Turbulent Wake of a Stack and the Influence of Velocity Ratio

2006 ◽  
Author(s):  
M. S. Adaramola ◽  
D. Sumner ◽  
D. J. Bergstrom
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Reynolds Shear Stress ◽  
Velocity Ratio ◽  
Cross Flow ◽  
Turbulent Wake ◽  
Mean Velocity ◽  
Distinct Structure ◽  
Flow Reynolds Number ◽  
Cross Flow Velocity

The effect of the jet-to-cross-flow velocity ratio, R, on the turbulent wake of a cylindrical stack of AR = 9 was investigated with two-component thermal anemometry. The cross-flow Reynolds number was ReD = 2.3×104, the jet Reynolds number ranged from Red = 7×103 to 4.6×104, and R was varied from 0 to 3. The stack was partially immersed in a flat-plate turbulent boundary layer, with a boundary layer thickness-to-height ratio of δ/H = 0.5 at the location of the stack. The flow around the stack was broadly classified into three flow regimes depending on the value of R, which were the downwash (R < 0.5), cross-wind dominated (0.5 < R < 1.5), and jet-dominated (R > 1.5) regimes. Each flow regime had a distinct structure to the mean velocity (streamwise and wall-normal directions), turbulence intensity (streamwise and wall-normal directions), and Reynolds shear stress fields.

Wall region of a relaxing three-dimensional incompressible turbulent boundary layer

1978 ◽  
Vol 85 (1) ◽  
pp. 33-56 ◽  
Author(s):  
K. S. Hebbar ◽  
W. L. Melnik
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Velocity Ratio ◽  
Three Dimensional ◽  
Cross Flow ◽  
Wall Region ◽  
Mean Flow ◽  
Mean Velocity ◽  
Flow Angle ◽  
The Mean

An experimental investigation was conducted at selected locations in the wall region of a three-dimensional turbulent boundary layer relaxing in a nominally zero external pressure gradient behind a transverse hump (in the form of a 30° swept, 5 ft chord, wing-type model) faired into the side wall of a low-speed wind tunnel. The boundary layer (approximately 3·5 in. thick near the first survey station, where the length Reynolds number was 5·5 × 106) had a maximum cross-flow velocity ratio of 0·145 and a maximum cross-flow angle of 21·9° close to the wall. The hot-wire data indicated that the apparent dimensionless velocity profiles in the viscous sublayer are universal and that the wall influence on the hot wire is negligible beyond y+= 5. The existence of wall similarity in the relaxing flow field was confirmed in the form of a log law based on the resultant mean velocity and resultant friction velocity (obtained from the measured skin friction).The smallest collateral region extended from the point nearest to the wall (y+≈ 1) up to y+= 9·7, corresponding to a resultant mean velocity ratio (local to free-stream) of 0·187. The unusual feature of these profiles was the presence of a narrow region of slightly decreasing cross-flow angle (1° or less) that extended from the point of maximum cross-flow angle down to the outer limit of the collateral region. A sublayer analysis of the flow field using the measured local transverse pressure gradient slightly overestimated the decrease in cross-flow angle. It is concluded that, in the absence of these gradients, the skewing of the flow could have been much more pronounced practically down to the wall (limited only by the resolution of the sensor), implying a near-wallnon-collateralflow field consistent with the equations of motion in the neighbourhood of the wall.The streamwise relaxation of the mean flow field based on the decay of the cross-flow angle was much faster in the inner layer than in the outer layer. Over the stream-wise distance covered, the mean flow in the inner layer and the wall shear-stress vector relaxed to a two-dimensional state in approximately 10 boundary-layer thicknesses whereas the relaxation of the turbulence was slower and was not complete over the same distance.

scholarly journals The effects of arbitrary injection angle and flow conditions on venturi-jet mixer

2012 ◽  
Vol 16 (1) ◽  
pp. 207-221
Author(s):  
S. Sundararaj ◽  
V. Selladurai
Keyword(s):  
Reynolds Number ◽  
Pressure Drop ◽  
Velocity Ratio ◽  
Cross Flow ◽  
Multivariate Linear Regression Analysis ◽  
Jet Injection ◽  
Injection Angle ◽  
Flow Reynolds Number ◽  
Jet Velocity ◽  
The Cross

This paper describes the effect of jet injection angle, cross flow Reynolds number and velocity ratio on entrainment and mixing of jet with incompressible cross flow in venturi-jet mixer. Five different jet injection angles 45o, 60o, 90o, 125o, 135o are tested to evaluate the entrainment of jet and mixing performances of the mixer. Tracer concentration along the downstream of the jet injection, cross flow velocity, jet velocity and pressure drop across the mixer are determined experimentally to characterize the mixing performance of the mixer. The experiments show that the performance of a venturi-jet-mixer substantially improves at high injection angle and can be augmented still by increasing velocity ratio. The jet deflects much and penetrates less in the cross flow as the cross flow Reynolds number is increased. The effect could contribute substantially to the better mixing index with moderate pressure drop. Normalized jet profile, concentration decay, jet velocity profile are computed from equations of conservation of mass, momentum and concentration written in natural co-ordinate systems. The comparison between the experimental and numerical results confirms the accuracy of the simulations. Correlations for jet trajectory and entrainment ratio of the mixer are obtained by multivariate-linear regression analysis using power law.

