Papers by Keyword: Dimples

Abstract: Reduction of drag and flow resistance in systems containing moving fluids is a prominenttool to increase energy efficiency. Besides active flow control – such as moving surfaces or boundarylayer suction – passive techniques such as surface patterning by means of dimples are promising sinceno additional energy consumer is introduced into the system. Even though the effect of drag reductiondue to dimples has often been observed, the physical principals responsible for this effect are not yetunderstood. Most of the research concerning dimples and drag reduction published so far has beencarried out experimentally, not many numerical investigations on this topic have been done. The mainreason for this is that the tiny, transient flow structures generated in direct vicinity of dimples can noteasily be resolved in simulations. Even in case of time dependent numerical investigations it is notclear, whether and with which method of sub-grid scale modeling Large Eddy Simulations are capableof modeling these structures sufficiently. In this work we investigated different surfaces with dimpledepth to diameter ratios h/D reaching from 0.01 to 0.1 in channels of height H = 0.417D at Reynoldsnumbers ReD 5 830 and ReD 11 650 using steady state simulations with a k-omega-SST turbulencemodel. Drag reductions were observed for all setups h/D < 0.08 compared to the smooth channel.The best results were obtained with dimple depths of 4-5 % of diameter showing a slight dependenceof Re which is in good agreement with literature. As the experimental investigation of the flow overdimpled surfaces is limited in spacial and temporal resolution we could demonstrate that numericalinvestigations give the possibility to overcome this drawback. However the solution of simulationsstrongly depends on numerous factors such as the discretization scheme, the numerical models andthe grid used to obtain results which might be a reason for slightly varying results of such simulationsfound in literature.

Abstract: Multi-point Sheet metal forming is a flexible technique in which a discrete punches are used to generate the continuous 3-D surface . The proposed die comprises a set of matrices of a closely stacked discrete punches arranged to form a cavity in which a free form surface can be formed .The working surface of the multi-point die are modeled by a uniform B-spline and constructed by adjusting the height of each punch manually. Finite element simulation for the process was performed to study the effect of punch number on the shape error and thickness variation. Experimental work include the building of multi-point die to form the aluminum sheets. The effect using rubber interpolators on dimpling defect were also studied . The results revealed that thickness variation error between the numerical and experimental results is equal to 5% .

Abstract: As a new technique to form sheet metal parts, Multi-point forming (MPF) also can be used on bending aluminum alloy profile. Since the Multi-Point Die (MPD) which replaces the traditional solid bending die is composed of many discrete punch elements, dimples always occur on the plate of profiles. To eliminate the dimpling defects, numerical simulation of the bending process with A6N01S-T5 aluminum alloy hollow profile using MPD were conducted. By comparing the bending effects on MPD with different size of punch elements and with different kind of elastic cushion, reasonable forming parameters were obtained. Pressing of Aluminum alloy profile with different radii on the MPD and solid die were simulated. The cross-section distortion indicated that the aluminum alloy profile can be formed with MPF technique while it has the advantage of flexibility. For the profile with large deformation, multi-step MPF method is a better choice due to its rapid reconfigurable characteristic.

Abstract: The composite materials with 2, 4 and 6 % Cu were developed using a noble stir casting method. Tensile test was conducted using universal tensile test machine according to ASTM-2002 (E8M-01) standard and the fracture surface was analysed using scanning electron microscope (SEM). The result showed that the ultimate tensile strength (UTS) was increased due to the percentage increment of Cu in the Al-Cu matrix. A significant change in UTS was observed from 2 to 4 % Cu whereas slight improvement was seen from 4 to 6 % Cu addition. The SEM micrograph of the fracture surface reveals that the cracks were propagated in the fibrous zone resulting from the initiation of micro voids between the matrix and particle interfaces. The number and size of dimples for 2% Cu were considerably lower and the facet features were noticeably higher than higher Cu content composition, whereas, the dimple and facet size and number for the 4% Cu are very close to the 6% Cu content composite which may play important role in the improvement of tensile strength. These findings tinted for the potential application of SiC_p reinforced Cu influenced Al-Cu-SiCp composite for automotive engine components and other similar applications as well.

Abstract: Multi-point stretch forming (MPSF) is a new technique to form aircraft outer skin panel. Since multi-point die is composed by the discrete punches, the result of the MPSF aircraft outer skin panel need to study in depth. The thickness of elastic cushion and free length are two important factors to affect the accuracy, and they must be chosen reasonably. A series of numerical simulations on typical MPSF processes were carried out to an aircraft outer skin panel part. The results show that the thicker the elastic cushion is, the more valid the dimple will be suppressed .The longer free length is, the smaller the equivalent strain and thinning and more uniform the distribution of thickness will be. When the free length is shorter, the degree of effect is relatively obvious on the equivalent strain and thinning and the distribution of thickness; when the free length is longer than a certain value, the degree of effect is small.

Abstract: As a flexible manufacturing technique, Multi-point stretch forming (MPSF) is a suitable method for forming aircraft outer skin part. The traditional solid stretching die is replaced by the discrete multi-point die (MPSD), and the sheet metal is stretch-formed over the MPSD generated by serial adjusting mode or parallel adjusting mode. The MPSF can be used to form the parts of different shape and reduce the cost and leading time of stretching die fabrication for aircraft outer skin part. A series of numerical simulations on typical MPSF processes of aircraft outer skin part were carried out. The thickness of elastic cushion and free length are important factors to influence on the stretch forming results of stretch-formed parts. The numerical simulation results show that the thicker the elastic cushion is, the more valid the dimple will be suppressed .The longer the free length is, the easier the wrinkle will be brought.

Abstract: Vertical section features in bonding point were produced by ion-sputter thinning, and were tested by using TEM-F30. Lift-off characteristics at the interface of Ultrasonic bond are observed by using SEM (JSM-6360LV). Results show that thickness of Au/Al atomic diffusion interface was about 500 Nanometer under ultrasonic and thermal energy. Ultrasonic vibration activates dislocations at metal crystal lattice. Fracture morphology of lift-off interface was dimples. Tensile fracture appeared by pull-test not in bonded interface but in basis material, and bonded strength at interface was enhanced by diffused atom from the other side.