Academic literature on the topic 'Unsteady computations'

Turbocharging the internal combustion (IC) engine is a common technique to increase the power density. If turbocharging is used with the downsizing technique, the fuel consumption and pollution of green house gases can be decreased. In the turbocharger, the energy of the engine exhaust gas is extracted by expanding it through the turbine which drives the compressor by a shaft. If a turbocharged IC engine is compared with a natural aspirated engine, the turbocharged engine will be smaller, lighter and will also have a better efficiency, due to less pump losses, lower inertia of the system and less friction losses. To be able to further increase the efficiency of the IC engine, the understanding of the highly unsteady flow in turbochargers must be improved, which then can be used to increase the efficiency of the turbine and the compressor. The main objective with this thesis has been to enhance the understanding of the unsteady flow in turbocharger and to assess the sensitivity of inflow conditions on the turbocharger performance. The performance and the flow field in a radial turbocharger turbine working under both non-pulsatile and pulsatile flow conditions has been assessed by using Large Eddy Simulation (LES). To assess the effects of different operation conditions on the turbine performance, different cases have been considered with different perturbations and unsteadiness of the inflow conditions. Also different rotational speeds of the turbine wheel were considered. The results show that the turbine cannot be treated as being quasi-stationary; for example,the shaft power varies for different frequencies of the pulses for the same amplitude of mass flow. The results also show that perturbations and unsteadiness that are created in the geometry upstream of the turbine have substantial effects on the performance of the turbocharger. All this can be summarized as that perturbations and unsteadiness in the inflow conditions to the turbine affect the performance. The unsteady flow field in ported shroud compressor has also been assessed by using LES for two different operational points. For an operational point near surge, the flow field in the entire compressor stage is unsteady, where the driving mechanism is an unsteadiness created in the volute. For an operational point far away from surge, the flow field in the compressor is relatively much more steady as compared with the former case. Although the stable operational point exhibits back-flow from the ported shroud channels, which implies that the flow into the compressor wheel is disturbed due to the structures that are created in the shear layer between the bulk flow and the back-flow from the ported shroud channels.<br>QC20100622

<p>Non-pulsatile and pulsatile flow in bent pipes and radial turbine has been assessed with numerical simulations. The flow field in a single bent pipe has been computed with different turbulence modelling approaches. A comparison with measured data shows that Implicit Large Eddy Simulation (ILES) gives the best agreement in terms of mean flow quantities. All computations with the different turbulence models qualitatively capture the so called Dean vortices. The Dean vortices are a pair of counter-rotating vortices that are created in the bend, due to inertial effects in combination with a radial pressure gradient. The pulsatile flow in a double bent pipe has also been considered. In the first bend, the Dean vortices are formed and in the second bend a swirling motion is created, which will together with the Dean vortices create a complex flow field downstream of the second bend. The strength of these structures will vary with the amplitude of the axial flow. For pulsatile flow, a phase shift between the velocity and the pressure occurs and the phase shift is not constant during the pulse depending on the balance between the different terms in the Navier- Stokes equations.</p><p>The performance of a radial turbocharger turbine working under both non-pulsatile and pulsatile flow conditions has also been investigated by using ILES. To assess the effect of pulsatile inflow conditions on the turbine performance, three different cases have been considered with different frequencies and amplitude of the mass flow pulse and different rotational speeds of the turbine wheel. The results show that the turbine cannot be treated as being quasi-stationary; for example, the shaft power varies with varying frequency of the pulses for the same amplitude of mass flow. The pulsatile flow also implies that the incidence angle of the flow into the turbine wheel varies during the pulse. For the worst case, the relative incidence angle varies from approximately −80° to +60°. A phase shift between the pressure and the mass flow at the inlet and the shaft torque also occurs. This phase shift increases with increasing frequency, which affects the accuracy of the results from 1-D models based on turbine maps measured under non-pulsatile conditions.</p><p>For a turbocharger working under internal combustion engine conditions, the flow into the turbine is pulsatile and there are also unsteady secondary flow components, depending on the geometry of the exhaust manifold situated upstream of the turbine. Therefore, the effects of different perturbations at the inflow conditions on the turbine performance have been assessed. For the different cases both turbulent fluctuations and different secondary flow structures are added to the inlet velocity. The results show that a non-disturbed inlet flow gives the best performance, while an inflow condition with a certain large scale eddy in combination with turbulence has the largest negative effect on the shaft power output.</p>

