CE 576 Environmental Flows
Spring 2012
Homework 7
*33. A river has a width of 30 m, a slope of 2.5 × 10−4 , a Manning coefficient of 0.036, and
a discharge of 3.6 m3 /s. The geometry is shown below.
a. Compute the depth in steady uniform flow with Manning’s equation. (Because
we have not discussed this calculation in class, let me know if you need guidance.)
b. Will sediment with a diameter of 0.1 mm move in this flow?
c. What is the largest sediment that will move in this flow?
1m
20 m
10 m
*34. Problem 33 dealt with the initiation of motion of sediment. Now you will compute a
profile of suspended sediment concentration in a channel flow.
a. In equilibrium, sinking of sediment particles (with velocity ws ) is balanced by
turbulent mixing with an eddy diffusivity Ks . Conservation of sediment mass
yields
d
dC
dC
=
−ws
Ks
,
dz
dz
dz
where z is the coordinate normal to the channel bottom. Argue that this equation
is correct for this situation.
b. A simple approach to estimating the eddy diffusivity is to assume that it is proportional to the eddy viscosity: Ks = ανt . In class, we took νt = κu∗ z. Here
take
z
νt = κu∗ z 1 −
,
h
where h is the water depth. Explain why this formulation might be more realistic
than the one we used in class.
c. Take C(za ) = Ca and show that
C
=
Ca
z (h − za )
za (h − z)
where P = ws /κu∗ is the Rouse number.
− Pα
,
d. Compute the Rouse number for the sediment in problem 33b.
e. Plot C/Ca vs. z/h for P = 0.1, 0.5, and 1; α = 1; and za /h = 0.01. Provide a
physical explanation for the effect of the Rouse number P .
35. Yet another way to estimate the shear velocity in channel flow is to use the turbulent
kinetic energy. Find the paper by Kim, Friedrichs, Maa, and Wright from the Journal
of Hydraulic Engineering in 2000.
a. Explain the TKE method in general as discussed on p. 400.
b. Explain why Kim et al. (2000) use the vertical velocity variance (i.e., w 2 ).
c. Briefly summarize the authors’ assessment of the TKE method.
d. Apply the TKE method to the data from the lab exercise (i.e., problem 32). Does
the vertical velocity variance work better than the full TKE?
36. Derive the equation for the turbulent kinetic energy:
∂k
∂u
∂u
1
g
∂
∂k
i
= −ui uj
− ρ u3 −
p uj + 12 ui 2 uj − νui i − ,
+ uj
∂t
∂xj
∂xj
ρ0
∂xj ρ0
∂xj
where
ν
=
2
∂uj
∂ui
+
∂xj
∂xi
∂uj
∂ui
+
.
∂xj
∂xi
*37. Munk (1996, Deep-Sea Res.) estimated that explaining the historic temperature profiles in the deep ocean would require vertical mixing with an eddy diffusivity on the
order of 10−4 m2 /s. Much of the last 45 years of physical oceanography has been
devoted to finding mixing strong enough to support Munk’s estimate. For example, oceanographers infer the vertical eddy diffusivity K from measurements of the
dissipation and a simplified version of the equation for turbulent kinetic energy.
a. Osborn (1980, J. Phys. Oceanogr.) devised a way to measure the buoyancy flux
b, a measure of vertical mixing. He assumed that the turbulence is stationary
and homogeneous (i.e., that the statistics vary neither in space nor time), defined
the flux Richardson number as Rf = b/P (where P is production), and used the
TKE equation to find
b = Γ,
where Γ = Rf /(1 − Rf ). Derive that result.
b. Show that a gradient transport assumption yields
K = Γ 2.
N
c. Assume that Γ takes its maximum estimated value of 0.2 and use Fig. 5 of
Moum (1997), which is on the website, to determine the types of flows for which
K > 10−4 m2 /s.
d. Discuss whether the assumptions of stationarity and homogeneity are likely to be
valid for turbulence in the ocean.
38. The k- model uses the following model for the Reynolds stress:
−ui uj
= νt
∂uj
∂ui
+
∂xj
∂xi
2
− kδij
3
a. Write down an expression for −u1 u3
b. Show that the model reproduces the correct turbulent kinetic energy.
c. The Wikipedia page for “Turbulence kinetic energy” states, “k- models assume
isotropy of turbulence whereby the normal stresses are equal:
u 2 = v 2 = w 2 .
Is this statement correct?