Lennart Löfdahl

Chalmers University of Technology · Division of Vehicle Engineering and Autonomous Systems

An integrated silicon pressure-shear stress sensor has been designed, fabricated and tested in a turbulent wall-boundary layer . The piezoresistive pressure sensor is based on polysilicon diaphragm technology and the thermal shear stress sensor on the gas cooling of a polyimide insulated heated chip. The pressure sensor diaphragm area is 100 × 100...

Large-scale commercialization of electric vehicles (EVs) seeks to develop battery systems with higher energy efficiency and improved thermal performance. Integrating simulation-based design optimization in battery development process expands the possibilities for novel design exploration. This study presents a dual-stage multiphysics simulation opt...

Large-scale introduction of electric vehicles (EVs) to the market sets outstanding requirements for battery performance to extend vehicle driving range, prolong battery service life, and reduce battery costs. There is a growing need to accurately and robustly model the performance of both individual cells and their aggregated behavior when integrat...

Energy efficient vehicles are essential for a sustainable society and all car manufacturers are working on improved energy efficiency in their fleets. In this process, an optimization of aerodynamics and thermal management is most essential. The objective of this work is to improve the energy efficiency using encapsulated heat generating units by f...

The flow around passenger cars is characterized by many different separation structures, typically leading to vortices and areas of reversed flow. The flow phenomena in these patches show a strong interaction and the evolution of flow structures is difficult to understand from a physical point of view. Analyzing surface properties, such as pressure...

Improving the aerodynamics of heavy trucks is an important consideration in the strive for more energy-efficient vehicles. Cooling drag is one part of the total aerodynamic resistance acting on a vehicle, which arises as a consequence of air flowing through the grille area, the heat exchangers, and the irregular under-hood area. Today cooling packa...

Increasingly tough regulations for emission levels and a growing demand for an environmentally clean motor industry impose high requirements in modern automotive development. During recent decades, carmakers have been utilizing various strategies to minimize energy losses in the powertrain to meet legislative and market demands. A great part of res...

Quantification of heat exchanger performance in its operative environment is in many engineering applications an essential task, and the air flow rate through the heat exchanger core is an important optimizing parameter. This paper explores an alternative method for quantifying the air flow rate through compact heat exchangers positioned in the und...

As the target figures for CO2 emissions are reduced every year, vehicle manufacturers seek to exploit all possible gains in the different vehicle attributes. Aerodynamic drag is an important factor that affects the vehicle’s fuel consumption, and its importance rises with the shift from the New European Driving Cycle to the Worldwide harmonized Lig...

This paper presents an experimental study of aeroacoustical sound sources generated by the turbulent flow around the side mirror of a Volvo V70. Measurements were carried out at the Volvo Cars aerodynamical wind tunnel (PVT) and at the aeroacoustical wind tunnel of Stuttgart University (FKFS). Several different measurement techniques were applied i...

Presented are results from numerical investigations of buoyancy driven flow in a simplified representation of an engine bay. A main motivation for this study is the necessity for a valid correlation of results from numerical methods and procedures with physical measurements in order to evaluate the accuracy and feasibility of the available numerica...

This study presents an implementation of delayed detached-eddy simulation (DDES) on a full-scale passenger vehicle for three configurations with the use of commercial software harpoon (mesher) and ansys fluent (solver). The methodology aims to simulate the flow accurately around complex geometries at relevantly high Re numbers for use in industrial...

This paper presents pressure drops and heat transfer rates for compact heat exchangers, where the heat exchangers are angled 90°, 60°, 30° and 10° relative to the incoming airflow. The investigation is based on two heat exchangers with different thicknesses, 19mm and 52mm. The experiments were performed for five airflow rates and five coolant flow...

The flow around a car is three-dimensional and turbulent. The main part of the dragis caused by pressure losses generated by various ow separations. In general a large anddistinct wake structure dominates the flow field at the base. Furthermore, diverse vertical structures are present at the backlight and the upper trunk region. The investigation o...

Modern trucks have a reasonably optimised cab shape , 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 vehic...

The phenomenon of three-dimensional flow separation is and has been in the focus of many researchers. An improved understanding of the physics and the driving forces is desired to be able to improve numerical simulations and to minimize aerodynamic drag over bluff bodies. To investigate the sources of separation one wants to understand what happens...

