The dynamic behaviour of the vortex structures occurring in a cavity in
interaction with a boundary layer controls the flow confinement inside
the cavity. The vortex structures present small scales as well as large
scales related to the cavity geometry. Theses eddies are usually
non-stationary. One of main applications of this study concerns the
pollutant dispersion within an urban street canyon when ambient winds
aloft are perpendicular to the street. Experimental and numerical
characterisations of the vortex structures are developed at moderate
Reynolds numbers. The results concern low velocities close to the
laminar-turbulent transition of the outside flow, for a given geometry
of cavity H/L=0.5. Flow velocity measurements inside and outside the
cavity are performed by Laser Doppler Velocimetry and Particle Image
Velocimetry based on the Optical Flow method. The Reynolds number, based
on the height of cavity, is close to 3000 for the simulation and PIV
measurements and ranging between 2000 and 8000 for LDV measurements.

2D direct numerical simulations are obtained from a code solving
Navier-Stokes equations (code OLORIN developed at LIMSI) for unsteady
isothermal incompressible flow. Equations are discretized following a
finite volumes approach with schemes of order 2 in space and time.
The flow is computed for the same geometrical and hydrodynamic
conditions as the experimental ones.

The interaction between a flow and a cavity is a test configuration
for confrontations between numerical simulation and experiments. However
a quantitative confrontation "step by step" makes no sense in the case
of a non-stationary, non-linear and therefore unpredictable problem.
We propose a first simple approach comparing temporal and spatial
modes.

For measurements and simulation show the existence of three main
frequency modes, quantitatively in very good agreement. Measurements
of the velocity fields by 2D PIV, associated with 2D visualisations
show the existence of instabilities in the shear layer at the boundary
between cavity and the main stream. One notes that the vertical velocity
component grows apparently linearly after the upstream edge of the cavity.
The fit of the evolution of according to with a physical description of
instability growth, afford us to measure spatial amplification rate and
wavelength. The spatial modes properties obtained from the experimental
velocity field and the numerical one, again are in very good agreement.