Combined application of surface flow visualization and laser-Doppler anemometry to engine intake flows

Flow visualization using oil streak techniques and laser-Doppler anemometry were carried out to provide detailed information on the flow through the intake valve of a research (model) engine head under steady flow conditions. The work was partially undertaken to develop the techniques as useful tools for engine research. On the other hand, variations of the flow field with valve lift and with valve location were of interest. In the present paper it is shown that a symmetric geometry does not necessarily result in symmetric flow patterns inside the cylinder; the tendency to asymmetry increases with increasing valve lift. These characteristics of the flow should be taken into account when flow computations are performed necessitating the use of three dimensional codes in the entire flow field, not in a symmetrical half-geometry.     

Leonardo's vision of flow visualization

Leonardo's studies of cardiovascular systems, in more than 50 surviving pages from two phases of his research (around 1508-1509 and 1513), are a clear demonstration of his observational genius and progressive deduction of cardiac mechanics and the vascular system. He carried out a detailed hemodynamic study of the aortic valve motion and the role of the Sinus of Valsalva in the closure dynamics of the aortic valve, and he accurately correlated the formation of vortices with the separation of a retarded (shear) layer from the lips of the leaflets. In-vivo verification of vortex formation in the Sinus of Valsalva during the systolic phase awaited the application of modern phase-averaged magnetic resonance imaging techniques. Did Leonardo actually build the glass model he twice mentioned, thus performing the first scientific flow visualization of impulsive vortex formation or other fluid mechanical phenomena? Evidence in support of this possibility can be found in both the unusually schematic style he employed for this suite of drawings and the recent flow imaging results obtained in our laboratory through laser-based imaging techniques.     

Instantaneous visualization of surface flows

An experimental system is described for visualizing the surface flow of a wing, using an oil smoke tracer technique. The method leads to the determination of the instantaneous velocity direction at the output of surface injectors. A preliminary investigation is made on a flat plate to optimize the conditions of oil smoke injection. Then, the visualization is performed on the upperside of a sweptback wing in the vicinity of the reattachment of the vortex flow. This visualization technique can be applied to other types of wall flows — separated or not — around various bodies.List of symbols b wing span - c _n normal (to leading edge) chord - c _r streamwise (or root) chord - d diameter of the injectors - distance from the apex along the leading edge - relative distance from the apex along the leading edge ( = /C _d) - sweep angle - e injector geometric parameter (e = d/l) - angle of attack - K injection parameter - l length of the injectors - v kinematic viscosity - P _t, P_s total and static pressure of the flow - P _inj injection pressure - P _r reduced pressure (P _r = (P_inj – P_t)/(P_t – P_s)) - Re flow Reynolds number (Re = V ·c _n/v) - Re _i injector Reynolds number (Re = V ·d/v) - s curvilinear distance along c _d - s relative curvilinear distance along c _d(s = s/c _d) - V infinite upstream flow velocity     

Digital interferometry for flow visualization

Digital holographic interferometry is a hybrid optical-digital technique for determining the phase of an interferogram. This technique improves the accuracy of interferometric measurement of fluid properties and enhances the utility of interferometric flow visualization. Displays of the interferometric phase produce excellent images of weakly refracting two-dimensional flows and can be used to produce integral projection images of three dimensional flows which differ from and complement schlieren and shadowgraph images. The technique is explained herein and examples of its use in both continuous wave and pulsed interferometry are presented.This work was presented in part at the 1985 Optical Society of America Annual Meeting     

Bullseye color schlieren flow visualization

In this variant of schlieren flow visualization a tricolored filter pattern made up of concentric rings is used instead of a knife edge at the cut-off plane. The method is particularly useful in combustion studies since it provides the range necessary to visualize flames while retaining the sensitivity necessary to detect fluid motion in the surrounding gasses. That utility is demonstrated in this paper in terms of basic principles and by application. Photographs presented include images of both premixed and diffusion flames in an unsteady swirling flow within a cylinder.     

Measurement of flow-induced pressures on the surface of a model in a flow visualization water tunnel

