Search

The fluid-structure interactions of a blunt fin mounted over a compliant panel in Mach 2 flow were investigated. The test geometry consisted of a thin, flexible panel fixed on all edges with a blunt fin with a hemispherically rounded leading edge mounted in a manner that the leading edge of the fin overhung the panel. The blunt fin was also pivoted to several angles of attack with respect to the oncoming flow. This allowed for the study of the dynamic system created by the interactions between the compliant panel and the blunt fin geometries. Both rigid and compliant panel models were used in order to provide a comparison between the flow with and without the dynamics of the compliant panel. High-speed schlieren and surface oil flow visualizations showed that the time-averaged flow remained almost entirely unchanged between the rigid and compliant panels. The instantaneous flow fields, however, showed a highly dynamic system where the compliant panel induced oscillatory shock waves that were both stationary and freely moving. Simultaneous high-speed schlieren visualization and stereo digital-image correlation showed a strong linkage between the motion of the shock waves and the deformation of the panel. This was demonstrated through frequency analysis and modal decomposition of the panel deformation and shock motions. [ABSTRACT FROM AUTHOR]

In order to cryopreserve functional engineered tissues (ETs), the microstructure of the extracellular matrix (ECM) should be maintained, as well as the cellular viability since the functionality is closely related to the ECM microstructure. Since the post-thaw ECM microstructure is determined by the deformation of ETs during cryopreservation, freezing-induced deformation of ETs was measured with a newly developed quantum dot (QD)-mediated cell image deformetry system using dermal equivalents as a model tissue. The dermal equivalents' were constructed by seeding QD-labeled fibroblasts in type I collagen matrices. After 24 h incubation, the ETs were directionally frozen by exposing them to a spatial temperature gradient (from 4[degrees]C to -20[degrees]C over a distance of 6 mm). While being frozen, the ETs were consecutively imaged, and consecutive pairs of these images were two-dimensionally cross-correlated to determine the local deformation during freezing. The results showed that freezing induced the deformation of ET, and its magnitude varied with both time and location. The maximum local dilatation was 0.006 [s.sup.-1] and was always observed at the phase change interface. Due to this local expansion, the unfrozen region in front of the freezing interface experienced compression. This expansion-compression pattern was observed throughout the freezing process. In the unfrozen region, the deformation rate gradually decreased away from the freezing interface. After freezing/thawing, the ET experienced an approximately 28% decrease in thickness and 8% loss in weight. These results indicate that freezing-induced deformation caused the transport of interstitial fluid, and the interstitial fluid was extruded. In summary, the results suggest that complex cell-fluid-matrix interactions occur within ETs during freezing, and these interactions determine the post-thaw ECM microstructure and eventual post-thaw tissue functionality. [DOI: 10.1115/1.4000875] Keywords: quantum dot-mediated tissue deformation measurement, cryopreservation, cryomedicine, dermal equivalents, post-thaw functionality

An iterative eduction procedure has been developed to study coherent structures in blunt-base cylinder wakes at 0 and 10 deg angles of attack to an M = 2.46 freestream flow. The eduction procedure uses an initial wavelet pattern to find candidate coherent structures of a particular scale and then uses the average intensity pattern of the educed structures to create an updated pattern. This iterative procedure continues until the average structure converges, indicating that a self-consistent intensity pattern has been described. Using this eduction procedure, the average appearance of the structures is described. Moreover, a convection velocity measurement is made by directly tracking individual coherent structures, unlike conventional imaging-based convection velocity measurements, which are not conditioned on coherent structures. The evolutionary trends of the structures are also quantified. The coherent structure averages show many of the same trends found in more conventional autocorrelation analysis. For example, the structures are inclined toward the freestream flow direction, and the angle of this inclination is affected by the recompression and reattachment processes. Convection velocity measurements show that the convection velocity is dependent on the transverse position of the structure in the shear layer, particularly for small structures. Finally, the coherent structures evolve in such a way that they become poorly representative of the average coherent structure pattern within 5-15 [micro]s after being first identified.

