For the numerical analysis, an open source Lattice BoltzmannMethod (LBM) tool called OpenLB is used. The tool combines the Boltzmann equation together with Large-Eddy Simulation (LES), as turbulence model, to simulate the fluid behavior. The combination of LBM-LES which is used by OpenLB is relatively new within the CFD realm. To asses, the performance of OpenLB in the field of flow characteristics, drag prediction and pressure patterns a generic heavy duty bluff vehicle called, Generalised European Transport Model (GETS), is used. The inherent unsteady nature of both LBM and LES are complementary to each other, where LES is accurate in prediction of vortex shedding behind bluff bodies and with the easy parallelization of LBM
computational time can be saved. A bluff vehicle such as the GETS model, where separation is expected, is the
perfect model to test the performance of OpenLB and compare it with established phenomena of bluff bodies
in literature and wind tunnel tests. By changing the configuration of the GETS model, variation in frontedge
radii and additional boat tails, a better insight into emerging trends can be observed and compared,
to validate the open source CFD tool. Furthermore, to investigate if there is potential in OpenLB for future
projects.
In addition to numerical analysis and a wind tunnel experiment is performed to obtain drag coefficients
for the various model configurations. The experimental analysis was performed at Reynolds number varying
from 8000 to 60000, based in the square root of the frontal surface. This means that a 1:50 scale model of the
original GETS model is used. Mainly, the drag force of themodelwith different configurations are measured to
validate the numerical results. The relative low Reynolds number tested in this study is due to the limitations
set byOpenLB. The absence of grid refinement, the use of LES as turbulencemodel and lack of a wall function,
constrains the Reynold numbers that can be simulated with keeping computational time in mind. Therefore
a choice was made to only simulate the Reynolds number of 8000, 24000 and 48000 and compare the effect it
has on the flow characterizer, drag coefficient and pressure.
It was shown that the model with the smallest front-edge radius had the highest drag coefficient. This was
the effect of a separation bubble over the front of the model. With increasing the radius the drag is also reduced,
observed both by numerical analysis and conducted experiments in literature. The flow is guided over
the rounding which gives a more favorable pressure gradient and therefore reduces the separation bubble
that arises over the front-edge. Additionally, the effect of applying a simple Bounce-Back or Bouzidi boundary
condition on the model is investigated. The interpolative nature of Bouzidi approximates the staircase
shape of the rounding with a curve, in contrary to the bounce-back boundary condition. The drag difference
between the two boundary conditions was 7.3%, which is moderate but it highly affects the flow over
the front. The flow characteristics are simulated at two different heights, one at ground proximity and the
other at a higher distance fromthe ground. The comparison is made because with the experimental analysis
the model was placed somewhat higher from the wind tunnel floor to avoid interference with thewind tunnel
boundary layer. Normally themodel is situated at the ride height of a real-life truck. The position of themodel
influences the location of unequal sized vortices aft of the model. At ground proximity, the largest vortex is
at the top whereas at a higher ride height this is vice versa. Which complies with Particle Image Velocimetry
(PIV) performed by van Raemdonck [78]. The ride height did slightly affect the drag coefficient but was
within a reasonable difference of less than 3%. Another observation that is made is, with increasing Reynolds
the drag coefficient decreases. This is valid for all the front-edge radii for both numerical and experimental
analysis.
The addition of the tail lowered the drag of the models, this trend is ascertained by both numerical and
experimental analysis. With increasing tail angle the drag reduction also increaseswhich is caused by increasing
pressure over the rear part of themodel. There are some exceptions. Experimental results show that with
the largest tail deflection of 18± the drag is increased. This can be the influence of how the tail is attached to
themodel and the material used to create the model. A CADmodel that is used in the numerical simulation,
every angle and dimension is perfect to the specifications. However, with wind tunnel models this not the
case and therefore has an influence on the results. In general, the numerical and experimental drag difference
is rather high, varying from 50% at low Reynolds numbers to 10% at higher Reynolds numbers. This is
mainly the cause of the force balance used to measure the forces in the wind tunnel. The balance is designed for larger models at higher inlet velocities which generate a larger force. So, at smaller forces, the balance is
not that sensitive which gives a wider spread in results at lower Reynolds numbers.
