Sivaram Gogineni

Low-temperature supersonic boundary layer control using repetitively pulsed magnetohydrodynamic forcing

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short-pulse duration, high-voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization results show that as the Reynolds number increases, the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using laser differential interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0∼200–250Torr. A crossed discharge (pulser+dc sustainer) in M=3 flows of air and nitrogen produced a stable, diffuse, and uniform plasma, with the time-average dc current up to 1.0A in nitrogen and up to 0.8A in air. The electrical conductivity and the Hall parameter in these f...

Low-temperature M=3 flow deceleration by Lorentz force

This paper presents results of cold magnetohydrodynamic (MHD) flow deceleration experiments using repetitively pulsed, short pulse duration, high voltage discharge to produce ionization in M=3 nitrogen and air flows in the presence of transverse direct current electric field and transverse magnetic field. MHD effect on the flow is detected from the flow static pressure measurements. Retarding Lorentz force applied to the flow produces a static pressure increase of up to 17%–20%, while accelerating force of the same magnitude results in static pressure increase of up to 5%–7%. The measured static pressure changes are compared with modeling calculations using quasi-one-dimensional MHD flow equations. Comparison of the experimental results with the modeling calculations shows that the retarding Lorentz force increases the static pressure rise produced by Joule heating of the flow, while the accelerating Lorentz force reduces the pressure rise. The effect is produced for two possible combinations of the magne...

Low-Temperature Supersonic Boundary Layer Control Using Repetitively Pulsed MHD Forcing

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short pulse duration, high voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5 T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization shows that side wall boundary layers in the supersonic test section are considerably thicker near the center plane of the flow. The results also show that as the Reynolds number increases from Rex=2.7·10 5 to 8.1·10 5 , the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using Laser Differential Interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0~200-250 torr. Operation of a crossed discharge (pulser + DC sustainer) in M=3 flows of air and nitrogen demonstrated that such a discharge produces a stable, diffuse, and uniform plasma. The time-average DC current achieved in such discharges is up to 1.0 A in nitrogen (conductivity of σ=0.073 mho/m) and up to 0.8 A in air (σ=0.072 mho/m). The electrical conductivity and the Hall parameter in nitrogen and air flows are inferred from the current voltage characteristics of the sustainer discharge. LDI measurements detected MHD effect on the ionized boundary layer density fluctuations at these conditions. Retarding Lorentz force applied to M=3 nitrogen, air, and N2-He flows produces an increase of the density fluctuation intensity by up to 2 dB (about 25%), compared to the accelerating force of the same magnitude. The effect is demonstrated for two possible combinations of the magnetic field and current directions producing the same Lorentz force direction (both for accelerating and retarding force). Comparison with the LDI spectra measured with no MHD force applied showed that the effect on the density fluctuations is produced only by the retarding Lorentz force, while the Joule heat effect appears insignificant.

Measurement of Flow Conductivity and Density Fluctuations in Supersonic Nonequilibrium Magnetohydrodynamic Flows

A new blowdown nonequilibrium plasma magnetohydrodynamic (MHD) supersonic wind tunnel operated at complete steady state has been developed and tested at Ohio State. The wind tunnel can be operated at Mach numbers up to M = 3-4 and mass flow rates of up to 45 g/s at a stagnation pressure of 1 atm

Flow field in a low-speed axial fan: a DPIV investigation

Abstract The characteristics of the flow in a low-speed axial fan were investigated using digital particle image velocimetry (DPIV). Instantaneous and time-averaged velocity measurements were made at the leading-edge, trailing-edge, and suction and pressure sides of the blade by synchronizing the passage of the blade with the laser and camera system. These measurements revealed steady and unsteady flow features at several operating points and allowed a composite DPIV image around the entire fan blade to be made. This composite image was compared with a panel code solution, and the main differences found between them were attributed to local viscous effects such as flow separation and wake unsteadiness that are not included in the present panel code implementation.

