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PaulP
Member Since: 04 May 2007
Location: Barcelona
Posts: 4317
  
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AndrewW wrote:I'd guess muzzle velocity of about 200mph. Dropping very rapidly over a few inches.
Are you talking about the Karcher or........................  2006 Discovery 3 TDV6 SE Auto Buckingham Blue
2007 Golf GT DSG
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 17th Aug 2008 12:25 am |
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DG
Site Moderator
Member Since: 12 Dec 2005
Location: The Gaff
Posts: 50934
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I have been searching for a decent calculator ...but I can't imaging it's 200 mph ...even if it were that would be diminishing rapidly from the end of the wand (as you mention from personal experience )
Anyway ...you can probably tell that the leader in my question is that if you are driving at 100mph in the rain with a 30 mph headwind is this exerting a similar water pressure as a pressure washer ???
...or do I need a pointy D hat  21 year LR veteran > D2 GS 2003 > D3 S 2006 > D3 HSE 2009 > D4 HSE 2013 > D4 HSE 2015 > D5 HSE 2018 > DS HSE R-Dynamic P300e 2021
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| 17th Aug 2008 12:40 am |
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crews control
Member Since: 18 Mar 2007
Location: Dorset
Posts: 5011
  
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This has got me thinking....
Isn't the pressure also a value derived from the volume of water and the surface area being hit? It's not just the velocity surely. driving in the rain at 100mph in a 30mph headwind would be a water velocity of 130mph but the volume of water would be spread across the frontal area of the disco (large). If the Karcher can produce the same volume of water as heavy rain (and at the same speed) wouldn't the problem be that it's all hitting a very small area of the paintwork. Doesn't the damage come from making the spray to narrow or using a needle jet.... (ie. velocity+volume per sq metre)
Or do I need to stand in the corner with a pointy hat next to DG?
2005 D3 HSE
2011 D4 Landmark
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2017 Ford Tourneo VR46
Club Waitomo
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| 17th Aug 2008 12:56 am |
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AndrewW
Member Since: 06 Aug 2007
Location: Saddleworth
Posts: 2302
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I stand to be corrected - in fact please correct these sums.....
Volume flow stated by Karcher = 7.3 litres per minute
= 438 litres per hour
= 438,000,000 cubic millimetres per hour 
Edit Diameter (d) of nozzle = 0.8mm
Area of nozzle = (pi x d²)/4 = (3.142 x 0.8 x 0.8 ) / 4 = 0.50272 sq.mm.
Velocity through nozzle = FLOW (cu.mm./hr) / AREA (sq.mm)
= 438,000,000 / 0.50272
= 871,260,340 mm./hr
= 871 km/hr
= 543 m.p.h.
Of course the jet is then shrouded and shaped, increasing the frontal area over a short distance, and any doubling of the jet width is a quartering of its frontal area and thus a quartering of velocity.
Down to about 200mph, would you say? 2006 D3 finally swapped for a 2016 D4 Graphite in Graphite grey. No mods
Last edited by AndrewW on 17th Aug 2008 12:34 pm. Edited 1 time in total
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| 17th Aug 2008 11:35 am |
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AndrewW
Member Since: 06 Aug 2007
Location: Saddleworth
Posts: 2302
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BTW the problem with the seal around the PDC sensor is the classic "instability within a flow" thing that lifts a carpet or picnic blanket in a wind. Nothing happens until the edge lifts, then the flow goes under and lifts it higher. It goes as high as it can then flops back leaving a slight hump which acts like an aerofoil section, increasing the lift potential for the next cycle. So it lifts and flops again, very rapidly - the more rapidly, the stiffer the material is.
OR the more slowly, the slacker the material is. (FART theory  ) 2006 D3 finally swapped for a 2016 D4 Graphite in Graphite grey. No mods
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| 17th Aug 2008 11:53 am |
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NJF
Member Since: 05 Oct 2007
Location: Gone
Posts: 2466
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Andrew, why is the area of the nozzle divided by 4?
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| 17th Aug 2008 12:09 pm |
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AndrewW
Member Since: 06 Aug 2007
Location: Saddleworth
Posts: 2302
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Area of a circle = pi x r² or = pi x d²/4
so if you have a 0.8mm square the area is 0.8 x 0.8 = 0.64 sq.mm.
a circle 0.8mm across is less in area - about 3/4, or in fact pi/4... or 0.50272sq.mm.
A 2006 D3 finally swapped for a 2016 D4 Graphite in Graphite grey. No mods
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| 17th Aug 2008 12:31 pm |
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caverD3
Member Since: 03 Jul 2006
Location: Oberon, NSW
Posts: 6922
 
