Fei-Bin Hsiao

Forcing level effects of internal acoustic excitation on the improvement of airfoil performance

The effects of internal acoustic excitation on the leading-edge, separated boundary layers and the aerodynamic performance of NACA 633-018 cross section airfoil are examined as a function of forcing level and forcing frequency of the introduced acoustics. Tests are separately conducted in two suction, open-typed wind tunnels at the Reynolds number of 3.0 x 10 s for the measurements and 1.0 x 10 4 for the visualization. Results indicate that the flow separation is delayed at the angles of attack higher than the stalled angle of small level excitation with the forcing frequency fe near the shear layer instability frequency ft. As the forcing level is increased to some extent, the velocity fluctuations around the slot exit are demonstrated to be the primary governing parameter for modifying the separated boundary layers. Data also show that the effective forcing frequency (and the Strouhal number, 50 extends over wider range as compared to the lower level excitation. Meanwhile, the pressure distributions on the airfoil surface exhibit recovery behaviors with different forcing frequencies. The corresponding boundary layers are visualized to be reattached to the surface to form a recirculation region when the airfoil is around at the stalled angles.

Generalized guidance law for homing missiles

The concept of a generalized guidance law is presented, and the closed-form solution for a homing missile pursuing a maneuvering target according to generalized guidance laws is given. It is shown that the guidance laws appearing in the literature are merely special cases of the one proposed by the authors. The derived generalized forms of capture area, missile acceleration, and homing time duration that are derived provide insight into the performance of the guidance laws being considered and lead to the discovery of new ones. The problem of finding a nonlinear optimal guidance law for a homing missile with controlled acceleration, applied so as to capture a maneuvering target with a predetermined trajectory while minimizing a weighted linear combination of time of capture and energy expenditures, is solved in closed form. The derived optimal control law is equal to the LOS (line of sight) rate multiplied by a trigonometric function of the aspect angle. Numerical simulation shows that the resulting guidance law appears to yield a significant advantage over true proportional navigation. >

Development of a Low-Cost Attitude and Heading Reference System Using a Three-Axis Rotating Platform

A development procedure for a low-cost attitude and heading reference system (AHRS) with a self-developed three-axis rotating platform has been proposed. The AHRS consists of one 3-axis accelerometer, three single-axis gyroscopes, and one 3-axis digital compass. Both the accelerometer and gyroscope triads are based on micro electro-mechanical system (MEMS) technology, and the digital compass is based on anisotropic-magnetoresistive (AMR) technology. The calibrations for each sensor triad are readily accomplished by using the scalar calibration and the least squares methods. The platform is suitable for the calibration and validation of the low-cost AHRS and it is affordable for most laboratories. With the calibrated parameters and data fusion algorithm for the orientation estimation, the self-developed AHRS demonstrates the capabilities of compensating for the sensor errors and outputting the estimated orientation in real-time. The validation results show that the estimated orientations of the developed AHRS are within the acceptable region. This verifies the practicability of the proposed development procedure.

INFLUENCE OF INTERNAL ACOUSTIC EXCITATION UPON FLOW PASSING A CIRCULAR CYLINDER

The flow past a circular cylinder under internal acoustic excitation is studied by measurements of the velocity and pressure fields. Acoustic waves are introduced internally from a slot on the surface of the cylinder. Pressure distributions around the cylinder, the pressure drag and lift of the cylinder, the energy spectra of flow structures, and the downstream wake profiles are carefully investigated. Results indicate that the separated shear layers around the cylinder are sensitive to the excitation frequency in the regime of shear layer instability and to the forcing location around the laminar separation point. The excitation on the cylinder causes an increase in lift and a dramatic reduction in drag. Also, the wake defect profile is narrowed due to the reduction of large-scale vortices to smaller eddies. The vortex shedding frequency is slightly altered when the flow is excited at the effective frequency. The Strouhal number for the most effective excitation is found to be of the order of one in the operating Reynolds number range. The result is verified with a flow visualization study using the smoke-wire technique.

ON THE MODE DEVELOPMENT IN THE DEVELOPING REGION OF A PLANE JET

The development of the mode arrangement in the developing region of an acoustically excited plane jet is extensively studied using hot-wire measurements. Two single-wire probes are located within the two shear layers of the jet to detect the flow structure patterns simultaneously. The jet is excited at its fundamental frequency in either varicose mode (m=0) or sinuous mode (m=1) condition. Experimental results show that different excitation mode conditions lead to different spreading rates and velocity fluctuation distributions in the developing flow field. The frequency spectral distributions for the m=1 excitation exhibit double peaks of fa and fb, rather than a single subharmonic f0/2 for the m=0 excitation. These three components constitute a relationship of (fa+fb)/2=f0/2. The mode switching phenomenon is found to be prominent under the m=0 case, while less pronounced for the m=1 case. The mode development is mainly governed by the evolution of the primary instabilities in the jet shear layers.

