Redha Wahidi

Experimental Investigation of the Boundary Layer and Pressure Measurements on Airfoils with Laminar Separation Bubbles

In an effort to understand the behavior of the laminar separation bubbles on NACA 0012 and LA2573a airfoils at different Reynolds numbers and angles of attack, the boundary layer over the airfoils was investigated by measuring the mean and fluctuating components of the velocity profiles over the upper surfaces of the airfoils. Also, surface pressure measurements were carried out to complete the mapping of the laminar separation bubble and to calculate the lift generated by the airfoils. The surface pressure distributions were in good agreement with the results obtained from XFLR5. The locations of separation, transition and reattachment were determined as functions of angle of attack and Reynolds numbers for the two airfoils.

Control of Laminar Separation Bubbles with Distributed Suction- Preliminary Studies

Operational requirements for modern uninhabited aerial vehicles (UAVs) include extended loiter at high altitudes. The combination of high altitude, small size and typically low flight speeds for long endurance means that the UAV airfoil will operate in a low Reynolds number regime, where laminar separation bubbles (LSBs) are known to create airfoil performance problems. This paper describes preliminary studies into the suction distributions required to prevent or control laminar separation.

Effects of grooves on the formation of the LEV of an impulsively-started flat plate

This experimental investigation examines the effects of longitudinal and transverse square grooves on the growth of the leading edge vortex produced by a two-dimensional flat plate impulsively started to constant velocity in quiescent water. The experimental conditions include Reynolds numbers based on the chord length and average velocity of 1518, 3036 and 6072 and angles of attack of 90° and 75°. The vortex formation time defined by the travel distance of the plate and chord length is 0.6. The DPIV data are reduced to compare the vorticity fields and circulation of the smooth and grooved plates, and to examine the Reynolds number and angle of attack dependency. The comparison is carried out in context of circulation growth rate and LEV size and magnitude. Moreover, the effect of the secondary vortex of opposite sign on the growth of the LEV is also discussed. The smooth plate results indicate that the circulation growth rate is sensitive to the angle of attack only for the higher Reynolds number cases where the growth rates begin to decrease at t/T > 0.5. The decrease in the growth rate suggests reductions in the size and magnitude of the vortex. The results also show that the LEV separates sooner at the higher angle of attack. Negative vorticity induced by the LEV is found in all cases and is conjectured to be associated with a secondary vortex. This secondary vortex plays a role in modifying the growth of the size and magnitude of the LEV. The longitudinal grooves cause this secondary vortex to increase in size and magnitude, whereas the transverse grooves have opposite effect. However, the transverse grooves have a more pronounced effect on the circulation growth rate.

Volumetric Three-Component Measurements of Air Jet Flows of Different Diffuser Designs

The effects of three diffuser configurations on the near-field region have been experimentally evaluated with the goal to reduce the losses and noise of an existing large subsonic wind tunnel. Preliminary analysis were performed on over thirty diffuser designs and the most promising designs were selected for the experimental investigation. The two most promising diffusers are a 3.5°half-apex angle diffuser with a constant-area at the end, and a modified version of this diffuser with the addition of an annular conical diffuser after the flat section. The annular conical section consists of three concentric truncated cones. The air jet flow was measured with a volumetric three-component velocimetry. This experimental investigation is considered one of the first flow measurements using the volumetric three-component velocimetry in air flow. The measurement technique was assessed with a round free jet flow. The results of the round free jet show that the potential core region is shorter relative to results in literature. The results also show that in the nearfield region, the variations in the mean axial velocity decrease before the variations in the streamwise component of the Reynolds normal stresses, whereas the variations in the other components of the Reynolds normal stresses persist for a longer distance. Comparison between the existing diffuser and the modified diffusers shows that the modified diffusers reduce the velocity in the jet which supports the preliminary computational results. Also, the Reynolds shear stresses in the shear layer of these diffusers are suppressed. The modified diffuser with the an annular conical diffuser produces more reduction in the jet velocity suggesting its effectiveness in reducing the head losses. Also, this diffuser produces less turbulence, hence less noise.

Suction Effects on the Transition and Reattachment of a Transitional Bubble

The effects of distributed suction on the transition and reattachment of the transitional bubble on an airfoil at a Reynolds number of 250,000 are investigated by analyzing the mean and fluctuating velocity profiles. It is found that suction delays separation and transition and decreases the turbulence intensity at these locations. Suction also affects the development of the turbulent boundary layer downstream of reattachment by causing the mean velocity profiles to be “fuller” and reducing the turbulence intensity in this region. Investigating the power spectra of the streamwise velocity fluctuations shows that suction suppresses the freestream disturbances upstream of the suction distribution. However, once suction begins, these disturbances amplify significantly.

Investigation of Boundary Layer Flows Over a Flat Plate With Different-Size Transverse Square Grooves at s/w = 10

Turbulent boundary layer flows over a flat plate with multiple transverse square grooves spaced 10 element widths apart were investigated. Mean velocity profiles, turbulence intensity profiles, and the distributions of the skin-friction coefficients (Cf) and the integral parameters are presented for two grooved walls. The two transverse square groove sizes investigated are 5mm and 2.5mm. Laser-Doppler Anemometer (LDA) was used for the mean velocity and turbulence intensity measurements. The skin-friction coefficient was determined from the gradient of the mean velocity profiles in the viscous sublayer. Distribution of Cf in the first grooved-wall case (5mm) shows that Cf overshoots downstream of the groove and then oscillates within the uncertainty range and never shows the expected undershoot in Cf. The same overshoot is seen in the second grooved-wall case (2.5mm), however, Cf continues to oscillate above the uncertainty range and never returns to the smooth-wall value. The mean velocity profiles clearly represent the behavior of Cf where a downward shift is seen in the Cf overshoot region and no upward shift is seen in these profiles. The results show that the smaller grooves exhibit larger effects on Cf, however, the boundary layer responses to these effects in a slower rate than to those of the larger grooves.Copyright