scholarly journals The Effects of Stratification on Flow Separation

2005 ◽  
Vol 62 (7) ◽  
pp. 2618-2625 ◽  
Author(s):  
Maarten H. P. Ambaum ◽  
David P. Marshall
Keyword(s):  
Boundary Layer ◽  
Aspect Ratio ◽  
Linear Theory ◽  
Flow Separation ◽  
Nonlinear Deformation ◽  
Stratified Flow ◽  
Two Dimensional ◽  
Critical Aspect ◽  
Streamline Curvature ◽  
Scaling Relationships

Abstract Separation of stratified flow over a two-dimensional hill is inhibited or facilitated by acceleration or deceleration of the flow just outside the attached boundary layer. In this note, an expression is derived for this acceleration or deceleration in terms of streamline curvature and stratification. The expression is valid for linear as well as nonlinear deformation of the flow. For hills of vanishing aspect ratio a linear theory can be derived and a full regime diagram for separation can be constructed. For hills of finite aspect ratio scaling relationships can be derived that indicate the presence of a critical aspect ratio, proportional to the stratification, above which separation will occur as well as a second critical aspect ratio above which separation will always occur irrespective of stratification.

Turbulent Boundary Layer Heat Transfer on Curved Surfaces

1979 ◽  
Vol 101 (3) ◽  
pp. 521-525 ◽  
Author(s):  
R. E. Mayle ◽  
M. F. Blair ◽  
F. C. Kopper
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Convex Surface ◽  
Concave Surface ◽  
Turbulent Shear ◽  
Two Dimensional ◽  
Streamline Curvature ◽  
Turbulent Shear Stress ◽  
Two Dimensional Flow

Heat transfer measurements for a turbulent boundary layer on a convex and concave, constant-temperature surface are presented. The heat transferred on the convex surface was found to be less than that for a flat surface, while the heat transferred to the boundary layer on the concave surface was greater. It was also found that the heat transferred on the convex surface could be determined by using an existing two-dimensional finite difference boundary layer program modified to take into account the effect of streamline curvature on the turbulent shear stress and heat flux, but that the heat transferred on the concave surface could not be calculated. The latter result is attributed to the transition from a two-dimensional flow to one which contained streamwise, Taylor-Go¨rtler type vortices.

Prediction of Flow Separation in a Diffuser by a Boundary Layer Calculation

1977 ◽  
Vol 99 (2) ◽  
pp. 379-386 ◽  
Author(s):  
Y. Senoo ◽  
M. Nishi
Keyword(s):  
Boundary Layer ◽  
Flow Separation ◽  
Shape Factor ◽  
Limit Relation ◽  
Performance Data ◽  
Separation Point ◽  
Pressure Recovery ◽  
Two Dimensional ◽  
Blockage Factor ◽  
Mean Pressure

From a consideration of flow stability, it is shown that the onset of separation in a diffuser depends upon the local blockage factor. Using performance data for two-dimensional diffusers from the literature, the shape factor of the boundary layer at the separation point Hs is related to the blockage factor Bs at that section, and the formula Hs = 1.8+3.75Bs is deduced as the separation-limit relation. It is proved that this separation-limit relation is also applicable to conical diffusers. Furthermore, a simple theory is derived to evaluate the time-mean pressure recovery in the separated region. Using this method, it is possible to predict whether a separation occurs in a diffuser and to evaluate the pressure recovery.

scholarly journals What Flow Conditions are Conducive to Banner Cloud Formation?

2016 ◽  
Vol 73 (6) ◽  
pp. 2385-2402 ◽  
Author(s):  
Isabelle Prestel ◽  
Volkmar Wirth
Keyword(s):  
Parameter Space ◽  
Flow Separation ◽  
Three Dimensional ◽  
Stratified Flow ◽  
Cloud Formation ◽  
Two Dimensional ◽  
Dimensionless Numbers ◽  
Dimensional Flow ◽  
Flow Past ◽  
Three Dimensional Simulations

Abstract Banner clouds are clouds that are attached to the leeward slope of a steep mountain. Their formation is essentially due to strong Lagrangian uplift of air in the lee of the mountain. However, little is known about the flow regime in which banner clouds can be expected to occur. The present study addresses this question through numerical simulations of flow past idealized orography. Systematic sets of simulations are carried out exploring the parameter space spanned by two dimensionless numbers, which represent the aspect ratio of the mountain and the stratification of the flow. The simulations include both two-dimensional flow past two-dimensional orography and three-dimensional flow past three-dimensional orography. Regarding flow separation from the surface, both the two- and the three-dimensional simulations show the characteristic regime behavior that has previously been found in laboratory experiments for two-dimensional orography. Flow separation is observed in two of the three regimes, namely in the “leeside separation regime,” which occurs preferably for steep mountains in weakly stratified flow, and in the “postwave separation regime,” which requires increased stratification. The physical mechanism for the former is boundary layer friction, while the latter may also occur for inviscid flow. However, flow separation is only a necessary, not sufficient condition for banner cloud formation. The vertical uplift and its leeward–windward asymmetry show that banner clouds cannot form in the two-dimensional simulations. In addition, even in the three-dimensional simulations they can only be expected in a small part of the parameter space corresponding to steep three-dimensional orography in weakly stratified flow.

