Hot-film sensors have been used to measure the boundary-layer fluctuations and subsequently to determine the boundary-layer states. In this paper, it is proposed to use hot-film sensors as a tool to measure the effects of plasma-induced ionic wind on the boundary-layer fluctuations which can be used to find the influence of the plasma actuator far downstream of the actuator. Time history and frequency response of the hot-film output voltage are investigated to explore the effects of plasma-induced wind downstream of the actuator. Also, the effects of the peak-to-peak voltage of a DBD plasma actuator on the fluctuations of the hot-film output voltages are discussed. It is showed that as far as the effects of ionic wind can be sensed by the hot-films, the control authority of the actuator is higher. It is concluded that induced velocity in the vicinity of the plasma actuator alone does not indicate the control authority of the plasma actuator.
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The boundary-layer control authority of a DBD plasma actuator using surface mounted hot-film sensors is evaluated. Wind tunnel experiments on a wind-turbine blade section were established at a Reynolds number of 0.27× 106. Aerodynamic performance of the wind-turbine blade section for both plasma-ON and plasma-OFF modes are evaluated using measurements made by both surface pressure and wake survey behind the model. Two distinct boundary-layer states are recognized. A state which occurs at the onset and in proximity of the deep stall, which is affected by the low-frequency instabilities of the separated flow. In this case, the steady actuation of plasma imparts local momentum on the nearby flow, eliminating the instabilities, hence, reattaching the detached flow. The other state happens beyond the static stall angle of attack of the airfoil where the flow over the suction side of the airfoil is fully separated and coexistence of both the leading edge and the trailing edge shear-layer instabilities and natural trailing edge vortex shedding is the underlying mechanism. In this case, although the plasma actuator eliminates the instabilities, to some extent, but the corresponding momentum injection is not efficient to stabilize and reattach the flow. ... Read more

Well-known polars in classical shock wave theory, that is, flow deflection angle-shock angle (θ-β), hodograph (u*,v*), and pressure deflection (θ-P*) diagrams, are investigated for the rarefied gas flows using a recently proposed shock wave detection technique by Akhlaghi and coworkers. The agreement between the obtained polars with the analytical relations in classical shock wave theory has been shown in the continuum limit for the cases of supersonic flow over the wedge and cylinder geometries. Investigations are performed using the RGS2D direct simulation Monte Carlo solver for supersonic gas flows over a circular cylinder at continuum limit and Kn = 10−4, 10−3, 0.01, 0.03, 0.07, and 0.10. Two species of nitrogen and argon at various Mach numbers of 1.5, 3.0, and 10.0 are considered. The shock polars are investigated along bow shock waves in front of the cylinder. The results indicate that rarefaction significantly affects the shock polars. As Knudsen number increases, shock angle, maximum flow deflection angle, and aft shock pressure increase. However, velocity components after the shock wave decrease as the flow becomes more rarefied. These effects are stronger for θ-β polar under the weak shock condition. Meanwhile, they are stronger for θ-P* and hodograph polars in strong shock situations. ... Read more

This paper introduces a shock-wave-detection technique based on the schlieren imaging for continuum and rarefied-gas flows. The scheme is applicable for any existing two-dimensional flowfields obtained by experimental or numerical approaches. A Gaussian distribution for a schlieren function within the shock-wave region is considered. This enables the authors to access any desired locations through the shock (e.g., shock center, or leading- and trailing-edge locations). The bow shock-wave profile is described via a rational function, which could be employed for the estimation of shock angle. The relation between pre- and postshock flow properties along the shock wave with a high resolution can be investigated by using this technique. The technique is verified in a novel way based on the well-known gas dynamics curves (i.e., flow-deflection angle vs shock angle and shock polar diagrams). High-speed continuum, near continuum, and rarefied-gas flows over a wedge and cylinder are considered for evaluation of the proposed technique. The results show a very good agreement with existing analytical relations for continuum flow. ... Read more

Experiments were conducted to study various kinds of shock wave/boundary layer interaction (SBLI) in an axisymmetric mixed-compression inlet. Further, some of the experimental findings were compared and verified by numerical solutions where possible. A classification of different types of SBLI relevant to the mixed-compression inlets is performed. Interactions of expelled normal shock wave/boundary-layer at subcritical and at buzz condition is investigated using Schlieren and shadowgraph flow visualization as well as unsteady pressure recordings. The data is further compared with the CFD prediction. Interactions of cowl lip reflected oblique shock and the terminal normal shock with the spike boundary-layer at both critical and supercritical operations that leads to shock trains and pseudo-shock phenomena are also studied. In this case numerical simulation results were used to illustrate the flow field. Experimental pressure recordings are used for validation and further discussion. The structure of SBLI flow in an inlet depends highly on the throttling value. For near critical throttling values, interaction of internal compression oblique shocks with boundary-layer and pseudo-shock phenomenon is dominant. Increasing the inlet back pressure pushes normal shock wave out of the inlet duct. Formation of lambda shock due to interaction of separated boundary-layer with normal shock wave is also investigated. The numerical and experimental results show that there exist different kinds of shock wave boundary-layer interactions relevant to the supersonic inlets. Each of these flow interaction phenomena has different effects on the stability and on the performance of the inlet. Interaction of terminal normal shock with internal duct boundary-layer causes pseudo-shock phenomenon that leads to increase of flow distortion and reduction of total pressure recovery. In addition interaction of normal shock wave with external cone boundary-layer causes buzz instability and degrades inlet performance. ... Read more

Effects of various boundary-layer bleed locations on the stability of an axisymmetric supersonic inlet has been investigated experimentally. The bleeds were located on the inlet compression cone and were designed to improve the stability margin of the inlet. The main objective of the study was to identify the boundary-layer bleed location and its effects on the improvement of the stability margin of the inlet. In addition, a buzz precursor detection methodology based on the rms level has been introduced. Experiments have been carried out on a mixed-compression inlet with a design Mach number of 2.0 and three different bleed locations at three freestream Mach numbers of 1.8, 2.0, and 2.2. All tests were conducted at an angle of attack of 0 deg. The results show that, based on the individual design of an inlet, a location can be found that the spilled normal shock stands at this location and consequently separation starts at the buzz onset. If the bleed slot is located in this place, the subcritical performance and stability margin of the inlet can be enhanced. ... Read more

The performance of a supersonic mixed compression air intake has been investigated experimentally. The intake is of axisymmetric one and has been designed for a free stream Mach number of 2.0. The present work has two main goals, first to investigate the performance of the intake without boundary layer bleed at design and at off-design conditions and second to study the effects of a bleed slot on the intake performance. The intake has been tested at free stream Mach numbers of 1.8, 2.0, and 2.2 and at zero degrees angle of attack. Total pressure recovery, mass flow ratio, and flow distortion have been selected to assess the intake performance. The bleed slot is located upstream of the intake throat perpendicular to the compression ramp surface. The suction is applied by the natural pressure difference between the entrance and the exit faces of the bleed duct. Results show that applying the boundary layer suction upstream of the intake throat can considerably improve the intake performance at its design and off-design conditions while it does not affect the intake mass flow rate. ... Read more