Two typical Silver Arrows from the thirties, one Mercedes-Benz W25 and one Auto Union Type C, were used to illustrate the simplicity and the capabilities of modern commercial CFD (Computational Fluid Dynamics) tools to mimic and compare the flow fields around these cars. In a Bachelor thesis carried out during the spring 2014 at Chalmers University...

Previous research on both small-scale and full-scale vehicles shows that base extensions are an effective method to increase the base pressure, enhancing pressure recovery and reducing the wake size. These extensions decrease drag at zero yaw, but show an even larger improvement at small yaw angles. In this paper, rear extensions are investigated o...

The current work investigates a method in 1D modeling of cooling systems including discretized cooling package with non-uniform boundary conditions. In a stacked cooling package the heat transfer through each heat exchanger depends on the mass flows and temperature fields. These are a result of complex three-dimensional phenomena, which take place...

This article presents Computational Fluid Dynamics (CFD) simulations fo one period of a louvered fin, for a crossflow compact finned heat exchanger, where the incoming airflow was inclined relative to its core. Four inclinations were investigated: 90°, which was when the air flowed perpendicular to the heat exchanger, 60°, 30° and 10° angles relati...

In the wind tunnel a common method for wake analysis is to attach a pressure probe to a traversing unit in order to retrieve flow field information. However, the approach is intrusive and has a direct impact on the surrounding flow field, affecting the investigated flow characteristics. In measurements conducted in the Volvo Cars Aerodynamic Wind t...

One main goal in the aerodynamic development of passenger vehicles is reduced fuel consumption. As vehicles are bluff bodies, drag is dominated by pressure drag, which is mainly caused by detached flow. To enable further reductions of the drag, it is of great importance to understand the physical phenomena behind separation.

Wheel aerodynamics has long been identified as a major contributor to the aerodynamic forces acting on vehicles. Lately, tyre design has been taking more and more attention as it shows potential of drag reduction. This paper looks into different methods of simulating tyre rotations in CFD and their effects on the aerodynamic forces acting on the ve...

To prevent test vehicles from movement during experiments in modern aerodynamic wind tunnels, fastening struts are typically used for a rigid connection between the model and the force balance underneath the wind tunnel floor. A weakness of this experimental set-up is that such struts limit the vertical movement of the vehicle.

There are a number of numerical and experimental studies of the aerodynamic performance of wheels that have been published. They show that wheels and wheel-housing flows are responsible for a substantial part of the total aerodynamic drag on passenger vehicles. Previous investigations have also shown that aerodynamic resistance moment acting on rot...

Automotive aerodynamics measurements and simulations now routinely use a moving ground and rotating wheels (MVG&RW), which is more representative of on-road conditions than the fixed ground-fixed wheel (FG&FW) alternative. This can be understood as a combination of three elements: (a) moving ground (MVG), (b) rotating front wheels (RWF) and (c) rot...

Nowadays, much focus for vehicle manufacturers is directed towards improving the energy efficiency of their products. The aerodynamic drag constitutes one major part of the total driving resistance for a vehicle travelling at higher speeds. In fact, above approximately 80km/h the aerodynamic drag is the dominating resistance acting on a truck. Henc...

The investigation showed that a more inclined heat exchanger resulted in lower static pressure drop and at the same time achieved a higher heat transfer rate, for a specific mass airflow rate. This result was obtained for all three heat exchangers. When analysing the parameters at the same core speed it was seen that the static pressure drop was in...

Under a global impulse for less man-made emissions, the automotive manufacturers search for innovative methods to reduce the fuel consumption and hence the CO2-emissions. Aerodynamics has great potential to aid the emission reduction since aerodynamic drag is an important parameter in the overall driving resistance force. As vehicles are considered...

This paper presents pressure-drop and airflow characteristics for compact heat-exchangers, where the relative airflow is angled 90° and 30°. The investigation is based on two heat-exchangers with different thicknesses, investigated for a number of airflow rates. The results are obtained from experiments and CFD simulations, where both a part of a d...

This paper presents results and a Computational Fluid Dynamics (CFD) method for simulation of a detailed louvered fin for a multi-louvered compact heat-exchanger. The airflow was angled at 90°, +30° and -30° relative to the heat-exchanger to evaluate changes in static pressure drop and airflow characteristics. The investigation was based on three h...

With the increase in fuel prices and the increasingly strict environmental legislations regarding CO2 - emissions, reduction of the total energy consumption of our society becomes more important. Passenger vehicles are partly responsible for this consumption due to their strong presence in the daily life of most people. Therefore reducing the impac...

