Dr. Roger E.A. Arndt

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Viscous effects in tip vortex cavitation and nucleation
(1994) Arndt, Roger E.A.; Maines, Brant H.
This paper is concerned with the physics of cavitation in trailing vortices. The research was aimed at investigating the interrelated effects of the vortex structure and bubble dynamics. The experimental phase utilizes a series of hydrofoils and includes lift and drag measurements, oil flow visualization of the boundary layer flow, and observation of both cavitation inception and disinence in strong and weak water. The complex bubble dynamics inherent in the inception process have been studied using an improved photographic technique. The bubble growth process is strongly dependent on the size and number of nuclei in the free stream and on the strength of the vortex. Numerical simulations indicate that the minimum pressure in the vortex is very close to the tip of the lifting surface, in agreement with the observation that the inception process also occurs very close to the tip under most conditions.
The proper orthogonal decomposition of pressure fluctuations surrounding a turbulent jet
(Cambridge University Press, 1997) Arndt, Roger E.A.; Long, D.F.; Glauser, M.N
It is shown that the pressure signal measured at the outer edge of a jet mixing layer is entirely hydrodynamic in nature and provides a good measure of the large-scale structure of the turbulent ﬂow. Measurement of the pressure signal provides a unique opportunity to utilize proper orthogonal decomposition (POD) to deduce the streamwise structure. Since pressure is a scalar, a signiﬁcant reduction in the numerical and experimental complexity inherent in the analysis of velocity vector ﬁelds results. The POD streamwise eigenfunctions show that the structure associated with any frequency–azimuthal mode number combination displays the general characteristics of ampliﬁcation–saturation–decay of an instability wave, all within about three wavelengths. High-frequency components saturate early in x and low-frequency components saturate further downstream, indicative of the inhomogeneous character of the ﬂow in the streamwise direction. Application of the POD technique allows the phase velocity to be determined taking into account the inhomogeneity of the ﬂow in the streamwise direction. The phase velocity of each instability wave (POD eigenvector) is constant and equal to 0.58Uj , indicating that the jet structure is non-dispersive. Using the shot-noise decomposition, a characteristic event is constructed. This event is found to contain evidence of both pairings and triplings of vortex structures. The tripling results in a rapid increase in the ﬁrst asymmetric (m = 1) component. On average, pairing occurs once every four Uj/D while tripling occurs once every 13Uj/D.
Longitudinal Motion Control of a High-Speed Supercavitation Vehicle
(Sage Publishing, 2006-06-15) Arndt, Roger E.A.; Balas, Gary J.; Bokor, Jozsef; Vanek, Balint
This article focuses on theoretical developments in modeling and control of High-Speed Supercavitating Vehicles (HSSV). A simplified model of longitudinal dynamics is developed for control, and a dynamic inversion based inner-loop control technique is proposed to handle the switched, time-delay dependent behavior of the vehicle. Two outer-loop control schemes are compared for guidance level tracking. Various aspects of disturbance characteristics and actuator dynamics are investigated and analyzed.
Hydroturbine Cavitation
(2009-05-06) Arndt, Roger E.A.
This is an intensive, one week course on the fundamentals of cavitation and its consequences in the operation of hydropower projects. The course will cover the factors that contribute to cavitation, including turbine design, operation, turbine setting, and other factors such as surface roughness and water quality. The topics to be covered include a review of turbomachinery fundamentals and cavitation principles. The discussion of cavitation principles will include factors leading to cavitation inception such as boundary shape, turbulence, secondary flow patterns, dissolved gas and nuclei content of the water. The features of cavitating flows that lead to erosion, noise and vibration will also be discussed.
Further Studies of Tip Vortex Cavitation
(1994-04) Arndt, Roger E.A.; Maines, Brant H.
Cavitation in vertical flows is an important problem. In particular, cavitation in the vortices trailing from lifting surfaces such as propellers and hydrofoils has been studied extensively in the past few years. Factors which affect tip vortex cavitation include water quality (which relates to the amount of tension that can be supported before cavitation occurs), the details of vortex roll-up close to the tip, and flow unsteadiness. An experimental and numerical investigation has been conducted to examine these effects. The experimental phase reported herein includes lift and drag measurements, oil flow visualization of the boundary layer flow on lifting surfaces, and observation of cavitation inception in strong and weak water. An improved photographic technique has been developed to study the complex bubble dynamics inherent in the inception process. Preliminary results indicate that the growth process is strongly dependent on the size and number of nuclei in the free stream. [missing text] vortices that occur at the tips of lifting surfaces and at the hubs of propellers and Francis turbines. Intermediate between turbulent eddies and tip and hub vortices are a variety of secondary flow phenomena such as the horseshoe vortices that form around bridge piers, chute blocks and struts, and the secondary vortices that are found in the clearance passages of turbomachinery. The focus of this paper is on tip vortex cavitation. Recent research on tip vortex cavitation is discussed. This problem was first studied in detail by McCormick (1954, 1962). Subsequently, little further attention was given to this topic until recently, when there has been a resurgence of interest in the problem. Our understanding of the physics has been considerably enhanced in the past decade (1983-1993). The emphasis of this paper is on the details of the inception process. Developed cavitation is also briefly described.
