Open-field scale-model experiments of fire whirls over L-shaped line fires

This paper presents the results of open-field scale-model experiments of fire-whirl formation over line fires. L-shaped line fires were burned in crosswinds, and the processes of fire-whirl formation were observed. The flame height was measured using an image-processing technique, while two-dimensional velocity components were measured at two different locations using ultrasonic anemometers. Two tests were selected for comparison: test A, in which intense fire whirls repeatedly formed, and test B, in which no whirls were observed. In test A, the wind flow was bent by the fire plume, creating swirling flows near the burning area, thereby forming fire whirls. On the other hand, the crosswind in test B was too fast to be affected by the fire plume. These results confirmed the existence of critical wind velocity to form intense fire whirls. The critical wind velocity, approximately 1 m/s, agreed with the scaling law on the critical wind velocity which was previously developed based on similar experiments of a smaller scale

An Effective Three-Dimensional Layout of Actuation Body Force for Separation Control

We conducted large eddy simulations of the control of separated flow over an airfoil using body forces and discuss the role of a three-dimensional vortex structure in separation control. Two types of cases are examined: (1) the body force is distributed in a spanwise uniform layout and (2) the body force is distributed in a spanwise intermittent layout, with three-dimensional vortices being expected to be generated in the latter cases. The flow fields in the latter cases have a shorter separation bubble than those in the former cases although the total momentum of the body force in the latter cases is the same as or half of the former cases. In the flow fields of the latter type, the three-dimensional vortices, which are not observed in the former cases, are generated by the body force downstream of the body force distributed. Thus, three-dimensional vortices are considered to be effective in controlling the separated flow

In-Flight Demonstration of Stall Improvement Using a Plasma Actuator for a Small Unmanned Aerial Vehicle

The flow control capability (especially for separation control) of a dielectric-barrier-discharge plasma actuator (DBD-PA) has been investigated extensively. However, these studies have been conducted under ideal conditions, such as wind tunnels and computational environments, and limited studies have investigated the effects of plasma actuators in an actual environment. In this study, the flow control capability of a DBD-PA under natural and in-flight conditions was investigated via field flight tests using an unmanned aerial vehicle (UAV). The DBD-PA driving system was constructed with a small high-voltage power supply on a 2-m-span UAV. With the support of an autonomous flight system, the pitch angle gradually increased as the airspeed decreased, and the stall occurred from the cruise state. This flight procedure was conducted with the DBD-PA on or off, and 246 pairs of flights were operated. The results revealed that a flight state with a higher pitch angle and lower airspeed occurred when DBD-PA was switched on. In addition, the moment of stall was quantitatively determined from the flight log, and it was confirmed that the maximum pitch angle when DBD-PA was switched on tended to be larger than that when DBD-PA was switched off. These results indicate that flow control with a DBD-PA on a 2-m-span UAV was effective in natural and in-flight situations

Dynamic Burst Actuation to Enhance the Flow Control Authority of Plasma Actuators

A computational study was conducted on flows over an NACA0015 airfoil with dielectric barrier discharge (DBD) plasma. The separated flows were controlled by a DBD plasma actuator installed at the 5% chord position from the leading edge, where operated AC voltage was modulated with the duty cycle not given a priori but dynamically changed based on the flow fluctuations over the airfoil surface. A single-point pressure sensor was installed at the 40% chord position of the airfoil surface and the DBD plasma actuator was activated and deactivated based on the strength of the measured pressure fluctuations. The Reynolds number was set to 63,000 and flows at angles of attack of 12 and 16 degrees were considered. The three-dimensional compressible Navier–Stokes equations including the DBD plasma actuator body force were solved using an implicit large-eddy simulation. Good flow control was observed, and the burst frequency proven to be effective in previous fixed burst frequency studies is automatically realized by this approach. The burst frequency is related to the characteristic pressure fluctuation; our approach was improved based on the findings. This improved approach realizes the effective burst frequency with a lower control cost and is robust to changing the angle of attack

Flow Control around NACA0015 Airfoil Using a Dielectric Barrier Discharge Plasma Actuator over a Wide Range of the Reynolds Number

In this study, an experimental investigation of separation control using a dielectric barrier discharge plasma actuator was performed on an NACA0015 airfoil over a wide range of Reynolds numbers, angles of attack, and nondimensional burst frequencies. The range of the Reynolds number was based on a chord length ranging from 2.52 × 105 to 1.008 × 106. A plasma actuator was installed at the leading edge and driven by AC voltage. Burst mode (duty-cycle) actuation was applied, with the nondimensional burst frequency ranging between 0.1–30. The control authority was evaluated using the time-averaged distribution of the pressure coefficient Cp and the calculated value of the lift coefficient Cl. The baseline flow fields were classified into three types: (1) leading-edge separation; (2) trailing-edge separation; and (3) the hysteresis between (1) and (2). The results of the actuated cases show that the control trends clearly depend on the differences in the separation conditions. In leading-edge separation, actuation with a burst frequency of approximately F+= 0.5 creates a wide negative pressure region on the suction-side surface, leading to an increase in the lift coefficient. In trailing-edge separation, several actuations alter the position of turbulent separation