Per Elofsson

EXPERIMENTS ON THE STABILITY OF STREAMWISE STREAKS IN PLANE POISEUILLE FLOW

The development and stability of streamwise streaks are studied in an air-flow channel experiment at subcritical Reynolds numbers. The streaks were generated by continuous suction through small slots at the wall. The streak amplitude first grows algebraically, and if the amplitude exceeds a certain threshold secondary instability in the form of travelling waves is observed. These waves give rise to high urms values in the region of large spanwise mean flow gradient. Measurements with two hot-wire probes indicate that velocity fluctuations are 180° out of phase at two neighboring peaks at each side of a low velocity region and implies the existence of a sinuous type instability. Measurements were also made with controlled disturbances where earphones were used to force the secondary instability. Phase averaged data clearly show the oscillation of the low velocity region and also provides the growth rate, phase speed as well as amplitude and phase distributions of the secondary instability. Several of these...

An experimental study of oblique transition in plane Poiseuille flow

Interactions of oblique waves have recently been investigated theoretically and numerically and found to give rise to rapid transition in flows subcritical to linear wave disturbances. The transition scenario consists of the formation and transient growth of streamwise streaks of high and low velocity and later a rapid growth of highfrequency disturbances leading to breakdown. The present study is the first extensive experimental investigation of oblique transition. The experiments were carried out in a plane Poiseuille flow air channel in which the oblique waves were generated, one at each wall, by vibrating ribbons and the development of the flow was mapped with hot-wire anemometry. The experiments consist both of low- and high-amplitude wave disturbances; in both cases streaky structures are created. For the low-amplitude case these structures decay, whereas for the high amplitude the flow goes towards breakdown. This study has confirmed and extended previous theoretical and numerical results showing that oblique transition may be an important transition scenario

An experimental study of oblique transition in a Blasius boundary layer flow

Abstract Transition initiated by a pair of oblique waves was investigated experimentally in a Blasius boundary layer flow by using hot-wire measurements and flow visualisation. The oblique waves were generated by periodic blowing and suction through an array of pipes connecting to the flow through a transverse slit in the flat plate model. The structure of the flow field is described and the amplitude of individual frequency-spanwise wave number modes was determined from Fourier transforms of the disturbance velocity. In contrast to results from investigations of oblique transition at subcritical flow conditions, the transition process at the present conditions suggests the combined effect of non-modal growth of streaks and a second stage with exponential growth of oblique waves to initiate the final breakdown stage.

Experimental and Numerical Investigation of Wheel Housing Aerodynamics on Heavy Trucks

Wheel and underbody aerodynamics have become important topics in the search to reduce the aerodynamic drag of the heavy trucks. This study aims to investigate, experimentally as well as numerically, the local flow field around the wheels and in the wheel housing on a heavy truck; and how different approaches to modelling the wheel rotation in CFD influences the results. Emphasis is on effects due to ground simulation, and both moving ground and wheel rotation were requirements for this study. A 1:4-scale model of part of a heavy truck geometry has been developed. During the model design numerical simulations were used to optimise the shape, in order to replicate the flow field near the wheel of a complete truck. This was done by changing the flow angles of the incoming and exiting flows, and by keeping the mass flow rates in to, and out of, the wheel housing at the same ratios as in a reference full size vehicle. To reduce blockage effects, the model was sectioned to reduce both height and width. In the experiments, pressure sensors and static pressure taps located in the wheel housing were utilised, and the simulations replicated the boundary conditions of the wind tunnel experiments, both in terms of the geometry of the model and wind tunnel as well as the modelling of the ground simulation. It was found that the wheel wake structures changed significantly when ground simulation was utilised. The main outflow through the wheel housing was influenced by the wheel rotation and took place further upstream, which resulted in large differences in the flow field downstream of the wheel. The influence of different strategies for modelling the wheel rotation in CFD was investigated and it was found that the Sliding Mesh approach was the most accurate method.

Wheel Housing Aerodynamics on Heavy Trucks

Modern trucks have a reasonably optimised cabshape, and there exist several OEM and aftermarket devices for drag reduction for heavy trucks as well. To further reduce the aerodynamic drag major changes to the current layout of the vehicle are required, or the focus must be shifted from the cab and tractor trailer gap to other regions of the vehicle. The drag of the underbody, including wheel housings, wheels and engine compartment, represents a significant proportion of the aerodynamic drag and there has not been much investigation in this specific area on heavy trucks. To be able to reduce the fuel consumption and to fulfil the legislated emission standards for heavy trucks it is important to take all areas of the vehicle under consideration, and even though the individual improvements may be small, the total drag reduction will be substantial. In order to study the flow close to the vehicle underbody it is important to utilise the correct boundary conditions, that is, moving ground and rotating wheels. This work has focused on the flow in the front wheel housings. The flow field around the front wheels under the influence of ground simulation on a heavy truck of standard European configuration was investigated using numerical simulations. The in- and outflow to the wheel housing was located and the vortices originating from the front wheels were identified. This information was then used to identify which areas of the wheel housing having the greatest potential for aerodynamic improvements by changing the front wheel housing design. Furthermore, several wheel housing design parameters were defined, and their influence on the flow field and aerodynamic drag were investigated. Examples of these parameters are the shape of the wheel housing opening and implementation of wheel housing ventilation. It was found that there is potential for reducing the aerodynamic drag by applying these geometric changes to the wheel housing, and several of the configurations could be implemented on current production vehicles.

