Open and closed-loop control of transonic buffet on 3D turbulent wings using fluidic devices

Abstract

This paper presents an overview of the work performed recently at ONERA on the control of the buffet phenomenon. This aerodynamic instability induces strong wall pressure fluctuations and as such limits aircraft envelope; consequently, it is interesting to try to delay its onset, in order to enlarge aircraft flight envelop, but also to provide more flexibility during the design phase. Several types of flow control have been investigated, either passive (mechanical vortex generators) or active (fluidic VGs, fluidic trailing-edge device (TED)). It is shown than mechanical and fluidic VGs are able to delay buffet onset in the angle-of-attack domain by suppressing the separation downstream of the shock. The effect of the fluidic TED is different, the separation is not suppressed, but the rear wing loading is increased and consequently the buffet onset is not delayed to higher angles of attack, but only to higher lift coefficient. Then, a closed loop control methodology based on a quasi-static approach is defined and several architectures are tested for various parameters such as the input signal, the objective function or, the tuning of the feedback gain. All closed loop methods are implemented on a dSPACE device calculating in real time the fluidic actuators command from the unsteady pressure sensors data.

Postponing the Onset and Alleviating the Load of Transonic Buffet by Using Steady and Periodic Tangential Slot Blowing

“…To save time and energy, high-speed flights are usually done by modern, large commercial aircraft, which always cruise between Mach 0.7 and 0.9 by employing supercritical wings [1][2][3]. These Mach numbers fall in the typical transonic flow range; consequently, unsteady shockwave/boundary layer interactions could induce intensive, large-scale, unsteady lift, and transonic buffet will be the result [4][5][6][7][8][9][10]. This transonic buffet not only decreases riding comfort [10][11][12], but structural fatigue could also be triggered, and disastrous accidents could even result [2,4,9,[13][14][15].…”

“…This transonic buffet not only decreases riding comfort [10][11][12], but structural fatigue could also be triggered, and disastrous accidents could even result [2,4,9,[13][14][15]. Thus, transonic buffet is deemed one of the most important factors limiting the flight envelope [2,7,9,14,16,17], since a margin of 30% on the lift coefficient at cruising conditions must be respected by design standards [7].…”

“…To delay the buffet onset margin, alleviate buffet loads, and widen the flight envelope, a lot of effort has been devoted to the development, including unveiling buffet mechanisms [4,8,[18][19][20][21][22], of functional control techniques [2,7,9,10,14,16,23,24], and so on. Meanwhile, studies have also been conducted to improve measuring techniques employed in wind tunnel tests [25], buffet onset prediction methods [17,26,27], numerical simulation techniques of shock/boundary layer interactions [6,11,25,[28][29][30][31], and so on.…”

“…They found that the time-averaged location of the shockwave was pushed downstream; thus, the root-mean-square of the oscillating force in the buffeting zone decreases, and the total lift is enhanced. Molton et al [36] and Dandois et al [7] tested mechanical, continuous, and pulsed fluidic vortex generators located upstream from the shock foot to reduce the extent of the separated area, and fluidic actuators as well as mechanical vortex generators have proven to be very efficient in postponing buffet onset. Ogawa et al [37] proposed a method of using three-dimensional bumps on upper wing surfaces to alleviate the strength of flow interference between the shockwave and boundary layers, and the results indicated that a 30% decrease of total drag can be achieved for the best control case.…”

A CFD benchmark of active flow control for buffet prevention

“…In the case of transonic buffet phenomenon, it has been shown that fluidic vortex generators located upstream of the shock foot can reduce the extent of the separated area and are very efficient to postpone the buffet onset [22]. However, the effect of the fluidic TED is different, the separation is not suppressed, but the rear wing loading is increased and consequently the buffet onset is not delayed to higher angles of attack, but only to higher lift coefficient [13]. Fluidic TED have also found applications on nozzle engines: fluidically enhanced chevrons have also shown promising results for supersonic jet noise reduction [20].…”