Potential flow in engine valves Page: 20 of 29
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N.A.C.A. Technical Memorandum No. 343
iments recently performed by Schrenk in Darmstadt. He observed
the noteworthy phenomenon that, on lifting the valve head, flow
II occurred first and then changed suddenly to flow I.
It is a question as to what the physical reason is for
the fact- that, with a certain valve lift, instability occurs
when there is no spring, as in Schrenk's arrangement0 It is
not to be assumed that the course of the forces acting on the
valve head, as stated above, has any effect on this phenome-
non, although the minimum force at a valve lift is of the same
drder of magnitude as that of Schrenk in the rough calculation
of the flow.
It is more probable that the friction on the stationary
wall is responsible for this phenomenon0 From the point E
on, the velocity along the wall decreases from n = c to
n = 1 in infinity. In an actual fluid, the flow along a sta-
tionary wall is such that, ts a result of the friction, the
fluid adheres to the wall ard the velocity then gradually in-
creases up to the value which follows from the potential flow.
The layer in which this increase occurs is very thin and is
called the boundary or marginal layer0 If, as in the above
case, the fluid flows along a wall wtih increasing velocity,
then the particles in the boundary layer come to rest much
sooner than the more distant ones. The pressure increase
therefore takes place much more quickly in the boundary layer,
A point is finally reached Nhare the pressure in the boundary
layer even generates a counter-current. In most cases, the
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Eck, Bruno. Potential flow in engine valves, report, December 1, 1925; (digital.library.unt.edu/ark:/67531/metadc65148/m1/20/: accessed February 21, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.