Rodney D. W. Bowersox

Two-component molecular tagging velocimetry utilizing NO fluorescence lifetime and NO 2 photodissociation techniques in an underexpanded jet flowfield

We report the application of molecular tagging velocimetry (MTV) toward two-component velocimetry as demonstrated in an underexpanded free jet flowfield. Two variants of the MTV technique are presented: 1) electronic excitation of seeded nitric oxide (NO) with gated fluorescence imaging (fluorescence lifetime) and 2) photodissociation of seeded NO2 followed by NO fluorescence imaging (NO2 photodissociation). The seeded NO fluorescence lifetime technique is advantageous in low-quenching, high-velocity flowfields, while the photodissociation technique is useful in high-quenching environments, and either high- or low-velocity flowfields due to long lifetime of the NO photoproduct. Both techniques are viable for single-shot measurements, with determined root mean squared results for streamwise and radial velocities of ~5%. This study represents the first known application of MTV utilizing either the fluorescence lifetime or the photodissociation technique toward two-component velocity mapping in a gaseous flowfield. Methods for increasing the spatial resolution to be comparable to particle-based tracking techniques are discussed.

Molecular Tagging Using Vibrationally Excited Nitric Oxide in an Underexpanded Jet Flowfield

We report a laser diagnostic technique which relies on planar laser-induced fluorescence of vibrationally excited nitric oxide (NO v=1 ) molecules produced from the 355 nm photodissociation of seeded NO 2 for molecular tagging velocimetry applications. The technique was applied toward an axisymmetric highly underexpanded jet flowfield to yield single-component (streamwise) velocity maps. Detection of the photodissociated NO v=1 molecules would be valuable in flow environments where molecular tagging velocimetry would be highly desirable, but where there are also significant background concentrations of NO. The technique would also be valuable in high-quenching and/or low-velocity flow conditions due to the long-lived nature of the photodissociated NO molecules. Single-shot streamwise velocity uncertainties were about 5% and could be lowered by increasing signal to noise. In addition, the vibrational relaxation of NO was explored in support of a U.S. Air Force Office of Scientific Research Multidisciplinary University Research Initiative project and it was found that the vibrational decay of NO was heavily dependent on collisional vibrational relaxation with oxygen atom formed through NO 2 photodissociation.

Simultaneous velocity and temperature measurements in gaseous flow fields using the VENOM technique

We present an initial demonstration of simultaneous velocity and temperature mapping in gaseous flow fields using a new nitric oxide planar laser-induced fluorescence-based method. The vibrationally excited NO monitoring (VENOM) technique is an extension of two-component velocimetry using vibrationally excited NO generated from the photodissociation of seeded NO(2) [Appl. Opt. 48, 4414 (2009)], where the two sequential fluorescence images are obtained probing two different rotational states to provide both velocity and temperature maps. Comparisons to computational fluid dynamics simulations show that the initial VENOM measurements provide good velocity and temperature maps in the relatively high-density regions of the flow, where the rms uncertainties are approximately 5% for velocity and 9% for temperature.

Flow Properties of a Supersonic Turbulent Boundary Layer with Wall Roughness

Anexperimental study ofthe ine uenceofsurfaceroughness on themean and turbulente owproperties of a highspeed (M =2:9;Re/m=2:0 ££ 10 7 ) turbulent boundary layer e ow was performed. Six wall topologies, including a smooth and e ve rough surfaces consisting of three random sand-grain plates and two uniformly machined plates (k + =100‐570), were tested. Mean e ow measurements included surveys of the velocity and density. Turbulence quantities included direct measurements of the kinematic velocity turbulence intensities, mass e ux turbulence intensities, thekinematicReynoldsshearstress,thecompressibleReynoldsshearstressandthedensity-transversevelocity e uctuation correlation. The trends in the mean e ow, observed for incompressible e ow, were found to hold forthepresentstudywhenVanDriestIIscalingwasused.Kinematicstatisticalturbulente owpropertieswerefound to scale by local mean quantities. Conversely, turbulent e ow statistical properties with an explicit thermodynamic dependence did not scale by local mean quantities and had a strong linear dependence on roughness height. Roughnesswasfound to extend theregion where inner scaling held toward larger values of y + for thecompressible Reynolds shear stress, the x- and y-velocity component turbulence intensities, the x-component of the mass e ux turbulence intensity, and the density-transverse-velocity correlation.

Sonic Injection into a Mach 5.0 Freestream Through Diamond Orifices

An experimental study to characterize the near-field (x/d<8.0) flow for sonic air injection through 15-deg half-angle diamond-shaped orifices at four incidence angles (10, 27.5, 45, and 90 deg) and two total pressures (0.10 and 0.46 MPa) into a high Reynolds number (53 x 10 6 ) Mach 5.0 freestream was performed. A 90-deg circular injector, with the same exit port area and total pressures, was examined for comparative purposes. The experimental methods included surface oil flow visualization, shadowgraph photography, Mie scattering flow visualization, pressure-sensitive paint, and a pitot-cone five-hole pressure probe. Flowfield documentation, jet penetration, and shock-induced total pressure loss were derived from these data. Attachment of the interaction shock wave was found to depend on both incidence angle and injector pressure. Penetration correlations were developed for both the diamond and circular injectors. An approximate analysis indicated that the shock-induced total pressure loss decreased with decreasing incidence angle and injection pressure, and the largest losses were incurred by the 90-deg circular injector.

