Publications

Abstract not provided.

Abstract not provided.

Abstract not provided.

A subscale experiment has been constructed using fins mounted on one wall of a transonic wind tunnel to investigate the influence of fin trailing vortices upon downstream control surfaces. Data were collected using a fin balance instrumenting the downstream fin to measure the aerodynamic forces of the interaction, combined with stereoscopic particle image velocimetry to determine vortex properties. The fin balance data show that the response of the downstream fin essentially is shifted from the baseline single-fin data dependent upon the angle of attack of the upstream fin. Freestream Mach number and the spacing between fins have secondary effects. The velocimetry shows the increase in vortex strength with upstream fin angle of attack, but no variation with Mach number can be discerned in the normalized velocity data. Correlations between the force data and the velocimetry indicate that the interaction is fundamentally a result of an angle of attack superposed upon the downstream fin by the vortex shed from the upstream fin tip. The Mach number influence arises from differing vortex lift on the leading edge of the downstream fin even when the impinging vortex is Mach invariant. Copyright Clearance Center, Inc.

The low-frequency meander of a trailing vortex shed from a tapered fin installed on a wind tunnel wall has been studied using stereoscopic particle image velocimetry in the near-wake at Mach 0.8. Distributions of the instantaneous vortex position reveal that the meander amplitude increases with downstream distance and decreases with vortex strength, indicating meander is induced external to the vortex. Trends with downstream distance suggest meander begins on the fin surface, prior to vortex shedding. Mean vortex properties are unaltered when considered in the meandering reference frame, apparently because turbulent fluctuations in the vortex shape and strength dominate positional variations. Conversely, a large peak of artificial turbulent kinetic energy is found centered in the vortex core, which almost entirely disappears when corrected for meander, though some turbulence remains near the core radius. Turbulence originating at the wind tunnel wall was shown to contribute to vortex meander by energizing the incoming boundary layer using low-profile vortex generators and observing a substantial increase in the meander amplitude while greater turbulent kinetic energy penetrates the vortex core. An explanatory mechanism has been hypothesized, in which the vortex initially forms at the apex of the swept leading edge of the fin where it is exposed to turbulent fluctuations within the wind tunnel wall boundary layer, introducing an instability into the incipient vortex core.

An experiment using fins mounted on a wind tunnel wall has examined the proposition that the interaction between axially-separated aerodynamic control surfaces fundamentally results from an angle of attack superposed upon the downstream fin by the vortex shed from the upstream fin. Particle Image Velocimetry data captured on the surface of a single fin show the formation of the trailing vortex first as a leading-edge vortex, then becoming a tip vortex as it propagates to the fin's spanwise edge. Data acquired on the downstream fin surface in the presence of a trailing vortex shed from an upstream fin may remove this impinging vortex by subtracting its mean velocity field as measured in single-fin experiments, after which the vortex forming on the downstream fin's leeside becomes evident. The properties of the downstream fin's lifting vortex appear to be determined by the total angle of attack imposed upon it, which is a combination of its physical fin cant and the angle of attack induced by the impinging vortex, and are consistent with those of a single fin at equivalent angle of attack.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Particle image velocimetry (PIV) data have been acquired using three different experimental configurations in the far-field of the interaction created by a transverse supersonic jet exhausting from a flat plate into a transonic crossflow. The configurations included two-component PIV in the centerline streamwise plane at two overlapping downstream stations, as well as stereoscopic PIV in both the same streamwise plane and in the crossplane. All measurement planes intersected at a common line. Data from both two-component measurement stations and the stereoscopic streamwise configuration agreed to within the estimated uncertainty, but data from the crossplane exhibited reduced velocity and turbulent stress magnitudes by a small but significant degree. Subsequent reprocessing of the data in nominally the same manner using a newer software package brought all values into close agreement with each other, but produced turbulent stresses substantially higher than those from the first software package. The error source associated with the choice of software was traced to the use of image deformation in the newer software to treat velocity gradients, which synthetic PIV tests show yields a more accurate result for turbulence measurements even for gradients within the recommended limits for classical PIV. These detailed comparisons of redundant data suggest that routine methods of uncertainty quantification may not fully capture the error sources of an experiment.

Abstract not provided.

Abstract not provided.

A sub-scale experiment has been conducted to study the trailing vortex shed from a tapered fin installed on a wind tunnel wall to represent missile configurations. Stereoscopic particle image velocimetry measurements have been acquired in the near-field for several locations downstream of the fin tip and at different fin angles of attack. The vortex's tangential velocity is found to decay with downstream distance while its radius increases, but the vortex core circulation remains constant. Circulation and tangential velocity rise greatly for increased fin angle of attack, but the radius is approximately constant or slightly decreasing. The vortex axial velocity is always a deficit, whose magnitude diminishes with downstream distance and smaller angle of attack. No variation with Mach number can be discerned in the normalized velocity data. Vortex roll-up is observed to be largely complete by about four root chord lengths downstream of the fin trailing edge. Prior to this point, the vortex is asymmetric in the tangential velocity but the core radius stays nearly constant. Vortical rotation draws low-speed turbulent fluid from the wind tunnel wall boundary layer into the vortex core, which appears to hasten vortex decay and produce a larger axial velocity deficit than might be expected. Self-similarity of the vortex is established even while it is still rolling up. Attempts to normalize vortex properties by the fin's lift coefficient proved unsuccessful.

Error in Particle Image Velocimetry (PIV) interrogation due to velocity gradients in turbulent flows was studied for both classical and advanced algorithms. Classical algorithms are considered to be digital cross-correlation analysis including discrete window offsets and, for the present work, advanced algorithms are those using image deformation to compensate for velocity gradients. Synthetic PIV simulations revealed substantial negative biases in the turbulent stress for classical algorithms even for velocity gradients within recommended PIV design limits. This bias worsens if the distribution of velocity gradients has a nonzero mean, and error in the mean velocity may be introduced as well. Conversely, advanced algorithms do not exhibit this bias error if the velocity gradients are linear. Nonlinear velocity gradients increase the error in classical algorithms and a significant negative bias in the turbulent stress arises for the advanced algorithm as well. Two experimental data sets showed substantially lower turbulent stresses for the classical algorithm compared with the advanced algorithm, as predicted. No new experimental design rules for advanced algorithms are yet proposed, but any such recommendation would concern second-order velocity derivatives rather than first order.

A sub-scale experiment has been constructed using fins mounted on one wall of a transonic wind tunnel to investigate the influence of fin trailing vortices upon downstream control surfaces. Data are collected using a fin balance instrumenting the downstream fin to measure the aerodynamic forces of the interaction, combined with stereoscopic Particle Image Velocimetry to determine vortex properties. The fin balance data show that the response of the downstream fin essentially is shifted from the baseline single-fin data dependent upon the angle of attack of the upstream fin. Freestream Mach number and the spacing between fins have secondary effects. The velocimetry shows that the vortex strength increases markedly with upstream fin angle of attack, though even an uncanted fin generates a noticeable wake. No variation with Mach number can be discerned in the normalized velocity data. Correlations between the force data and the velocimetry suggest that the interaction is fundamentally a result of an angle of attack superposed upon the downstream fin by the vortex shed from the upstream fin tip. The Mach number influence arises from differing vortex lift on the leading edge of the downstream fin even when the impinging vortex is Mach invariant.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.