Comparing the flow characteristics of the model with the different configurations a few things can be
noticed. That the separation bubble is reduced with increasing radius. Also, the strength of the recirculation
region of the bubble is reduced. This is one of the reasons why the drag coefficient is reduced with increasing
front-edge radius. Furthermore, the effect of the additional tail on the separation bubble is also visible. The
flow over the rear is accelerated by the tilted plates of the tail, which affects the boundary layer over the entire
model, it re-energizes regions of low-velocity flow reducing the size of the separation bubble. Comparing
the simulated flow behavior of boat tails with a slant angle of 6± 12± and 18± to literature shows satisfactory
results. The addition of the tail has three major effects on the flow aft of the tail. First, reduced the wake size.
Second, pushed the wake more aft so it has less influence in the model and third, delaying separation over
the tail and guiding flow more inward to reduce the vortex strength. All of these core functions of the boat tail
are simulated correctly. At each simulated angle the flow characteristic match that described in literature.
The pressure coefficient could not be compared because the position of measurement was not equal.
Due to an absence of a wall function the pressure at the wall of the OpenLB simulation are zero therefore no
true comparison with literature can be made. In literature the pressure is often measured at the face of the
body. Hence, no qualitative comparison could be made about the magnitude and the shape of the pressure
distribution. However, certain trends are visible with the pressure plots. The larger boundary layer that is
caused by a sharp front-edge radius decreases the pressure over the aft of the model, whereas, with a larger
front rounding the pressure is increased. The addition of a tail clearly increases the base drag and therefore
reduces the drag contribution. The pumping effect, that is very common with bluff bodies, is also observed
with the help of the pressure plot. The pumping effect is, in fact, a periodic motion that sheds rear-end
vortices generating a longitudinal oscillatory motion. This is reflected in the pressure coefficient plot which
shows the back and forth motion with increasing and decreasing pressure coefficient.
In conclusion, it can be stated that OpenLB is not yet ready to be used for the mainstream engineering
problems. The absence of certain key features limits the tool in many ways. Using LES without grid refinement
and wall function only low Reynolds numbers can be simulated. Overall, the flow characteristics that
are simulated with the various model configurations are accurate even if the Reynolds numbers are not the
same order of magnitude. The drag prediction of the simulations are underestimated if compared to the experimental
results. This could be due to the choice of the LES model or the effect of too much dissipation
which reduced the drag coefficient. The force balance used in the wind tunnel has also a major influence
on the results, by being not sensitive enough at lower Reynolds numbers. Although the pressure coefficient
did not match with that of literature some important bluff body phenomenon could be observed from the
trends.

The global fuel consumption by Heavy duty vehicles is an important issue in every climate change summit. Aerodynamic drag consumes majority of fuel at highway speeds. Thus, it is important to strive hard to reduce drag using different techniques. WABCO's SideWing (side-skirt) is an air dam (aerodynamic device) placed along the sides of the trailer to prevent low and high momentum flow being mixed and eventually increase drag.

WABCO's track test data suggests that the side-skirt is under-performing in dual rear axle condition, that is in 6X4 tractor configuration. This thesis is aimed at understanding the flow physics responsible for fuel savings in single axle condition (4X2) and further extend the knowledge to 6X4 condition. In order to do so, flow physics at the front wheel (available in literature) is studied initially and validated using CFD simulations in this thesis. The side-skirt behavior is studied in single axle condition. Furthermore, the influence of rear wheel of tractor on drag and flow features is understood first. Lastly, the changes in flow features observed on the addition of an axle are understood.

The sidek-skirt showed reduced drag reduction in 6X4 condition in this thesis. This is positive trend in the sense that CFD simulation is conforms with the track test data. It is understood that the additional axle creates a under pressure region lower than compared to single axle case.