Reducing resistance forces all over the vehicle is the most sustainable way to reduce fuel consumption. Aerodynamic drag is the dominating resistance force at highway speeds, and the power required to overcome this force increases by the power three of speed. The exterior body and especially the under-body and rear-end geometry of a passenger car a...

Passenger car fuel consumption is a constant concern for automotive companies and the contribution to fuel consumption from aerodynamics is well known. Several studies have been published on the aerodynamics of wheels. One area of wheel aerodynamics discussed in some of these earlier works is the so-called ventilation resistance. This study investi...

A collection of illustrations of swept-wing boundary layer secondary instability at low subsonic flow velocities is presented in this paper. The experimental results, obtained by hot-wire "visualization," provide detailed insight into the spatio-temporal structure and evolution of oscillations in the three-dimensional laminar boundary layer disturb...

Constantly lowering emissions legislation and the fact that fuel prices have increased tremendously over recent years, have forced vehicle manufacturers to develop more and more energy-efficient vehicles. The aerodynamic drag is responsible for a substantial part of the total driving resistance for a vehicle, especially at higher velocities; thus i...

Due to stricter emissions legislation and increasing fuel prices vehicle efficiency must be improved. A trend that has been seen in recent years is increased engine power for heavy vehicles. Different exhaust gas after-treatment systems have also been introduced to be able to meet the emission demands. Though, these implementations do not only affe...

To achieve more energy efficient transportation we have to reduce losses and resistance forces all over the vehicle. Aerodynamic drag is one of the primary resistance forces a passenger vehicle has to overcome and the force increases exponentially with increased speed. The under-body and rear-end geometry of a passenger car is a significant contrib...

Landström, C. ; Löfdahl, L. ; Tim, W. et al. (2012). Aerodynamic effects of different tyre models on a sedan type. SAE International Journal of Passenger Cars– Mechanical Systems. July 2012

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 i...

Targets for reducing emissions and improving energy efficiency present the automotive industry with many challenges. Passenger cars are by far the most common means of personal transport in the developed part of the world, and energy consumption related to personal transportation is predicted to increase significantly in the coming decades. Improve...

The results illustrates that the MRF methodology is consistent with test data for axial flow conditions up to fan transition, i.e. this methodology could be used for applications with a correctly installed axial fan. However after fan transition the methodology is not consistent any more.

Today CFD is an important tool for engineers in the automotive industry who model and simulate fluid flow. For the complex field of Underhood Thermal Management, CFD has become a very important tool to engineer the cooling airflow process in the engine bay of vehicles. To model the cooling airflow process accurately in CFD, it is of utmost importan...

Today there are a large variety of drag reducing devices for heavy trucks that are commonly used, for example roof deflectors, cab side extenders and chassis fairings. These devices are often proven to be efficient, reducing the total aerodynamic resistance for the vehicle. However, the drag reducing devices are usually identical for a specific pul...

Amongst the aerodynamic devices often found on race cars, the diffuser is one of the most important items. The diffuser can work both to reduce drag and also to increase downforce. It has been shown in previously published studies, that the efficiency of the diffuser is a function of the diffuser angle, ground clearance and most importantly, the ba...

This investigation is a continuing analysis of the cooling performance and aerodynamic properties of a rear mounted cooling module on a semi-generic commercial vehicle, which was carried out by Larsson, Löfdahl and Wiklund [1]. In the previous study two designs of the cooling package installation were positioned behind the rear wheelhouse and the r...

Efforts towards ever more energy efficient passenger cars have become one of the largest challenges of the automotive industry. This involves numerous different fields of engineering, and every finished model is always a compromise between different requirements. Passenger car aerodynamics is no exception; the shape of the exterior is often dictate...

The aim with this investigation was to study the aerodynamic properties of truck-trailer combinations of varying lengths. The aerodynamic properties of the combinations were evaluated in order to study similarities and differences in the flow field between different configurations. By the use of Computational Fluid Dynamics (CFD) six different type...

Computational Fluid Dynamics (CFD) is today an important tool in the design process of fuel and energy efficient vehicles. Under hood Thermal Management (UTM) is one of the design fields which CFD has proven itself to be a very competitive design tool over other available tools and techniques. To simulate the under hood environment it is very impor...

Passenger cars represent the largest part of all means of personal transportation today. Thus, it is important to work towards reduced energy consumption of cars if a sustainable mobility is to be achieved. This involves many aspects of vehicle engineering; one of them being aerodynamics. This study focuses on aerodynamic drag and the contributions...