Compressible turbulence measurements in a high-speed high-Reynolds-number mixing layer

To assess the significant physics associated with compressible turbulence, extensive multiple overheat cross/normal-wire, shadowgraph image processing and conventional probe surveys were obtained in a two-dimensional, supersonic, free mixing layer, which consisted of Mach 1.8 air (Re/m=7×10 6 ) injected tangentially into a Mach 4.0 freestream (Re/m=67×10 6 ). A turbulence transformation was developed that allowed direct measurement of the total Reynolds shear stress. Profiles of three-dimensional turbulent shear, apparent mass, and heat flux data were acquired.

Combined Laser Doppler Velocimetry and Cross-Wire Anemometry Analysis for Supersonic Turbulent Flow

A synergistic laser Doppler velocimetry and multiple overheat cross-film anemometry analysis was developed to allow for direct measurements of the mean and turbulent flow properties in a supersonic turbulent flow. The present technique facilitated, for thin-layer, nonreacting flows, the measurement of the Reynolds (time) and Favre (mass-weighted time) averaged turbulent shear stress, heat flux, and apparent mass flux, without the usual ad hoc assumption of negligible static pressure fluctuations. The mean velocity, mass flux, and density were also acquired. The directly measured mean flow and turbulence results, in a Mach 2.8 wall boundary layer (Re θ = 1.12 x 10 4 ), were found to compare very well with numerical predictions based on the k-w two-equation (for the Favre approach) and the compressible apparent mass mixing length extension algebraic (for the Reynolds approach) turbulence models.

Influence of Curvature-Driven Favorable Pressure Gradient on Supersonic Turbulent Boundary Layer

The influence of a wall curvature-driven favorable pressure gradient on the turbulent and mean flow properties of a Mach 2.9 boundary layer (Re x = 1.23 × 10 7 ) were investigated using laser Doppler velocimetry. A zero-pressure-gradient boundary layer (Re x = 1.35 x 10 7 ) was also mapped for comparative purposes. In addition to the typical mean and turbulent statistical properties, the data in this study were acquired with the specific goal of resolving the mean strain rates in all three coordinate directions; these data allowed for measurement of the extra Reynolds shear-stress production terms. The maximum magnitude of the distortion was 0.1, which indicated that the pressure gradient was strong. The expected stabilizing effect on the turbulence intensities was observed. Near the wall, the kinematic Reynolds shear stresses were reduced by approximately 75%. Above y/δ 0.4 the favorable pressure gradient induced a negative Reynolds shear stress, whereas the main strain rate remained positive. The reduced shear-stress levels were attributed to negative overall production and the use of a body-intrinsic coordinate system for data collection

Experimental Investigation of the Role of Downstream Ramps on a Supersonic Injection Plume

An experimental study was conducted to investigate penetration and plume expansion enhancement of a discrete low-angled (25 deg) supersonic (M =1.9) injection into a supersonic (M =2.9) crosse ow. The enhancement was achieved by injecting the low-angled jet parallel to a compression ramp. Seven compression ramp cone gurations were studied. The jet-ramp interaction enhancement mechanisms included baroclinic torque vorticity, ramp spillage vorticity, bulk compression, and the Magnus force. Shadowgraph photography was used to identify shock structures. Measurements of mean e ow properties quantie ed the e owe eld total pressure losses. Mie scattering images were used to qualitatively assess the e owe eld and to quantify the plume size, trajectory, and concentration decay rate. The results indicated that up to a 22% increase in penetration, a 39% plume expansion (» mixing), and a 27% increase in the concentration decay rate, with a corresponding 17% increase in total pressure loss, can be achieved by injection over a compression ramp as compared with low-angled injection alone.

Transverse Injection Through Diamond and Circular Ports into a Mach 5.0 Freestream

Sonic transverse gaseous injection into a Mach 5.0 freestream flow was numerically simulated using two-equation and detached-eddy turbulence models. Circular- and diamond-shaped injectors were investigated in this study. The numerical simulations were compared with available experimental results and it was determined that both the Reynolds-averaged Navier-Stokes and detached-eddy simulation models captured the secondary flow structure. A detailed comparison of the secondary structures was performed for both injectors. Two new vortex structures of practical importance were observed in the diamond-injector flowfield. First, a leading-edge mixing mechanism was identified. Second, a trapped lateral counter-rotating vortex pair was produced. These new structures were observed in both Reynolds-averaged Navier-Stokes and detached-eddy simulation simulations. The detached-eddy simulations indicated that the large-scale structures observed in the plume/wake region of the flowfield were more organized in the diamond-injector test case. To better understand the secondary flow advection mechanism, the magnitudes of the terms in the compressible vorticity transport equation were compared. The diamond-injector structured-grid Reynolds-averaged Navier-Stokes solution was used as a baseline for this study. The inviscid compressibility, vortex-stretching, and baroclinic-torque terms were dominant. Downstream of the barrel-shock region, the baroclinic term was found to diminish when compared with the other inviscid terms. Planar-averaged results for the transport quantities confirmed this behavior. Vortex stretching was found to persist the longest.