Norman Wood

Towards a practical piezoceramic diaphragm based synthetic jet actuator for high subsonic applications - Effect of chamber and orifice depth on actuator peak velocity

An investigation on the effect of geometry and actuation variables on peak jet velocity of a piezoelectric diaphragm synthetic jet actuator was conducted using hotwire anemometry. Two distinct experiments were conducted; the first at constant excitation amplitude with the aim of identifying the SJA geometry that provides the highest jet velocity. This geometry was then used in a second experiment with constant geometry to explore the effects of excitation amplitude on actuator jet velocity. The results show that the actuator jet velocity is inversely proportional to both the orifice and chamber depth over the range of parameters tested. The actuator parameters that provide the highest peak jet velocity were identified to be H/Do = 0.6 and h/Do = 2.1, where H is the chamber depth, h is the orifice depth and Do is the orifice diameter. Excitation at the mechanical diaphragm resonance frequency resulted in jet velocities double the magnitude of those observed at cavity resonance. The Helmholtz theory consistently over estimated the cavity resonance frequency by ≈ 18%. Peak actuator velocities of around 130m/s from a 1.2mm diameter orifice can now be reliable achieved. The outcome of this investigation is considered to be a positive step on the understanding on how design and actuation parameters influence the performance of a synthetic jet actuator.

Experimental investigation of twin-fin buffeting and suppression

An investigation has been undertaken to study the characteristics of vertical fin buffeting for various twin-fin configurations at high angles of attack. Tests were performed in the 2.1 X 1.5 m low-speed wind tunnel at the University of Bath, using three 60-deg cropped generic delta wings of 0.5-m span. To sense unsteady pressures at the fin surface, a rigid fin instrumented with miniature pressure transducers was fabricated, whereas a flexible fin of similar planform and size was used to measure the buffeting response. It was found that the fin configurations were typified by two maximum buffeting conditions. The first peak response was effectively a vortex/fin interaction in isolation, whereas the second peak response was the result of a vortex shear-layer oscillation between the fins. A novel flow concept (tangential leadingedge blowing) was then used to decrease the levels of buffeting response for all fin configurations. By reducing the effective angle of attack of the vortical flow, peak buffeting responses were shifted to higher angles of attack.

Dye visualisation of inclined and skewed synthetic jets in a cross flow

Dye visualisation of both inclined synthetic jets and skewed synthetic jets was undertaken in a cross-flow experiment and the results were compared with those of normal synthetic jets. The process of vortex roll-up near the orifice exit and how the structure develops and interacts with the cross-flow as it propagates downstream was investigated so as to obtain an understanding of the effect of orifice orientation on the behaviour of synthetic jets. The effects of varying Reynolds number, velocity ratio and Strouhal number due to changes in diaphragm displacements and freestream velocities on the characteristics of synthetic jets were also examined. It is observed that in comparison to the normal jets vortical structures produced by both inclined and skewed jets tend to stay closer to the near wall region where maximum flow control effect is required. In both cases, at a relatively low Reynolds number and velocity ratio the active structures produced by the synthetic jet appear to be hairpin vortices which turn into vortex rings that migrate away from the wall as the Reynolds number and velocity ratio increase. These hairpin vortices are persistent in the near wall region hence are believed to be desirable structures for delaying flow separation.

A PIV study of synthetic jets with different orifice shape and orientation

*† ‡ § PIV measurements have been undertaken on synthetic jets with rectangular orifices of different aspect ratios and circular orifices of different inclination angles in quiescent conditions. The results were compared with those from normal circular synthetic jets so as to examine the effects of orifice shape and orientation on the characteristics of synthetic jets. The results show that increasing aspect ratio leads to an initially higher axial velocity at the exit of the orifice. Although the synthetic jet with a high aspect ratio appears to be twodimensional when they just form, they are highly unstable and experience subsequent “axis swapping” between the major and minor axis of the jet. It is also found that an increase in orifice inclination angle results in a higher jet exit velocity and more asymmetrical roll-up of vortex rings which undergo rapidly self reorganization to form axisymmetric structures further downstream. Overall, although synthetic jets with a higher aspect ratio or inclination angle possess higher initial momentum and vorticity than equivalent normal circular jets, this additional energy tends to be dissipated rapidly due to the need for the distorted vortex rings to rearrange themselves into more stable structures. Further investigations will be carried out to extend the present study to boundary layers and separated flows so as to assess the potential of rectangular jets and inclined jets in producing more effective flow structures that can delay flow separation on high lift systems.

Investigation of Phase and Spacing effects in Synthetic Jet Actuator arrays

Experiments have been undertaken with two small-scale Synthetic Jet Actuators in a zero-pressure gradient boundary-layer, in order to investigate the effect of relative input signal phase. CTA anemometry techniques were used, demonstrating that changes in the downstream flow structure could be observed. Time-averaged streamwise velocity profiles demonstrated that a case with a 270 o phase difference between the upstream and downstream actuator resulted in a more concentrated movement of near-wall fluid towards the vortex core than a 90 o case. Significant differences in the PSD analyses of downstream streamwise velocity time histories were found, suggesting that input signal phase could influence the stability and hence effectiveness of flow structures used in flowcontrol applications. Further investigation using phase-locked CTA data acquisition showed that the relative positions of the two adjacent vortex rings were reversed between cases, suggesting that the resulting induced velocities could affect the stability of the combined synthetic jet.

