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- Ask, Jonas, 1970, et al.
(författare)
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A Numerical Investigation of the Flow Past a Generic Mirror and its Impact on Sound Generation
- 2009
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 131:061102, s. 0621011-06110212
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Konferensbidrag (refereegranskat)abstract
- The case investigated is the flow past a generic side mirror mounted on a flat plate at the Reynolds number of Re(D)=5.2x10(5) based on the mirror diameter. The present work studies both flow and acoustic sources by evaluating two second-order advection schemes, different levels of turbulence modeling, and three different grids. The advection schemes discussed in the present study are a second-order upwind scheme and a monotonic central scheme. The turbulence models investigated cover three levels of modeling. These are the original formulation of the detached eddy simulation (DES) model, the Smagorinsky-Lilly sub-grid scale (SGS) model with near-wall damping, and a dynamic Smagorinsky model. The different grids are as follows: a primary grid where all parameter studies are conducted and a second grid with significantly higher wake resolution and to some extent also increased plate resolution, while maintaining the resolution at the front side of the mirror. The final grid uses a significantly higher plate resolution and a wake resolution similar to that of grid two, but a comparably lower mirror front side resolution as compared with the two other grids. The general outcome of this work is that the estimation of the grid cutoff frequency through a relation of the velocity fluctuation and the grid size matches both the experimental results and trend lines perfectly. Findings from the flow field show that the horseshoe vortex in front of the mirror causes pressure fluctuations with a magnitude exceeding the maximum levels at the rear side of the mirror. Its location and unsteady properties are perfectly captured in the final simulation as compared with the experiments conducted by Daimler-Chrysler. A laminar separation at the front side of the mirror is more or less found for all wall resolved cases except the DES simulation. The third grid fails to predict this flow feature, but it is shown that this effect has no significant effect on either the static pressure sensors at the mirror surface or at the dynamic sensors located downstream of the mirror. The simulation also supports the fundamental frequency based on the eddy convection in the mirror shear layer, which is shown to be twice as high as the frequency peak found in the lateral force spectra.
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| 4. |
- Ask, Jonas, 1970, et al.
(författare)
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Flow and dipole source evaluation of a generic SUV
- 2007
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Ingår i: 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference). - Reston, Virigina : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- Accurately predicting both average flow quantities and acoustic sources at the front side window of today's ground vehicles is still a considerable challenge to automotive companies world-wide. One of the most important aspects for obtaining trustworthy results, but also the most tedious one and therefore perhaps overlooked, is the control and outcome of the mesh generation process. Generating unstructured volume meshes suitable for Large Eddy Simulations with high level representation of geometrical details is both a time consuming and an extremely computer demanding activity. This work investigates two different mesh generation processes with the main aim to evaluate their outcome with respect to the prediction of the two dominating dipole sources in a temporal form of the Curie's equation. Only a handful of papers exists with high level representation of the vehicle geometry and the aim of predicting the fluctuating exterior noise sources. To the author's knowledge no studies have been conducted in which both these source terms are evaluated quantitatively against measurements. The current paper investigates the degree to which the amplitude of these two source terms can be predicted by using the traditional law-of-the-wall and hex-dominant meshes with isotropic resolution boxes for a detailed ground vehicle geometry. For this purpose the unstructured segregated commercial FLUENT Finite Volume Method code is used. The flow field is treated as incompressible, and the Smagorinsky-Lilly model is used to compute the sub-grid stresses. Mean flow quantities are measured with a 14-hole probe for 14 rakes downstream of the side mirror. Dynamic pressure sensors are distributed at 16 different positions over the side window to capture the fluctuating pressure signals. All measurements in this work were conducted at Ford's acoustic wind tunnel in Cologne. All simulations accurately predict the velocity magnitude closest to the side window and downstream of the mirror head recirculation zoner. Some variations in the size and shape of this recirculation zone are found between the different meshes, most probably caused by differences in the detachment of the mirror head boundary layer. The Strouhal number of the shortest simulation was computed from the fundamental frequency of the mirror lift force component. The computed Strouhal number agrees well with the corresponding results from similar objects and gives an indication of an acceptable simulation time. Dynamic pressure sensors at 16 different locations at the vehicle side window were also used to capture the levels of the two dipole source terms. These results are compared against the three simulations. With the exception of three positions, at least one of the three simulations accurately captures the levels of both source terms up to about 1000Hz. The three positions with less agreement as compared to measurements were found to be in regions sensitive to small changes in the local flow direction.
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| 5. |
- Ask, Jonas, 1970
(författare)
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Predictions of Aerodynamically Induced Wind Noise Around Ground Vehicles
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the last decade wind noise has with few exceptions consistently generated a constant or even growing level of customer complaints to automotive companies world-wide. This can partly be explained by a relatively greater focus on engine, power-train and tire noise combined with a growing need to reduce weight in future cars.One class of wind noise problems commonly referred to as air-rush noise relates to turbulent pressure fluctuations caused by separated or vortical flows, which is addressed in this thesis.Sound generation and to some extent sound propagation are here evaluated fromincompressible flow fields, since the unsteady flow hydrodynamics at low Mach numbers is often the dominant sources of sound. A temporal form of Curle's equation is used in these evaluations.It is shown that the noise level measured inside the compartment of a production vehicle has a dipole character. Accordingly,sound generation is evaluated from the dipole terms in the acousticanalogy and, for one specific case, the radiated sound is evaluated by the surface integral of the sameterms.Three different cases are investigated, ranging from the laminar flow past an open two-dimensional cavity to the turbulent flow past a representative ground vehicle.It is shown that the dipole terms in Curle's equation can with sufficient accuracy be predicted for the regions of interest even for the full vehicle case.A potential problem is highlighted, which is that, even if the acoustic source magnitudes are correctly predicted in an incompressible field, the sound directivity may be erroneous due to distortions in the pressure field and small differences in the flow field for flows with few but dominant structures.
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| 6. |
- Ask, Jonas, 1970, et al.
(författare)
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Sound Generation and Radiation of an Open Two-Dimensional Cavity
- 2009
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Ingår i: AIAA Journal. - : American Institute of Aeronautics and Astronautics (AIAA). - 1533-385X .- 0001-1452. ; 47:6, s. 1337-1349
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Tidskriftsartikel (refereegranskat)abstract
- This work studies differences in sound generation and radiation between incompressible and compressible flowfields at the corresponding Mach number of 0.15 by evaluating the two dominant dipóle terms in a temporal form of Curie's equation. The present work adds incompressible simulation results to a previously reported direct simulation of an open two-dimensional cavity oscillating in wake mode. The length-to-depth ratio of the cavity is L/D = 4, and the flow is considered to be laminar. The Reynolds number based on the cavity depth is Re D = 1500. At these conditions, the flow is characterized by large variations in the streamwise force coefficient, showing similarities to a bluff-body wake flow, which makes the present case an attractive candidate for an incompressible approach. The root-mean-square levels of the two acoustic source terms show good agreement in the vicinity of the cavity, and the agreement in the radiated sound is almost perfect when isolating the cavity walls in the surface integration. The agreement in the radiated sound for the two flowfields is equally good when isolating the downstream wall extending from the cavity trailing edge and 10D downstream. However, when the surface integration comprises both the cavity walls and the downstream wall, a major mismatch in the directivity arises, and it is shown that this mismatch is primarily caused by an almost complete cancellation of the sources at the cavity bottom and the downstream wall in the incompressible simulations.
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