Heavy Vehicle Wheel Housing Flows - a Parametric Study

The drag from the underbody, including wheels and wheel housing, constitutes a significant amount of the total aerodynamic drag of heavy vehicles. A correct simulation of the underbody boundary conditions, including rotating wheels and moving ground, has turned out to be of great importance in the minimising of the aerodynamic drag. In the current study several front wheel housing design parameters have been evaluated using Computational Fluid Dynamics (CFD). Design concepts, like enclosed inner wheel housings, underbody panel and wheel housing ventilation, were evaluated by flow analysis and comparison of the drag force contribution. It was shown that changes to the wheel housing geometry had an important impact on the local flow field and force distribution. The total drag of the vehicle decreased with reduced wheel housing volume and wheel housing ventilation can reduce the aerodynamic drag significantly provided it is designed properly. It was also found that even though there were large differences in the drag force of each component the change in total drag was small.

An Investigation of the Aerodynamic Drag Mechanisms Due to Ground Simulation in Yawed Flow Conditions for Heavy Trucks

The drag from the underbody, including wheels and wheel housings, constitute a significant amount of the total aerodynamic drag of heavy trucks. A correct simulation of the underbody boundary conditions, including rotating wheels and moving ground, has turned out to be of great importance in the minimising of the aerodynamic drag. Earlier studies on passenger cars have described the drag mechanisms involved when implementing proper ground simulation. However, model scale wind tunnel tests of heavy trucks have shown an opposite trend on the drag coefficient with ground simulation compared to passenger cars. An important aspect of truck aerodynamics is the yaw dependency of the drag coefficient when the vehicle is exposed to crosswinds. Therefore it is of outmost importance to evaluate the performance of the vehicle in yawed flow conditions during the development process. In the current study the influence of ground simulation, including moving ground and rotating wheels, on the flow has been investigated at several yaw angles using Computational Fluid Dynamics. For the simulations a tractor trailer geometry corresponding to a standard European configuration was used, and the geometry included a fully detailed underbody and engine compartment. It was found that there was a significant difference in the structure of the wheel wakes and this was mainly due to the implementation of wheel rotation. It was also shown that the drag coefficient increased due to the utilisation of ground simulation in yawed flow conditions.

Plasma Streamwise Vortex Generators for Flow Separation Control on Trucks

An experimental study of the effect of Dielectric Barrier Discharge plasma actuators on the flow separation on the A-pillar of a modern truck under cross-wind conditions has been carried out. The experiments were done in a wind tunnel with a 1:6 scale model of a tractor-trailer combination. The actuators were used as vortex generators positioned on the A-pillar on the leeward side of the tractor and the drag force was measured with a wind-tunnel balance. The results show that the effect at the largest yaw angle (9 degrees) can give a drag reduction of about 20% and that it results in a net power reduction. At lower yaw angles the reduction was smaller. The present results were obtained at a lower Reynolds number and a lower speed than for real driving conditions so it is still not yet confirmed if a similar positive result can be obtained in full scale.

Numerical Investigation of Blockage Effects on Heavy Trucks in Full Scale Test Conditions

The effect of blockage due to the presence of the wind tunnel walls has been known since the early days of wind tunnel testing. Today there are several blockage correction methods available for cor ...

Accurate Drag Prediction Using Transient Aerodynamics Simulations for a Heavy Truck in Yaw Flow

Aerodynamic development of a full-scale truck presents a challenge for experimental testing due to the scale of the vehicle relative to most wind-tunnel test facilities. Numerical simulation is becoming more prevalent for assessing design changes and improving vehicle aerodynamic drag. In this process, the cumulative effects of small design changes are needed. Furthermore, the drag must be considered both at zero crosswind and with five degrees crosswind yaw angle in order to properly represent typical driving conditions. It is well-known that the aerodynamics of heavy trucks are complicated by a very transient wake flow that causes large fluctuations in base pressure, and therefore in the drag coefficient. This effect is often even more prevalent at non-zero yaw angles. The transient wake flow presents a challenge for effectively using simulation tools to predict the drag effects of small design changes, which may have some influence on the wake flow and base pressure.