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MethodologyMethodology
Calibration of wind tunnelCalibration of wind tunnel
Calibration of rainfall simulatorCalibration of rainfall simulator
ConclusionsConclusions
LiteratureLiterature
Portable wind and rainfall simulatorPortable wind and rainfall simulator
Main objectivesMain objectivesIntroductionIntroduction
Laboratory results of a portable wind and rainfall simulatorLaboratory results of a portable wind and rainfall simulatorW. Fister, J. B. Ries, RW. Fister, J. B. Ries, R--G. Schmidt, T. IserlohG. Schmidt, T. Iserloh
Department of Physical Geography, Trier University, GermanyDepartment of Physical Geography, Trier University, Germany
Contact:Contact: Dipl.Dipl.--Geogr. Wolfgang FisterGeogr. Wolfgang Fister; Department of ; Department of Physical Geography, Physical Geography, Trier University; Behringstrasse, 54286 Trier, Germany; PhoneTrier University; Behringstrasse, 54286 Trier, Germany; Phone: +49 / (0)651 201: +49 / (0)651 201--4512; E4512; E--mail: [email protected]: [email protected]
Soil degradation and desertification is often caused by wind and water erosion. For many years researchers have been studying both processes separately. Laboratory investigations with wind tunnels that include rainfall simulators have shown the existence of very complex interactions between wind and water erosion. These results were confirmed in recent field research, especially for semi-arid regions. For future quantification and prediction of soil desertification both processes need to be measured together.
(1)(1) Major limiting factors that prevent the development of a Major limiting factors that prevent the development of a ““naturalnatural”” turbulent turbulent boundary layer and boundary layer and ““naturalnatural”” drop fall velocities are short tunnel length and drop fall velocities are short tunnel length and low tunnel height.low tunnel height.
(2)(2) Due to the properties of full cone nozzles a homogeneous spatialDue to the properties of full cone nozzles a homogeneous spatial drop drop distribution can only be created by using more than one nozzle.distribution can only be created by using more than one nozzle.
General objective of this study is to simultaneously measure erosion rates by wind and water erosion with a combined wind and rainfall simulator operational in the field. To reach this goal following calibration measurements were conducted:
(1) Homogeneity of airflow direction(2) Homogeneity of wind speed(3) Drop size distribution and drop fall velocity(4) Spatial distribution of rainfall
Integrated rainfall simulatorWind tunnel (1)
Fig. 10: Results of airflow direction measurement near surface (1); x-axis (tunnel length); y-axis (tunnel width)
Fig. 11: Results of wind speed measurements above tarboard; 11x22 raster points; test duration 10 seconds, interval 1 second; black arrow (airflow direction); black line (position of profile)
(2) (3) (4)
Fig. 6: Woolen wiresairflow direction (1)
Fig. 7: Pietot tube wind speed (2)
Fig. 8: Laser Disdrometerdrop size/fall velocity (3)
Fig.9: Collecting pansspatial distribution (4)2 m3 m5 m
honeycomb & transition section
fan
working section
sediment catching area
Combined sediment trap
Fig. 3: Dimensional sketch of combined sediment catcher
Fig. 5: Modif. Wilson and Cooke Sampler (MWAC)Wilson & Cooke 1980• passive trap• horizontally installed• low cost• good calibration results
Fig. 1: Built up wind tunnel Fig. 2: a) Preliminary nozzle mounting b) Nozzle inside tunnel
a)
b)
Fig. 4: Guelph-Trent Wedge Trap (GTW)Nickling & McKenna-Neuman 1997• passive trap, vertically integrating• side can be opened for cleaning• low cost• good calibration results
0.13 ma)
0.35 m
0.22 m
0.02 m
wind tunnel
flow direction(air and water) 0.7 m
0.7 m
working sectionrunoff inletundercutting protection
0.6 m
gutter system
0.2 m
MWAC trapsGTW traps
a)
b)
b)
0.04 m0.01 m
drop number
drop size [mm]
fall
velo
city
[m/s
]
Drop size distribution vs. drop fall velocity
Figure 12: Drop size distribution and drop fall velocity measured by Thies Laser Disdrometer (3); test duration 1 min; intensity 130 mm/h; pressure 0.4 bar; fall heigth 0.7 m; Lechler full cone nozzle 460.608
Figure 13: Spatial Distribution of rainfall test inside tunnel with a single nozzle (4); test duration 12 min; intensity 130 mm/h; pressure 0.4 bar; fall heigth 0.7 m; Lechler full cone nozzle 460.608
tunnel sidewall
0.07 m
0.07 m collecting pan
• Complete naturally occuring drop spectra is covered
• Especially larger drops are too slow (max. fall velocity ~6 m/s) compared to natural rainfall droplets (green line)
• Concentric pattern of rainfall amount with high variations between center and rim (~50%)
• Runoff from upper sidewall increases water amount in collecting pansSpatial distribution of simulated rainfall is rather inhomogeneous
NICKLING, W. G. & MCKENNA-NEUMAN, C. (1997): Wind tunnel evaluation of a wedge-shaped aeolian sediment trap. – Geomorphology, 18, p. 333-345.
WILSON, S. J. & COOKE, R. U. (1980): Wind erosion. – In: Kirkby, M. J. & Morgan, R. P. C.: Soil erosion, Chichester.
• Deflection of airflow from parallel by sidewalls (maximum 3°)Homogeneous, parallel airflow between y=20 and y=50 cm
• Wind speed variation between 6.5 and 9 m/s
• Lowest wind speeds near floor and between x = 40-60 cm and y = 10-50 cm
• Nearly logarithmical vertical wind profile until 18 cm height
tunnel length [cm]
tunnel width [cm]
tunnel height [cm]
wind profilewind speed [m/s]
heig
ht [c
m]
Regression
Despite the problems to simulate Despite the problems to simulate ““naturalnatural”” wind and rainfall conditions wind and rainfall conditions we we expect to be able to obtain valuable quantitative information reexpect to be able to obtain valuable quantitative information regarding the garding the relative impact of wind and water erosion on soil degradation. Erelative impact of wind and water erosion on soil degradation. Especially the specially the high mobility and reproducability make this device suitable for high mobility and reproducability make this device suitable for relative inrelative in--situ investigationssitu investigations
