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Michael S. Tsurikov and Noel T. Clemens
Recent experiments in turbulent flows, particularly in the field of combustion, have used simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) to examine the structure of the velocity field and the scalar field. In such experiments, an important issue is the resolution required to compute such quantities as strain rate and scalar dissipation accurately. To do this, one must resolve the finest turbulent scale actually present in the flow. According to classical turbulence theory, this scale is the Kolmogorov scale, h, at which the kinetic energy of the flow is dissipated into heat. Later work postulates two other scales, the strain-limited vorticity (ln) and diffusion (lD) scales. For the case of a round jet, for instance, ln is approximately equal to 6h. There is no current consensus, however, whether it is really necessary to resolve these finest scales to compute the previously mentioned quantities accurately.
The present study aims to address this issue by using simultaneous PIV and PLIF to study a flow where the finest scales are large enough to be resolved by the optical diagnostics. The schematic below shows the experimental setup and the fine-scale turbulence facility. This facility produces an axisymmetric coflowing turbulent jet, and measurements are taken at a downstream location where the turbulent scales are large. Velocity information is obtained from 2-D particle image velocimetry (PIV). The tracer particles are generated by a theatrical fog machine and seeded into the coflow; they are subsequently entrained by the jet as it develops. The conserved scalar (concentration) field is obtained by planar laser induced fluorescence (PLIF) imaging of acetone vapor; the vapor is seeded into the jet in a three-stage bubbler. The jet gas is saturated with acetone vapor, and is heated to equalize densities at the jet exit to avoid negative buoyancy effects. Two laser beams, one green (532nm) and one ultraviolet (266nm), are used for the optical diagnostics. These are combined, formed into vertical sheets approximately 1.5 inches high, and passed into the jet facility. Two cameras, one for PIV and one for PLIF, acquire data and store it on personal computers for later processing.
Measurements were conducted in a jet with a local Reynolds number of 8700 (based on the 5% velocity full width). The PIV images were processed to yield two components (x and y) of the velocity field. This field was then differentiated to obtain contour plots of out-of-plane vorticity, principal strain rate, and kinetic energy dissipation. The PLIF images were processed to yield the scalar dissipation rate field. A sample set of data is shown below.
Strong correlation is evident between high scalar dissipation, areas of compressive strain, and areas of high kinetic energy dissipation. This is to be expected, since it is known that scalar dissipation layers align orthogonal to compressive strain, and kinetic energy dissipation can be expected to accompany scalar dissipation. No such correlations are evident in the vorticity field, since vorticity tends to align itself with extensive strain (which could be orthogonal to the plane of measurement). The vorticity is concentrated in thick, somewhat circular regions, suggesting the presence of tube-like vortical structures (which are "sliced" in cross- section by the laser sheet), and "sandwiching" of scalar dissipation layers between regions of positive & negative vorticity is evident. The thickness of the scalar dissipation structures is at least 2h, while the corresponding kinetic energy dissipation structures are much thicker. The PIV data were processed at resolutions of 1, 2, and 4 Kolmogorov scales to examine the effect of processing resolution on the statistics. No change in vorticity statistics was found as the resolution was worsened. However, the highest magnitudes of strain and kinetic energy dissipation are underpredicted at the coarser resolutions. The structures found in the kinetic energy dissipation field exhibit a wide range of topologies, from line-like to almost circular, while the scalar dissipation field is found to consist of only thin, line-like structures. The latter is evidence of the sheet-like topology of the 3-D scalar dissipation field that has been observed previously; the line-like structures seen in the above imaging is the 2-D projection of the 3-D structure.
Publications:
- The structure of dissipative scales in axisymmetric turbulent gas-phase jets. M.S. Tsurikov and N.T. Clemens, 40th AIAA Aerospace Sciences Exhibit, Reno, NV, 2002.
- Scalar/velocity imaging of the fine scales in gas-phase turbulent jets. M.S. Tsurikov and N.T. Clemens, 39th AIAA Aerospace Sciences Exhibit, Reno, NV, 2001.
- High-resolution PIV/PLIF measurements of a gas-phase turbulent jet. M.S. Tsurikov, J.E. Rehm, and N.T. Clemens, 37th AIAA Aerospace Sciences Exhibit, Reno, NV, 1999.
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