Control of low-speed turbulent separated flow using jet vortex generators

A parametric study has been performed with jet vortex generators to determine their effectiveness in controlling flow separation associated with low-speed turbulent flow over a two-dimensional rearward-facing ramp. Results indicate that flow-separation control can be accomplished, with the level of control achieved being a function of jet speed, jet orientation (with respect to the free-stream direction), and jet location (distance from the separation region in the free-stream direction). Compared to slot blowing, jet vortex generators can provide an equivalent level of flow control over a larger spanwise region (for constant jet flow area and speed).Nomenclature C _p pressure coefficient, 2(P-P_)/V ² - C _Q total flow coefficient, Q/ v - D ₀ jet orifice diameter - Q total volumetric flow rate - R Reynolds number based on momentum thickness - u fluctuating velocity component in the free-stream (x) direction - V free-stream flow speed - VR ratio of jet speed to free-stream flow speed - x coordinate along the wall in the free-stream direction - jet inclination angle (angle between the jet axis and the wall) - jet azimuthal angle (angle between the jet axis and the free-stream direction in a horizontal plane) - boundary-layer thickness - momentum thickness - lateral distance between jet orifices A version of this paper was presented at the 12th Symposium on Turbulence, University of Missouri-Rolla, 24–26 Sept. 1990     

Investigations of mean and turbulent flow characteristics of a two dimensional plane diffuser

This paper reports the investigation of mean and turbulent flow characteristics of a two-dimensional plane diffuser. Both experimental and theoretical details are considered. The experimental investigation consists of the measurement of mean velocity profiles, wall static pressure and turbulence stresses. Theoretical study involves the prediction of downstream velocity profiles and the distribution of turbulence kinetic energy using a well tested finite difference procedure. Two models, viz., Prandtl's mixing length hypothesis and k- model of turbulence, have been used and compared. The nondimensional static pressure distribution, the longitudinal pressure gradient, the pressure recovery coefficient, percentage recovery of static pressure, the variation of U _max/U _bar along the length of the diffuser and the blockage factor have been valuated from the predicted results and compared with the experimental data. Further, the predicted and the measured value of kinetic energy of turbulence have also been compared. It is seen that for the prediction of mean flow characteristics and to evaluate the performance of the diffuser, a simple turbulence model like Prandtl's mixing length hypothesis is quite adequate.List of symbols C ₁ , C ₂ ,C turbulence model constants - F _x body force - k kinetic energy of turbulence - l _m mixing length - L length of the diffuser - u, v, w rms value of the fluctuating velocity - u, v, w turbulent component of the velocity - mean velocity in the x direction - _A average velocity at inlet - U _bar average velocity in any cross section - U _max maximum velocity in any cross section - V mean velocity in the y direction - W local width of the diffuser at any cross section - x, y coordinates - dissipation rate of turbulence - _m eddy diffusivity - Von Karman constant - mixing length constant - _l laminar viscosity - _eff effective viscosity - v kinematic viscosity - density - _k effective Schmidt number for k - effective Schmidt number for - stream function - non dimensional stream function     

Numerical prediction of turbulent flow in a conical diffuser usingk-ε model

Fully developed incompressible turbulent flow in a conical diffuser having a total divergence angle of 8° and an area ratio of 4∶1 has been simulated by ak-ε turbulence model with high Reynolds number and adverse pressure gradient. The research has been done for pipe entry Reynolds numbers of 1.16×10⁵ and 2.93×10⁵. The mean flow velocity and turbulence energy are predicted successfully and the advantage of Boundary Fit Coordinates approach is discussed. Furthermore, thek-ε turbulence model is applied to a flow in a conical diffuser having a total divergence angle of 30° with a perforated screen. A simplified mathematical model, where only the pressure drop is considered, has been used for describing the effect of the perforated screen. The optimum combination of the resistance coefficient and the location of the perforated screen is predicted for high diffuser efficiency or the uniform velocity distribution.     

Effect of bubbles on turbulent kinetic energy transport in downward flow measured by time-resolved PTV

A time-resolved particle tracking velocimetry (PTV) system and a shape projection imaging system were used to investigate the turbulence modifications by bubbles in a downward bubbly flow. Two bubble sizes and three mean void fractions were tested at a Reynolds number of about 20,000. The strong modifications in the mean velocity, turbulent kinetic energy (TKE) budget, and velocity spectra are observed in the central region of the pipe that has a high local void fraction. In particular, kinetic energy production decreased, whereas the TKE dissipation rate increased. This suggests that the transfer of energy due to bubbles has a very large effect on the TKE budget. Moreover, velocity spectra reveal that the presence of bubbles modifies the length scales of turbulent eddies, which contain, transfer, and dissipate energy.     

