Relationship between Fourier-Modes and Spatial-Structures in a Cavity-Boudary Layer Interaction at Moderate Reynolds Numbers, postscript, pdf, François Lusseyran, Allecsandra Rambert, Pierre Gougat, Yann Fraigneau, Afif Elcafsi and Georges M. Quénot, 11th International Symposium on the Applications of Laser Techniques to Fluid Mechanics, paper 34.3, Lisbon, Portugal, 8-11 July, 2002

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.