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Bump followed by hydraulic jump, find bump height

Short answer

Apply continuity and energy between sections 1 and 3 to get v1 = 1.17 m/s, v3 = 3.89 m/s. Sequent depth of supercritical y3 yields y4 = 0.82 m. Critical depth at crest gives bump height h ≈ 0.29 m.

Water in a horizontal channel accelerates smoothly over a bump and then undergoes a hydraulic jump as in figure below. If y1y_1 = 1 m, y3y_3 = 30 cm, estimate v1v_1, v3v_3, y4y_4 and bump height h. Neglect friction. [Figure: section (1) upstream with depth y1y_1; section (2) on the bump crest of height h; section (3) after the bump with supercritical depth y3y_3; a hydraulic jump leading to section (4)]

Given: horizontal channel, y1=1y_1 = 1 m (upstream, subcritical), y3=0.3y_3 = 0.3 m (supercritical, after the bump), then a hydraulic jump to y4y_4. Friction is neglected. Section 1 and section 3 are at the same bed level.

v1v_1 and v3v_3

Continuity: v1y1=v3y3v_1y_1 = v_3y_3, so v3=v1(1/0.3)=3.333 v1v_3 = v_1(1/0.3) = 3.333\,v_1.

Energy (smooth flow over the bump, no loss between 1 and 3):

y1+v122g=y3+v322gy_1 + \frac{v_1^2}{2g} = y_3 + \frac{v_3^2}{2g} 1−0.3=v122g[(10.3)2−1]  ⇒  v12=2(9.81)(0.7)10.111=1.3581 - 0.3 = \frac{v_1^2}{2g}\left[\left(\frac{1}{0.3}\right)^2 - 1\right] \;\Rightarrow\; v_1^2 = \frac{2(9.81)(0.7)}{10.111} = 1.358 v1=1.165 m/s,v3=3.885 m/sv_1 = 1.165\ \text{m/s},\qquad v_3 = 3.885\ \text{m/s}

y4y_4 (after the jump)

Fr3=3.8859.81×0.3=2.265Fr_3 = \frac{3.885}{\sqrt{9.81 \times 0.3}} = 2.265 y4=0.32(1+8(2.265)2−1)=0.822 my_4 = \frac{0.3}{2}\left(\sqrt{1 + 8(2.265)^2} - 1\right) = 0.822\ \text{m}

Bump height hh

The flow changes from subcritical to supercritical smoothly over the bump, so it passes through critical depth at the crest (section 2). With q=v1y1=1.165 m2/sq = v_1y_1 = 1.165\ \text{m}^2/\text{s}:

yc=(1.16529.81)1/3=0.517 m,Ec=1.5 yc=0.776 my_c = \left(\frac{1.165^2}{9.81}\right)^{1/3} = 0.517\ \text{m},\qquad E_c = 1.5\,y_c = 0.776\ \text{m} E1=1+1.16522g=1.069 mE_1 = 1 + \frac{1.165^2}{2g} = 1.069\ \text{m} h=E1−Ec=1.069−0.776=0.293 mh = E_1 - E_c = 1.069 - 0.776 = 0.293\ \text{m}

Answer: v1=1.17v_1 = 1.17 m/s; v3=3.89v_3 = 3.89 m/s; y4=0.82y_4 = 0.82 m; h≈0.29h \approx 0.29 m (critical flow assumed at the crest).

✓ Answer checked by Claude Opus 5.5 (high effort)

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