Engineering topics

Parikh, P. G.

Publications and source records attributed to Parikh, P. G..

12 records

NASA NTRS1990

Hybrid laminar flow control tests in the Boeing Research Wind Tunnel

The hybrid laminar flow control (HLFC) concept has undergone wind tunnel testing at near full-scale Reynolds number on an infinite wing of 30-deg sweep on which boundary-layer suction was furnished over the first 20 percent of chord of the upper surface. Depending on the external pressure distribution, the HLFC extended the laminarity of the boundary layer as far back as 45 percent of chord; this corresponds to a transition Reynolds number of about 11 million. The maximum chordwise extent of laminar run was found to be insensitive to the suction level over a wide range.

Parikh, P. G.↗

NASA NTRS1990

Application of boundary layer control to HSCT low speed configuration

The feasibility of using boundary layer control (BLC) on a high speed civil transport (HSCT) high lift configuration for low speed performance improvement is studied. The possibility of using a part of the suction system previously designed for cruise laminar flow control (LFC) for low speed BLC mode was explored. A suction scheme for BLC was devised for the HSCT high lift configuration. The extent of the suction regions and levels were determined via panel method based inviscid flow analysis coupled with three-dimensional boundary layer analyses. The compatibility of the BLC suction requirements was assessed, and the modifications necessary to operate the system in both modes of operation, were determined. An assessment of the potential aerodynamic performance improvement resulting from an implementation of the BLC concept compared to an optimized simple flap configuration without BLC was made by an Euler code analysis of the simple flap configuration.

Parikh, P. G.↗

NASA NTRS1990

Application of laminar flow control to supersonic transport configurations

The feasibility and impact of implementing a laminar flow control system on a supersonic transport configuration were investigated. A hybrid laminar flow control scheme consisting of suction controlled and natural laminar flow was developed for a double-delta type wing planform. The required suction flow rates were determined from boundary layer stability analyses using representative wing pressure distributions. A preliminary design of structural modifications needed to accommodate suction through a perforated titanium skin was carried out together with the ducting and systems needed to collect, compress and discharge the suction air. The benefits of reduced aerodynamic drag were weighed against the weight, volume and power requirement penalties of suction system installation in a mission performance and sizing program to assess the net benefits. The study showed a feasibility of achieving significant laminarization of the wing surface by use of a hybrid scheme, leading to an 8.2 percent reduction in the cruise drag. This resulted in an 8.5 percent reduction in the maximum takeoff weight and a 12 percent reduction in the fuel burn after the inclusion of the LFC system installation penalties. Several research needs were identified for a resolution of aerodynamics, structural and systems issues before these potential benefits could be realized in a practical system.

Parikh, P. G.↗

NASA NTRS1989

Stability of 3D wing boundary layer on a SST configuration

Tollmien-Schlichting (TS) and cross-flow (CF) instability growth characteristics were studied in three dimensions, for the case of a Mach 2.4 SST with double-delta planform whose inboard leading-edge is subsonic and outboard leading-edge is supersonic. Attention is given to the requirements for supersonic speed laminarization of both highly swept, rounded leading-edge wings and moderately-swept, sharp leading-edge wings. Suction requirements for the control of both TS and CF instabilities are calculated; it is found that while mild suction and surface cooling are effective in TS-instability damping, the CF influence of such techniques is rather weak. CF instability control must be via pressure-distribution tailoring and suction.

Parikh, P. G.↗

NASA NTRS1985

Improved Exhaust Diffuser for Jet-Engine Testing

High-altitude simulator reduced power requirements. Test cell uses its exhaust-capture duct only to remove gases from engine; cooling air evacuated through separate path by auxiliary suction system. This way, capture duct cross-sectional area kept close to exhaust jet area, leading to greatly improved recovery performance.

Parikh, P. G.↗

NASA NTRS1984

A new concept for exhaust diffusers of altitude test cells

A new exhaust diffuser concept for jet engine altitude test cells which greatly reduces operating power and cost requirements for exhausters is discussed. The concept utilizes the capture duct as an efficient diffuser only, while evacuating the secondary air via a separate path using an auxiliary suction system. Implementation of the concept would reduce the peak exhauster power requirement during a TF-30 altitude test by 48 percent and the overall exhaust power cost of the test program by 41 percent. The design accommodates various engine sizes and can achieve optimum pressure recovery performance during both A/B and IRP modes of engine operation. The pressure recovery performance of the proposed exhaust diffusers does not deteriorate with increasing cooling air fraction. The disadvantages of the proposed scheme are: increased mechanical complexity of the extended variable geometry diffuser duct and the need for an auxiliary suction system for evacuating cell-cooling air.

