Aerodynamics

Enhanced aerodynamic performance that avoids flow separation on wing surfaces has been traditionally achieved by appropriate aerodynamic design of airfoil sections.  However, when the wing design is driven by non-aerodynamic constraints (stealth, payload, etc.) the forces and moments of the resulting unconventional airfoil shape may be much smaller than on a conventional airfoil.  Therefore, either active or passive flow control techniques can be used to enhance aerodynamic performance throughout the flight envelop.

Although passive control devices, such as vortex generators, have proven, under some conditions, to be quite effective in delaying flow separation, they offer no proportional control and introduce a drag penalty when the flow does not separate (or when they are not needed).

In contrast, active flow control enables coupling of the control input to flow instabilities that are associated with flow separation and thus may enable substanial control authority at low actuation levels.  Furthermore, active actuation is largely innocuous except when activated and has the potential for delivering variable power.  In previous studies, active control efforts have employed a variety of techniques including external and internal acoustic excitation, vibrating ribbons or flaps, and steady or unsteady blowing.

Over the past couple of decades, the synthetic jet actuator has emerged as a versatile actuator for active flow control.  The formation and evoluation of synthetic jets are described in detail in the work of Smith & Glezer (1998), Glezer & Amitay (2002), Amitay and Cannelle (2006), Van Buren et al. (2014).  The effectiveness of fluidic actuators based on synthetic jets is derived from the interaction of these jets with the flow near the flow boundary that can lead to the formation of a quasi-closed recirculating flow region, resulting in a virtual modification of the shape of the surface.

The aerodynamic research at CeFPaC has several objectives:  (1) understand the flow physics of the flow field of the system in question, (2) understand the flow mechanisms associated with the interaction between the flow and the actuators, (3) explore, experimentally and numerically, the feasibility of using active flow control for flight control, (4) develop low order models of the flow, and (5) develop a closed-loop control schemes.

Projects in Aerodynamics

sample TS cancellation

A turbulent boundary layer greatly increases the drag on a wing, and therefore aircraft fuel consumption.  By delaying the transition of organized, laminar flow into disorganized, turbulent flow, billions a year can be saved in fuel costs.  One way that this transition can be delayed is by using a vibrating surface element to suppress the Tollmien-Schlichting (TS) waves responsible.

In high speed helicopters, the reverse flow region present in the retreating side of the rotor disc can cause adverse effects on helicopter blades. During forward flight, the reverse flow region can form in the inboard section of the retreating blades due to the forward airspeed of the helicopter overcoming the local flow over the blades.

Sample OFV results over a NACA0015 airfoil using static tips

Drones and High Altitude Long Endurance vehicles typically operate at moderate to high Reynolds numbers based on airfoil chord length, i.e., Rec ≈ 105 to 106.  These vehicles are becoming increasingly important to applications like national security-related surveillance, search and rescue in dangerous terrain, scientific research, and animal conservation, among others. As such, the understanding flow conditions in such a way to ensure the safety of these aircraft is of paramount importance.

Separation is an adverse aerodynamic effect resulting in loss of aerodynamic performance. Previous studies on separation have mainly dealt with a two-dimensional analysis due to the assumption that the third dimension was negligible or that it was too complicated to analyze, leading to an incomplete analysis on separation. However, literature has indicated that this is insufficient in understanding separation and needs to be studied as such.

Previously tested 1/19th scale vertical tail model based on Boeing 767 tail with synthetic jet actuators

The addition of active flow control devices, such as synthetic jet actuators, on three-dimensional aerodynamic surfaces (i.e. vertical tail, wings, etc.) can lead to significant flowfield modification for beneficial improvements in aerodyanmic performance.  Previous work by Dr. Nicholas Rathay and collaborators on this project focused on augmenting the side force generated by synthetic jets through separation control on scaled vertical tail models.  Since commercial airplane tails are sized based on a single engine failure situation, they are larger than necessary for normal flight.

Sketch of SIA concepts (a) single-disk configuration (b) dual-disk configuration

Synthetic jet actuators (SJA) are zero net mass flux (ZNMF) devices traditionally constructed by forming a cylindrical cavity with an orifice in either its side or one of its bases (See Figure 1).  Piezoelectric disks are used as either one (a) or both (b) of the bases.  The disks are then vibrated by means of AC voltages.  As the disks move inward and outward, the volume of the cavity changes and air is rapidly ingested and ejected through the orifice.