Fundamental investigation of the flow around a finite span low aspect ratio pin and its application to flow control

Abstract

The fundamental flow physics associated with the interaction of a low aspect ratio, cantile-vered cylindrical pin with a crossflow were investigated experimentally using several measurement techniques. The flow field sensitivity to varied geometric parameters such as, the pin’s aspect ratio (ratio of pin’s height to its diameter) and its relative submergence (the ratio of pin’s height to the local boundary layer) was studied using stereoscopic particle image veloci-metry (SPIV), surface pressure measurements, hotwire anemometry, and qualitative oil flow visualizations. The flow field associated with a finite span, cantilevered pin is highly three-dimensional due to the strong dependence between the pin’s free end flow and its wake, impacting the entire span of the pin. Moreover, the present geometric conditions facilitate a sub-critical aspect ratio, such that the dominant mode of vortex shedding is symmetric. The strong end effects also contribute large downwash in the form of entrained momentum to the near wake, decreasing the pressure drag and re-energizing the flow near the wall.

A separate study was conducted to evaluate the global impacts from the low aspect ratio pins over a deflected control surface of a NACA 0012 airfoil as a potential means of separation control using SPIV and surface pressure measurements. An array of pins, fully submerged within a turbulent flow, and placed upstream of a strong adverse pressure gradient and severely separated flow, demonstrated significant reduction in separation and total drag, while increasing the global circulation (i.e., lift). Most notably, the influences from a single static pin were compared to that of a dynamic pin and results indicated an ability to lock on and amplify the shedding frequency associated with the wake of the airfoil. These results provide further evidence of the potential uses for low aspect ratio pins (static or dynamic) as effective devices for a wide-range of applications, and more specifically, for separation control.

From these results, further investigations were conducted on effective control mechanisms to exploit the narrow band of frequencies for which the flow is most receptive to. The dynamic pin, driven at the wake’s fundamental frequency, produced a lock-on effect such that the coherent large-scale structures were augmented, effectively promoting enhanced mixing and net momentum into the near wake. Under the excitation of the JASMA, at frequencies related to the convective shear layer and absolute instabilities, the added momentum from the synthetic jet demonstrated an ability to vector and narrow the wake, re-distribute the turbulent kinetic energy, and generate a large-scale vortex near the rear base of the cylinder.

In general, it was shown that the surrounding flow field is highly sensitive to the subtle changes in the pin’s aspect ratio and relative submergence, specifically within the tested range of ~ O(1). Moreover, these flow structures are also heavily influenced by competing mechanisms of self-induction and the shear flow due to outer fluid entrainment and the proximity to the wall. In particular, the salient vortical features observed in the mean flow field (i.e., the tip, horseshoe, arch, and trailing vortices), exhibit relatively strong interdependencies. The tip vortices, which emanate from the pin’s free end, reveal variations in their formation and strength based on the incoming flow (i.e., laminar or turbulent) as well as the relative submergence within the boundary layer. Moreover, as the aspect ratio is increased (i.e., the diameter is decreased) these features become less distinctive, and likewise, so do the streamwise trailing vortices. The inverse was demonstrated for lower aspect ratio pins (with increasingly bluff cross-sections), such that the level of interaction between the vortical features is increased due to reduced mean downwash and secondary vorticity in the wake. These behaviors promote the formation of a base pair of vortices, initiated at the pin base-wall junction.

Overall, the present study has demonstrated a means by which to achieve significant flow field modifications and transport mechanisms throughout the flow, while minimizing losses via implementation of these active techniques to low aspect ratio, cantilevered cylindrical pins.

Subsequently, two active mechanisms, specifically in the form of dynamic motion and inte-grated synthetic jet, were incorporated with the low aspect ratio pin. The former was tested to achieve large-scale vertical oscillatory motion of the pin at prescribed frequencies to yield lock-on phenomenon, while the latter, known as the Jet Assisted Surface Mounted Actuator (JASMA), focused on unsteady vorticity and momentum addition. These novel methods were tested for the same geometric and flow conditions as the baseline (static) cylindrical pin, in order to evaluate the efficacy and overall potential for system performance enhancement applications.