The EU has committed itself to reduce the greenhouse gas emissions with 20% by 2020 with respect to the 1990 level. This means that all industry sectors including the transport sector have to become more energy efficient. The long and short haul trucks are, with 4.95% of the total CO2 equivalent emissions, a major contributor in the transport sector. In order to achieve a reduction in emissions, the European Commission is proposing new regulations that allow for front elongation of trucks. This elongation might improve the aerodynamic efficiency and hence reduce the emissions and fuel costs. Reducing aerodynamic drag for tractor-trailer combinations has been researched for a long period of time. In 1965 Hoerner published the results of his experimental research on 3D bluff bodies and identified that the drag coefficient at the front was related to the total drag coefficient. In 1985 Cooper determined that an optimal front edge radius based on the Reynolds number and the frontal area of the vehicle exists. These two studies concerning bluff bodies formthe basis of numerous follow up studies and this thesis. This thesis analyses how the geometry of an elongated bluff body can be optimized in order to reduce the aerodynamic drag coefficient Typically the flow around a bluff body is characterized instable and separated flow. At the front of the truck the incoming flow velocity reduces to zero at the stagnation point. A high pressure region is formed at this location. Further downstream the flow reaches the front edge corners of the truck, where the flow is accelerated. Flow separation might occur at these front edge corners. At the rear a large low pressure region is formed which contributes to the total drag coefficient. In this thesis numerical simulations are performed in order to identify the relative importance of several front-end design parameters. The numerical calculations are performed by means of the steady-state RANS k ¡! turbulence model. The domain is refined near the surface model and its wake using two density boxes around the model with different cell dimensions and an inflation layer around the model. A wall model is used to approximate the flow behavior in the boundary layer. The wall model and the local refinements reduce the computational time while retaining a high level of accuracy of the numerical solution. The numerical simulation is validated using the data from an already existing wind tunnel experiment. This wind tunnel experiment has been performed on a 1:15 scale model in the Low Turbulence wind tunnel at the TU Delft. First the influence of the mesh cell size is determined by varying the cell sizes of the two density boxes surrounding the model. One density box is close to the model and has small cell dimensions, while the second density box is encapsulating a larger volume. This resulted in chosen cell dimensions with a ratio of 1:4. The discretization error is 4.1%, while the number of cells used is 4.47¤106. The second part is the validation itself. The drag coefficient of the model is 0.297 for the wind tunnel experiment and 0.300 for the simulation,while the base pressure is slightly lower for the numerical simulation. The pressure distribution at the base is not correctly captured in the simulation results. The final part of the validation is the comparison of the boundary layer development. The results of the simulation obtained in this thesis show great similarity with the results found in the wind tunnel experiment. To explore the real world effects the 1:15 scale results...

In the European Union greenhouse gas emissions of heavy duty vehicles make up 30% of the total caused by road transport. By placing vehicles in a platoon configuration the aerodynamic drag can be heavily reduced. The effect of platooning has been studied on both American and European type vehicles. However many of these studies only disclose the drag reductions and do not fully explain the flow behaviour between the models which in the end is what is causing the reduction in drag. Next to this studies done on European type vehicles mostly used time-averaged simulations so a next step is understanding the effect of unsteady flow on the vehicles. In this study the flow field between two vehicles in a platooning configuration is analysed numerically and experimentally for varying intervehicle distance and front- and rear drag reduction devices. The GETS model was used in this study which is a simplified model of a European heavy duty vehicle. For the numerical analysis Exa PowerFLOW is used which is a transient CFD package based on the LBM. Turbulence is modelled using VLES together with a RNG of the κ - ϵ equations. A study of the mesh size showed the sensitivity on the drag coefficient of the single model.
The experimental analysis was performed in the OJF of the TU Delft at a Reynolds number of 3.9 x 105 based on the square root of the model. Next to balance measurements CVV measurements in the gap between the two models were taken in order to visualize the flow field.