The near wake of a blunt-base cylinder at 10[degrees] angle-of-attack to a Mach 2.46 free-stream flow is visualized at several locations to study unsteady aspects of its structure. In both side-view and end-view images, the shear layer flapping grows monotonically as the shear layer develops, similar to the trends seen in a corresponding axisymmetric supersonic base flow. The interface convolution, a measure of the tortuousness of the shear layer, peaks for side-view and end-view images during recompression. The high convolution for a septum of fluid seen in the middle of the wake indicates that the septum actively entrains fluid from the recirculation region, which helps to explain the low base pressure for this wake compared to that for a corresponding axisymmetric wake. [DOI: 10.1115/1.2062727]

Acetone planar laser-induced fluorescence (PLIF) was used to study the supersonic flow over an axisymmetric base with mass bleed. Detailed side-view and global end-view images were obtained in the recirculation regions, shear layer, and trailing wake. For the three bleed rates studied, the instantaneous behavior of the flowfield deviated substantially from the expected mean flow, especially for bleed rates below the optimal case. Large pockets of unmixed bleed fluid dominated the flowfield for the low and intermediate bleed rates, even in the trailing wake, highlighting the unsteady nature of the flowfield. The power-on condition for the high-bleed-rate case was the most temporally stable flowfield, due to the high momentum of the bleed, jet. A multiplateau stirring effect was prevalent for all cases, consistent with previous compressible shear layer findings, and the effect increased with increasing bleed rate and distance downstream. Probability density functions (PDFs) of PLIF intensity indicated a similar mixing character between the low and intermediate bleed rates in the near wake and for all bleed rate cases in the trailing wake. The multiplateau stirring effect and unmixed bleed fluid pockets were represented well in the mixing PDFs.

The near wake of a blunt-base cylinder at 10-deg angle of attack to a Mach 2.46 freestream flow is studied using Mie scattering flow visualization. Large-scale structures are clearly evident in side-view images of the windward and lateral parts of the shear layer but not in the leeward shear layer. Autocorrelation analysis shows that the average structures grow substantially in side views as the shear layer develops on the lateral plane and the windward shear layer, but not the leeward shear layer. End-view images show that a septum originates from the leeward part of the shear layer and extends to the windward part of the shear layer during reattachment, dividing the wake into two lobes after reattachment and causing a short reattachment length. The structures in end-view images are largest in the windward part of the shear layer and increase in size as the shear layer develops. The structures are inclined toward the leeward direction in the lateral plane, indicating circumferential flow from windward to leeward in the main shear layer. The appearance of the images suggests that this circumferential flow causes mass buildup on the leeward side of the wake, prompting the formation of the septum.

Skin-friction measurements for a normal-shock/boundary-layer interaction with several recirculating flow control methods have been conducted in a planar Mach 1.4 wind tunnel. The skin friction has been measured along the spanwise centerline and downstream of the interaction using the laser interferometry skin-friction (LISF) technique, which optically detects the rate of thinning of an oil film applied to the test surface. Velocity profiles measured by laser Doppler velocimetry at the same locations as the LISF measurements in the interaction were also used to evaluate the skin friction. Comparison of the two measurement techniques for skin friction is found to show reasonable agreement. Various configurations of the mesoflap arrays of different shapes and thicknesses were examined, and the results were compared to cases of both a solid wall with no control mechanisms and conventional recirculating passive flow control with a porous plate. Of the various mesoflap arrays tested, one mesoflap array provided higher skin friction downstream of the interaction, and as such tends to have better recovery from flow separation. However, all values of skin friction for the mesoflap arrays and the porous plate were found to be lower than for the solid-wall reference case. As such, the flow downstream of these control systems can be more susceptible to separation for this particular condition, although the control systems may reduce viscous drag in external flows.

