Race Engine Forum UK

Is there any simple way to measure velocity. I have seen all sorts of expensive pitot tubes for sale.

I say this as i have started to mess around placing objects such as deflectors in ports(spare time of course) and i want to see what effect they have on velocity. I can place these objects in such a way that flow does not drop.

No, there isn't Chris.

Certainly, if you want to do a velocity survey around the port the pitot probe is the tool, and types supplied by say, Superflow do require backup equipment to turn pitot pressure into a velocity measurement.

The problem being that because we're applying suction (so to speak) to the port the probe has to have a U bend in it to measure the total (static + dynamic) pressure of the incoming air so there also has to be a port on the probe 90 deg to the airstream so static pressure can be measured as well. Then correction can be applied to the total (pressure) head to deduce the air velocity. All rather involved.

Of course you can work out the air velocity on your flowrig at any point in the port easily enough if you know the mass flow (which is constant from valve throat to port entry) from the equation:

massflow (kg/sec) = density (kg/cu m) x cross sectional area (sq m) x velocity (m/sec).

..naturally all this will tell you is how the velocity drops as you enlarge the port, which we all know anyway. How that affects engine performance is another matter.

GC

Guy Croft wrote:Of course you can work out the air velocity on your flowrig at any point in the port easily enough if you know the mass flow (which is constant from valve throat to port entry) from the equation:

massflow (kg/sec) = density (kg/cu m) x cross sectional area (sq m) x velocity (m/sec).

Just remember that this is the average velocity, in reality the velocity varies from nothing at the port wall to a maximum in the centre of the port and back to zero. how steep the profile is depends on whether the flow is laminar or turbulent.

Quite so, average velocity.

G