Acoustical Absorption and Scattering Cross-Sections of Spherical Bubble Clouds

The present work investigates the acoustical absorption and scattering cross-sections of spherical bubble clouds subject to harmonic far field pressure excitation. Bubble dynamics effects and energy dissipation due to viscosity, heat transfer, liquid compressibility and relative motion of the two phases are included. The equations of motion for the average flow and for the bubble radius are linearized and a closed form solution is obtained. Due to the presence of natural oscillatory modes and frequencies, the acoustical cross-sections of the cloud are very different from those of each individual bubble in the cloud, as well as from the acoustical cross-sections of a single large bubble with the same volume of vapor and gas. In general the acoustical properties of any given volume of the dispersed phase depend strongly on the degree of dispersion because of the complex interactions of the dynamics of the bubbles with the whole flow

Rapid Interactive Design-to-Performance of Mixed-Flow Space Inducers

The article illustrates the invited lecture given by the author at International Symposium on Pump and Fan Technology, September 26-28, 2018, ShenYang, China, on the development and experimental validation of a reduced order model for preliminary design and noncavitating performance prediction of mixed-flow tapered-hub inducers for space propulsion applications. The model expresses the 3D incompressible, inviscid, irrotational flow in the blade channels by superposing a 2D axial vorticity correction to a fully-guided axisymmetric flow with radially uniform axial velocity. Suitable redefinition of the diffusion factor for bladings with non-negligible radial flow simultaneously allows for the control of the blade loading and the estimate of the boundary layer blockage and viscous blade losses at the specified design flow coefficient, providing a simple criterion for matching the hub profile to the axial variation of the blade pitch angle. Carter’s rule is employed to account for flow deviation at the inducer trailing edge. Mass continuity, angular momentum conservation and the Euler equation are used to derive a simple 2nd order boundary value problem whose numerical solution defines the far-field axisymmetric flow velocity at the inducer discharge. The noncavitating pumping characteristic is then obtained using suitably adapted semi-empirical corrections for incidence, casing and tip clearance losses. The model has been verified to closely approximate the geometry and noncavitating head characteristics of two space inducers tested in the Cavitating Pump Rotordynamic Test Facility, as well as those of a number of tapered-hub inducers documented in the literature

Turbomachinery Developments and Cavitation

After a brief review of flow-induced instabilities in turbopumps for liquid propellant feed systems of modern rocket engines, the lecture illustrates some recent results of the work carried out at Alta on the hydrodynamics and unsteady cavitation phenomena of these machines. A reduced order model for preliminary design and noncavitating performance prediction of tapered axial inducers is illustrated. In the incompressible, inviscid, irrotational flow approximation the model expresses the 3D flow field in the blade channels by superposing a 2D cross-sectional vorticity correction to a fully-guided axisymmetric flow with radially uniform axial velocity. Suitable redefinition of the diffusion factor for bladings with non-negligible radial flow allows for the control of the blade loading and the estimate of the boundary layer blockage at the specified design flow coefficient, providing a simple criterion for matching the hub profile to the axial variation of the blade pitch angle. Carter’s rule is employed to account for flow deviation at the inducer trailing edge. Mass continuity, angular momentum conservation and Euler’s equation are used to derive a simple 2nd order boundary value problem, whose numerical solution describes the far field axisymmetric flow at the inducer discharge. A closed form approximate solution is also provided, which proved to yield equivalently accurate results in the prediction of the inducer performance. Finally, the noncavitating pumping characteristic is obtained by introducing suitably adapted correlations of pressure losses and flow deviation effects. The model has been verified to closely approximate the geometry and noncavitating performance of a number of tapered-hub high-head inducers for space application. The results of a series of tests conducted in water under similarity conditions on the four-bladed DAPAMITO4 inducer, designed and manufactured by means of the above reduced-order model, are illustrated. Several non-synchronous instabilities have been observed on the inducer, including an axial surge, a backflow oscillation and, at higher temperatures, incipient rotating cavitation and backflow vortex instability. In addition, synchronous rotating cavitation (leading to the characteristic “one step” shape of the cavitating performance curve near head breakdown conditions) has been detected at all the flow conditions investigated. It has been found that the amplitude of the flow oscillations associated to this instability generally tends to decrease at higher water temperatures. The characterization of the rotordynamic forces acting on a whirling four-bladed, tapered-hub, variable-pitch high-head inducer, under different load and cavitation conditions is presented. The results have obtained in the Cavitating Pump Rotordynamic Test Facility at Alta by means of a novel experimental technique, allowing for the continuous measurement of the rotordynamic force spectra as functions of the whirl ratio. Comparison with simultaneous high-speed movies of the inducer inlet flow highlighted the relationship between the cavitation dynamics in the inducer backflow and the spectral behavior of the rotordynamic force as functions of the whirl ratio. Finally, the future perspectives of the work carried out at Alta on the hydrodynamics and unsteady cavitation phenomena of high performance turbopumps for liquid propellant feed systems of modern rocket engines are briefly illustrated

