EXPERIMENTAL EVALUATION OF ACCELERATION CORRELATIONS FOR LOCALLY ISOTROPIC TURBULENCE

The two-point correlation of the fluid-particle acceleration is the su m of the pressure gradient and viscous force correlations. The pressur e-gradient correlation is related to the fourth-order velocity structu re function. The acceleration correlation caused by viscous forces is formulated in terms of the third-order velocity structure function. Ve locity data from grid-generated turbulence in a wind tunnel are used t o evaluate these quantities. The evaluated relationships require only the Navier-Stokes equation, incompressibility, local homogeneity, and local isotropy. The relationships are valid for any Reynolds number. F or the moderate Reynolds number of the wind-tunnel turbulence, the acc eleration correlation is roughly three times larger than if it is eval uated on the basis of the assumption that velocities at several points are joint Gaussian random variables. The correlation of components of acceleration parallel to the separation vector of the two points, is negative near its minimum at spacings close to 17 times the microscale . Its value near this minimum implies that fluid particles at those sp acings have typical relative accelerations of one-half that of gravity in the directions toward and away from one another. For large Reynold s numbers, the two-point correlation of acceleration is dominated by t he two-point correlation of the pressure gradient, The data verify tha t the acceleration correlation caused by viscous forces is much smalle r than that caused by the pressure gradient.