Hydraulic Analogy

The equations of unsteady two dimensional compressible inviscid flows are analo­gous to the shallow water equations where the height of the water corresponds to the density of the compressible fluid. To show such analogy, we will start with the isentropic Euler equations in conservative form

Подпись:Подпись:Hydraulic Analogy
+ dU + d-f. = о

dx dy

dpu dpu2 dpuv dp

dt + dx + dy dx

dpv dpuv dpv2 d p

dt + dx + dy dy

Hydraulic Analogy



The above equations imply conservation of mass, momentum and entropy. Energy is not conserved in general. Only for smooth flow, energy will be conserved.

Next, consider a thin layer of water over a flat smooth plate, where the height of the water is given by h (x, y, t).

Hydraulic Analogy Подпись: (13.23)

The conservation of mass requires

Since pw = const, it cancels out and the equation becomes similar to the com­pressible flow continuity equation, where p is replaced by h.

Hydraulic Analogy Подпись: dpwhuv d y Подпись: d h pw9h rn Подпись: 1 dh2 2 pw 9~dX Подпись: (13.24)

The momentum equations for the column of water with height h is given, ignoring the viscous effects, by

Подпись:dpwhuv dpwhuv dpwhv2 dh 1 dh2

+ + = —pwgh = — pw9

dt dx dy dy 2 dy

Once again, pw cancels out.

Assuming small variations of the water surface and ignoring the vertical acceler­ation, the pressure is governed by the hydrostatic equation

p = Patm + pw9(h — z), 0 < z < h (13.26)

where z is the elevation of a point above the flat plate.


On the other hand, from isentropic relation of compressible flows,

dp = dp d£ = 2 df_ dp = dp dp _ 2dL (13 28)

dx dp dx a dx ’ d y dp d y a d y ‘

where a2 = yRT. Hence the analogy holds if a2 corresponds to gh. In this case, the Mach number M = V/a corresponds to the Froude number F = V/л/gh of the shallow water waves where л/gh is the speed of surface waves due to small disturbances.

Notice the pressure of compressible flow corresponds to h2, hence the analogy holds only for y = 2. In this case the temperature of the compressible flow corre­sponds to h.

To summarize these relations, the different non dimensional quantities for com­pressible and shallow water flows are given in Table 13.1, where subscript r stands for reference quantities

Подпись: Table 13.1 Analog quantities for compressible flow and shallow water models Подпись: Compressible flow Shallow water P/Pr h/hr P/ Pr (h/hr )2 T / Tr h/hr a/ar -Jh/hr M F

There are several special cases of interest including

(i) Quasi-one dimensional unsteady flows in nozzles,

(ii) Two dimensional steady external and internal flows,

(iii) irrotational flows.

The above derivation shows that the analogy is valid for rotational flows where vorticity may be generated due to the variation of the total enthalpy according to Crocco’s relation. One can consider the special case where vorticity vanishes and a potential flow exists for both steady as well as unsteady phenomena. In all cases, the flow is assumed isentropic. In the following, hydraulic jumps (corresponding to shock relations of compressible flows) will be studied.

The propagation of sound corresponds to the propagation of an infinitesimal grav­ity wave, since

Подпись: Du ~Dt (13.29)

Hydraulic Analogy Подпись: (13.30)

corresponds to

with a2 = gh.

Подпись: d2 h dt2 Подпись: gh0V2h = 0 Подпись: (13.31)

Also, the linearization of the shallow water equation yields the classical wave equation

where a0 = gh0.

Real time observation is possible since g = 9.81 m/s2 and for h = 0.01 m, a = 0.3 m/s. In fact, shallow water experiment can be performed at the kitchen sink!

Подпись: P_ P0 Hydraulic Analogy Подпись: (13.32)

For smooth steady flows, one can obtain the isentropic relation and corresponding shallow water equations. For example

Supersonic (M > 1) and subsonic (M < 1) flows correspond to supercritical (F > 1) and subcritical (F < 1) flows.

The analogs of shock waves are called hydraulic jumps. Moving with the discon­tinuity and considering the relative normal velocity, one can derive the following steady relations

u 1 h 1 = u2h2 = m (13.33)

To evaluate the momentum balance, a control volume is used as shown in Fig. 13.3. Friction on the bottom surface is neglected.

