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CROSS-SECTION PERIODICITY OF TURBULENT GRAVEL-BED RIVER FLOWS. M.J. Franca & U. Lemmin RCEM, 6 th October 2005. OUTLINE. Instrumentation (ADVP) River measurements Bed forms Mean velocity Turbulence production Conclusions. ACOUSTIC DOPPLER VELOCITY PROFILER (ADVP). ADVP configuration. - PowerPoint PPT Presentation
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M.J. Franca & U. Lemmin
RCEM, 6th October 2005
CROSS-SECTION PERIODICITY OF TURBULENT GRAVEL-BED RIVER FLOWS
OUTLINE
- Instrumentation (ADVP) - River measurements- Bed forms- Mean velocity- Turbulence production- Conclusions
ACOUSTIC DOPPLER VELOCITY PROFILER (ADVP)
- Acoustic sonar is based on the echo backscattered by moving targets.
- It allows the measurement of quasi-instantaneous 3D velocity profiles.
ADVP configurationdeployable structure
RIVER MEASUREMENTSInvestigation in the Swiss river Chamberonne under stationary and shallow water conditions.
Ven
og
e 1
Ch
am
bero
nn
eVen
og
e 2
Q
Mean slope – S
(%)
Discharge – Q
(m3/s)
Mean water depth
(m)
Width – B (m)
Re
(x104)
Fr D50 from the bottom (mm)
D84 from the bottom (mm)
0.67 0.55 0.29 5.75 4.5 – 12.3 0.24 – 0.44 49 81
measuring section
measuring grid – across the section
BED FORMS
- Periodic bed shape across the section - b,y ≈ 2h ≈ 10%B
- Signatures of streamwise sediment stripes produce during high water events when
bed load transport occurs
- Prandtl’s secondary motion of the second type may take place during high water
events due to the decrease in the aspect ratio
cross-section of the riverbed
MEAN VELOCITY DISTRIBUTION: U
- The structure of the flow is 3D
- Mean flow distribution and flow resistance are strongly form-dependent
- Periodically distributed high and low velocity regions were detected in the surface
layer: CH and CL regions or cells
- More intense CL cells coincide with the deeper profiles - periodicity CL+,y ≈ 2h
- The coexistence of CL and CH cells implies compensatory secondary motion
contour lines of the mean streamwise velocity across-section
TRANSVERSAL MEAN VELOCITY FIELD: V and W
- Permanent organized structure: SLOM – Surface Layer Organized Motion
- Lateral mass transfer between CH and CL regions
- A rotating movement is induced by the lateral transfer - streamwise vorticity
- SLOM vortical cells scale with the water depth (density of 4 cells per meter)
“detrended” mean transversal velocities
THE VELOCITY DIP
dU(y) umax(y)
u(y,z 0.90h)
Fr(y) U(y)
gh(y)
- D-shaped profiles correspond to the
CL regions
- The occurrence of the d-shaped
profiles is related to the local flow
regime
- For Fr<≈0.35 the dip phenomenon is
important
relation between the velocity dip and local Froude number
PERIODICITY OF THE TURBULENCE PRODUCTION
- The spectral dynamics is also conditioned by the bed form periodicity
- The extent of the productive plateau varies as function of the bed forms
power spectrum density variation across the section
≈ 2 cm from the bottom
≈ 2 cm from the surface
CROSS-SECTION PERIODICITY OF THE FLOW STRUCTURE
- All flow characteristics are bed-form dependent
- The flow is influenced by the macro-scale roughness until the surface (undulation the dU curve)
- Bed forms: b,y ≈ 2h
- Roughness: k,yu*,y ≈ 2h, in
phase
- Momentum: q,y ≈ 2h, out of phase
- Velocity dip: CL+,y ≈ 2h out of phase
periodicity of different flow characteristics
CONCLUSIONS
- Existence of periodically distributed streamwise coarse sediment ridges formed
during flood events - The flow structure is essentially 3D, hence 2D concepts are to be used with care- The hydraulic characteristics of the river flow are conditioned by the periodic bed
forms: mean and turbulent flow- Existence of an organized 3D flow in the surface layer, SLOM, conditioned by the
bed forms/local flow regime- A general wake effect induced by the large-scale roughness may confine the flow
response to the layer z/h>0.80- These results are important in respect to transport and mixing processes in rivers
FLOW RESISTANCE
- The influence of the bed forms is also visible in the roughness parameterization of
the flow (k and u*)
- The Nikuradse equivalent roughness and the friction velocity are in phase with the
bed forms: k,y = u*,y≈ 2h
- The flow resistance has a strong form dependence
CROSS-SECTION PERIODICITY OF THE FLOW STRUCTURE
All flow characteristics are influenced by the bed forms; in the presence of macro-scale roughness elements like these ones, the bed forms control the flow character up to the surface; the inviscid response of the flow is important all the way to the surface, as can be demonstrated by the undulation form of the dU curves.
b,y ≈ 2h
k,yu*,y ≈ 2h, in phase
q,y ≈ 2h, out of phase
CL+,y ≈ 2h out of phase