A procedure is presented which allows to compute in a
non-invasive manner, blood viscosity through flow measurements
obtained at a fixed vessel cross-section. The data set is made of
measurements (artery radius and spatially discrete velocity profiles)
performed at given time intervals for which the signal to noise ratio
is typical of U.S. Doppler velocimetry in clinical situation. This
identification approach is based on the minimization, through a
backpropagation algorithm, of a cost function quantifying the distance
between numerical data obtained through Navier-Stokes simulations and
experimental measurements. Since this cost function implicitly
depends on the value of viscosity used in numerical simulations, its
minimization determines an effective viscosity which is shown to be
robust to measurement errors and sampling time. Such an approach is
shown to work in an in vitro experiment, and seems to be
suitable for in vivo measurements of viscosity by the atraumatic
techniques of Doppler echography.