| 42741. |
- Marklund, Ture Jesper, 1970, et al.
(författare)
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Influence of a Diffuser to the Wake Flow of a Passenger Car
- 2012
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Ingår i: American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FEDSM. - 0888-8116. - 9780791844755 ; :PARTS A AND B, s. 53-62
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Konferensbidrag (refereegranskat)abstract
- 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 contributor to the overall aerodynamic drag and the shape of it is normally a compromise between styling, cost and other properties. To reduce the aerodynamic drag it is very important to have a good pressure recovery at the rear-end; to end up with a base pressure as high as possible. It is not necessarily the case that an optimized lower part of the rear-end for a square-back car has the same performance as a notch-back or fast-back car.This work investigates the rear-end flow and aerodynamic performance of a sedan and wagon car with varying rear-end under-body design parameters. The study is a numerical analysis using a standard CFD approach commonly used in the automotive industry. A parameter study of under-body covers with varying rear angles, making the rear floor act like a diffuser. The function of the rear floor working as a diffuser is similar regardless of the upper geometry, but its function as a drag reduction device can be very different. Results from this study show a potential to reduce aerodynamic drag of the sedan car approximately 10%, with the best diffuser angle and cover plates over the floor. The best drag reduction for the wagon car was 2-3 % and the optimum was at a smaller diffuser angle. Flow analysis of the wake shows how important it is the wake is balanced.
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| 42742. |
- Marklund, Ture Jesper, 1970
(författare)
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Minimize Vortex Drag of a Passenger Car
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aerodynamic drag force is the single biggest resistance force for a passenger car toovercome at highway speeds. Since all driving resistance forces can be directly linked tofuel consumption this has become a significant priority in recent years. Increasing fuelprices and environmental issues are strong drivers for reducing these resistance forces.Vehicle manufacturers are today struggling to develop more energy efficient vehicles thatwill meet future emissions targets of CO2 (carbon dioxide). To do so it is essential toimprove both efficiency of driveline and reduce resistance such as inertia and drag forces.Pressure forces from the exterior body of a passenger car are the dominating forces of thetotal drag force. This will classify a passenger car aerodynamically as a bluff body. Thebiggest pressure forces are associated to wake formations at the rear end of a bluff body.This is the reason for this study of simplified vehicle like bluff bodies focusing on therear end.Detailed flow field investigations of wake flows behind the models and boundary layerflows close to the surfaces have been performed. The measurements were carried outwith stationary ground simulation in the L2 scale model wind tunnel at ChalmersUniversity of Technology in Gothenburg. The wake flow was measured with a smallscale 12-hole omniprobe that is capable of capturing almost reversed flows. The purposewas to measure the wake flow of small vehicle-like bodies in ground proximity todetermine preferred rear end bodywork geometries. The testing was carried out on fourdifferent rear end type of models labeled boat-tailed, fastback and square-back rear end.It is important to have a small and balanced wake to reduce drag. It is preferable to have ahigh pressure recovery to the rear part of the body and minimum vortices. The drag forceis due to the pressure difference between the front and rear of the body.Computational Fluid Dynamics (CFD) simulations have been performed on the samebodies with the same boundary conditions as the wind tunnel tests. The numericalsettings were selected to compare standard simulation methods generally used forexternal vehicle aerodynamics.
