CU06997 Fluid dynamicsHydraulic structures13.1 Hydraulic structures (page 430)13.2 Thin plate weirs (page 430-440)13.3 Long based weirs (page 440-445)13.4 Flumes (page 445-452) Just read13.5 Spillways (page 452-465) Just read1
Weir?? What?? Why??http://en.wikipedia.org/wiki/Weir• Regulation• Overflow• Flow measuring• Sometimes it is just a obstacle• Thin plate (sharp crested) weirs [korte overlaat]• Long based (broad crested) weirs [lange overlaat]1
Free or submerged flowVolkomen of onvolkomen overlaat• Free flow (volkomen)If the water level downstream changes,the water level upstream does not change• Submerged flow (onvolkomen / verdronken)If the water level downstream changes,the water level upstream also changes• http://vimeo.com/168473891
Thin plate (sharp-crested) weirsFree flowWater level upstream >> Water level downstreamUse specific energy=total head measured from the crest2
Formula’s Thin plate weirsThere are a lot of different formula’s which try to beas accurate as possible.Most important to remember is that in the formula’swater level and total head (specific energy) ismeasured from the crest and the formula’s can only beused with free flow (volkomen)Next slides some examples, you will find more in thebook.2
Basic formula 𝑄 = 𝑚 ∙ 𝐵 ∙ 𝐻32Q = discharge overflow [m3/s]m = runoff coefficient [m1/2/s]B = Width crest overflow [m]H = Head at overflow [m]measured from top crest!!HTotal head2
Long based (broad crested) weirs[Lange overlaat]Most of the time these weirs are not meant formeasuring discharge.More for regulating purposes or just because there isa object in the river which act as long based weirFree flow (volkomen) and submerged (onvolkomen)flow can occur3
Submerged flow [onvolkomen]Free flow [volkomen]Total Head or Energy line HTotal Head or Energy line HSuper critical flowHydraulic JumpBottom eddyLong-based (broad-crested) weirs3
Remarks• When the flow downstream is supercritical it isalways a free flow at the weir• If the water level downstream is lower than 2/3 H ,it is a free flow.• If the water level downstream is higher than 2/3 H,it is a submerged flow.• With free flow, the water level at the end of thecrest is 2/3 H.• H and water level are measured from the top of thecrest. H is measured upstream of the weir3
Submerged broad crested weir flow)(2 33 hHghBcq olv col=discharge coefficient submerged broad crested weir B = width weirh3 = water level downstreamTotal Head or Energy line HSubmerged broad crested weir [onvolkomen lange overlaat]Bottom eddy3
Free flow broad crested weir23HBcq vv Cv=discharge coefficient free flow broad crested weir [m1/2/s]B = width weirh3 = water level downstreamTotal Head or Energy line HFree flow broad crested weir [volkomen lange overlaat]Super critical flowHydraulic Jump3
Partially filled Culvertacts as a broad crested weir4DownstreamUpstreamCross-sectionLength CulvertFlow velocity
Partially filled Culvertacts as a broad crested weir4• The bed level of the culvert is the crest level• The width of the culvert is the width of the crest (B)• The water level (h3) and total head (H) are measuredfrom the bed level of the culvert.• If the water level (h3) downstream is lower than 2/3 H ,it is a free flow.• If the water level (h3) downstream is higher than 2/3 H,it is a submerged flow.• When submerged flow :totolc1
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