At the intake of hydropower plants, air-core vortex formation is known to cause severe damage. In order to study how to prevent and reduce the origin of the vortex, Vattenfall has built a scale model of the Akkats hydropower plant dam, where scale testing is possible. This thesis work consists of discerning whether Computational Fluid Dynamics (CFD) in terms of solving the Unsteady Reynolds Average Navier-Stokes equations (URANS) can be used as a complement to scale testing. For this work, the RNG k-epsilon turbulence model is chosen, and the flow field is solved with implicit time discretization using a pressure-based solver, for three different inlet flow conditions. Despite significant differences in the inflow of these three cases, the resulting flow fields are surprisingly similar. A main result is that no vortex is formed in any of the cases. The cause of this is discussed, but the number of possible answers is large. The main purpose of the report has therefore become to lay the foundation for further research. Amongst the top priorities in parameters to investigate lies the choice of turbulence model, the surface height, the pressure discretization scheme and to perform calculations on a more expensive mesh.<br>Virvlar som uppstår vid intaget i vattenkraftverk kan orsaka stora skador. För att kunna göra studier om hur man bäst motverkar virveln och förhindrar dess uppkomst, har Vattenfall AB byggt en småskalig modell av dammen vid Akkats vattenkraftverk. Det här arbetet behandlar frågeställningen huruvida Computational Fluid Dynamics (CFD) med lösning av ekvationerna för Unsteady Reynolds Average Navier-Stokes (URANS) kan användas som ett komplement till dessa modell-tester. I det här arbetet har turbulensmodellen RNG k−epsilon valts och flödesfältet löses för tre olika tillstånd för flödet vid inloppet, med hjälp av implicit tidsdiskretisering tillsammans med en tryckbaserad ekvationslösare. Trots betydande skillnader för inflödet för dessa tre fall är de resulterande flödesfälten överraskande lika. Ett huvudresultat är att ingen virvel formas för någon av dessa fall. Anledningen till detta har diskuterats, men antalet möjliga anledningar är många. Huvudsyftet med den här rapporten har därför blivit att lägga en grund för framtida efterforskningar på området. Några av de viktigaste parametrarna att undersöka är valet av turbulensmodell, höjden på vattenytan, tryckdiskretiserings-schema samt att genomföra beräkningar för en finare mesh.

The effects of shock wave impact on unsteady turbine heat transfer are investigated. A numerical approach is developed to simulate the flow physics present in a previously performed unsteady wind tunnel experiment. The windtunnel experiment included unheated and heated flows over a cascade of highly loaded turbine blades. After the flow over the blades was established, a single shock with a pressure ratio of 1.1 was introduced into the wind tunnel test section. A single blade was equipped with pressure transducers and heat flux microsensors. As the shock wave strikes the blade, time resolved pressure, temperature, and heat transfer data were recorded.<br>Ph. D.

<p>Price, Jennifer Lou. Unsteady Measurements and Computations on an Oscillating Airfoil with Gurney Flaps. (Under the direction of Dr. Ndaona Chokani)The effect of a Gurney flap on an unsteady airfoil flow is experimentally and computationally examined. In the experiment, the details of the unsteady boundary layer events on the forward portion of the airfoil are measured. In the computation, the features of the global unsteady flow are documented and correlated with the experimental observations.The experiments were conducted in the North Carolina State University subsonic wind tunnel on an oscillating airfoil at pitch rates of 65.45 degrees/sec and 130.9 degrees/sec. The airfoil has a NACA0012 cross-section and is equipped with a 1.5% or 2.5% chord Gurney flap. The airfoil is tested at Reynolds numbers of 96,000, 169,000 and 192,000 for attached and light dynamic stall conditions. An array of surface-mounted hot-film sensors on the forward 25% chord of the airfoil is used to measure the unsteady laminar boundary layer separation, transition-to-turbulence, and turbulent reattachment. In parallel with the experiments incompressible Navier-Stokes computations are conducted for the light dynamic stall conditions on the airfoil with a 2.5%c Gurney flap at a Reynolds number of 169,000.The experimental measurements show that the effect of the Gurney flap is to move the separation, transition and reattachment forward on the airfoil. This effect is more marked during the airfoil's pitch-down than during pitch-up. The computational results verify these observations, and also show that the shedding of the dynamic stall vortex is delayed. Thus the adverse effects of dynamic stall are mitigated by the Gurney flap.<P>