Simulation of Viscous Diffusion for Extension of the Surface Vorticity Method to Boundary Layer and Separated Flows

1981 ◽  
Vol 23 (3) ◽  
pp. 157-167 ◽  
Author(s):  
D. T. C. Porthouse ◽  
R. I. Lewis
Keyword(s):  
Boundary Layer ◽  
Flow Separation ◽  
Point Vortex ◽  
Fluid Flows ◽  
Reynolds Numbers ◽  
Separated Flows ◽  
Two Dimensional ◽  
Drag Forces ◽  
Boundary Layer Parameter ◽  
Lift And Drag

A numerical method for two-dimensional incompressible viscous fluid flows is tested on the diffusion of a point vortex. It is then applied to the boundary layer to reconstruct the Blasius profile, to demonstrate flow separation, and to simulate turbulence. The significance of Thwaites' boundary layer parameter for flow separation is explained. The Kelvin-Helmholtz instability, which is responsible for two-dimensional turbulence, is represented by the motion of an array of point vortices after an initial disturbance. The formation of the Von Karman vortex street downstream of a circular cylinder is described by computer simulation, and the influence of viscous diffusion is shown. For two different cylinder Reynolds numbers the vortex shedding frequencies and oscillating lift and drag forces are evaluated.

The Departure from Equilibrium of Turbulent Boundary Layers

1968 ◽  
Vol 19 (1) ◽  
pp. 1-19 ◽  
Author(s):  
H. McDonald
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Kinetic Energy ◽  
Stress Distribution ◽  
Boundary Layers ◽  
Turbulent Boundary Layers ◽  
Two Dimensional ◽  
Pressure Distributions ◽  
Momentum Integral ◽  
State Examination

SummaryRecently two authors, Nash and Goldberg, have suggested, intuitively, that the rate at which the shear stress distribution in an incompressible, two-dimensional, turbulent boundary layer would return to its equilibrium value is directly proportional to the extent of the departure from the equilibrium state. Examination of the behaviour of the integral properties of the boundary layer supports this hypothesis. In the present paper a relationship similar to the suggestion of Nash and Goldberg is derived from the local balance of the kinetic energy of the turbulence. Coupling this simple derived relationship to the boundary layer momentum and moment-of-momentum integral equations results in quite accurate predictions of the behaviour of non-equilibrium turbulent boundary layers in arbitrary adverse (given) pressure distributions.

scholarly journals Aspect Ratio Driven Relationship between Nozzle Internal Flow and Supersonic Jet Mixing

Aerospace ◽  
2021 ◽  
Vol 8 (3) ◽  
pp. 78
Author(s):  
Kalyani Bhide ◽  
Kiran Siddappaji ◽  
Shaaban Abdallah
Keyword(s):  
Boundary Layer ◽  
Aspect Ratio ◽  
Nozzle Exit ◽  
Supersonic Jet ◽  
Internal Flow ◽  
Shear Stresses ◽  
Loss Mechanism ◽  
Potential Core ◽  
Jet Mixing ◽  
Aspect Ratios

This work attempts to connect internal flow to the exit flow and supersonic jet mixing in rectangular nozzles with low to high aspect ratios (AR). A series of low and high aspect ratio rectangular nozzles (design Mach number = 1.5) with sharp throats are numerically investigated using steady state Reynolds-averaged Navier−Stokes (RANS) computational fluid dynamics (CFD) with k-omega shear stress transport (SST) turbulence model. The numerical shadowgraph reveals stronger shocks at low ARs which become weaker with increasing AR due to less flow turning at the throat. Stronger shocks cause more aggressive gradients in the boundary layer resulting in higher wall shear stresses at the throat for low ARs. The boundary layer becomes thick at low ARs creating more aerodynamic blockage. The boundary layer exiting the nozzle transforms into a shear layer and grows thicker in the high AR nozzle with a smaller potential core length. The variation in the boundary layer growth on the minor and major axis is explained and its growth downstream the throat has a significant role in nozzle exit flow characteristics. The loss mechanism throughout the flow is shown as the entropy generated due to viscous dissipation and accounts for supersonic jet mixing. Axis switching phenomenon is also addressed by analyzing the streamwise vorticity fields at various locations downstream from the nozzle exit.

Sign in / Sign up

Export Citation Format

Share Document