Three-dimensional Unsteady Computations Research Articles

Computations of Flapping Flow Propulsion for Unmanned Underwater Vehicle Design

Three-dimensional unsteady computations of the flow past a flapping and deforming fin are performed. The computed unsteady lift and thrust force-time histories are validated with experimental data and are in good agreement. Several fin parametric studies are performed for a notional unmanned underwater vehicle. The parametric studies investigated the force production of the fin as a function of varying the flexibility, the bulk amplitude of fin rotation, the vehicle speed, and the fin stroke bias angle. The results of these simulations are used to evaluate the hydrodynamic performance of the vehicle and to support controller development. Computations are also performed to map out the hydrodynamic characteristics of a new test vehicle, designed and built at Naval Research Laboratory to demonstrate the hovering and low-speed maneuvering performance of a set of actively controlled curvature fins.

Computations of Insect and Fish Locomotion with Applications to Unconventional Unmanned Vehicles

Living creatures such as insects, birds, and fish generate lift and thrust most often by executing large-amplitude wing flapping, possibly with substantial shape deformation from root to tip. The flow for these motions is unsteady, and conventional steady-state aerodynamics is unable to correctly compute the time history of their flapping-force generation. Three-dimensional unsteady computations of flapping about the deforming wing or fin surface are necessary to correctly predict the lift and thrust throughout the flapping cycle. It is only by executing such computations for creatures or vehicles with moving and deforming surfaces that we can gain insights into the time-varying pressure distribution on all surfaces and how that results in flapping-force generation. This can be coupled with visualization of the origination and evolution of body, wing, and wake vorticity. Three-dimensional unsteady flow computations of the flapping flights of the fruit fly, a pectoral-fin swimmer (the bird wrasse), and a variety of unmanned air vehicles were carried out in pursuit of this information. The performance of these flapping wings under gust conditions was also investigated. The effect of fin deformation on the force production was studied. Novel biomimetic vehicles, incorporating information gained from these computations, were designed and built and their performance is described.

A Computational Fluid Dynamics (CFD) analysis of an undulatory mechanical fin driven by shape memory alloy

Many fishes use undulatory fin to propel themselves in the underwater environment. These locomotor mechanisms have a popular interest to many researchers. In the present study, we perform a three-dimensional unsteady computation of an undulatory mechanical fin that is driven by Shape Memory Alloy (SMA). The objective of the computation is to investigate the fluid dynamics of force production associated with the undulatory mechanical fin. An unstructured, grid-based, unsteady Navier-Stokes solver with automatic adaptive remeshing is used to compute the unsteady flow around the fin through five complete cycles. The pressure distribution on fin surface is computed and integrated to provide fin forces which are decomposed into lift and thrust. The velocity field is also computed throughout the swimming cycle. Finally,a comparison is conducted to reveal the dynamics of force generation according to the kinematic parameters of the undulatory fin (amplitude, frequency and wavelength).

Fluid dynamics of flapping aquatic flight in the bird wrasse: three-dimensional unsteady computations with fin deformation.

Many fishes that swim with the paired pectoral fins use fin-stroke parameters that produce thrust force from lift in a mechanism of underwater flight. These locomotor mechanisms are of interest to behavioral biologists, biomechanics researchers and engineers. In the present study, we performed the first three-dimensional unsteady computations of fish swimming with oscillating and deforming fins. The objective of these computations was to investigate the fluid dynamics of force production associated with the flapping aquatic flight of the bird wrasse Gomphosus varius. For this computational work, we used the geometry of the wrasse and its pectoral fin, and previously measured fin kinematics, as the starting points for computational investigation of three-dimensional (3-D) unsteady fluid dynamics. We performed a 3-D steady computation and a complete set of 3-D quasisteady computations for a range of pectoral fin positions and surface velocities. An unstructured, grid-based, unsteady Navier-Stokes solver with automatic adaptive remeshing was then used to compute the unsteady flow about the wrasse through several complete cycles of pectoral fin oscillation. The shape deformation of the pectoral fin throughout the oscillation was taken from the experimental kinematics. The pressure distribution on the body of the bird wrasse and its pectoral fins was computed and integrated to give body and fin forces which were decomposed into lift and thrust. The velocity field variation on the surface of the wrasse body, on the pectoral fins and in the near-wake was computed throughout the swimming cycle. We compared our computational results for the steady, quasi-steady and unsteady cases with the experimental data on axial and vertical acceleration obtained from the pectoral fin kinematics experiments. These comparisons show that steady state computations are incapable of describing the fluid dynamics of flapping fins. Quasi-steady state computations, with correct incorporation of the experimental kinematics, are useful when determining trends in force production, but do not provide accurate estimates of the magnitudes of the forces produced. By contrast, unsteady computations about the deforming pectoral fins using experimentally measured fin kinematics were found to give excellent agreement, both in the time history of force production throughout the flapping strokes and in the magnitudes of the generated forces.

Influence of Unsteady Effects on the Measurements in a Transonic Axial Compressor

Interaction phenomena between rotor and stator are unavoidable in advanced compressors and their effects increase with the performance of the turbomachines. Until now, it was not possible to quantify the interaction effects, but with the development of three-dimensional unsteady computation codes in a complete stage, it is possible to know, in detail, the flow field through the machine and to make evident and to explain the difficulties encountered in measuring the flow parameters. A study has been conducted in this way at ONERA, on an axial transonic compressor stage. The computations have been made with a simulation of the losses; in this manner, the overall computed and measured performances of the compressor are the same. A detailed analysis of the unsteady computation results makes evident, between rotor and stator, large variations of some parameters of the flow as a function of time, but also as a function of the axial and tangential relative position of steady probes and stator blades. Unsteady measurements made on another transonic machine confirm the indications given by these computations.