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If you use it in the engine bay there is a damn good chance of damaging the transmission module behind the battery. “There are only three sports: bullfighting, motor racing, and mountaineering; all the rest are merely gamesâ€
Ernest Hemmingway
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| 17th Aug 2008 12:59 pm |
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DG
Site Moderator
Member Since: 12 Dec 2005
Location: The Gaff
Posts: 50934
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AndrewW wrote:
Down to about 200mph, would you say?
Fair play chap so taking into acoount gravity and air resistance once the water has left the nozzle....what speed would it be about 18" away from the bodywork   21 year LR veteran > D2 GS 2003 > D3 S 2006 > D3 HSE 2009 > D4 HSE 2013 > D4 HSE 2015 > D5 HSE 2018 > DS HSE R-Dynamic P300e 2021
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| 17th Aug 2008 3:58 pm |
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AndrewW
Member Since: 06 Aug 2007
Location: Saddleworth
Posts: 2302
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WAA-A-a-a-a-ay down!
You can spray my (simple) car washing nozzle onto your hand 6-8" from the nozzle with no problem. Wouldn't do it to a cut/graze but the effective speed is (guessing frantically) about a tenth.
It will very much depend on the nozzle design - I have an Alto one with two side paddles that you can bring in or move out of the way of the jet. Without the paddles it will bring off paint and would certainly penetrate skin at a few inches - with the paddles adjusted in, it's a pussy-cat - a fluffy, wide triangle of fairly forceful flow.
So - another guess - we must be in the region of 20-40mph with a typical nozzle at 18" - certainly blasts leaves out of the gutter at about that sort of lick... 
 I've forgotten why we were asking now! It didn't start with PDC sensors, did it? 2006 D3 finally swapped for a 2016 D4 Graphite in Graphite grey. No mods
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| 17th Aug 2008 4:31 pm |
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BLFarrar
Member Since: 02 Aug 2006
Location: Deepest, Dankest, Darkest, Dingiest......Le Halifax, West Yorkshire...with strong links to Ireland
Posts: 6222
 
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by your approach to work out the speed / velocity of water........interesting line of thought
AndrewW wrote:I stand to be corrected - in fact please correct these sums.....
I had delved into the data available on water jet cutting, shear stresses & how paint / coatings are tested by high pressure water jets without much success........we use an HP water jet cutting company at work & was eager to speak to them on Monday as to where data on water as a cutting media was found...I know they use very much higher pressures & the velocity of water doesn't decay as rapidly as you would think (this is how they cut thick materials).
Where I got stuck was the effect the pressure (in mPa) had on the velocity in the actual nozzle ??? & whether this was stepped up...water before nozzle at pump pressure enters the very much reduced nozzle of 0.8mm dia..... BREXIT - done properly.
Right now ...We need Government - not Politics
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D3 HSE-perfectly formed, passenger friendly...has real DIPSTICK
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| 17th Aug 2008 4:37 pm |
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npinks
Member Since: 31 Jan 2008
Location: Leeds
Posts: 1943
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i have used a pressure washer on lots of cars, spraying from about 18 - 24" away with the nozzle set to a flat fan.
Spray the car at a 45degree angle to wash the dirt off the car as spraying at 90degrees will force surface contaminates into the paint, leaving tiny particles of dirt that will come loose at the hand wash stage causing swirl marks
Don't spray scratches or parts where the bodywork were there is a chance of water getting beneath the paint work and causing the lacquer to bubble or paintwork to fly off (happen on the wifes old Pug 106  )
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| 17th Aug 2008 4:40 pm |
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paul
Member Since: 12 Feb 2006
Location: Midlands
Posts: 594
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I used one on the disco and it washed some of the letters off the number plate!
How good is that for quality? 2006 TDV6 S Auto, Buckingham Blue, for the wife and kids.
2013 BMW 535d M sport touring.
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| 17th Aug 2008 4:42 pm |
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BLFarrar
Member Since: 02 Aug 2006
Location: Deepest, Dankest, Darkest, Dingiest......Le Halifax, West Yorkshire...with strong links to Ireland
Posts: 6222
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| .....this is my experience |
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npinks wrote:where there is a chance of water getting beneath the paint work and causing the lacquer to bubble or paintwork to fly off (happen on the wifes old Pug 106 )
....only my wreckage was a Volvo 760, especially the underseal.....in my attempts to clean under the car....here you sometimes don't have the space to get the safe distance.... BREXIT - done properly.
Right now ...We need Government - not Politics
Save the Dipstick Flagbearer-keep it simple, less likely to fail campaign-agenda items:Starting Handles, Acetylene Lamps.
Founder: Dipsticks-R-Us Inc
D3 HSE-perfectly formed, passenger friendly...has real DIPSTICK
Jag XK-but sadly no DIPSTICK...HUGE design fault
FL2 has DIPSTICK..."real comfort in rear seats"
VW Golf wondermobile (?)..has real DIPSTICK
Morris Minor..original DIPSTICK technology..and a real KEY.
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| 17th Aug 2008 4:44 pm |
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chesters
Member Since: 03 Oct 2005
Location: The Toon
Posts: 698
 
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If you are going to use this on your Disco, keep it well back especially on the tyres.
As it can do damage that may weaken the sidewall. Arctic Frost Discovery 3 TDV6 SE, Active Rear Diff, Adaptive Head Lights, Tow Pack, Ebony Leather, Roof Rails, Mantec Guard. Tasmod Stainless Sill plates. Grabber AT's. Home made mud flaps. Side Steps back on until the next time.
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| 17th Aug 2008 4:50 pm |
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