**3.2. Passive control using a slot**

The first case study uses a simple convex surface and the second computational case uses the same convex surface with a slot between the over-pressure point and the under-pressure point on the surface (placed in separation boundary layer region). The tendency of equalizing the pressures leads to a blow in the first orifice of the slot, while in the second one the suction phenomenon occurs. The jet (*Uj*=25 m/s) is developed in a rectangular channel with 75 mm (height) x 20 cm (width) and passes over a convex surface (25 cm length). The shape of the surface is given by two elliptical fillet surfaces.

Mathematical Modelling and Numerical Investigations on the Coanda Effect 117

**Figure 10.** Contours of vorticity magnitudes (maximum expulsion).

**Figure 11.** Velocity vectors at maximum ingestion (a), and maximum expulsion (b).

**4. Numerical analyis of turbulent flow in a Coanda ejector** 

pressure ratios on the Coanda ejector performance.

induced by the ejector.

The task of this study is to investigate the influence of various geometric parameters and

The Coanda ejector is an axisymmetric device that uses the injected primary flow on the inner curved surface and entrains the secondary flow. The main purpose of the Coanda ejector is to provide a high ratio of the induced mass flow rate to the primary mass flow rate. A primary flow is supplied from a high pressure reservoir. The primary flow follows the curved contour of the ejector after a sonic throat, due to the Coanda effect, and expansion waves/compression waves are created depending on the pressure at the outlet section of the primary nozzle. The turbulent mixing of the primary flow with the ambient air near the entrance of the ejector transfers the momentum of the primary jet to the stagnant air in the ejector throat. The secondary, or induced flow is thus dragged by the turbulent shear stress along the viscous effects towards the ejector exit while being mixed with the primary flow by the persistence of a large turbulent intensity throughout the ejector. There are a few works [21, 22, 23] which examine the basic mechanism by which the secondary flow is

The experimental model has 11 pressure probes disposed on the median plane of the Coanda surface and connected to a digital pressure scanner.

For computations we use steady RANS with a *k-* SST *c.c.* turbulence model and the computation grid has 219,300 nodes. The suction-blowing phenomenon has a beneficial effect on keeping the boundary layer attached on 82% of the surface compared to the case without the slot when the boundary layer is attached to 58% of the surface. Figure 9 shows the velocity field in the computational domain and the pressure distribution on the surface for each of the aforementioned situations. The jet is deflected by 20 degrees from the original direction. Using a hydraulic resistance on the slot we can control the separation point of the jet and the jet orientation (the problem will be investigated in future work).

For an active control using synthetic jet concept [19], we use the same configuration as in the first case, but the configuration has an actuator with a lateral slot placed at the point of the detached boundary layer. The diaphragm oscillates in a sinusoidal way, with a frequency of 100 Hz and amplitude of 1 mm ( / <sup>1</sup> *<sup>j</sup> F fL U* ). For simulation purposes, an unsteady RANS, *k-* SST turbulence model with curvature correction is used [20]. The computation grid has 160,000 nodes and the y+ values of the wall-next grid points are between 0.05 and 1, and the x+ values between 10 and 100. In this investigation the separation was not completely suppressed and the boundary layer was not enough energized by the generated vortices structures (see Figure 10). A small unsteady deviation on the jet of about 3 degrees was noticed (see Figure 11).

**Figure 9.** Velocity vectors without (a) and with (b) slot.

Mathematical Modelling and Numerical Investigations on the Coanda Effect 117

**Figure 10.** Contours of vorticity magnitudes (maximum expulsion).

The first case study uses a simple convex surface and the second computational case uses the same convex surface with a slot between the over-pressure point and the under-pressure point on the surface (placed in separation boundary layer region). The tendency of equalizing the pressures leads to a blow in the first orifice of the slot, while in the second one the suction phenomenon occurs. The jet (*Uj*=25 m/s) is developed in a rectangular channel with 75 mm (height) x 20 cm (width) and passes over a convex surface (25 cm

The experimental model has 11 pressure probes disposed on the median plane of the

computation grid has 219,300 nodes. The suction-blowing phenomenon has a beneficial effect on keeping the boundary layer attached on 82% of the surface compared to the case without the slot when the boundary layer is attached to 58% of the surface. Figure 9 shows the velocity field in the computational domain and the pressure distribution on the surface for each of the aforementioned situations. The jet is deflected by 20 degrees from the original direction. Using a hydraulic resistance on the slot we can control the separation point of the

For an active control using synthetic jet concept [19], we use the same configuration as in the first case, but the configuration has an actuator with a lateral slot placed at the point of the detached boundary layer. The diaphragm oscillates in a sinusoidal way, with a frequency of 100 Hz and amplitude of 1 mm ( / <sup>1</sup> *<sup>j</sup> F fL U* ). For simulation purposes, an unsteady RANS, *k-*

 SST turbulence model with curvature correction is used [20]. The computation grid has 160,000 nodes and the y+ values of the wall-next grid points are between 0.05 and 1, and the x+ values between 10 and 100. In this investigation the separation was not completely suppressed and the boundary layer was not enough energized by the generated vortices structures (see Figure 10). A small unsteady deviation on the jet of about 3 degrees was noticed (see Figure 11).

a. b.

SST *c.c.* turbulence model and the

length). The shape of the surface is given by two elliptical fillet surfaces.

jet and the jet orientation (the problem will be investigated in future work).

Coanda surface and connected to a digital pressure scanner.

For computations we use steady RANS with a *k-*

**Figure 9.** Velocity vectors without (a) and with (b) slot.

**3.2. Passive control using a slot** 

**Figure 11.** Velocity vectors at maximum ingestion (a), and maximum expulsion (b).
