Image space based visualization of unsteady flow on surfaces

We propose unsteady flow advection-convolution (UFAC) as a novel visualization approach for unsteady flows. It performs time evolution governed by pathlines, but builds spatial correlation according to instantaneous streamlines whose spatial extent is controlled by the flow unsteadiness. UFAC is derived from a generic framework that provides spacetime-coherent dense representations of time dependent-vector fields by a two-step process: 1) construction of continuous trajectories in spacetime for temporal coherence; and 2) convolution along another set of paths through the above spacetime for spatially correlated patterns. Within the framework, known visualization techniques-such as Lagrangian-Eulerian advection, image-based flow visualization, unsteady flow LIC, and dynamic LIC-can be reproduced, often with better image quality, higher performance, or increased flexibility of the visualization style. Finally, we present a texture-based discretization of the framework and its interactive implementation on graphics hardware, which allows the user to gradually balance visualization speed against quality.

In this paper, we propose a new technique to visualize dense representations of time-dependent vector fields based on a Lagrangian-Eulerian Advection (LEA) scheme. The algorithm produces animations with high spatio-temporal correlation at interactive rates. With this technique, every still frame depicts the instantaneous structure of the flow, whereas an animated sequence of frames reveals the motion a dense collection of particles would take when released into the flow. The simplicity of both the resulting data structures and the implementation suggest that LEA could become a useful component of any scientific visualization toolkit concerned with the display of unsteady flows.

A new method for the synthesis of dense, vector-field aligned textures on curved surfaces is presented, called IBFVS. The method is based on image based flow visualization (IBFV). In IBFV two-dimensional animated textures are produced by defining each frame of a flow animation as a blend between a warped version of the previous image and a number of filtered white noise images. We produce flow aligned texture on arbitrary three-dimensional triangular meshes in the same spirit as the original method: texture is generated directly in image space. We show that IBFVS is efficient and effective. High performance (typically fifty frames or more per second) is achieved by exploiting graphics hardware. Also, IBFVS can easily be implemented and a variety of effects can be achieved. Applications are flow visualization and surface rendering. Specifically, we show how to visualize the wind field on the earth and how to render a dirty bronze bunny.

Recent advances in unsteady flow visualization

An investigation of the unsteady flow effects in a supersonic compressor stage is reported. Semiconductor transducers mounted in the casing, a stroboscopic schlieren-technique and probes equipped with semiconductor transducers were used to obtain unsteady static pressure, flow visualization and three-dimensional flow information, respectively. The throttling effect during the starting procedure of the stage was found to produce changes of the quasi-steady and unsteady rotor flow pattern as well as changes in both amplitude and frequency of the flow disturbances at the inlet. The conditions which are generated in this way at design speed are different from those reached by back-pressure imposed on the started supersonic rotor at the same speed. The relative position of rotor and stator has an influence on the flow field only below design speed. One can conclude from the measurements of the rotor outlet angle that these effects generated by throttling the rotor flow by a stator during the starting procedure might be avoided with movable stator blades.

The study of fluid behaviors has been a challenging topic. Flow visualization enables us to visually acquire qualitative and quantitative flow information. There exist various software tools performing different flow visualization tasks. However, we lack tools that help students learn important flow field concepts. In this paper, we present a visualization app, named FlowVisual which runs on iOS devices, to illustrate basic flow field concepts in 3D. In order to meet a comprehensive learning goal for students, we integrate a number of techniques into FlowVisual design, including field-line tracing, field-line comparison, critical point detection and classification, template-based seeding, and surface visualization. We evaluate and demonstrate the effectiveness of FlowVisual by conducting a formal user study including an introduction and training session, an auto-grading test, and a post-questionnaire survey.

