observed downstream of the reattachment line, indicating a spatial periodicity in shedding.

Dominant flow structures in the wake of a cyclist

In this wind tunnel investigation the time-averaged wake structure is analyzed for a full scale cyclist mannequin over a complete crank cycle. At typical elite level road cycling speeds, detailed velocity field measurements were performed by traversing a four-hole dynamic pressure probe in planes behind the mannequin for 15° increments in crank position. They highlight the complexity of flows associated with cyclist geometries and show that variations in drag with leg position are primarily attributed to changes in the flow regime and not frontal surface area. The wake is shown to be highly three-dimensional with the primary flow structures consisting of multiple streamwise vortices. The formation strength and interaction of these structures about the center plane of the mannequin depend on whether each leg is in an up or down position around the crank cycle. This dependence on the position of each leg results in an asymmetrical wake configuration for the majority of crank positions tested. This is further highlighted in a series of flow visualization studies showing the origin and asymmetry in the formation of these structures over the upper body with crank position. This work highlights the importance of considering multiple flow regimes when investigating the aerodynamic performance of cyclists.

Optimisation of Boat-Tails for Heavy Vehicles

Boat-tails offer significant promise in reducing long haul heavy vehicle aerodynamic drag, and hence fuel consumption and greenhouse gas emissions. This paper presents results from a basic numerical and experimental investigation of drag coefficient reductions for various boat-tail configurations. The vehicle chosen is an arbitrary streamlined front end with a width to height ratio of 64%. No tractor to trailer gap is modeled. Approximate model scale is 12% and test Reynolds Number is ∼850,000. Experimental data and numerical simulations are presented for the zero yaw angle condition. For boat-tail angles where the flow remains largely attached a relationship is observed between drag reduction and the minimum boat-tail area. An optimum boat-tail angle (in the range of 15 degrees) is identified for 0 degrees yaw from both experimental and numerical data. For boat-tail angles greater than optimum a distinct increase in drag is observed in the experimental data, which is associated with flow separation. Comparison of experimental and numerical results show reasonable agreement for attached flow cases, and indicate a similar optimal boat-tail angle.Copyright

Flow Characteristics of a Three Dimensional Bluff Body With a Single Rotating Cylinder

This work investigated the application of a rotating cylinder to the upper leeward edge of a three dimensional bluff body in ground proximity.Aerodynamic drag measurements, base pressure contours and wake velocity profiles were obtained in a closed jet wind tunnel for Reynolds Numbers in the range of approximately 220,000 to 660,000. The cylinder of diameter 0.1H was mounted on the upper edge of the leeward face of the body. The ratio of cylinder surface velocity to freestream velocity was varied from 0 to 2.0. A computational model of the geometry was developed and results are presented for various velocity ratios and cylinder diameters.The results of this work demonstrated that, even at low velocity ratios, the cylinder rotation has a large effect on the flow structures in the body wake region. A large downwash is observed that creates two large counter-rotating vortices and a resultant significant increase in drag.The aerodynamic drag changes are presented as a function of velocity ratio and are shown to be Reynolds Number insensitive over the range tested. Aerodynamic drag was shown to increase with increasing velocity ratio over the velocity ratio range 0.25 to 2.0.Copyright

Numerical and Experimental Investigation of the effect of Multiple Rotating Cylinders on Base Pressure of a Three Dimensional Bluff Body in Ground Proximity

This paper details An experimental and numerical investigation of the application of rotating cylinder combination at the lee-ward four edges of a three dimensional bluff body in ground proximity. Aerodynamic drag measurements and wake velocity profiles were obtained in a closed jet wind tunnel at a Reynolds Numbers of approximately 400,000. Drag results are presented for cylinder configurations: top only, bottom only, both sides, top and sides, and all cylinders. Cylinder surface velocity to freestream velocity ratio is systematically varied from 0 to 1. The influence of the rotating cylinders on wake flow characteristics is observed, and changes to the wake structure, size, shear layer, separation region, and vorticity and their effect on base pressure are presented. It is shown that at low rotation rates a net drag reduction can be achieved for a relatively low energy input, while higher rotation rates lead to the formation of dominant, streamwise structures and an associated increase in drag.

Siting Wind Turbines near Cliffs: The Effect of Ruggedness

Wind farms have often been located in close proximity to coastal cliffs to take advantage of the consistent wind regimes associated with many coastal regions, as well as to extract any available increase in flow speed that might be generated by such cliffs. However, coastal cliffs are often rugged as a result of erosion and the natural shape of the landform. This research explores the impact of the three-dimensional cliff topography on the wind flow. Specifically, wind tunnel testing is conducted, modeling the naturally occurring ruggedness as sawtooth lateral variations of various amplitudes applied to a forward facing step (FFS). Surface shear stress visualization techniques have been employed to derive the flow topology associated with different topographies, while pressure probe measurements are used to measure the development of wind speed and turbulence intensity (TI). Pressure probe measurements and surface pressure taps also assist to determine the lateral and vertical extents of the vortex structures identified. In particular, flow fields characterized by the probe measurements were consistent with vortex bursting that is described by various researchers in the flow over delta wings. Such bursting is observed as a stagnation and corresponding expansion of the vortex. Based on these observations, recommendations are provided for the siting of wind turbines near analogous cliffs. [DOI: 10.1115/1.4041231]

A thermistor-based instrument for measuring vehicle cooling airflow

This paper details the development and use of a thermistor-based instrument to measure cooling inlet airflow velocity. A 160-point instrument was used to measure flow profiles through a vehicle radiator in a full-scale automotive wind tunnel. It was found that the repeatability of the mean radiator velocity measurements was +/− 0.3% and the repeatability of the inhomogeneity of the profiles was +/− 0.4%. The effect of turbulence and flow angle on the thermistor accuracy was also considered.