The single model showed a toroidal recirculation region in which a vortex ring can be seen. The drag force fluctuates at a Strouhal number of 0.073, side and lift force at 0.058 and 0.102 and 0.130 and 0.160. These fluctuations are caused by pressure fluctuations in the wake where the highest magnitudes are seen in the shear layer. The models with a smaller front radius saw a rise in drag with flow separation occurring for the smallest radius for the numerical results and for both smaller radii in the experiment. The addition of a boat tail lowered the drag of the models. With increasing tail angles the drag reduction also increases which is caused by the increase in base pressure. The tails also decrease the force fluctuations due to decreased pressure fluctuations in the shear layer.
At the closest spacing of 0.10 times the vehicle length all tested configurations benefit from the platoon. The flow between the two models is made up of a toroidal recirculation region much smaller in size compared to the single model. In general the flow takes an S-shaped path between the two models. From the underbody of the leading model it either stagnates on the front of the trailing model or moves into the gap where it ends up in the upper of lower vortex or stagnates on the front or rear of one of the models. When a sharper front edge radius is applied to the trailing model more flow is deflected into the gap leading to lower pressure vortices and a higher stagnation pressure. A small vertical misalignment of the trailing model, which was seen during the experimental campaign, leads to a higher stagnation pressure on the bottom of the trailing model, also here more flow is deflected into the gap. At this spacing the side-force fluctuations are a bit higher due to the vortices that leave the domain passing over the rounded edges of the model.
At the middle spacing of 0.45 times the vehicle length not all configurations benefit from the platoon. The trailing models with the baseline frontal radius saw an increase in drag due to the lack of thrust generated by the rounded edges. The other models did see a drag decrease. At this distance the wake of the leading model is quite similar to that of the single model. Flow enters from the top, bottom and sides of the model and ends up in the recirculation region or it stagnates on the rear of the model before it leaves the wake and stagnates on the front of the trailing model or flows over the rounded edges where it accelerates and leaves the gap. The effect of a sharper radius applied to the trailing model has little effect on the flow field. When a tail is applied to the leading model the recirculation region has been reduced as was seen for the single models. Due to the upwash the stagnation pressure on the bottom front has been increased but the drag has decreased compared to the platoon without a tail due to the increased suction of the rounded edges. At this distance the force fluctuations are much higher compared to the single model. The magnitude of drag and side force can go up to 2 and 6.5 times the values of the single model depending on the configuration. This is due to the stronger vortices from the leading model which pass over the trailing model.
For the last vehicle spacing of 0.91 s/L all the configurations benefit from the platoon again. For the baseline models the gains are only a few percent but for the trailing model with a sharper front radius the gains are higher than those at the middle intervehicle distance. This is due to the reduced stagnation pressure and almost undisturbed suction coming from the rounded edges. At this distance any effect on the wake of the leading vehicle has vanished. The unsteady forces still show an increased amplitude compared to the single model however compared to the middle intervehicle distance they are much lower.
The comparison between the results from the numerical and experimental analysis is quite good. The drag and flow field results are very similar. For the pressure and the unsteady forces this is less the case. The mismatch seen in the pressure comparison can be caused by alignment errors that were observed during the experiment. Next to this while reconstructing the flow field from the experimental measurements the resulting coordinates were outside the expected range. A manual coordinate shift was applied which may have resulted in additional errors. For the unsteady forces it is assumed that the ground plate and its interaction with the model as well as additional vibrations are the cause of the much higher fluctuations seen from the experimental data.

To limit the threat posed by global climate change it is vital to reduce the emissions of the transport sector in the short term. Two existing aerodynamic add-on devices for the rear-end of a heavy duty vehicle, a tail and guide vanes, are combined in an attempt to achieve a larger drag reduction. A research is performed using numerical simulations and a wind tunnel experiment in the Open Jet Facility to gain insight in the interaction between the tail and the guide vane. It was found that the guide vane is able to further increase the base pressure of the vehicle, but that the drag reduction is sensitive to the drag of the vane. Due to the interaction between the tail and the guide vane the largest drag reduction is achieved when the guide vane is operating at its minimum drag condition.

