Cody Brownell

Quantitative planar imaging of turbulent buoyant jet mixing

Planar Rayleigh scattering provides quantitative mixing measurements in the developing region of axisymmetric turbulent helium jets issuing into air. The measurements focus on the relatively near field, in which the jets are primarily momentum driven. The imaging parameters are specified to ensure high spatial resolution. The mean jet fluid concentration fields attain self-similarity within the measurement region, though the forms of the mole fraction profiles indicate a reduction in turbulent transport at the jet outer boundary, arising from the reduced jet fluid density. Nevertheless, jet-like scaling pertains for the concentration fields. Mass fraction fluctuations on the jet centreline attain the expected asymptotic value of ≈23 % of the centreline mass fraction values. The scalar dissipation rates, however, show an axial decay rate that is slower than theoretical predictions. The two-dimensional extent of the measurements also allows spatial filtering similar to that inherent in large-eddy simulations (LESs). The results confirm that fluctuation levels and scalar dissipation rates determined for the filtered fields are reduced as the effective resolution is reduced, but while the fluctuation profiles for the filtered fields are similar for the different filter sizes, the forms of the scalar dissipation profiles are highly dependent on filter size. These latter results in particular are of a form that will be useful for grid-dependent assessments of LES results.

Planar laser imaging of differential molecular diffusion in gas-phase turbulent jets

Planar laser Rayleigh scattering yields quantitative, two-dimensional measurements of differential diffusion in a turbulent propane-helium jet issuing into air. The jet exit Reynolds number ranges from 1000 to 3000, corresponding to estimated outer-scale Reynolds numbers from 4300 to 13 000. Using a technique originally proposed by Bilger and Dibble [Combust. Sci. Technol. 28, 161 (1982)], the imaging measurements allow direct determination of a normalized scalar difference quantity ξ. For the lower Re, significant differential diffusion develops in the pretransitional portion of the flow. Downstream of the turbulent transition, radial profiles of mean ξ take on a characteristic form, with an excess of the less-diffusive propane on the jet boundary. This characteristic form is independent of Reynolds number, and is thus apparently independent of the degree of differential diffusion in the pretransition range. Evolution of the ξ fields in the turbulent part of the flow is surprisingly consistent with the m...

In Situ Velocity Measurements in the Near-Wake of a Ship Superstructure

Velocity measurements in a ship airwake are obtained in situ aboard a 108 ft naval training vessel. The measurements and analyses aremotivated by the need for validation data for airwake computational fluid dynamics simulations. Three-component anemometers are placed above the bow of the ship and at numerous locations above a flight deck at the stern of the ship. Data are presented for a direct headwind (nominally 0 deg wind-over-deck). The mean velocity field shows a clear structure to the flow, dominated by a recirculation region in the near-wake of a hangar-like backward-facing step. The location of this primary vortex and the reattachment point on the flight deck are estimated. Reynolds stresses are presented to quantify the turbulent fluctuations, which are required for the prediction of unsteady loading on rotorcraft operating in this environment. Significant anisotropy is measured in the wake, both within the primary vortex and in the far field. The peak Reynolds shear stress is located in the recirculation region, while the streamwise normal stress is found to increase with height throughout the measurement domain. Finally, autoand two-point velocity correlations from the flight deck provide an estimate of flow scales, showing the potential influence of turbulence on piloted helicopter operations.

USNA Ship Air Wake Program Overview (Invited)

This paper provides an overview of a multi-year research project that involves the systematic investigation of ship air wakes using an instrumented United States Naval Academy (USNA) YP (Patrol Craft, Training). The objective is to validate and improve Computational Fluid Dynamics (CFD) tools that will be useful in determining ship air wake impact on naval rotary wing vehicles. This project is funded by the Office of Naval Research and includes extensive coordination with Naval Air Systems Command. Currently, ship launch and recovery wind limits and envelopes for helicopters are primarily determined through at-sea in situ flight testing that is expensive and frequently difficult to schedule and complete. The time consuming and potentially risky flight testing is required, in part, because computational tools are not mature enough to adequately predict air flow and wake data in the lee of a ship with a complex superstructure. The top-side configuration of USNA YPs is similar to that of a destroyer or cruiser, and their size (length of 108 ft and above waterline height of 24 ft) allows for collection of air wake data with a Reynolds number that is the same order of magnitude as that of modern naval warships, an important consideration in aerodynamic modeling. A dedicated YP has been modified to add a flight deck and hangar structure to produce an air wake similar to that on a modern destroyer. Three axis acoustic anemometers, fog generators and an inertial measurement unit have been installed. Repeated testing on the modified YP is being conducted in the Chesapeake Bay, which allows for the collection of data over a wide range of wind conditions. Additionally, a 4% scale model of the modified YP has been constructed and tested in the 42×60×120 inch USNA wind tunnel. The project involves USNA midshipmen who are participating in test planning, collecting and analyzing data, and in CFD modeling, providing the midshipmen with valuable professional and research experience. Comparison of YP in situ data with similar data from wind tunnel testing and CFD simulations shows reasonable agreement for a headwind condition and for wind 15° off the starboard bow.

