Engineering topics
Publications and source records attributed to Ayoub, A..
A spectral study of a hot-wire investigation in the near wake of a finite circular cylinder of high-aspect ratio is reported. The measurements included frequency spectra and cross correlations in spanwise and streamwise directions. The study identifies four spanwise regions, in terms of frequency, in the immediate wake of the finite cylinder.
The flow past a delta wing of aspect ratio one is examined at angles of attack ranging from 25 deg to 90 deg. On the basis of detailed observations of the flow in a water channel, using laser-induced fluorescence, four angle of attack flow regimes are identified, in addition to the well known regime of a stable pair of leading edge vortices. These are: a regime where the two leading edge vortices break down independently, primarily into the spiral form; a regime where spiral and bubble breakdowns alternate periodically on the two sides of the wing, accompanied by an antisymmetric motion in the chordwise direction of the two points of breakdown; a regime where a single streamwise vortex springing from the wing apex alternates in sign at irregular intervals; and finally, a regime dominated by a wake bubble and the relatively small scale vortices arising from the instability of its boundary.
The flow field generated by a two-dimensional airfoil with a statically deflected flap type spoiler has been experimentally studied. Subsonic tests ascertained the effect of varying spoiler deflection and angle of attack on the flow field at two Reynolds numbers. Measurements of the mean and fluctuating surface pressures and velocities in the wake, along with schlieren flow visualization of the near wake, were made. The results pertaining to the unsteady characteristics of the spoiler flow field are presented. It is shown that vortex shedding characterized the spoiler wake and determines the unsteady nature of the overall flow field; the character of the vortex shedding changes with spoiler deflection; and the fluctuating surface pressure field (vortex shedding frequency component) is of appreciable magnitude at large spoiler deflections.
The complicated flow in the tip region of a finite circular cylinder in uniform cross flow has been examined at the Reynolds numbers 85,000, 180,000, and 770,000. Simultaneous measurements of the surface-pressure and wake-velocity fluctuations have revealed the existence of a shedding regime in the tip region that is distinct from the one prevailing on the main body of the cylinder. In particular, this regime can be unstable and intermittent, can have a cellular structure in the wake, or can be subcritical when the main flow is supercritical.
Results of hot wire measurements made in the near wake at a Reynolds number of 9955 are reported. The measurements include the mean velocity profiles, root mean square values of the velocity fluctuations, frequency spectra, and velocity cross correlations. The mean velocity profiles were used to determine the wake width, whose variation in the downstream and spanwise directions was examined. It is observed that close to the cylinder, the wake is narrower toward the free end than it is away from it, while further downstream the wake is wider toward the tip than it is away from it. It is found that the flow over the span can be characterized by four regions: a tip region where vortex shedding occurs at a lower frequency than that prevalent for away from the tip; an intermediate region adjacent to the first one where a frequency component of a nonshedding character is present; a third region characterized by a gradually increasing shedding frequency with increasing distance from the tip; and a two dimensional region where the shedding frequency is constant.
The problem of determining the pressure fluctuations induced on the surface of a cylinder by the fluctuating wake behind it is formulated. A formal solution relating the unsteady surface pressure field to the velocity field in the wake is derived and used to obtain general results independent of cylinder shape and Reynolds number. The case of the circular cylinder is then examined in detail.