Hydraulic Jump

[owillis28]

The first time I took the PE exam, I struggled with Hydraulic Jump questions. A question on the exam dealt with two different hydraulic jumps and trying to find the length of the hydraulic jump (at least that is what I remember)

Can anyone post some addition example problems for review from an older exam or different sample exam not sold with the CERM. I would be extremely grateful.

owillis

 

[frazil]

Well here's one example from hydraulics class. It's in SI units -- I think the equation is slightly different in english units. Off the top of my head, I don't think I have a hydraulic jump question that asked for the length -- only the depth on either side.

 

[Guest]

owillis --

I have some textbook problems that I worked out as part of preparing for the exam. I will need to dig them out though - I can do that after work today for you.

JR

_______________________________________________________________

Problems

1. A rectangular channel for which Q=64 cfs, b=4-ft, n=0.013, So=0.01. A weir extends across the channel. The weir has a height of 4-ft. The discharge for the weir is given by the equation Q=3.4LH3/2

Find:

A. Depth upstream from weir;

B. Normal depth yn in channel;

C. Critical depth yc in channel;

D. Depth immediately downstream from the jump; and

E. Length of channel from the depth in part (d) to the depth in part (a).

Also, the two attached problems.

JR

 

[owillis28]

Well here's one example from hydraulics class. It's in SI units -- I think the equation is slightly different in english units. Off the top of my head, I don't think I've ever had a hydraulic jump question that asked for the length -- only the depth on either side.

My first notion was to use equation 19.90 from the CERM. Isn't this a conjugate depth problem? I read the problem and thought to myself "OK, they are giving me the flow rate, the dimension of the rectangular channel with the depth before the hydraulic jump". Just find the depth on the otherside. Plug and chug, move on to the next one.

Where did I go wrong? Help?

owillis

 

[ingluis]

Answers to attached jpg. #1:

a. d1 = 2.2 ft

b. dc = 6.8 ft

c. spuercritical dn<dc

d. d2 = 13.0 ft

 

[frazil]

owillis -

I apologize for even posting this example. I copied it straight out of my class notes without working the problem, and it appears to be completely wrong! :brickwall:

First of all the equation to compute the ratio should be just like eq 19.92 with an 8(Fr)^2. Second the answer isn't even right. I get 2.62 for the downstream depth.

You're right -- it is a plug and chug problem. You can use eq 19.91 to get the same answer for the downstream depth. Most hydraulic jump problems I've seen are plug and chug -- just beware of UNITS.

Anyway, sorry if I've confused you even more. You'll be fine. Just the fact that you're hanging out here at eb.com means you have like an 85% chance of passing. lusone:

 

[ktulu]

Anyway, sorry if I've confused you even more. You'll be fine. Just the fact that you're hanging out here at eb.com means you have like an 85% chance of passing. :brickwall:

frazil,

I am going to remember that you said that..... lusone:

ktulu

 

[Guest]

ktulu --

I wasn't able to pass until I STARTED hanging out at EB.com !! That's why it rocks !!!!!!! :bananalama: :th_rockon: :brickwall: lusone: :appl:

JR

 

[ktulu]

Well, I hope that trend continues, b/c I came on board after the October exam...

 

[djm120]

owillis --

I have some textbook problems that I worked out as part of preparing for the exam. I will need to dig them out though - I can do that after work today for you.

JR

_______________________________________________________________

Problems

1. A rectangular channel for which Q=64 cfs, b=4-ft, n=0.013, So=0.01. A weir extends across the channel. The weir has a height of 4-ft. The discharge for the weir is given by the equation Q=3.4LH3/2

Find:

A. Depth upstream from weir;

B. Normal depth yn in channel;

C. Critical depth yc in channel;

D. Depth immediately downstream from the jump; and

E. Length of channel from the depth in part (d) to the depth in part (a).

Do you have the answers for these or have them worked out?

 

[jfusilloPE]

owillis --
I have some textbook problems that I worked out as part of preparing for the exam. I will need to dig them out though - I can do that after work today for you.

JR

_______________________________________________________________

Problems

1. A rectangular channel for which Q=64 cfs, b=4-ft, n=0.013, So=0.01. A weir extends across the channel. The weir has a height of 4-ft. The discharge for the weir is given by the equation Q=3.4LH3/2

Find:

A. Depth upstream from weir;

B. Normal depth yn in channel;

C. Critical depth yc in channel;

D. Depth immediately downstream from the jump; and

E. Length of channel from the depth in part (d) to the depth in part (a).

Do you have the answers for these or have them worked out?

Sometimes, no most times, it is better for you, the applicant, to work the problem and post your solution here. Learning by failure is probably the most effective way to learn, and thereby providing your own first thought on the solution we can better help you define where you may be going wrong.