Aero-Optical Mitigation of Shocks Around Turrets at Transonic Speeds Using Passive Flow Control

Experimental studies of the aero-optical effects around a partially-protruding cylindrical turret for a range of incoming transonic Mach numbers are presented and discussed. Spatially-temporally resolved wavefronts were collected using a high-speed Shack- Hartmann sensor and flow visualization was performed with a Schlieren system. Different flow regimes with a local shock, either a steady or an unsteady one, were described for the baseline case and the shock dynamics was found to be sensitive to a local flow speed. In addition, several passive flow control devices, consisted of a single spanwise row of vertically-placed small-diameter pins or porous screens, were tested in order to mitigate detrimental unsteady-shock-related aero-optical effects. It was found that passive flow control devices with large blockage values slowed the flow near the cylinder surface down to subsonic speeds by introducing total pressure losses in the wall region upstream of the cylinder, thus eliminating the shock formation over a wide range of transonic Mach numbers and significantly improving aero-optical environment at some elevation angles.

Experimental Observations of Instability Modes in a Rectangular Jet

The instability modes of a jet issuing from a rectangular nozzle of aspect ratio 4 have been studied experimentally at exit Mach numbers ranging from 0.03 to 1.5. Depending on the exit Mach number, several distinct characteristics are identified according to the arrangement of the flow structure with respect to the jet centerline. In the very low velocity range, U ≤ 20 m/s, a symmetric mode prevails. An antisymmetric mode dominates at all other Mach numbers, except in the range 0.6 ≤ M ≤ 0.85, where both symmetric and antisymmetric modes exist and there is a continuous switching between them

Estimation of Aero-Optical Wavefronts Using Optical and Non- Optical Measurements

6Air Force Research Laboratory, Directed Energy Directorate, Kirtland AFB, NM 87117 In this paper we present two algorithms for estimating the aero-optical aberration of a transonic flow around a 2-D turret based on Malley probe signals or pressure signals from few selected points. These two algorithms use Artificial Neural Networks and Linear Stochastic Estimation of varying model orders to estimate Proper Orthogonal Decomposition modal coefficients. These estimated coefficients are then used to reconstruct an estimated wavefront. This estimated wavefront is subtracted from the true wavefront to obtain a simulated reduction in the overall level of optical aberration. Reductions of up to 48% are achieved for both models. A robustness analysis is also performed, in which it is found that the algorithm is not sufficiently robust to changing flow conditions. Solutions are proposed for further investigation.

Investigation of Shock Dynamics on a Hemisphere Using Pressure and Optical Measurements

The aero-optical environment and pressure field around a hemispherical turret were experimentally studied in a wind tunnel between Mach numbers 0.5 and 0.65. The shock motion based on timeresolved wavefronts and shadowgraphs were analyzed and the average shock location, its spatial extension and temporal spectra were extracted. The typical shock frequency was at StD ~ 0.3 for M=0.63; these results correlate well with flight tests. When the shock was present on the hemisphere, pressure data both on the hemisphere and in its wake were found to be correlated in range of StD = 0.1..0.3, suggesting a global lock-in mechanism. The shock strength was found to be larger when the shock moves in the upstream direction, compared to the downstream motion. Possible links between the pressure around the hemisphere and shock dynamics were also discussed.

Identifying coherent structures in a 3-stream supersonic jet flow using time-resolved schlieren imaging

We analyze time-resolved schlieren images of the near-field of a 3-stream supersonic jet. The primary jet operates in the vicinity of Ma = 1.6, and the images are collected at the rate of 50 to 400 kfps. We analyze transverse-axial images by constructing time series from more than 400 points selected for their possible significance, based on a qualitative analysis of the schlieren images. The points are grouped along the various shear layers and in the near-field outside the jet. We examine in turn the power spectra and cross-correlations between points. Overall qualitative and quantitative trends in both spectra and correlation are noted, revealing a strong dependence of both on transverse and axial location in the flow field. Defining features in the spectra give insight into the frequency bands which will be more closely analyzed in future phases of this study. The results from this preliminary study point to the validity of using time-resolved schlieren imaging as a non-intrusive experimental method to generate time series, to which a range of analysis methods is applicable.