AbstractWind-tunnel data on velocity perturbations evolving in a laminar swept-wing flow under low subsonic conditions are reported. The focus of the present experiments are secondary disturbances of the boundary layer which is modulated by stationary streamwise vortices. Both the stationary vortices and the secondary oscillations of interest are g...

Automotive wind tunnel testing is a key element in the development of the aerodynamics of road vehicles. Continuous advancements are made in order to decrease the differences between actual on-road conditions and wind tunnel test properties and the importance of ground simulation with relative motion of the ground and rotating wheels has been the t...

The most common fan model to use in commercial CFD software today is the Multiple Reference Frame (MRF) model. This is at least valid for automotive under hood applications. Within the industry, for this typical application, this model is commonly known to under predict performance. This under prediction has been documented by the authors’ of this...

This study focus on the aerodynamic influence of the engine bay packaging, with special emphasis on the density of packaging and its effect on cooling and exterior flow. For the study, numerical and experimental methods where combined to exploit the advantages of each method. The geometry used for the study was a model of Volvo S60 sedan type passe...

A common fan model to use in automotive underhood simulations is the Multiple Reference Frame (MRF) model and within the industry, for this specific application, this model is well known to under-predict performance. In a former paper, referenced 2009-01-0178, a simple 'speed correction' of the MRF model was proposed by the authors. The correctio...

Moving ground systems with rotating wheels have been used in wind tunnel tests during the last decades. Several studies on the effects of rotating wheels and the importance of wheel aerodynamics have been published. It is well known that both the local flow field and the global aerodynamic forces are affected by rotation of the wheels. Different st...

A common fan model to use in automotive under hood simulations is the Multiple Reference Frame (MRF) model and within the industry, for this specific application, this model is well known to under predict performance. In this paper we have examined the possibilities of correcting this deficiency with a simple ?speed correction?. This is done by tes...

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...

In this paper 3D numerical simulations of turbulent incompressible flows are validated against experimental data from the linear low pressure turbine/outlet guide vane (LPT/OGV) cascade at Chalmers in Sweden. The validation focuses on the secondary flow-fields and loss developments downstream of a highly loaded OGV. The numerical simulations are pe...

The flow field around bluff bodies constitutes a classic area within fluid dynamics and has been the topic for much research through the years. However, in the use for road vehicles with the effect of the ground, the behavior is changed very much from more classical aviation usage. In this paper we are investigating the drag force reduction on a ve...

To decrease the lead time and hence cost for an aerodynamic development programme for a new car model, extended use of down scaled wind tunnel models is one possibility. This however, requires that the models carry many of the exterior, engine bay and underbody details that are present on the full scale counterpart. An important issue in scale test...

Future demands on passenger cars consist to a large extend of making them more energy efficient. Reducing the driving resistance by reducing the aerodynamic drag will be one important part in reducing fuel consumption. In most cases during passenger car development, early experimental investigations are performed in scale model wind tunnels. Consid...

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 o...

In this case study the cooling airflow of an existing open-wheeled racecar has been improved with the use of Computational Fluid Dynamics. The race team in context had at several occasions experienced overheating of their racecar and was looking for ways to improve the cooling performance without changing the bodywork radically. As the car is used...

In the 1955 Le Mans race the worst crash in motor racing history occurred and this accident would change the face of motor racing for decades. After the crash numerous investigations on the disaster were performed, and fifty years after some interesting books were launched on the subject. However, a number of key questions remain unsolved; and one...

An experimental investigation was carried out to assess the drag reducing potential of active flow control in conjunction with flat panel flaps attached to the trailer of a generic tractor–trailer model. The experiments were carried out in a wind tunnel with a 1/10th scale generic tractor-trailer model at Reynolds numbers up to 640,000 based on the...

During the development of the aerodynamic properties of fore coming road vehicles down scaled models are often used in the initial phase. However, if scale models are to be utilised even further in the aerodynamic development they have to include geometrical representatives of most of the components found in the real vehicle. As the cooling package...

The need for low fuel consumption and lower emissions is for every automotive OEM a reality. With a cooling fan in today's trucks that typically consumes of the order of magnitude 50bHp fully engaged - reducing this loss is very important. In addition a lot of the new after treatment systems requires a higher airflow through the engine bay, and new...