On the structure of jets in a crossflow

1985 ◽  
Vol 157 ◽  
pp. 163-197 ◽  
Author(s):  
J. Andreopoulos
Keyword(s):  
Boundary Layer ◽  
Velocity Ratio ◽  
Cross Flow ◽  
Sampling Technique ◽  
Reynolds Numbers ◽  
Jet Flow ◽  
Turbulent Jet ◽  
Turbulent Jet Flow ◽  
Cross Flow Velocity ◽  
Intermittency Factor

Spectral analysis and flow visualization are presented for various velocity ratios and Reynolds numbers of a jet issuing perpendicularly from a developing pipe flow into a crossflow. The results are complete with conditional averages of various turbulent quantities for one jet-to-cross-flow velocity ratio R of 0.5. A unique conditional-sampling technique separated the contributions from the turbulent jet flow, the irrotational jet flow, the turbulent crossflow and the irrotational crossflow by using two conditioning functions simultaneously. The intermittency factor profiles indicate that irrotational cross-flow intrudes into the pipe but does not contribute to the average turbulent quantities, while the jet-pipe irrotational flow contributes significantly to them in the region above the exit where the interaction between the boundary-layer eddies and those of the pipe starts to take place. Further downstream, the contributions of the oncoming boundary-layer eddies to the statistical averages reduce significantly. The downstream development depends mainly on the average relative eddy sizes of the interacting turbulent fields.

Effects of alternating elliptical chamber on jet impingement heat transfer in vane leading edge under different cross-flow conditions

2021 ◽  
pp. 1-17
Author(s):  
K. Xiao ◽  
J. He ◽  
Z. Feng
Keyword(s):  
Heat Transfer ◽  
Velocity Ratio ◽  
Cross Flow ◽  
Leading Edge ◽  
Longitudinal Vortices ◽  
Impingement Jet ◽  
Flow And Heat Transfer ◽  
Impingement Heat Transfer ◽  
Cross Flow Velocity ◽  
The Cross

ABSTRACT This paper proposes an alternating elliptical impingement chamber in the leading edge of a gas turbine to restrain the cross flow and enhance the heat transfer, and investigates the detailed flow and heat transfer characteristics. The chamber consists of straight sections and transition sections. Numerical simulations are performed by solving the three-dimensional (3D) steady Reynolds-Averaged Navier–Stokes (RANS) equations with the Shear Stress Transport (SST) k– $\omega$ turbulence model. The influences of alternating the cross section on the impingement flow and heat transfer of the chamber are studied by comparison with a smooth semi-elliptical impingement chamber at a cross-flow Velocity Ratio (VR) of 0.2 and Temperature Ratio (TR) of 1.00 in the primary study. Then, the effects of the cross-flow VR and TR are further investigated. The results reveal that, in the semi-elliptical impingement chamber, the impingement jet is deflected by the cross flow and the heat transfer performance is degraded. However, in the alternating elliptical chamber, the cross flow is transformed to a pair of longitudinal vortices, and the flow direction at the centre of the cross section is parallel to the impingement jet, thus improving the jet penetration ability and enhancing the impingement heat transfer. In addition, the heat transfer in the semi-elliptical chamber degrades rapidly away from the stagnation region, while the longitudinal vortices enhance the heat transfer further, making the heat transfer coefficient distribution more uniform. The Nusselt number decreases with increase of VR and TR for both the semi-elliptical chamber and the alternating elliptical chamber. The alternating elliptical chamber enhances the heat transfer and moves the stagnation point up for all VR and TR, and the heat transfer enhancement is more obvious at high cross-flow velocity ratio.

