The ionized stagnation region flow at low Reynolds number
Low Reynolds number viscous flow at stagnation region of blunt bodies during hypersonic reentry
Lew, H.↗
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HEAT TRANSFER MODELS FOR HIGH MACH NUMBER - LOW REYNOLDS NUMBER RESEARCH
Description of hemisphere cylinder models to measure heat flux in low density wind tunnels
WIND TUNNEL↗
Design of low Reynolds number airfoils. II
Low Reynolds number airfoils were designed for high lift-to-drag ratios. Design considerations of low Re-airfoils are discussed and design examples are presented. It is shown that the performance of advance low Reynolds airfoils critically depends on proper laminar separation and transition control for the minimum profile drag coefficient with minimum turbulator device drag. This transition control close to the laminar separation is possible by enforcing transition on the upper surface either directly by means of spanwise rows of three-dimensional roughness elements, bleed or suction holes, or indirectly by means of destabilizing the upper surface boundary layer by means of backward facing surface steps, two-dimensional or zig-zag tapes or weak boundary layer blowing. Pneumatic turbulators are found to be advantageous because they can be adjusted to different external flow conditions better than mechanical ones.
Pfenninger, W.↗
Design of low Reynolds number airfoils. I
The low Reynolds number airfoils designated ASM-LRN-003 and -007 have been designed for high section L/D ratios using Drela's (1985) design-and-analysis code; close to 70-percent laminar flow is maintained on the upper surfaces, and 100-percent on the lower, at coefficients of lift of 1.0-1.3, assuming optimum laminar separation and transition control on the upper surface by means of suitable turbulators. If peak performance is critical, airfoils of this type with an undercut front lower surface and a correspondingly sharper leading edge may be resorted to.
Pfenninger, W.↗
Flowfield and acoustic properties of a Mach number 0.9 jet at a low Reynolds number
An anechoic vacuum chamber test facility is used to assess the flow field and acoustic properties of a jet with a low Reynolds number of about 3600 and a Mach number of 0.9. A conventional Pitot pressure probe is used to monitor the mean flow field, a hot-wire probe is employed to detect flow fluctuations, and the acoustic field is probed by condenser microphones. The jet is operated at the low Reynolds number in order to examine the behavior of the large-scale 'orderly' structure with particular emphasis on its noise-generating capabilities. It is shown that a nonlinear mechanism involving the large-scale flow instability is responsible for a dominant portion of the noise generated from the jet.
Stromberg, J. L.↗
Flow field and acoustic properties of a Mach number 0.9 jet at a low Reynolds number
An experimental study of the flow field and acoustic properties of a low Reynolds number M = 0.9 jet has been performed in a low pressure anechoic test chamber. The mean flow field was surveyed with a conventional pitot pressure probe and flow fluctuations were detected with a normal hot wire probe. Also, condenser microphone measurements were made in the acoustic field. The flow fluctuations within the jet were found to be dominated initially by a relatively discrete, large-scale, wave-like instability centered around a Strouhal number of 0.44. The axial wavelength of this instability was determined to be 1.45 jet diameters and its azimuthal character includes the n = 0 and n = plus or minus 1 modes. The growth of this instability coupled with its nonlinear breakdown are major contributors to the termination of the potential core region of the jet. The acoustic field of the jet, in contrast to the flow field, has a broad frequency spectrum with a peak amplitude near a Strouhal number of St = 0.2. The results indicate that a nonlinear mechanism involving the large-scale flow instability is responsible for a dominant portion of the noise generated from this jet.
Stromberg, J. L.↗
Accurate measurements of sphere drag at low Reynolds numbers.
Sphere drag measurements at low Reynolds number
FLOW RESISTANCE↗
On the drag of model dendrite fragments at low Reynolds number
An experimental study of low Reynolds number drag on laboratory models of dendrite fragments has been conducted. The terminal velocities of the dendrites undergoing free fall along their axis of symmetry were measured in a large Stokes flow facility. Corrections for wall interference give nearly linear drag vs Reynolds number curves. Corrections for both wall interference and inertia effects show that the dendrite Stokes settling velocities are always less than that of a sphere of equal mass and volume. In the Stokes limit, the settling speed ratio is found to correlate well with the primary dendrite arm aspect ratio and a second dimensionless shape parameter which serves as a measure of the fractal-like nature of the dendrite models. These results can be used to estimate equiaxed grain velocities and distance of travel in metal castings. The drag measurements may be used in numerical codes to calculate the movement of grains in a convecting melt in an effort to determine macrosegregation patterns caused by the sink/float mechanism.