An investigation into the dynamic nature of supersonic base-pressure fluctuations was conducted. Two axisymmetric models, a blunt and a boattailed afterbody, were studied at a freestream Mach number of 2.46 and zero angle of attack. High-frequency pressure measurements were recorded with both radially and circumferentially positioned transducers. Blunt afterbody measurements showed rms pressure fluctuations on the order of 5% of the average base pressure. Power spectral density (PSD) estimates at the two outermost radial locations for the blunt afterbody revealed that a significant portion of the pressure fluctuation energy is contained in a peak centered near 850 Hz (Sr = 0.094). RMS pressure fluctuations on the boattalled afterbody were on the order of 4.3% of the average base pressure. The PSD estimates at the two outermost locations contained a more defined and larger peak centered near 800 Hz (Sr = 0.089) as compared to the blunt base. The two outermost radial locations on both afterbodies are subjected to similar pressure histories and, hence, display large values of coherence at low frequencies. The pressure history at the center position is not well correlated to those at the two outermost locations, resulting in low values of coherence. Possible mechanisms for the substantial pressure fluctuation energy contained at low frequencies are pulsing or flapping of the recirculation region, whereas higher-frequency energy could be attributed to large-scale turbulent structures in the shear layer.

We have performed high-resolution [N.sub.2] coherent anti-Stokes Raman spectroscopy (CARS) measurements using a modeless dye laser (MDL) as the Stokes beam source to determine the effects of a reduction in mode noise on the accuracy and precision of the method. These results are compared with previous research that employed a conventional broadband dye laser (CBDL) as the Stokes beam source. A new spectral-fitting procedure was developed to avoid starting-point bias in the least-squares fitting results, which possibly had altered the previous measurements. Single-shot measurements of pressure were performed in a static-pressure vessel over the range of 0.1-4.0 atm to examine the pressure sensitivity of the technique. The precision of these measurements is a measure of the baseline noise level of the system, which sets the detection limit for flow-field pressure fluctuations. Centerline measurements of pressure and temperature in an underexpanded jet ([M.sub.j] = 1.85) were also used to determine the performance of the technique in a compressible flow field. Our study represents the first known application, to our knowledge, of a MDL CARS system in a low-temperature, low-pressure supersonic environment. Improvements in accuracy for mean single-shot measurements and increased precision were found for pressure vessel conditions above 1.0 atm. For subatmospheric pressure vessel conditions (0.1-1.0 atm) and the underexpanded jet measurements, there was a decrease in accuracy and precision compared with the CBDL results. A comparison with the CBDL study is included, along with a discussion of the MDL system behavior. OCIS codes: 280.2490, 300.6230, 300.6420, 300.6450.

The velocity field in the base region of a circular cylinder with a length-to-radius ratio of 3.0 aligned at a 10-deg angle of attack to a nominal Mach 2.5 freestream has been investigated experimentally. The objective is to better understand the mechanisms that govern the characteristics and development of three-dimensional, compressible base flows. Laser Doppler velocimetry was used to measure both mean velocity components and turbulence statistics. The results reveal all expected base flow features, including a low-speed recirculation region, a separated shear layer, a reattachment region, and a trailing wake. The distance from the base to the rear stagnation point in this three-dimensional base flow is reduced by 55% as compared to the corresponding axisymmetric base flow. The shear layer in the leeward region of the flow turns sharply toward the radial centerline, resulting in a shift of the flow stagnation location toward the windward side of the base flow. In addition, the shear layer in the leeward plane thickens at a substantially faster rate than in either the windward or side planes. The turbulent stresses in the windward region of the shear layer are larger in magnitude than observed in either the leeward or side regions. The peak stresses measured in this three-dimensional base flow are located in the reattachment region, in direct contrast to axisymmetric results, where peak stresses were measured in the shear layer just upstream of reattachment.

