Engineering topics

Choudhari, Meelan

Publications and source records attributed to Choudhari, Meelan.

50 records · Page 3

NASA NTRS1994

Localized and distributed boundary-layer receptivity to convected unsteady wake in free stream

Receptivity to a model convected disturbance in the presence of localized and distributed variations in wall geometry and wall-suction velocity is examined. The model free-stream disturbance corresponds to the time-harmonic wake of a vibrating ribbon that is placed at a suitable distance above the surface of a thin airfoil. The advantages of using this disturbance for experiments on receptivity to convected disturbances are outlined. A brief parametric study is presented for a flat-plate boundary layer. The study quantifies the effect of wake position as well as wake width; in addition, it should be helpful in the choice of an optimal setting for a controlled experiment of the above type, which the above parametric study shows as feasible.

Choudhari, Meelan↗

NASA NTRS1994

Long-wavelength asymptotics of unstable crossflow modes, including the effect of surface curvature

Stationary vortex instabilities with wavelengths significantly larger than the thickness of the underlying three-dimensional boundary layer are studied with asymptotic methods. The long-wavelength Rayleigh modes are locally neutral and are aligned with the direction of the local inviscid streamline. For a spanwise wave number Beta much less than 1, the spatial growth rate of these vortices is O(Beta(exp 3/2)). When Beta becomes O(R(exp -1/7)), the viscous correction associated with a thin sublayer near the surface modifies the inviscid growth rate to the leading order. As Beta is further decreased through this regime, viscous effects assume greater significance and dominate the growth-rate behavior. The spatial growth rate becomes comparable to the real part of the wave number when Beta = O(R(exp -1/4)). At this stage, the disturbance structure becomes fully viscous-inviscid interactive and is described by the triple-deck theory. For even smaller values of Beta, the vortex modes become nearly neutral again and align themselves with the direction of the wall-shear stress. Thus, the study explains the progression of the crossflow-vortex structure from the inflectional upper branch mode to nearly neutral long-wavelength modes that are aligned with the wall-shear direction.

Choudhari, Meelan↗

NASA NTRS1993

Roughness-induced generation of crossflow vortices in three-dimensional boundary layers

The receptivity theory of Goldstein and Ruban is extended within the nonasymptotic (quasi-parallel) framework of Zavol'skii et al to predict the roughness-induced generation of stationary and nonstationary instability waves in three-dimensional, incompressible boundary layers. The influence of acoustic-wave orientation, as well as that of different types of roughness geometries, including isolated roughness elements, periodic arrays, and two-dimensional lattices of compact roughness shapes, as well as random, but spatially homogeneous roughness distributions, is examined. The parametric study for the Falkner-Skan-Cooke family of boundary layers supports our earlier conjecture that the initial amplitudes of roughness-induced stationary vortices are likely to be significantly larger than the amplitudes of similarly induced nonstationary vortices in the presence of acoustic disturbances in the free stream. Maximum unsteady receptivity occurs when the acoustic velocity fluctuation is aligned with the wavenumber vector of the unsteady vortex mode. On the other hand, roughness arrays that are oriented somewhere close to the group velocity direction are likely to produce higher instability amplitudes. Limitations of the nonasymptotic theory are discussed, and future work is suggested.

Choudhari, Meelan↗

NASA NTRS1993

Inviscid vortex motions in weakly three-dimensional boundary layers and their relation with instabilities in stratified shear flows

We consider the inviscid instability of three-dimensional boundary-layer flows with a small crossflow over locally concave or convex walls, along with the inviscid instability of stratified shear flows. We show how these two problems are closely related through the forms of their governing equations. A proposed definition of a generalized Richardson number for the neutrally stable inviscid vortex motions is given. Implications of the similarity between the two problems are discussed.