Zakhem, R.↗
On the drag of model dendrite fragments at low Reynolds number
An experimental study of low Reynolds number drag on laboratory models of dendrite fragments has been conducted. The terminal velocities of the dendrites undergoing free fall along their axis of symmetry were measured in a large Stokes flow facility. Corrections for wall interference give nearly linear drag vs Reynolds number curves. Corrections for both wall interference and inertia effects show that the dendrite Stokes settling velocities are always less than that of a sphere of equal mass and volume. In the Stokes limit, the settling speed ratio is found to correlate well with primary dendrite arm aspect ratio and a second dimensionless shape paremeter which serves as a measure of the fractal-like nature of the dendrite models. These results can be used to estimate equiaxed grain velocities and distance of travel in metal castings. The drag measurements may be used in numerical codes to calculate the movement of grains in a convecting melt in an effort to determine macrosegregation patterns caused by the sink/float mechanism.
Zakhem, R.↗
Decay of Isotropic Turbulence at Low Reynolds Number
Decay of isotropic turbulence is computed using direct numerical simulations. Comparisons with experimental spectra at moderate and low Reynolds numbers (R(sub lambda) less than 70) show good agreement. At moderate to high Reynolds numbers (R(sub lambda) greater 50), the spectra are found to collapse with Kolmogorov scaling at high wave numbers. However, at low Reynolds numbers (R(sub lambda) less than 50) the shape of the spectra at the Kolmogorov length scales is Reynolds number dependent. Direct simulation data from flowfields of decaying isotropic turbulence are used to compute the terms in the equation for the dissipation rate of the turbulent kinetic energy. The development of the skewness and the net destruction of the turbulence dissipation rate in the limit of low Reynolds numbers are presented. The nonlinear terms are found to remain active at surprisingly low Reynolds numbers.
Mansour, N. N.↗
The Penguin: a Low Reynolds Number Powered Glider for Station Keeping Missions
The Penguin is a low Reynolds number (approx. 100,000) remotely piloted vehicle (RPV). It was designed to fly three laps indoors around two pylons in a figure-eight course while maximizing loiter time. The Penguin's low Reynolds number mission is an important one currently being studied for possible future flights in the atmospheres of other planets and for specialized military missions. Although the Penguin's mission seemed quite simple at first, the challenges of such low Reynolds number flight have proven to be quite unique. In addition to the constraint of low Reynolds number flight, the aircraft had to be robust in its control, highly durable, and it had to carry a small instrument package. The Penguin's flight plan, concept, performance, aerodynamic design, weight estimation, structural design, propulsion, stability and control, and cost estimate is detailed.
Costello, J. K.↗
On an acoustic field generated by subsonic jet at low Reynolds numbers
An acoustic field generated by subsonic jets at low Reynolds numbers was investigated. This work is motivated by the need to increase the fundamental understanding of the jet noise generation mechanism which is essential to the development of further advanced techniques of noise suppression. The scope of this study consists of two major investigation. One is a study of large scale coherent structure in the jet turbulence, and the other is a study of the Reynolds number dependence of jet noise. With this in mind, extensive flow and acoustic measurements in low Reynolds number turbulent jets (8,930 less than or equal to M less than or equal to 220,000) were undertaken using miniature nozzles of the same configuration but different diameters at various exist Mach numbers (0.2 less than or equal to M less than or equal to 0.9).
Yamamoto, K.↗
Low-Reynolds-number k-epsilon model for unsteady turbulent boundary-layer flows
An assessment of the near-wall and low-Reynolds-number functions used in low-Reynolds-number k-epsilon models suggests that they are not suitable for the near-wall region of unsteady turbulent boundary layers, where the flow is characterized by rapid changes in phase. An improved low-Reynolds-number k-epsilon model is developed in this paper. The near-wall and low-Reynolds-number functions in this model are formulated as functions of the local turbulent Reynolds numbers instead of the inner variable y(+). The present model also has the correct asymptotic behavior in the near-wall region. The turbulence model has been incorporated in an unsteady boundary-layer code and validated for unsteady turbulent boundary layers with and without adverse pressure gradients. The predictions agree well with the experimental data and the theoretical analysis. For the cases tested, the present model correctly predicts the unsteady near-wall flow and the unsteady shin friction at various frequencies.
