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.

Surface oil flow visualization for a single pin

The goal of this research is to design and develop a new type of piezoelectric actuator based on dynamic surface modification in the form of low-aspect-ratio, dynamic, cylindrical pins.  The major objectives are to implement an array of these dynamic, discrete elements onto a flapped airfoil model, scaled for realistic conditions, as a means of controlling the separation over the deflected control surface.  This project encompasses both fundamental research as well as larger scale application.

This project focuses on the development and application of piezoelectric linear actuators in different examples of active flow control. The primary goal of this research is to build and quantify custom piezoelectric bending beam actuators; the piezoceramic used is Lead Zirconate Titanate. Different actuators with varying parameters such as piezoelectric thickness and beam length are being fabricated and tested. These devices are actuated with a periodic function, resulting in an oscillating platform on which to mount different flow control devices.

Currently many vertical tails on commercial aircraft are oversized in order to compensate for an extremely rare and specific emergency scenario: A single engine out in high crosswind during takeoff and landing. Our goal is to improve the performance of a smaller vertical tail, which would allow for higher deflection angles, and therefore higher sideforce, in order to compensate for the high yaw produced during these emergency scenarios. This would allow for a significant decrease in weight and drag, since the majority of an airplanes flight time is spent at cruise conditions.

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.

Compact inlets with an S-shape have become widely used in aircrafts due to the numerous advantages compared to conventional ducts.  However, due to their low length to diameter ration and low aspect ration, there are some flow phenomena, which are undesirable such as massive separation leading to losses in total pressure and secondary flow structures causing distortion at the Aerodynamic Interface Plane (AIP).  These drawbacks need yet to be overcome in order to apply these ducts on a wider range of vehicles.

In the design of aerodynamic vehicles, there must be some care in the design process that would allow the vehicle to move through air or water with the least resistance (smallest amount of drag). As the body can move easier through the flow, the amount of fuel used will be minimized and the carbon footprint can be reduced. 

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.

Applying bio-mimicry intelligence to the aerodynamic performance of tall slender buildings has potential to lead to not only improved response to wind loading, but generate savings in material and construction costs, affect energy consumption by providing self-shading and controlling local air flow to promote local wind energy generation and ventilation strategies.

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.