The transport sector is making a significant contribution to the global CO2 emissions causing Global Warming. A large portion of this is caused by heavy-duty vehicles like tractor semi-trailer combinations. Since tractor semi-trailer combinations are often operated at relatively high speeds for long periods of time their fuel consumption, and with that their emissions, can be strongly reduced by reducing the aerodynamic drag. This can be done by improving the aerodynamics of individual vehicles, by carefully rounding their leading edges or application of drag reduction devices, like boat tails and side skirts. Another option is to use the benefits of drafting by operating two or more vehicles closely together in a platoon. The benefits of this have already been proven in multiple studies and real world experiments. When platooning will be implemented on a large scale, it will be beneficial to optimise the platoons for maximum drag reduction. To be able to do this, the effect of different truck design parameters have to be investigated. This study focuses on the influence of underhood flow on the drag of a tractor semi-trailer in isolation and in a platoon. This is done by using simplified models adapted to have a underhood model consisting of one porous medium and four ducts to replicate the mass flow, pressure drop and flow field of a real underhood area. Simulations were performed on full-size and highway speeds using the commercially available PowerFLOW solver, based on the Lattice Boltzmann Method. For an isolated vehicle it was found that an increased underhood mass flow gives a higher total drag, mainly because of the drag contribution of the porous medium. Due to the mass flow entering the underhood, the suction over the leading edges is slightly reduced. On the other hand, smaller leading edge radii with higher suction give less mass flow through the underhood. Besides this the underhood flow actually has beneficial effects on the parts surrounding the tractor-trailer gap and in the trailer underbody area. The highest total drag was found for the models with the most underhood flow and the smallest leading edge radius. Platoons of two vehicles were tested with three different inter vehicle distances, 3.75, 7.5 and 15 meters. The leading vehicle, which did not have underhood flow in all cases, has the strongest drag reduction for the shortest distance. The trailing vehicle has the lowest drag at the largest tested distance, while it is highest for the middle distance. This can be explained by the reduction in pressure in front of the vehicle. This reduces the drag contribution of the front surface, but also reduces the suction over the leading edges. The models with underhood flow experienced a stronger drag reduction, meaning that the absolute drag values were closer than for the isolated vehicle. This is caused by the reduced underhood mass flow in a platoon. At the shortest inter vehicle distance only 35% of the mass flow of an isolated vehicle is available, while this is 50 and 70% when the distance is increased. The beneficial effects of underhood flow on an isolated vehicle are still present in a platoon, although they are reduced in strength. When a boat tail is mounted on the back of the trailer of the leading vehicle the drag of this vehicle is strongly reduced due to the increased back pressure. However, this might not be beneficial for the trailing vehicle. The increased stagnation pressure indeed increases the total drag of the trailing vehicle at an inter vehicle distance of 3.75 m. As discussed before, an increased stagnation pressure also gives increased leading edge suction. Therefore the drag is actually reduced for the two larger inter vehicle distances. It was also found that the tail gives higher flow speeds over the top of the trailing vehicle, and lower flow speeds around the bottom. This reduces the drag for the underbody parts like the wheels and slightly decreases the underhood mass flow. When the platoon is placed at a yaw angle, the total drag of the leading vehicle is increased. The contribution of the front part is decreased, but this is more than compensated for by the drag increase for the rear and all other parts, which are no longer perfectly aligned with the flow. The drag increase of the trailing vehicle is stronger, due to an increased contribution of the front part. The underhood mass flow is also increased compared to the platoons without yaw, this effect is strongest at small inter vehicle distances. The leading vehicle causes the flow to be more aligned for the trailing vehicle, therefore the drag increase for most other parts is less strong. The same effect can be seen for the side force, which is way lower for the trailing vehicle and increases for increasing inter vehicle distance.