David A. Boger

A preconditioned Navier–Stokes method for two-phase flows with application to cavitation prediction

Abstract An implicit algorithm for the computation of viscous two-phase flows is presented in this paper. The baseline differential equation system is the multi-phase Navier–Stokes equations, comprised of the mixture volume, mixture momentum and constituent volume fraction equations. Though further generalization is straightforward, a three-species formulation is pursued here, which separately accounts for the liquid and vapor (which exchange mass) as well as a non-condensable gas field. The implicit method developed here employs a dual-time, preconditioned, three-dimensional algorithm, with multi-block and parallel execution capabilities. Time-derivative preconditioning is employed to ensure well-conditioned eigenvalues, which is important for the computational efficiency of the method. Special care is taken to ensure that the resulting eigensystem is independent of the density ratio and the local volume fraction, which renders the scheme well-suited to high density ratio, phase-separated two-fluid flows characteristic of many cavitating and boiling systems. To demonstrate the capabilities of the scheme, several two- and three-dimensional examples are presented.

Performance Analysis of Cavitating Flow in Centrifugal Pumps Using Multiphase CFD

A multi-phase CFD method is used to analyze centrifugal pump performance under developed cavitating conditions. The differential model employed is the homogeneous two-phase Reynolds-Averaged-Navier-Stokes equations, wherein mixture momentum and volume continuity equations are solved along with vapor volume fraction continuity. Mass transfer modeling is provided for the phase change associated with sheet cavitation. Quasi-three-dimensional (Q3D) and fully-three-dimensional analyses are performed for two impeller configurations. Using Q3D analysis, steady and time-dependent analyses were performed across a wide range of flow coefficients and cavitation numbers. Characteristic performance trends associated with offdesign flow and blade cavitation are observed. The rapid drop in head coefficient at low cavitation numbers (breakdown) is captured for all flow coefficients. Local flow field solution plots elucidate the principal physical mechanisms associated with the onset of breakdown. Results are also presented which illustrate the full three dimensional capability of the method.

High Reynolds Number, Unsteady, Multiphase CFD Modeling of Cavitating Flows

A preconditioned, homogeneous, multiphase, Reynolds Averaged Navier-Stokes model with mass transfer is presented. The model is preconditioned in order to obtain good convergence and accuracy regardless of phasic density ratio or flow velocity. Engineering relevant validative unsteady two and three-dimensional results are given. A demonstrative three-dimensional, three-field (liquid, vapor, noncondensable gas) transient is also presented. In modeling axisymmetric cavitators at zero angle-of-attack with 3-D unsteady RANS, significant asymmetric flow features are obtained

Propeller Cavitation Breakdown Analysis

A Reynolds-averaged Navier-Stokes computational model of homogeneous multiphase flow is presented. Cavitation driven thrust and torque breakdown over a wide range of advance ratios is modeled for an open propeller. Computational results are presented as a form of validation against water tunnel measured thrust and torque breakdown for the propeller. Successful validation of the computational model is achieved. Additional observations are made with regards to cavity size and shape as well as cavitation breakdown behavior.

Improvements to SUGGAR and DiRTlib for Overset Store Separation Simulations

SUGGAR is a general overset grid assembly capability that is targeted at moving body simulations. DiRTlib is a library that encapsulates the functionality required to perform the overset interpolation and communications required in an overset composite grid solution. This paper describes enhancements that have been made to SUGGAR and DiRTlib to improve the performance and capability for moving body problems. A procedure to adapt the grids to overlap requirements is demonstrated and found effective in reducing or eliminating the orphans due to insufficient overlap. In addition, SUGGAR can also be linked into the flow solver as a library to provide a capability integral to the flow solver. The paper documents the library interface and discusses modifications required to the flow solver to utilize SUGGAR as a library. Finally, a new overset domain connectivity code, Suggar++, is being developed, and this paper briefly discusses the improvements it offers relative to SUGGAR.

CFD Design and Analysis of a Passively Suspended Tesla Pump Left Ventricular Assist Device

This article summarizes the use of computational fluid dynamics (CFD) to design a novel suspended Tesla left ventricular assist device. Several design variants were analyzed to study the parameters affecting device performance. CFD was performed at pump speeds of 6500, 6750, and 7000 rpm and at flow rates varying from 3 to 7 liters per minute (LPM). The CFD showed that shortening the plates nearest the pump inlet reduced the separations formed beneath the upper plate leading edges and provided a more uniform flow distribution through the rotor gaps, both of which positively affected the device hydrodynamic performance. The final pump design was found to produce a head rise of 77 mm Hg with a hydraulic efficiency of 16% at the design conditions of 6 LPM through flow and a 6750 rpm rotation rate. To assess the device hemodynamics the strain rate fields were evaluated. The wall shear stresses demonstrated that the pump wall shear stresses were likely adequate to inhibit thrombus deposition. Finally, an integrated field hemolysis model was applied to the CFD results to assess the effects of design variation and operating conditions on the device hemolytic performance.

