Jinzhang Feng

Cavity Flow Predictions Based on the Euler Equations

An Euler solver based on artificial-compressibility and pseudo-time stepping is developed for the analysis of partial sheet cavitation in two-dimensional cascades and on isolated airfoils. The computational domain is adapted to the evolution of the cavity surface and the boundary conditions are implemented on the cavity interface. This approach enables the cavitation pressure condition to be incorporated directly without requiring the specification of the cavity length or the location of the inception point. Numerical solutions are presented for a number of two-dimensional cavity flow problems, including both leading edge cavitation and the more difficult mid-chord cavitation conditions. Validation is accomplished by comparing with experimental measurements and nonlinear panel solutions from potential flow theory. The demonstrated success of the Euler cavitation procedure implies that it can be incorporated in existing incompressible CFD codes to provide engineering predictions of cavitation. In addition, the flexibility of the Euler formulation may allow extension to more complex problems such as viscous flows, time-dependent flows and three-dimensional flows.

Numerical Modeling of the Thermodynamic Effects of Cavitation

A Navier-Stokes solver based on artificial compressibility and pseudo-time stepping, coupled with the energy equation, is used to model the thermodynamic effects of cavitation in cryogenic fluids. The analysis is restricted to partial sheet cavitation in two-dimensional cascades. Thermodynamic effects of cavitation assume significance in cryogenic fluids because these fluids are generally operated close to the critical point and also because of the strong dependence of the vapor pressure on the temperature. The numerical approach used is direct and fully nonlinear, that is, the cavity profile evolves as part of the solution for a specified cavitation pressure. This precludes the necessity of specifying the cavity length or the location of the inception point. Numerical solutions are presented for two-dimensional flow problems and validated with experimental measurements. Predicted temperature depressions are also compared with measurements for liquid hydrogen and nitrogen. The cavitation procedure presented is easy to implement in engineering codes to provide satisfactory predictions of cavitation. The flexibility of the formulation also allows extension to more complex flows and/or geometries.

A Preconditioned Dual-Time, Diagonalized ADI Scheme for Unsteady Computations

The diagonalized ADI algorithm of Pulliam and Chaussee has been extended to include a timederivative preconditioning that permits efficient computation of flows over a wide range of Mach numbers. The algorithm has also been modified so that time-accurate analyses are possible through the use of dual-time stepping. The dual-time stepping has been incorporated in such a way that no additional factors are needed to handle the dual-time term in the left-hand-side inversion process. The preconditioning can accommodate general equations of state such as constant density (incompressible), perfect gas, or supercritical fluids. Unsteady vortex shedding from a circular cylinder is presented as an example of the capability of the algorithm.

CFD analyses of coolant channel flowfields

The flowfield characteristics in rocket engine coolant channels are analyzed by means of a numerical model. The channels are characterized by large length to diameter ratios, high Reynolds numbers, and asymmetrical heating. At representative flow conditions, the channel length is approximately twice the hydraulic entrance length so that fully developed conditions would be reached for a constant property fluid. For the supercritical hydrogen that is used as the coolant, the strong property variations create significant secondary flows in the cross-plane which have a major influence on the flow and the resulting heat transfer. Comparison of constant and variable property solutions show substantial differences. In addition, the property variations prevent fully developed flow. The density variation accelerates the fluid in the channels increasing the pressure drop without an accompanying increase in heat flux. Analyses of the inlet configuration suggest that side entry from a manifold can affect the development of the velocity profile because of vortices generated as the flow enters the channel. Current work is focused on studying the effects of channel bifurcation on the flow field and the heat transfer characteristics.

SIMULATIONS OF PLANAR FLAPPING JETS IN CONFINED CHANNELS

Computational analyses are used to provide a more complete understanding of the mechanisms that contribute to the development of oscillating planar jets. The geometry considered is a two-dimensional jet exhausting into a blind channel, whose open end is opposite to the initial direction such that the jet must turn through 180 deg to exit. The resulting flowfields exhibit three distinct characters that depend on the channel expansion ratio and the Reynolds number. At low Reynolds numbers the flow is steady and symmetric. A symmetry-breaking bifurcation at intermediate Reynolds numbers produces steady asymmetric flows. A Hopf bifurcation at higher Reynolds numbers yields unsteady flows. Predicted critical Reynolds numbers and oscillation frequencies are presented for different expansion ratios. Solutions are obtained from the time-dependent Navier-Stokes equations by means of an incompressible formulation based on dual-time stepping via artificial compressibility