Influence of surface flow on aerodynamic loads of a cantilever wing

The properties of the characteristic surface flow modes and their influence on the aerodynamic loads of a cantilever wing at low chord Reynolds numbers were experimentally studied. The chordwise variations of the separation and reattachment locations as well as the length and center position of the separation bubble in the domain of the chord Reynolds number/root angle of attack heavily depend on the characteristic surface flow modes. The aerodynamic loads, e.g., the lift, drag, and moment coefficients, are profoundly influenced by the characteristic behaviors of suction surface flow. The lift coefficient increases with the increase of root angle of attack in the regimes of laminar separation, separation bubble, and transition. The curve of lift coefficient has a largest slope in the laminar separation regime. The increase rate of the lift coefficient decreases when the separation bubble is formed. The stall happens in the turbulent separation regime. The drag coefficient slightly decreases in the laminar separation regime, remains almost a constant in the separation bubble regime, and increases in the transition regime.

On the evolution of instabilities in the near field of a plane jet

The dynamic behaviors of coherent structures in the near field of a plane jet are extensively studied by hot‐wire measurements. The instability evolution for both streamwise and transverse velocity fluctuations are investigated simultaneously. The shear stresses contributed by the individual instabilities are detected by using the cross‐spectrum technique. Experimental results show that the jet spreading is governed by the vortex interaction mechanism, and the evolution of the instabilities for the two velocity components is found to perform distinct patterns. The characteristics for the instability evolution cannot be adequately identified from only streamwise velocity fluctuations. The vortex formation and merging locations should be determined from the saturation condition of the transverse velocity fluctuations v’( f ). Results also indicate that the instabilities cannot only absorb energy from the mean flow, but also transfer their energy to the subharmonics, or even back to the mean flow. The variat...

Double row vortical structures in the near field region of a plane wall jet

The qualitative and quantitative behaviour of double row vortical structures in the near field region of a plane wall jet are studied experimentally by flow visualization and hot-wire measurements. Ensemble averaging is employed to investigate the interaction of vortices with the wall. In the flow visualization study, a double row vortical structure, which includes a primary vortex formed in the outer layer region and a secondary vortex induced in the inner layer region, and the vortex lift-off phenomenon are clearly observed during the development of the wall jet. The phase averaged results of the velocity measurements show that the instability leading to induction of the secondary vortex is stimulated by the primary vortex. In the early stage of wall jet transition, the inflection point of the inner layer velocity profile moves transversely from the wall surface to the inner layer region due to passage of the well-organized primary vortex in the outer layer region. The inner layer instability is thus induced and the instability wave rolls up to form the secondary vortex. Furthermore, the secondary vortex will convect downstream faster than the primary vortex, and this difference in convective speed will lead to the subsequent phenomenon of vortex lift-off from the wall surface.

Experimental studies on flow transition of a plane wall jet

Flow transition in the developing region of a plane wall jet is studied experimentally by means of hot-wire measurements. The Reynolds number, based on the jet exit velocity and nozzle exit height, varies from 3 x 10 2 to 3 x 10 4 in the investigations. The results indicate that the most important factor causing wall jet flow to change from the initial exit state to the final turbulent state is the rapid increase of turbulent intensity from the formation of vortical structures and their interactions with the wall. Data also shows that the transition process of the wall jet depends on the Reynolds number in the operating range. When the Reynolds number is greater than 2000, the mean velocity distribution of the wall jet changes directly from a top-hat profile at the nozzle exit to a turbulent profile in the turbulent developed region. When the Reynolds number is smaller than 2000, Glauerts laminar velocity profile is found between the nozzle exit and the turbulent region. It is also found that the transition will be delayed and the transition region prolonged with the decrease of the Reynolds number.

Experimental study of cellular shedding vortices behind a tapered circular cylinder

Abstract The dynamic behavior of shedding vortices in turbulent wake behind a tapered circular cylinder is experimentally studied by hot-wire measurements in a low-speed wind tunnel. Four tapered cylinders with the same fineness ratio but different tapered ratio are used in the investigations. The Reynolds number based on the cylinders mean diameter ranges from 4.0×103 to 1.4×104. Data show that two to three constant-frequency vortex cells of the shedding vortices along the spanwise direction are observed in the operating Reynolds number range. The corresponding Strouhal number based on the vortex shedding frequency and the cylinders local diameter varies with respect to the taper ratio of the cylinder. It is also found that most of the spanwise shedding vortices are well aligned across the intersection region of the vortex cells except the vortex dislocation occurs to cause vortex lines discontinuous, in which the phase difference between the adjacent vortex cells jumps from π to −π. By applying the pseudo-visualization technique in conjunction with the peak-valley counting method, a simplified model is proposed to explain the relationship between the velocity modulation and the vortex dislocation.