High-vector density planar velocity fields were obtained for an incompressible mixing layer using particle image velocimetry (PIV) for the purpose of determining spatial correlations of velocity fluctuations and linear stochastic estimates of the large-scale structures. The linear stochastic estimates were calculated based on the deformation tensor. The velocity ratio of the mixing layer was 0.575, and the density ratio was unity. At the location where the PIV images were obtained, [Re.sub.x] = 1.8 x [10.sup.5], [Re.sub.[delta].sub.[omega]] = 1.1 x 104, and the pairing parameter was Rx/[lambda] = 8. Preliminary hot-film measurements showed the mixing-layer mean velocity and turbulence profiles to be self-similar at this location. The mixing layer was found to be largely two-dimensional with well-organized Brown-Roshko roller structures and braids (Brown, G. L., and Roshko, A., 'On Density Effects and Large Structures in Turbulent Mixing Layers' Journal of Fluid Mechanics, Vol. 64, 1974, pp. 775-814). Measured velocity fluctuations and Reynolds shear stress data agree well with previous experimental results. The [R.sub.u'u'] correlation is a horizontally oriented ellipse, with a slight inclination from horizontal. The [R.sub.v'v'] correlation is a vertically oriented ellipse. The linear stochastic estimate of a roller suggests that these structures are very slightly elliptical, with a horizontal major axis. The linear stochastic estimate of a braid suggests that the braids are obliquely oriented with stagnation occurring at a point.

The flow characteristics of a three-dimensional, compressible, turbulent boundary layer have been investigated experimentally. The three dimensionality was generated by inclining a cylindrical afterbody at 10-deg angle of attack to a Mach 2.45 freestream. The objective was to determine the mechanisms that govern the growth and behavior of pressure-driven, three-dimensional, compressible, turbulent boundary layers. Laser Doppler velocimetry was used to determine mean velocity components and turbulence statistics. The results show a significantly thicker boundary layer on the leeward side of the body than in the windward region. This circumferential variation in boundary-layer thickness is creased by the pressure-driven circumferential flow, which provides a mass surplus in the low-pressure, leeward region and a mass deficit in the high-pressure, windward portion of the boundary layer. In addition, the pressure discontinuity at the angular junction and the axial pressure gradient also play a role in the boundary-layer growth. Turbulent normal and shear stresses peak very near the wall, with an initial streamwise peak forming at the interaction of the oblique shock/expansion fan with the boundary layer. The highly turbulent fluid on the windward side of the body is transported toward the leeward region by the circumferential flow in the boundary layer.

It is possible to show the mean and time-resolved pressure measurement capability of the dual-pump coherent anti-Stokes Raman scattering (CARS) technique by undertaking measurements in a room-temperature gas cell at pressures from 0.1 atm to 20 atm. Simultaneous pressure and temperature measurement capability can be demonstrated in an underexpanded jet. Temperature can be simultaneously measured on both time-averaged and single-shot bases with very good results throughout a range of 100 K to 300 K.

Interactions between the expansion-compression waves and the free shear layer increase fluctuations and angular orientation of the large-scale structures in the shear layer of a supersonic wind tunnel. High-speed cinematography is used to resolve compressible planar mixing layer structures at different pressure conditions. The fluctuations in the unmatched pressure conditions are more than that in the matched conditions. A nondimensional pressure gradient parameter influences the degree of angular fluctuations.

A sophisticated particle image velocimetry system is versatile in estimating the time-resolved flowfield structure in the wake of a base cavity of a two-dimensional base in a subsonic flow. Among other things, the system analyzed the base cavity drag reduction to be because of movement of the base surface beyond the low-pressure wake vortices.

The fully developed region of the mixing layer reveals that peak values of streamwise and spanwise turbulence intensities are similar. Decreased peak values of transverse turbulence intensity, normalized primary Reynolds shear stress and normalized turbulent kinetic energy are also obtained from the results.

Offers suggestions and hints to help new engineering faculty in their quest for tenure. Areas addressed include: (1) teaching; (2) research (considering publishing and obtaining funding); (3) service; and (4) letters of recommendation. (JN)

The motion of the counter-rotating, crossflow vortices dominates the downstream velocity field of the transverse jet injected into a supersonic flow (TJISF) jet plume. The streamwise mean flow vorticity field and the in-plane velocity vectors of the crossflow planes indicate that the motion of each crossflow vortex is, at first, constrained by the other vortex and by the wall. As the vortices move apart and rise away from the wall, the crossflow vortices behave more like two free vortices.

