The flow on the centerline of grid-generated turbulence is characterised via hot-wire anemometry for 3 grids with different geometry: a regular grid (RG60), a fractal grid (FSG17) and a single square grid (SSG).Thanks to a higher value of the thickness t0 of its bars, SSG produces greater values of turbulence intensity T u than FSG17, despite SSG having a smaller blockage ratio. However the higher T u for SSG is mainly due to a more pronounced vortex shedding contribution. The effects of vortex shedding suppression along the streamwise direction x are studied by testing a new 3D configuration, formed by SSG and a set of four splitter plates detached from the grid (SSG+SP). When vortex shedding is damped, the centerline location of the peak of turbulence intensity x peak moves downstream and T u considerably decreases in the production region. For FSG17 the vortex shedding is less intense and it disappears more quickly, in terms of x/x peak , when compared to all the other configurations. When vortex shedding is attenuated, the integral length scale Lu grows more slowly in the streamwise direction, this being verified both for FSG17 and for SSG+SP.In the production region, there is a correlation between the vortex shedding energy and the skewness and the flatness of the velocity fluctuations. When vortex shedding is not significant, the skewness is highly negative and the flatness is much larger than 3. On the opposite side, when vortex shedding is prominent, the non-Gaussian behaviour of the velocity fluctuations becomes masked.

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Hydrodynamic bearings have a key role in the functioning of heavy duty gas turbines: they join a great vibration absorption with an efficient power dissipation by means of a film oil inserted between the turbo machine axes and the bearing case. A classical approach for studying the functioning and the performance of this kind of bearings is to solve the so called Reynolds’ equation, which is obtained from the Navier-Stokes equations under simplifying assumptions. As a result the pressure field is derived, the fluid film being considered isothermal: dissipation effects have to be estimated a posteriori in a postprocessing procedure. On the other hand a fluid environment having to be taken into account, a direct approach is carried out by the time consuming CFD analysis. After defining an appropriate mesh and choosing the appropriate solver, an almost exact solution of the entire flow field is obtained, that is pressure, velocity and temperature distributions. The main drawback is that the required time is several order greater than that required for the solution of the Reynolds’ equation. In the present work an alternative strategy is proposed, which consists of an iterative procedure: at each step the Reynolds’ equation is solved in order to obtain the pressure field; a 1D energy balance is then applied along the length of the bearing for computing the temperature field. In this way the close relationship between pressure and temperature is modelled, the former depending on the oil viscosity locally changing with temperature, and the latter depending on the local oil mass flow and on dissipated power strictly correlated to the pressure distribution. The upgrading of both the entities ends when the convergence is reached. Comparisons with literature test cases reveal the efficiency of the proposed technique: treating the interaction between pressure and temperature gives a solution which is very close to industrial configurations investigated, and at the same time the computational load is as light as that needed for the solution of the only Reynolds’ equation. The performance of the above coupled solver can be greatly emphasised applying it to the bearing design. An integration with a multi-objective genetic optimization process is proposed, taking as objects to be optimized both geometrical and environmental variables. Application examples are shown about an industrial Ansaldo Energia lemon bore hydrodynamic bearing: given a currently applied configuration, possible improvements are suggested. Results are presented.

Extended AbstractThe effects of grid-generated turbulence on the local heat transfer coefficient around the circumference of a cylinder in crossflow are investigated experimentally in a wind tunnel. A thin Inconel foil (25 μm thick) is wrapped around the central portion of the cylinder and is resistively heated with a direct electrical current, a technique approximating a uniform heat flux boundary condition on the cylinder's wall [1]. The wall temperature Tw is measured around the cylinder with thermocouples installed underneath the heater foil. Three grids with different geometry and different blockage ratios σg are placed at the inlet of the wind tunnel: a regular square-mesh grid (RG60) with σg = 32%, a fractal square grid (FSG17) with σg = 25%, and a single square grid (SSG) with σg = 20%. The grid-generated turbulent flows are documented first without cylinder in terms of turbulence intensity Tu and integral length scale Lu using hot-wire anemometry along the centreline. The cylinder is subsequently placed at several streamwise distances x from each grid and the heat transfer measurements are performed around the centreline circumference. The Reynolds number Re, based on the cylinder's diameter D, varies approximately between 10 800 and 48 800.For the same turbulence parameter TuRe 0.5 [2], the Frossling number Nu0/Re 0.5 (Nu0 is the Nusselt number at the cylinder's front stagnation point) is highest for RG60, as a result of the lowest ratios Lu/D produced by this grid, in agreement with [3]. The maximum enhancement of the circumferentially averaged Nusselt number Nuavg, with respect to its value under laminar free-stream conditions Nulam, occurs in the proximity of xpeak, where Tu is maximum, and it increases with Re. The maximum value of Nuavg/Nulam is higher for SSG than for FSG17, despite SSG having a lower σg. This is explained by the larger values of Tu in the production region of SSG, which are achieved by increasing the ratio t0/L0 [4] with respect to FSG17, where t0 and L0 are the thickness and the length of the largest bars of the grid respectively. However the rate of decrease of Nuavg/Nulam with x is lower for FSG17, thus reflecting the slower decay of Tu along x for this grid. At large distances from the grids, the values of Nuavg are appreciably higher for SSG and for FSG17 than for RG60, despite the latter having a higher σg.The SSG, and to a greater extent the FSG17, exhibit a prolonged turbulence production region, where the turbulence intensity Tu increases with x before decaying. This allows the unprecedented opportunity of comparing the angular heat transfer profiles in the production and in the decay regions for same values of Tu. It is found that on the forward part of the cylinder, where a laminar boundary layer exists, the ratio Nu/Re 0.5 is lower in the production region of the grids, especially for SSG. This can be related to (i) a higher energy of the vortex shedding from the bars of the grids, especially for SSG for which the shedding is stronger, and to (ii) a higher intermitten...

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