Freestream

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

Experimental and Numerical Investigation of Flow Field and Downstream Surface Temperatures of Cylindrical and Diffuser Shaped Film Cooling Holes1

Abstract: An experimental and numerical study of the flow field and the downstream film cooling performance of cylindrical and diffuser shaped cooling holes is presented. The measurements were conducted on a flat plate with a single cooling hole with coolant ejected from a plenum. The flow field was investigated by means of 3D-PIV as well as 3D-LDV measurements, the downstream film cooling effectiveness by means of infrared thermography. Cylindrical and diffuser holes without lateral inclination have been examined, varying blowing ratio and density ratio as well as freestream turbulence levels. 3D-CFD simulations have been performed and validated along with the experimental efforts. The results, presented in terms of contour plots of the three normalized velocity components as well as adiabatic film cooling effectiveness, clearly show the flow structure of the film cooling jets and the differences brought about by the variation of hole geometry and flow parameters. The quantitative agreement between experiment and CFD was reasonable, with better agreement for cylindrical holes than for diffuser holes.Copyright © 2011 by Siemens Energy, Inc.

The effect of flow speed and body size on Kármán gait kinematics in rainbow trout

Abstract: We have little understanding of how fish hold station in unsteady flows. Here, we investigated the effect of flow speed and body size on the kinematics of rainbow trout Karman gaiting behind a 5 cm diameter cylinder. We established a set of criteria revealing that not all fish positioned in a vortex street are Karman gaiting. By far the highest probability of Karman gaiting occurred at intermediate flow speeds between 30 and 70 cm s(-1). We show that trout Karman gait in a region of the cylinder wake where the velocity deficit is about 40% of the nominal flow. We observed that the relationships between certain kinematic and flow variables are largely preserved across flow speeds. Tail-beat frequency matched the measured vortex shedding frequency, which increased linearly with flow speed. Body wave speed was about 25% faster than the nominal flow velocity. At speeds where fish have a high probability of Karman gaiting, body wavelength was about 25% longer than the cylinder wake wavelength. Likewise, the lateral (i.e. cross-stream) amplitude of the tail tip was about 50% greater than the expected lateral spacing of the cylinder vortices, while the body center amplitude was about 70% less. Lateral body center acceleration increased quadratically with speed. Head angle decreased with flow speed. While these values are different from those found in fish swimming in uniform flow, the strategy for locomotion is the same; fish adjust to increasing flow by increasing their tail-beat frequency. Body size also played a role in Karman gaiting kinematics. Tail-beat amplitudes of Karman gaiting increased with body size, as in freestream swimming, but were almost three times larger in magnitude. Larger fish had a shorter body wavelength and slower body wave speed than smaller fish, which is a surprising result compared with freestream swimming, where body wavelength and wave speed increased with size. In contrast to freestream swimming, tail-beat frequency for Karman gaiting fish did not depend on body size and was a function of the vortex shedding frequency.

Conical Similarity of Shock/Boundar y-Layer Interactions Generated by Swept and Unswept Fins

Abstract: A parametric experimental investigation has been made of the class of three-dimensi onal shock wave/turbulent boundary-layer interactions generated by swept and unswept leading-edge fins. The fin sweepback angles were 0-65 deg at 5, 9, and 15 deg angles of attack. Two equilibrium two-dimensional turbulent boundary layers with a freestream Mach number of 2.95 and a Reynolds number of 6.3 x 107/m were used as incoming flow conditions. AH of the resulting interactions were found to possess conical symmetry of the surface flow patterns and pressures outside of an initial inception zone. Further, these interactions were found to obey a simple conical similarity rule based on inviscid shock wave strength irrespective of fin sweepback or angle of attack. This is one of the first demonstrations of similarity among three-dimensional interactions produced by geometrically dissimilar shock generators. Nomenclature a = exponent in Reynolds number scaling law h = fin height, cm LI = length along inviscid shock wave trace on test surface for the inception of conical flow, cm Ls = length along inviscid shock wave trace from fin leading edge, cm LuN = upstream influence length normal to inviscid shock wave trace on test surface, cm MO, = freestream Mach number MN =M00sinj80, component of freestream Mach number normal to inviscid shock wave trace on test surface p = surface static pressure, N/m2 Pec = static pressure of the incoming freestream flow, N/m2 Red = Reynolds number based on boundary-layer thickness

Reduction of freestream turbulence at low velocities

Abstract: Controlling freestream turbulence (FST) in low-turbulence wind or water channels is a common challenge and often difficult to achieve. Particularly at low velocities, design guidelines from literature may not fulfill their purpose and thus require alternative strategies. In this study, we propose the installation of a fine-meshed screen downstream of the contraction and not in the settling chamber as typically advised in literature. With this strategy, the lower operational limitation of our facility could be extended below $$U_{\infty }= 0.06\,\text {ms}^{-1}$$ and the turbulence intensity reduced by $$60 \%$$ at 0.04 ms $$^{-1}$$ . This is not only an improvement of freestream conditions but also the key to experiments in the laminar boundary layer, which is highly sensitive to FST. In fact, two fundamentally different transition mechanism can be distinguished with this approach.

The Structure of Shock Cell Noise from Supersonic Jets

Abstract: Shock cell noise is created by supersonic jets operating at off-design conditions due to the interaction between the evolving turbulence and the quasi-fixed shock cell structure. Transonic wind tunnel tests are conducted to identify the structure of the noise produced by this process in the presence of external flow. The data is compared to a simple kinematic model that incorporates the freestream velocity. It predicts the relationship between frequency and radiation angle and also predicts the screech frequency as the limiting case of upstream propagation. Results indicate that the freestream flow has a significant effect on the noise production. The noise level decreases and the radiation angle rotates toward the upstream axis as the external flow increases. But, when convective effects are considered the direction of noise is 90 o to 100 o for all conditions.