The accelerated fully rough turbulent boundary layer

1977 ◽  
Vol 82 (3) ◽  
pp. 507-528 ◽  
Author(s):  
Hugh W. Coleman ◽  
Robert J. Moffat ◽  
William M. Kays
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Stress Tensor ◽  
Skin Friction ◽  
Shape Factor ◽  
Reynolds Shear Stress ◽  
Smooth Wall ◽  
Mean Velocity

The behaviour of a fully rough turbulent boundary layer subjected to favourable pressure gradients both with and without blowing was investigated experimentally using a porous test surface composed of densely packed spheres of uniform size. Measurements of profiles of mean velocity and the components of the Reynolds-stress tensor are reported for both unblown and blown layers. Skin-friction coefficients were determined from measurements of the Reynolds shear stress and mean velocity.An appropriate acceleration parameterKrfor fully rough layers is defined which is dependent on a characteristic roughness dimension but independent of molecular viscosity. For a constant blowing fractionFgreater than or equal to zero, the fully rough turbulent boundary layer reaches an equilibrium state whenKris held constant. Profiles of the mean velocity and the components of the Reynolds-stress tensor are then similar in the flow direction and the skin-friction coefficient, momentum thickness, boundary-layer shape factor and the Clauser shape factor and pressure-gradient parameter all become constant.Acceleration of a fully rough layer decreases the normalized turbulent kinetic energy and makes the turbulence field much less isotropic in the inner region (forFequal to zero) compared with zero-pressure-gradient fully rough layers. The values of the Reynolds-shear-stress correlation coefficients, however, are unaffected by acceleration or blowing and are identical with values previously reported for smooth-wall and zero-pressure-gradient rough-wall flows. Increasing values of the roughness Reynolds number with acceleration indicate that the fully rough layer does not tend towards the transitionally rough or smooth-wall state when accelerated.

scholarly journals Global energy fluxes in turbulent channels with flow control

2018 ◽  
Vol 857 ◽  
pp. 345-373 ◽  
Author(s):  
Davide Gatti ◽  
Andrea Cimarelli ◽  
Yosuke Hasegawa ◽  
Bettina Frohnapfel ◽  
Maurizio Quadrio
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Energy Dissipation ◽  
Velocity Field ◽  
Flow Control ◽  
Reynolds Shear Stress ◽  
Power Input ◽  
Energy Fluxes ◽  
Mean Velocity ◽  
The Mean

This paper addresses the integral energy fluxes in natural and controlled turbulent channel flows, where active skin-friction drag reduction techniques allow a more efficient use of the available power. We study whether the increased efficiency shows any general trend in how energy is dissipated by the mean velocity field (mean dissipation) and by the fluctuating velocity field (turbulent dissipation). Direct numerical simulations (DNS) of different control strategies are performed at constant power input (CPI), so that at statistical equilibrium, each flow (either uncontrolled or controlled by different means) has the same power input, hence the same global energy flux and, by definition, the same total energy dissipation rate. The simulations reveal that changes in mean and turbulent energy dissipation rates can be of either sign in a successfully controlled flow. A quantitative description of these changes is made possible by a new decomposition of the total dissipation, stemming from an extended Reynolds decomposition, where the mean velocity is split into a laminar component and a deviation from it. Thanks to the analytical expressions of the laminar quantities, exact relationships are derived that link the achieved flow rate increase and all energy fluxes in the flow system with two wall-normal integrals of the Reynolds shear stress and the Reynolds number. The dependence of the energy fluxes on the Reynolds number is elucidated with a simple model in which the control-dependent changes of the Reynolds shear stress are accounted for via a modification of the mean velocity profile. The physical meaning of the energy fluxes stemming from the new decomposition unveils their inter-relations and connection to flow control, so that a clear target for flow control can be identified.

Flow Characteristics of Transitional Boundary Layers on an Airfoil in Wakes

2000 ◽  
Vol 122 (3) ◽  
pp. 522-532 ◽  
Author(s):  
H. Lee ◽  
S.-H. Kang
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Skin Friction ◽  
Velocity Fluctuation ◽  
Plate Boundary ◽  
Flow Characteristics ◽  
Turbulence Models ◽  
Transitional Region ◽  
Mean Velocity ◽  
Measured Velocity

Transition characteristics of a boundary layer on a NACA0012 airfoil are investigated by measuring unsteady velocity using hot wire anemometry. The airfoil is installed in the incoming wake generated by an airfoil aligned in tandem with zero angle of attack. Reynolds number based on the airfoil chord varies from 2.0×105 to 6.0×105; distance between two airfoils varies from 0.25 to 1.0 of the chord length. To measure skin friction coefficient identifying the transition onset and completion, an extended wall law is devised to accommodate transitional flows with pressure gradient and nonuniform inflows. Variations of the skin friction are quite similar to that of the flat plate boundary layer in the uniform turbulent inflow of high intensity. Measured velocity profiles are coincident with families generated by the modified wall law in the range up to y+=40. Turbulence intensity of the incoming wake shifts the onset location of transition upstream. The transitional region becomes longer as the airfoils approach one another and the Reynolds number increases. The mean velocity profile gradually varies from a laminar to logarithmic one during the transition. The maximum values of rms velocity fluctuations are located near y+=15-20. A strong positive skewness of velocity fluctuation is observed at the onset of transition and the overall rms level of velocity fluctuation reaches 3.0–3.5 in wall units. The database obtained will be useful in developing and evaluating turbulence models and computational schemes for transitional boundary layer. [S0098-2202(00)01603-5]

scholarly journals Influence of Endwall Flow on Airfoil Suction Surface Mid-Height Boundary Layer Development in a Turbine Cascade

