Vittorio Michelassi

Aerodynamic Performance of a Transonic Turbine Guide Vane with Trailing Edge Coolant Ejection, Part I: Experimental Approach

Inlet guide vanes (IGV) of high-temperature gas turbines require an effective trailing edge cooling. But this cooling significantly influences the aerodynamic performance caused by the unavoidable thickening of the trailing edge and the interference of the cooling flow with the main flow. As part of a comprehensive research program, an inlet guide vane was designed and manufactured with two different trailing edge shapes. The results from the cascade tests show that the flow behavior upstream of the trailing edge remains unchanged. The homogeneous values downstream show higher turning and higher losses for the cut-back blade, especially in the supersonic range. Additional tests were conducted with carbon dioxide ejection, in order to analyze the mixing process downstream of the cascade.

Doppler spectra from contrast agents crossing an ultrasound field

When contrast agents are injected in a fluid, it is implicitly assumed that they move at the same velocity as the fluid itself. However, a series of in vitro tests performed by using air-filled microbubbles suspended in distilled water, have shown that the Doppler spectrum generated in this case may be notably different from that obtained from non-resonating scatterers. In this paper, we show, through a simple simulation model, that the actual movement of microbubbles may be predicted as the result of the complex balance between two forces: the ultrasound radiation force, which tends to move the particles along the sound beam direction, and the fluid drag force, which tends to move the particles along the fluid stream. The contrast agents turn out to be displaced only during the passage of the ultrasound burst; during the remaining time, they are maintained at the fluid velocity by the drag force. Based on the total particle displacement estimated between consecutive pulses, a series of Doppler spectra corresponding to different intensity levels was computed. This series was shown to be in excellent agreement with the experimental spectra obtained in vitro using Levovist(R) (Schering AG, Berlin, Germany) particles suspended in distilled water flowing at a steady rate.

On the interaction between ultrasound and contrast agents during Doppler investigations

Knowledge of interaction mechanisms between ultrasound (US) and contrast agents (CA) suspended in blood is important for a correct interpretation of clinical investigation results. Experiments performed in different laboratories have shown that, as a consequence of primary radiation force, CA tend to move away from the US transducer. Accordingly, Doppler spectra produced by particles suspended in moving water turn out to be significantly altered from what is theoretically expected. The purpose of this paper is twofold. First, an original model describing the bubble dynamics as the outcome of the balance between US radiation force and fluid drag force is validated for the case in which bubbles are suspended in blood. The high fluid viscosity is shown to prevent significant bubble deviations from the unperturbed fluid streamlines so that, in large vessels, a residual spectral distortion may exist only at the highest intensity levels permitted by current regulations. Finally, the relative importance and differences between the effect of primary radiation force and streaming mechanisms that, in principle, could lead to similar effects, are discussed.

Automatic computational fluid dynamics-based procedure for the optimization of a centrifugal impeller

Abstract A typical centrifugal impeller characterized by a low flow coefficient and cylindrical blades is redesigned by means of an intelligent automatic search program. The procedure consists of a feasible sequential quadratic programming algorithm (Fletcher, R. Practical Methods of optimization, 2000 (Wiley)) coupled to a lazy learning (LL) interpolator 1 to speed-up the process. The program is able to handle geometric constraints to reduce the computational effort devoted to the analysis of non-physical configurations. The objective function evaluator is an in-house developed structured computational fluid dynamics (CFD) code. The LL approx-imator is called each time the stored database can provide a sufficiently accurate performance estimate for a given geometry, thus reducing the effective CFD computations. The impeller is represented by 25 geometric parameters describing the vane in the meridional and s-0 planes, the blade thickness, and the leading edge shape. The optimization is carried out on the impeller design point maximizing the polytropic efficiency with nearly constant flow coefficient and polytropic head. The optimization is accomplished by maintaining unaltered those geometrical parameters which have to be kept fixed in order to make the impeller fit the original stage. The optimization, carried out on a cluster of 16 PCs, is self-learning and leads to a geometry presenting an increased design point efficiency. The program is completely general and can be applied to any component which can be described by a finite number of geometrical parameters and computed by any numerical instrument to provide performance indices. The work presented in this paper was done under the METHOD EC funded project for the implementation of new technologies for optimization of centrifugal compressors.

