Stereo Particle Image Velocimetry System Research Articles

A novel caudal-fin-inspired hourglass-shaped self-agitator for air-side heat transfer enhancement in plate-fin heat exchanger

A novel caudal-fin-inspired hourglass-shaped self-agitator installed inside a plate-fin heat exchanger has been experimentally investigated to characterize its thermo-hydraulic performance. Because of fluid-structure interaction between the air and the caudal-fin-inspired hourglass-shaped self-agitator, there is periodic conversion between fluid kinetic energy and elastic strain energy. Vortices generated by the self-sustained vibration of the self-agitator enhance flow mixing and thus heat transfer performance. Experiments were conducted at a wide range of Reynolds numbers from 400 to 10,000, which covered the laminar-transitional-turbulent regime. Experimental correlations of pressure drop as a function of a dimension parameter, friction factor, and Nusselt number have been proposed. Mutual coupling motions and effects of multiple-row flapping caudal-fin-inspired hourglass-shaped self-agitators in parallel and tandem configurations were studied by using a high-speed camera. A stereo Particle Image Velocimetry system was used to conduct detailed flow field measurements to quantify the flow mixing level. Parameter studies for the stream-wise distance (i.e., the pitch) between the neighboring caudal-fin-inspired hourglass-shaped self-agitators have been investigated to exploit the best performance of the heat exchanger equipped with caudal-fin-inspired hourglass-shaped self-agitators. For the chosen plate-fin heat exchanger and assigned working conditions, the best heat transfer performance was obtained with twelve-row self-agitators with a pitch of 12.5 mm, which caused a 200% increase in the Nusselt number over the clean channel at the same Reynolds number. However, the best overall performance was obtained with six-row self-agitators with a pitch of 25 mm, which caused a 68% enhancement in thermal-hydraulic characteristics compared to the clean channel at the same Reynolds number.

Laboratory experiments on three-dimensional deformable granular landslides on planar and conical slopes

Landslides of subaerial and submarine origin may generate tsunamis with locally extreme amplitudes and runup. While the landslides themselves are dangerous, the hazards are compounded by the generation of tsunamis along coastlines, in enclosed water bodies, and off continental shelves and islands. Tsunamis generated by three-dimensional deformable granular landslides were studied on planar and conical hill slopes in the three-dimensional NEES tsunami wave basin at Oregon State University based on the generalized Froude similarity. A unique pneumatic landslide tsunami generator (LTG) was deployed to control the kinematics and acceleration of the naturally rounded river gravel and cobble landslides to simulate broad ranges of landslide shapes and velocities along the slope. Lateral and overhead cameras are used to measure the landslide shapes and kinematics, while acoustic transducers provide the shape of the subaqueous deposits. The subaerial landslide shape is extracted from the camera images as the landslide propagates under gravity down the hill slope, and surface reconstruction of the landslide is conducted using the stereo particle image velocimetry (PIV) system on the conical hill slope. Subaerial landslide surface velocities are measured with a planar PIV system on the planar hill slope and stereo PIV system on the conical hill slope. The submarine deposits are characterized by the runout distances and the deposit thickness distributions. Larger cobbles are observed producing hummock type features near the maximum runout length. These unique laboratory landslide experiments serve to validate deformable landslide models as well as provide the source characteristics for tsunami generation.

Flow Control by Dynamic Vane Vortex Generators Based on Piezoceramic Actuators

This paper describes a study of dynamical vane vortex generators in a flow over a flat plate. Fluidic vortex generators are more effective when operated dynamically. Thus, it is the aim of this study to find out whether mechanical vortex generators are also superior under dynamic operating conditions. The motion of the vortex generators is generated by piezoceramic actuators constructed in a bimorph configuration, which consists of a carbon-fiber bar covered with piezoceramic face actuators. The actuators exploit the longitudinal piezoelectric effect (d 33 effect), they are operated in resonance to reach the required displacement and generate a sinusoidal motion of the vortex generators. Vortex generators and actuators were integrated into a flat plate in a low-speed wind tunnel. A stereo particle image velocimetry system was used to record phase-locked flowfields that were analyzed using vortex classification methods. It was found that the transient development of the vortex core position and circulation is very different from that of static vanes. While vortices from static vortex generators are able to survive over a considerable distance, the vortices from dynamically driven ones decay faster. It is argued that the dynamic vortices have a greater ability to reorganize the momentum in the turbulent boundary layer.