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| 42743. |
- Marklund, Ture Jesper, 1970
(författare)
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Under-body and Diffuser Flows of Passenger Vehicles
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Energy efficient vehicles will be required to meet future emission and fuel consumption requirements. Customers require reduced fuel consumption due to increasing fuel prices and the environmental issues, are drivers to reduce CO2. It is essential to improve the drivelines, but improving resistance forces of the vehicle is also an efficient and sustainable way to improve energy efficiency. Aerodynamic drag is the dominating resistance force for passenger and commercial vehicles at highway speeds.A passenger car is a bluff body aerodynamically, with pressure forces at the rear that dominate the aerodynamic drag. This is due to a relatively square shape, with a length / height ratio of approximately three, and a truncated rear-end that generates a wake. About 60 % of the aerodynamic drag forces of a passenger vehicle are related to the exterior body, upper and under-body; the rest being related to wheel, wheel house and cooling drag.This work focuses on the aerodynamics of the rear-end and under-body of bluff bodies in general, but also applied to passenger cars. Firstly, simplified bluff bodies, that represent different vehicle types, were used to study and map the general behaviour of the bodies. The findings were then tested and applied to full–size vehicles, with the focus on under-body flows and the effect of under-body diffusers. Both experimental and numerical tools were used, and scale model as well as full-size test bodies have been investigated.A unique feature with road vehicle aerodynamics are the boundary conditions: ground proximity and moving ground; relative the body. Also, rotating wheels and a cooling flow that re-distributes the flow around the body have to be considered. The Chalmers L2 wind tunnel is equipped with a moving ground system, and the simulations were set up with moving ground, rotating wheels and a cooling flow. The rotating wheels were simulated with the MRF approach and the cooling flow was tuned by measuring the cooling flow of a full-sized car and using this data in the simulations.A significant difference in the flow in an under-body diffuser, depending on upper body, was noticed in the bluff body experiments. In particular, drag was reduced more for a sedan or fastback upper body, compare to a wagon or square-back. This difference was confirmed in simulations of full–size vehicles, under road-vehicle boundary conditions, with under-body diffusers applied. It was found that it is very important to have flow symmetry around the vehicle and especially at the wake, to optimize pressure recovery at the rear end and reduce drag.
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| 42744. |
- Marklund, Ture Jesper, 1970, et al.
(författare)
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WAKE MEASUREMENT OF A SIMPLE BLUFF BODY WITH VARYING REAR END DESIGN AND CLOSE PROXIMITY TO GROUND
- 2010
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Ingår i: ASME, FEDSM2010, August 1-5, 2010, Montreal, CANADA.
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Konferensbidrag (refereegranskat)abstract
- The flow field around simplified bluff bodies constitutes a classic area within vehicle aerodynamics. In this study we are looking at the wake flow of a vehicle-like simplified model with and without a rear open diffuser in close proximity to ground. The measurements were done with stationary ground simulation in the L2 scale model wind tunnel at Chalmers University of Technology in Gothenburg, Sweden. The wake flow was measured with a 12-hole omniprobe that has a spherical probe head of 6.35 mm. The purpose was to test how well this probe suits wake measurements of small vehicle-like bodies in ground proximity. The testing was done to a boat tailed/fast back model of small scale. Reynolds number for the experiments was about 1,01 million based on the model length and will be considered low compared to a full scale passenger vehicle. Since the main objective was to test the measurement method, the result is still valid. Measurements are compared to simulations done in the commercial CFD code Fluent. The task was to study the rear end of a simplified bluff body by measuring the close wake of a model with and without a diffuser. A diffuser reduces the drag of this model significantly, and the flow measurements show the difference in wake size and vortex formations.
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| 42745. |
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| 42746. |
- Marouf Rashid, Hawal, 1982, et al.
(författare)
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Wideband passive circuits for sideband separating receivers
- 2018
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Ingår i: 2018 29th IEEE International Symposium on Space Terahertz Technology, ISSTT 2018. ; , s. 195-200
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Konferensbidrag (refereegranskat)abstract
- In this work, we present the design and characterization a new generation of compact wideband quadrature hybrids operating both at room and cryogenic temperatures for prospective use in the IF chain of 2SB THz receivers. The compact wideband design employs a multi-section topology (coupled line coupler - Lange coupler- coupled line coupler) and thin-film technology with gold plated transmission lines and air bridges to connect the fingers of the Lange coupler (middle section). The hybrids were designed to have the amplitude and phase imbalance better than 0.6 dB and ±3° respectively over a 3.5-12.5 GHz and 4-16 GHz frequency bands. The experimental verification of the assembly at 293 K and 4 K shows very good agreement between the measurements and simulations with amplitude imbalance better 0.5 dB and maximum phase imbalance of ±1.5°.
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| 42747. |
- Marstorp, Linus, et al.