The results of detailed experimental investigations of the effect of riblets on the development of a Λ vortex structure and its transformation into a turbulent spot, is presented. The experiments were performed using modern methods experimental data acquisition, processing, and representation. The results of the experiments showed that the Λ struct...

In the present experimental work, which utilizes a hot-wire visualization method, an influence of external pressure gradient on the development of secondary instabilities in the straight wing boundary layers has been demonstrated for the first time. The comparison of experimental data and direct numerical simulation results of this process has been...

The nonlinear stage of breakdown to turbulence is a strongly three-dimensional process and represents a difficult task for experimental studies. Investigation of laminar-turbulent transition in aerospace applications additionally involves a complex base-flow with pressure gradients and secondary velocity components resulting in successive increase...

Results of experimental investigations of the nonlinear stage of sinusoidal and varicose instability of a streaky structure, which leads to multiplication of streaky structures and origination of coherent structures (such as Λ-structures), are presented. Riblets suppress the intensity of streaky structures, stabilize the flow against the developmen...

A detailed experimental study on the formation of crossflow vortex mode packets and their secondary instability in a swept wing boundary layer was carried out. As a result, two modes of nonstationary perturbations traveling along the vortices were found.

To fine tune the aerodynamic properties of road going vehicles the flow along the underbody is of outmost importance. Hence moving ground facilities has been introduced. In full-width mono-belt facilities the test vehicle is most often suspended over the ground plane by a sting system. To mount the wheels, two possibilities exist. The designer of t...

This paper presents 2D and 3D-numerical simulations compared with experimental data from a linear Low Pressure Turbine/Outlet Guide Vane (LPT/OGV) cascade at Chalmers in Sweden. Various performance characteristics for both on and off design cases were investigated, including; pressure distributions, total pressure losses and turning. The numerical...

Nonlinear varicose instability of a streaky structure is experimentally investigated in a wing boundary layer. Spatial evolution of the streaky structure perturbed by high-frequency traveling waves is compared at zero and adverse streamwise pressure gradients. The process of streak multiplication and generation of �-shaped vortices is under examina...

A plane wall-jet flow is numerically investigated and compared to experiments. The measured base flow is matched to a boundary-layer solution developing from a coupled Blasius boundary layer and Blasius shear layer. Linear stability analysis is performed, revealing high instability of two-dimensional eigenmodes and non-modal streaks. The nonlinear...

A detailed experimental study on the formation of crossflow vortex mode packets and their high-frequency secondary instability in a swept-wing boundary layer was carried out. Stationary vortex packets are most likely to be generated under natural flight conditions and transition to turbulence is quickest within these disturbances. In the present ex...

Recent progress in micro-fluid dynamics has identified an increased demand for efficient mixing of highly viscous fluids in small channels and cavities. One way to do this is through the steady streaming generated by the vibration of solid boundaries. In this paper we investigate the mixing properties of such streaming flows in an infinite channel....

In this paper, we present a micromachined metal hotwire anemometer sensor for use in wall shear-stress measurements. We describe its design and fabrication. A novel hybrid assembly method has been developed to make it possible to measure close to the surface without contacting leads interfering with the flow. Experimental results illustrate the beh...

The nonlinear stage of banded structure (Λ-, Ω-structures, hairpin vortices) sinusoidal and varicose instability in Blazius boundary layer was experimentally studied in low-turbulent aerodynamic tube. Investigation was more detailed as compared with previous reported experiment (thermoanemometric measurements of velocity axial component and velocit...

This paper presents a detailed experimental evaluation of the flow field in a state of the art linear cascade with boundary-layer suction. The linear cascade was designed using a new design-process as described by Hjärne et al. [1]. From the measurements presented in this paper it can be concluded that all of the features of the test-rig work as ex...

During the last decade MEMS devices have had major impacts on both industrial and medical applications. Examples of the former group are accelerometers for automobile airbags, scanning electron microscope tips to image single atoms, microheatexchangers for electronic cooling, and micromirrors used for light-beam steering. Micropumping is useful for...

An analytical study of the flow induced by vibrating solid walls is presented. Transverse motions in the form of standing waves vibrated at small amplitudes are considered and the governing equations are perturbed using the slope as a small parameter. Steady streaming occurs at second order in the perturbation solution involving the formation of ce...

The investigation results of the controlled experiment of a streaky structure varicose instability in a swept wing boundary layer are presented. The picture of a streaky structure spatial evolution and secondary high-frequency disturbances generated on it is obtained using hot-wire measurements. It is shown that exactly a varicose instability is re...