Blackaby, Nicholas D.↗

NASA NTRS1993

Boundary-layer receptivity due to distributed surface imperfections of a deterministic or random nature

Acoustic receptivity of a Blasius boundary layer in the presence of distributed, 2D surface irregularities is investigated analytically. It is shown that, out of the entire spatial spectrum of the surface irregularities, only a narrow band of Fourier components can lead to an efficient conversion of the acoustic input at any given frequency to an unstable eigenmode of the boundary-layer flow. Random irregularities which are spatially homogeneous in nature, and also possess a continuous spectrum in the streamwise direction, lead to instability amplitudes that are intermediate to those caused by the periodic and isolated irregularities, respectively, being, in fact, of the same order as the geometrical mean of the amplitudes in the latter two cases. A physical explanation for these asymptotic scalings is given, in addition to providing an analytical expression for the expected value of the instability amplitude for an ensemble of statistically irregular surfaces with random phase distributions.

Choudhari, Meelan↗

NASA NTRS1992

A finite Reynolds-number approach for the prediction of boundary-layer receptivity in localized regions

Previous theoretical work on the boundary layer receptivity problem has utilized large Reynolds number asymptotic theories, thus being limited to a narrow part of the frequency-Reynolds number domain. An alternative approach is presented for the prediction of localized instability generation which has a general applicability, and also accounts for finite Reynolds number effects. This approach is illustrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer due to the interaction of a free stream acoustic wave with a region of short scale variation in the surface boundary condition. The specific types of wall inhomogeneities studied are: regions of short scale variations in wall suction, wall admittance, and wall geometry (roughness). Extensive comparison is made between the results of the finite Reynolds number approach and previous asymptotic predictions, which also suggests an alternative way of using the latter at Reynolds numbers of interest in practice.

Choudhari, Meelan↗

NASA NTRS1992

On the spatial evolution of long-wavelength Goertler vortices governed by a viscous-inviscid interaction

The generation of long-wavelength, viscous-inviscid interactive Goertler vortices is studied in the linear regime by numerically solving the time-dependent governing equations. It is found that time-dependent surface deformations, which assume a fixed nonzero shape at large times, generate steady Goertler vortices that amplify in the downstream direction. Thus, the Goertler instability in this regime is shown to be convective in nature, contrary to the earlier findings of Ruban and Savenkov. The disturbance pattern created by steady and streamwise-elongated surface obstacles on a concave surface is examined in detail, and also contrasted with the flow pattern due to roughness elements with aspect ratio of order unity on flat surfaces. Finally, the applicability of the Briggs-Bers criterion to unstable physical systems of this type is questioned by providing a counterexample in the form of the inviscid limit of interactive Goertler vortices.

Choudhari, Meelan↗

NASA NTRS1992

Distributed acoustic receptivity in laminar flow control configurations

A model problem related to distributed receptivity to free-stream acoustic waves in laminar flow control (LFC) configurations is studied, within the Orr-Sommerfield framework, by a suitable extension of the Goldstein-Ruban theory for receptivity due to localized disturbances on the airfoil surface. The results, thus, complement the earlier work on the receptivity produced by local variations in the surface suction and/or surface admittance. In particular, we show that the cumulative effect of the distributed receptivity can be substantially larger than that of a single, isolated suction strip or slot. Furthermore, even if the receptivity is spread out over very large distances, the most effective contributions come from a relatively short region in vicinity of the lower branch of the neutral stability curve. The length scale of this region is intermediate to that of the mean of these two length scales. Finally, it is found that the receptivity is effectively dominated by a narrow band of Fourier components from the wall-suction and admittance distributions, roughly corresponding to a detuning of less than ten percent with respect to the neutral instability wavenumber at the frequency under consideration. The results suggest that the drop-off in receptivity magnitudes away from the resonant wavenumber is nearly independent of the frequency parameter.

Choudhari, Meelan↗

NASA NTRS1992

Boundary-layer receptivity due to distributed surface imperfections of a deterministic or random nature

Acoustic receptivity of a Blasius boundary layer in the presence of distributed surface irregularities is investigated analytically. It is shown that, out of the entire spatial spectrum of the surface irregularities, only a small band of Fourier components can lead to an efficient conversion of the acoustic input at any given frequency to an unstable eigenmode of the boundary layer flow. The location, and width, of this most receptive band of wavenumbers corresponds to a relative detuning of O(R sub l.b.(exp -3/8)) with respect to the lower-neutral instability wavenumber at the frequency under consideration, R sub l.b. being the Reynolds number based on a typical boundary-layer thickness at the lower branch of the neutral stability curve. Surface imperfections in the form of discrete mode waviness in this range of wavenumbers lead to initial instability amplitudes which are O(R sub l.b.(exp 3/8)) larger than those caused by a single, isolated roughness element. In contrast, irregularities with a continuous spatial spectrum produce much smaller instability amplitudes, even compared to the isolated case, since the increase due to the resonant nature of the response is more than that compensated for by the asymptotically small band-width of the receptivity process. Analytical expressions for the maximum possible instability amplitudes, as well as their expectation for an ensemble of statistically irregular surfaces with random phase distributions, are also presented.