(författare)
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A stochastic subgrid model with application to turbulent flow and scalar mixing
- 2007
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 19:3, s. 035107-
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Tidskriftsartikel (refereegranskat)abstract
- A new computationally cheap stochastic Smagorinsky model which allows for backscatter of subgrid scale energy is proposed. The new model is applied in the large eddy simulation of decaying isotropic turbulence, rotating homogeneous shear flow and turbulent channel flow at Re-tau=360. The results of the simulations are compared to direct numerical simulation data. The inclusion of stochastic backscatter has no significant influence on the development of the kinetic energy in homogeneous flows, but it improves the prediction of the fluctuation magnitudes as well as the anisotropy of the fluctuations in turbulent channel flow compared to the standard Smagorinsky model with wall damping of C-S. Moreover, the stochastic model improves the description of the energy transfer by reducing its length scale and increasing its variance. Some improvements were also found in isotropic turbulence where the stochastic contribution improved the shape of the enstrophy spectrum at the smallest resolved scales and reduced the time scale of the smallest resolved scales in better agreement with earlier observations.
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| 42748. |
- Marstorp, Linus, et al.
(författare)
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Explicit algebraic subgrid stress models with application to rotating channel flow
- 2009
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Ingår i: Journal of Fluid Mechanics. - 0022-1120 .- 1469-7645. ; 639, s. 403-432
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Tidskriftsartikel (refereegranskat)abstract
- New explicit subgrid stress models are proposed involving the strain rate and rotation rate tensor, which can account for rotation in a natural way. The new models are based on the same methodology that leads to the explicit algebraic Reynolds stress model formulation for Reynolds-averaged Navier-Stokes simulations. One dynamic model and one non-dynamic model are proposed. The non-dynamic model represents a computationally efficient subgrid scale (SGS) stress model, whereas the dynamic model is the most accurate. The models are validated through large eddy simulations (LESs) of spanwise and streamwise rotating channel flow and are compared with the standard and dynamic Smagorinsky models. The proposed explicit dependence on the system rotation improves the description of the mean velocity profiles and the turbulent kinetic energy at high rotation rates. Comparison with the dynamic Smagorinsky model shows that not using the eddy-viscosity assumption improves the description of both the Reynolds stress anisotropy and the SGS stress anisotropy. LESs of rotating channel flow at Re-tau = 950 have been carried out as well. These reveal some significant Reynolds number influences on the turbulence statistics. LESs of non-rotating turbulent channel flow at Re-tau = 950 show that the new explicit model especially at coarse resolutions significantly better predicts the mean velocity, wall shear and Reynolds stresses than the dynamic Smagorinsky model, which is probably the result of a better prediction of the anisotropy of the subgrid dissipation.
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| 42749. |
- Marstorp, Linus, 1978-
(författare)
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Modelling of subgrid-scale stress and passive scalar flux in large eddy simulations of wall bounded turbulent flows
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of the thesis is to develop and validate subgrid-scale models that are relevant for large eddy simulations of complex flows including scalar mixing. A stochastic Smagorinsky model with adjustable variance and time scale is developed by adding a stochastic component to the Smagorinsky constant. The stochastic model is shown to provide for backscatter of both kinetic energy and scalar variance without causing numerical instabilities. In addition, new models for the subgrid-scale stress and passive scalar flux are derived from modelled subgrid scale transport equations. These models properly account for the anisotropy of the subgrid scales and have potentials wall bounded flows. The proposed models are validated in wall bounded flows with and without rotation and show potential or significantly improve predictions for such cases.
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| 42750. |
- Marstorp, Linus, et al.
(författare)
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Stochastic SGS modelling in homogeneous shear flow with passive scalars
- 2006
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Ingår i: Direct and Large-Eddy Simulation VI. - DORDRECHT, NETHERLANDS : SPRINGER. - 1402049099 ; , s. 167-174
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Konferensbidrag (refereegranskat)abstract
- A new stochastic Smagorinsky model for the subgrid stress and subgid scalar flux is proposed. The new model is applied in LES of rotating homogeneous shear flow, which is an excellent case for developing and testing subgrid scale models. The proposed model provides for backscatter of energy and scalar variance, and reduces the length scale of the subgrid dissipation compared to the standard Smagorinsky model. At the same time, the flatness factor of the subgrid dissipation obtained from the stochastic model is of the same order of magnitude as for the Smagorinsky model.
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