1982 ◽  
Author(s):  
O. P. Sharma ◽  
R. A. Graziani
Keyword(s):  
Boundary Layer ◽  
Reynolds Shear Stress ◽  
Cross Flow ◽  
Prediction Method ◽  
Conservation Equation ◽  
Turbine Cascade ◽  
Suction Surface ◽  
Extra Term ◽  
Boundary Layer Development ◽  
Layer Development

This paper presents the results of an analysis to assess the influence of cascade passage endwall flow on the airfoil suction surface mid-height boundary layer development in a turbine cascade. The effect of the endwall flow is interpreted as the generation of a cross flow and a cross flow velocity gradient in the airfoil boundary layer, which results in an extra term in the mass conservation equation. This extra term is shown to influence the boundary layer development along the mid-height of the airfoil suction surface through an increase in the boundary layer thickness and consequently an increase in the mid-height losses, and a decrease in the Reynolds shear stress, mixing length, skin friction, and Stanton number. An existing two-dimensional differential boundary layer prediction method, STAN-5, is modified to incorporate the above two effects.

Effect of velocity ratio on the interaction between plasma synthetic jets and turbulent cross-flow

2019 ◽  
Vol 865 ◽  
pp. 928-962 ◽  
Author(s):  
Haohua Zong ◽  
Marios Kotsonis
Keyword(s):  
Boundary Layer ◽  
High Speed ◽  
Velocity Ratio ◽  
Vortex Pair ◽  
Synthetic Jets ◽  
Low Energy ◽  
Vortex Rings ◽  
Wall Region ◽  
Mean Velocity ◽  
Counter Rotating Vortex Pair

Plasma synthetic jet actuators (PSJAs) are particularly suited for high-Reynolds-number, high-speed flow control due to their unique capability of generating supersonic pulsed jets at high frequency (${>}5$  kHz). Different from conventional synthetic jets driven by oscillating piezoelectric diaphragms, the exit-velocity variation of plasma synthetic jets (PSJs) within one period is significantly asymmetric, with ingestion being relatively weaker (less than $20~\text{m}~\text{s}^{-1}$) and longer than ejection. In this study, high-speed phase-locked particle image velocimetry is employed to investigate the interaction between PSJAs (round exit orifice, diameter 2 mm) and a turbulent boundary layer at constant Strouhal number (0.02) and increasing mean velocity ratio ($r$, defined as the ratio of the time-mean velocity over the ejection phase to the free-stream velocity). Two distinct operational regimes are identified for all the tested cases, separated by a transition velocity ratio, lying between $r=0.7$ and $r=1.0$. At large velocity and stroke ratios (first regime, representative case $r=1.6$), vortex rings are followed by a trailing jet column and tilt downstream initially. This downstream tilting is transformed into upstream tilting after the pinch-off of the trailing jet column. The moment of this transformation relative to the discharge advances with decreasing velocity ratio. Shear-layer vortices (SVs) and a hanging vortex pair (HVP) are identified in the windward and leeward sides of the jet body, respectively. The HVP is initially erect and evolves into an inclined primary counter-rotating vortex pair ($p$-CVP) which branches from the middle of the front vortex ring and extends to the near-wall region. The two legs of the $p$-CVP are bridged by SVs, and a secondary counter-rotating vortex pair ($s$-CVP) is induced underneath these two legs. At low velocity and stroke ratios (second regime, representative case $r=0.7$), the trailing jet column and $p$-CVP are absent. Vortex rings always tilt upstream, and the pitching angle increases monotonically with time. An $s$-CVP in the near-wall region is induced directly by the two longitudinal edges of the ring. Inspection of spanwise planes ($yz$-plane) reveals that boundary-layer energization is realized by the downwash effect of either vortex rings or $p$-CVP. In addition, in the streamwise symmetry plane, the increasing wall shear stress is attributed to the removal of low-energy flow by ingestion. The downwash effect of the $s$-CVP does not benefit boundary-layer energization, as the flow swept to the wall is of low energy.

Shear Layer Development, Separation, and Stability Over a Low-Reynolds Number Airfoil

2018 ◽  
Vol 140 (7) ◽  
Author(s):  
Paul Ziadé ◽  
Mark A. Feero ◽  
Philippe Lavoie ◽  
Pierre E. Sullivan
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Base Flow ◽  
Mean Velocity ◽  
Laminar Separation ◽  
Low Reynolds ◽  
Layer Development

The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large eddy simulation (LES). Two angles of attack (AOAs) were considered: 5 deg and 12 deg. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 deg, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis (LSA) of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. LSA suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.

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