Engineering topics
Publications and source records attributed to Modarress, D..
Beam-shaping diffractive optical elements are used to create structured light patterns in fluid flows. Particle scattering results in detected signals that can be used to determine the particle size and velocity.
Light scattering off particles flowing through a two-slit interference pattern can be used to measure the shear stress of the fluid. We have designed and fabricated a miniature diffractive optic sensor based on this principle.
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A high-sensitivity interferometer has been developed and used to detect boundary-layer transitions over a symmetric airfoil. The tests, which included both natural and roughness-induced transitions, were performed in a transonic wind tunnel. The measurements showed a peak amplitude rms and higher energy in the spectrum of the signal associated with transition. The tests revealed that the interferometer system can be used to locate the transition region over three-dimensional aerodynamic models.
A new algebraic reconstruction technique (ART) code based on the iterative refinement method of least squares solution for tomographic reconstruction is presented. Accuracy and the convergence of the technique is evaluated through the application of numerically generated interferometric data. It was found that, in general, the accuracy of the results was superior to other reported techniques. The iterative method unconditionally converged to a solution for which the residual was minimum. The effects of increased data were studied. The inversion error was found to be a function of the input data error only. The convergence rate, on the other hand, was affected by all three parameters. Finally, the technique was applied to experimental data, and the results are reported.
A high-sensitivity differential interferometer has been developed to detect small density fluctuations over an optical path length of the order of the boundary layer thickness near transition. Two experimental configurations have been used to evaluate the performance of the interferometer: an open shear-layer configuration and a wind-tunnel turbulent spot configuration. In each experiment small temperature fluctuations were introduced as the signal source. Simultaneous cold-wire measurements have been compared with the interferometer data. The comparison shows that the interferometer is sensitive to very weak phase variations of the order of 0.001 of the laser wavelength.
A high sensitivity differential interferometer was developed to locate the region where the boundary layer flow undergoes transition from laminar to turbulent. Two laboratory experimental configurations were used to evaluate the performance of the interferometer: open shear layer, and low speed wind tunnel turbulent spot configuration. In each experiment, small temperature fluctuations were introduced as the signal source. Simultaneous cold wire measurements were compared with the interferometer data. The comparison shows that the interferometer is sensitive to very weak phase variations in the order of 0.001 the laser wavelength. An attempt to detect boundary layer transition over a flat plate at NASA-Langley Unitary Supersonic Wind Tunnel using the interferometer system was performed. The phase variations during boundary layer transition in the supersonic wind tunnel were beyond the minimum signal-to-noise level of the instrument.
A high sensitivity differential interferometer has been developed to locate the region where the boundary layer flow changes from laminar to turbulent. Two experimental configurations have been used to evaluate the performance of the interferometer, open shear layer configuration and wind tunnel turbulent spot configuration. In each experiment small temperature fluctuations were introduced as the signal source. Simultaneous cold wire measurements have been compared with the interferometer data. The comparison shows that the interferometer is sensitive to very weak phase variations in the order of .001 the laser wavelength.
A high sensitivity laser interferometry for continuous monitoring of optical path changes is presented. The sensitivity to small changes in the optical path is achieved by means of a feedback system which keeps the interferometer at its maximum (and linear) position. Two experiments were designed to verify the interferometer's performance, both in direct and in scattering configurations. In each case, small temperature fluctuations were introduced as the signal source. Simultaneous measurement of temperature were compared with the results of the interferometer. Good correlation between the results were obtained. Finally, the application of the high sensitivity interferometer for detection of boundary-layer transition is examined. Optical signal strength and possible sources of noise are examined. Some methods of model surface preparation for enhancement of signal-to-noise ratio are discussed.
This is a summary and discussion of the state of the art in the use of laser diagnostics in fluid and plasma dynamics. This will encompass: interferometry, scattering, spectroscopy, and velocimetry. Examples of such applications will be provided in the area of fluid dynamics of particle generation and breakup, analysis of three-dimensional density fields, and applications in combustion.
The existence of 'sampling bias' in individual-realization laser velocimeter measurements is experimentally verified and shown to be independent of sample rate. The experiments were performed in a simple two-stream mixing shear flow with the standard for comparison being laser-velocimeter results obtained under continuous-wave conditions. It is also demonstrated that the errors resulting from sampling bias can be removed by a proper interpretation of the sampling statistics. In addition, data obtained in a shock-induced separated flow and in the near-wake of airfoils are presented, both bias-corrected and uncorrected, to illustrate the effects of sampling bias in the extreme.