Pressure Fields and Cavitation in Turbulent Shear Flows
(National Academy of Sciences, 1979) Arndt, Roger E.A.; George, William K.
Cavitation in turbulent shear flows is the result of a complex interaction between an unsteady pressure field and a distribution of free stream nuclei. Experimental evidence indicates that cavitation is incited by negative peaks in pressure that are as high as ten times the rms level. This paper reviews the current state of knowledge of turbulent pressure fields and presents new theory on spectra in Lagrangian frame of reference. Cavitation data are analyzed in terms of the available theory on the unsteady pressure field. It is postulated that one heretofore unconsidered factor in cavitation scaling is the highly intermittent pressure fluctuations which contribute to the high frequency end of the pressure spectrum. Because of limitations on the response time of cavitation nuclei, these pressure fluctuations play no role in the inception process in laboratory experiments. However, in large scale prototype flows, cavitation nuclei are relatively mroe responsive to a wider range of the pressure spectrum and this can lead to substantially higher values of the critical cavitatino index. Unfortunately, this issue is coulded by the fact that higher cavitation indices can be found in prototype flows because of gas content effects. Some cavitation noise data are also examined within the ocntext of available theory. The spectrum of cavitation noise in free shear flows has some similarity to the noise data found by Blake et al. (1977) with the exception that there appears to be a greater uncertainty in the scaling of the rate of cavitation events which leads to a substantial spread in available data.
Instability of Partial Cavitation: A Numerical/Experimental Approach
(National Academies Press, 2000) Arndt, Roger E.A.; Song, C.C.S.; Kjeldsen, M.; He, J.; Keller, A.
Sheet cavitation and the transition to cloud cavitation on hydrofoils and marine propellers results in a highly unstable flow that can induce significant fluctuations in lift, thrust and torque. In order to gain a better understanding of the complex physics involved, an integrated numerical/experimental investigation was carried out. A 2D NACA 0015 hydrofoil was selected for study, because of its previous use by several investigators around the world. The simulation methodology is based on large eddy simulation (LES), using a barotropic phase model to couple the continuity and momentum equations. The complementary experiments were carried at two different scales in two different water tunnels. Tests at the Saint Anthony Falls Laboratory (SAFL) were carried out in a 19 cm square water tunnel and a geometrically scaled up series of tests were carried out in the 30 cm square water tunnel at the Versuchsanstalt fur Wasserbau (VAO) in Obernach, Germany. The tests were designed to complement each other and to capitalize on the special features of each facility.
Experimental and Numerical Investigation of Large Scale Structures in Cavitating Wakes
(American Institute of Aeronautics and Astronautics, 2006-06) Arndt, Roger E.A.; Wosnik, Martin
Cavitation is a design consideration for a broad variety of devices handling liquids. In many cases, unstable operation is caused by cavitation-induced flow instabilities. Complex cavitation characteristics are observed in many types of fluid machinery. Examples range from the high- pressure fuel pumps in the Space Shuttle Main Engine to a variety of hydroturbines. In addition there is an increasing interest in very high performance marine vehicles that must operate in the cavitating regime. Associated with the deleterious effects of performance breakdown, noise, and vibration, there is a possibility of erosion. The purpose of this research is to investigate the two- phase flow structure in the wake of a hydrofoil undergoing unsteady partial cavitation using an integrated experimental/numerical approach. This topic provides both numerical and experimental challenges. A two-dimensional NACA 0015 hydrofoil was selected for study, because of its previous use by several investigators around the world. The simulation methodology is based on a Large Eddy Simulation (LES), using a barotropic phase model to couple the continuity and momentum equations. The complementary experiments were carried out at two different scales in two different water tunnels. Tests at the St. Anthony Falls Laboratory (SAFL) were carried out in a 0.19x0.19m2 water tunnel and a geometrically scaled up series of tests was carried out in the 0.3x0.3 m2 water tunnel at the Versuchsanstalt für Wasserbau (VAO) in Obernach, Germany. The tests were designed to complement each other and to capitalize on the special features of each facility.