In the case of a cascade, the diagram shown above is at the base of the cavity of baseplate where the jet impinges marked by the red elipes in the diagram below

as the fluid gets further away from the axis of the jet, it gets slower and the boundarylayer gets thicker

The effective water velocity remains pretty much unchanged at the edges of the "cone" of impingement. It's only after the water "bounces" that it slows down.

The trick to the Cascade was to guage the cup diameter to present the cup wall just after the point of "rebound" from the cup base. This of course is dependent upon the jet velocity but I believe that I managed to pick a jet/cup width ratio that applies to the general flow rates that people use. In general it was okay to have the cup wall being slightly too far away as opposed to being too close. This allowed me to design to higher flow rates and not suffer terribly at lower flow rates.

If I were designed a low-flow only block (<2LPM), I would have done quite a number of things differently.

Speaking of that...

Do you know what the relative durability of silver and copper would be?

So the diameter of the cavities are based on the geometry of the jet which in turn is based on the velocity of fluid and ratio of jet height to jet nozzle diameter

You have designed the cascade so that it does something like in the modified diagram below.,..?

Sorry..
left out the pic

Well, according to the Wolverine handbook on cooling, copper is able to withstand 6fps (~2m/s) velocity without any form of long-term erosion. The Wolverine book is talking about ultra-conservative long-life scenarios.

An Eheim 1048 plus heater-core will give about a 2.5m/s jet velocity.

As to the scale of the erosion, my personal belief is that it is extremely low even at 10m/s velocities. So long as there is no gritty particulate matter being thrust against the metal I figure it should be pretty safe.

Hard to put an exact figure on it, but have been running this silver block with ~5.5m/s jet velocities with the Iwaki MD-30RZ for close 3 months now. About every month I pull it apart and inspect it. Last time I checked the fine-level machining marks left at the base of the cups, which numbers in the very small number of microns in scale, were still plainly visible. If there's any erosion happening, it certainly doesn't look like it'll be an issue within 10 years at the least, even with this fairly powerful pump and soft silver.

Something like that. Not sure I agree with the "scale" of various parts of that picture, but yeah, something like that.

As only the bottom part walls of the cavity is involved in the secondary impingement...so i was thingking if making helical groves on the walls will give added performance..?

The groves could be easily made by using a small inner threadmaker (taps)..

Theoriticaly, this will surely thin the boundary layer of the fluid flowing out of the cavity walls because of the turbulance created by the whirling effect

Cathar, I was thinking about how you said that your cup:jet ratio was such that the "rebounded" water would hit the wall just after rebound. For some reason I got to thinking about parabolas and hyperbolas and focusing the rebounded water to a certain area to acheive added cooling after rebound. Haven't gone any further with that thought because I also wondered, do you know if the rebounded water is rebounding straight back out of the cup or could the rebounded water also be going straight back into the jet and slowing it down? Obviously your block works great and you've spent much time designing it but I haven't seen anyone mention where the exiting water is going exactly. I hope you see where I'm going with this :rolleyes:
I was just wondering if exiting the rebounded water to somewhere else besides back out the cup might help the impingement effect some.

Yes, I had considered tapping the holes like you say. Aside from the difficulty of tapping into copper into such small holes and not breaking taps when doing so, I was concerned that the pitch of the thread would be such that no real swirling would develop as the vertical velocity is just too quick. Instead there may be a more dangerous scenario developing whereby the water inside the thread grooves remains fairly static and the water just rushes up the middle. I never directly experimented with it though.

Part of the rebounding is useful in that it creates extra turbulence as it shears against the force of the incoming jet. Remeber what I was saying above about how having some jet shear is actually very useful, and one can see improved performance as a result, and this is true up to a point of when the jet loses too much power for the extra shear turbulence to compensate for.

Guess I should have stated my thoughts more clearly as I was assuming the rebounded to be too much interference to the point of slowing the jet down.

I guess the clearer question about the jet shear would have been, have you done flow testing to chech this or are there computer models that will show what the flow of your cup/jet is?
You obviously know alot about fluid dynamics it seems ( I know basically nothing as if you couldn't tell) so was just curious if the amount of shear happening was a guess aor calculated. I'm assuming calculated from the amount of study you've put into this.

Most of my data is a mix of study of research, and experimental evidence. Went through quite a large number of variations/prototypes of the Cascade. I don't have access to the sort of computational fluid dynamics software that would be needed to simulate this sort of flow, or if even the models of such are good enough to do it properly? :shrug:

Nah, it's all gross design evaluations, nothing calculated. Cathar did all the experimenting, to find the optimal solution, and the rest is history.

For straight water, this design is going to be extremely difficult to beat. All from a simple concept: double inpingement.

As I've stated, you could easily build 30 variations of this block, to find an optimal solution: hole spacing, hole diameter, jet diameter, jet distance, fluid density, thermal conductivity of the baseplate material and coolant, and jet velocity. Those are the variables.

I believe that Cathar may have used a home made simulator, but I don't know how useful it was here.

(BTW Cathar, how many variations did you have to build/try?)

8 different plate/jet pairs for which each was modified 3-5 times, exploring cup depth, inter-cup geometry, cup-jet width ratio, jet height, cup/jet density effects, base-plate thickness, etc. By the time I finished each pair was basically useless.

Sounds like something that Im doing....

Only that im not using the protruding nozzles....

and im also doing some computational fluid dynamics simulation using CFD

In the simulation i will only model an array of 2 x 4 jet nozzles....
to minimize computing time.

I figured out that only a part of the block needs to be simulated in order to predict the characteristic of each specimen....

Cathar, thanks for your patience and your willingness to share your hard earned knowledge with us :)
This is and interesting subject and one I haven't been able to find much in depth coverage on. At least not that are readily available to the mass public.

Sorry if I'm a pest but this whole subject has my brain a buzzin :)

Have you done any research into what back pressure on the return flow side does to performance? Seems as if too much back pressure on the return side could cause hindrence of the jets velocity in the cups. In other words if the return flow flowed more freely, with less resistance than the pressure side, then the jet velocity would also be maximized?
Possibly larger ID on the return side dumping into a res that's not full.