ERDC/CHL CHETN-IV-55
March 2003
The flow table reproduces complex flow phenomena such as flow separation, flow entrainment,
turbulence, three-dimensional (3-D) flow structure, and cross-channel transport. For U.S. Army
Corps of Engineers (USACE) projects where these processes are thought to be significant, the flow
situation can be clarified by fabricating a scale model of the actual bathymetry and shore boundaries
for use on the flow table. Flow patterns are visualized using the aforementioned techniques so that
complex flow/boundary processes are better understood. Changes to bathymetry or upstream
boundaries are easily simulated, and the impact is immediately observed. This type of flow table
study helps assure that more extensive study tools, and corresponding proposed solutions, address
the dominant causative hydrodynamic factors. For example, if strong 3-D circulation is evident in
the flow table model, it may be necessary to employ a 3-D hydrodynamic numerical model rather
than a two-dimensional (2-D), depth-averaged numerical model to describe adequately the
consequences of engineering modifications. Also, the flow table can efficiently screen potential
project alternatives so that follow-on detailed studies are more focused and cost-effective.
In addition to studies supporting existing or planned USACE projects, the flow table can also be
used to study fundamental flow processes such as 3-D flow, boundary layers, and the velocity
structure in turbulent jets. Because many complex flow phenomena such as separation and
turbulence are reliably reproduced in small-scale physical models, the flow table can be used as a
validation tool in conjunction with development of advanced hydrodynamic numerical models that
incorporate these features.
FLOW TABLE DETAILS: The flow table, shown schematically in Figure 1, is approximately the
size of a billiards table. Flow of water from the constant head tank (HT) is controlled by a valve that
assures a steady flow rate feeding the upstream basin (IN). Water flows across the horizontal
(2.44 m H 1.22 m) glass bottom of the flow table and spills over the adjustable-height weir into the
catchment tank (OUT) which in turn overflows into the reservoir (RES). The reservoir is detached
from the flow table to isolate vibrations of the pump as water is recirculated to the head tank. The
discharge rate onto the flow table is controlled by an adjustable valve (A1) and flow meter. Under
operating conditions the flow table holds approximately 0.91 m3 (240 gal) of water plus an
additional 0.08 m3 (21 gal) for every inch of water depth over the glass bottom.
Flow velocities on the water table are measured with a two-component laser Doppler fiber-optic
probe mounted on a horizontal traversing system beneath the 19-mm-thick glass bottom. Laser
beams emanating from the probe pass through the bottom glass and intersect at a known position
(adjustable) in the water column. Velocities are determined at the beam crossing point; hence, the
measurement system is totally nonintrusive so the flow is in no way disturbed by the measuring
system. Traversing of the LDV probe is computer controlled in two horizontal directions allowing
automatic recording of velocity at precise, predetermined locations throughout the testing area.
Usually, velocity data are collected on a uniformly-spaced grid with the probe collecting a time
series of instantaneous velocities at each grid point before moving to the next location. The velocity
time series in the two orthogonal horizontal directions at each point are averaged to provide two
components of the velocity vector. Sampling rate and duration are adjustable, but typically data are
collected at 100 Hz for a duration of 10 sec at each point. Because the flow is quasi-steady, the final
result is a map of velocity vectors detailing the flow throughout the measurement region as
illustrated in Figure 2.
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