Interaction of a turbulent jet and a cocurrent confined incompressible flow

The effects induced in a coaxial circular channel flow by an axisymmetric turbulent jet are investigated for various values of the velocity and radius ratios 0.16m<1 and 2.5f30.9. The problem is solved by means of an e-L model of turbulence [1, 2]. The calculation scheme differs from the usual one for boundary layers, jets and wakes in that the pressure p is assumed to be unknown and is determined by assigning the boundary conditions for the radial velocity component and the transverse gradient of the longitudinal velocity component on both boundaries. On the basis of the calculations and the experimental data of [3, 4] generalized relations are obtained. These make it possible to estimate the turbulence characteristics of an axisymmetric jet in a confined cocurrent flow when the pressure is variable along the flow.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 14–19, September–October, 1986.In conclusion, the author wishes to thank G. S. Glushko for constructive discussion of the results and useful advice.     

Measurement of the turbulent kinetic energy budget of a planar wake flow in pressure gradients

Turbulent kinetic energy (TKE) budget measurements were conducted for a symmetric turbulent planar wake flow subjected to constant zero, favorable, and adverse pressure gradients. The purpose of this study is to clarify the flow physics issues underlying the demonstrated influence of pressure gradient on wake development, and provide experimental support for turbulence modeling. To ensure the reliability of these notoriously difficult measurements, the experimental procedure was carefully designed on the basis of an uncertainty analysis. Three different approaches were applied for the estimate of the dissipation term. An approach for the determination of the pressure diffusion term together with correction of the bias error associated with the dissipation estimate is proposed and validated with the DNS results of Moser et al (J Fluid Mech (1998) 367:255–289). This paper presents the results of the turbulent kinetic energy budget measurement and discusses their implications for the development of strained turbulent wakes.An erratum to this article can be found at     

Relationship between turbulent and kinetic energy in a mixed substance

Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, No. 1, pp. 38–42, January–February, 1992.     

Experimental analysis of transverse intermittency in a turbulent jet flow

The statistical properties of the velocity differences are experimentally investigated in a turbulent jet-flow at moderate Re _λ by X-probe hot wire anemometry measurements. It is found that the traverse velocity components show a more intermittent behavior with respect to the longitudinal ones. This result is obtained by the analysis of the longitudinal and transverse intermittency exponents measured by the Extended Self-Similarity form of scaling, and by the comparison of the longitudinal and transverse velocity difference Probability Distribution Functions. Received: 25 March 1996/Accepted: 15 August 1996     

Investigation of the flow structure in a plane turbulent jet

Results of an experimental investigation of a plane, submerged air jet are elucidated. The distribution of the mean velocity, the longitudinal and transverse velocity component pulsations, the tangential friction stress, and the correlation coefficient in jet cross sections are presented. The results of measurements are compared with the data of other authors.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 176–179, July–August, 1970.     

Numerical prediction of turbulent flow structure generated by a synthetic cross‐jet into a turbulent boundary layer

A detailed numerical study using large‐eddy simulation (LES) and unsteady Reynolds‐averaged Navier–Stokes (URANS) was undertaken to investigate physical processes that are engendered in the injection of a circular synthetic (zero‐net mass flux) jet in a zero pressure gradient turbulent boundary layer. A complementary study was carried out and was verified by comparisons with the available experimental data that were obtained at corresponding conditions with the aim of achieving an improved understanding of fluid dynamics of the studied processes. The computations were conducted by OpenFOAM C++, and the physical realism of the incoming turbulent boundary layer was secured by employing random field generation algorithm. The cavity was computed with a sinusoidal transpiration boundary condition on its floor. The results from URANS computation and LES were compared and described qualitatively and quantitatively. There is a particular interest for acquiring the turbulent structures from the present numerical data. The numerical methods can capture vortical structures including a hairpin (primary) vortex and secondary structures. However, the present computations confirmed that URANS and LES are capable of predicting current flow field with a more detailed structure presented by LES data as expected. Copyright © 2011 John Wiley & Sons, Ltd.     

Propagation of an underexpanded turbulent jet in a parallel supersonic flow

The characteristics of supersonic off-design turbulent jets discharging into parallel supersonic flows are investigated numerically using approximate equations for the effective turbulent viscosity which are derived by analysis of the experimental data.     

Coherent contribution to turbulent mixing of a jet in cross flow

In this work, an experimental study of a jet issuing in cross flow is reported. Support is offered to a theory on the origin of Karman-like vortices, in the wake of jets issuing in boundary layers. Further, a coherent structure with strong contribution to the energetics of the flow field is identified here. This structure has not been reported earlier, according to the present authors' knowledge, and it is shown to be related to engulfment of external fluid at the bottom of the jet, and to interesting stochastic and spectral characteristics of the flow field. Three-dimensional plots of the coherent quantities, based on experimental data, reveal a double-helical morphology of the coherent structure. The same morphology has recently been proposed for the far field of jets issuing in stagnant fluid (i.e., without cross flow). The results of this study are expected to support theoretical and numerical work on jets issuing in cross flow.     

