Chia Min Leong

Three-dimensional interaction of a finite-span synthetic jet in a crossflow

The formation and evolution of flow structures due to the interaction of a finite-span synthetic jet with a zero-pressure gradient laminar boundary layer were experimentally investigated using stereoscopic particle image velocimetry. A synthetic jet with three orifice aspect ratios of AR = 6, 12, and 18 was issued into a free-stream velocity of U∞ = 10 m/s (Reδ = 2000) at blowing ratios of Cb = 0.5–1.5. The interaction was found to be associated with two sets of flow structures: (1) a recirculation region downstream of the orifice due to virtual blockage, and (2) a steady streamwise vortex pair farther downstream. These two flow structures were characterized in detail. Tube-like velocity deficits in the free-stream were evident, as well as regions of increased velocity within the boundary layer. Reducing the aspect ratio of the orifice decreased the spacing of the edgewise vortices (generated due to the finite span of the orifice) as well as reducing the virtual blockage of the jet. A control volume analysis of the fluid streamwise momentum indicates that there is a momentum deficit just downstream of the jet orifice and the change in streamwise momentum is proportionally similar for all cases.

Impact of orifice orientation on a finite-span synthetic jet interaction with a crossflow

The formation and evolution of flow structures associated with a finite-span synthetic jet issued into a zero-pressure gradient boundary layer were investigated experimentally using stereoscopic particle image velocimetry. A synthetic jet with an aspect ratio of AR = 18 was mounted on a flat plate and its interaction with a free stream, having a velocity of U∞ = 10 m/s (Reδ = 2000) at momentum coefficients of Cμ = 0.08, 0.33, and 0.75, was studied. The effect of the orifice pitch (α = 20∘–90∘) and skew (β = 0∘–90∘) angles on vortex formation as well as the global impact of the synthetic jet on the flow field was explored in detail. It was found that the orifice orientation had a significant impact on the steady and unsteady flow structures. Different orifice skew and pitch angles could result in several types of vortical structures downstream, including: no coherent vortex structure, a single (positive or negative) strong vortex, or a symmetric vortex pair. In all cases, the velocity near the wall was increased; however, cases with higher blockage (i.e., more wall-normal, transverse orifice) resulted in a strong velocity deficit in the free stream where orifices with lower pitch angles yielded in an increase in velocity throughout. The analysis is concluded with a summary of quantitative metrics that allude to flow control effectiveness.

Interaction of a synthetic jet with the flow over a low aspect ratio cylinder

This paper discusses the interaction of a single synthetic jet with the flow over a finite span low aspect ratio cylinder. The synthetic jet was located at the mid-span of the cylinder, and was oriented such that the length of the jet orifice was parallel to the freestream direction. The investigation incorporated experimental techniques such as stereoscopic particle image velocimetry (SPIV) and hot-wire anemometry. The near wake of the cylinder was dominated by a non-negligible spanwise velocity, or downwash, from the cylinders free-end that interacted with the near wake resulting in a highly three-dimensional flow field with a strong spanwise dependence. SPIV measurements within the near wake showed that, due to the actuation of the synthetic jet, the wake was vectored and narrowed, resulting in a redirection of the downwash behind the cylinder. Furthermore, the synthetic jet actuation resulted in a localized region of inward-directed flow at a spanwise location outboard of the jet orifice leading to a...

Interactions of a Dynamic Vortex Generator with a Cross-flow: An Experimental Study

Vortex generators (VG) delay or prevent boundary layer separation, for example, on airfoils at high angles of attack, by promoting mixing. This mixing replaces the lowmomentum flow near the wall with the high-momentum flow from outside of the boundary layer region. This allows for the flow to overcome adverse pressure gradient and stay attached. However, a major disadvantage of a VG that is static (referred to as SVG) is the increased drag it causes even when the vortex generator is not needed (for example, during cruise condition). On the other hand, a vortex generator that is deployable can be retracted when not needed and thus, fully avoids the drag penalty when not in use. Furthermore, a dynamic vortex generator (DVG) uses periodic excitation in order to improve its effectiveness as well as to reduce the drag penalty when in use. A numerical study of a SVG and DVG in a laminar boundary layer was conducted. The VG was placed at a fixed angle of 18◦. The mean height was set to be the local boundary layer thickness and the corresponding Reynolds number at the VG location based on the local boundary layer thickness (Reδ) was about 2000. The actuation parameters, amplitude and frequency, were varied between 17% to 33% of the local boundary layer thickness and 38 Hz to 150 Hz, respectively.Time-averaged, phased-averaged and fluctuating velocity fields were recorded for different cases and compared at several downstream locations in the wake of the vortex generator. In the DVG cases, an increase was observed in the phaseaveraged and time-averaged streamwise velocity component at the core of the primary vortex. This increase became larger with an increase in the actuation parameters while change in the frequency resulted in higher changes in the streamwise velocity component. The amplitude of actuation was found to be an important factor in promoting mixing, where the larger actuation amplitude resulted in higher kinetic energy due to the fluctuating velocity components which persisted farther into the wake.

Interaction of a Single Synthetic Jet with a Finite Aspect Ratio Circular Cylinder

The interaction of a finite-span synthetic jet with a finite span low aspect ratio cylinder is explored experimentally using Stereoscopic Particle Image Velocimetry (SPIV) measurements in the near wake and along the surface of the cylinder in order to understand how local small changes to the near surface flow field result in large alterations to the wake structure. A single synthetic jet located at mid-span and azimuthally at  = 90˚ with respect to the freestream velocity was oriented such that the long dimension of its orifice was parallel to the freestream direction. Without flow control, downwash from the free end dominates the near wake of the cylinder, resulting in suppression of the von-Karman vortex street and the creation of a symmetrically-shedding arch-type vortex. Once the synthetic jet is actuated, the interaction between the synthetic jet and the local near-surface flow induces large-scale global changes to the flow field, altering the circulation and a narrowed wake, while also creating a vortex dislocation along one leg of the arch-type vortex outboard of the synthetic jet orifice. Above the jet orifice, a pair of counter-rotating streamwise vortices are formed, which entrain flow from the separated region inboard of the jet orifice and delay separation outboard of the jet orifice, creating the narrowed wake and vortex dislocation seen in the near wake measurements.

Stability and Control on a Low Aspect Ratio Cantilevered Circular Cylinder

The stability of the near wake of a low aspect ratio cantilevered cylinder, with and without the influence of synthetic jet forcing, was estimated using piecewise two-dimensional inviscid stability analysis. Using a cusp method, the size of the 3-D vortex formation length was estimated. When the synthetic jet was activated it decreased the vortex formation length, altering the spectral content of the near wake, in agreement with experimental measurements. Furthermore, the turbulence intensity within the near wake could be estimated from the eigenfunctions corresponding to the most amplified spatial instability modes.