Computational Fluid Dynamics Design and Analysis of a Passively Suspended Tesla Pump Left Ventricular Assist Device

This article summarizes the use of computational fluid dynamics (CFD) to design a novel suspended Tesla left ventricular assist device. Several design variants were analyzed to study the parameters affecting device performance. CFD was performed at pump speeds of 6500, 6750, and 7000 rpm and at flow rates varying from 3 to 7 liters per minute (LPM). The CFD showed that shortening the plates nearest the pump inlet reduced the separations formed beneath the upper plate leading edges and provided a more uniform flow distribution through the rotor gaps, both of which positively affected the device hydrodynamic performance. The final pump design was found to produce a head rise of 77 mm Hg with a hydraulic efficiency of 16% at the design conditions of 6 LPM through flow and a 6750 rpm rotation rate. To assess the device hemodynamics the strain rate fields were evaluated. The wall shear stresses demonstrated that the pump wall shear stresses were likely adequate to inhibit thrombus deposition. Finally, an integrated field hemolysis model was applied to the CFD results to assess the effects of design variation and operating conditions on the device hemolytic performance.

Overset Grid Applications in Hypersonic Flow Using the DPLR Flow Solver

This paper describes the addition of an overset grid capability to the DPLR flow solver for hypersonic flow in thermochemical nonequilibrium. Modifications to the preexisting flow solver were simplified through the use of DiRTlib, a “solver neutral” library of overset utilities. The new capability is demonstrated on a series of examples, including the Orion Crew Module and other reentry vehicles. For the overset grids used in these examples, the hole cutting and interpolation stencils were determined using SUGGAR, a generalized grid assembly code that can naturally accommodate both the three-dimensional and true two-dimensional cell-centered discretization schemes in DPLR. First a series of building-block examples are presented which highlight aspects of the new capability and assess the technique with comparisons to baseline, block-structured discretizations. The new capability is then exercised for the specific case of a tension tie geometry protruding from the Orion heatshield at both wind tunnel and flight conditions. The addition of overset capability to the DPLR flow solver is seen to be an essential feature for analyzing increasingly complex geometries in thermochemical nonequilibrium.

Computational Fluid Dynamics Design and Analysis of a Passively Suspended Tesla Pump Left Ventricular Assist Device: PASSIVELY SUSPENDED TESLA PUMP LVAD

This article summarizes the use of computational fluid dynamics (CFD) to design a novel suspended Tesla left ventricular assist device. Several design variants were analyzed to study the parameters affecting device performance. CFD was performed at pump speeds of 6500, 6750, and 7000 rpm and at flow rates varying from 3 to 7 liters per minute (LPM). The CFD showed that shortening the plates nearest the pump inlet reduced the separations formed beneath the upper plate leading edges and provided a more uniform flow distribution through the rotor gaps, both of which positively affected the device hydrodynamic performance. The final pump design was found to produce a head rise of 77 mm Hg with a hydraulic efficiency of 16% at the design conditions of 6 LPM through flow and a 6750 rpm rotation rate. To assess the device hemodynamics the strain rate fields were evaluated. The wall shear stresses demonstrated that the pump wall shear stresses were likely adequate to inhibit thrombus deposition. Finally, an integrated field hemolysis model was applied to the CFD results to assess the effects of design variation and operating conditions on the device hemolytic performance.

Structure- and Fluid-Borne Acoustic Power Sources in 90 Degree Piping Elbows Excited by Turbulent Flow

The structure-borne and fluid-borne vibro-acoustic power spectra induced by turbulent fluid flow over the walls of a continuous 90 degree piping elbow are computed. Although the actual power input by the wall pressure fluctuations to the piping is distributed throughout the elbow, equivalent total powers input to various structural wavetypes (bending, torsion, axial) and fluid (plane waves) at the inlet and discharge of the elbow are computed. The powers at the elbow ‘ports’ are suitable inputs to wave-based and statistically-based models of larger piping systems that include the elbow. Calculations for several flow and structural parameters, including pipe wall thickness, flow speed, and flow Reynolds number are shown. The power spectra are scaled on flow and structural-acoustic parameters so that levels for conditions other than those considered in the paper may be estimated, subject to geometric similarity constraints (elbow radius/pipe diameter). The approach for computing the powers, which links Computational Fluid Dynamics, Finite Element and Boundary Element modeling, and efficient random analysis techniques, is general, and may be applied to other piping system components excited by turbulent fluid flow, such as U-bends and T-sections.© 2006 ASME