The flow along the afterbody and in the base region of a circular cylinder with a length-to-radius ratio of 3.0 aligned at a 10-deg angle of attack to a nominal Mach 2.5 freestream has been investigated experimentally. The objective is to better understand the mechanisms that control base flow for supersonic bodies with a nonzero-angle-of-attack orientation. Laser Doppler velocimetry measurements were conducted in the incoming boundary layer to quantify the initial conditions at the onset of three-dimensional behavior. Schlieren and Mie scattering visualizations were obtained to discern governing flow features and to image the large-scale turbulent structures of this separated flow. Surface oil-streak visualizations were obtained to determine the three dimensionality of the afterbody surface flow and to deduce the base surface flowfield. Pressure-sensitive paint measurements were completed to determine the spatial evolution of surface pressure along the cylindrical body at angle of attack and to determine the change in base pressure caused by inclination of the body. Results provide evidence of expected mean-flow features, including base-corner expansions, separated shear layer development, recompression shocks, and a turbulent wake. No evidence of lee-side flow separation was detected along the afterbody. However, a strong secondary circumferential flow, which develops along the afterbody due to pressure gradients on its surface, results in the entrainment of fluid into the base region from the leeward portion of the flow. The average base pressure ratio measured for the angle of attack case is 48.4% lower than that measured for zero angle of attack, resulting in a significant increase in base drag for cylindrical objects inclined at angle of attack.

Mean and instantaneous measurements of pressure, temperature, and density have been acquired in an optically accessible gas cell and in the flow field of an underexpanded sonic jet by use of the high-resolution [N.sub.2] coherent anti-Stokes Raman scattering (CARS) technique. This nonintrusive method resolves the pressure- and temperature-sensitive rotational transitions of the v = 0 [right arrow] 1 [N.sub.2] Q-branch to within [Delta][Omega] = 0.10 [cm.sup.-1]. To extract thermodynamic information from the experimental spectra, theoretical spectra, generated by a [N.sub.2] spectral modeling program, are fit to the experimental spectra in a least-squares manner. In the gas cell, the CARS-measured pressures compare favorably with transducer-measured pressures. The precision and accuracy of the single-shot CARS pressure measurements increase at subatmospheric conditions. Along the centerline of the underexpanded jet, the agreement between the mean CARS P/T/[Rho] measurements and similar quantities extracted from a Reynolds-averaged Navier-Stokes computational fluid dynamic simulation is generally excellent. This CARS technique is able to capture the low-pressure and low-temperature conditions of the M = 3.4 flow entering the Mach disk, as well as the subsonic conditions immediately downstream of this normal shock. [C] 2000 Optical Society of America OCIS codes: 280.2490, 300.6450, 300.6420.

The mixing characteristics of an underexpanded sonic jet injected into a Mach 1.6 crossflow are studied experimentally. Shadowgraph photography and planar laser-induced fluorescence from acetone are used to obtain temporally resolved flowfield visualizations of a side view of the barrel shock region, a side view of the downstream plume, and an end view cross section of the plume. Instantaneous images, mean and standard deviation images, and image-intensity probability density functions (PDFs) are used to study the mixing produced by large-scale turbulent structures. PDFs show that time-averaged mixture fraction measurements greatly overestimate the actual level of instantaneous mixing in the flowfield. Comparisons of the instantaneous images and PDFs show the role of large-scale rolling structures and jetlike plumes in transporting coherent packets of fluid across the three-dimensional shear layer formed between the jet and crossflow. Significant instantaneous mixing in this flowfield occurs in the wake region downstream of the barrel shock region and below the jet centerline. The counter-rotating streamwise vortex pair in the jet plume plays an important role in the scalar mixing processes because it transports jet fluid down toward the wake and entrains crossflow fluid from below up into the jet.