Flow-induced pressures were measured on the surface of a delta wing in a flow visualization water tunnel, and at the same time the vortical flow over the wing was visualized using dye. For a free-stream velocity of 0.1 m/s used in the experiments, the pressures were small (less than 15 Pa). This is believed to be the first time that flow-induced pressures have been measured in a water tunnel at such a low velocity. Measured pressure distributions can now be properly interpreted in terms of observed detailed flow patterns, without having to undertake complementary wind tunnel experiments to measure model loading.Symbols   c   centreline chord of the delta wing (c=300.0 mm) - C _p   pressure coefficient on the surface of the delta wing - f   frequency (Hz) - p _ref   static pressure measured by the reference static-pressure probe (Pa) - P _ref   total pressure measured by the reference total-pressure probe (Pa) - p _wing   flow-induced pressure at a pressure tapping on the delta wing (Pa) - R   Reynolds number for the delta wing in water (R=Uc/v _w) - s   local semi-span of the delta wing (mm) - U   free-stream velocity in the test section of the water tunnel (m/s) - x   chordwise distance from the apex of the delta wing (mm) - y   spanwise distance from the centreline chord of the delta wing (mm) -   angle of attack of the delta wing (deg) -   angle of sideslip of the delta wing (deg) - v _w   kinematic viscosity of water (m²/s) - _a   density of air (kg/m³) - _w   density of water (kg/m³)

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Streakline flow visualization with neutron radiography

Although flow visualization is an extremely useful tool in fluid mechanics research, many practical fluid flow problems have limited optical access for visualization. A technique has been developed which permits fluid flow to be visualized even in cases where the flow is completely shrouded by metal. The technique employs real-time thermal neutron radiography, which is similar to x-ray radiography except that a collimated beam of neutrons is used. Neutrons can easily penetrate metal casings, but are attenuated by elements such as hydrogen, boron, cadmium, and gadolinium. Various combinations of neutron-opaque tracer particles moving in neutron-transparent ambient fluids were tested for image contrast, resolution, and ability to accurately track the flow. Experiments in a simple pipe flow have demonstrated the feasibility of the technique. Namely, it was possible to visualize the motion of streaklines within a flow field shrouded by metal, which would not have been possible with any other technique.     

In situ flow visualization of capillary flow of concentrated alumina suspensions

A new approach was taken to understand the flow behavior of concentrated particle suspensions in pressure-driven capillary flow. The flow of concentrated alumina suspensions in a slit channel was visualized and quantitatively analyzed with modified capillary rheometer. The suspensions showed complex flow behaviors; unique solid–liquid transition and shear banding. At low flow rates, 55 vol% alumina suspension showed a unique transient flow behavior; there was no flow at first and continuous change of flow profile was observed with time. At low shear rates in particular, the suspensions exhibited shear-banded flow profile which could be divided into three regions: the region with low flow rate near the wall, the region with rapid increase of flow velocity to maximum, and the region of velocity plateau. Based on both flow visualization and measurement of shear stress, it was found that the shear-banded flow profile in pressure-driven slit channel flow was strongly correlated with shear stress. The banding in pressure-driven flow was different from that in Couette flow. The banding of concentrated alumina suspensions was unique in that sluggish velocity profile was pronounced and two inflection points in velocity profile was exhibited. In this study, shear banding of concentrated alumina suspensions in slit channel flow was visualized and quantitatively analyzed. We expect that this approach can be an effective method to understand the flow behavior of particulate suspensions in the pressure-driven flow which is typical in industrial processing.     

A three-dimensional visualization technique applied to flow around a delta wing

The visualization of flows in two dimensions by using planar laser light sheets is a commonly used technique. We extend this technique to three dimensions by rapidly scanning the laser light sheet to obtain a set of slices of the flow around a full span delta wing. The leading edge vortices, which are marked with smoke, are unburst by tangential blowing around the leading edges at angles of attack in excess of 25°. Since the measurement period is on the order of the smallest convective time scale, we obtain a virtually instantaneous set of planar cross sections of the flow. Software based on the marching cubes algorithm is used to stack the slices and reconstruct a three-dimensional surface of the smoke-seeded fluid. This surface, which corresponds to the vortices, clearly shows the qualitative effects of blowing on the delta wing flow.     

Quantitative flow visualization using the hydraulic analogy

The current work describes the development of a non-intrusive optical method for the quantitative determination of water heights along a hydraulic jump in shooting water flows on a water table. The technique involves optically superimposing a series of alternating dark and clear fringes on the water flow. It is proposed that the fringe deviations seen under a hydraulic jump can be simulated using a series of optical prisms oriented along the direction of the hydraulic jump. The height of each prism gives the local maximum water height at the fringe location. Three types of theoretical prism configurations (isosceles flat-topped prism, scalene flat-topped prism and rounded-topped prism models) have been studied for two flow systems: shooting flow around a wedge and around a cylinder. Equations relating the physical characteristics of the deviated fringes to the height of the theoretical prism and hence the local water height are presented. The variation in water height along a hydraulic jump for flow around a wedge obtained using the optical technique has been compared with heights obtained using a depth gauge. The results were in good agreement for the range of Froude numbers studied (Fr=1.9−3.6). The rounded-topped prism model led to the best agreement with the physical measurements, within 11% throughout the range of conditions studied. The uncertainty associated with the water height determination using the optical technique is ±10%. Received: 15 September 1998/Accepted: 16 April 1999     

Vortex flow adjacent to a stationary surface

The radius of transition from an inner core of solid body rotation to an outer free vortex motion was determined via the momentum integral method.     