Fan, Sixin↗
Critical Evaluation of the Conference on Low Reynolds Number Aerodynamics at the University of Notre Dame
As a result of the continued interest in designing efficient low Reynolds number systems, the University of Notre Dame decided to organize a Conference on Low Reynolds Number Aerodynamics in June 1989. This Conference followed the 1986 International Conference in London by about three years and the first Conference on Low Reynolds Number Airfoil Aerodynamics at Notre Dame in 1985 by four years. The emphasis of the 1989 Conference was to assess the state-of-the-art in the chord Reynolds number range from about 10,000 to about 700,000. Applications of current interest include high altitude remotely or robotically piloted vehicles, ultra-light, and human powered vehicles as well as mini-RPVs at low altitudes. Other examples include small axial-flow fans used to cool electronic equipment in the unpressurized sections of high-altitude aircraft and gas turbine blades. High Reynolds number airfoil design strategies attempt to control the onset and development of turbulent boundary layers. This is difficult at low Reynolds numbers because of the increased stability of attached laminar boundary layers. Therefore, laminar separation is common even at small angles of attack at low Reynolds numbers. Under these conditions, the development of a turbulent boundary layer usually depends on the formation of a transitional separation bubble. The purpose of this Conference on Low Reynolds Number Aerodynamics was to bring together those researchers who have been active in areas closely related to this subject. It is clear from the papers presented that a great deal of progress has been made in understanding the occurrence and behavior of laminar separation and transition as well as their overall effect on the performance of airfoils at low chord Reynolds numbers. This progress has brought us closer to our goal of improving analytical methods for the design and evaluation of a variety of practical applications.
Mueller, Thomas J.↗
Mixing length in low Reynolds number compressible turbulent boundary layers
The paper studies the effect of low Reynolds number in high-speed turbulent boundary layers on variations of mixing length. Boundary layers downstream of natural transition on plates, cones and cylinders, and boundary layers on nozzle walls without laminarization-retransition are considered. The problem of whether low Reynolds number amplification of shear stress is a result of transitional flow structure is considered. It is concluded that a knowledge of low Reynolds number boundary layer transition may be relevant to the design of high-speed vehicles.
Bushnell, D. M.↗
Low-Reynolds-number effects in a fully developed turbulent channel flow
Low-Reynolds-number effects are observed in the inner region of a fully developed turbulent channel flow, using data obtained either from experiments or by direct numerical simulations. The Reynolds-number influence is observed on the turbulence intensities and to a lesser degree on the average production and dissipation of the turbulent energy. In the near-wall region, the data confirm Wei and Willmarth's (1989) conclusion that the Reynolds stresses do not scale on wall variables. One of the reasons proposed to account for this behavior, namely, the 'geometry' effect or direct interaction between inner regions on opposite walls, was investigated in some detail by introducing temperature at one of the walls, both in experiment and simulation. Although the extent of penetration of thermal excursions into the opposite side of the channel can be significant at low Reynolds numbers, the contribution these excursions make to the Reynolds shear stress and the spanwise vorticity in the opposite wall region is negligible. In the inner region, spectra and cospectra of the velocity fluctuations u and v change rapidly with the Reynolds number, the variations being mainly confined to low wavenumbers in the u spectrum.
Antonia, R. A.↗
Effect of low Reynolds number turbulence amplification on the Galileo probe flowfield
The amplification of turbulence at low Reynolds numbers is analyzed as it affects the Jupiter-entry flowfield, surface-heating rate, and mass-loss rate of the 335-kg Galileo probe now being designed. The constant k(2) in the Clauser-Klebanoff outer law of the two-layer algebraic eddy-viscosity model of Cebeci (1970), as used in earlier models of the Galileo flowfield (Moss and Simmonds, 1982), is modified to increase at low Reynolds numbers, as found experimentally by Varner and Adams (1980). Calculations were performed for peak heating conditions (at 49.13 sec of entry), using a turbulent Prandtl number of 0.9 and turbulent Lewis numbers (LeT) of 0.8, 1.0, and 1.2. The low-Reynolds-number effect is found to produce mass-loss-ratio increases of from 4 to 50 percent, while an LeT of 1.2 produced an increase of from 4 to 10 percent as compared to an LeT of 1.0. While these findings are based on experimental data obtained under conditions somewhat different from those probably present on Jupiter, their importance for a conservative probe design is stressed.
Gupta, R. N.↗
Analysis and Design of Rotors at Ultra-Low Reynolds Numbers
Design tools have been developed for ultra-low Reynolds number rotors, combining enhanced actuator-ring / blade-element theory with airfoil section data based on two-dimensional Navier-Stokes calculations. This performance prediction method is coupled with an optimizer for both design and analysis applications. Performance predictions from these tools have been compared with three-dimensional Navier Stokes analyses and experimental data for a 2.5 cm diameter rotor with chord Reynolds numbers below 10,000. Comparisons among the analyses and experimental data show reasonable agreement both in the global thrust and power required, but the spanwise distributions of these quantities exhibit significant deviations. The study also reveals that three-dimensional and rotational effects significantly change local airfoil section performance. The magnitude of this issue, unique to this operating regime, may limit the applicability of blade-element type methods for detailed rotor design at ultra-low Reynolds numbers, but these methods are still useful for evaluating concept feasibility and rapidly generating initial designs for further analysis and optimization using more advanced tools.
Kunz, Peter J.↗