Diameter Water Tunnel Research Articles

Visualization of a helicopter rotor hub wake

Experiments were conducted on a 1:17 model-scale helicopter rotor hub in the Pennsylvania State University Applied Research Laboratory 12-inch diameter Water Tunnel. Qualitative methods were used to investigate the interaction between the rotor hub and a canonical engine pylon upper surface. Oil paint visualization showed flow separation on the engine pylon and a high degree of interaction between the hub and pylon flows. Fluorescent dye injection revealed significant differences in the unsteady flow character between advancing and retreating sides of the rotor hub. Stereo particle image velocimetry was used in the wake of an isolated hub configuration to quantify the spatial and phase-dependent variations in wake velocity, revealing a wake bias towards the advancing side and, unexpectedly, a phase-averaged unsteadiness bias towards the retreating side. In general, a previously unavailable broad view of the rotor hub flowfield was obtained.

Acoustic cavitation localization in reverberant environments

Cavitation detection and localization techniques generally require visual access to the fluid field, multiple high-speed cameras, and appropriate illumination to locate cavitation. This can be costly and is not always suitable for all test environments, particularly when the bubble diameter is small or duration is short. Acoustic detection and localization of cavitation can be more robust and more easily implemented, without requiring visual access to the site in question. This research utilizes the distinct acoustic signature of cavitation events to both detect and localize cavitation during experimental water tunnel testing. Using 22 hydrophones and the processing techniques plane-wave beamforming and Matched-Field Processing (MFP), cavitation is accurately and quickly localized during testing in a 12” diameter water tunnel. Cavitation is induced using a Nd:YAG laser for precise control of bubble location and repeatability. Accounting for and overcoming the effects of reflections on acoustic localization in acoustically small environments is paramount in water tunnels, and the techniques employed to minimize error will be discussed.

Combined polymer and microbubble drag reduction on a large flat plate

Drag-reduction experiments with combined injection of high-molecular-weight long-chained polymers and microbubbles were conducted on a 3.1 m long flat plate model in the 1.22 m diameter water tunnel at the Applied Research Laboratory of the Pennsylvania State University. Combined gas injection upstream of polymer injection produced, over a wide range of test conditions, higher levels of drag reduction than those obtained from the independent injection of polymer or microbubbles alone. These increased levels of drag reduction with combined injection were often greater than the product of the drag reductions obtained by the independent constituents, defined as synergy. We speculate that the synergy is a result of the gas-layer-induced extension of the polymer-alone initial diffusion zone in combination with the increased drag reduction by microbubbles. This increased length of the initial zone layer, consistent with high drag reduction, can significantly increase the persistence of the drag reduction and may improve the outlook for practical application.

Microbubble skin friction reduction on an axisymmetric body under the influence of applied axial pressure gradients

The influence of both a favorable and an adverse applied axial pressure gradient on microbubble-induced skin friction reduction was examined. An 87 mm diameter, 632 mm long model equipped with a 273 mm long cylindrical force balance was employed. Experiments were carried out in a 305 mm diameter water tunnel, at free-stream speeds of 4.6, 7.6, 10.7, 13.7, and 16.8 m/sec. Air was injected at rates as high as 12×10−3 m3/sec. Measurement of the static pressure along the body with gas injection demonstrated that gas injection did not alter the pressure gradient and that the flow remained axisymmetric. Reductions in skin friction for the zero pressure gradient case agreed well with the earlier results of Deutsch and Castano [Phys. Fluids 29, 3590 (1986)]. The adverse-gradient-induced separation of the boundary layer for speeds at and above 7.6 m/sec, for air injection rates in excess of 5.0×10−3 m3/sec. The favorable gradient strongly inhibited the drag reduction mechanism [47].

The Structure of a Three-Dimensional Tip Vortex at High Reynolds Numbers

The tip vortex structure of a three-dimensional hydrofoil at high Reynolds number was measured experimentally in both the 48 in. (1.22 m) diameter water tunnel and the 48 in. (1.22 m) wind tunnel at the Applied Research Laboratory, Penn State. The flow on and near the hydrofoil was measured in both facilities using a number of flow visualization techniques and laser velocimetry. The downstream tip vortex was measured with a three-component laser velocimeter at a number of streamwise positions. A detailed planar mapping of the flow in the water tunnel was completed near the region of cavitation inception. The effect of roughness on the downstream tip vortex was investigated. In this paper, these measurements are presented with discussion relative to the occurrence of cavitation.

