Scielo RSS <![CDATA[R&D Journal]]> http://www.scielo.org.za/rss.php?pid=2309-898820180001&lang=en vol. 34 num. lang. en <![CDATA[SciELO Logo]]> http://www.scielo.org.za/img/en/fbpelogp.gif http://www.scielo.org.za <![CDATA[<b>CFD Investigation of the Transonic Flow-field for a Decelerating Axisymmetric Cylinder</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S2309-89882018000100001&lng=en&nrm=iso&tlng=en This study is a fundamental investigation to demonstrate the effect of significant deceleration on the transonic flow field around a blunt cylindrical projectile through a numerical approach implemented in ANSYS Fluent. The projectile geometry and order of acceleration magnitude is based on a study by Jiang et al. [1], who investigated the near blast flow-field of a supersonic projectile emerging from a barrel into ambient air using an Euler solver. This study considers the region of flight once the projectile has travelled away from the barrel outlet. Two projectile shapes is considered (25x25 mm and 50x25 mm cylinders) and two deceleration magnitudes (10 000 m/s² and 5 000 m/s²). The projectiles are deceleratedfrom steady state Mach 1.2 to 0.8, at zero-incidence. The bow shock and wake recovery compression waves are shown to propagate forward relative to the body where the wake recovery compression waves overtake the body and appear ahead of the nose. Strong coupling exists between the near wake flow field and outer region of the separation bubble on the cylinder for an aspect ratio of 1, influencing the shock dynamics during deceleration. <![CDATA[<b>Time-accurate transonic CFD Simulation of a generic store release case</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S2309-89882018000100002&lng=en&nrm=iso&tlng=en Store release from a parent aircraft in the transonic regime is a complex transient interaction to simulate, due to both compressibility effects and the strong interference flow field generated between the parent and store bodies. This work presents the results from a time-accurate transonic numerical simulation of a generic store release case, with detailed attention to ejector force profiles. Confidence limits for time-accurate trajectory calculations with STAR-CCM+® have been improvedfor this standard benchmark case. <![CDATA[<b>Four-point bending fatigue test specimen design by FEA</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S2309-89882018000100003&lng=en&nrm=iso&tlng=en The FEA method is utilised in the geometric design of test specimens for use in four-point plane beam bending fatigue testing. A purpose-designed and -made testing jig is employed in the test routines. The specimen geometry is adjusted from a standard, prismatic, rectangular cross-section to a unique or nonstandard, non-prismatic, complex geometry that nearly ensures fatigue crack initiation on the free surface of the specimen away from areas of contact. The proposed method enables the engineer to retain the in-service or prevailing surface condition of the specimen material, while evaluating fatigue properties of interest at full force reversal. Due to the relatively complex specimen geometry, simple analytical methods typically used for average stress calculations in mechanical testing routines are substituted by the FEA method, allowing for accurate predictions of bending stresses, as well as reliable predictions of the location of fatigue crack initiation. The process of interaction between analysis and testing is demonstrated, resulting in a specimen design that is easy to manufacture and simple to test. Several test specimens are subjected to fatigue testing, demonstrating the accuracy and predictability of results. <![CDATA[<b>Wind tunnel testing considerations in expansion tan/shock wave interaction studies</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S2309-89882018000100004&lng=en&nrm=iso&tlng=en In the research into expansion fan/shock wave interactions, wind tunnel testing is an essential tool allowing controlled freestream conditions and variation of parameters of interest. However, certain considerations must be accounted for in the wind tunnel model design, data collection, and interpretation of the results in order to ensure good quality results and accurate interpretation thereof. Model design involved sizing of the model such that the tunnel is able to start, design of an attachment mechanism to allow mounting into the tunnel, prediction of the aerodynamic loads encountered by the model both on tunnel startup and during operation, and ensuring that the material strength of the model and support structure was sufficient to prevent failure. Schlieren flow visualization is the appropriate tool for the study of compressible flows, and careful attention was paid during setup to reduce the schlieren angle and place the cut-off at the correct location to minimise the optical aberrations of coma and astigmatism. When studying the schlieren results, visualization of the three-dimensional shock waves as regions, rather than lines, and the shock wave emanating from the lower surface of the expansion generator were taken into account for accurate interpretation. Adhering to these considerations allowed the physics involved in an expansion fan/shock wave interaction to be investigated. <![CDATA[<b>Composite tube testing and failure theory computational comparison</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S2309-89882018000100005&lng=en&nrm=iso&tlng=en Composites are used in lightweight structural designs. This paper describes experimental tests conducted on composite tubes and a comparison with computational results. A test method was developed which involves an applied axial compressive load on tubes fabricated using Unidirectional (UD) carbon fibre set at +35°, to induce compressive and shear stresses along thefibres. Four major failure criteria were compared against test results: Tsai-Wu, Hoffman, Tsai Hill and Maximum Strain. The Hoffman and Tsai-Wu criteria were shown to be accurate and conservative. The Tsai-Hill criteria produced high strength ratios. The Maximum Strain criteria had the highest strength ratio, proving to be the least conservative and deviated most from computational results. This paper shows that the Hoffman and Tsai-Wu failure criteria may be used confidently in applications such as filament winding and continuous pultrusion methods, which are widely used in producing closed sections. Additionally, elastic and shear moduli were varied in simulations to show that small inaccuracies in those properties do not substantially change the maximum failure index output. <![CDATA[<b>Microstructure and material properties of LENS fabricated Ti-6Al-4V components</b>]]> http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S2309-89882018000100006&lng=en&nrm=iso&tlng=en Cylindrical components of dimensions 15 mm in diameter and 11 mm in height were laser printed by additive manufacturing using a LENS!™ system. Process parameters used in layer by layer build-ups showed that energy densities in the range of 200 - 300 J/mm³ were sufficient to produce adequate builds with improved physical structures and mechanical properties of Ti-6Al-4V ELI alloy, and subsequently used to produce tensile specimen that showed encouraging yield and tensile strengths in excess of 900 MPa. An approach to estimating the achievable tensile properties is proposed and investigated against experimental results as a means of identifying parameters to be used in studies aimed at tailoring mechanical properties of materials.