Freestream

About: Freestream is a research topic. Over the lifetime, 3428 publications have been published within this topic receiving 56147 citations.

Drag reduction of airplane fuselages through shaping by the inverse method

Abstract: The axial singularity solution for the axisymmetric inverse problem has been extended to utilize doublet elements with linear intensity distribution. The solution converges faster than the source-based method, and is therefore quite promising. A procedure based on this solution has been used to design low-drag laminar fuselage shapes for small aircraft applications with a volumetric Reynolds number range of 10-30 million. A profile with a fineness ratio of 6, transition at 40% of body length, and volumetric drag coefficient of 0.012 at a nominal /?v of 15 million, has been developed. The present inverse procedure was shown to be a powerful alternative to optimization methods. Several transition criteria were investigated in the course of the study. The Crabtree criterion appears to be the most consistent. Experimental transition data for axisymmetric bodies at high (flight) Reynolds numbers are urgently needed. Nomenclature CDV = volumetric drag coefficient based on V 2/3 as characteristic area /,. = fineness ratio, body length/maximum diameter H = boundary-layer shape factor, 8*/9 L = body length / = length of an axial-singularity element n = number of doublet elements q = local velocity on body surface (at the edge of the boundary layer) RL = length Reynolds number based on UM and L Rs = Reynolds number based on local velocity and s Rx = Reynolds number based on local velocity and jc Re = Reynolds number based on local velocity and 9 7?v = volumetric Reynolds number based on Ux and V1/3 r. — local body radius and radial coordinate 5 = length along body surface starting at nose Ux = freestream velocity u = axial component of q v = radial component of q X = nondimensional axial coordinate, x/L

Transition Effects on Secondary Flows in a Turbine Cascade

Abstract: The regions of laminar and turbulent flow have been investigated in a linear cascade of a high tuming HP rotor blades. Measurements of intermittency close to the blade and end wall surfaces have shown substantial areas of laminar and transitional flow. The implications for turbulence modelling are important, and Navier-Stokes computations have been performed to investigate how well transition can be modelled in such a flow. Using the intermittency data to specify transitional areas, the mixing length model of turbulence produces excellent results, although there is some sensitivity to the assumed freestream length scale. High Reynolds k-e model results show too much turbulence and loss using the measured high inlet length scale, but the results are improved with the Kato-Launder modification. A low Reynolds number model does not seem to predict the transition effects, although more work is required with this model.Copyright © 1996 by ASME

Effects of nonuniform incident velocity on a dynamic wind turbine airfoil

Abstract: For further insight into the performance of a horizontal axis wind turbine blade section under yaw loads, a 2D numerical simulation of a pitching S809 airfoil under dynamic stall with an unsteady incident velocity is presented. The streamwise incident velocity and pitch angle of incidence oscillated with the same frequency but with a range of phase differences, − π ≤ Φ ≤ π. Changing Φ caused variation of the results, which can be highlighted as significantly augmented and dramatically damped dynamic stall loads, both increasing and decreasing trends for vortex growth time during Φ increase, a shifted location of the maximum loads and a change in the order of the vortex pair circulation in each cycle. The results showed strong dependency on the velocity and acceleration of the freestream during dynamic stall, which categorized the results in four individual subdomains with different behaviors. Copyright © 2014 John Wiley & Sons, Ltd.

Separated Flow Transition Mechanism and Prediction With High and Low Freestream Turbulence Under Low Pressure Turbine Conditions

Abstract: A correlation for separated flow transition has been developed for boundary layers subject to initial acceleration followed by an unfavorable pressure gradient. The correlation is based on the measured growth of small disturbances in the pre-transitional boundary layer. These disturbances were identified and quantified through spectral analysis of the wall normal component of velocity. Cases typical of low pressure turbine airfoil conditions, with Reynolds numbers (Re) ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) were considered at low (0.5%) and high (8.7% inlet) freestream turbulence levels. In some cases, two-dimensional rectangular bars were placed at the beginning of the adverse pressure gradient region as passive flow control devices. The dimensionless magnitude of the initial disturbance which begins to grow at the suction peak depends on the freestream turbulence level and the size of any bar applied to the surface. The growth rate depends on the Reynolds number. When the pre-transitional disturbances grow to a sufficient magnitude, transition begins. The new correlation is based on the physics observed in the turbulence spectra, but allows transition prediction using only the Reynolds number, freestream turbulence level and bar height. The correlation has been checked against experimental data from the literature, and allows transition location prediction to within the uncertainty of the experimental measurements. The correlation represents an improvement over previous correlations which accounted for Reynolds number or freestream turbulence effects, but not both.Copyright © 2004 by ASME

Numerical study of transitional separated-reattached flow over surface-mounted obstacles using large-eddy simulation

Abstract: Large-eddy simulation (LES) of transitional separatingreattaching flow on a square surface mounted obstacle has been performed. The Reynolds number based on the uniform inlet velocity and the obstacle height is 4.5 103. A dynamic subgrid-scale model is employed in this work. The mean LES results compare favourably with the available experimental and direct numerical simulation (DNS) data. Extensive analysis of the time series signals of the velocity and pressure fields at different locations including positions close to solid surfaces, at the centre and edge of the separatedreattached boundary layer using the windowed Fourier transform (WFT) and the wavelet transform was performed. The spectra analysis revealed the nature of the amplified frequencies at all the important locations of the flow field. Excited modes that could be due to the movement (shedding) of large-scale structures and pairing of such types of structures are identified. A clear frequency peak was captured just upstream of the separation line. The value of the frequency peak and the low percentage of the back flow velocity compared to the freestream velocity in the current case strongly support the idea that this amplified frequency is most likely due to the KelvinHelmholtz (KH) instability mechanism of the shear layer forming in the boundary of the small upstream separated region rather than being attributed to the flapping of the shear layer