Measurements for Lagrangian characteristics of turbulence, including mean-square value of lateral diffusion from a point source
Y2, rms value of fluctuating velocity
v', integral scale corresponding to mixing length of eddy Λ
L, velocity auto-correlation coefficient
RL (τ),
etc., were made in isotropic turbulent flow behind a grid, with the tracer-particle tracking method proposed by present authors previously.
Comparison of these Lagrangian data with Eulerian integral scale Λ
f and the double velocity correlation coefficient
f (
r) led to the following results :
1) For turbulent Reynolds number of
Rλ=20 to 70, β (=Λ
L/Λ
f) is within the values of 0.6 to 0.3, becoming lower as
Rλ increases.
2) Distribution of
RL against τ is fairly similar to that of
f against β
r/
v'.
Further, it was confirmed by both theoretical analysis and experiments that the growth of
Y2 (
t) was weakened by turbulent energy decay, and remarkably so for long diffusion times.
View full abstract