On the Hydrodynamics of Rocket Propellant Engine Inducers and Turbopumps

The lecture presents an overview of some recent results of the work carried out at Alta on the hydrodynamic design and rotordynamic fluid forces of cavitating turbopumps for liquid propellant feed systems of modern rocket engines. The reduced order models recently developed for preliminary geometric definition and noncavitating performance prediction of tapered-hub axial inducers and centrifugal turbopumps are illustrated. The experimental characterization of the rotordynamic forces acting on a whirling four-bladed, tapered-hub, variable-pitch high-head inducer, under different load and cavitation conditions is presented. Future perspectives of the work to be carried out at Alta in this area of research are briefly illustrated

Rapid Interactive Design-to-Performance of Mixed-Flow Space Inducers

The article illustrates the invited lecture given by the author at International Symposium on Pump and Fan Technology, September 26-28, 2018, ShenYang, China, on the development and experimental validation of a reduced order model for preliminary design and noncavitating performance prediction of mixed-flow tapered-hub inducers for space propulsion applications. The model expresses the 3D incompressible, inviscid, irrotational flow in the blade channels by superposing a 2D axial vorticity correction to a fully-guided axisymmetric flow with radially uniform axial velocity. Suitable redefinition of the diffusion factor for bladings with non-negligible radial flow simultaneously allows for the control of the blade loading and the estimate of the boundary layer blockage and viscous blade losses at the specified design flow coefficient, providing a simple criterion for matching the hub profile to the axial variation of the blade pitch angle. Carter’s rule is employed to account for flow deviation at the inducer trailing edge. Mass continuity, angular momentum conservation and the Euler equation are used to derive a simple 2nd order boundary value problem whose numerical solution defines the far-field axisymmetric flow velocity at the inducer discharge. The noncavitating pumping characteristic is then obtained using suitably adapted semi-empirical corrections for incidence, casing and tip clearance losses. The model has been verified to closely approximate the geometry and noncavitating head characteristics of two space inducers tested in the Cavitating Pump Rotordynamic Test Facility, as well as those of a number of tapered-hub inducers documented in the literature

Stability of Parallel Bubbly and Cavitating Flows

This paper examines the bubble dynamic effects on the stability of parallel bubbly and cavitating flows of low void fraction. Inertial effects associated with the bubble response and energy dissipation due to the viscosity of the liquid, the heat transfer between the two phases, and the liquid compressibility are included. The equations of motion are linearized for small perturbations and a modified Rayleigh equation for the inviscid stability of the two-dimensional parallel flow is derived. Numerical solutions of the characteristic problem for the modified Rayleigh equation of a free shear layer are obtained by means of a multiple shooting method. Depending on the dispersion of the gaseous phase in the bubbly mixture, the ambient pressure and the free stream velocities, the pressure of air bubbles can induce significant departures from the classical solution for a single phase fluid. Results are presented to illustrate the influence of the relevant flow parameters

The Fed and the Stock Market

The Fed closely monitors the stock market and the stock market continuously forms expectations about the Fed decisions. What does this imply for the relation between the fed funds rate and the S&P500? We find that the answer depends on the conditions prevailing on the financial market. During periods of high (low) volatility in asset price inflation an unexpected 5 fall in the stock market index implies that the Fed cuts the interest rate by 19 ($6$) basis points while an unanticipated policy tightening of 50 basis points causes a 4.7 (2.3) decline in the S&P500. The Fed reaction to asset price return is however statistically different from zero only in the high volatility regime, whereas the fall in asset price return following an interest rate rise is highly significant during normal times onlyasset price volatility, nonlinear policy, threshold SVAR, system GMM.

Assessing the Effects of Military Expenditure on Growth

Military spending is an expenditure by governments that has influence beyond the resources it takes up, especially when it leads to or facilitates conflicts. This chapter provides an overview of the issues involved in analysing the effects of military spending on growth. It considers the alternative general economic theories that inform the development of models to undertake empirical analyses, and estimation issues in undertaking those analyses. The Feder-Ram model, the modified Solow and the endogenous growth models, are discussed in detail, before being estimated to illustrate the issues involved in estimating the models and to compare their performance.Military spending; growth; panels spending,semi-parametric estimation

Corruption, Military Spending and Growth

This paper considers the complementary effect of corruption and military spending on economic growth, analyzing both the direct impact of public spending and effect of allocating resources between categories of public spending within the framework of an endogenous growth model. The non-linearities that emerge from are the result of the links between the components of public spending, corruption and economic growth. The main findings of the empirical analysis confirm the expectation that corruption and military burden lower the growth rate of GDP per capita. They also suggest that when the the complementarity effect between military spending and corruption is omitted, as in most studies, the impact of military burden on economic performance is underestimated.corruption, military spending, development economics

Optimal Military Spending in the US: A Time Series Analysis

This paper extends previous work on the optimal size of government spend- ing by including nested functional decompositions of military spending into consumption and investment. Post World War II US data are then used to estimate nested non-linear growth models using semiparametric methods. As expected, investment in military and non-military expenditure are both found to be productive expenditures. Moreover there is little evidence to suggest that current military spending is having a negative impact on economic growth in the US, while civilian consumption only tends to have only a weak impact. This does not imply that society will necessarily bene?t from a reallocation of more spending to the military sector, nor that it is the best way to achieve economic growth. It does suggest that the US economy is not necessarily being hindered by its current military burden.Economic growth; productive state spending; military spending,semi-parametric estimation