PIV measurements of the effect of pitch and skew on a circular orifice synthetic jet in a turbulent boundary layer

-1 , the velocity ratio between the freestream and the exit peak velocity of the jet was set to 1.2, the Strouhal number to 0.069 and the boundary layer thickness was measured to be 9 times the diameter of the orifice. Differences in the roll-up process and in the evolution of the structures formed in the interaction with the boundary layer are presented at a location 10D downstream of the orifice for normal, pitched and pitched and skewed jets. For the normal and pitched synthetic jet it is shown that vorticity in upstream edge of the ejected structure is suppressed and that truncated hairpin like structures are produced consistent with the flow structures typically observed in flow visualization studies. For the pitched and skewed jet, the resulting hairpin structure is asymmetric about the streamwise vertical plane, with significantly greater vorticity in the upper leg of the hairpin. This type of structure is similar to that produced by conventional vane vortex generators or steady air jet vortex generators and shows promise as an optimal flow control configuration.

Development of a Dynamic Wind Tunnel Model for Demonstration of Flow Control Maneuver Effectors

Flow control maneuver effectors offer the potential for flight vehicle control with out the use of conventional hinged control surfaces. This paper describes the design, development and test of a dynamic wind tunnel model to demonstrate two specific flow control technologies for vehicle control, namely Fluidic Thrust Vectori ng (FTV) for pitch control and Compliant Aerodynamic control (CAT) for roll control. The latter technology is based on the concept of designing regions of flow that are close to separation, such that they are receptive to small flow control inputs. The dem onstrator model is based on a generic blended wing UCAV planform and can be configured to be free in pitch or free in roll. A key challenge in the model design was porting high pressure air for the FTV system into the model across a rotational degree of fr eedom. This was solved using bespoke high pressure sealed bearings. Experiments were conducted in the University of Manchester 2.1x2.8m wind tunnel at a free stream speed of 30m/s. Preliminary qualitative results show that the model can be successfully con trolled in both pitch and roll using the flow control technologies under investigation, paving the way for future quantitative experiments.

Effect of Sweep on Buffet over Novel Wing Planforms

The results of wind tunnel testscarried outon two semispan models, with sweep angles of 40 and 60 deg, arepresented. The planforms of the models have features consistent with likely cone gurations of future low-observable aircraft. An examination of pressure e uctuations over such aircraft is of particular importance because of the dife culties involved in altering such aircraft late in their design to address problems of excessive buffeting. The dynamic calibration of surface pressure tappings has allowed both mean and unsteady pressures to be measured. Unsteady pressure readings have been analyzed to derive both root-mean-square pressures and spectral information. Peaks in the spanwise root-mean-square pressure distribution are found inboard of the leading edge vortex for each wing. Two additional peaks are present over the more highly swept wing. One of these peaks is coincident with the secondary separation; another is found outboard of the leading edge vortex.

The Application of Synthetic Jet Actuators for the Modification of the Characteristics of Separated Shear Layers

This paper presents an experimental investigation related to controlling the unsteady characteristics of the separated shear layers occurring over highly swept wings, and in particular focuses on application of synthetic jet actuators for modification of unsteady dynamic loading on the wing surface due to the phenomenon referred to as vortex breakdown (burst). In the post burst flow region the surface pressure measurements reveal the presence of certain characteristic spectral peaks that are thought to represent the presence of a spiralling filament of vorticity inside the expanded vortex that is known to be present in the burst flow over swept wings. This paper details an investigation into how the use of an array of 18 discrete synthetic jet actuators, distributed along the leading edge of a delta wing with a 60° sweep angle, can be used to alter the spectral content of this unsteadiness and reduce the level of unsteady pressure found in the post-burst region toward the wing trailing edge by up to 40%. Measurements of the surface pressure spectral distributions over the wing are presented together with PIV measurements of the vortex cross-section, conducted in the successive planes parallel to the wing trailing edge. Additional surface flow visualisation indicates that the effect of the actuators on the leading edge boundary layer is to induce local separation delays close to each actuator orifice, which introduce “ripples” into the shear layer as it separates. The results obtained are used to formulate an interpretative hypothesis attempting to explain the mechanisms responsible for modification of the spectral content and the level of excitation measured on the wing surface.Copyright

An Investigation into the Use of Synthetic Jets for the Control of Surface Unsteadiness due to Vortex Breakdown

This paper details the findings of experiments that have attempted to identify the fluid mechanical mechanisms that allowed an array of synthetic jet actuators to alter the unsteadiness levels in the post-burst region of the flow over a 60° angle of sweep, delta wing configuration. The experiments made use of an array of synthetic jet actuators embedded in the wing leading edge to try to alter the shear layer that separates from the leading edge before rolling up to form the vortex which subsequently bursts at a location downstream of the wing apex. The frequencies used to actuate the flow were related to, and were an order of magnitude larger than the dominant frequencies found in the burst region that was to be altered. Following on from earlier experiments that had shown that this technique was able to reduce the surface rms pressures in the post-burst region by up to 30%, this paper details the findings of a series of PIV and surface flow visualisation experiments that examined the burst vortex itself and the associated leading edge separation line. The findings showed that the effect of actuation was in fact to produce time-averaged separation delays in the leading edge separation line, local to each actuator. These delays produced kinks (or ripples) in the separated shear layer that, in turn, led to a series of variations in the vortex diameter. From these results it has been postulated that this variable diameter feature of the vortex leads to the coherent spiral structure known to exist within the burst flow being de-stabilized due to the production of self induced Biot-Savart velocities. Additionally it is shown that actuation directly into the post-burst region is less effective at producing the rms reductions previously seen, than actuation of the pre-burst area of the vortex.