The modified ghost fluid method for coupling of fluid and structure constituted with hydro-elasto-plastic equation of state

10.1137/050647013SIAM Journal on Scientific Computing3031105-1130SJOC

Kavitaatioeroosion karakterisointi Francis-turbiinin juoksupyörän materiaalille

In this study, the cavitation erosion resistance was characterized for a steel type used in Francis turbine runner blades. The goal of the study was to define the cavitation erosion rate of the runner blade steel and to compare the results to those of previously studied materials. The previously studied materials were aluminium alloy 7075, nickel aluminium bronze alloy C95400 and stainless steels A2205 and 304L. The material was tested in a cavitation tunnel in which sheet and cloud cavitation appears in the test section. Acoustic emission signal was measured from the sample experiencing cavitation erosion. Cavitation erosion can be divided into four distinct stages; the incubation period, the acceleration period, the deceleration period and the steady state period. Each of these periods is connected to the material response of a sample in a cavitation field. The material response is linked to the stage of work hardening and material cavity formation, as cavitation modifies the material surface. The cavitation tunnel used in this study circulates water and it has variable upstream and downstream pressures. The test section is a radially diverging channel in which cavitation inception occurs in the beginning of the radial section and cavitation closure occurs several millimeters downstream. The sample was cylindrical with one face experiencing cavitation. The cavitation erosion evolution was measured with a contact profilometer. The volume loss and the maximum depth of penetration were calculated from the surface profiles and they were compared to results from other materials. The acoustic emission results were compared with the erosion evolution. Material characteristics were obtained by elemental analysis, by macroscopic compression tests, by nanoindentation tests and by split Hopkinson pressure bar tests. The results show that the studied steel is as resistant to cavitation erosion as aluminium alloy 7075 and less resistant to cavitation erosion than nickel aluminium bronze alloy C95400 and stainless steels A2205 and 304L. The reason for the lower resistance compared to the stainless steels is corrosion, lower quality level or both. The voltage root mean square value of the acoustic emission signal reduced with increasing cumulative erosion time. Acoustic emission monitoring was found to be a potential method in estimating cavitation erosion evolution in hydraulic machines.Tässä työssä tutkittiin Francis-turbiinin juoksupyörän siivessä käytettävän terästyypin kavitaatiokestävyyttä. Työn tavoitteena oli tutkia materiaalin eroosionopeutta sekä verrata sitä aiemmin tutkittuihin materiaaleihin. Aiemmin tutkitut materiaalit olivat alumiiniseos 7075, nikkeli alumiini pronssiseos C95400 sekä ruostumattomat teräkset A2205 ja 304L. Materiaalia tutkittiin kavitaatiotunnelissa joka tuottaa taso- sekä pilvimäistä kavitaatiota testausosiossaan. Näytekappaleesta mitattiin akustista emissiota kun se oli kavitaatioeroosion vaikutuksen alaisena. Kavitaatioeroosiolla on neljä toisistaan erotettavaa vaihetta: alkuvaihe, kiihtymisvaihe, hidastumisvaihe sekä tasaisen eroosionopeuden vaihe. Jokainen vaihe on yhteydessä materiaalin kavitaatiovasteeseen. Tämä vaste on yhteydessä materiaalin muokkauslujittumisen sekä onkaloiden syntymisen tasoihin, koska kavitaatio muokkaa materiaalin pintaa. Tutkimuksessa käytetty kavitaatiotunneli kierrättää vettä ja sen ylä- sekä alavirran paineita voidaan muuttaa. Testausosio on radiaalisesti laajeneva kanava, jossa kavitaatio syntyy radiaalisen osion alussa ja romahtaa muutaman millimetrin päässä alavirtaan. Näytekappale oli sylinterimäinen ja sen yksi sivu altistui kavitaatioeroosiolle. Kavitaatioeroosion eteneminen mitattiin kontaktiprofilometrillä. Pintaprofiileista laskettiin tilavuushäviö sekä suurin eroosiosyvyys ja näitä tuloksia verrattiin toisten materiaalien vastaaviin tuloksiin. Akustisen emission mittaustuloksia verrattiin eroosion kehittymiseen. Materiaalin ominaisuudet selvitettiin alkuaineanalyysillä, makroskooppisilla puristustesteillä, nanoindentaatiolla sekä split Hopkinson pressure bar -testeillä. Tulokset osoittavat, että tutkittu teräslaatu on kavitaatiokestävyydeltään yhtä hyvä kuin alumiiniseos 7075, mutta huonompi kuin nikkeli alumiini pronssiseos C95400 tai ruostumattomat teräkset A2205 sekä 304L. Syy ruostumatonta terästä heikompaan kestävyyteen on joko korroosiossa, matalammassa laatutasossa tai molemmissa. Akustisen emission signaalin ulostulojännitteen neliöllisen keskiarvon kokonaiskeskiarvon havaittiin laskevan kavitaatioeroosion edetessä. Tämän havainnon vuoksi akustisen emission arvioitiin olevan potentiaalinen menetelmä hydraulisten koneiden kavitaatioeroosion vaiheiden tunnistamiseen

A novel multi-dimensional Eulerian approach to computational solid dynamics

Many problems in solid dynamics involve moving boundaries, finite elastoplastic deformations, and strong non-linear waves. Continuum modelling of such events is difficult on account of these characteristics, and there exist a number of inadequacies in current numerical algorithms. Furthermore, a comprehensive understanding of certain underlying processes is yet to be achieved which places a limit on the derivation of engineering models to simulate these occurrences. Much needed atomistic studies, capable of revealing much about the governing physical processes, remain limited by current computational resources. This thesis is devoted to targeting these difficulties by proposing new continuum numerical schemes and a means of studying both micro- and macro-scale behaviours via a dynamic coupling of continuum mechanics and molecular dynamics theory. Eulerian shock-capturing schemes have advantages for modelling problems involving complex non-linear wave structures and large deformations in solid media. Various numerical methods now exist for solving hyperbolic conservation laws that have yet to be applied to solid dynamics. A three-dimensional finite-volume scheme on fixed grids is proposed for elastoplastic solids. The scheme is based upon the Godunov flux method and thus requires solution of the Riemann problem. Both exact and approximate solutions are proposed for the special case of non-linear elasticity. An implicit algorithm is developed to allow for resolving rate-dependent inelastic deformations. The methods are tested against exact solutions in one-dimension, and symmetrical polar solutions in two- and three-dimensions. To account for multiple immiscible materials it is necessary to include some means of tracking material boundaries within a numerical scheme. A moving grid scheme is a simple means of accommodating transient boundaries. Interface tracking based on the use of level set functions is an attractive alternative for problems with sliding interfaces since it allows discontinuous velocity profiles at the material boundaries whilst employing fixed grids. Both of these methods are explored in the current context. A series of one-dimensional testcases have been carried out that demonstrate the ability of the numerical schemes to accurately resolve complex boundary conditions between interacting free surfaces. Where singularities occur in a system comprising solid materials, atomistic studies are invaluable for achieving a fundamental insight into the governing physical processes. However where non-linear waves are generated, domain size proves to be a limiting factor in achieving solutions free from numerical artifacts. A domain decomposition multi-scale modelling strategy is developed that couples the Eulerian shock capturing scheme with a molecular dynamics solver. The method is demonstrated for one-dimensional testcases involving strong shear waves and multiple components. Attention is devoted to resolving transient wave propagation free from spurious wave reflections through investigation of the numerical parameters.EThOS - Electronic Theses Online ServiceGBUnited Kingdo