Characteristics of Tip-Clearance Flows of a Compressor Cascade and a Propulsion Pump

Tip-leakage flows for a linear compressor cascade and a one-stage shrouded pump rotor are discussed in this paper. A numerical method solving the Reynolds averaged Navier Stokes equations is used to explore various detail features of the tip-leakage flows. Calculation results for the cascade provide an assessment for predicting flow past a non-rotating blade passage with zero and 2% chord clearances. On the other hand, the pump rotor configuration provides a swirling passage flow with the complication of a trailing-edge separation vortex mixed with the tip-clearance and passage vortices and produces a very complex three-dimensional flow in the rotor wake. The physical aspects of the tip-clearance flows are discussed including suction-side reloading and pressure-side unloading due to a tip clearance and formation and transportation of the tip-leakage vortex. Detailed velocity comparisons in the blade passage and the tip gap region are shown to indicate the difficulty of predicting tip-leakage flow. The pressure at the core of the tip vortex is also examined to evaluate the strength of the tip-leakage vortex. Some computational guidelines for design usage are provided for these tip-leakage flow calculations.Copyright

Application of a distributed network in computational fluid dynamic simulations

A general-purpose 3-D, incompressible Navier-Stokes algorithm is implemented on a network of concur rently operating workstations using PVM and com pared with its performance on a CRAY Y-MP and on an Intel iPSC/860. The problem is relatively computa tionally intensive, and has a communication structure based primarily on nearest-neighbor communication, making it ideally suited to message passing. Such problems are frequently encountered in CFD, and their solution is increasingly in demand. The communica tion structure is explicitly coded in the implementation to fully exploit the regularity in message passing in order to produce a near-optimal solution. Results are presented for various grid sizes using up to eight pro cessors.

Implementation of a parallel algorithm on a distributed network

The objective of this research is to investigate the potential of using a network of concurrently operating workstations for solving large computer-intensive problems typical of Computational Fluid Dynamics. Such problems have a communication structure based primarily on nearest-neighbor communication and are therefore ideally suited to message passing. The implementation of a 3D Navier-Stokes code on a network of IBM RISC/6000s is described. The performance of this code is compared with that on conventional high-performance machines such as the Cray-YMP and the Intel iPSC/860. The results suggest that a cluster of workstations presents a viable and economical resource for this purpose. Additionally a workstation network has the advantage of easy accessibility, fault tolerance and a simple environment for program development. 12 refs.

Practical Applications of Parallel Processing in Computational Fluid Dynamics

The onset of parallel computing has had a profound impact on CFD, both in terms of its utility as an analysis and design tool, and its capability in adequately resolving flowfields that exhibit complex physical phenomena. The resources provided by a distributed network of processors cohesively working together provides an opportunity to tackle some of the most challenging problems in fluid dynamics, that have proved elusive to serial supercomputing. Turbomachinery and combustion are two such areas of fluid dynamics that have benefited immensely from the increase in resources afforded by parallel machines. In this paper, we discuss the parallel implementation of conventionall y popular implicit and explicit serial CFD algorithms and utilize them to solve problems of increased complexity in turbomachinery and combustion. Specific applications in turbomachiner y that are discussed in this paper include the rotor-stator interaction problem, analysis of tip vortex generation, multiple passage simulations for non-periodic distortions and cavitation. Also discussed in the paper are problems related to combusting flowfields in rocket injectors that require fine local resolution necessary to capture the wide range of dominant characteristic scales.

Prediction of Vortex and Linear Cascade Interaction Noise

This paper is concerned with increasing the understanding of the noise generation mechanism assoicated with the interaction between foils and shed vorticity. An inviscid computational method is developed that predicts the flow interaction unsteadiness and the resultant acoustic pressures due to the acceleration and deceleration of vortices in the proximity to foils. This time-accurate prediction method is used to study the generation of shed vorticity and the interaction between a vortex and a foil or linear cascade of blades in terms of flow interaction phenomena, forces acting on foils, instability of free traveling vortices, and radiated acoustic pressure. The results show that the predicted acoustic pressure is proportional to the closeness of the free vortices to the foil or the cascade blades. The number of acoustic pulses, generated when a free vortex encounters a foil, depends on the inflow angle of attack. When compared to a single foil, a cascade produces effects that suppress the vortex/blade flow interaction. However, the acoustic response depends heavily on the free vortex location in relation to the cascade blades.Copyright