Chaotic motions of forced and coupled galloping oscillators

Numerical simulations of the behavior of a periodically forced square galloping oscillator yielded results showing that the behavior of this system has similarities with the behavior of the circle map. Lock-in regions were found to be ordered as rational numbers obtained by the Farey construction. At the transition from quasiperiodic to chaotic motion corresponding to a winding number equal to the golden mean, the fractal dimension of the critical line was found to be 0.864, that is, to within 0.5% of the theoretical value for the circle map. Numerical studies were also performed on an autonomous system consisting of two elastically coupled galloping oscillators. Preliminary tests conducted in the 0.3 m diameter water tunnel of the David Taylor Research Center and in the CBT wind tunnel demonstrated the feasibility of the experimental study of both the forced oscillator and the autonomous coupled oscillators described in the paper. Research on these systems is being conducted in collaboration with the Center for Computational and Applied Mathematics, NIST, and the David Taylor Research Center, U.S. Department of the Navy.

Chaotic motions of forced and coupled galloping oscillators

Numerical simulations of the behavior of a periodically forced square galloping oscillator yielded results showing that the behavior of this system has similarities with the behavior of the circle map. Lock-in regions were found to be ordered as rational numbers obtained by the Farey construction. At the transition from quasiperiodic to chaotic motion corresponding to a winding number equal to the golden mean, the fractal dimension of the critical line was found to be 0.864, that is, to within 0.5% of the theoretical value for the circle map. Numerical studies were also performed on an autonomous system consisting of two elastically coupled galloping oscillators. Preliminary tests conducted in the 0.3 m diameter water tunnel of the David Taylor Research Center and in the CBT wind tunnel demonstrated the feasibility of the experimental study of both the forced oscillator and the autonomous coupled oscillators described in the paper. Research on these systems is being conducted in collaboration with the Center for Computational and Applied Mathematics, NIST, and the David Taylor Research Center, U.S. Department of the Navy.

Laminar boundary-layer transition on a heated underwater body

A large (3.05 m long × 0.32 m diameter) heated-surface, axisymmetric body, designed for transition research in a 1.22 m diameter water tunnel is described. Boundary-layer transition data are presented as functions of the heating power supplied to the body and the total concentration of free-stream particulate matter in the water. Body surface temperatures range from 0 to 25°C over the ambient water temperature, and the total heat supplied ranges from 0 to 93.3 kW. Transition-arclength Reynolds numbers are found to vary from 4.5 × 106 for the body operating cold to 3.64 × 107 for the maximum heat level considered. The concentration of free-stream particles is shown to affect the transition Reynolds number. These particles range in diameter from 10 to 70 μm and their concentration ranges from less than 5 to 198 particles per cm3. The decrease in transition Reynolds number due to to the higher concentration of particles is of order 30%.

Boundary-layer transition on a body of revolution

The laminar-to-turbulent transition region was measured over a body length Reynolds number range of 9.55- 47.7 million on a large, streamlined, axisymmetric body sting mounted in the Applied Research Laboratory's 1.22 m diameter water tunnel. Theoretical calculations of the transition Reynolds numbers for this body shape were carried out and the results were correlated with the experimental data. It is shown that good correlation can be achieved if en is chosen as the transition criterion in the linear stability calculations, where n is an empirical number, which depends on the freestream turbulence intensity. It was found that this number approached 9 for those test velocities where the freestream turbulence intensity is on the order of 0.1%. OUNDARY-layer transition on axisymmetric bodies is of practical importance because its location influences the total skin frictional drag of the body. It is well recognized that moving the transition point downstream to larger body arc length positions results in drag reduction. Consequently, much of the current research is dealing with boundary-layer control concepts such as temperature differentials at the wall and surface suction. The Applied Research Laboratory at The Pennsylvania State University is actively engaged in transition research from both the analytical and experimental points of view. Experiments are generally designed for the 1.22 m diameter water tunnel located at this laboratory. This facility is par- ticularly suited for such work because of the high water velocities that can be attained (up to 19.8 m/s), the ability to test large bodies ( — 0.50 m in diameter and -3.5 m long), and the fact that settling section turbulence management results in test section turbulence intensities that are quite low ( — 0.1 %). Under normal operating water temperatures (25 °C) this water tunnel has an upper unit Reynolds number of approximately 21 million/m. By way of preliminary preparation for planned experiments on bodies with various types of boundary-laye r control, the work described in this paper was carried out. After a can- didate body shape was selected and a test model fabricated, experiments were performed in the 1.22 m diameter water tunnel to acquire baseline transition data and in-tunnel body pressure distributions. Except for the hydrodynamic shape of this body, no boundary-laye r control was employed in these experiments. The results of the pressure distribution measurements are presented elsewhere.! In addition to the experimental results, theoretical calculations for the growth of linear disturbances in the laminar boundary layer are also described for the test body. These calculations were performed using the methods of Gentry and Wazzan,2 where the body pressure distribution (which is a required input) was calculated from potential flow theory and corrected for inviscid tunnel interference ef- fects.3'4 The transition location may be deduced from this