The net acting force per unit span in the x-direction due to pressure is

J Pwg(h1 – z) dz – J Pwg(h2 – z) dz = 1 pwg (h2 – h^ (13.34)

Hence, the rate of change of x-momentum reads

Pwu2h2 – Pwu2h1 = 2Pwg ^2 – h^ (13.35)


u2hi + – ghf = uh2 + – gh2 = n (13.36)

The above equations can be solved for (u, h) = (u2, h2) given (u1, h 1) as follows uh = m, and u2h + 1 gh2 = n (13.37)

Hydraulic Analogy

Fig. 13.3 Hele-Shaw flow past a rectangular block (from en. wikipedia. org)

Подпись:Hydraulic Analogy(13.44)

The solution with F1 < 1 and h/ h 1 < 1 is excluded. It corresponds to an expansion shock. To see that analogy, consider the specific energy (see White [7])


Подпись: h h 1
Подпись: -2 + 4 + 2 F2
Подпись: (13.43)

Подпись:Подпись: (13.42)

Hydraulic Analogy
Hydraulic Analogy
Подпись: The trivial root is h = h1, i.e. no jump. Other roots satisfy the relation
Подпись: 1 2 1 1 2 2 gh + 2 gh1 h + 2 ghi - n = 0
Подпись: Since Подпись: A solution of this quadratic equation is given by

1 2 1 m2

E = gh + 2 U = gh + 2 H2

For a given m (discharge), there are two possible states for the same E, see the sketch in Fig. 13.4 and their relation is plotted in Fig. 13.5.

There is a minimum value of E at a certain value of h called the critical value. Setting dE/dh = 0, Emin occurs at g – m2/h3 = 0, or hc = [m2/g)1/3 and Emin = ghc + m2/(2hC).

Hydraulic Analogy

At the critical depth, m2 = ghl = c2h2, where c0 = Vghc and F = 1.

For E < Emi„, no solution exists and for E > Emin two solutions are possible with h > hc and u < co as well as h < hc and u > c0. The analogy with Mach waves is clear with the angle of the wave given by

sin g = — (13.46)


Подпись: E1 Подпись: (h2 - h1)3 E2 = g Подпись: (13.47)

Now, across the jump

This equation shows that dissipation loss is positive only if h2 > h 1, a requirement implied by the second law of thermodynamics.

A more consistent analysis would be based on the total enthalpy (and not just potential and kinetic energy)

h _ p/pr 1 1 u 2

Подпись:Подпись: (13.49)u2 ~ (Y – 1)p/Pr M2 + 2 Mr

The corresponding quantity for shallow water is

H _ h 1 1 / u 2

m2 hr F2 + 2 ur

The total enthalpy should decrease for steady isentropic Euler shocks (see [8]) and hence expansion shocks are excluded and the same is true for hydraulic jumps. Again, from the conservation of mass across the hydraulic jump

u і h і = u2h2 = m (13.50)

and from the momentum balance

2 19 9 19

u2h1 + gh2 = u2h2 + gh2 = n (13.51)

one can show, upon elimination of h that

1 Подпись: 11 M2 - M1 + gm 2 2 u2 Подпись: 0 (13.52)Hydraulic Analogy m2 1 m2

mu1 + g 2 = mu2 + g 2

2 u 21 2 u 22

Introducing the jump notation [a] = a2 – a1, one can write

< u > + gm = 0 (13.53)

Hydraulic Analogy

2 u2


Подпись: (13.55)Подпись: = — M1M2 = —gh<u>


The latter is analogous to the Prandtl relation for normal shock of compressible fluid flow.

Hydraulic analogy can provide experimental demonstration of compressible flow phenomena using a device called water table, which consists of an inexpensive sheet of glass with suitable sides. Either a model is moved in a thin layer of water or the
model is fixed and the water flows around it. The analogy is particularly useful for transonic flows including shocks. Examples are:

(i) Transonic flows in convergent/divergent nozzle at design and off-design con­ditions,

(ii) Steady two dimensional supersonic flows over airfoils (flat plate, parabolic or diamond airfoils). Compression shocks and expansion fans are easily identified. Attached and detached shocks as well as fish tail shocks can be observed, including wave reflected from the walls,

(iii) Unsteady flows, including accelerated and decelerated as well as oscillating airfoils.

There are limitations, of course. Strong shocks are not well represented. The assump­tions that vertical accelerations are small, the viscous force exerted by the horizontal bottom is negligible and the vertical variation of the velocity can be neglected in the continuity equation are questionable if quantitative results are required. In general, separated flows are not representative since the Reynolds numbers are very different in water and compressible flows. For more details, the reader is referred to Oswatitsch [9],Loh [10], Thompson [11] and White [7].

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