Freestream

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

Görtler Vortices in Hypersonic Flow on Compression Ramps

Abstract: Direct numerical simulation of Gortler vortices in hypersonic compression ramp flow is performed. Three ramp angles of 15, 20, and 25 deg are considered for a freestream Mach number of 7.7 and a un...

Asymmetrical wake generated by a spinning cylinder

Abstract: An extended set of measurements has been carried out on a developing turbulent wake behind a spinning circular cylinder immersed in a uniform stream. Various moments of the velocity signals have revealed the effect of the rotation upon characteristic parameters of the flowfield. For small values of rotation, where the circumferential velocity is less than the freestream velocity, a quasiconventional turbulent wake behavior obtains. At high values of rotation, the magnitudes of all dynamic turbulent fluctuations decrease substantially due to the suppression of the Kantian vortex street. As well, it was observed that a region of negative production was generated in the developed flow for those cases where the circumferential velocities were comparable to the freestream velocity. This aspect is explored in considerable detail.

Navier-Stokes predictions of pitch damping for finned projectiles using steady coning motion

Abstract: Previous theoretical investigations have proposed that the side force and moment acting on a body of revolution in steady coning motion could be related to the pitch-damping force and moment. In the current research effort, this approach has been applied to produce the first known Navier-Stokes predictions of the pitch damping for finned projectiles. The flow field about finned kinetic energy projectiles in steady coning motion has been successfully computed using a parabolized Navier-Stokes computational approach. The computations make use of a rotating coordinate frame in order to solve the steady flow equations. From the computed flow field, the side moment due to coning motion is used to determine the pitchdamping coefficient. The computational predictions of the slope of the side moment coefficient with coning rate normalized by the sine of the angle of attack have been compared with pitch damping coefficients determined from range firings for two finned projectile configurations. The predictions show good agreement with the range data. This computational approach provides a significant predictive capability for the design of kinetic energy projectiles whose terminal ballistic performance can be degraded by moderate levels of yaw at the target. Nomenclature a00 freestream speed of sound c m pitching moment coefficient c m , slope of the pitching moment coefficient with angle of attack Cm, + Cm, pitch damping moment coefficient c n side moment coefficient * Senior Member, AIAA t Associate Fellow, AIAA Cn, slope of the side moment coefficient mith angle of attack c n slope of the side moment coefficient with coning rate c n , Magnus moment coefficient Chra slope of the normal force coefficient with angle of attack CN, + CN, pitch damping force coefficient slope of the side force coefficient with coning rate Magnus force coefficient projectile diameter total energy per unit volume flux vectors in transformed coordinates source term in Navier-Stokes eqs. jacobian characteristic length, typically D freestream Mach number pressure, as used in N-S eqs. spin rate, as used roll equations freestream static pressure Reynolds number, amp, Dip, distance downrange center of gravity shift, calibers viscous flux vector reference area of projectile, 7 r ~ ~ / 4 time velocity components in x,y,z directions freest ream velocity Cartesian coordinates w.r.t. body axial location of body center of gravity Note: Force coefficients are scaled, F / ; ~ , a& M& Srej ; Moment coefficients are scaled, M / f p, a& M& DS,,~

Integrating Mach–Zehnder interferometry with TPIV to measure the time-resolved deformation of a compliant wall along with the 3D velocity field in a turbulent channel flow

Abstract: A system combining tomographic PIV (TPIV) and Mach–Zehnder interferometry (MZI) simultaneously measures the time-resolved 3D flow field and 2D distribution of wall-normal deformation in a turbulent channel flow over a transparent compliant surface. This paper focuses on the experimental techniques and data analysis procedures, but includes sample results. Standard TPIV analysis resolves the log layer of the mean velocity and the linear decrease in total shear stress with distance from the wall. Single-pixel ensemble correlations reveal the buffer layer and top of the viscous sublayer. Analysis of the MZI data consists of two steps, namely critical spatial filtering of interferograms to remove noise and phase demodulation to calculate the surface shape. A new technique to improve the filtration of noise from interferograms based on spatial correlations of small windows is introduced and optimized. Taking advantage of this enhancement, the phase/deformation distribution is calculated directly from arccosines of the intensity, which avoids edge artifacts affecting spectral calculations. Validations using synthetic noisy interferograms indicate that errors associated with correlation-based enhancement are consistently lower and much less sensitive to fringe shape than spectral band-pass filtering. The experimental wavenumber–frequency spectra show that the deformation consists of patterns that are larger than the field of view, surface waves and small-scale patterns. Some of the latter are advected at the freestream velocity, but mostly at 70 % of the freestream, the mean speed at 10 % of the channel half height. Indeed, spatial correlations of the deformation with velocity components peak at this elevation.