Measurement of the turbulent flow field in a free-surface jet of water

Measurements of the mean velocity and turbulence intensity are presented for a rectangular jet of water ejecting into a gaseous ambient. Data are reported for streamwise locations up to 30 nozzle widths from the discharge and spanwise locations covering the inner 80% of the jet width. The flow conditions at the nozzle discharge were controlled by using different nozzle designs (parallel-plate and converging) and flow manipulators (wire grid and screens). The results track the mean velocity and turbulence intensity profiles with streamwise distance, highlighting changes in both the profile shapes and magnitudes for both measured quantities. Independent of nozzle configuration, the mean velocity profile was shown to be most nonuniform and the turbulence intensity most nonhomogeneous at the nozzle discharge. With increasing streamwise distance, the mean velocity profile underwent a gradual transition to a completely uniform condition, while the turbulence field decayed and became homogeneous. The rate of viscous dissipation was shown to depend strongly on the nozzle exit condition. This work was supported by the National Science Foundation under grant numbers CTS-8912831 and CTS-9307232     

Interaction of a gas-droplet turbulent jet with a cocurrent high-velocity high-temperature gas flow

A mathematical model and a method for calculating a gas-droplet turbulent jet with allowance for velocity nonequilibrium and virtual mass of the condensed phase during turbulent fluctuations and also heat and mass transfer within the three-temperature scheme are developed. Methodical calculations are performed. The results of these calculations are in reasonable agreement with available experimental data. The structure of the gas-droplet jet in a cocurrent high-velocity high-temperature gas flow is studied by numerical methods. The ratio of intensities of heat and mass transfer between the phases and turbulent diffusion transfers of substances is found to be different at the initial, transitional, and basic segments of the jet. This difference is responsible for the nonmonotonic axial distribution of vapor density and the lines of the halved mass flow of the condensed phase. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 3, pp. 85–94, May–June, 2008.     

Mean flow characteristics of a turbulent plane jet with buoyancy induced curvature

An experimental investigation of heated vertical and inclined plane air jets discharged into quiescent surroundings is described. A unique feature of this data is that Pilot tube measurements were used to define the mean trajectory of the inclined jets so that subsequent hot-wire traverses could be made normal to the curved path. While the mean velocity and temperature profiles are self-similar for the range of exit conditions studied, other aspects of the mean jet development depend on the exit Reynolds and Froude numbers, or the discharge angle. It is noted that variations between this study and other published data suggest further measurements of this flow situation are needed, with particular attention to specific features of the jet apparatus and ambient surroundings, and to the exit Reynolds number. Presently with Dept. of Mechanical Engineering, University of Alexandria, Alexandria, Egypt     

Control of turbulent separated flow over a backward-facing step by local forcing

An experimental study was made of the flow over a backward-facing step. Excitations were given to separated flow by means of a sinusoidally oscillating jet issuing from a thin slit near the separation line. The Reynolds number based on the step height (H) varied 13000 Re _H 33000. Effect of local forcing on the flow structure was scrutinized by altering the forcing amplitude (0 A ₀ 0.07) and forcing frequency (0 St _H 5.0). Small localized forcing near the separation edge enhanced the shear-layer growth rate and produced a large roll-up vortex at the separation edge. A large vortex in the shear layer gave rise to a higher rate of entrainment, which lead to a reduction in reattachment length as compared to the unforced flow. The normalized minimum reattachment length (x _r )_min/x _x0 was obtained at St 0.01. The most effective forcing frequency was found to be comparable to the shedding frequency of the separated shear layer.List of symbols a ₀ forcing amplitude=(Q _forced–Q _unforced)/U ₀ - AR aspect ratio=W/H - C _p wall-pressure coefficient=(P-P ₀)/(l/2) U ₀ ² - ER expansion ratio=(2H+H)/2H - f _f forcing frequency, Hz - f _s shedding frequency, Hz - g slit width = 1.0 ± 0.1 mm - H step height = 50 mm - P wall-static pressure, Pa - P ₀ wall-static pressure at x/H= -2.0, Pa - Q _forced total velocity measured at reference position for forced flow, m/s - Q _unforced total velocity measured at reference position for unforced flow, m/s - Re _H Reynolds number based on H and U ₀,= U ₀ H/v - St _H Reduced forcing frequency, Strouhal number = f _f H/U ₀ - St Reduced forcing frequency based on the momentum thickness = f _f /U ₀ - U, V streamwise and vertical time-mean velocity, m/s - u streamwise fluctuation velocity, m/s - U ₀ free-stream velocity, m/s - r.m.s. intensity of streamwise velocity fluctuation, m/s - x _r reattachment length, m - X _{r 0} reattachment length for A ₀ = 0, m - x, y, z distance of streamwise, vertical and spanwise respectively, m - W width of test section = 625 mm Greek symbols boundary-layer thickness, cm - ^* displacement thickness, cm - _p forward-flow time fraction - density of air for measurement, kg/m³ - v kinematic viscosity of air for measurement, m²/s - momentum thickness, cm