Boundary-layer measurements realized by laser velocimetry are presented for a Much 2.9, two-dimensional, shock-wave/turbulent boundary-layer interaction containing an extensive region of separated flow. Mean velocity and turbulent intensity profiles were obtained from upstream of the interaction zone to downstream of the mean reattachment point. The superiority of the laser velocimeter technique over pressure sensors in turbulent separated flows is demonstrated by a comparison of the laser velocimeter data with results obtained from local pilot and static pressure measurements for the same flow conditions. The locations of the mean separation and reattachment points as deduced from the mean velocity measurements are compared to oil-now visualization results. Representative velocity probability density functions obtained in the separated now region are also presented. Critical to the success of this investigation were: the use of Bragg cell frequency shifting and artificial seeding of the now with submicron light-scattering particles.
Calculations of boundary layer flows past blunt bodies at angles of incidence are presented. Using the method of integral relations together with the method of lines, the full three-dimensional boundary layer equations are reduced to a system of first-order ordinary differential equations. The streamwise shear stress function and the cross-flow velocity component are represented as suitable functions of the streamwise velocity component. The role of the zone of dependence is automatically satisfied by the choice of differencing in the method of lines. Solutions correct to the second order are obtained in the positive shear region for flow over an ellipsoid at 30-deg incidence. The results are compared with corresponding finite difference solutions.
The method of integral relations is extended to general three-dimensional compressible laminar boundary layer flows. The transformation employed to transform the basic three-dimensional compressible boundary layer equations into quasi-incompressible form is an extension of the Howarth transformation. The resulting system of differential equations is integrated numerically by the method of integral relations as proposed by Dorodnitsyn. To demonstrate the accuracy of the method, it is applied to calculation of the parabolic flow over a flat plate and the boundary flow over an infinite yawed cylinder, for which solutions are known. It is then applied to the flow over a flat plate disturbed by a cylinder normal to the plate, for which a finite-difference solution is available for comparison. It is finally applied to calculating the crossflow velocity variation for supersonic flow over a swept wedge.
Results of experimental investigations into turbulent boundary-layer behavior under the influence of pressure gradients and with separation are presented for transonic and supersonic flow fields. In the transonic case, an axisymmetric model was implemented consisting of an annular circular arc bump affixed to a circular cylinder aligned with the flow direction. For the supersonic separation study, an oblique shock wave impinging on the wind tunnel wall boundary layer was employed to cause separation. The mean streamwise and normal velocity components as well as the respective turbulence intensities were obtained with a two-color frequency shifted laser velocimeter.
Boundary-layer measurements realized by laser velocimetry are presented for a Mach 2.9, two-dimensional, shock-wave/turbulent boundary-layer interaction containing an extensive region of separated flow. Mean velocity and turbulent intensity profiles were obtained from upstream of the interaction zone to downstream of the mean reattachment point. The superiority of the laser velocimeter technique over pressure sensors in turbulent separated flows is demonstrated by a comparison of the laser velocimeter data with results obtained from local pitot and static pressure measurements for the same flow conditions. The locations of the mean separation and reattachment points as deduced from the mean velocity measurements are compared to oil-flow visualization results. Representative, velocity probability density functions obtained in the separated flow region are also presented. Critical to the success of this investigation were: (1) the use of Bragg cell frequency shifting and (2) artificial seeding of the flow with submicron light-scattering particles.
As an alternative to pure finite-difference techniques, the Method of Integral Relations (MIR) is formulated for three-dimensional boundary-layer flows with separation, and applied to two problems. The first concerns an incompressible laminar flow over a plate with an attached cylinder, where the potential solution of the flow round the circular cylinder is used as the boundary condition. In the second application the solution to the three-dimensional boundary-layer flow over blunt bodies is discussed and results for an ellipsoid of revolution at 30 deg incidence are presented. In both cases the results are found to be in satisfactory agreement with those obtained by finite-difference methods.
Wind tunnel investigations of a shock-induced separated flow and the flow about a two-dimensional airfoil section at transonic speeds using laser velocimetry techniques are described. Laser velocimeter data are presented and compared with conventional pressure type measurements (pitot and static pressure probe measurements for the shock-induced separation, and surface pressure measurements for the two-dimensional airfoil section). In addition to these data comparisons, a simple optical arrangement for the incorporation of a Bragg cell in a laser velocimeter system is described. The effects of particle concentration gradients within a turbulent boundary layer on laser velocimeter measurements are also demonstrated.