Application of a Stereo PIV System for Investigations of Flow Fields in Towing Tank and Cavitation Tunnel

A newly installed submersible stereo PIV system, designed for the use in the towing tank and operating perpendicular to the main flow direction, is described. This Stereo Particle Image Velocimetry system allows the contactfree, 3-dimensional determination of velocity fields in the towing tank and cavitation tunnel. Flow conditions can be analyzed with a resolution of a few thousand points simultaneously, with a frame rate of up to 15 pictures per second, providing a good insight into flow characteristics without disturbing the flow. The paper will describe two applications of the system that have already been carried out (propeller in open water condition and in the cavitation tunnel) and will discuss merits and disadvantages of the new system. The potentials for validation of CFD calculations will be shortly discussed. Future plans for further applications of the system will be introduced.

A211 軸対称衝突噴流の三次元渦構造の抽出

A scanning stereo particle image velocimetry system was applied to measure the three-dimensional distribution of three-component velocity in an axisymmetric impinging jet. The jet formed by a round nozzle with an exit diameter of D=5 mm impinges vertically on the flat plate that was located at 45D from the nozzle. The jet Reynolds number was set at Re D ?? 1000 and a measurement region was set to contain the stagnation region. Appling Linear-Stochastic-Estimation to the measurement result, characteristic vortex structure in impinging region and time variations of that was estimated. It was confirmed that the structure was convected to streamwise direction with time passage through the vicinity of stagnation point, and was compressed axially and stretched to radial direction.

Inclined cross-stream stereo particle image velocimetry measurements in turbulent boundary layers

This work can be viewed as a reprise of Head & Bandyopadhyay's (J. Fluid Mech. vol. 107, p. 297) original boundary-layer visualization study although in this instance we make use of stereo particle image velocimetry (PIV), techniques to obtain a quantitative view of the turbulent structure. By arranging the laser light-sheet and image plane of a stereo PIV system in inclined spanwise/wall-normal planes (inclined at both plane, instantaneous swirl patterns exhibit a predisposition for counter-rotating vortex pairs (arranged with an ejection at their confluence). Such arrangements are consistent with the hairpin packet model. Correlation and scaling results show outer-scaling to be the correct way to quantify the characteristic spanwise length scale across the log and wake regions of the boundary layers (for the range of Reynolds numbers tested). A closer investigation of two-point velocity correlation contours indicates the occurrence of a distinct two-regime behaviour, in which contours (and hence streamwise velocity fluctuations) either appear to be ‘attached’ to the buffer region, or ‘detaching’ from it. The demarcation between these two regimes is found to scale well with outer variables. The results are consistent with a coherent structure that becomes increasingly uncoupled (or decorrelated) from the wall as it grows beyond the logarithmic region, providing additional support for a wall–awake description of turbulent boundary layers.

Reconstruction of large coherent structures from SPIV measurements in a forced turbulent mixing layer

A procedure is suggested here for reconstructing the time variation of a three-dimensional (3D) coherent velocity field, based on applying Least Square Method to a very limited number of phase-locked measurements. The measurements are performed in a spanwise plane of a forced turbulent mixing layer by employing the stereo particle image velocimetry system. The forcing is produced by oscillating two- and three-dimensional (3D) flappers placed at the edge of the splitter plate. The feasibility and validity of the procedure for velocity field reconstruction are checked by using Hot Wire Measurements. Very clear patterns are observed of two types of coherent structures: spanwise K-H billows (rolls) and streamwise vortices. These are due to primary and secondary instabilities and their time evolution is presented.

C305 スキャニング・ステレオPIVによる軸対称衝突噴流の三次元速度場計測

A scanning stereo particle image velocimetry system was applied to measure the three-dimensional distribution of three-component velocity in a round impinging jet. A water jet formed by a round nozzle with an exit diameter of D=5 mm was diagnosed by the current technique. The jet impinges vertically on the flat plate that was located at 45D from the nozzle. The jet Reynolds number was set at Re D≈l000 and a measurement region was set to contain the stagnation region. A measurement volume (-80 x 80 x 80 mm 3) containing 50 velocity planes was scanned in 131.25 ms, which was sufficiently short to capture the instantaneous vortical structures. The iso-vorticity surfaces depict vortical structures in the stagnation region. The budget equation for the mean-square vorticity fluctuations was evaluated.