Influence of the Atmospheric Surface Layer on a Turbulent Flow Downstream of a Ship Superstructure

AbstractThis paper describes a set of turbulence measurements at sea in the area of high flow distortion in the near-wake and recirculation zone behind a ships superstructure that is similar in geometry to a helicopter hangar/flight deck arrangement found on many modern U.S. Navy ships. The instrumented ship is a 32-m-long training vessel operated by the United States Naval Academy that has been modified by adding a representative flight deck and hangar structure. The flight deck is instrumented with up to seven sonic anemometers/thermometers that are used to obtain simultaneous velocity measurements at various spatial locations on the flight deck, and one sonic anemometer at bow mast is used to characterize inflow atmospheric boundary conditions. Data characterizing wind over the deck at an incoming angle of 0° (head winds) and wind speeds from 2 to 10 m s−1 obtained in the Chesapeake Bay are presented and discussed. Turbulent statistics of inflow conditions are analyzed using the Kaimal universal turbu...

Measurements of multiple mole fraction fields in a turbulent jet by simultaneous planar laser-induced fluorescence and planar Rayleigh scattering

This paper presents two-dimensional measurements of all individual mole fractions in a three-species, non-reacting turbulent flow, using planar laser-induced fluorescence (PLIF) and planar laser Rayleigh scattering. The flow is an axisymmetric jet of acetone and helium in an air coflow. PLIF measures the acetone mole fraction, while Rayleigh scattering measures a linear combination of the acetone and helium mole fractions. The simultaneous implementation of these techniques allows for the calculation of the helium and air mole fraction fields. The results of this diagnostic method are being used for the study of multicomponent molecular transport effects in turbulent fluid mixing.

Velocity measurements in a ship airwake with crosswind

In situ air velocity measurements in the near wake of a Navy training ship are presented for an inflow of 15 to starboard. This data is required for the validation of ship airwake simulations, which are used to determine the launch and recovery envelopes for shipborne rotorcraft and for use in piloted flight simulations. The measurements are taken primarily above an aft flight deck, which sits immediately behind a step-like hangar structure. A description of the mean flow structure is included, as well as the Reynolds stresses at numerous points along the ship centerline. Comparisons are made between the present 15 case and the case of a direct headwind, presented previously. Compared to the 0 inflow condition, the flow symmetry is clearly broken with a cross-wind. The port and starboard sides of the deck have very different mean flow profiles and turbulent stress components. An updraft is visible over much of the starboard side of the flight deck, which is not found on the port side, or on either side under a headwind. Along the centerline, the streamwise normal component of the turbulent stresses are much larger in the cross-wind case than in the headwind case, while the shear components have similar magnitudes. This suggests that the wake turbulence is similar, but that in the cross-wind case the flight deck is more heavily burdened by inflow fluctuations from the atmosphere.

Experimental investigations of mixing in turbulent jets with buoyancy

This paper presents measurements of molecular mixing in turbulent buoyant jets of helium issuing into air, using planar laser imaging of the jet uid mole fraction elds. The o ws considered are nominally momentum-driven, so buoyancy eects are presumed to be conned to the small scales of the o w. The measurements focus on the developing region of the jet, which is of particular interest to o ws with combustion. The results suggest that buoyancy aects the details of the evolution of the mixing eld even while the mean eld maintains scaling properties consistent with non-buoyant jets. Specically , the mean jet uid mole fraction proles, and the proles of mole fraction uctuations, show a sharper jet/ambient uid interface relative to non-buoyant jets, possibly indicative of reduced entrainment. The mole fraction uctuations along the jet centerline are also weaker than those reported in non-buoyant jets. Statistics of jet uid mole fractions, conditional on spatial location within the jet, provide one view of the spatial evolution of the jet, while the eld nature of the measurements allows us to investigate the eect of spatial ltering, such as that inherent in large-eddy simulations (LES), on the elds of the turbulent uctuations. The results are intended to inform ongoing eorts to model the mixing process in o ws with density dierences, such as combustion systems.

Scale Relations and Spatial Spectra in a Differentially Diffusing Jet

Planar Rayleigh scattering is used to study differential diffusion in a turbulent propanehelium jet. Using a technique proposed by Bilger & Dibble, the differential diffusion variable ξ, a normalized scalar difference quantity, is extracted from the imaging data [1]. Both statistical and structural aspects of differential diffusion in turbulence are studied, including the distribution of the mean and variance of ξ, the ξ power spectra, and the thickness and distribution of differential diffusion dissipation structures. Comparisons are made to existing data from numerical simulations, particularly in the case of the scalar spectra.

Planar imaging measurements of three scalars in a turbulent jet

Two-dimensional measurements of all individual mole fractions in a three-species nonreacting turbulent o w are obtained using planar Rayleigh scattering and planar laser-induced uorescence (PLIF). The o w is an axisymmetric jet of acetone and helium in an air coo w. PLIF measures the acetone mole fraction, while Rayleigh scattering measures a linear combination of the acetone and helium mole fractions. The simultaneous implementation of these techniques allows for the calculation of the helium and air mole fraction elds. The acetone and helium are initially mixed, but have diering distributions downstream. This allows investigation of the eect of molecular transport properties, namely dierences in molecular diusivit y and density, on turbulent mixing.