Wind tunnel tests with using CLC- CLC-system

Wind tunnel tests program with using the CLC-System for active flow control included the following issues:

Influence of the value of pressure coefficient (Cpc) used by the CLC-System as a signal for opening or closing the valves, on the pressure distribution along the airfoil chord, on the airfoil aerodynamic characteristics and on the air jets volume flow rate.

Influence of the airfoil angle of attack and undisturbed flow velocity on the airfoil aerodynamic characteristics.

All the wind tunnel tests were performed with the same sequence of changes in the angle of flap deflection, i.e. initially the flap was deflected from the = 0⁰ up to = 40⁰with angular speed = 1.4-1.5deg/s, then was kept deflected ( = 40⁰) for about 20 seconds and finally restored to the starting position ( = 0⁰) with approximately same angular speed. During all these studies CLC System operated and control the opening or closing the valves. With a fully open electromagnetic valves, the proportional valve (contained in air supplying system, Fig. 6) was so positioned, that total (from 36 nozzles) air volume flow rate was VFR ≈ 120 m³/h (which correspond to air jet velocity Vj ≈ 90 m/s).

The example of wind tunnel test results with usage of the CLC-System for active flow control are presented in Fig. 12. This figure shows changes in the total airfoil lift coefficient during the flap deflection. The next 10 plots (Fig. 13) present the changes in the pressure distribution on the upper and lower surface of the flap and rear part of the airfoil main body over time (during the CLC-System operation at = 40⁰) . These plots are shown in the order,

that first presents the moment, when due to CLC System operation, it begins to growth the negative pressure on the upper flap surface in the leading edge area. The last plot presents approximately the same moment after one full cycle of the CLC System operation (i.e. plots cover the entire cycle of the CLC-System action).

Fig. 12 Airfoil lift coefficient during the CLC-System operation.

An Experimental Study of a Separation Control on the Wing Flap Controlled by Close Loop System

CEAS 2013 The International Conference of the European Aerospace Societies

Fig. 13 Changes in the pressure distribution over time.

5 Conclusion

In the project ESTERA complete Closed Loop Control System for fluidic active flow control was designed and manufactured together with the necessary controller unit. The CLC System prototype has been tested experimentally on a two dimensional airfoil model NACA 0012 equipped with movable flap. The tests were performed in the low speed wind tunnel T-1 in the Institute of Aviation for Mach numbers M = 0.1, 0.075 and 0.05.

Performed wind tunnel tests of the CLC-System prototype showed the following:

Blowing is an effective way to increase the lift coefficient achieved by the airfoil with strongly deflected slotted flap. It was recorded maximally 30% increase in the lift coefficient. The lift increased not only due to reduction at a separation zone on the flap but also due to increasing a negative pressure on the upper surface of the main airfoil body (suction effect of the air blowing).

The tests confirmed hypothesis, that the measurement of the pressure on upper-aft part of the flap (in one point only) allows to detect the separation.

The investigation confirmed an efficiency of the CLC System as a way to increase lift with relatively low volume flow rate of the compressed air. Using a pulsed jets controlled by CLC System, the volume flow rate was diminished from VFR ≈ 120 m³/h (steady blowing) to VFR ≈ 68 m³/h (for Cpc = 0.0 ) and to VFR ≈ 33 m³/h (for Cpc = -0.4).

The increase in airfoil CL value due to CLC System operation was generally independent of the value of pressure coefficient, used by the CLC-System as a signal for opening or closing the valves, i.e.

Cpc.

Duration of the one complete cycle of the CLC-System operation was about t ≈ 65 ms. Since the opening of the valves to the full flow attachment on the flap passes about 12 ÷ 14 ms. On the other hand since the closing of the valves to the full flow

separation on the flap passes about 27 ÷ 28 ms.

During the flap deflection (the increase of the flap deflection from = 0⁰ to = 40⁰ and decrease from = 40⁰ to = 0⁰ was tested), the angle of attack significantly affects the beginning and end of the CLC System operation. The increase in the airfoil angle of attack delays the start of the CLC-System operation.

References

[1] Gad-el-Hak M., Flow Control – Passive, Active, and Reactive Flow Management, Cambridge University Press, 2000.

[2] Gad-el-Hak M., “Flow control - The future”, Journal of Aircraft, No. 38, 2001, pp. 402-418.

[3] Nishri A., Wygnanski I., “Effects of periodic excitation on turbulent flow separation from a flap”, AIAA Journal, No. 36, 1998, pp. 547–556.

[4] Melton L., Yao C. and Seifert A., “Active control of separation from the flap of a supercritical foil”, AIAA Journal, No. 44, 2005, pp. 34–41.

[5] Seifert A., Greenblat D., Wygnanski I., “Active Separation Control: an Overview of Reynolds and Mach Numbers Effects”, Aerospace Science and Technology, No. 8, 2004, pp. 569–582.

[6] Seifert A., Pack L., “Compressibility and Excitation Location Effects on High Reynolds Numbers Active Separation Control”, Journal of Aircraft, No. 40, 2003, pp. 110–126.

[7] Krzysiak A., Narkiewicz J., “Aerodynamic Loads on Airfoil with Trailing-Edge Flap Pitching with Different Frequencies”, Journal of Aircraft, No.2, 2006, pp. 407-418.

[8] Alam M., Liu W., Haller G., “Close-Loop Separation Control - An Analytic Approach”, Physics of Fluids, No. 18, 2006, (043601) [9] Bright M., Culley D., Braunscheidel E., Welch

G., ”Closed Loop Active Flow Separation Detection and Control in a Multistage Compressor”, NASA/TM 213553, 2005.

[10] Krzysiak A,. Hanzlik A., Ruchała P., “Final report of the ESTERA project”, Institute of Aviation, Poland, 2013.

CEAS 2013 The International Conference of the European Aerospace Societies Abstract

CFD has become increasingly important in the design of systems for store separation. It offers opportunities to investigate complex flow physics interacting with the separated store, which is a basis for the design of the store release unit (ERU). An integrated system for numerical simulation of store separation by solving, quasi-steady or unsteady, Euler or Navier-Stokes equations is presented. The flow computations are coupled to a 6-DOF rigid body motion module. The grid is deformed in order to conform to the moving boundaries and remeshed when the grid deformation module fails to achieve sufficient grid quality. The computational method is assessed against experimental data for an AGARD test case, separation of a generic finned-store shape at transonic speed from a wing-sting-pylon configuration. The computational results compares well with wind tunnel measurements.

1 Introduction

In the last decades, computational fluid dynamics coupled to 6-DOF simulation have been applied to analyze store separation scenarios. The three main components of the

computational model consists of flow solver, grid system and flight mechanics model.

Quasi-steady Euler computations have been successfully applied to simulate separation of external weapons Ref. [1-3]. If stores are separated from weapons bays, the flow around the weapons is unsteady and the computations must be time accurate viscous simulations in order to capture relevant flow physics, Ref. [4-5].