Choudhari, Meelan↗

NASA NTRS1991

A finite Reynolds number approach for the prediction of boundary layer receptivity in localized regions

Previous theoretical work on the boundary layer receptivity problem has utilized large Reynolds number asymptotic theories, thus being limited to a narrow part of the frequency - Reynolds number domain. An alternative approach is presented for the prediction of localized instability generation which has a general applicability, and also accounts for finite Reynolds number effects. This approach is illustrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer due to the interaction of a free stream acoustic wave with a region of short scale variation in the surface boundary condition. The specific types of wall inhomogeneities studied are: regions of short scale variations in wall suction, wall admittance, and wall geometry (roughness). Extensive comparison is made between the results of the finite Reynolds number approach and previous asymptotic predictions, which also suggests an alternative way of using the latter at Reynolds numbers of interest in practice.

Choudhari, Meelan↗

NASA NTRS1991

A finite Reynolds number approach for receptivity due to rapid variations in surface boundary conditions

A new approach to the prediction of instability generation due to the interaction of free-stream disturbances with regions of short-scale variations in surface boundary conditions is presented which accounts for the finite Reynolds number effects while providing a framework for studying receptivity in a variety of flows. The approach is demonstrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer due to the interaction of a free-stream acoustic wave with a localized wall inhomogeneity. The results obtained by the finite Reynolds number approach are compared with predictions of the asymptotic approach.

Choudhari, Meelan↗

NASA NTRS1991

A general approach for the prediction of localized instability generation in boundary layer flows

The present approach to the prediction of instability generation that is due to the interaction of freestream disturbances with regions of subscale variations in surface boundary conditions can account for the finite Reynolds number effects, while furnishing a framework for the study of receptivity in compressible flow and in 3D boundary layers. The approach is illustrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer, due to the interaction of a freestream acoustic wave with a localized wall inhomogeneity. Results are presented for the generation of viscous and inviscid instabilities in adverse pressure-gradient boundary layers, supersonic boundary layer instabilities, and cross-flow vortex instabilities.

Choudhari, Meelan↗

NASA NTRS1990

Boundary layer receptivity phenomena in three-dimensional and high-speed boundary layers

The process by which the boundary layer internalizes the environmental disturbances in the form of instability waves is known as the boundary-layer receptivity. The paper discusses the importance of receptivity in transition research. The receptivity scenario for three-dimensional and high-speed boundary layers is examined. It is found that, while receptivity mechanisms present in the low-speed case are also operative in these complex flows, certain uniquely 'compressible' receptivity mechanisms may come into play as well. Both numerical, and where convenient, asymptotic procedures are utilized to develop quantitative predictions of the localized generation of a variety of instability types (Tollmien-Schlichting, inflectional, higher modes, crossflow vortices) in boundary layer flows relevant to the National Aero-Space Plane (NASP).

Choudhari, Meelan↗

NASA NTRS1990

Boundary layer receptivity due to three-dimensional convected gusts

Compressible fully three-dimensional interactions involving convected free-stream disturbances are analyzed by utilizing a rapid distortion theory in conjunction with triple-deck concepts. Only one class of regions where the boundary layer is receptive is considered; these regions are downstream from the leading edge, where a local feature, such as a wall hump, forces the boundary layer to make a short-scale adjustment. The results obtained are applicable to a variety of situations such as receptivity due to shallow three-dimensional roughness elements or three-dimensional wall-suction distributions. The influence of the gust orientation, frequency, and the type of mean flow disturbance on the amplitudes of the generated instability waves is demonstrated. It is found that a three-dimensional instability wave can be generated by the interaction of an oblique gust with a two-dimensional mean flow disturbance.

Choudhari, Meelan↗