An overset grid thin-layer Navier-Stokes code has been extended to include dynamic motion of helicopter rotor blades through relative grid motion. The unsteady flowfield and airloads on an AH-IG rotor in forward flight were computed to verify the methodology and to demonstrate the method's potential usefulness towards comprehensive helicopter codes. In addition, the method uses the blade's first harmonics measured in the flight test to prescribe the blade motion. The solution was impulsively started and became periodic in less than three rotor revolutions. Detailed unsteady numerical flow visualization techniques were applied to the entire unsteady data set of five rotor revolutions and exhibited flowfield features such as blade vortex interaction and wake roll-up. The unsteady blade loads and surface pressures compare well against those from flight measurements. Details of the method, a discussion of the resulting predicted flowfield, and requirements for future work are presented. Overall, given the proper blade dynamics, this method can compute the unsteady flowfield of a general helicopter rotor in forward flight.

In flow visualization, the streamline has always been used to display the vector fields, but only using streamlines cannot reveal the nature of flowing intuitively. The dynamic glyphs technology is an important method to show flowing 3D vector fields. However, existing methods of dynamic glyphs are hard to convey the global patterns and local entities at the same time. In this paper, we present a practical method for dynamic visualization of 3D vector fields which combine dynamic glyphs and streamlines, which are rendered respectively according to the information of 3D vector fields. Our method integrates virtues of these two usual techniques, and provides high performance visualization of flowing 3D vector fields directly. Experiments with practical datasets certify that our method could not only depict the local and the global patterns, but more importantly reveal the dynamic nature of flowing 3D vector data.

The two-mass approximation is often presented as a simple but efficient model for the purpose of numerical synthesis of voiced sounds. However, the quality and the naturalness of the generated signal is still to be improved. A simple modification of the two-mass model is proposed in which the mechanical dynamics of that of two point masses. The movement of these point masses drives a two-parameter curve describing the smooth surface of the vocal folds. This representation allows description of unsteady flow separation within the glottis. A semiempirical boundary layer theory is used to calculate the position of flow separation. This theory is experimentally tested using stationary flow measurements and unsteady flow visualization. Furthermore, computed volume flux and Bernoulli force are presented and compared with classical models. The description also includes an inelastic collision which yields a prediction of an additional volume flux due to the deformation of the vocal folds after the through flux has been stopped by the closure of the glottis. This additional deformation appears to have a significant influence on the voiced sound production.

We experimentally investigate unsteady aerodynamic mechanisms of insect flapping wings. Measurements of unsteady aerodynamic forces and flow visualization around the flapping wing using PIV are conducted with dynamically scaled mechanical models in a water tunnel, which simulates hovering and forward flapping flights. We experimentally clarify the effects of three unsteady aerodynamics on the aerodynamic characteristics of a flapping wing: delayed stall, rotational circulation, and wake capture. The contribution of delayed stall to the total lift accounts for about 90%. The feathering rotation during the flapping translation contributes to an increase of power rather than lift. The wake capture is dependent on the reduced frequency and the timing of feathering rotation. In addition, we investigate the flow interaction between fore- and hindwings like a dragonfly. The phase difference between the fore- and hindwings are effective on the aerodynamic characteristics of the tandem wings.

Visualization of coronary blood flow by means of a slice-selective inversion pre-pulse in concert with bright-blood coronary MRA. Coronary magnetic resonance angiography (MRA) of the right coronary artery (RCA) was performed in eight healthy adult subjects on a 1.5 Tesla MR system (Gyroscan ACS-NT, Philips Medical Systems, Best, NL) using a free-breathing navigator-gated and cardiac-triggered 3D steady-state free-precession (SSFP) sequence with radial k-space sampling. Imaging was performed with and without a slice-selective inversion pre-pulse, which was positioned along the main axis of the coronary artery but perpendicular to the imaging volume. Objective image quality parameters such as SNR, CNR, maximal visible vessel length, and vessel border definition were analyzed. In contrast to conventional bright-blood 3D coronary MRA, the selective inversion pre-pulse provided a direct measure of coronary blood flow. In addition, CNR between the RCA and right ventricular blood pool was increased and the vessels had a tendency towards better delineation. Blood SNR and CNR between right coronary blood and epicardial fat were comparable in both sequences. The combination of a free-breathing navigator-gated and cardiac-triggered 3D SSFP sequence with a slice-selective inversion pre-pulse allows for direct and directional visualization of coronary blood flow with the additional benefit of improved contrast between coronary and right ventricular blood pool.