Parikh, P. G.↗

NASA NTRS1983

Transient response of a turbulent boundary layer

A unique feature of the present ensemble-averaged measurements of a turbulent boundary layer's transient response to a spontaneous change in the free stream velocity distribution, is that the test boundary layer is a standard, steady, flat plate turbulent boundary layer at the entrance to the unsteady region, and is then subjected to sudden changes in free stream velocity distribution in the test section. These water tunnel tests were controlled by minicomputer. It is noted that the boundary layer development was relatively slow, with a characteristic time that was greater than the free stream time-of-flight by a factor of as much as 3. Response varied dramatically across the boundary layer, and the evolution of the turbulent stress field occurred on the same time scale as that of the ensemble-averaged velocity field.

Parikh, P. G.↗

NASA NTRS1982

Resonant entrainment of a confined pulsed jet

This paper reports the discovery of a new resonant entrainment phenomenon associated with a confined, pulsed jet flow. It was found that a confined jet, when pulsed at an organ-pipe resonant frequency of the confinement tube, experiences greatly enhanced entrainment and mixing near the exit end of the confinement tube compared to a steady confined jet. The mixing and entrainment rates for the resonantly pulsed confined jet approach, and in some cases slightly exceed, those for an unconfined pulsed jet. Both visual and quantitative evidence of this phenomenon is presented. The new effect should be of considerable interest in ejector and combustor design, both of which benefit from any enhancement in mixing between a primary and a secondary flow

Parikh, P. G.↗

NASA NTRS1982

Dynamics of an unsteady turbulent boundary layer

The present investigation is concerned with the response of a well-defined, steady, flatplate turbulent boundary layer subjected subsequently to two types of oscillations of the free-stream velocity in the test section. It is found that the mean velocity profile in the boundary layer is unaffected by imposed free-stream oscillations in the range of frequencies employed. It is the same as the one measured with a free-stream velocity distribution held steady at its mean value. The behavior of the mean velocity field is a consequence of two observations. The time-averaged Reynolds stress distribution across the boundry layer is unaffected by the imposed oscillations. The Reynolds stresses arising from the organized velocity fluctuations under imposed oscillatory conditions are negligible compared to Reynolds stress due to the random fluctuations.

Parikh, P. G.↗

NASA NTRS1981

Dynamic behavior of an unsteady trubulent boundary layer

Experiments on an unsteady turbulent boundary layer are reported in which the upstream portion of the flow is steady (in the mean) and in the downstream region, the boundary layer sees a linearly decreasing free stream velocity. This velocity gradient oscillates in time, at frequencies ranging from zero to approximately the bursting frequency. For the small amplitude, the mean velocity and mean turbulence intensity profiles are unaffected by the oscillations. The amplitude of the periodic velocity component, although as much as 70% greater than that in the free stream for very low frequencies, becomes equal to that in the free stream at higher frequencies. At high frequencies, both the boundary layer thickness and the Reynolds stress distribution across the boundary layer become frozen. The behavior at higher amplitude is quite similar. At sufficiently high frequencies, the boundary layer thickness remains frozen at the mean value over the oscillation cycle, even though flow reverses near the wall during a part of the cycle.

Parikh, P. G.↗

NASA NTRS1981

Dynamic behavior of an unsteady turbulent boundary layer

Experiments on an unsteady turbulent boundary layer are reported in which the upstream portion of the flow is steady (in the mean) and in the downstream region, the boundary layer sees a linearly decreasing free stream velocity. This velocity gradient oscillates in time, at frequencies ranging from zero to approximately the bursting frequency. For the small amplitude, the mean velocity and mean turbulence intensity profiles are unaffected by the oscillations. The amplitude of the periodic velocity component, although as much as 70 percent greater than that in the free stream for very low frequencies, becomes equal to that in the free stream at higher frequencies. At high frequencies, both the boundary layer thickness and the Reynolds stress distribution across the boundary layer become frozen. The behavior at higher amplitude is quite similar. At sufficiently high frequencies, the boundary layer thickness remains frozen at the mean value over the oscillation cycle, even though flow reverses near the wall during a part of the cycle. Previously announced in STAR as N81-28392

Parikh, P. G.↗

NASA NTRS1974

Free convection over a vertical porous plate with transpiration

The problem of free convection over an isothermal vertical porous plate with transpiration is studied both numerically and experimentally. Numerical solutions to the variable-property transpired free-convection boundary layer equations have been obtained using the finite difference procedure of Patankar and Spalding (1967). The effects of uniform transpiration on heat transfer and on temperature and velocity profiles are predicted. Interferometrically measured nondimensional temperature profiles for the uniform wall temperature and transpiration case agreed closely with these numerical predictions.

Parikh, P. G.↗