Unexpected Doppler effects from microbubbles moving through an ultrasound beam

Encapsulated microbubbles crossing an ultrasound field are known to be sensitive to radiation force, i.e. they tend to move along the direction of the incident beam. In this paper, a simple simulation model is introduced and shown capable of predicting the displacements imposed to microbubbles when they simultaneously experience the ultrasound radiation force and the drag force of the fluid where they are suspended. Based on the total particle displacement estimated between consecutive transmitted pulses, Doppler spectra corresponding to different intensity levels were computed. They were then compared to the experimental spectra obtained in vitro by using Levovist ((R)Schering, Berlin, Germany) particles suspended in distilled water flowing at a steady rate. The excellent agreement between experimental and simulation results confirm the validity of the proposed model.

Secondary Flow Decay Downstream of Turbine Inlet Guide Vane With End-Wall Contouring

The secondary flows development downstream of a turbine stator blade with non-symmetric endwall contours is measured and analyzed. Measurements are carried out in a subsonic linear cascade with a large test section. The measurements are carried out using micro-pneumatic five hole probes which allow a detailed description of the vortical structures. The effect of secondary flows on the blade load is discussed. The passage vortices are found to collide and produce two pairs of counter-rotating vortices. The collision is caused by the mutual influence of the endwall contours and the pressure distribution on the blade. The measurements in 13 planes downstream of the trailing edge also show the presence of an extra vortex which appears because of sudden channel expansion. The 13 measurement planes orthogonal to the mean flow allow a detailed description of the secondary flow decay. The large flow distortion in terms of pitch and yaw angles is analyzed and discussed.Copyright

Preconditioned Scalar Approximate Factorization Method for Incompressible Fluid Flows

The artificial compressibility method for solving incompressible flows is combined with a general preconditioning technique for complex laminar and turbulent flows, with or without separation. The effect of preconditioning on the solution and on the convergence rate is investigated. The original block-algoril lim, based on the approximate factorization technique, is modified to include a scalar solver which requires less operations. The applicability of the scalar form to incompressible flow problems preconditioned by means of Turkels scheme is demonstrated. The effect of the diffusive terms is discussed for several flow cases. Results indicate that the general preconditioning can improve convergence rates even in complex flows with a careful choice of parameters. The generalized preconditioned scalar solver for turbulent flows, in the absence of significant separation regions, is practically equivalent to the block solver, but it is still not competitive for laminar flows, because of the approximate tre...

Using Viscous Calculations in Pump Design

A viscous computer code for designing the meridional channels of high-performance pumps is presented. An averaging technique is used to reduce the three-dimensional flow to a two-dimensional model. The code, based upon an implicit finite difference method for steady two-dimensional incompressible flows, was validated in complex flow geometries prior to application in the design analysis of an actual pump. Viscous effects are taken into account by two different turbulence models

Efficient Solution of Turbulent Incompressible Separated Flows

A computational method for incompressible separated flows based on two-dimensional approximate factorization is presented. Turbulence effects are accounted for by low-Reynolds number forms of the k-e model. Mass conservation is enforced by the artificial compressibility method. Decoupling and coupling of the equations of motions with the turbulence model equations are investigated. Testing of the coupled solver showed no improvement in convergence or accuracy in comparison to the classical decoupled approach. The solver was then applied to several large-recirculation flows using a modified version of the low-Reynolds-number form of the k-e model proposed by Chien and a two-layer version of the k-e model introduced by Rodi. Both versions gave fast convergence rates and good agreement with experiments.

Unsteady Acoustic Forcing on an Impeller Due to Coupled Blade Row Interactions

This article contains an investigation of the unsteady acoustic forcing on a centrifugal impeller due to coupled blade row interactions. Selected results from an aeromechanical test campaign on a GE Oil and Gas centrifugal compressor stage with a vaneless diffuser are presented. The most commonly encountered sources of impeller excitation due to upstream wake interaction were identified and observed in the testing campaign. A 30/rev excitation corresponding to the sum of upstream and downstream vane counts caused significant trailing edge vibratory stress amplitudes. Due to the large spacing between the impeller and the return channel vanes, this 30/rev excitation was suspected to be caused by an aero-acoustic excitation rather than a potential disturbance. The origin of this aero-acoustic excitation was deduced from an acoustic analysis of the unsteady compressor flow derived from CFD. The analysis revealed a complex excitation mechanism caused by impeller interaction with the upstream vane row wakes and subsequent acoustic wave reflection from the downstream return channel vanes. The findings show it is important to account for aero-acoustic forcing in the aeromechanical design of low pressure ratio centrifugal compressor stages.