We propose a new technique to visualize dense vector fields associated with unsteady fluid flows. This technique is based on a Lagrangian-Eulerian Advection (LEA) scheme, and it enables animations with high spatio-temporal correlation at interactive rates. We demonstrate the efficiency and efficacy of the technique in applications to numerical simulation of a shock interacting with a longitudinal vortex, and of ocean circulation in the Gulf of Mexico. The simplicity of the data structures and the facility of implementation suggest that LEA could become a useful component of any scientific visualization toolkit concerned with the display of unsteady flows.

A 3D vector field flow topology extraction algorithm based on critical point analysis is presented for CFD (Computational Fluid Dynamics) data sets. It operates on curvilinear, structured grids, including large multi-zone grids. In many CFD computations, no-slip boundary conditions are imposed on the velocity field. On these boundaries, the fluid velocity vector is zero. The algorithm analyzes this special case by examining the skin friction field. Using glyphs to locate critical points, we can interactively mark vortex cores. This is drawn by selecting glyphs that indicate spiraling flow and integrating a streamline in the direction of the eigenvector associated with the real eigenvalue. The test results show clearly the fluid flow critical features.

A current project is underway to create a prototype of an anatomically correct seagull with biologically accurate flight kinematics. The presented work is focused on the computational fluid dynamics (CFD) analysis of bird flight kinematics. A finite volume approach, using Fluent, was used to attempt to model the kinematics of bird flight with varying degrees of freedom to analyze the lift, drag, pressure, and vortices magnitude associated with a range of flight kinematics. Dimensional analysis has been performed to analyze the effects of angle of incidence on the different sections of a seagull wing. Validated CFD analysis has been performed to identify optimal degree of freedom for generating maximum amount of lift while minimizing drag. The analysis benefitted from dynamic meshing and a user defined function to model the seagull wing, profiles of which were approximated by the S1223 airfoil. The user defined function allowed for variation of degrees of freedom to model the flight in the current bird prototype and to assess the effects of changing angles of incidence and inlet velocity on lift and drag. Difficulties were encountered when trying to accurately analyze unsteady aerodynamics over a flapping motion. The appropriate grid resolution, the user defined function, as well as the appropriate grid and dynamic mesh parameters within Fluent were all possible areas of concern. The grid resolution was determined by analyzing a steady state case and determining the variation in lift and drag values calculated by increasing the grid density. A user defined function was created that accurately represents the kinematics associated with the bird wing. A triangular grid was utilized for the dynamic mesh with re-meshing procedure activated at every iteration during the analysis. The final geometry provided an accurate method for dynamic re-meshing and overcame the problem of negative cell volume associated with re-meshing using a rectangular mesh configuration. It was determined that maximum cell volume, number of time steps, and time step interval were all important criteria when determining parameters for the unsteady flight analysis. Results indicate that the unsteady dynamics of bird flapping motion can be effectively represented with modified CFD analysis with updated finite volume scheme. Data indicates that values associated with varying angles of attack at a steady state cannot be used to model flapping flight. The paper will report on further validation to analyze the pressure, lift and drag associated with flapping flight in a three-dimensional study.

The effect of aerodynamic detuning on the supersonic steady and unsteady blade passage flow field is experimentally investigated on a free surface water table by means of color Schlieren and shadowgraph flow visualization techniques. Two aerodynamic detuning mechanisms are considered: (1) alternate circumferential spacing of adjacent airfoils; (2) the replacement of alternate airfoils with splitters. The steady flow visualization demonstrates the significant effect of aerodynamic detuning on the passage flow field and, in particular, the shock wave-airfoil surface intersection locations. The unsteady flow visualization studies show the importance of the interblade phase angle. A mathematical model is also described and utilized to demonstrate the enhanced aeroelastic stability associated with the altered cascade passage shock wave structure due to these aerodynamic detuning mechanisms.