Characterization of frequency mode inside an open cavity for different shape ratio

INTRODUCTION

In urban environment, the dynamic spatio-temporal of the fluid movements is there very complex, he occurred of movements of vortex, of very variable scale. The turbulence that occurred can have several reasons. Indeed, the presence of the obstacles, their geometry and their orientation are as many the factors that modify the dynamics and the level of the turbulence. The presence of buildings to the neighborhood of the urban setting, modify considerably the level of the turbulence and therefore the structure of the flow inside the street canyon. The dynamic comportment of the flow inside a cavity was the subject of a big number of experimental studies during the last forty years.

The vortex, as the wakes that developed inside a cavity, are the seat of instabilities and detachment phenomenon. These instabilities have consequences displayed on the structure of the flow; the amplification of the mixture plays a very important role in the thermal and mass transfer's phenomena. The possibility of monitoring this instability becomes a topic of nowadays a topic of actuality. Among the techniques utilized are those of active controls of the instabilities. Indeed, the formation of the vortex structure, emerging from the interaction between a boundary layer and a cavity, is broken down into two distinct phases; the first one corresponds to a process of amplification-convection of small disturbances in the flow initially existing near the cavity downstream corner, In fact, the mixing layer are developing above a cavity presenting a velocity profile having an inflection point which makes it unstable according to the hydrodynamic theory (1). Consequently, within the mixing layer, the Kelvin-Helmholtz instability amplifies the initial disturbances which continue to grow in size until they form a large vortex structure that escapes towards the upstream which characterizes the vortex escape frequencies. he second phase is related to a retroaction to a distance generated by the interaction of the vortex with the upstream corner which has created new disturbances near the downstream corner. The exploitation of the display images in the latter zone highlights the vortex passage. These latter have been identified as it they were on a translation move defined by a velocity more or less equal to U (2,3) Kelvin-Helmholtz instabilities have been made visible in the cutting layer by the rolling up of the current lines. The big vortex structures that escapes toward the flow getting round the recirculation created by the upstream edge of the cavity (4). The data processing by the technique of measure Laser Doppler Velocimetry, in cavity upstream, has proved the existence of a well defined vortex escape frequency.

[FIGURE 1 OMITTED]

We are presenting in this paper the study of this frequency variation using the cavity upstream as well as its variation according to the cavity shape ratio. These results analysis reveals the existence of many modes within the flow which characterizes every cavity constant and by changing the length L of the cavity and keeping the height [h.sub.2] constant.

EXPERIMEN TAL DEVICE

The experimental configuration is presented in Fig. 1. The cavity characteristic dimensions are indicated in meters in Table 1 and 2 were H is the height, L the length and l the width of the cavity.

Table 1: Characteristic dimension of the cavities having constant L

               Cavity1/2[h.sub.2]  Cavity 1[h.sub.2]  Cavity 2[h.sub.2]

[h.sub.2] (m)        0.05            0.1               0.2

L (m)                0.1             0.1               0.1

Table 2: Characteristic dimension of the cavities having constant h

               Cavity 1  Cavity 3L4  Cavity1L2

[h.sub.2] (m)    0.5        0.5         0.5
L (m)            0.1        0.75        0.5

Velocity profiles in the boundary layer were obtained by hot wire measurements. The interaction region between the boundary layer and the cavity and the vortex burst from the cavity were characterized by Laser Doppler Velocimetry measurements.

Measurements were performed for different cavity shape ratio and for different mean velocity. The different shape ratio was obtained by changing the height of the cavity [h.sub.2] and keeping the length L.

RESULTS

To improve the characterization of the vortex structures were performed by PIV An example of visualization is presented in Fig. 2.

[FIGURE 2 OMITTED]

Two counter rotation vortex are observed with eddies of smaller dimensions turning around them. The vortex has a diameter equal to the height of the cavity

Measurements make after the cavities showed that the boundary layer was no longer laminar. A spectral frequency analysis of the hot wire signals was realized which showed that the frequencies were in the flow frequency domain (200Hz). This perturbation of the boundary layer was due to the exist of the vortex structures from the cavity. The flow visualization (Fig. 3) confirms the perturbation of the laminar boundary layer.