Modern developments in shear flow control with swirl

Passive and active control of swirling turbulent jets is experimentally investigated. Initial swirl distribution is shown to dominate the free jet evolution in the passive mode. Vortex breakdown, a manifestation of high intensity swirl, was achieved at below critical swirl number (S = 0.48) by reducing the vortex core diameter. The response of a swirling turbulent jet to single frequency, plane wave acoustic excitation was shown to depend strongly on the swirl number, excitation Strouhal number, amplitude of the excitation wave, and core turbulence in a low speed cold jet. A 10 percent reduction of the mean centerline velocity at x/D = 9.0 (and a corresponding increase in the shear layer momentum thickness) was achieved by large amplitude internal plane wave acoustic excitation. Helical instability waves of negative azimuthal wave numbers exhibit larger amplification rates than the plane waves in swirling free jets, according to hydrodynamic stability theory. Consequently, an active swirling shear layer control is proposed to include the generation of helical instability waves of arbitrary helicity and the promotion of modal interaction, through multifrequency forcing

Transverse parton momenta in single inclusive hadron production in e+e−{e^ + }{e^ - } annihilation processes

We study the transverse momentum distributions of single inclusive hadron production in ${e^ + }{e^ - }$ annihilation processes. Although the only available experimental data are scarce and quite old, we find that the fundamental features of transverse momentum dependent (TMD) evolution, historically addressed in Drell-Yan processes and, more recently, in Semi-inclusive deep inelastic scattering processes, are visible in ${e^ + }{e^ - }$ annihilations as well. Interesting effects related to its non-perturbative regime can be observed. We test two different parameterizations for the $p_\perp$ dependence of the cross section: the usual Gaussian distribution and a power-law model. We find the latter to be more appropriate in describing this particular set of experimental data, over a relatively large range of $p_\perp$ values. We use this model to map some of the features of the data within the framework of TMD evolution, and discuss the caveats of this and other possible interpretations, related to the one-dimensional nature of the available experimental data

Large amplitude acoustic excitation of swirling turbulent jets

A swirling jet with a swirl number of S = 0.12 is exited by plane acoustic waves at various Strouhal numbers (St = fD/U sub alpha). The maximum forcing amplitude of excitation was at 6.88 percent of the time-mean axial velocity at a Strouhal number of St = 0.39. The maximum time-mean tangential and axial velocities at the nozzle exit were 18 and 84 m/sec respectively. It was observed that the swirling jet was excitable by plane acoustic waves and the preferred Strouhal number based on the nozzle diameter and exit axial velocity of the jet was about 0.39. As a result of excitation at this frequency, the time-mean axial velocity decayed faster along the jet centerline, reaching about 89 percent of its unexcited value at x/D = 9. Also the half velocity radius and momentum thichness, at 7 nozzle diameters downstream, increased by 13.2 and 5.8 percent respectively, indicating more jet spread and enhanced mixing. To our knowledge, this is the first reported experimental data indicating any mixing enhancement of swirling jets by acoustic excitation

Effect of initial tangential velocity distribution on the mean evolution of a swirling turbulent free jet

An existing cold jet facility at NASA-Lewis was modified to produce swirling flows with controllable initial tangential velocity distribution. Distinctly different swirl velocity profiles were produced, and their effects on jet mixing characteristics were measured downstream of an 11.43 cm diameter convergent nozzle. It was experimentally shown that in the near field of a swirling turbulent jet, the mean velocity field strongly depends on the initial swirl profile. Two extreme tangential velocity distributions were produced. The two jets shared approximately the same initial mass flow rate of 5.9 kg/s, mass averaged axial Mach number and swirl number. Mean centerline velocity decay characteristics of the solid body rotation jet flow exhibited classical decay features of a swirling jet with S = 0.48 reported in the literature. It is concluded that the integrated swirl effect, reflected in the swirl number, is inadequate in describing the mean swirling jet behavior in the near field

Controlled excitation of a cold turbulent swirling free jet

Experimental results from acoustic excitation of a cold free turbulent jet with and without swirl are presented. A flow with a swirl number of 0.35 (i.e., moderate swirl) is excited internally by plane acoustic waves at a constant sound pressure level and at various frequencies. It is observed that the cold swirling jet is excitable by plane waves, and that the instability waves grow about 50 percent less in peak rms amplitude, and saturate further upstream compared to corresponding waves in a jet without swirl having the same axial mass flux. The preferred Strouhal number based on the mass-averaged axial velocity and nozzle exit diameter for both swirling and nonswirling flows is 0.4. So far no change in the mean velocity components of the swirling jet is observed as a result of excitation

Rectangular jet with shear layer swirl: Rotation & mixing enhancement

A rectangular jet emerging from a nozzle with embedded swirl vanes in its exit boundary layer is studied. The swirling shear layer imposes an external torque on the jet boundary where it causes jet rotation in the direction of axis switching, skew deformation as well as enhanced mixing. In moderate aspect ratio rectangular nozzles, e.g. AR = 5:1, a pair of co-rotating streamwise vortices is formed on the narrow boundary of the jet that dominates its dynamics in the near field. A vortex-induced model is developed that accounts for the rotation of the rectangular jet with embedded shear layer swirl. The model also shows that the jet rotation is diminished with increasing aspect ratio, as AR−2. The higher entrainment rate in the rotating jet with skew deformation causes the jet mass flow rate with shear layer swirl to be 20–25% higher than the corresponding plain rectangular nozzle of the same aspect ratio (x/De > 1). The proposed model is validated using computational simulation results of previous investigations that appeared in the literature

Interior Point Decoding for Linear Vector Channels

In this paper, a novel decoding algorithm for low-density parity-check (LDPC) codes based on convex optimization is presented. The decoding algorithm, called interior point decoding, is designed for linear vector channels. The linear vector channels include many practically important channels such as inter symbol interference channels and partial response channels. It is shown that the maximum likelihood decoding (MLD) rule for a linear vector channel can be relaxed to a convex optimization problem, which is called a relaxed MLD problem. The proposed decoding algorithm is based on a numerical optimization technique so called interior point method with barrier function. Approximate variations of the gradient descent and the Newton methods are used to solve the convex optimization problem. In a decoding process of the proposed algorithm, a search point always lies in the fundamental polytope defined based on a low-density parity-check matrix. Compared with a convectional joint message passing decoder, the proposed decoding algorithm achieves better BER performance with less complexity in the case of partial response channels in many cases.Comment: 18 pages, 17 figures, The paper has been submitted to IEEE Transaction on Information Theor