Application of a high accuracy finite difference technique to steady,free surface flow problems

The spatially third-order accurate QUICK finite difference technique is applied to the solution of the depth-integrated equations of motion for steady, subcritical, free surface flow in a wide, shallow, rectangular channel with and without an abrupt expansion. The conservative, control-volume discretization of the equations of motion and the use of QUICK in approximating required cell and cell face average quantities is discussed. Results presented show that it is possible to obtain stable solutions for advective free surface flows without resorting to implicit numerical smoothing.     

On interpretation of flow visualization of unsteady shear flows

Flow visualization of an unsteady wake is simulated by computer-generated streaklines and timelines, the two most commonly employed markers in laboratory flows. The resultant visualization patterns are compared with prescribed vorticity fields having constant or decreasing strength. Effects on the visualized flow of thickness and location of the injected marker, discrete (as opposed to continuous) marker elements, and different reference frames are addressed.     

Calibrated multichannel electronic interferometry for quantitative flow visualization

Calibrated multichannel electronic interferometry, a new technique for quantitative flow visualization of transient phenomena, is discussed. This technique uses an interferometer combined with diffraction gratings to generate three phase shifted interferograms simultaneously which are used to perform multichannel phase shifting. The optical system is calibrated with no phase object present using standard piezoelectric phase shifting, and this calibration information is stored as an electro-optic hologram. The calibration information is used along with the three phase-shifted interferograms that exist with a phase object present to perform time-resolved phase shifting. Examples using natural convection and separated flows are presented.     

A flow visualization technique for low-speed wind-tunnel studies

The paper describes a flow visualization technique (direct injection method) used in low-speed wind-tunnel studies. Mixture of titanium tetrachloride and carbon tetrachloride is used to produce closely spaced parallel streaklines of white smoke of long duration. The technique is described in detail and representative photographs of various flow patterns are presented.     

Clean seeding for flow visualization and velocimetry measurements

Flow visualization, particle image velocimetry (PIV), and laser Doppler velocimetry (LDV) are among the most useful tools available for experimental aerodynamics studies. Implementation of these techniques, however, requires that seed material be introduced into the flow. The undesirable qualities of the seeding material often prevent the use of flow visualization and velocimetry techniques in many test environments. This is particularly true for large-scale, closed-circuit tunnels where facility operators must weigh the risks of facility contamination, sensor damage, and safety concerns that might result from the introduction of seed particles. Identification of a practical clean seeding material that minimizes or eliminates these concerns would enable flow visualization and velocimetry techniques to be deployed in these facilities. Here, we demonstrate two seeding systems that have the potential to provide such a solution. The first system is a new concept which uses liquid carbon dioxide that can be made to form discrete particles as it expands from a high-pressure tank. PIV measurements are demonstrated in several flows, including supersonic and subsonic tunnels, using these residue-free seed particles. The second system utilizes a combination of steam and liquid nitrogen to produce an aerosol or fog that serves as flow seeding. Water- or steam-based seeding has been previously demonstrated for flow visualization in subsonic tunnels; here however, we utilize this seed material for PIV and LDV measurements as well as for flow visualization in a large supersonic tunnel.     

Ensemble-averaging and correlation techniques for flow visualization images

Flow visualization using marking techniques such as timelines provides a basis for quantitative analysis of macroscale features of unsteady flows by global ensemble-averaging and correlation techniques. In the visual-ensemble-averaging technique described herein, the timeline positions are tracked and averaged in successive images. The phase reference for the averaging process can take the form of an analog pressure, velocity, or displacement signal, or a recurring coherent portion of the image. Global correlations of the timeline patterns are obtained using the same timelines defined for the ensemble-averaging process. A new type of visual correlation function, giving the correlation between two timelines in a given image or successive images, is proposed. Preliminary results are given.     

Point-to-plane corona discharge for high-speed reacting flow visualization

We present the results of a novel technique for the high-speed visualization of a flame reaction zone using a streamer-initiated point-to-plane unipolar pulsed corona discharge. Our results show images of the flame front under conditions of natural hydrodynamic flame instability, as well as external air flow modulation induced flame instability. This technique can potentially be used as a high-speed 2-D flow visualization diagnostic tool to monitor flow instabilities in reacting and non-reacting fluids that have a density gradient. We also show that this technique does not modify the flame characteristics in any measurable way, if the high electric field region of the streamer/corona discharge is located in the downstream region.