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Ground-based radar observations of snow stratigraphy and melt processes in the percolation facies of the Greenland ice sheet
- I.H.H. Zabel, K.C. Jezek, P.A. Baggeroer, S. P. Gogineni
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- Journal:
- Annals of Glaciology / Volume 21 / 1995
- Published online by Cambridge University Press:
- 20 January 2017, pp. 40-44
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Summer melt is a primary source of mass loss on the Greenland ice sheet. An understanding of melt processes on the ice sheet, their connection with atmospheric processes, and the redistribution of meltwater is important for ascertaining the mass balance of the ice sheet. High-resolution radar measurements made in the percolation zone of the Greenland ice sheet reveal the evolving radar signature of summer surface melting and subsequent refreezing of meltwater. A traverse over the snow surface has resulted in the first radar map of snow stratigraphy over an extended distance. The dominant sources of back-scatter in the study area are the snow surface and effectively continuous annual ice layers. We suggest applications of our results to help define the extent of the percolation zone and to discriminate between regions where surface melt is lost to the sea and those where melt refreezes nearly in place.
10 - Transition and turbulence
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- By A. Prasad, C. H. K. Williamson, W. S. Saric, T. Peacock, T. Mullin, A. Drake, R. V. Westphal, R. A. Kennelly, JR., D. M. Driver, J. H. Duncan, V. Philomin, H. Qiao, J. Kimmel, M. A. Rutgers, X.-L. Wu, W. I. Goldburg, G. Zocchi, E. Moses, A. Libchaber, D. R. Sabatino, T. J. Praisner, S. Gogineni, R. Rivir, D. Pestian, L. Goss, Y.-B. Du, P. Tong
- M. Samimy, Ohio State University, K. S. Breuer, Brown University, Rhode Island, L. G. Leal, University of California, Santa Barbara, P. H. Steen, Cornell University, New York
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- Book:
- A Gallery of Fluid Motion
- Published online:
- 25 January 2010
- Print publication:
- 12 January 2004, pp 97-107
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Summary
A new mechanism for oblique wave resonance
Despite the large body of research concerned with the near wake of a circular cylinder, the far wake, which extends beyond about 100 diameters downstream, is relatively unexplored, especially at low Reynolds numbers. We have recently shown that the structure of the far wake is exquisitely sensitive to free-stream noise, and is precisely dependent on the frequency and scale of the near wake; indeed it is shown that the presence of extremely low-amplitude peaks in the free-stream spectrum, over a remarkably wide range of frequencies, are sufficient to trigger an “oblique wave resonance” in the far wake.
We show, in the upper photograph of Fig. 1, a nonlinear interaction between oblique shedding waves generated from upstream (to the left) and 2–D waves amplified downstream from free-stream disturbances (in the central region). We use the “smoke-wire” technique (placed 50 diameters down-stream), and the wake is viewed in planview, with flow to the right. This two-wave interaction triggers a third wave, namely an “oblique resonance wave” at a large oblique angle, to grow through nonlinear effects (in the right half of the photograph), in preference to the original two waves. If smoke is introduced 100 diameters downstream, in the lower photograph (under slightly different conditions), then all that is seen is a set of such large-angle oblique resonance waves.
This work is supported by the Office of Naval Research.
Visualization of different transition mechanisms
The sequence of photos in Figs. 1(a)-1(d) illustrates the different types of boundary-layer transitions that occur as a function of Tollmien-Schlichting (T-S) wave amplitude and fetch.
1 - Jets and mixing layers
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- By M. M. Koochesfahani, P. E. Dimotakis, M. Gharib, P. Derango, E. Villermaux, H. Rehab, E. J. Hopfinger, D. E. Parekh, W. C. Reynolds, M. G. Mungal, T. Loiseleux, J.-M. Chomaz, T. F. Fric, A. Roshko, S. P. Gogineni, M. M. Whitaker, L. P. Goss, W. M. Roquemore, S. Wernz, H. F. Fasel, S. Gogineni, C. Shih, A. Krothapalli
- M. Samimy, Ohio State University, K. S. Breuer, Brown University, Rhode Island, L. G. Leal, University of California, Santa Barbara, P. H. Steen, Cornell University, New York
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- Book:
- A Gallery of Fluid Motion
- Published online:
- 25 January 2010
- Print publication:
- 12 January 2004, pp 1-10
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Summary
Laser-induced fluorescence (LIF) diagnostics and highspeed, real-time digital image acquisition techniques are combined to map the composition field in a water mixing layer. A fluorescent dye, which is premixed with the lowspeed freestream fluid and dilutes by mixing with the highspeed fluid, is used to monitor the relative concentration of high-speed to low-speed fluid in the layer.
The three digital LIF pictures shown here were obtained by imaging the laser-induced fluorescence originating from a collimated argon ion laser beam, extending across the transverse dimension of the shear layer, onto a 512–element linear photodiode array. Each picture represents 384 contiguous scans, each at 400 points across the layer, for a total of 153 600 point measurements of concentration. The vertical axis maps onto 40 mm of the transverse coordinate of the shear layer, and the horizontal axis is time increasing from right to left for a total flow real time of 307 msec. The pseudocolor assignment is linear in the mixture fraction (ξ) and is arranged as follows: red-unmixed fluid from the low-speed stream (ξ=0); blue-unmixed fluid from the high-speed stream (ξ=1); and the rest of the spectrum corresponds to intermediate compositions.
Figures 1 and 2, a single vortex and pairing vortices, respectively, show the composition field before the mixing transition. The Reynolds number based on the local visual thickness of the layer and the velocity difference across the layer is Re=1750 with U2/U1=0.46 and U1=13 cm/sec. Note the large excess of high-speed stream fluid in the cores of the structures.