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5.1. NON-PULSATILE AND PULSATILE FLOW IN CURVED PIPES 39 The comparison of the computed velocity profiles with the measured pro- files has been conducted at 3 different locations; half a diameter upstream of the bend, at the exit of the bend and half a diameter downstream of the bend. Half a diameter upstream of the bend, the deviations between the computed and measured velocity profiles are smaller than the measurement error. At the evaluation stations downstream the bend, the deviation is larger and the RANS computation could not capture the behaviour of the flow field, espe- cially at the inner wall, where the computed streamwise velocity is lower for all cases than it is in the measurement, see Figure 5.3. The time mean velocity profiles from the LES computation without a subgrid scale model gives better agreement with the measured velocity profiles at all evaluation stations, but the LES computations over-predict the axial velocity at the outer part of the bend and under-predict the velocity in the centre part of the cross-section of the bend. Despite the fact that the results from the RANS method give the largest deviation from the measured velocities, this model is used to investigate the effects of perturbations at the inlet. The reason is the long computation times that are required for the LES approach. Different inlet velocity profiles are applied at the inlet to the single bend pipe. The imposed eccentricity and swirl are used to assess the sensitivity of the results to inlet perturbations. These types of perturbations are often found in experiments where the inlet pipe is short and no fully developed turbulent pipe flow can develop. When comparing the velocity profiles for the different cases with measured velocity profiles, it is clear that the case with the swirling motion gives the best agreement at the exit of the bend. One way to give a measure of the inlet effects is to compare the Root Mean Square (RMS) value of the ∂U/∂ζ, where ∂U is the difference between the velocity profile obtained with a symmetric velocity profile and a non-symmetric or swirling velocity profile at a certain station. ∂ζ is the difference in kinetic energy at the inlet for the symmetric inlet profile and a non-symmetric or swirling inlet profile. Table 1 shows the results from the analysis at the exit of the bend and it is clear that inlet condition with swirling flow gives the largest effect. In this case the flow field is rotated due to the swirling motion before the bend and hence, the low velocity region is turned a few degrees in the clockwise direction. This is also the inlet profile which gives the best agreement with the measured velocity profiles downstream of the bend, indicate that there is probably a swirling secondary flow component in the experimental case too.PDF Image | Numerical computations of the unsteady flow in a radial turbine
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